Fire Science

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Production Credits Chief Executive Officer: Ty Field President: James Homer Chief Product Officer: Eduardo Moura Executive Publisher: Kimberly Brophy Vice President of Sales, Public Safety Group: Matthew Maniscalco Director of Sales, Public Safety Group: Patricia Einstein Executive Acquisitions Editor: William Larkin Associate Managing Editor: Janet Morris

Production Editor: Cindie Bryan Senior Marketing Manager: Brian Rooney Art Development Editor: Joanna Lundeen VP, Manufacturing and Inventory Control: Therese Connell Composition: diacriTech Cover Design: Kristin E. Parker Photo Research and Permissions Associate: Ashley Dos Santos Cover and Title Page Image: Glen E. Ellman Printing and Binding: Courier Companies Cover Printing: Courier Companies Library of Congress Cataloging-in-Publication Data Gann, Richard G., 1944– Principles of fire behavior and combustion / Richard G. Gann, Raymond Friedman.—Fourth edition. pages cm Includes bibliographical references and index.

ISBN-13: 978-0-7637-5717-5 (pbk.)

ISBN-10: 0-7637-5717-9 (pbk.)

1. Fire. 2. Combustion. 3. Flammable materials. 4. Fire prevention. I. Friedman, Raymond. II. Title.

TP265.G36 2015 363.37’7—dc23 2013033274 6048 Printed in the United States of America 17 16 15 14 13 10 9 8 7 6 5 4 3 2 1

BRIEF CONTENTS Chapter 1 Fire Measurement and the SI System of Units Chapter 2 Chemical Elements and Compounds: Atoms and Molecules Chapter 3 Physical and Chemical Change Chapter 4 Flow of Fluids Chapter 5 Heat Transfer Chapter 6 Combustion, Fire, and Flammability Chapter 7 Fire Characteristics: Gaseous Combustibles Chapter 8 Fire Characteristics: Liquid Combustibles Chapter 9 Fire Characteristics: Solid Combustibles Chapter 10 Combustion Products Chapter 11 Smoke and Heat Hazards Chapter 12 Movement of Fire Gases Chapter 13 Fire Fighting Chemicals Chapter 14 Computational Modeling of Fires Appendix A FESHE Correlation Guide Appendix B Imperial and Metric Conversions Glossary Index

TABLE OF CONTENTS Chapter 1 Fire Measurement and the SI System of Units About Measurement Length, Area, and Volume Units Mass and Density Units Time Units Force and Pressure Units Energy and Enthalpy Units Power Units Temperature Units Conversion Factors

Chapter 2 Chemical Elements and Compounds: Atoms and Molecules Atoms Stability of Atoms Atomic Mass and Dimension Molecules and Compounds Chemical Bonds and Valence Organic Chemistry Nomenclature Isomers Ions Free Radicals and Free Atoms

Chapter 3 Physical and Chemical Change States of Matter Characterization of Phases Properties of Gases Properties of Liquids Properties of Solids Physical and Chemical Change Physical Changes Chemical Changes Principle of Combining Proportions Energetics of Chemical Change Chemical Equilibrium and Chemical Kinetics

Chapter 4 Flow of Fluids Laws Governing Motions of a Rigid Body Momentum and Acceleration of a Rigid Body

The Effect of Gravitation on a Rigid Body Potential Energy and Kinetic Energy: Mechanical Work Basic Elements of Fluid Behavior Force and Pressure Viscosity Buoyancy

Chapter 5 Heat Transfer Temperature and Heat Modes of Heat Transfer Conductive Heat Transfer Convective Heat Transfer Radiative Heat Transfer Hazards from Heat Transfer Life Safety Endurance of Structures: Fire Resistance

Chapter 6 Combustion, Fire, and Flammability Combustion Flaming and Nonflaming Combustion Fire Initiation Fire Spread Fire Ventilation Fire Termination Two Examples of Room Fires Flammability Fire Consequences, Hazard, Risk, and Flashover

Chapter 7 Fire Characteristics: Gaseous Combustibles Categorization of Flames Premixed versus Diffusion Flames Laminar versus Turbulent Flames Ignition of Gases Flammability Limits and Propagation Rates of Premixed Flames Flammability Limits Burning Velocity Explosions, Deflagrations, and Detonations Chemical Mechanisms of Combustion of Gases Elementary Chemistry Hydrogen Oxidation Premixed Methane–Oxygen Flame Chemistry Combustion of Larger Hydrocarbon Fuels Specific Hazardous Gases Hydrogen (H2)

Acetylene (C2H2) Methane (CH4) Ethylene (C2H4) Ammonia (NH3)

Chapter 8 Fire Characteristics: Liquid Combustibles Ignition of Liquids: Flash Point, Fire Point, and Autoignition Temperature Burning Rates of Liquid Pools Flame Spread Rates over Liquid Surfaces Hazards of Liquid Fuel Fires

Chapter 9 Fire Characteristics: Solid Combustibles Fire Stages and Metrics Solids versus Gases and Liquids Materials and Products Pyrolysis Ignition to Flaming Combustion Ignition to Nonflaming Combustion Char Formation and Melting Mass Burning and Flame Spread Combustible Solids Cellulosic and Other Natural Materials Synthetic Polymeric Materials Fire Retardants Composite Materials and Furnishings Acid–Base Pairs Metals Exothermic Materials

Chapter 10 Combustion Products Smoke Aerosols General Nature Soot Formation Aerosol Mist Formation Measurement of Aerosol Yields Quantity of Smoke Particles Produced Visibility through Smoke Gaseous Combustion Products CO2 and H2O CO Partially Oxidized Organic Molecules Hydrogen Halides HCN

Nitrogen Oxides Other Combustion Gases Smoke Alarms

Chapter 11 Smoke and Heat Hazards Hazards of Smoke Exposure Toxicity of Prominent Fire Gases Carbon Monoxide Carbon Dioxide Hydrogen Cyanide Hydrogen Chloride and Hydrogen Bromide Nitrogen Oxides Organic Irritants Other Toxic Species Oxygen Deficiency Smoke Toxic Potency Measurement Nonthermal Smoke Damage Thermal Damage The Limiting Hazard Concept

Chapter 12 Movement of Fire Gases Structure of a Fire Plume in the Open Fire Plume under a Ceiling Filling of a Fire Compartment by Smoke Smoke Flow from a Compartment with an Opening Smoke Movement in Buildings

Chapter 13 Fire Fighting Chemicals Categories of Fire Suppressants Aqueous Agents Water Enhanced Water Aqueous Foams Nonaqueous Agents Inert Gases Active Halogenated Agents Dry Chemical Agents Special Considerations for Fire Extinguishment Extinguishment of Flowing Gas Flames Extinguishment of a Shallow Liquid Fuel Spill Fire Extinguishment of a Deep Tank Liquid Fuel Fire Ultrafast Extinguishment of Fires

Chapter 14

Computational Modeling of Fires Types of Models Users of Models Zone Models The Zone Approximation The Consolidated Model of Fire and Smoke Transport Zone Model Field Models Characteristics of Field Models The Fire Dynamics Simulator Computational Modeling and the Limiting Hazard Concept Values and Limitations of Models

Appendix A FESHE Correlation Guide

Appendix B Imperial and Metric Conversions

Glossary Index

INSTRUCTOR, STUDENT, AND TECHNOLOGY RESOURCES Instructor Resources Instructor’s ToolKit CD Preparing for class is easy with the resources on this CD, including: •

PowerPoint Presentations Provides you with a powerful way to make presentations that are educational and engaging to your students. These slides can be modified and edited to meet your needs.

• Lesson Plans Provides you with complete, ready-to-use lesson plans that include all of the topics covered in the text. Offered in Word documents, the lesson plans can be modified and customized to fit your course.

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Test Bank Contains multiple-choice questions, and allows you to create tailor-made classroom tests and quizzes quickly and easily by selecting, editing, organizing, and printing a test along with an answer key, including page references to the text.

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Image and Table Bank Provides you with a selection of the most important images and tables found in the textbook. You can use them to incorporate more images into the PowerPoint presentations, make hand outs, or enlarge a specific image for further discussion.

Technology Resources Navigate Course Manager Combining our robust teaching and learning materials with an intuitive and customizable learning platform, Navigate Course Manager gives you the tools to build a solid, knowledgeable foundation with world-class content. With Navigate Course Manager, learning is no longer confined to the four walls of the classroom. Now you can learn anytime and anywhere, when it is ideal for you.

World-class content joins instructionally sound design in a user-friendly online interface to give students a truly interactive, engaging learning experience with: •

eFolio, an engaging eBook that offers study advantages far beyond the print textbook. With anytime access to complete textbook content, interactive eBooks allow students the flexibility to navigate between the text and enhanced activities for greater understanding of core concepts.

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Course Management tools that simplify the management and delivery of curriculum and assessments to students enabling anytime, anywhere access to learning. Instructors can track real-time progress, manage assignments, and view results in the grade book.

ACKNOWLEDGMENTS Over the course of my career in fire science, I have benefited from numerous collaborations, initially with colleagues at the U.S. Naval Research Laboratory, mostly at the National Institute of Standards and Technology (formerly the National Bureau of Standards), and continuously from interactions with fire professionals both in the United States and around the world. Special thanks to Rick Peacock for his assistance with resources for computer modeling. Preparing the latest edition of this book was a pleasure due to the quality of Ray Friedman’s earlier edition. During the preparation of this manuscript (and the years preceding it), I have benefited from the patience, love, and support of my wife, Debbie Gann.

Richard G. Gann, PhD Montgomery Village, MD September 2013

Reviewers and Contributors Brian Bagwell, Psy.D. Metropolitan State University of Denver Denver, Colorado Timothy W. Baker Lansing Community College Lansing, Michigan David A. Budde EMS & Fire Science Technology Director Lake Land College Mattoon, Illinois Melvin Byrne Virginia Department of Fire Programs Fairfax, Virginia Kevin L. Hammons IRIS Fire Investigations Englewood, Colorado Gary Johnson Central Ohio Technical College Newark, Ohio Sherry LaQua-Hanchett Portland Community College Portland, Oregon Stephen S. Malley Weatherford College Public Safety Professions Weatherford, Texas Byron Matthews Cheyenne Fire and Rescue Cheyenne, Wyoming Larry Perez, Program Director

New Mexico State University at Dona Ana Las Cruces, New Mexico Mike Richardson St Matthews Fire Department Louisville, Kentucky Christopher M. Riley Portsmouth Fire, Rescue, and Emergency Services Portsmouth, Virginia John Shafer Green Maltese Greencastle Fire Department Greencastle, Indiana Douglas Smith Portland Community College Portland, Oregon Robert Solomon, PE NFPA Quincy, Massachusetts Kenneth Staelgraeve Macomb Community College Clinton Township, Michigan Peter J. Struble Practitioner in Residence Fire Science Program Wallingford, Connecticut Michael Wolever Toledo Fire and Rescue (ret.)

Bowling Green State University Bowling Green, Ohio

INTRODUCTION How Do Chemistry and Physics Relate to Fire Protection?

The Evolution of Fire and Fire Science Personal and public safety is enhanced by familiarity with the science of how fires start, grow, and are controlled. This is the premise of this book. Over the millennia of human existence, the evolution of this understanding has curtailed the impact of unwanted fire on a societal scale. Our application of this knowledge today can reduce the threat to our own lives and possessions. As recently as 1985, unwanted fires in the United States cost 6200 lives, 28,400 injuries, and $14.8 billion in property damage. Modern fire science has reduced these numbers, but in 2010, unwanted fires in the United States still cost 3100 lives, 17,700 civilian injuries, 72,000 fire fighter injuries, $11.6 billion in property damage, and 10,000 square miles of burned forests and wildlands [1, 2]. The total cost to the economy was a staggering $350 billion.

Fire is older than civilization. The first fuels, in the form of vegetation, appeared on our planet some 500 million years ago, and the first wildfires were ignited by lightning and volcanoes. Our first hominid ancestors appeared approximately 5 million years ago. These nomadic creatures lived through great destruction from the uncontrolled spread of fires. They also learned by observation that rain falling on a fire could limit harm to them and ignition of the surrounding plant life [3, 4].

It was not until about 400,000 years ago that our forebears learned how to get hold of something burning. They found great value in controlled fire: it provided warmth, made food easier to eat, and kept away wild animals. They still did not know how to start a fire, so skill at keeping a fire burning at all times was schooled and valued, and someone who allowed a fire to go out was subject to punishment.

Less than 1000 generations ago, the species we refer to as Cro-Magnon had begun living in established clusters and locations. Within their small enclaves, they raised crops and engaged in hunting and gathering. More importantly, they had learned how to start fires. Fire was used to clear land for farming, to capture and keep livestock, and to bake clay and work metal. The development of more permanent homes and greater possessions also meant these early humans now had more to lose from a fire.

As the centuries passed, the number of large, dense, urban centers grew; being constructed of wood and other flammable or combustible materials, these nascent cities were especially vulnerable to fire. In what is now Europe, more than 40 recorded conflagrations occurred between 31 BC and 410 AD. Successive cultures developed penal codes to deter arson, building codes to mitigate large fires, and permanent water supplies and fire brigades to fight fires.

And yet, inexorably, as recently as the beginning of the 20th century, conflagrations continued to destroy large portions of cities, such as Baltimore (Figure I-1) and Chicago. Rapid fire growth in single buildings, such as the 1911 Triangle Waist Company in New York, continued to claim many lives.

The latter part of the 19th century had seen the emergence of modern chemistry as a science that could be used to explain many natural phenomena. By this time, Newton’s 17th-century formulation of the basic laws of physics had also been expanded greatly. The field of combustion science was born, and fuel-burning engines were developed to provide power to cities and motorized transport.

At this point in human history, it was realized that fire is a form of combustion. As such, fire is a chemical process that behaves according to the laws of physics. It follows that understanding of the pertinent chemical and physical principles provides the basis for preventing and controlling fire.

Figure I-1 Map of the Baltimore, Maryland, area destroyed by the 1904 fire [5].

Map courtesy of the University of Texas Libraries, The University of Texas at Austin.

The chemistry of fire encompasses the chemical make-up of the items that burn, the chemical reactions that give rise to flames and other fire products, the chemical reactions that retard or suppress burning, and the harmful chemical reactions of the fire products with people and property. Certain physical principles are also important in the understanding of fire. Notably, the laws governing momentum and energy apply. They underlie the rate of mixing of air into the flames, the buoyant rise of the fire gases to the ceiling and the subsequent motion under the ceiling, the escape of smoke from a burning room into connecting compartments, and the rate at which heat is transferred from the flames to not yet ignited material or to people trying to escape the fire.

The Role and Contents of This Book This text introduces the scientific concepts and principles needed to understand fire and its consequences, and how it is controlled. In essence, it provides the basics of what could be called fire literacy. The text is directed at people who are embarking on a fire science curriculum and at those who would simply like to learn more about this fascinating, yet threatening, phenomenon. It is intended to stimulate thinking about such questions as these: • • • •

What is a fire?

How do fires start, grow, and go out?

Which fire hazards are of concern?

What can a computer model of a fire do?

Principles of Fire Behavior and Combustion, Fourth Edition is a highly expanded and updated successor to Principles of Fire Protection Chemistry and Physics, Third Edition. It addresses all the course objectives and learning outcomes for the National Fire Academy FESHE Model Curriculum Associate’s (Core) course called Fire Behavior and Combustion.

The first five chapters of this text are an elementary review of the formalism and fundamentals of chemistry and physics that govern fire behavior. Effort has been made to show the relevance of what might appear, at first glance, to be esoteric material. Many of the specialized words in the text are defined in the glossaries at the end of each chapter and again at the end of the text. Even if you are already well versed in chemistry and physics, you should at least skim this material; it will both serve as a refresher and establish a common basis for the material to follow. If you have never taken courses in chemistry or physics, you might benefit from obtaining an introductory textbook on chemistry (e.g., C. H. Corwin’s Introductory Chemistry: Concepts and Critical Thinking, sixth edition, Pearson Prentice Hall, 2010) and physics (e.g., D. Halliday, R. Resnick, and J. J. Walker’s Fundamentals of Physics Extended, ninth edition, Wiley, New York, 2010). I also found that a web search was useful in filling in some gaps.

The subsequent nine chapters of this text describe combustion; the fire characteristics of materials (gases, liquids, and solids); the properties, movement, and effects of combustion products (temperature, smoke, toxicity, and corrosivity); and fire extinguishing agents and procedures. The principles behind the hardware and tactics of firefighting are included, but the applications are left to

other references, examples of which are cited. The text provides information on special situations that might confront the fire fighter or be of interest to the fire protection engineer (e.g., spontaneous ignition, exothermic materials, and fires in abnormal environments) and on the computer modeling of fires.

In the new edition, the text has been changed to be friendlier to the reader who is encountering many of these subjects for the first time. Each chapter contains introductory material that identifies the importance and context of the chapter content, as well as the capabilities the reader will develop from that content. There are new examples relevant to fires, additional data and figures to reinforce the text, and extensive references for those who might want to learn more about any of the subjects.

Compared to the third edition, the fourth edition contains new material throughout.

• This introduction outlines the history of fire and its role in society, the early realization that fire is a chemical phenomenon, and indicates how the understanding of fire principles leads to enhanced ability to prevent and control unwanted fires.

• Chapter 1, “Fire Measurement and the SI System of Units” now explains how different sets of units arose, why it is important to be able to convert among alternate units for the same property, how to report and use numbers to the degree of precision that is appropriate; and why fires are characterized by enthalpy rather than energy.

• Chapter 2, “Chemical Elements and Compounds: Atoms and Molecules,” now explains how the molecules in common materials are named and shows the different ways that molecules can be portrayed, depending on the properties that the viewer needs to see.

• Chapter 3, “Physical and Chemical Change,” has additional information on the states of matter; explanations of how molecular behavior leads to the material properties we sense; extended descriptions of phase changes, with examples related to fires; and presentation of the equivalence ratio, which determines the heat generation and the nature of combustion products from a fire.

• Chapter 4, “Flow of Fluids,” now contains an expanded presentation of Newton’s laws of motion and gravitation; calculation of pressure drops in a standpipe and a stairwell; and a revised discussion of viscosity, buoyancy, and turbulence, including their roles in fires.

• Chapter 5, “Heat Transfer,” now contains expanded presentations of conduction, convection, and thermal radiation; and a new section on burn and structural hazards.

• Chapter 6, “Combustion, Fire, and Flammability,” is a new chapter. It presents the National Fire Incident Reporting System (NFIRS) and how it enables identifying the most common and the most dangerous fire types; the fire tetrahedron; the definitions of combustion and flammability; the stages of fires; and the concepts of fire initiation, spread, ventilation, backdraft, extinguishment, hazard, and risk. It includes two examples of room fires that demonstrate the progression from a small flame to room flashover.

• Chapter 7, “Fire Characteristics: Gaseous Combustibles,” now relates fire stages to generic types of flames and contains an enhanced presentation of ignition.

• Chapter 8, “Fire Characteristics: Liquid Combustibles,” contains expanded text relating vapor pressure and temperature to ignitability and flammability hazard, as well as enhanced text regarding boilover and its hazards.

• Chapter 9, “Fire Characteristics: Solid Combustibles,” now contains differentiation between the burning of solid fuels and other fuel states; explanation of the difference between materials and products; their testing and evaluation, including the expanding use of heat release rate; and expanded description of the types of pyrolysis, gasification, and ignition. There are also sections on smoldering combustion, ignition of secondary burning items, commercial uses of different types of synthetic polymers, and the rationale for the use of fire retardants and the current public debate that is leading to re-examination of the benefits and proper use of these additives.

• The splitting of the presentation on combustion products into two chapters, Chapters 10 and 11, “Combustion Products” and “Smoke and Heat Hazards,” respectively, reflects the major advances in the knowledge of fire smoke and its hazards. There is a new section on the importance of smoke aerosols, additional text on the measurement and characterization of aerosols, presentation of different criteria for visibility through smoke, and an expansion and update on the principles of smoke alarms. New sections on smoke toxicity include discussion of the incapacitating effects of smoke components on people, the way these effects are measured and quantified for use in fire safety assessments, a brief introduction to thermostructural damage, and the concept of the limiting fire hazard.

•

Chapter 12, “Movement of Fire Gases” contains expanded text on the filling of rooms by fire smoke, the flow of smoke from a room, and the physics that governs smoke movement throughout a building.

• Chapter 13, “Fire Fighting Chemicals” now contains an expanded section on terminology, a new section on the response of automatic sprinklers to a fire, and additional text on the mechanisms of fire suppression using water. Environmental impacts have completely changed the landscape for non-aqueous fire suppression. The phenomenology of the global environmental effects is explained, and new extensive sections describe the migration to different gaseous fire suppressants. There is expanded discussion of flame extinguishment using water mist and dry chemical powders, and a new section on ultrafast flame suppression.

• Chapter 14: “Computational Modeling of Fires” reflects the transition from innovative research to tools that have become the norm for engineering practice. New text includes the elements of a fire model, whether it be a simple equation or complex mathematics requiring a computer for solution and a section on the uses and users of such models. There are also sections on CFAST and FDS, the two most commonly used computer models, each of which can be downloaded at no charge.

Additional Thoughts for the Reader Throughout this book, you will find citations to two references to which every student of fire science should have access: the NFPA’s Fire Protection Handbook (two volumes), now in its 20th edition, and the SFPE Handbook of Fire Protection Engineering, now in its fourth edition. These are the “go to” resources for dealing with fire safety matters where more complexity is evident or suspected. While each successive edition has improved significantly over the prior edition, any edition of these references provides background material and detailed exposition on the components of fire protection science, as well as graphs and tables of supporting data. D. Drysdale’s An Introduction to Fire Dynamics (third edition, John Wiley, New York, 2011) is also a solid resource.

The provision of fire safety is a mission in motion. In a few short decades, the advances have been remarkable. Smoke alarms—once an expensive curiosity—are now installed in nearly all homes.

Automatic sprinklers are the norm in commercial buildings. Less fire-prone cigarettes and ground-fault circuit interrupters are decreasing the number of ignitions from these sources. Oxygen consumption calorimetry is enabling the commercialization of products, such as mattresses, that do not burn as vigorously as the versions they replace. Computational fire models are facilitating innovative building designs that are newly functional, yet still safe. Our fire incidence data system and its analysis have grown in content and credibility; this system shows that our national fire problem is decreasing and identifies our substantial cost savings as a result of this trend.

All of these achievements have been accomplished in the presence of headwinds. Our growing affluence over the past decades has increased the combustible fire load in our homes and in those structures where we work and play. New materials that may be superior in other ways are ignited more readily and burn more vigorously. Recognition of health hazards has forced replacement of PCBs and asbestos, two families of materials that had been providing fire safety benefits. Most recently, new understanding of our global environment has altered our perception of fire safety, which has traditionally been provided locally or regionally. Notably, the threats of ozone depletion and global warning have led to restrictions being placed on the use of the highly effective halogenated fire suppressants.

Old fire problems are solved; new ones emerge. Fire morphs, but it remains a bane of our world.

What is not changing is the need for an intellectually curious, practically oriented fire safety community, ready to protect an evolving society.

References 1.

2.

3.

4.

5.

Karter, M. J. Jr. (2011). Fire Loss in the United States during 2010. Quincy, MA: National Fire Protection Association.

National Interagency Fire Center. www.nifc.gov.

Goudsblom, J. (1992). Fire and Civilization. New York, NY: Penguin Press.

Grun, B. (1982). The Timetables of History. New York, NY: Simon and Schuster.

Lyons, P. R. (1976). Fire in America! Quincy, MA: National Fire Protection Association.

CHAPTER 1

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Fire Measurement and the SI System of Units OBJECTIVES After studying this chapter, you should be able to: • • • •

Explain the importance of measurement in understanding fire behavior.

Name the basic SI units of measurement and covert between values in SI units and English units.

Understand the precision of a measurement and the reduced precision used in estimations.

Explain the differences between mass and weight and among energy, heat, and enthalpy.

Introduction In 1999, the Mars Climate Orbiter miscalculated the distance to the planet’s surface and disintegrated in the planetary atmosphere. The cause was human error. The spacecraft was programmed with English units, but NASA used metric units. The difference in measuring units led to the incorrect transfer of the navigational information between the spacecraft’s manufacturer team in Denver, Colorado, and the flight team in Pasadena, California—and the ultimate destruction of the spacecraft. Input data for engineering calculations can be tabulated in a variety of units. To avoid serious consequences, it is critical that the input data, the calculation method, and the calculated values all use a consistent set of units.

About Measurement Measurement is the key to understanding fire phenomena and to translating that understanding into fire safety practice. To help understand the phenomena, it is important to ask when the fire started, how rapidly it grew, how hot it became, and how severe the threat to the population was. The answers to all of these (and many other) questions are rooted in an ability to quantify. The meaning of relative terms, such as “fast moving” or “big,” varies widely depending on people’s experiences and perceptions. To a gardener, a big fire may involve a large pile of leaves; in contrast, to an insurance company, a big fire may be one that destroys a house.

Given the many different languages of the world, it is not surprising that the early cultures made up their own methods to measure objects, frequently cast in terms of properties of the human body. (That way, you always had your “yardstick” with you.) The units of measurement varied from region to region and often from person to person. For example, the Chinese measured length using the bu (about 1.67 m), the Anglo-Saxons used the ell (about 1.14 m), and the Spaniards used the vara (about 0.86 m). As cultures expanded to the point of geographical contiguity, and as trade among multiple cultures began to prosper, the need for a common set of measurements grew.

The current international measurement system, also known as the metric system, was introduced in France by Napoleon at the beginning of the 19th century. It was refined further in the 1960s, and certain units, referred to as SI units, were agreed upon. SI comes from the French name Système International d’ Unités.

All industrialized countries, with the exception of the United States and to some extent the United Kingdom, have chosen SI units to express mass, length, time, electrical current, temperature, and other measures. Adoption of the SI system facilitates the following: •

Quantitative communication regarding nearly everything, from the weather to the multitudinous forms of life.

• Exchange of manufactured products among countries.

• Computations, due to the use of factors of 10 for each unit. Instead of 12 inches in 1 foot and 5280 feet in a mile, SI uses 1000 millimeters in 1 meter and 1000 meters in 1 kilometer.

In the United States, the primary users of SI units today are scientists and engineers. In other countries, both scientists and ordinary citizens primarily use SI units or are in the process of changing

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over to their use. Because the data compiled in different engineering references are in either SI or English units, it is important that the U.S. reader learn to convert between the two systems.1 When a quantity is measured, there is a limit to the precision of the value that is obtained. If a length measurement is performed with an inexpensive ruler, it may be possible to read only the nearest millimeter marking. A value might then be reported as 147 mm, in which case the length is represented by three significant figures. With a more meticulously marked ruler and a magnifying glass, it would be possible to estimate the length more precisely and report the value to four significant figures—for example, 147.3 mm. One should report a value to the number of significant figures. Using an electronic measuring device with a 10-digit display does not increase the number of significant figures. Similarly, entering a number into a computer spreadsheet in which the cells are set to display 10 digits does not increase the number of significant figures in the value.

When estimating a calculated value, it is acceptable to speed the calculation by using fewer than the actual number of significant figures. Thus, in estimating the total surface area of the Earth, one might assume that the planet is a perfect sphere with a radius of about 6000 km. The surface area is given by the formula 4πr2 (where r = the planet’s radius), and the value of π is close to 3. The magnitude of the surface area can then be estimated at approximately 500,000,000 km2, reported to one significant figure.

Length, Area, and Volume Units The basic SI unit of length is the meter (m). Originally, the meter was selected as 1/10,000,000 of the distance from the Earth’s equator to the North Pole. Toward the end of the 19th century, however, it was redefined as the distance between two lines on a standard bar composed of an alloy of 90 percent platinum and 10 percent iridium, measured when the bar is at the melting temperature of ice. The meter is currently defined as the length of the path traveled by light in 1/299,792,458 of 1 second.

Table 1-1 shows various SI “meter” units as they relate to the English equivalents. References [1] and [2] at the end of the chapter contain more conversions among length (and other) units.

Table 1-1 SI Length Units as Related to the Meter, with English Equivalents

Notice that “in.” is the abbreviation for inches. The period is included to distinguish it from the preposition “in”; it is the only abbreviation that is followed by a period.

Formally, SI dimensions are given in multiples of 1000 (km, m, mm, and so on). Nevertheless, some intermediate factors of 10 (e.g., cm, dm) are widely used.

Area is two-dimensional and, for a rectangular flat surface, is the length of the surface times its width. In the SI system, small areas can be expressed in square meters (m²), square centimeters (cm²), and so on. In the English system, areas of similar size are expressed in square inches (in.2) or square feet (ft2). Larger areas, such as tracts of land, are expressed in hectares (ha) in the metric system; 1 hectare is 10,000 m². The English equivalent of one hectare is 2.47 acres. In fire dynamics, the crosssectional area of a vent is used to calculate the flow through the vent, and the area of a hot surface is used to calculate the heat transferred to a colder object.

Volume is three-dimensional. For a rectangular space, such as a room, it is the length times the width times the height. Volume can be expressed in cubic meters (m³), cubic centimeters (cm³), and so

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on. The liter (L) is commonly used as a unit of liquid and gas volume, and is the same as 1 cubic decimeter (dm³) or 1000 cubic centimeters (1000 cm³). One liter is equivalent to 0.264 U.S. gallon or 1.056 quarts.

Mass and Density Units The basic SI unit of mass is the kilogram (kg). The kilogram was selected because it is approximately the mass of 1 liter of water. (The mass of a volume of water varies because water expands or contracts slightly as its temperature changes.) The gram (g), also widely used, is 1/1000 of a kilogram, and is approximately the mass of 1 cubic centimeter (cc) of water. Table 1-2 shows the relationship of various mass units to the kilogram (with their English equivalents). To convert from metric units to English units, multiply the metric value by the number in the right column. To convert from English units to metric units, divide the English value by the number in the right column.

Table 1-2 SI Mass Units as Related to the Gram, with English Equivalents

The concepts of mass and weight are often confused. The mass of an object is a fundamental property of the object, representing the quantity of matter in the object. An object’s mass is invariant (except in a nuclear bomb explosion, when mass changes into energy). By comparison, weight refers to the force acting on an object because of gravity attraction and is a convenient way to measure mass on Earth at sea level. If an object were on the moon, its weight would be only about one-sixth of its weight on Earth, and if the same object were in an orbiting space station; it would be nearly weightless.

However, its mass would be the same in all three cases.

Density is the mass of a substance in a unit volume. It is generally is expressed in grams per cubic centimeter (g/cm³), kilograms per cubic meter (kg/m³) or, in English units, pounds per cubic foot (lb/ft3).

The term specific gravity refers to the ratio of the density of a substance to that of a reference substance. For liquids and solids, the reference substance is usually water; for gases, the reference substance is air. Especially for gases and liquids, the temperature and pressure must also be specified, because the densities of the substance of interest and the reference substance depend on the temperature and pressure (Table 1-3). The densities of most solids are less sensitive to temperature and pressure.

Table 1-3 Densities of Selected Materials [2]

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For mixtures of two or more substances, there are multiple ways of denoting the relative prevalence of each component.

• Concentration: the mass of a component per unit volume.

• Volume fraction (gases): the ratio of the volume that a gas in the mixture would occupy (at standard temperature and pressure) to the total volume of the system. This is sometimes multiplied by 100 to obtain the volume percent. Thus the volume fraction of oxygen in dry air is 0.209, and the volume percent of oxygen in dry air is 20.9 percent. (There is more on the composition of air in the Physical and Chemical Change chapter.)

• Mass fraction: the ratio of the mass of a component in a mixture to the total mass of the mixture.

This can also be multiplied by 100 to obtain the mass percent. Thus, for dry air, the mass fraction of oxygen is 0.233 and the mass percent is 23.3 percent.

Note Historically, there has been extensive use of units like ppm (parts per million), such as to indicate a concentration of a toxic gas in fire smoke, or pph (parts per hundred), such as to indicate the amount of fire retardant added to a plastic material. There is a critical ambiguity in these units: “ppm” might refer to 1 g of material X in 1000 kg material Y or 1 3

cm of material X in 1000 L of material Y. As a result, the use of these types of units is discouraged. The units to be used instead of ppm are µL/L for volume fractions and mg/kg for mass fractions. These are numerically identical: 1 mg/kg = 1 ppm by mass.

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Time Units Units for time are the same in the SI system and the English system. The basic unit is the second (s).

Table 1-4 shows abbreviations for related time units.

Speed is the rate at which an object is moving, with typical metric units being m/s or km/h. Velocity is speed in a chosen direction. Thus, if a train is moving to the northeast at a speed of 150 km/h, its velocity in the east direction is 106 km/h (150/√2). (An alternative wording is that the train is moving eastward at 106 km/h.) Colloquially, when the speaker and the audience both understand the direction of movement, the terms may be used synonymously.

Table 1-4 Time Units (SI and English) Time Unit

Abbreviation

hour

h

minute

min

second

s

millisecond

ms

5

microsecond

µs

nanosecond

ns

Acceleration is the rate of change of speed. Typical metric units are m/s2 and km/h.

Note SI units named for a person are abbreviated using a capital letter. When the unit is spelled out, it begins with a lowercase letter except when it appears at the beginning of a sentence or in a title.

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Force and Pressure Units The basic unit of force in the SI system is the newton (N). A newton is the force needed to accelerate a mass of 1 kg at the rate of 1 m/s2. In the English system, 1 lb of force is the force that will accelerate 1 lb of mass at the rate of 32.2 ft/s2. This definition was selected so that 1 lb of mass at sea level would feel a gravitational attraction of 1 lb of force. (Note the use of the same term, lb, to denote two different types of units.)

From the relation between the pound of mass and the kilogram, and the relation between the foot and the meter, it is easy to show that 1 newton is equal to 0.224 pound of force. The gravitational force on 1 kg at sea level is 9.81 N. Pressure is force per unit area. The basic SI unit of pressure is the pascal (Pa), which is 1 N/m².

One Pa is a very low pressure, so a unit called the bar is also used. A bar is defined as 100,000 Pa or 100 kilopascals (kPa). One bar is only 1.3 percent greater than normal atmospheric pressure at sea level (101.3 kPa); therefore, for approximate calculations, 1 bar is often equated to 1 atmosphere (atm).

Several English units of pressure arose out of convenience in particular applications. The following describes the more common ones: •

Testing of the fracture or deformation condition for materials gave rise to the unit of pounds per square inch (psi). The pressure of compressed gases in their storage cylinders is commonly monitored in psig, where the “g” stands for “gauge.” This is the pressure above atmospheric pressure. Pressures in psig are 14.7 psi lower than pressures in psia, where “a” stands for “absolute.”

• Manometers (glass U-shaped tubes filled with a fluid) were frequently used to measure pressure differences or absolute atmospheric pressure. When using a manometer, the measured height of the liquid column is proportional to the gas pressure. The two commonly used fluids were mercury (Hg) and water (H2O). The conversion factors from SI units are as follows: 101 kPa = 760 mm Hg (also referred to as torr) 101 kPa = 4020 in. H2O The latter units are often used to measure the small pressure differences that arise within buildings due to the heating and air conditioning systems. Manometers are no longer in general use: mercury is toxic and must be disposed as hazardous waste, while a water manometer can be very large.

SI pressure units are becoming more widely used in these applications, but they have not fully displaced these English units in engineering practice and reference tables.

Energy and Enthalpy Units A fire in a closed, constant-volume system generates energy. The increase in energy transfers heat to the system, raising the pressure within the volume, and increasing the temperature of the gases, the combustibles, and the “box” itself. Of course, most fires occur in an environment of nearly constant pressure, as even a small pressure increase breaks windows and otherwise spreads the combustion products out of the room of fire origin. Additional energy release is needed to expand the gases to keep

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the system at the starting pressure. This augmented energy release is defined as the increase in enthalpy, and it is the parameter that is properly considered in characterizing a fire. (The use of the term “energy release” has been widely misused to be synonymous.) If the enthalpy increase as a whole simply raises the temperature and expands any gases that are present, then the enthalpy change is equal to the heat released.

The basic SI unit of enthalpy, energy, or heat is the joule (J). A joule is the quantity of energy expended when applying a force of 1 N through a distance of 1 m. Thermal energy as well as mechanical energy can be expressed in joules. One joule equals 0.239 calorie (cal), or 4.187 J equals 1 cal. A calorie is the energy needed to heat 1 g of water by 1 °C. (A dietary “calorie” is actually 1000 calories.)

A particularly important type of enthalpy or heat release is the heat of combustion. This quantity is the maximum heat that can be generated in the burning of a material. As such, it represents the upper limit of the contribution of a combustible item to a fire. Under some fire conditions, a combustible item will burn inefficiently, releasing less than the full heat of combustion; this smaller value is called the effective heat of combustion. The units for both heat releases are typically kJ/g. This subject will be developed further in the Physical and Chemical Change chapter.

In English units, enthalpy is expressed in foot-pounds (ft-lb) or British thermal units (Btu). One ft-lb is equal to 1.355 J, and 1 Btu is equal to 1055 J or 252 cal.

Power Units Power is the rate at which enthalpy or energy is expended. In SI units, power is expressed in watts (W). One watt is 1 J/s. The kilowatt (1000 W) and the megawatt (MW; equal to 1,000,000 W) are used frequently. The heat generated in a fire is generally expressed in kW or MW. In English units, horsepower (hp) and British thermal units (Btu) are still used. One horsepower equals 745 W. One Btu/s is equal to 1.055 kW; thus, when estimating power generation or consumption, it is reasonable to approximate 1 kW was being equal to 1 Btu/s.

Temperature Units Temperature is a measure of the warmth or coldness of a substance. Two temperature scales are used in the SI system: the Celsius scale (°C) and the Kelvin scale (K).

On the Kelvin scale, sometimes called the thermodynamic temperature scale, negative temperatures do not occur. The zero point on the Kelvin scale is called absolute zero and equals −273.15 °C on the Celsius scale. No temperature colder than this is possible.

Other features of the Kelvin scale indicate its basic nature: • The volume occupied by a gas is proportional to its temperature on the Kelvin scale, as long as its pressure is maintained constant.

• The thermal radiation emitted by a flame or a hot surface is proportional to the fourth power of the Kelvin temperature.

• The velocity of sound through a gas is proportional to the square root of its Kelvin temperature.

Because of these and other scientific facts, it would be logical to use only the Kelvin scale for temperature. However, the Celsius scale (previously called the Centigrade scale) was used for more than a century before these facts were discovered, so the world continues to use both scales. On the Celsius scale (at sea level), water freezes at 0 °C and boils at 100 °C. Negative temperatures as low as –273.15 °C are possible.

Temperature using the Kelvin scale is expressed in “kelvins,” not “degrees Kelvin.” The magnitude of 1 K is the same as 1 °C. To convert from K to °C, subtract 273.15 (generally simplified as 273). To convert from °C to K, add 273 (Equation 1-1):

For example, 20 °C = 293 K, and −10 °C = 263 K. The English system uses the Fahrenheit scale (°F), where 0 °F was chosen as the temperature at which a brine solution (reached by mixing water and salt) would freeze. On this scale (at sea level),

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water freezes at 32 °F and boils at 212 °F. Conversions from °F to °C are computed using the formula in Equation 1-2:

Conversions from °C to °F are computed using the formula in Equation 1-3:

For example, 86 °F = (86 – 32)/1.8 = 30 °C, and 25 °C = (1.8 × 25) + 32 = 77 °F.

Conversion Factors References [1] and [2] at the end of this chapter contain extensive conversion factors among SI units and other units. Some pocket calculators and websites also offer conversion factors. Table 1-5 provides conversion factors for most of the quantities discussed in this text.

In practice, it is helpful to use a consistent set of denominations for units. This reduces the likelihood of error from mixing two units of mass (e.g., g and kg) in a calculation. Two sets of metric units are commonly used: •

The meter–kilogram–second system, known as the “MKS” system. Lengths are expressed in meters (m), mass in kilograms (kg), and time in seconds (s).

• The “cgs” system. Lengths are expressed in centimeters (cm), mass in grams (g), and time in seconds (s).

Temperature units are the same (K and °C) in both systems.

MKS units are generally preferred, but the magnitude of the calculated quantity can guide the choice of system. For example, automobile velocity (tens of meters per second) might be expressed in MKS units, while the dimensions of a human finger are conveniently expressed in cm.

Table 1-5 Conversion Factors among Common Units. To convert Column A to Column B, multiply Column A by Column C. To convert Column B to Column A, divide Column B by Column C.

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WRAP-UP Chapter Summary •

The ability to quantify, or measure, is essential to understanding fire phenomena and to performing fire safety calculations.

• The basic measurements for fire phenomena are time, length, area, volume, mass, density, force, pressure, enthalpy and energy, power, and temperature.

• There are multiple units for each of these measurements. Metric units are most widely used worldwide; English units remain in use in the United States.

• Familiarity with these various units and their interconversion can minimize the chance of making a serious calculation error.

Key Terms absolute zero The lowest possible temperature, at which all molecular motion has ceased. This temperature is 0 K, −273.15 °C, and −459.67 °F. accuracy The degree of closeness of measurements of a quantity to that quantity’s actual (true) value. bar A unit of pressure equal to 105 Pa or 102 kPa. Celsius scale (°C) A temperature scale in which 0 °C and 100 °C are the freezing point and boiling point of water, respectively. concentration The quantity of a substance in a mixture per unit volume of the mixture. density Usually, the mass of a substance per unit volume. However, when an extinguishing agent is applied to a surface, the term density is used to mean the mass rate of application of agent per unit of surface. energy The capacity to do work or effect a change within a system at constant volume.

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enthalpy The capacity to do work or effect a change for a system at constant pressure. The enthalpy released is equal to the energy released plus the change in the product of the temperature and pressure. force The influence on a body that causes it to accelerate if it is free to move. heat The enthalpy or energy that travels from a higher temperature source to a lower temperature sink. heat of combustion (at constant pressure) The enthalpy released when 1 mole of a combustible item reacts completely with oxygen at atmospheric pressure and 298 K to form combustion products at 298 K. joule (J) The basic SI unit of energy or enthalpy. It is equal to a force of 1 N acting through a distance of 1 m.

Kelvin scale (K) A temperature scale in which 0 K is absolute zero and 1 K equals 1 °C. kilogram (kg) The basic unit of mass in the SI system. Its magnitude is defined as the mass of an object called the international prototype kilogram, made of an alloy (90 % platinum and 10 % iridium by mass), machined into a right-circular cylinder, 39.17 mm in both diameter and height. mass The fundamental inertial property of an object. mass fraction The mass of a substance in a mixture per unit mass of the mixture. meter (m) The basic SI unit of length. It is defined as the length of the path traveled by light in 1/299,792,458 of 1 second. newton (N) The basic SI unit of force. It is the force needed to accelerate a mass of 1 kg at the rate of 1 m/s2. pascal (Pa) The basic SI unit of pressure, or force per unit area. It is equal to a force of one N exerted over an area of one square meter. power The rate at which enthalpy or energy is expended. precision The degree to which the correctness of a quantity is expressed. pressure Force per unit area.

SI units The units used in the metric system of measurement. significant figures In a number, those digits that carry meaning contributing to its precision. specific gravity The ratio of the density of a substance to the density of a reference substance at a specified temperature and pressure. For gases, the reference substance is generally taken to be dry air. For liquids, the reference substance is water. volume fraction The volume of a gas in a mixture of gases per unit volume of the mixture. watt (W) The fundamental SI unit of power. It is equal to the expenditure of one joule for one second.

Challenging Questions 1. Using the units presented in this chapter, list 10 quantities that might be useful in describing a fire.

2. Normal body temperature is 98.6 °F, and a 5 °F increase represents a serious fever. Convert these values to °C and then to kelvins.

3. A fire truck 12 m long is traveling at 90 km/h. Convert these values to English units.

4.

A fire pump pressurizes water at 60 psi above atmospheric pressure and pumps it at the rate of 300 U.S. gal/min. Convert these values to SI units.

5.

An electric motor self-heats to 15 °C above ambient temperature when operating steadily. If the ambient temperature is 70 °F, what is the operating temperature of the motor in °F?

6. An automatic sprinkler applies water at a “density” of 0.3 gal/min/ft2. Convert this to SI units.

7. If the heat of combustion of benzene is 40 kJ/g, what is it in cal/g? In Btu/lb?

8. A 170 lb fire fighter is carrying 75 lb of equipment up (10 in.) stairs. There are 15 stairs per story in this building. How many dietary calories does the fire fighter burn in climbing 10 flights of stairs?

9. If the same fire fighter were part of a colony on Jupiter, where the force of gravity is 2.5 times that on Earth, what would the fire fighter’s mass be? What would his weight be?

References

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1.

DiNenno, P. J., ed. SFPE Handbook of Fire Protection Engineering, 4th ed. Quincy, MA: National Fire Protection Association, 2008: Appendix A. 2. Haynes, W. M., ed. Handbook of Chemistry and Physics, 92nd ed. Boca Raton, FL: CRC Press, 2011: Section 1.

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The relationships among the units of these fundamental properties are quite precise. The conversion factors in this chapter have been rounded to a number of significant figures that provides sufficient precision while maintaining ease of computation. For sources of conversion factors of higher precision, type “SI units� in a web browser and visit the presented sites or see References [1] and [2] at the end of this chapter.

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CHAPTER 2

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Chemical Elements and Compounds: Atoms and Molecules OBJECTIVES After studying this chapter, you should be able to: • • • • •

List the chemical elements that are especially important in fires.

Describe atomic mass and dimension.

Describe molecules, compounds, free radicals, and ions.

Recognize the bonding features of an organic fuel from its name.

Find further information about atomic and molecular properties.

Introduction Concepts of invisibly small building blocks of matter are more than 2500 years old. Theories as to the nature of these particles abounded, based entirely on philosophy and faith. Around 1800, science-based theories began to emerge; observations of chemical reactions led to inferences of how atoms can combine to form molecules. Throughout the 19th and 20th centuries, accumulated experimental evidence, supported by mathematically rigorous theories, evolved the structure of atoms and molecules that we know today. Nevertheless, all of this evidence was indirect, because no could see an atom or a molecule. Then, in the early 1980s, following the invention of the scanning tunneling electron microscope, scientists began to produce images of the electric fields around individual atoms and molecules. A subsequently developed field of study deposited molecules onto a surface that was extremely uniform at the atomic level and allowed scientists to see their structure and shapes. And, yes, they looked like they were supposed to look.

Atoms All physical substances in the world are made of chemicals, and all chemicals are made of elements or combinations of elements. There are 118 known chemical elements. Some of the more widely known elements are carbon, oxygen, hydrogen, helium, chlorine, iron, copper, lead, silver, and gold. Each element consists of one type of atom, a tiny particle that gives the element its distinctive properties.

Each atom comprises a very small, relatively heavy nucleus having a positive electric charge and a larger surrounding cloud of orbiting electrons, which are negatively charged and relatively light. The positive charge of the nucleus is balanced by the negative charge of the electrons; thus, from a distance, the atom appears neutral. More than 99.9 percent of an atom’s mass is concentrated in the nucleus.

Each element consists of atoms unique to that element. Each element has also been assigned an atomic number. The atomic number of hydrogen (abbreviated as H) is 1; the hydrogen atom consists of a nucleus with an electric charge designated as +1. A single electron, with a charge of −1, orbits the nucleus. Helium (He) is the second element, with an atomic number of 2; its atoms each have a nucleus with an electric charge of +2 (i.e., one more than the electric charge of hydrogen). Two electrons orbit around each helium nucleus. The atoms of the third element, lithium (Li), have a nuclear charge of +3 and three orbiting electrons per atom, and so forth, through the list of elements.

Table 2-1 presents a partial list of the chemical elements, including most elements mentioned in this text. The properties of these 25 elements and others can be found in the Handbook of Chemistry and Physics [1], along with historical information on their discovery.

One might infer, from the description presented so far, that a He atom should be two times as heavy as an H atom, a Li atom three times as heavy as an H atom, and so on. Examination of the atomic masses shown in Table 2-1 makes it clear that this is not the case. For example, a Li atom is about seven times as heavy as an H atom, even though the electric charge within its nucleus is only three times as large. This discrepancy remained difficult to explain until the neutron was discovered in 1930.

A neutron is a particle with a mass that is extremely close to the mass of the nucleus of an H atom (which is called a proton); however, the neutron is uncharged (electrically neutral), while the proton has

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a positive unit charge. The relative masses of the various kinds of atoms become understandable if each nucleus is considered to consist of a combination of protons and neutrons, which are bound together very tightly.

Table 2-1 Abridged List of Chemical Elements [1]

For example, if a He nucleus consisted of two protons and two neutrons, it would have twice the electric charge and four times the mass of the H nucleus (a single proton). This is, indeed, the case.

Likewise, a Li nucleus consists of three protons and four neutrons and has seven times the mass of a hydrogen nucleus.

An atom of the heaviest naturally occurring element, uranium (U), with atomic number 92, has 92 protons and about 146 neutrons in its nucleus and a mass 238 times that of a hydrogen nucleus. Notice the words “about 146 neutrons� in the previous sentence. Actually, a small percentage of U atoms have 92 protons and either 143 or 142 neutrons in the nucleus. These three possible atoms, which are slightly different forms of the same element, are called isotopes. The three naturally occurring isotopes of uranium are designated 238U, 235U, and 234U, respectively. The properties of these isotopes are quite different: 238U is quite stable, while 235U is the explosive component in an atomic bomb. In this notation for isotopes, the superscript to the left of the elemental symbol is the sum of the number of protons and neutrons in the nucleus of the isotope. Sometimes, the notation also includes the atomic number of the element, which appears as a subscript to the left of the elemental symbol.

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Most elements are made up of multiple isotopes. Typically, 99 percent or more of the atoms of an element belong to a single isotope. The atomic mass for each element shown in Table 2-1 includes proportional contributions from all the naturally occurring isotopes, relative to the mass of the 12C isotope of carbon, which by definition is exactly 12.

Note While shown as a single value, the atomic weight of an element can vary depending on the location on Earth where the sample was obtained. This is due to small local differences in the proportions of the various isotopes. Because the effect of these differences occurs well to the right of the decimal point, they are not discussed further in this text.

However, the differences have been used for such purposes as identifying whether a nomadic tribe brought an idol with them or made it in their new location.

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The number of atoms in 12.000 g of 12C is 6.022 × 1023. This number, which is called Avogadro’s number, is the number of atoms in the gram atomic mass of any element. The unit defined as Avogadro’s number of particles is the mole (mol).

There are a few exceptions to the “rule” that the population of an atom is dominated by a single isotope. The most commonly encountered is the element chlorine, which is a mixture of two isotopes, 35 Cl and 37Cl. The nucleus of each of these two isotopic forms has an electric charge of +17. The two isotopes contain 18 and 20 neutrons, respectively. The chlorine found on Earth contains 75.8 percent 35 Cl and 24.2 percent 37Cl by mass, so the gram atomic mass of chlorine is 35.45 g.

As is discussed in the Physical and Chemical Change chapter, we must know atomic masses to calculate the combining proportions of reactants when chemical reactions, such as those occurring in fires, take place. It is useful to memorize approximate gram atomic masses of certain commonly encountered elements, in particular the following: Hydrogen = 1 g Carbon = 12 g Nitrogen = 14 g Oxygen = 16 g The column in Table 2-1 labeled “Usual Valence” is explained later in this chapter.

There is one particular group of elements whose atoms are important in fire retardant and fire suppression chemistry. These are the halogens: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

(Astatine is also a halogen, but is not important for our purposes.) Compounds containing these elements will be discussed in the chapters Fire Characteristics: Solid Combustibles and Fire Fighting Chemicals.

Stability of Atoms If an atom participates in an ordinary physical or chemical process, it can experience one of the following changes: 1. Lose one or more of its electrons 2. Share one or more of its electrons with a neighboring atom to form a chemical bond 3. Gain one or more extra electrons Its nucleus, however, is completely unaffected. For example, if an atom is passed through a flame, an electric arc, or a laser beam, its nucleus will be unchanged. Atoms cannot be changed by fire, which produces peak temperatures on the order of 1800 °F (1000 °C).

There are three types of processes by which an atom can be changed. First, if an atom is heated to a temperature of hundreds of millions of degrees, such as in a thermonuclear explosion or in the interior of the sun, then the nucleus can be changed. The original atom of element A can become an atom of element B, or perhaps it can split into two atoms of elements C and D. Second, a nucleus can be changed by striking it with a subatomic particle that has been accelerated to nearly the velocity of light (3 · 108 m/s) by applying an electric field in the range of millions of volts. Third, certain atoms, such as

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those of radium and 235U, are radioactive, which means that their nuclei are naturally unstable. When these nuclei spontaneously emit charged particles, they become atoms of a different element. This text does not address the chemistry of nuclear transformations.

Atomic Mass and Dimension Atoms are not infinitesimally small, but rather have finite, measurable size and mass. The hydrogen atom (1H) is the lightest and smallest atom. It has a mass of 1.673 × 10–24 g and a diameter of 1.06 × 10–10 m (0.106 nm, 1.06 Å). The largest atoms are approximately seven times the diameter and 150 times the mass of the hydrogen atom.

It is useful to put these very small values of atomic properties into perspective. A very tiny droplet of water, produced by an atomizer, with a diameter of 1 µm and requiring a microscope to be seen clearly, would have the same mass as 330,000,000,000 hydrogen atoms. If 10,000 hydrogen atoms could be lined up in a row, touching each other, then the row would be about 1 µm long. For reference, a human hair is about 20 µm to 200 µm thick.

Molecules and Compounds Atoms often bind to one another to form a molecule, defined as a stable combination of atoms. In the bonding process, the electrons of atoms become shared, forming one electron cloud and pulling the atoms together. The two nuclei, both positively charged, repel each other. These opposing forces result in a stable configuration of the nuclei. For example, if two hydrogen (H) atoms come together under the right conditions, they will form a hydrogen molecule, the formula of which is H2. The distance between the centers of the two nuclei is 0.074 nm. This molecule is stable in the sense that it must be heated to a temperature of 5400 °F (3000 °C) or higher before it will dissociate (split apart) into two hydrogen atoms. However, the molecule H2 can react chemically with other kinds of atoms or molecules at much lower temperatures—even at room temperature in some cases—to form different molecules. This process occurs in the fuel cells in a hydrogen-powered car, for example, where hydrogen and oxygen are combined to form water and release enthalpy.

When it comes to forming molecules, each element has its own “personality.” Some elements exist as stable diatomic (two-atom) molecules, including oxygen (O2), nitrogen (N2), and chlorine (Cl2). At ordinary temperatures, other elements exist only as single atoms. Thus only at extremely low temperatures do helium atoms (He) or argon atoms (Ar) combine to form He2 or Ar2. As another example of different behavior, three atoms of oxygen can combine to form ozone (O3), the compound that protects Earth’s surface from the intense ultraviolet light emitted by the sun. However, ozone is not as stable as O2 and will spontaneously decompose to O2 within a few hours at room temperature.

A molecule that consists of two or more atoms of different elements is called a chemical compound, or simply a compound. For example, a hydrogen atom can combine with a chlorine atom to form the chemical compound known as hydrogen chloride (HCl). Hydrogen chloride is stable at ordinary temperatures, but will decompose back into its elements at temperatures greater than approximately 4500 °F (2500 °C). Water (H2O) is a compound formed from hydrogen and oxygen. The H2O molecule is also quite stable and will decompose only at temperatures greater than approximately 4900 °F (2700 °C). Another hydrogen–oxygen compound, hydrogen peroxide (H2O2), is much less stable than water and can decompose to form hydrogen and oxygen at room temperature. Yet another combination of H and O atoms, OH, is not a chemical compound at all. It is called a free radical because it is extremely unstable. Free radicals are discussed in more detail later in this chapter.

The element carbon (C), a solid, can react with oxygen, a gas, to form either the gaseous chemical compound carbon monoxide (CO) or the gaseous chemical compound carbon dioxide (CO2). Both of these compounds are stable by themselves up to quite high temperatures, but either compound can react with free radicals to form new substances, even at lower temperatures.

This chapter has thus far concentrated on gaseous molecules; however, liquids and solids can also consist of molecules. Water vapor (H2O) can condense to a liquid by cooling, and the liquid form of water can freeze to ice. Ethanol and aqueous film-forming form (AFFF; added to water for effective suppression of fuel spill fires) are examples of liquid molecules. Table salt (NaCl) and table sugar (C12H22O11) are examples of molecular solids.

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Generally for similar compounds, the smaller molecules tend to exist as gases at room temperature and the larger molecules tend to exist as liquids. Consider the family of hydrocarbons—that is, compounds consisting only of carbon and hydrogen atoms, with a generic molecular formula of CxHy. CH4 (methane), C2H6 (ethane), and C3H8 (propane) are gases at room temperature and pressure.

C4H10 (butane) is a liquid that evaporates quickly if not confined, as in a cigarette lighter. The hydrocarbons containing five or more carbon atoms are liquids, with some of the heaviest being tar-like.

Another way of describing the smaller liquid hydrocarbons is to say that they are more volatile; that is, they evaporate and boil at lower temperatures than their larger chemical “cousins.� As discussed in the Fire Characteristics: Liquid Combustibles chapter, this has a direct bearing on the fire hazard of these compounds.

Chemical Bonds and Valence The concepts of chemical bonds and valence provide a means of determining which molecules might form and, of these possibilities, which are likely to be stable and which are likely to be unstable. As mentioned earlier in this chapter, chemical bonds hold atoms together in a molecule. Valence is the number of bonds an atom can form with other atoms.

As seen in the last column of Table 2-1, some atoms have only a single valence state. For example, a hydrogen or chlorine atom can form only one bond and has a valence of 1, whereas an oxygen atom (valence of 2) forms two bonds, and an aluminum atom (valence of 3) forms three bonds. Some atoms, such as nitrogen and phosphorus, have multiple valence states.

Atoms that have valence states larger than 1 can form either single bonds or multiple bonds. Carbon is the central element in the molecules in nearly all the combustibles encountered in unwanted fires as well as the molecules that make up living bodies. With its valence of 4, a carbon atom can form four single bonds, one double bond plus two single bonds, two double bonds, or one triple bond plus one single bond.

The only molecule in which a carbon atom has a valence other than 4 is carbon monoxide (CO).

The only bond in this compound is with the oxygen atom, which has a valence of 2. Thus the CO molecule might be expected to behave like a free radical, looking to form two more bonds with other atoms. In reality, CO is quite stable up to several thousand degrees Celsius, and it can be stored in a compressed-gas cylinder for years at room temperature without undergoing chemical change. As will be seen in the Combustion Products chapter, CO is a toxic gas generated in all fires, and its toxicity is a result of this unused valence.

Figure 2-1 shows six small molecules that contain only single chemical bonds. The bonds are represented by lines connecting the atoms, and the number of such lines from an atom equal to the valence state of that atom.

Figure 2-1 Six stable molecules with single bonds.

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From the simplified renditions in Figure 2-1, it might appear that all the atoms in each of these molecules lie in a plane. Indeed, three of these six molecules (H2, H2O, and HCl) are planar. The arrangement of the atoms in each of the other three molecules is three-dimensional (3-D). This is a result of the positively charged nuclei repelling each other and staying as far apart from each other as possible.

Chemists have developed different ways of portraying molecules, depending on which information they wish to convey. Five of these portrayals of methane, the simplest organic molecule, are shown in Figure 2-2, and the same types of portrayals of glucose, a common sugar, are shown in Figure 2-3.

The five portrayals are explained here: 1. Chemical formula. This rendition shows only the number of each type of atom that is present, although for simple molecules, some information about the layout of the atoms might be inferred.

Figure 2-2 Five portrayals of the methane molecule.

2. Planar structure. This rendition shows the alignment of the atoms and the location of multiple bonds.

3. Perspective structure. This rendition indicates which atoms are located in, behind, and in front of the plane of the paper or computer screen. In the methane portrayal, the four H atoms are at the four corners of a tetrahedron, with the C atom at the center. This keeps the four H nuclei as far apart from each other as possible. This is the fundamental geometry of all those carbon-based compounds in which the carbon bonds are all single bonds.

4. Ball-and-stick model. This rendition is another 3-D portrayal. The nuclei are shown as balls and the bonds are shown as sticks. Typically, each type of atom has a different color, the length of a stick is a qualitative indicator of the length of the chemical bond (separation of the nuclei), and a multiple bond is indicated by replacing the stick with two or three parallel lines.

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Figure 2-3 Five portrayals of the glucose molecule.

5. Balloon model. This 3-D portrayal indicates the boundaries of the space occupied by the electrons.

Figure 2-4 shows the perspective structures of five additional stable molecules. Each of these molecules contains double or triple bonds, as well as single bonds in some cases. Geometrically, the atoms in the carbon dioxide and acetylene molecules lie on a straight line. In the ethylene molecule, the atoms are all in the same plane. The carbon atoms in the benzene molecule are all in the same plane, with the hydrogen atoms alternating slightly into and out of that plane. In the propylene molecule, the atoms connected to the two carbons on the left are nominally in the same plane as the three carbon atoms, while the geometry of the atoms connected to the carbon on the right is roughly tetrahedral.

Organic Chemistry Nomenclature Nearly all the fuels of concern to practitioners of fire safety are organic molecules—that is, they are based on carbon chemistry. Because many of these compounds will be used as examples throughout this text, it is necessary to present the basic rules by which they are named.

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Figure 2-4 Five stable molecules with double or triple bonds.

Table 2-2 presents the basic groups of organic compounds. Later in the text, we will introduce some compounds with more complex structures. These will be named at their locations in the text, and a bonding diagram will be presented, if that is relevant.

The alkanes contain only single carbon–carbon bonds. Alkanes are also called saturated compounds. The name of each compound in this group ends in -ane.

The alkenes (also called olefins) and alkynes are referred to as unsaturated compounds. The double bond (=) in an alkene is stronger than a carbon–carbon single bond, and the triple bond (≡) in an alkyne is stronger still. The names of alkenes end in -ene, and the names of alkynes end in -yne, If, in one of these molecules, there is an alternating sequence of multiple and single bonds, such as −C=C−C=C− or −C≡C−C≡C−, the overall carbon–carbon bonding is very strong. These are called conjugated bonds.

The names of aromatic compounds also end in -ene. These compounds contain rings of six carbon atoms, with the carbon–carbon bonding depicted as alternating single and double bonds. In reality, the six conjugated carbon–carbon bonds share all the electrons equally and become equivalent and very strong.

Table 2-2 Some Groups of Organic Compounds

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a

These names are common names. They do not follow the naming rules given previously.

The last four rows in Table 2-2 illustrate molecules that also contain oxygen atoms. In alcohols, whose names end in -ol, one or more of the carbon atoms is bound to an OH group—that is, an O atom whose other bond is to an H atom. In aldehydes, one or more of the carbon atoms is double bonded to an oxygen atom and singly bonded to an H atom. In a ketone, one or more of the carbon atoms is double bonded to an oxygen atom and not to any hydrogen atoms. The other two carbon bonds are to other carbon atoms. The organic acids contain one or more carbon atoms double bonded to an oxygen atom and single bonded to an OH group.

In Table 2-2, “R” denotes an organic group, which could be any of the previously mentioned types of molecules with a hydrogen atom removed. The R bond to the rest of the molecule is a carbon–carbon bond. For example, propyl acetylene is H7C3–C≡C–H.

Isomers For most organic molecular formulae that have three or more carbon atoms, it is possible for the atoms to bind in more than one order. These different arrangements of the same atoms are called isomers, and they often have different combustion properties. Examples of several isomers are depicted in Figure 2-5 In Figure 2-5a, the two octanes (both C8H18) show two (of several) different ways in which the 8 carbon atoms and 18 hydrogen atoms can be bonded together, while satisfying the rule that carbon has a valence of 4 and hydrogen has a valence of 1. The configuration with 8 carbon atoms in a row is termed normal octane (n-octane); the other configuration is termed isooctane (or iso-octane), or more precisely 2,2,4-trimethylpentane. The physical and chemical properties of these two isomers are somewhat similar, but are not identical. For example, the heats of combustion are within 0.1 percent of each other [2] and n-octane boils at 259 °F (126 °C), whereas isooctane boils at 210 °F (99 °C) [3].

However, there is a large difference in performance if these compounds are used as the fuel in a typical automobile engine: isooctane burns very well. In fact, it defines the high end of the Research Octane Number scale, with an assigned value of 100. By contrast, burning n-octane in a car results in very poor performance. It has a zero or negative Research Octane Number.

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Figure 2-5 Four examples of isomers.

The second pair of isomers, shown in Figure 2-5b, comprises dimethyl ether (CH3OCH3) and ethanol or ethyl alcohol (CH3CH2OH). Each of these molecules has the formula C2H6O, but the structure of each molecule is unique. In dimethyl ether, the oxygen atom is bonded between two carbon atoms; in ethanol, the oxygen atom is bonded between a carbon atom and a hydrogen atom. The boiling point of dimethyl ether is –11 °F (–24 °C), whereas the boiling point of ethyl alcohol is a much higher 172 °F (+78 °C). The chemical reactivities of the two molecules are also very different. For example, the flash points (ignition temperatures) in air have been reported as –41.8 °F (–41 °C) and 55.4 °F (+13 °C), respectively [4].

The third pair of isomers, shown in Figure 2-5c, consists of cyclopropane and propylene, each with the formula C3H6 but with different molecular architecture. Both are flammable gases at room temperatures. Propylene is the chemical building block for polyolefin fibers used in carpets and upholstery fabrics, as well as some hard plastics. Cyclopropane was formerly used as an anesthetic, but currently has little commercial use.

The fourth isomer example, shown in Figure 2-5d, consists of the three isomers of dichlorobenzene (C6H4Cl2), with the chlorine atoms in different relative positions on the benzene ring. The three isomers are as follows: These isomers differ dramatically in that p-dichlorobenzene (“moth” crystals) is a solid at room temperature, while the other two isomers are liquids at even well below room temperature. The difference in melting point between the ortho and para forms is 126 °F (70 °C).

Melting Point Isomer

(°F)

Orthodichlorobenzene (o-dichlorobenzene)

1.4

Metadichlorobenzene (m-dichlorobenzene)

−13

22

(°C) [3] −17 −25

Paradichlorobenzene (p-dichlorobenzene)

127.4

+53

Ions The atoms and molecules discussed so far are all electrically neutral; that is, the positive charges in the nuclei of the atoms are exactly balanced by the negative charges on the electrons associated with each atom or molecule. However, it is possible for an electron to become detached from an atom, converting the atom to a positive ion or cation (pronounced cat-eye-on). Alternatively, a free electron can attach itself to an atom, creating a negative ion or anion (an-eye-on). A molecule can also be converted to a cation or an anion by electron detachment or attachment, respectively.

If an ion has an excess or a deficiency of a single electron, it is referred to as singly ionized. If the ion has an excess or deficiency of two or more electrons, it is multiply ionized.

Ions can exist in the gas phase, in liquid solutions, or in solid crystals. For example, common table salt (NaCl) consists of sodium cations (Na+) and chloride anions (Cl-) in a regular, cubic array. i.e., a crystal (Figure 2-6). When this salt dissolved in water, the salt water consists of water molecules, Na+ ions, and Cl– ions. Similarly, a solution of baking soda, (sodium bicarbonate, NaHCO3, a crystalline powder), consists of water molecules, Na+ ions, and HCO3– (bicarbonate) ions.

To produce ions in the gas phase, high temperatures, energetic radiation, or high-velocity particles are required. All of these conditions exist in an electric arc; some ions are also found in ordinary flames.

Once the source of ionization is removed or the system is cooled, the ions and free electrons in a gas at atmospheric pressure and temperature will recombine within a fraction of a second to form neutral species. Ions can exist indefinitely in solution or in crystals.

Free Radicals and Free Atoms Within flames, there are fragments of molecules that are highly reactive and are important participants in the chemistry that converts fuels into combustion products, heat, and light (discussed further in the Fire Characteristics: Gaseous Combustibles chapter). These unstable species are called free radicals.

One large group of free radicals is based on carbon atoms with one or more bonds missing. Examples include the methyl radical, CH3, and the phenyl radical (a benzene molecule (Figure 2-7) with the hydrogen atom removed from one of the carbon atoms). Another free radical that is central to ignition and flame propagation is the hydroxyl radical (OH). An OH free radical is capable of removing the H atom from a stable fuel molecule, rendering the resulting fuel fragment open to further reaction.

Figure 2-6 The atomic arrangement in sodium chloride (NaCl), a cubic crystal, showing the sodium ions in red and the chloride ions in tan.

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Also important in flame chemistry are unattached atoms (e.g., H, O, N, F, and Cl) that normally form stable diatomic molecules (H2, O2, N2, F2, and Cl2, respectively). These unstable species are called free atoms.

At room temperature, any of these species will, within a fraction of a second, combine with other available molecular fragments to form stable species.

Figure 2-7 Three free radicals.

WRAP-UP Chapter Summary • All physical substances are made of chemicals, all chemicals are made of elements or combinations of elements, and all elements consist of atoms.

• The building blocks of atoms are the nucleus, which is composed of neutrons and protons, and electrons. Bonds between atoms are the result of shared electrons.

• In a fire, chemical bonds are broken and formed, but the atomic nuclei (and thus the elements in the burning materials) are not changed.

• Most of the combustible materials that are involved in fires are composed of organic chemicals.

• There are standard ways to name organic compounds. Some of the compounds have acquired an additional name.

• Isomers are compounds with the same elemental formula that have different chemical structures and different properties that could affect the compounds’ behavior in a fire.

• Free radicals and free atoms are unstable species that are central to ignition and the continued flaming in fires.

Key Terms alcohol An organic compound containing one or more hydroxyl (–O–H) groups attached to carbon atoms. aldehyde An organic compound in which a carbon atom has a double bond to an oxygen atom and a single bond to a hydrogen atom. alkane An organic compound in which all the carbon-carbon bonds are single bonds. alkene (olefin) An organic compound in which there are one or more carbon-carbon double bonds. amorphous solid A solid that lacks the geometric order of its atoms, molecules, or ions that is inherent in a crystal. anion A negatively charged atom or molecule. aromatic compound An organic compound containing a ring of conjugated unsaturated bonds. atom The smallest characteristic unit of a chemical element, consisting of a nucleus with a positive electric charge, surrounded by negatively charged electrons. atomic mass The average mass of atoms of an element, calculated using the relative abundance of the naturally occurring isotopes. atomic number An integer equal to the positive electric charge on the nucleus of an atom of each of the 118 elements.

Avogadro’s number The number of atoms (6.022 · 1023) in the gram atomic mass of any element. cation A positively charged atom or molecule.

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chemical bond The attractive force between two atoms that allows the formation of chemical substances that contain two or more atoms. chemical compound A pure chemical substance consisting of molecules containing atoms of two or more different chemical elements. conjugated bond A sequence of double (or triple bond) alternating with single bonds, leading to notably higher bond strengths. crystal A solid consisting of atoms, molecules, or ions fixed in a regular geometric pattern extending in all three spatial dimensions. electron The negatively charged particle common to all atoms. element Any of the 118 kinds of atoms of which all matter is composed. Each element consists of atoms unique to that element and different from the atoms of all other elements. free atom An atom that would be stable when combined with another atom of the same type, but which at the moment is unattached, either because it has just been formed by a chemical reaction, or because it is at very low pressure or frozen in an inert matrix. free radical A molecular fragment with unsatisfied chemical valences, i.e., the capacity to form one or more covalent bonds. halogen Atoms or molecules of the elements fluorine, chlorine, bromine, iodine, or astatine. hydroxyl radical (OH) A free radical consisting of a hydrogen atom and an oxygen atom, —OH. ion An atom or molecule in which the number of protons is not equal to the number of electrons. isomers Two or more molecules, each consisting of the same number and kinds of atoms that are bound together in different ways. isotopes Two or more atoms of the same chemical element, which have the same number of protons but different numbers of neutrons in the nucleus. methyl radical (CH3) A free radical containing three hydrogen atoms bound to a single carbon atom, — CH3. mole (mol) The amount of a substance that contains Avogadro’s number of atoms or molecules. molecule The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms. neutron A electrically neutral particle with nearly the same mass as the proton. Neutrons are part of the nuclei of all atoms except the most common isotope of hydrogen. nucleus The positively charged mass at the center of each atom, composed of protons and (with the exception of 1H), neutrons and containing more than 99.9 percent of the mass of the atom. organic compound A molecule whose structure is based on carbon. phenyl radical An aromatic ring of six carbon atoms, five of which are bonded to hydrogen atoms, with the sixth carbon atom having a free valence, C6H5—. proton The positively charged particle that is common to the nuclei of all atoms and that comprises the nucleus of the most common isotope of hydrogen. The number of protons in a nucleus defines the element to which the atom belongs. saturated compound An alkane. unsaturated compound A carbon compound containing one or more double or triple bonds between carbon atoms. valence The number of bonds an atom can form with other atoms. Some atoms have multiple valences.

Challenging Questions 1. Are all the atoms of a given element identical? Explain.

2. There are three stable isotopes of magnesium: 24Mg, 25Mg, and 26Mg. The fractional abundances of the isotopes are 0.79, 0.10, and 0.11, respectively. Calculate the gram atomic mass of magnesium.

3.

Calculate the gram molecular masses of the following species and label those that are compounds: H2O (water), C12H22O11 (table sugar), —(C3H6)1000—(polypropylene plastic), S6 (sulfur powder), Al2O3 (aluminum oxide), CHCl3 (chloroform).

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4. If the volume of a typical organic molecule can be approximated by a cube that is 0.5 nm on each side, how many such molecules would fit in a 1-gallon gasoline can (to one significant figure)?

5. Explain the difference between a stable molecule, a free radical, and an ion.

6. Draw five possible isomers of the compound C4H8.

7.

The following problem might be quite difficult, but is worth trying if you like logic puzzles. Draw structures for the following organic compounds: isopentane, propenone, and biphenyl. (Hint: This requires drawing analogies from some of the organic compound structures presented in this chapter.)

References 1. Haynes, W. M., ed. Handbook of Chemistry and Physics, 92nd ed. Boca Raton, FL: CRC Press, 2011: Section 4.

2. Handbook of Chemistry and Physics, 92nd ed., op. cit., Section 5.

3. Handbook of Chemistry and Physics, 92nd ed., op. cit., Section 3.

4. Drysdale, D. D. “Ignition of Liquids.� In: SFPE Handbook of Fire Protection Engineering, 4th Edition, DiNenno, P. J., ed. Quincy, MA: National Fire Protection Association, 2008.

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CHAPTER 3

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Physical and Chemical Change OBJECTIVES After studying this chapter, you should be able to: • • • • • • • •

Name the three basic states of matter found in the material world and explain how they are characterized.

Describe the phase changes among these states and the change in enthalpy associated with each.

Write and use the ideal gas law.

Balance a chemical equation for the combustion of a material during a fire.

Estimate the heat released during burning based on the balanced chemical equation using the mass of oxygen consumed.

Understand the meaning of fuel-lean, stoichiometric, and fuel-rich combustion.

Explain why the outcome of a combustion reaction is determined by thermodynamics, while the rate of the reaction is determined by chemical kinetics.

Describe ideal and realistic flame temperatures.

Introduction In the 17th century, a German physician and alchemist named Becher proposed that things that burned contained an element called phlogiston. During burning, the phlogiston moved from the fuel to the air; when the air could absorb no more phlogiston, the burning stopped. This theory became the prevailing explanation of the day. More than a century later, the Frenchman Lavoisier showed that the opposite was true: oxygen from the air reacted with the fuel to generate heat and light. Today, our broad knowledge of thermal and chemical behavior of molecules enables us to understand the reasons why some burn, some do not, and some are particularly good at putting fires out.

States of Matter Characterization of Phases Three basic classifications of matter are found in the material world: gases, liquids, and solids. These states, or phases, can usually be characterized by sight, smell, touch, and/or mobility. The following paragraphs describe these properties for the three states.

As a rule, gases are colorless—that is, not visible to the human eye. Some gases, such as air, are odorless. Gasoline vapor is colorless but has a distinct smell. Chlorine gas is uncommon in that it is both visible and odorous: it appears green, and the human nose is sensitive to it. (Think of the smell near a swimming pool.) Generally, we are not conscious of touching gases, but the sensors in our skin react to their temperature and speed. Gas molecules are very mobile. They spontaneously expand throughout a container and mix with other gases in the container. In addition, they expand and contract greatly in proportion to changes in pressure and temperature. A vapor is a gas that is readily condensable to a liquid. (Note that all gases are condensable if cooled sufficiently.)

There is no broad generalization about the color of liquids. Some are colorless, like water and distilled vinegar, yet we can see them due to the difference in the way light passes through them relative to the way it passes through air. Other liquids are colored, such as the reddish elemental bromine and the silver-colored mercury. The human nose is sensitive to the vapors of some liquids, such as acetic acid (the active ingredient in vinegar) and gasoline, but is not sensitive to vapors from other liquids, such as glycerin. Most people are immediately aware of touching a liquid; if you immerse a finger in a liquid, the liquid is displaced but its surface conforms to the shape of your finger. Liquids are less mobile than gases. An amount of a liquid can be described by its volume, which changes only slightly in response to changes in pressure and temperature. While liquids can disperse into each other, this process typically takes time. Thus, rather than wait for a martini to taste uniformly of gin and vermouth, we shake or stir it to homogenize the mixture.

Nearly all solids are colored, with ice and clear window glass being commonly encountered exceptions. (Even these solids can be visible for the same reason that water is visible.) The vapors

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above solids are even more dilute than the vapors above liquids; thus, at ordinary temperatures, solids, such as steel or cotton, are generally odorless. (Paradichlorobenzene, the active ingredient in moth balls, is one exception.) However, when heated in a fire, many solids, such as wood and carpeting, emit characteristic odors. If you push on a solid with your finger, it resists the pressure. (The soft pile of a carpet resists the pressure by moving away up to the point where it cannot move farther; it does not envelop your finger like a liquid would.) Solids do not mix with each other by themselves. They need to be either ground and physically mixed or dissolved and then resolidified.

Many substances are combinations of more than one state of matter. Carbonated beverages, for example, consist of gaseous carbon dioxide in water. Apple cider is a suspension of fine apple particles in apple juice. Oil shale is liquid petroleum sequestered in porous rock.

Properties of Gases Gases consist of individual atoms or molecules moving in random directions at high velocities. At normal atmospheric pressure (1 atm, 101 kPa) and temperature 68 °F (20 °C), gas molecules themselves take up only about 0.04 percent of the volume they occupy—the remaining 99.96 percent of the space is empty. At this temperature and pressure, the molecules in air move with an average velocity of 460 m/s (1030 mi/h), and each molecule collides with others and changes direction about 1010 (10 billion) times per second. The pressure of a gas is a result of the collisions of the molecules with a surface.

Note The average speed of the molecules in a gas is proportional to the square root of the absolute temperature (kelvins).

Accordingly, the higher the temperature, the more violent the collisions between gas molecules, and the higher the likelihood that collisions will cause one or both of the collision partners to decompose. At very high temperatures (greater than about 5000 K to 6000 K), the collisions are so violent that molecules can no longer exist, such that the gas consists of only free atoms, ions, and free electrons.

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For gases, the interrelationships between pressure, volume, and temperature are fixed and are described by the ideal gas law, Equation 3-1, which can be written in three equivalent forms:

in which: ρ = density in (g/L) m = mass of the gas (g) M = atomic or molecular mass of the gas (g) n = number of moles of the gas, equal to m/M P = absolute pressure (atm) R = the gas constant (0.08205 L · atm/mol · K or 8.314 (J/K · mol) T = absolute temperature (K = °C + 273) V = volume occupied by the gas (L or m3, consistent with the value of R being used) Note that throughout this text, symbols in italics represent variables. Constants are presented in normal text.

Note Both the fresh air entrained into a fire and the gas that leaves the fire zone (air that is somewhat depleted of oxygen and laden with combustion products) follow the ideal gas law. If the temperature of the gas is increased at constant pressure, the average velocity of the molecules increases and the gas expands. If, at constant pressure, the number of moles of gas is increased by the combustion process, the volume of the gas is increased. If the volume is fixed and the temperature is increased, the pressure will increase.

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The air of Earth’s atmosphere is the most important gas affecting fire phenomena, so it is a good subject for examining a gas mixture. Air is primarily composed of nitrogen and oxygen, but it also contains small proportions of argon, carbon dioxide, and—especially on a humid day—water vapor. For simplicity, in this book, dry air is considered to consist of only nitrogen and oxygen.

At a given temperature and pressure, 1 L of air contains the same number of molecules as 1 L of pure nitrogen, oxygen, or any other gas. Out of every 100 molecules of dry air, 79 molecules are nitrogen and 21 molecules are oxygen. Thus the mole fraction of oxygen in air is 0.21, and the mole fraction of nitrogen in air is 0.79. An alternative way of representing this is in terms of volume percent. Dry air consists of 79 percent nitrogen by volume and 21 percent oxygen by volume; we can also say that the volume fractions of nitrogen and oxygen in air are 0.79 and 0.21, respectively. (See the Fire Measurement and the SI System of Units chapter.)

Sometimes it is important to express a gas mixture in terms of mass fractions. These values are not numerically the same as volume fractions because the masses of different molecules are almost always different. The mass of molecular oxygen is 32 because the atomic mass of oxygen is 16, and the oxygen molecule, O2, is diatomic. Similarly, the mass of molecular nitrogen is 28. Because the oxygen molecule is 32/28 times as heavy as the nitrogen molecule, dry air consists of more than 21 percent oxygen by mass. Thus, in dry air, the mass fraction of oxygen is (21 · 32)/[(21 · 32) + (79 · 28)] = 0.233. By difference, the nitrogen mass fraction in air is 0.767.

Properties of Liquids If the temperature of a gas is reduced, which in turn reduces the molecular speed, or if the pressure is increased, which in turn forces the molecules closer together, a point can be reached at which the gas condenses into a liquid. This is the point at which the weak attractive forces between the molecules overcome their tendency to separate after a collision.

In a liquid, the molecules touch each other, but they continue to move relative to each other. If a drop of ink is put into a glass of water, the ink molecules will diffuse slowly through the water because of the molecular motion.

Every liquid has a vapor pressure, which increases with increasing temperature. The vapor pressure of a liquid is the pressure of the gaseous molecules over the liquid at equilibrium—that is, when the rate of evaporation is equal to the rate of condensation. At the freezing point of a liquid, the vapor pressure of the liquid is essentially zero. At the boiling point of a liquid, the vapor pressure of the liquid is equal to the atmospheric pressure.

As presented in the Fire Measurement and the SI System of Units chapter, the SI unit of pressure is the pascal, Pa, and 1 atm = 101 kPa. Liquid water has a vapor pressure of 2.34 kPa at 68 °F (20 °C). If water vapor at a pressure greater than 2.34 kPa exists above the liquid water, then the water vapor will condense. If water vapor is present at a pressure less than 2.34 kPa, or if no water vapor is present (dry air), then liquid water at 68 °F (20 °C) will evaporate until the pressure of the water vapor reaches 2.34 kPa, or until the temperature of the liquid water drops below 68 °F (20 °C) because of evaporative cooling. If dry air exists over liquid water at 68 °F (20 °C) at a pressure above 2.34 kPa, the evaporation will occur slowly. However, if the pressure of the dry air is less than 2.34 kPa, the water will boil, resulting in rapid evaporation. If the temperature of the water is 212 °F (100 °C), boiling will occur unless the pressure of the dry air is greater than 1 atm.

Table 3-1 shows the vapor pressures of water and two combustible liquids for a series of temperatures. Notice that the changes in vapor pressure are more gradual at the lower temperatures, but increase more rapidly as the temperature approaches the boiling point (Tbp). Additional data on vapor pressure (as well as all the phase properties discussed in this chapter) can be found in various reference books, such as the Handbook of Chemistry and Physics [1] and Perry’s Chemical Engineers’ Handbook [2]. Figure 3-1 is a plot of the data in Table 3-1.

Note The relationship between the equilibrium vapor pressure of a liquid, Pv, and the ambient temperature, T, is given by the Clausius-Clapeyron equation: −L/RT

Pv = C e

or ln Pv = ln C – L/RT

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–3

where L is the heat of vaporization of the liquid (kJ/kg). L divided by –R (the ideal gas constant, 8.314 · 10 (kJ/K · mol), is the slope of a plot of the logarithm of the vapor pressure versus inverse temperature. The logarithm of the constant C is the intercept.

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Table 3-1 Vapor Pressures of Three Liquids at Different Temperatures

Properties of Solids When the temperature of a liquid is reduced, generally a freezing point will be reached at which the liquid changes into a solid. In a crystalline solid, or crystal, the atoms, ions, or molecules are fixed in regular geometric positions (a lattice) and cannot move through the solid. However, they can vibrate—that is, they can move back and forth on either side of their equilibrium positions in the crystalline lattice. When a crystal, such as ice or sodium chloride, is heated sufficiently, the force of these vibrations exceeds the forces holding the atoms, ions, or molecules in place, and the solid melts. The melting point is the same temperature as the freezing point.

Some liquids do not have a sharp freezing point and do not form crystals upon cooling. Instead, these liquids become progressively more viscous as they are cooled, so that the molecules are less and less free to move about, and finally the substance entirely loses its capability of flowing and becomes a solid.

The liquids capable of such transitions generally consist of relatively large molecules. When they solidify, the molecules are trapped in a random arrangement, as contrasted with the orderly arrangement found in a crystal. Such solids are called amorphous substances or glasses. Ordinary window glass is a familiar example. When window glass is heated, it gradually softens over a range of hundreds of degrees, rather than having a sharp melting point, as do crystals of ice and sodium chloride. Other examples of glassy solids include tar, asphalt, cold molasses, waxes, and many synthetic polymers, such as nylons and polystyrenes. (See the Fire Characteristics: Solid Combustibles chapter.) Some polymers can exist stably in both crystalline and amorphous forms. Paper is a crystalline form of cellulose, for example, while cellophane is an amorphous form of the same molecules.

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Figure 3-1 Temperature dependence of vapor pressure for three liquids.

Metals are a special kind of solid. They generally consist of positively charged atomic ions in a geometrically defined crystal lattice, with a matching number of electrons free to move through the lattice.

The fact that metals conduct heat and electricity far better than other solids is due to the high mobility of the electrons through the crystal lattice.

A metal can consist of a single pure element such as copper, aluminum, iron, or 24-carat gold, or it can be an alloy of two or more elements. Brasses are alloys predominantly of copper and zinc. Steels are alloys of iron, perhaps other metals, and carbon. White gold is an alloy of gold and a white metal, such as nickel, manganese, or palladium.

Physical and Chemical Change When a material is heated during a fire, it may undergo both physical and chemical changes. The next sections provide details of these two processes.

Physical Changes When a solid, initially at room temperature, is heated sufficiently, it proceeds through a sequence of thermophysical steps. These steps need not involve any chemical change in the molecular make-up of the material.

1.

2.

3.

4.

5.

The temperature of the solid rises.

The solid melts, or fuses, becoming a liquid.

The temperature of the liquid rises.

The liquid vaporizes, or gasifies.

The gas temperature increases.

Each of these steps requires an influx of enthalpy (heat) from the surroundings. The chemical heat from the fire provides that influx. As the room cools after the fire passes or is extinguished, the general process is reversed, and enthalpy is removed until the matter cools and solidifies. In the purely physical process being discussed here, this enthalpy removal is identical to the enthalpy influx. (If the solid had changed chemically during the fire, then the enthalpy removed during cooling would differ from the enthalpy influx during heating.) A material that is initially a liquid follows the same sequence, minus the steps involving a solid phase.

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The second and fourth steps in this sequence involve a phase change, also called a change of state. In these steps, the material changes from the solid state to the liquid state, and then from the liquid phase to the gaseous phase.

In all cases, the enthalpy change for a step that releases enthalpy is negative; the enthalpy change for a step that absorbs enthalpy is positive.

Examination of the energetics of these steps begins with the heat required to raise the temperature of a substance from a lower to a higher value when no change of state (phase change) occurs within this temperature range, ΔT. Equation 3-2 can be used to calculate the heat (or enthalpy) required to change the temperature of a given mass:

in which: ΔH = heat required (J) m = mass (g) cp = heat capacity of the substance (J/g · K), also referred to as specific heat (Table 3-2 provides values of the heat capacities of some substances.) ΔT = temperature change (°C or K) Table 3-2 Heat Capacity of Common Substances State

Substance

Heat Capacity (J/g-K)a

Gas

air

1.007b

argon

0.521c

water vapor

2.04 (373 K)b

n-hexane

2.26b

methanol

2.53b

water

4.18b

ice

4.2b

copper

0.385b

steel

0.46d

gypsum plaster

0.108c

aluminum

0.897b

concrete (normal weight)

0.9c

polystyrene

1.2e

polyethylene

1.9e

wood (oak)

2.4d

wood (pine)

2c

Liquid

Solid

a

Heat capacity values vary with temperature. These values are for 298 K except as noted. See the references for more complete coverage.

b

Values from Reference [4].

c

Values from Reference [3].

d

Values from Reference [5].

e

Values from Reference [6].

Note While the listed gases and liquids are unique substances, many types of each of the listed alloyed metals and polymers exist, and wide variations in such aggregate materials as concrete and plaster are possible. For these solid materials, the tabulated values are indicative rather than generic.

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The complement to simple heating of a solid is the energetics of phase changes. The transition between the solid state and the liquid state (melting or freezing) of a pure substance is characterized by a heat of fusion. Its value is numerically the same as the heat of solidification, but the sign will be different: melting absorbs heat, while freezing releases heat. For nearly all calculations, it is sufficient to assume this transition takes place, with no temperature change, at the substance’s melting point or, for glasses, the glass transition temperature.1 For a variety of materials, Reference [3] provides values for enthalpies (heats) of fusion and the temperatures at which melting occurs. However, many variants of a material characterized by a single name (e.g., “steel”) may exist, and there may not be values available for a particular formulation of interest.

The transition between the liquid state and the gaseous state (vaporizing or condensing) of a pure substance is characterized by the heat of vaporization, which is numerically equal, but opposite in sign, to the heat of condensation. Generally, one assumes all vaporization or condensation takes place at the boiling point. This is a good approximation for fire applications. As can be seen in Table 3-1, only small fractions of these liquids evaporate at temperatures less than 68 °F (20 °C) (room temperature), so only a small error is introduced by assuming that all the material is a liquid at room temperature.

However, if the room had been preheated and, therefore, significant evaporation had occurred at temperatures below the starting temperature, then a more complex calculation of the energetics of vaporization might be warranted.

For a variety of liquids, Reference [3] provides values for heats of vaporization and boiling points. The environmental pressure can affect values of both these parameters. However, except at high altitude, using data for 101 kPa gives reasonable results.

Phase change calculations can have important practical value. Consider a chemical storage area containing the two flammable liquid fuels identified in Table 3-2. To convert liquid ethanol at its boiling point 172 °F (78 °C) to ethanol vapor, the heat of vaporization is 837 J/g. By comparison, the heat of vaporization of n-hexane at its boiling point 156 °F (69 °C) is only 365 J/g. The boiling points are similar, but the heats of evaporation differ substantially. Thus n-hexane evaporates more readily than ethanol and poses a greater ignition hazard.

It is also possible for a solid to vaporize directly, without any liquid forming. For example, paradichlorobenzene crystals (C6H4Cl2, the active ingredient in moth balls) will, over a period of time, vaporize into air at room temperature. This process is called sublimation; the heat of sublimation of a solid is approximately equal to the sum of its heat of fusion and its heat of vaporization.

The reverse of sublimation—that is, the direct change from a vapor to a solid—can also occur. Water vapor in the atmosphere, for example, can change into snow or frost. Similarly, when gaseous carbon dioxide (at high pressure) is discharged from a hand-held fire extinguisher, the expansion causes cooling, and white particles of solid carbon dioxide form.

For mixtures of substances, the transitions between the gas, liquid, and solid states depend on the relative proportions of the substances as well as the temperature and pressure. The principles governing these transitions are complex. For example, the freezing point of a mixture of propylene glycol and water (like that found in common automotive antifreeze) is much lower than the freezing point of either pure propylene glycol or pure water. Similarly, the vapor pressures of liquid mixtures follow complex laws.

Textbooks on physical chemistry cover this subject.

Real combustibles are almost always assemblies of materials. A carpet might consist of a nylon fiber and a jute backing. Either or both of these materials are blended with a fire retardant to meet flammability requirements. A piece of upholstered furniture might be composed of a wooden frame, multiple padding materials, and a cover fabric. The last two components might themselves be mixtures or be made of multiple layers.

This composite nature is especially challenging to people who perform computer modeling of fires for building design or fire reconstruction. (See the Computational Modeling of Fires chapter.) The technology for modeling such an assembly does not yet exist; therefore, fire scientists have simplified the situation by using a value called the heat of gasification. This term encompasses all the heat input needed to take the test specimen from an intact entity at room temperature to a fully vaporized state—in other words, to proceed through all five steps listed at the beginning of this section, plus any chemical decomposition that might occur during the sequence. Reference [7] contains a table of heats of gasification for some materials.

In summary, during a fire, physical changes from state to state occur among the three states of matter (solid, liquid, gas). In a physical process, the molecules do not undergo any chemical change. Going

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from a gas to a liquid to a solid releases enthalpy, while going from a solid to a liquid to a gas—the common sequence in a fire—absorbs enthalpy.

Chemical Changes Fire is a type of combustion process. Combustion also takes place in jet engines, fossil fuel power plants, and the burners in gas stoves. In all these cases, any physical changes in the fuels are accompanied by chemical changes in which the burning molecules change into other molecules, releasing enthalpy in the process. The atoms themselves do not change, of course, but they react to combine in different ways with other atoms. Chemical bonds are broken and new chemical bonds are formed.

In a combustion process, the gross nature of the chemistry that occurs depends on the relative abundance of combustible material (fuel) and oxidizer (generally oxygen from the air). A stoichiometric mixture is a mixture of fuel and oxygen for which the masses of these two components are exactly those needed for complete combustion, with no residue of either component. For a combustible containing carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur, for example, the primary products of complete combustion in well-mixed fuel and air are carbon dioxide (CO2), water (H2O), nitrogen (N2), hydrogen chloride (HCl), and sulfur dioxide (SO2).

Note The big difference between fires and other types of combustion is that fires are unwanted and unexpected. Neither the fuel nor the burning environment is preselected and repeatably controlled as it is in, for example, an automobile engine or a power plant.

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To the extent that oxygen remains when all the fuel is consumed (i.e., there is excess air), the original mixture is referred to as fuel lean. Fuel-lean mixtures also tend to form the products of complete combustion. To the extent that unburned fuel remains after the oxygen is fully consumed (i.e., there is excess fuel), the mixture is referred to as fuel rich. Combustion under fuel-rich conditions leads to some formation of the products of incomplete combustion, such as carbon monoxide (CO), soot (mostly carbon atoms, with a small number of hydrogen atoms), and hydrogen cyanide (HCN). These are accompanied by some unburned fuel and some partially combusted molecules.

These three types of fuel–air mixtures are denoted by the equivalence ratio:

where mfuel is the mass of fuel and m is the mass of air and Φ < 1: fuel lean Φ = 1: stoichiometric Φ > 1: fuel rich Combustion is typically incomplete in large fires, which tend to be fuel rich in their most hazardous stages. In cases where insufficient oxygen is available for complete reaction to final products, additional information is needed to know which combustion products are formed and how prevalent those products are.

Fortunately, extensive research into this problem has yielded some general answers. In particular, for the formation of toxic CO, it has been established that the yield of CO rises from nearly zero at very fuellean mixtures to values of about 0.2 g of CO per gram of fuel consumed for fuel-rich mixtures (Figure 32) [8].

Even the simplest fire involves hundreds of chemical reactions. The rest of this section provides the basic information to characterize these reactions and focuses on the capability to answer three questions: • Which chemical change is occurring?

• How much enthalpy does the change absorb or emit?

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• How fast are the chemical reactions?

The following classes of chemical changes (with one or more examples of each) can occur in a fire: 1.

2.

3.

4.

Dissociation: H2 → 2 H Association (or recombination): 2 H → H2 Bimolecular exchange reaction: CO + OH → CO2 + H Condensation (the formation of a larger molecule from smaller species): 2 C2H4 (ethylene) → C4H8 (butylene)

5. Oxidation: In each of these examples, a series of individual reaction steps is involved. Here, only the overall, global reaction is shown for each fuel, which is highlighted in boldface. a. b. c. d.

Hydrogen: 2 H2 + O2 → 2 H2O Carbon monoxide: 2 CO + O2 → 2 CO2 Methane: CH4 + 2 O2 → CO2 + 2 H2O Propane: C3H8 + 5 O2 → 3 CO2 + 4 H2O

Figure 3-2 Yield of carbon monoxide as a function of the fuel/air equivalence ratio.

6. Decomposition (again, showing only the sum of the individual reaction steps): a. CH4 (methane) → C (solid carbon) + 2 H2 b. (C2H4)n (polyethylene) → n C2H4 (ethylene) The rate at which any chemical change takes place depends on the temperature: the higher the temperature, the more rapidly the chemical change occurs. Many chemical systems are unreactive at room temperature but react very rapidly at higher temperatures. For example, a piece of acid-free paper exposed to air at room temperature for 100 years will undergo only a very slight degree of chemical reaction, as evidenced by gradual yellowing and brittleness. However, if this piece of paper is placed in a furnace containing air at 1100 °F (600 °C), it will burst into flames and be consumed in a few seconds.

Flames are the result of sequences of gas-phase chemical reactions. Liquid fuels vaporize and solid fuels undergo pyrolysis (thermal decomposition) first, with both processes producing vapors that then react with oxygen. The vaporization, decomposition, and pyrolysis processes almost always absorb heat.

Reactions that absorb heat are called endothermic reactions. (Solid pyrotechnic mixtures and solidpropellant rocket fuels constitute exceptions to this rule; they decompose exothermically.) Fires almost

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always involve oxidation reactions between various combustibles and the oxygen in the air. These reactions release heat, and are referred to as exothermic reactions.

Principle of Combining Proportions When describing an oxidation process, we often say something like, “Propane burns by reacting with the oxygen in air.” Translated into chemical language, this might be written as: C3H8 + O2 → CO2 + H2O However, in the examples earlier in the “Chemical Changes” section of this chapter, this was written as C3H8 + 5 O2 → 3 CO2 + 4 H2O The latter equation has been balanced so that the number of each type of atom is the same on the left side of the equation as it is on the right side. This is consistent with the fact that, in non-nuclear reactions, atoms are neither created nor destroyed—they just change partners. Balancing is important because it allows calculation of important quantities such as the following: • • • •

The mass of oxygen (or volume of air) required to completely burn a chosen mass of fuel The residual volume percent of oxygen after a certain mass of fuel has been consumed The maximum heat that can be released from burning of the fuel The maximum temperature that might be reached in the fire room

Here are the steps that were followed to balance the propane equation: 1.

2.

3.

4.

Write C3H8 + x O2 → y CO2 + z H2O. (x, y, and z are unknown for now.)

Assign y = 3 to make the carbon balance correct.

Assign z = 4 to make the hydrogen balance correct.

Knowing y and z, study the oxygen balance. The right side has (3 · 2) + (4 · 1), or 10 oxygen atoms; therefore, the left side must have 5 O2 molecules (10 oxygen atoms). Thus x must be 5.

Propane often burns in air, rather than in pure oxygen. Given this fact, it is useful to create a balanced chemical equation that includes the nitrogen in the air, even though it does not enter into the reaction.

Because 1 mole of any gas occupies the same volume as 1 mole of any other gas, the volume of a mixture of gases at a given temperature and pressure is the sum of the volumes of the component gases at the same temperature and pressure. Because air contains 21 percent oxygen by volume, completely combusting one volume of propane will require 5/0.21 = 23.8 volumes of dry air. Alternatively, 1 volume of dry air is sufficient to combust 0.042 volume of propane completely. The resulting balanced equation is 0.79 N2 + 0.042 C3H8 + 0.21 O2 → 0.79 N2 + 0.126 CO2 + 0.168 H2O If less air than this amount is available, only partial combustion can occur. Balanced equations can be written to represent these conditions, if the combustion products and their relative concentrations are known.

The elements on the left side of the equation are gaseous, as are the first two molecules in the right side of the equation. But what about the water molecules? The volume fraction of water is 0.168/(0.79 + 0.126 + 0.168) = 0.15. At 101 kPa (1 atm) total pressure, the vapor pressure of the water would be 15 kPa. Looking at Table 3-1, we see that this value is similar to the vapor pressure of water at 130 °F (55 °C). Thus, in the hot vicinity of the fire, where the temperature exceeds 130 °F (55 °C), all of the generated water would be in the form of vapor. At a distance from the fire where the combustion gases have cooled to less than 130 °F (55 °C), some of the water will have condensed to the liquid phase. It might be carried in the flow of water as droplets or deposited on the walls and floor.

For applications where masses of the gases are reacted, recall that the molecular mass of propane is 44 ([3 · 12] + [8 · 1]), that of oxygen is 32, that of CO2 is 44 (12 + [2 · 16]), and that of water is 18 (16 + [2 · 1]). The combining proportions of the propane oxidation reaction are (1 · 44) + (5 · 32) → (3 · 44) + (4 · 18), or 44 + 160 → 132 + 72

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Therefore, 44 g of propane will combine with 160 g of oxygen to form 132 g of carbon dioxide and 72 g of water. As expected from the conservation of matter, there are 204 g on both sides of the equation.

In summary, fires involve not only physical changes of state, but also substantial chemical changes.

The nature of these changes depends on the nature of the fuel and the equivalence ratio in the fire. The overall chemical change can be represented by a single, balanced chemical reaction. Use of the principles in this section permits calculation, either on a mass basis or, for gases, on a volume basis, of the combining proportions (or stoichiometry) in which any chemical substance reacts with oxygen, as long as the following elements are known: • The chemical formula of the combustible, in terms of its elements • The air availability • The combustion products that are expected to form

Energetics of Chemical Change Every chemical change is accompanied by a change in enthalpy (ΔH). This change usually is manifested in the form of heat, but it also can take the form of electrical energy (such as electrochemical processes in batteries) or mechanical energy (such as the movement of pistons in an automobile engine). Unless otherwise stated, this text presumes that the chemical reactions absorb or generate heat only.

Consider the following chemical reaction for the burning of carbon, perhaps in the form of charcoal:

This equation shows that if solid carbon, C(s), at 25 °C (298 K) reacts completely with gaseous oxygen O2(g) at 25 °C, and if the resulting hot carbon dioxide gas is cooled to the original temperature (25 °C), then the reaction generates 393.5 kJ of heat for each mole (12 g) of carbon consumed. The minus sign before 393.5 kJ indicates that the reaction is exothermic; that is, heat is released to the surroundings. In the combustion community, the negative of ΔH for a combustion reaction, which is then a positive number, is called the heat of combustion.

Now consider another chemical reaction, the oxidation or combustion of carbon monoxide (CO):

When the system is maintained at constant pressure, the heat of combustion of CO is 283.0 kJ. The phrase “at constant pressure” merits additional discussion. Any reaction occurring in the open takes place at approximately 1 atm of pressure.2 But consider the reaction occurring in a closed container. In the preceding example, 1.5 moles of reactant gases (1 mole of CO and ½ mole of O2) combine to form 1 mole of CO2. After the CO2 is cooled to the original temperature, the gas still fills the container, so the pressure is now two-thirds of the original value (i.e., 0.67 atm). To maintain the pressure at 1 atm, the ideal gas law says that the volume must be decreased, so the outside atmosphere must perform mechanical work to compress the gas to the smaller volume. This adds enthalpy to the system, with a magnitude given by ΔPV.3 For the equation given earlier, this enthalpy change is approximately 1.24 kJ. If the same process occurred at constant volume instead of constant pressure, the heat liberated would be equal to the energy of the reaction and would release 1.24 kJ less. This compression enthalpy is less than 1 percent of the heat of combustion and, for purposes of estimation, would be considered negligible. This will be the case for most fires burning in the open, and it is the reason that many practitioners use “energy” and “enthalpy” interchangeably (as discussed in the Fire Measurement and the SI System of Units chapter). For combustion occurring in a containment vessel or in a cylinder in an internal combustion engine, the compression term can be considerably larger (as discussed in the Fire Characteristics: Liquid Combustibles chapter) and should not be disregarded.

Another practical consideration is that, for most oxidation processes, two values of the heat of combustion are possible. To illustrate this, consider the combustion of the gas ethylene (C2H4) at constant pressure:

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The heat liberated by this reaction depends on whether the water in the products, after cooling to 298 K (25 °C) is assumed to be in the form of gas or liquid. At 25 °C and 1 atm, water will be primarily a liquid rather than a gas. This generality suggests that the larger value, 1411 kJ/mole, which is called the higher or gross heat of combustion, should be used. However, in fires, the water vapor in the products does not condense immediately because cooling is slow. Thus the smaller value, 1323 kJ/mole of ethylene, which is the lower or net heat of combustion, should be used. The difference between the higher and lower values is the heat of condensation of water, or 44 kJ/mole at 298 K. In the ethylene case, 2 moles of water, or 88 kJ, is involved per mole of ethylene that is burned.

Both higher (gross) and lower (net) heats of combustion of many substances are tabulated in handbooks, and it is important to use the value that is correct for the application at hand [3]. Table 3-3 presents the net heat of combustion for some common substances that might be involved in fires.

The ratio of the heat of combustion of a substance to the mass of air required by the principles of combining proportions is constant (to within about ±5 percent) for nearly all of the combustibles encountered in unwanted fires [10]. The value is 13.1 kJ per gram of oxygen required. This is an extremely valuable finding, in that one can estimate the heat generated by a fire without knowing exactly what was burning. (See the Fire Characteristics: Solid Combustibles chapter.)

Table 3-3 Net Heat of Combustion for Various Gases, Liquids, and Solids

a

The values are from Reference [9] except those noted by *, which are from Reference [1]. To calculate gross values of the heat of combustion, add 44 kJ times the moles of water formed per mole of combustible to the kJ/mole column, or 2.4 kJ times the grams of water formed per gram of combustible to the kJ/gram column.

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Most combustibles, when burned with just the stoichiometric amount of air, under conditions where no heat is lost, will produce flames at temperatures from 2100 to 2300 K, and chemical equilibrium would be achieved rapidly in such flames. For each fuel, this temperature is called the adiabatic flame temperature. It is the highest possible temperature for the combustible mixture because all the possible chemical heat is released and all of that heat is applied to raising the temperature of the mixture.

If the fuel is totally consumed and if all the combustion products are gases, then the adiabatic flame temperature (Tad) is given by the heat released (ΔH298, J/g) divided by the sum of the heat capacities (J/g · K) of all the gases on the right side of the balanced combustion equation:

Here, nT is the number of moles of gas on the right side of the equation, ni is the number of moles of one gas on the right side of the equation, and cp,i is the heat capacity of that gas (J/g). The symbol Σ indicates the sum of the contributions of all the gases on the right side of the chemical reaction equation.

Note that the nitrogen from the air must be included because it is present in large quantity and has a non-negligible heat capacity. The concept of this equation is straightforward, but the solution is more difficult because the heat capacities of the gases increase with increasing temperature.

In fires, combustion often occurs with a yellow luminous flame because of tiny, hot carbon particles that form and radiate heat away from the combusting system. As much as 30 percent to 40 percent of the heat of combustion can be lost from such a flame because of this radiation. Additional heat is absorbed by the walls and ceiling of the fire room (see the Heat Transfer chapter) as well as in gasifying additional fuel for the fire. Consequently, the maximum flame temperature in a fire will be near 1500 K to 1600 K.

Chemical Equilibrium and Chemical Kinetics If the right types of molecules were placed in a box and the temperature raised sufficiently, the molecules would begin to react. At some point in time, the reactions would slow and then cease because the atoms in the box would have rearranged themselves into the most stable combinations possible at the prevailing temperature and pressure. This final state is called chemical equilibrium. It is possible to calculate, by chemical thermodynamic procedures described elsewhere [10], the molecular composition that any mixture at a given temperature and pressure will attain when chemical equilibrium is reached.

The time required to reach this final state is controlled by chemical kinetics—that is, by the rates of the chemical reactions. It is much more difficult, but possible in some cases, to calculate the time needed for the process to reach equilibrium. If a reaction rate cannot be calculated, it often is possible to estimate the rate or to measure it in an experiment.

In many cases of practical combustion, the fuel and air do not remain at high temperatures long enough to reach equilibrium. As a result, some products of incomplete combustion will survive. We see the results as tailpipe emissions from automobiles and soot from fires.

To illustrate these important concepts, consider a mixture of 2 moles of hydrogen and 1 mole of oxygen, with any reaction occurring according to the following expression: 2 H2 + O2 → 2 H2O A thermodynamic calculation shows that when this system reaches chemical equilibrium (at 77 °F [25 °C] and 1 atm), more than 99.999 percent of the molecules present will be H2O molecules rather than H2 and O2 molecules.

However, when we perform this experiment and mix hydrogen and oxygen at 77 °F (25 °C), nothing observable happens. Even if the mixture is stored for a year, only a tiny fraction of the molecules will have reacted. Clearly, the rate of this reaction is extremely slow at 77 °F (25 °C), even though it is energetically favored. Now suppose the temperature of the hydrogen–oxygen mixture was raised steadily in a gas-tight container. The mixture would explode suddenly at 1040 °F (560 °C), changing completely to water within a fraction of a second.

If the mixture had been heated to only 930 °F (500 °C) at 1 atm, no reaction would have occurred. If the temperature were then kept constant at this value and the pressure raised gradually, an explosion would occur when the pressure reached 4 atm. Even more surprising, if the pressure had been lowered

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at 930 °F (500 °C) instead of being raised, an explosion would have occurred when the pressure dropped to 0.05 atm.

This reaction was studied extensively, and a Nobel Prize was awarded in 1956 to the two scientists who first unraveled this mystery. They found that chain reactions of the atoms and free radicals H, O, OH, and HO2 are involved. Today, the rate at which any mixture of hydrogen and oxygen (or air) will go to equilibrium (that is, form water) can be predicted accurately for any set of experimental conditions.

Similar chemical kinetic considerations apply to the ignition temperature of any combustible with air.

However, as the number of atoms in the fuel molecule increases, so does the number of reactions in the ignition process. While proven mechanisms for the ignition of some smaller hydrocarbon fuels have been identified, the mechanisms for larger fuel molecules and mixtures of these fuels (as in gasoline) have not yet been worked out fully.

A general finding in chemical kinetics is that rates of chemical reactions increase with increasing temperature. For many organic fuels, reaction rates double with every 50 °F (10 °C, or 10 K) increase in temperature. The explanation for this behavior is that molecules move faster at higher temperatures and, as a consequence, undergo more violent collisions with one another, causing chemical bonds to break more readily. At flame temperatures, the chemical reactions that consume the fuel are complete in a small fraction of a second. Figure 3-3 illustrates this phenomenon.

Figure 3-3 The relationship between the temperature of a gas and the fraction of molecular collisions capable of breaking chemical bonds and, therefore, causing a reaction to occur.

WRAP-UP Chapter Summary • The world is composed of gases, liquids, and solids. These states, or phases, can be characterized by their sight, smell, touch, and mobility.

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• In a gas, the molecules move independently. The bulk properties of a gas are related by the ideal gas law. Mixtures of gases can be described by fractions based on volume (number of molecules) or mass.

• The molecules in a liquid have limited mobility relative to gases, and the unaided mixing of liquids therefore occurs more slowly. A liquid generally gasifies by evaporation; in other words, the same molecules exist in the gas phase as in the liquid phase. The vapor pressure and vaporization rate of a liquid increase with temperature and are important factors in the ease of ignition of flammable fluids.

• When heated, solids generally pyrolyze, generating gaseous fragments that are different from the chemical structure of the solid.

• The heat required to raise the temperature of a liquid or solid depends on the heat capacity of the material and the temperature rise of interest. In addition, heat must be added to a material to produce a phase change.

• Fire is a combustion process. Its products can be grouped based on whether the combustion is fuel lean, stoichiometric, or fuel rich. The most damaging fires are fuel rich.

• Balancing the overall fuel–air reaction equation provides a tool for quantifying the enthalpy generated by the combustion. Such an equation reflects the nature of the combustion products. For fuel-lean or stoichiometric conditions, the products are mostly those of complete combustion. When fires burn in insufficient oxygen, additional information is needed to identify and quantify the combustion products.

• Chemical thermodynamics provides information regarding the combustion products. Chemical kinetics determines how quickly the combustion proceeds and whether there is time for the fuel-air mixture to reach equilibrium. Burning reactions tend to proceed at rates that increase exponentially with temperature.

• The heat released by the burning of an organic fuel per gram of oxygen consumed is nearly independent of the chemical structure of the fuel.

• The flame temperature is a function of the heat of combustion of the fuel, the heat capacity of the atmosphere, and heat losses to the surroundings. The temperature of flames in fires is several hundred kelvins lower than the maximum possible, or adiabatic, flame temperature.

Key Terms adiabatic flame temperature The maximum possible temperature for a combustible mixture, reached when there is complete combustion of the fuel and all of that heat is applied to raising the temperature of the mixture. alloy A mixture of two or more metals. balancing The process of adding coefficients to the atoms and molecules in a chemical equation, such that the number of each type of atoms is the same on the left (reactant) side of the equation as on the right (product) side of the equation. chain reaction A self-sustaining sequence of chemical reactions in which reactive atoms or free radicals are formed, leading to further reactions. change of state The transformation of a compound from one phase to another (e.g., liquid to gas) without a change in chemical composition. chemical equilibrium The state in which all chemical species are present at concentrations that have no further tendency to change with time. chemical kinetics The study of the rates at which chemical systems change their compositions in their approach to equilibrium. chemical thermodynamics The flow of enthalpy or energy associated with chemical reactions leading to a state of chemical equilibrium. endothermic reaction A chemical change that absorbs heat. exothermic reaction A chemical change that releases heat. fuel lean A mixture of fuel(s) and oxidant(s) in which there is an excess of oxidant, relative to a stoichiometric mixture. fuel rich A mixture of fuel(s) and oxidant(s) in which there is an excess of fuel, relative to a stoichiometric mixture. glass transition temperature A point within the temperature range over which an amorphous solid liquefies, corresponding to the substance attaining a specified value of a physical property, such as viscosity. gross heat of combustion The value of the heat of combustion when the water formed is condensed to liquid form; also called the higher heat of combustion.

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heat of combustion (at constant pressure) The enthalpy released when 1 mole of a combustible reacts completely with oxygen at 101 kPa and 298 K to form combustion products at 298 K. heat of fusion The energy absorbed when a unit mass of a solid melts without chemical change. heat of gasification (of a solid) The enthalpy absorbed by a solid when it vaporizes or decomposes to form gases. heat of solidification The enthalpy released when a unit mass of a liquid solidifies without chemical change. heat of sublimation The enthalpy absorbed when a unit mass of a solid gasifies directly, without forming a liquid and without chemical change. heat of vaporization The enthalpy absorbed when a unit mass of a liquid vaporizes without chemical change. net heat of combustion The value of the heat of combustion when the water formed remains as a vapor; also called the lower heat of combustion. oxidation The reaction of a compound with oxygen. (More generally, the loss of electrons from an atom or molecule during a reaction, with a resulting increase in its valence.) phase change A change of state. pyrolysis The thermal decomposition of a substance into other molecules. The verb form, both transitive and intransitive, is “pyrolyze.” stoichiometric mixture A mixture of reactants in which the quantities of the reactants are exactly those needed for the formation of the specified products, with no excess of any reactant remaining after the reaction has been completed. For combustion processes, the reactants are a fuel and oxygen. sublimation The evaporation of molecules from a solid to form a gas in the absence of a liquid and without chemical change. vapor pressure The pressure of gaseous molecules over their liquid phase at equilibrium—that is, when the rate of evaporation is equal to the rate of condensation.

Challenging Questions 1.

One mole of liquid water is in a closed vessel whose temperature is 273 K, whose initial internal pressure is 1 atm (the space above the liquid is filled with air), and whose interior volume is 50 times the volume of the water. Calculate the volume of the vessel. Now the vessel is heated to the boiling point of water. Assuming that the gases are ideal, calculate the pressure in the vessel.

2.

On a day when the temperature is 30 °C and the relative humidity is 90 percent, what is the percentage of oxygen by volume in the air?

3.

How many joules are needed to raise the temperature of 1 kg of gypsum plaster by 1 °C? How many joules are needed to raise 1 kg of oak to the same temperature? If identical, steady fires were burning in identical rooms, except that one room is lined with wood and one room is lined with gypsum plaster, in which room would the walls tend to be hotter? (Assume that the walls themselves are not burning.)

4. When water vapor condenses to a liquid, must heat be added or removed to maintain a constant temperature?

5. What is the difference between physical change and chemical change in a material?

6. What is the usual effect of temperature on the rate of chemical change in a combustion reaction?

7. Explain the following terms: stoichiometric mixture, mole, endothermic, net heat of combustion, and gross heat of combustion.

8.

Balance the equation for benzene (C6H6) reacting with oxygen to form carbon dioxide and water vapor in a fuel-lean fire. In the fuel-rich stage of a fire, 10 mole percent of the fuel carbon can form carbon monoxide. Revise and rebalance the benzene combustion equation to reflect this information.

9.

Estimate the final air temperature in a closed room (8 m × 4 m × 3 m) immediately after 5 kg of paper (cellulose) burns.

A. Balance the combustion equation. Assume that the combustion is complete when the mole fraction of oxygen reaches 0.15.

B. Calculate the heat generated by the complete combustion of this mass of paper.

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C. Calculate the mass and total heat capacity of the air in the room, assuming that the contribution of the combustion products is negligible.

D. Calculate the temperature rise of the air, neglecting any other thermal processes, and then add the starting temperature.

E. Repeat this calculation for 5 kg of a polystyrene. Which room was hotter and by how much?

10. List four reasons why the temperature of a flame is lower than the adiabatic flame temperature.

References 1. Haynes, W. M., ed. (2011). Handbook of Chemistry and Physics, 92nd ed. Boca Raton, FL: CRC Press. The Handbook is updated annually. For further information, see http://www.hbcpnetbase.com/.

2. Perry, R. W., and D. W. Green, eds. (1997). Perry’s Chemical Engineers’ Handbook, 7th ed. New York, NY: McGrawHill.

3. Perry and Green, Section 2.

4. DiNenno, P. J., ed. (2008). SFPE Handbook of Fire Protection Engineering, 4th ed. Quincy, MA: National Fire Protection Association.

5. Drysdale, D. D. (2008). Chemistry and Physics of Fire. In: Fire Protection Handbook, 20th ed., A. E. Cote, ed. Quincy, MA: National Fire Protection Association, Chapter 3.

6. Madrzykowski, D., and D. W. Stroup. (2008). Flammability Hazard of Materials. In: Fire Protection Handbook, 20th ed., A. E. Cote, ed. Quincy, MA: National Fire Protection Association, Chapter 3.

7. Tewarson, A. (2008). Generation of Heat and Gaseous, Liquid, and Solid Products in Fires. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 3–4.

8. Pitts, W. M. (1994). The Global Equivalence Ratio Concept and the Prediction of Carbon Monoxide Formation in Enclosure Fires (NIST Monograph 179). Gaithersburg, MD: National Institute of Standards and Technology.

9. Drysdale, D. D. (2008). Thermochemistry. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 1–5.

10. Friedman, R. (2008). Chemical Equilibrium. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 1–6.

1

Recall, from earlier in this chapter, that the transition of an amorphous solid to a liquid does not occur at a discrete temperature, but rather over a temperature range. The glass transition temperature is a point within this range at which a specified property value, e.g., a specified viscosity, occurs.

2

The pressure at high altitudes can be substantially lower. For instance, the average atmospheric pressure in Denver, Colorado, and Albuquerque, New Mexico, is approximately 0.8 atm. At all altitudes, there can be smaller variations in pressure due to prevailing weather systems.

3

A more complete description of this type of enthalpy change can be found in any physical chemistry textbook.

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CHAPTER 4

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Flow of Fluids OBJECTIVES After studying this chapter, you should be able to: • • • • •

Describe the basic laws of motion and gravitation.

Calculate pressures in a standpipe and a stairwell.

Calculate the velocity of a falling object and the time it takes to reach the ground.

Describe potential and kinetic energy.

Describe the effects of fluid viscosity and buoyancy on fire flows.

Introduction In February 1904, a fire spread from west to east across the city of Baltimore, Maryland. By the time the wind-aided fire was extinguished, approximately one-fourth of the city was destroyed. In 2013, 19 firefighters attacking the Yarnell Hills wildland fire in Arizona died when the fire enveloped them following a sudden change in wind direction. In the 1990 fire in the MGM Grand Hotel in Las Vegas, Nevada, people died in rooms as high as the 25th floor, even though the fire itself did not rise above the second floor. The highly toxic smoke had flowed up the elevator shafts.

Fluid flow considerations pervade the field of fire protection. Access to water at fire scenes is determined by the flow from the mains, through risers, and through hoses and automatic fire sprinklers. Air flow feeds both smoldering and flaming fires, and the flows from the fires transport smoke and toxic gases that threaten the safety of building occupants and emergency responders alike. The same fire-generated flows have beneficial effects, such as leading the products of combustion to trigger smoke alarms and heat to activate sprinklers.

The prior two chapters presented concepts and processes that inherently involve no net motion (i.e., no flow).

Although the molecules in a room full of air are moving very fast and colliding with one another frequently, there is no overall directionality to this activity; thus, the system as a whole is static. This chapter extends the concepts introduced earlier to dynamic systems, in which the entire system is on the move.

The chapter begins by introducing principles that govern the movement of ideal objects that are rigid—that is, objects that do not change as they move. (Think of the movement of a baseball.) All of these concepts are then combined in a discussion of the phenomena associated with the flow of fluids. These fluids, which can be liquids or gases, can change shape, density, and dimension as they flow. Of high interest in fires are the fresh air flow into a fire, the fire-driven air flow laden with combustion products, and the flow from a fire suppression system. More advanced material on fluid flow may be found in Reference [1].

Laws Governing Motions of a Rigid Body Momentum and Acceleration of a Rigid Body The subject of this chapter is motion, and there is no concept more fundamental to the effects that motion has on fires than the concept of momentum. At any given time, an object has momentum equal to mv, where m is the mass of the object and v is its velocity. (The total momentum is actually given by mass · speed, with the momentum in each of three orthogonal directions given by the mass times the velocity in that direction. In general, we will be talking about momentum in a particular direction, so the use of the term velocity is correct here.) When the object is at rest, its velocity is zero, as is its momentum.

In 1687, Sir Isaac Newton published his Philosophia Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), which established the basic laws of physics. This ground-breaking work identifies Newton’s three laws of motion.

Newton’s first law of motion states that an object at rest or in motion tends to stay that way unless acted upon by an outside force [2]. Alternative statements of this law are that momentum is conserved, that a body in motion stays in motion, and that an object’s velocity remains constant until the object is perturbed. Examples of such outside forces or perturbations include friction, headwinds, and tailwinds.

When a force or magnitude F acts on a body of mass m, the body accelerates in the direction of the applied force. The acceleration, a, is directly proportional to F and is inversely proportional to m. This is Newton’s second law of motion, as given by Equation 4-1:

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Using MKS units (see the Fire Measurement and the SI System of Units chapter), acceleration is given in units of m/s2, force is in newtons (N), and mass is in kilograms. The cgs unit of force is the dyne, with 1 N being equal to 100,000 dyn.

Newton’s third law of motion states that every action is balanced by an opposite and equal reaction.

Thus the force of water jetting from a fire hose is opposed by a backward force on the fire fighters holding the hose.

The Effect of Gravitation on a Rigid Body Also found in Philosophia Naturalis Principia Mathematica is Newton’s law of universal gravitation. This law states that any two (uncharged) masses are attracted to each other by a force acting along the straight line connecting the two masses. The magnitude of that force is proportional to the two masses and to the inverse square of the distance between the two masses, as given by Equation 4-2:

This equation is useful as is for estimating the orbits of planets and moons. It can also be simplified for calculating the downward effect on items in Earth’s gravitational field, which we simply abbreviate as gravity.

Objects near Earth’s surface are nominally one Earth radius from the center of Earth’s mass (ignoring the minor difference between an object at sea level and the same object atop Mount Everest), and M1, Earth’s mass, is nominally a constant (ignoring, for example, the occasional meteor that hits the surface). The resulting simplified equation is shown in Equation 4-3:

where the value of g is 9.8 N/kg (980 dyn/g). This equation looks very much like Equation 4-1.

Consider a bird feeder of mass m, hanging from a cord that is attached to a tree limb. The downward gravitational force of the feeder is exactly balanced by an upward tensile force in the cord. Thus there is no net force on the object, and it remains stationary. Now suppose that the cord is cut at time t0. The released object will accelerate downward with an acceleration, g, and its increasing velocity, v, may be calculated as shown in Equation 4-4:

Thus the velocity increases linearly with time and may be calculated at any time interval. (Equation 4-4 assumes that the frictional resistance force caused by the motion of the falling object through air is negligible. This is a good approximation for the first several seconds of free fall. To accurately describe the motion after this point, a solution based on calculus must replace the current algebraic one.)

The distance, d, that the object has fallen during the interval (t – to) may be calculated as follows.

Because the velocity increases linearly with time, with the initial velocity being zero and the final velocity being vf, the average velocity vav is vf/2. Therefore, as shown in Equation 4-5:

Elimination of vf between Equations 4-4 and 4-5 gives Equation 4-6:

The object’s mass does not appear in this equation; thus, all objects, regardless of their masses, will fall the same distance in a chosen time interval. In reality, the resistance from dropping through air is not always negligible, and a light object of large cross-section will be slowed to a greater extent than a heavy object of small cross-section.

Potential Energy and Kinetic Energy: Mechanical Work 47

The concept of a change in enthalpy or energy applies to mechanical systems as well as to the thermal systems presented in the Physical and Chemical Change chapter. If the change in a mechanical system does not result in expansion or compression, then the changes in energy and enthalpy are equal. In this light, we will use “energy” here, rather than repeating both terms.

Energy is conserved. It is neither created nor destroyed; it just changes form—thus states the first law of thermodynamics [1]. For example, the food energy expended to ride a bicycle is balanced by the mechanical movement of the bicycle and rider, minus the energy needed to overcome wind resistance, imperfect wheel bearings, and tire traction, as well as the energy dissipated by perspiration and stimulated breathing.

In addition to energy being described as chemical, mechanical, electrical, or some other form, it may be categorized as potential or kinetic. Potential energy is energy that is stored, waiting to be expended.

For example, the aforementioned bird feeder suspended by a cord has potential energy relative to the feeder after it has fallen to the ground. This difference in energy may be measured by the mechanical work that must be done to raise the feeder from the lower level to the higher level. Conversely, the feeder could be made to perform mechanical work while it descends from the higher to the lower level.

One joule of mechanical work consists of a force, F, of 1 N acting through a distance, d, of 1 m. In the cgs system, 1 erg of work is done by a force of 1 dyn acting through 1 cm. One joule equals 107 erg.

The force acting on an object in a gravitational field is g times the mass of the object. Let h be the height (vertical distance) between two locations. Then the difference in potential energy of an object of mass m at these two locations is given by Equation 4-7:

Mechanical potential energy can also reflect a pressure, P, ready to act through a volume, V. One joule of work is done by a pressure of 1 N/m² acting through a volume of 1 m³. An example is the potential energy of the compressed air in fire fighters’ breathing apparatus.

Kinetic energy is associated with motion. Suppose that a force F acts on a body of mass m, which is initially at rest. After the first instant, the mass is moving at some velocity. The force is still being applied, so in the second instant, the mass moves faster; that is, it is accelerated. At some time, the object will have moved a distance d, accelerating all the while. Assuming that no other force is acting on the object, the kinetic energy imparted to the object is F d, which is the same as the mechanical work done on the object, according to the principle of conservation of energy. From Equation 4-1, F = ma, and from generalization of Equation 4-6. We have Equation 4-8:

The acceleration in this example is constant in Equation 4-9:

If time, t, is eliminated between Equations 4-8 and 4-9, the result is shown in Equation 4-10:

Thus, from Equations 4-1 and 4-10, the increase in kinetic energy is given by Equation 4-11:

Returning to the bird feeder, as it drops from its original height, the potential energy is converted to kinetic energy. An instant before it hits the ground, the kinetic energy is equal to the original potential energy. Because kinetic and potential energy are quantified by Equations 4-11 and 4-7, respectively, it follows that

Rearrangement of Equation 4-12 to solve for v gives Equation 4-13:

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Notice that the mass of the falling object has cancelled; there is simply a relationship between the velocity of the object and the height it has fallen. Thus, if air resistance is neglected, an object dropped from height h will achieve a velocity v upon impact, independent of its mass. The impact velocity is proportional to the square root of the distance the object falls.

Conversely, an object projected upward with an initial velocity v will reach a maximum height (or apogee) of h. This height can be calculated using Equation 4-13.

Basic Elements of Fluid Behavior Force and Pressure A fluid is subject to change in shape and dimension during a physical or chemical process, so its characterization is different from that a rigid solid. For a fluid, the concept of pressure, P (force per unit area), is used instead of force, and the concept of density, ρ (mass per unit volume), is used instead of mass. The pressure may vary from one region in a fluid to another.

For gases, which are readily compressible, the density can also vary from one region of the fluid to another. When you exhale, for example, you decrease the volume of your lungs, increasing the pressure of the air within. Following the ideal gas law (see the Physical and Chemical Change chapter), the density of the air increases proportionately. Just after the air leaves your mouth, it is at room pressure and density.

For incompressible fluids such as water, the density is the same everywhere. When you drink water through a straw, your sucking on the straw reduces the pressure inside, while the pressure on the water surface in the glass remains at 1 atm. The water responds to the pressure difference by flowing through the straw from the higher pressure end to the lower pressure end. The density of the water remains the same, however; it is 1 g/cm3 both in the glass and in your mouth.

As a practical example of a pressure calculation, consider a vertical column of water of density, ρ, filling a standpipe of height, h, and cross-sectional area, A. The gravitational pull on the column squeezes the water near the bottom of the column. As a result, the pressure within the water is higher at the bottom and decreases with increasing height.

This pressure difference is easily calculated. The volume of the standpipe is given by hA. The mass of the water in the standpipe is equal to hAρ. The gravitational force acting on this mass is equal to ghAρ. The difference in pressure between the bottom of the column and the top of the column is ΔP. Pressure is force per unit area, so the force difference associated with this pressure difference is AΔP. Because the column of fluid is not moving, and no force other than gravity is acting on it, the pressure force must balance the gravitational force. When these two forces are equated, the cross-section A cancels, and the result is Equation 4-14:

For water in a standpipe, g and ρ are constants, and the pressure drop increases with increasing height.

This important equation, while derived for an incompressible fluid, is also approximately true for air at atmospheric pressure, as long as no temperature gradients are present, and as long as the pressure difference is a small fraction of the total pressure. Consider a stairwell in a three-story building.

Substituting g = 9.8 N/kg, h = 10 m (about three stories), and ρair = 1 kg/m3 in Equation 4-14 leads to a pressure difference of 98 Pa, which is only a 0.1 percent difference in pressure from the top to the bottom of the stairwell.

A second type of fluid calculation involves a fluid streaming from a hole in an otherwise closed volume. If the pressure of the fluid inside the volume, near the hole, is higher than the external pressure by an amount ΔP, a jet of fluid will emerge from the hole with velocity v. This velocity may be calculated as follows.

As the fluid leaves the volume, the potential energy of the stationary fluid inside the volume is converted to kinetic energy. Equation 4-11 tells us that the kinetic energy of a unit volume of fluid after accelerating to velocity v is ρv2/2. The potential energy that the unit volume of fluid had before emerging is equal to the work that would have to be done to push a unit volume of fluid back into volume against the adverse pressure gradient. This work is equal to the pressure difference times the volume of fluid (also taken as unit volume), so the change in potential energy is ΔPV. Equating potential and kinetic energy gives Equation 4-15:

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This equation may be rearranged as shown in Equation 4-16:

The excess fluid pressure in Equation 4-16 could arise from the effect of gravitation on the fluid (hydrostatic effect), as given by Equation 4-14, it could arise by the action of a pump, and/or it could be caused by thermal expansion of the fluid. If the pressure is solely hydrostatic, then Equation 4-14 may be substituted into Equation 4-16 to give Equation 4-13. Yes, the equation for the velocity of a fluid jet shooting from an orifice is exactly the same as the equation for the velocity achieved by a falling rigid body in the absence of friction. This similarity arises because both equations are statements of the equivalence of the kinetic energy as measured by velocity and the potential energy associated with position in a gravitational field.

Let us return to the first case of water in a vertical standpipe, but now adding a pump. The fluid is flowing and has kinetic energy associated with its velocity. Assume that the fluid element is moving along a streamline1 from point 1 to point 2. Then Bernoulliâ&#x20AC;&#x2122;s equation applies as shown in Equation 417:

The first term on each side of the equation represents the energy from the pump; the second term represents the potential energy associated with position in the gravitational field; and the third term represents the kinetic energy associated with the fluid movement within the tube. Equation 4-17 says that the sum of these three energies is constant along a streamline, as long as negligible energy is dissipated by friction.

Equation 4-17 reduces to Equation 4-15 when the flow is horizontal (h1 = h2). It reduces to Equation 4-16 when there is no pump and the pressure difference is caused only by hydrostatics.

Viscosity It is more difficult to pull some fluids through a tube than others. For example, it takes more effort to suck a milkshake through a straw than it does to suck water through the straw. It takes even less effort to inhale air through the straw. This effort, or force, is needed to overcome the frictional resistance offered by the fluid. The property of the fluid that gives rise to this friction is its viscosity.

Viscosity is important in fluid flow for three reasons: 1. When a fluid flows over an immersed object or through a duct, a drag force slows the flow. The magnitude of this drag depends on the viscosity of the fluid. This factor must be taken into account when considering the flow of water through a sprinkler pipe or the flow of fire effluent down a long corridor outside a burning hotel room.

2. At the perimeter of the moving fluid, adjacent to any surface or obstruction, is a slow-moving region of the fluid. The thickness of this boundary layer depends on the viscosity of the fluid and determines the temperature difference and thus the heat transfer between the fluid and the surfaces (discussed further in the Heat Transfer chapter). In the main fluid flow (i.e., outside the boundary layer), the flow is independent of viscosity and is governed by the principles of conservation of energy and momentum discussed earlier.

3. A fluid may flow either in a laminar (streamlined) or turbulent (fluctuating) fashion, and viscosity is a principal factor in determining which mode occurs. This behavior is discussed further in the next section. Higher viscosity tends to suppress turbulence.

To visualize viscous effects, imagine a cylinder moving at a constant velocity inside a slightly larger, fixed cylinder. The annular space between the two cylinders is filled with a fluid. A force must be applied to the inner cylinder to keep it moving at that velocity. This force is proportional to the velocity, v, the surface area of the inner cylinder, A, and the gap between the cylinders, x, as shown in Equation 4-18:

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The proportionality factor, η, is called the coefficient of viscosity of the fluid or simply the viscosity. In the MKS system, the unit of viscosity is the pascal-second (Pa · s), where 1 Pa · s is equal to 1 N · s/m2.

The more generally used unit is the poise (cgs system); 1 Pa · s equals 10 poise. The use of centipoise (for liquids) and micropoise (for gases) is a common convenience.

Values of η are determined experimentally, with these experiments being repeated with different values of A, v, and x. If the viscosity does not change as these parameters are varied, the fluid is said to be Newtonian. Most common fluids, including all gases and water, are Newtonian.

The viscosity of a fluid varies significantly with temperature, but is only very slightly dependent on pressure. As a liquid is heated, it becomes less viscous; as a gas is heated, it becomes more viscous.

Table 4-1 shows values of viscosity for a number of fluids.

Laminar and Turbulent Flow The easiest way to visualize laminar flow is as a set of wall-less concentric cylinders of a fluid that are flowing through a cylindrical pipe. The fluid in the outermost cylinder (1) moves slowly, being subject to friction with the (stationary) inner surface of the pipe, where the fluid velocity is zero. The outside of the next fluid cylinder (2) is touching the inside of the outermost cylinder. The fluid in cylinder 2 moves faster than the fluid in cylinder 1, as its friction is with the fluid in cylinder 1 and that fluid is moving. The fluid in cylinder 3 moves faster than the fluid in cylinder 2, and so on. As shown in the left-hand drawing in Figure 4-1, the velocity profile across the pipe is bullet shaped. The average velocity is exactly half of the peak velocity. Mixing within the flow is slow, accomplished by diffusion. If red ink were injected near the top of the pipe and blue ink were injected near the bottom of the pipe, the first appearance of purple fluid would occur well downstream.

By contrast, turbulent flow in the same pipe would appear more vigorous. Eddies would promote crosswise mixing among the concentric cylinders, so the pipe is soon filled with purple fluid. The velocity profile across the pipe approaches a flat shape, as shown in the right-hand drawing in Figure 4-1, with the average velocity equal to 80 percent or more of the peak velocity.

The laminar or turbulent nature of these flows is characterized by the dimensionless Reynolds number, Re = dvρ/η, in which d is a characteristic distance and the other symbols are as previously defined. The choice of d depends on the specific application. For example, for flow through a cylindrical pipe, d is the inner diameter of the pipe. The Reynolds number may be understood as being the ratio of the inertial force associated with the moving fluid to the viscous force restraining the motion.

Table 4-1 Viscosities of Some Newtonian Fluids

* Air and water data from Reference [3]; n-hexane, glycerin, and mercury data from Reference [4].

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Figure 4-1 Laminar and turbulent flows in a pipe.

Note Laminar flow is characterized by Reynolds numbers less than approximately 2000. Fully turbulent flows are characterized by Reynolds numbers greater than approximately 3200. In between these two values, there is a transitional region that is extremely difficult to characterize.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

If a flow is in the laminar or transition regions and a turbulent flow is desirable, there are four ways to accomplish this: • Decrease the fluid viscosity—for example, by heating the fluid, adding a flow-enhancing chemical, or changing the fluid • Increase the diameter of the pipe • Increase the roughness of the inner wall of the pipe • Increase any pressure fluctuations in the flow, perhaps by changing the pump Another common way for turbulence to be generated is for a laminar flow to encounter an object, so that the fluid must flow around the object. If the Reynolds number is large enough (depending on the shape of the object and especially on the absence of streamlining), a turbulent wake will form downstream of the object. Conversely, reversing these factors would promote laminar flow through a pipe.

Next, consider another simple geometry: a uniform laminar air flow at 300 K passing over a sharpedged flat plate with an initial velocity of 1 m/s, as shown in Figure 4-2.

A laminar boundary layer begins to form at the leading edge. This boundary layer is normally defined as the region where the velocity is less than 99 percent of the free stream velocity. For this geometry, it is the fluid velocity far enough from the plate that it is unaffected by viscous effects at the plate surface.

(For pipe flow, the free stream velocity is the volumetric flow of the fluid divided by the internal crosssection of the pipe.) Let the downstream distance be the characteristic distance in the Reynolds number formula. Using viscosity data from Table 4-1 and a density of air of about 1 kg/m3, a Reynolds number of 3000 will be reached about 60 mm from the leading edge of the plate. This outcome demonstrates that, even without friction, the transition to turbulence occurs fairly soon after the flow meets the plate (or any other obstruction).

As shown in Figure 4-2, the thickness of the boundary layer increases with (the square root of) the downstream distance from the leading edge, as long as the boundary layer remains laminar. At a given position, the thickness of the boundary layer in the laminar region is proportional to the square root of the viscosity and inversely proportional to the square root of the free stream velocity. The frictional drag on the flat plate is much greater in the region where turbulence is present.

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Figure 4-2 Transition from laminar to turbulent flow in a boundary layer on a sharp-edged flat plate.

The degree of turbulence and the thickness of the boundary layer also affect the extent to which smoke particles and some gases become deposited on the walls and ceiling as a fire-generated flow moves along a corridor. A flow with turbulent eddies or a thin laminar boundary layer is more likely to experience wall losses of combustion products. The easier it is to reach the surfaces, the more likely a condensable species in the effluent will stick to them. The same logic applies to cooling of the hot gas flow: the more hot gas that contacts the cooler walls and ceiling, the more efficient the heat transfer.

Buoyancy Hot air rises and light objects float. Both of these are examples of buoyancy. The principle and calculation of buoyancy was established in 212 B.C. by Archimedes in terms of forces: any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

This principle applies directly to the hot plume that rises from a fire and then spreads elsewhere in the fire room and into adjacent spaces. The fire gases have a density considerably less than the density of ambient air, because at constant pressure the density is inversely proportional to the absolute temperature. (This is the ideal gas law, as discussed in the Physical and Chemical Change chapter.) The buoyant upward force is proportional to the difference in the densities in the two flows. Thus, the upper part of the fire room is hotter than the lower layer, and this pattern continues as the hot gases flow out of the fire room into other volumes.

Note An explanation of buoyancy at the molecular level is as follows. The molecules within and emanating from the flame are at a higher temperature than the surrounding air. The hot molecules have more kinetic energy and, therefore, higher velocities with which to oppose the force of gravity. As a consequence, they rise relative to the molecules in the cooler ambient air.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

In fire plumes where the heated region is small and the plume narrow, such as the region over a candle or a smoldering cigarette, the upward flow will be laminar. Wider plumes are more likely to be turbulent than narrow plumes; if the heated region is as wide as 0.5 m, the upward flow will be turbulent.

A turbulent plume will entrain surrounding air much more rapidly than a laminar plume, analogous to the mixing that occurred in the earlier pipe flow example. This difference in mixing character has a major influence on the temperature, velocity, and smoke concentration of the fire-generated flow.

WRAP-UP Chapter Summary •

Newton’s three laws of motion describe how an object and an applied force react when they come together. Newton’s law of gravitation enables calculation of the movement of objects that are dropped

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or propelled upward.

Potential energy is energy that is stored, waiting to be expended. Kinetic energy is associated with objects in motion. Interchange between the two types of energy occurs, but the total energy of a system is always conserved.

• Simple equations may be used to express the relationships between height and pressure differential for the air in a stairwell or the water in a standpipe.

• The air flows associated with fires can be either laminar or turbulent. Viscosity and buoyancy play key roles in their nature. In such flows, there are often boundary layers in which the velocity is significantly lower than that of the bulk fluid.

• Viscous effects play an important role in water flow through pipes in fire control systems.

•

Key Terms boundary layer The slow moving layer of fluid in the immediate vicinity of a surface where the effects of viscosity are significant buoyancy The upward force exerted by a fluid that opposes the weight of an immersed object. free stream velocity The velocity in the part of a flow that is not disturbed by any object or boundaries. gravity The attraction of all physical bodies for each other, most commonly experienced as the force that gives weight to objects with mass and causes them to fall to the ground when dropped. kinetic energy The energy associated with the motion of an object or a fluid. laminar flow (streamline flow) The movement of a fluid in smooth, parallel layers, with no disruption among the layers. potential energy The energy in an object or fluid due to its position in a gravitational field. A compressible fluid may also have potential energy because of its pressure. turbulent flow Fluid movement characterized by irregular fluctuations. viscosity The thickness of a fluid due to friction between neighboring regions of the fluid that are moving at different velocities.

Challenging Questions 1. List Newton’s three laws of motion.

2. What is gravity?

3. In a 300-m-tall building, the stairwells are no more than 20 stories high. A story is approximately 3 m high. What is the maximum pressure differential in a stairwell? What is the atmospheric pressure difference between the top of the building and street level? What is the difference in static water pressure between consecutive floors?

4. For a freely falling object weighing 3 kg: A. B. C. D.

What is the object’s velocity 2 s after its release?

What is the kinetic energy of the object at that time?

How far will the object have fallen?

How does doubling the mass of the object affect these values?

5. A 100-m3 open tank contains water 3 m deep.

A. What is the pressure on the water at the bottom of the tank, relative to atmospheric pressure?

B. If a hole is drilled in the bottom of the tank, what is the velocity at which a water jet issues from the hole?

6. Why is the presence or absence of turbulence in a flowing fluid important?

7. Why do the flames from a fire rise?

References 1. Kandola, B. S. (2008). Introduction to Mechanics of Fluids. In: SFPE Handbook of Fire Protection Engineering, 4th ed.

P. J., DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 1-1.

2. Brennan, R. P. (1992). Dictionary of Scientific Literacy. New York, NY: John Wiley & Sons.

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3.

Perry, R. W., and D. W. Green, eds. Perry’s Chemical Engineers’ Handbook, 7th ed. New York, NY: McGraw-Hill, Section 2.

4. Haynes, W. M., ed. (2011). Handbook of Chemistry and Physics, 92nd ed. Boca Raton, FL: CRC Press.

1

The meaning of flow “along a streamline” will become clearer when turbulence is discussed later in this chapter.

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CHAPTER 5

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Heat Transfer OBJECTIVES After studying this chapter, you should be able to: • • • • • • •

Name and explain the three modes of heat transfer.

Explain why radiative heat transfer in fires is especially important.

Explain the difference between an intensive property and an extensive property of a material.

Calculate the heating rate of an object due to heat conduction and radiation.

Describe the difference between thermally thin and thermally thick materials.

Describe the structural hazards that can result from loss of fire resistance.

Calculate the burn hazards to people from exposure to convective and radiative heat.

Introduction A fire spreads by raising the temperature of new fuel to the point where it ignites. In the Oakland Hills, California, firestorm of 1991, this occurred by several different means. Flaming brands from burning trees were carried aloft, igniting fires on the other side of an eight-lane highway. Wind-aided flames from a burning house contacted and ignited adjacent houses. Some homes’ exteriors were barely singed, although they had significant fires inside, likely as a result of radiant heat from nearby flames that passed through the windows and ignited items inside. Similar phenomena occur in fires that start inside of a house, as a burning item ignites successive combustibles.

There are also tactics by which the ignition and burning rate of combustibles are diminished. For example, a variety of measures may be applied to keep them cool, such as the hydrated fire retardants in carpets, fire blankets, and automatic sprinklers.

This chapter introduces the physical modes by which heat is transferred to and from a substance, raising or lowering its temperature.

Temperature and Heat As described in the Physical and Chemical Change chapter, a gaseous, liquid, or solid substance can be characterized by a temperature. Temperature is an intensive property, which means that its value does not depend on the expanse of the object. Removing half of a hot object does not in itself change the temperature of the remaining mass. Pressure, density, concentration, boiling point, chemical formula, and odor are other intensive properties.

Heat transfer is an enthalpy flow from one location to another that occurs because of a temperature difference. Heat always flows from a higher temperature area to a lower temperature region; if two contiguous objects are at the same temperature, no heat will flow between them. The heat content of an object is an extensive property. It changes proportionately as the mass of the object is increased or decreased. As presented as Equation 2 in the Physical and Chemical Change chapter, the heat required to change the temperature of a given mass is Q = m · cp · ΔT. Other extensive properties include mass, volume, length, and enthalpy.

Heat is contained in matter—that is, assemblies of atoms or molecules. In a monatomic gas, the heat content is simply the sum of the kinetic energies of the individual atoms. This kinetic energy of an atom is proportional to the product of its mass and the square of its velocity (Equation 11 in the Flow of Fluids chapter). The temperature of a group of monatomic gas atoms is proportional to the kinetic energy of an average atom in the group. If there are twice as many atoms with the same average velocity, then the heat content (an extensive property) is twice as much, but the temperature (an intensive property) is unchanged.

This same description also holds for the kinetic energy of a polyatomic gas. However, a molecule has additional modes in which to store energy: a molecule rotates and its chemical bonds vibrate. The additional enthalpy stored within such a molecule means that, at a chosen temperature, a given number of molecules of a polyatomic gas will contain significantly more heat than the same number of atoms of a monatomic gas. This can be seen in Table 2 in the Physical and Chemical Change chapter, where the

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heat capacities of even simple molecules (air and water vapor) are considerably larger than the heat capacity of argon, an atomic gas. The intensity of the vibrations and rotations rises with increasing temperature; therefore, so does the molecular heat content.

Because the molecules in a solid are fixed in place, their enthalpy is not associated with a velocity.

Instead, their enthalpy is due to their vibration within the solid. The higher the temperature, the greater the amplitude of these vibrations.

The heat content of a liquid is more difficult to quantify. Within a liquid, each molecule is moving, but it remains in very close quarters with many other molecules. The potential energy of each molecule is, therefore, a function of the interactions of the array of molecules that are all moving relative to one another. Fortunately, the measurement of cp is straightforward, resulting in values such as those in Table 2 of the Physical and Chemical Change chapter. Note that the heat capacity of liquid water is significantly larger than the heat capacity of water vapor.

The references in the Physical and Chemical Change chapter contain tables of heat capacities for numerous gases, liquids, and solids.

Modes of Heat Transfer Heat is transferred between bodies in two ways: through conduction and through radiation.

Convection, also fundamental in describing fire phenomena, is not an independent mode of heat transfer, but rather is treated as conduction by means of a fluid (usually air), combined with a contribution from the movement of that fluid.

The intent of the material in this section is to provide the reader with sufficient knowledge to progress to the material in later parts of this text. For more detailed renditions, see Reference [1] for an overview of heat transfer, Reference [2] for conductive heat transfer, Reference [3] for convective heat transfer, and Reference [4] for radiative heat transfer.

Conductive Heat Transfer Heat is transferred by conduction (1) across a temperature gradient within a solid or (2) between two solids initially at different temperatures that are then placed in contact with each other. The atoms or molecules in the higher temperature locale will have higher enthalpies, on average, and heat will flow from the hotter area to the colder locale. At the molecular level, the larger amplitude vibrations of the atoms or molecules in the hotter locale pass some of their enthalpy to the neighboring colder atoms or molecules.

Heat can also be conducted through a stagnant fluid. The case where the bulk fluid flows during the heat transfer is treated as convection in the next section.

The rate of conductive heat flow is proportional to the temperature gradient. This relationship is expressed mathematically by the Fourier equation for steady-state heat conduction:

in which qx = the heat flow (J/s or W) in the x-direction

A = the area across which heat is flowing (m2) ΔT = the difference in temperature (K) between two points a distance Δx (m) apart κ = a proportionality constant characteristic of the material, called the thermal conductivity of the material; its dimensions are W/m-K Equation 5-1 is valid only as long as the temperature varies linearly with x, and κ is truly constant over the temperature range ΔT. This is the case for the glass or mineral fiber that insulates attics from the outdoors. Equation 5-1 is also relevant to the early heating of wood in contact with overloaded electrical wiring. At higher temperatures, however, the wood loses moisture and then chars. These phase and chemical changes alter the value of κ. Equation 5-1 also is not sufficient to describe unsteady heat transfer or even steady-state heat transfer through a curved object such as a cylinder or sphere. In such cases, calculus must be used to determine the amount of heat transferred, with the temperature gradient being expressed in differential form.

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Values of thermal conductivity for various common materials are shown in Table 5-1. Study of the table shows some interesting things.

•

The thermal conductivity of copper is hundreds of times greater than that of nonmetallic solids such as concrete, plaster, or wood, and more than 10,000 times as great as that of air. Some cookware is made of (or coated with) copper to promote even heating of the pot and, therefore, the food cooked in it. As is well known, metals are efficient conductors of electricity, whereas nonmetals and gases are not. It turns out that heat is also carried through electrically conductive solids by the movement of the electrons, which is a far more efficient heat transfer process than that carried out by the vibrational energy of molecules.

• The thermal conductivity of a metal decreases as the temperature increases, whereas the thermal conductivity of a gas increases with increasing temperature. Equation 5-1 can still be used to estimate the heat flow, using an average value of κ, if the change in κ is modest and smooth over the temperature range of interest.

Table 5-1. Thermal Conductivities of Various Materials [5]

Reprinted with permission from SFPE Handbook of Fire Protection Engineering, 4th edition; Society of Fire Protection Engineers, Boston, Appendix B, Copyright © 2008, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and official position of the NFPA or the referenced subject, which is represented only by the standard in its entirety.

When calculating the heat flow from one solid material to another, a common starting point is the assumption that the common surfaces of the solids are at the same temperature. In effect, the common surface of the colder material is treated as the heat source for that material. This is a good approximation if the thermal contact between the two materials is very good. If this is not the case, the heat transfer may be determined as much by the roughness of the common surfaces as by the thermal conductivity of the colder material. This scenario is generally remedied by treating the conduction as a two-step process: (1) heat transfer across the interface between the two materials, which is characterized by an experimentally obtained heat transfer coefficient, followed by (2) heat transfer through the colder material.

Once the magnitude of the conductive heat flow is known, two additional quantities of interest may be determined. The first quantity is the amount of heat required to raise an object to a desired temperature (or, alternatively, the maximum amount of heat that can be transferred without exceeding an undesirable temperature rise). A measure of this quantity is the product of the mass m (g) (kg/m3) and the heat capacity cp (J/g K)—that is, the amount of heat required to increase a unit mass of a material by 1 K. If the heat is equilibrated rapidly throughout the material and if there are no heat losses from the material, then the required heat, q (joules), to raise the material’s temperature is simply:

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For more complex situations, guidance may be found in Reference [4].

The potential for heat losses from the “target” object suggests a classification of materials based on this property. If, during the heat transfer process at one surface of an object, all of the other surfaces remain at ambient temperature, the object is described as being thermally thick. In such a case, there are no heat losses from these surfaces to the surroundings. Good insulating materials, such as 16 in. of glass fiber insulation in an attic space, are typically thermally thick. By contrast, if the temperature of one or more of the other surfaces of the object exceeds the ambient temperature, it will transfer heat to the surroundings. Such an object is described as thermally thin. A 3-in. layer of glass fiber insulation is likely to be thermally thin on a cold winter day. Fire fighters’ turnout gear is designed to be thermally thick for the duration of firefighting activity. However, if the fire fighter stays too long or if the fire gets too hot, the garment may become thermally thinner, leading to skin burns.

The second quantity that can be determined after the magnitude of the conductive heat flow is known as a characteristic time. For a fixed heat flow, surface area, and thermal conductivity, Equation 5-1 gives a temperature profile inside the object—that is, a relationship between temperature and distance from the surface. Equation 5-3 gives a time profile for temperature rise within the object:

where t is the time at which a noticeable temperature rise occurs at depth Δx. If Δx is the thickness of the object, then t is the time at which the object becomes thermally thin.

Convective Heat Transfer Heat is transferred by convection when a fluid flows over a solid whose surface temperature is different from that of the fluid. In such cases, a slow-moving boundary layer of the fluid usually separates the faster moving bulk of the fluid from the solid surface. The rate of heat transfer between the fluid and the surface depends on the thickness of this film or boundary layer, the temperature difference across the boundary layer, and the thermal conductivity of the fluid in the boundary layer. Equation 5-4 is widely used for this type of problem:

in which q′ = the heat flow (W) A = the area across which heat is flowing (m2) ΔT = the difference in the temperature of the bulk fluid and the temperature of the surface (K) h = the heat transfer coefficient (W/m2 K) The essential task in performing a calculation using this equation is getting a good value for h.

Engineers have created dimensionless groups for this purpose. These combinations of variables are constructed such that the units in the numerator and the denominator of the group cancel each other. In this case, a dimensionless group, hL/k, defines the Nusselt number (Nu). Here k is the thermal conductivity of the fluid and L is a characteristic length, which must be defined for each application. The resulting equation is h = Nu k/L. Thus, if Nu can be determined for a given case, and if k and L are known, it is easy to calculate h and then apply Equation 5-3.

The rate of heat transfer varies with the geometry of the interaction between the fluid and the solid and the nature of the fluid flow. It should not be surprising, then, to learn that different formulas for calculating Nu have been obtained from measurements. A number of these formulas are compiled in Reference [3]. They express the Nusselt number as functions of other dimensionless numbers, chiefly the Reynolds number (Re), the Grashof number (Gr), and the Prandtl number (Pr). Examples of these functions follow: • Nu = 0.68 (Re)0.466 (Pr)0.33 (Hot fluid flowing past a cold cylinder of diameter L, with the Reynolds number, based on L, being between 40 and 4000.)

• Nu = [0.037 (Re)0.8 – 871] (Pr)0.33 (Hot fluid flowing across a cold flat plate of length L in the downstream direction. The boundary layer is initially laminar but becomes turbulent before the end of the plate is reached.)

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• Nu = 0.14 [(Gr) (Pr)]0.33 (Horizontal hot plate of diameter L facing upward, with a buoyant upward flow of initially cold fluid resulting from contact with the hot plate. This formula is valid when [(Gr) (Pr)] is greater than about 10 million.)

The Prandtl number, involving the ratio of the fluid viscosity to its thermal conductivity, has a nearly constant value of about 0.7 for air or combustion product gases, independent of temperature. The fact that Pr appears with a fractional exponent makes it even less sensitive to temperature, and Pr0.33 = 0.70.33 = 0.9. As a result, the first two equations for the Nusselt number are sensitive only to the Reynolds number and, therefore, to the degree of turbulence in the flow. In the third example, the Grashof number is the ratio between the buoyancy force due to spatial variation in fluid density (caused by temperature differences) and the restraining force due to the viscosity of the fluid. It is given by Gr = g L ξ ΔT/υ2, where g is the gravitational constant (m/s2), L is a representative dimension, ξ is the coefficient of thermal expansion of the fluid (K−1), ΔT is the difference between the solid–fluid surface temperature and the fluid’s bulk temperature, and υ is the viscosity of the fluid (m2/s).

Note Empirical tools are available to estimate answers to problems for which exact solutions are difficult. Given this fact, it is less important to memorize the definitions of these dimensionless numbers and their relationships to heat transfer efficiency.

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Radiative Heat Transfer The other basic mode of heat transfer is thermal radiation. This mode is important for at least five reasons: 1. The primary energy feedback from the flame to the burning material often occurs primarily by radiation, rather than by convection (flow of hot gases) or conduction (as through a wall).

Accordingly, the flame radiation strongly influences the rate of burning.

2. The spread of a fire to nearby combustibles often takes place by radiative transfer. For example, a radiative flux of 35 kW/m2 impinging on a vertical particleboard (wood) surface will cause ignition in approximately 50 s. The more intense the radiation, the more rapid the fire spread.

3. The radiant emission from the flames and the hot upper layer of a room determines the time of flashover. (See the Combustion Fire, and Flammability chapter.)

4. The radiation from a sizable fire can be so intense that occupants are burned; even the protection from fire fighters’ turnout gear may be threatened.

5. The ability of a building (a) to localize a fire to a single compartment and (b) to remain standing can be compromised if the construction materials are overheated.

Radiative heat transfer does not require molecules to be in contact with one another. In fact, radiation can travel through a vacuum: the sun’s radiation reaches Earth through the vacuum of space.

Radiation can also pass through any transparent medium such as dry air or a semitransparent medium such as water or glass. Radiation travels at the speed of light, so radiative heat transfer occurs rapidly.

Radiation has a spectrum of wavelengths or frequencies, ranging from X-rays to radio waves (Figure 5-1). The parts of the radiation spectrum of greatest interest in the transfer of heat in fires are the infrared region and the adjacent part of the visible region (Figure 5-2).

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Figure 5-1 The electromagnetic spectrum.

Figure 5-2 The variation with wavelength of black body radiation at various temperatures.

The basic principle of radiation is that every object at any temperature above absolute zero continually emits radiant energy to its surroundings, and also continually absorbs radiant energy from its surroundings. The higher the objectâ&#x20AC;&#x2122;s temperature, the greater the rate at which this energy is emitted. The surface area A (m2) of any body at any finite absolute temperature T (K) will emit thermal radiation q (kW) in accord with the Stefan-Boltzmann law:

The radiant flux from the surface (kW/m2), Q, is given by

Recall that the rates of convective and conductive heat transfer are proportional to the temperature difference between the two objects. Thus radiative heat transfer, with its dependence on T4, becomes dominant when high temperatures are involved.

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In Equations 5-5 and 5-6, ε is a dimensionless factor between 0 and 1, known as the surface emissivity, or simply emissivity. An object with an emissivity of 1 at all wavelengths is referred to as a black body; it emits the maximum amount of radiant energy. A gray body is an object with an emissivity less than 1 at all wavelengths; it emits a proportionately lower fraction of the maximum possible radiant energy.

At temperatures pertinent to fires, Figure 5-2 shows that the thermal radiation is predominantly in the infrared portion of the spectrum, with some radiation in the visible region. In reality, many objects have emissivities that vary with wavelength and/or are less than unity.

• A number of solids have emissivities near to or greater than 0.9 over the visible/infrared range— for example, oak, paper, marble, plaster, gypsum, red brick, soot, and any surface with a flat dark coating. These are typically treated as black (or very dark gray) bodies.

• Many materials that may appear to be brightly colored actually have emissivities in the infrared that are near unity and lower values in the visible region. For such a material, we often make the gray body approximation, i.e., we treat the material as if its varying emissivity were constant.

• Some materials have low emissivities over the infrared and visible regions of the spectrum. For example, shiny metals generally have emissivities less than 0.1, and in some cases less than 0.02.

Reference [6] contains values for a variety of solid materials and products.

Note For practical purposes in estimating thermal radiation, it is convenient to set the emissivity equal to 1 to obtain an upper limit to the thermal radiation. If the material is known (or suspected) to have a lower emissivity, this value can then be applied to obtain a more realistic estimate. This leads to a convenient mnemonic “calibrator” for the radiant flux 2

from a surface. At 1000 K and ε = 1, the radiant flux from a black body is about 60 kW/m .

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Figure 5-2 shows the distribution of thermal radiative energy flux at various temperatures. At any temperature, there is a peak in the radiation intensity at some wavelength. As the temperature increases, the peak wavelength decreases. The product of the temperature and the wavelength corresponding to the peak is a constant (approximately 2800 µm · K). The area under the curve in the figure is proportional to the total radiative output. As seen in the figure, the radiative output increases substantially with increasing temperature consistent with Equation 5-5.

In a fire, the two major sources of radiant heat are the flames and the hot upper layer that forms in a room as a result of the flames. More specifically, this thermal radiation is attributable to the soot particles in the flame and the soot in the upper layer, which are black body emitters. (The yellow or orange appearance of the flame is due to the incandescence of the soot.) Another (usually smaller but not negligible) source of radiation is radiation from certain hot gas molecules in the fire effluent, primarily water vapor and carbon dioxide.

Note A number of reference books and websites cite relationships between the temperature of a hot object and its color. For example, a heating coil in an electric stove might appear red. Substantial variation among these color assignments occurs, presumably due to differences in the perception of color. Consequently, it is more useful to realize that increasing temperatures of hot objects follow the chromatic order of the light spectrum. Orange objects are hotter than red objects, and yellow flames are hotter than orange flames. White flames, such as from burning magnesium, are hotter than flames of ordinary hydrocarbons, but their perception as “white” is actually due to overload of the receptors in our eyes.

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The emissivity of a gas–soot mixture, which can be somewhat transparent to radiation, depends on the physical thickness of the mixture and the concentrations of soot and emitting gases. If the thickness and concentrations are great enough, the cloud is said to be optically thick, and the emissivity is unity.

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The soot concentration depends on the chemical nature of the combustible, the mass of fuel that has burned, and the equivalence ratio. (See the Fire Characteristics: Gaseous Combustibles and Combustion Products chapters.)

When incoming radiation strikes a nontransparent surface, a fraction of the radiation will be absorbed, heating the material, and the remainder will be reflected. The fraction absorbed, α, is called the absorptivity of the material. The emissivity and the absorptivity of a material at a given temperature are equal, but they can vary somewhat with temperature. Thus, even if the emitter and the receiver are made of the same material, if the emitter is much hotter than the receiver, the emissivity may differ from the absorptivity.

The intensity of flame radiation on a target depends on the following factors: • • • • •

Surface area of the flame facing the target.

Flame temperature.

Flame emissivity (which depends on its sootiness and its thickness).

Distance from the flame to the target.

Whether there are cooler smoke or fog droplets between the flame and the target. This interference scatters the radiation and reduces the intensity on the target.

A simple example of radiative transfer occurs every Thanksgiving—the roasting of a turkey.

Equation 5-3 shows that the net radiant heat transfer is proportional to the fourth power of the absolute temperature of a hot object minus reradiation from the cooler object, which is proportional to the fourth power of the absolute temperature of the cooler object. If the cooler object is much cooler than the hotter object, and if the surface of the cooler object is not larger than that of the hotter object, then the radiation from the cooler object can be neglected. For example, if the internal surface area of an oven is 4 m2 and the temperature is 400 °F (475 K), the emitted thermal radiation (Equation 5-3) is approximately 12 kW. The radiation from the turkey, whose exposed surface is about 0.2 m2 and whose temperature is 300 K, is only about 100 W. The following is an example of an estimation of the intensity of the flame radiation incident on a potential “target” combustible that is 10 m away from the center of the flames. The fire is consuming a combustible at the rate of 100 g/s, and the heat of combustion of the fuel is 50 kJ/g. In such a case, the theoretical rate of heat release is 100 · 50 = 5000 kJ/s, or 5000 kW. If 30 percent of this energy is radiated from the flame, 65 percent is convected upward with the fire products, and 5 percent is not released at all because of incomplete combustion, it follows that 30 percent of 5000 kW, or 1500 kW, is radiated by the flame. This radiation goes in all directions. A hypothetical sphere, centered on the flame and extending to the target, has a surface area of 4π r2 = 4(3.14)(10)2 = 1260 m². Because the radiation is distributed equally in all directions (i.e., is isotropic), the fraction of the 1500 kW that impinges on 1 m² of the target is 1/1260. Therefore, the target receives 1500/1260 = 1.2 kW/m². This radiation intensity is far from sufficient to ignite nearly any combustible material. If the target were 1 m from the flames, however, the irradiance would be 120 kW/m²—more than enough to ignite common household furnishings or interior finishes.

More refined methods of calculation exist, taking into account the flame shape, the effect of intervening smoke, and other factors. These methods are beyond the scope of this text.

Chemistry plays a role in this process by influencing the fraction of combustion enthalpy that appears as flame radiation (30 percent in the preceding example). Table 5-2 shows that this fraction of combustion enthalpy is very dependent on the chemical nature of the combustible, ranging from 9 percent (hydrogen) to 43 percent (1,3-butadiene).

Those flames that radiate intensively are characterized by a higher degree of incomplete combustion for two reasons: (a) the formation of soot that radiates less heat than does complete combustion of the carbon to carbon dioxide and (b) the loss of heat by radiation. Both of these effects lower the temperature of the flame, which in turn slows down the chemical oxidation reactions.

Table 5-2 Fractions of Combustion Enthalpy for Various Diffusion Flames of Gases in Air

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a

Estimate, based on Reference [7].

b

Reference [8].

c

Reference [9].

d

Authorsâ&#x20AC;&#x2122; estimate.

The hot smoke layer under the ceiling of a room in which fire is present serves as another important source of radiation, as does the ceiling itself, once it becomes hot. These radiation sources are never as hot as the flame, but they usually extend over a much larger area. In turn, the radiant flux from them is often important in promoting flame spread. Figure 5-3 shows the radiative flux emitted by a hot surface at various temperatures.

The precise calculation of radiative transfer between an emitter and a receiver of specified size, orientation, and separation distance often involves sophisticated geometric calculations. View factors are involved. These calculations have already been done for a number of cases, and the results are available as formulas or graphs [4, 6].

Hazards from Heat Transfer Life Safety The hot gases from a fire can threaten the lives of people who are exposed to them owing to the elevated temperatures and the radiant energy incident on their skin. Generally, second-degree burns of the face and skin occur prior to burns of the respiratory tract. ISO 13571 presents consensus equations for the conductive and radiative effects involved in such injuries [10].

People can tolerate radiant exposure of bare skin at levels up to 2.5 kW/m2, but incapacitation1 due to skin burning can occur at higher irradiance levels. Incapacitation is often equated to the time to second-degree burning, trad,burn (min), as given by Equation 5-7:

where Q is the radiant heat flux (kW/m2). At lower exposures to radiant heat, experiencing pain can have a behavioral effect, such as a decreased capacity for making sound decisions while trying to escape from a burning building. The time to pain is given by Equation 5-8:

Figure 5-4 presents Equations 5-7 and 5-8 in graphical form.

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Figure 5-3 Radiation flux emitted by a nonreflecting surface versus temperature.

People may also experience pain from immersion in the hot, humid fire effluent. The time to incapacitation for fully clothed people is given by Equation 5-9:

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Figure 5-4 Time to pain or second-degree skin burns as a function of thermal radiation flux.

where the temperature is in °C. For unclothed or lightly clothed people, the corresponding equation is

Figure 5-5 presents Equations 5-9 and 5-10 in graphical form.

These four equations have been empirically fitted to experimental data with an estimated uncertainty of Âą25 %. They reflect the susceptibility of an average person, so some people will feel pain or be burned at lower levels, whereas others will be affected only at higher levels.

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Figure 5-5 Time to pain from exposure to humid, convective heat for clothed and unclothed skin as a function of temperature of the surrounding air.

Endurance of Structures: Fire Resistance A building’s columns, beams, and other structural components are tested for their ability to withstand the elevated temperatures that could result from convective and radiative heating in a fire. Structural metals weaken as they are heated. For example, steel loses about half of its ability to support mass when heated to temperatures near 1100 °F (600 °C), well below its melting point. Beams and columns expand as their temperatures rise, stressing the structural assemblies that make up the building.

At elevated temperatures, concrete can undergo explosive spalling. This phenomenon occurs when the moisture in the pores of the concrete is heated to the point where it cannot escape sufficiently quickly through the fine pores. The vapor pressure builds up until it ruptures the pore walls, leading to potentially violent cracking of the bulk concrete. The result is fracturing and loss of strength of the concrete structural member.

Walls, floors, and ceilings are also susceptible to failure from heating in a fire. When one of these partitions reaches too high a temperature, three types of failure can happen: ignition of combustibles on the “cold” side of the partition, passage of smoke through cracks, and passage of flames through more extensive openings. Generally, the high temperature on the “cold” side of the partition occurs first.

Testing for all of these phenomena is performed according to ASTM E-119 [11] and the related ISO 834 [12]. The item to be tested is placed in a furnace, and the temperature of the furnace is raised according to a prescribed formula. The rating of the specimen is related to the time that the failure condition is reached.

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WRAP-UP Chapter Summary • • • • • • •

Intensive properties of materials are independent of their mass or volume. Extensive properties depend on the expanse of the material.

The temperature of a colder object can be heated by conduction or thermal radiation from a warmer body. Convection is a special case of conduction in which a fluid acts as an intermediary between the heat source and the cool object.

Rates of conduction and convection depend linearly on the temperature difference between the warmer and colder objects. The rate of radiative heating depends on the fourth power of the temperature, so it dominates heat transfer from flames.

Straightforward equations have been developed to describe the simple heating of an object due to heat conduction and radiation. Calculations for heating by convection involve a heat transfer coefficient that depends on the geometry of the thermal process.

When one surface of a thermally thick material is heated, all of the other surfaces remain at their initial temperature. For a thermally thin material, one or more of the other surfaces is elevated, and heat is lost to the surroundings.

When heated by a fire, building structural components (beams and columns) and partitions (wall, floors, and ceilings) can weaken.

Exposure of skin to thermal radiation or hot gases from a fire can compromise a person’s ability to escape from or survive a fire.

Key Terms absorptivity The fraction of radiant energy striking a surface that is absorbed, rather than reflected or passed through the material. black body An energy emitter (absorber) that radiates (absorbs) the maximum energy per unit area at all wavelengths. conduction The transfer of heat from a warmer region to a cooler region of a material or between two contacting materials at different initial temperatures by means of molecular or atomic diffusion and collisions, and without any motion of either material as a whole. convection The transfer of heat from a warmer region to a cooler region by the movement of one or more fluids. emissivity (surface emissivity) The fraction of radiative energy emitted by an object, relative to that emitted from a black body at the same temperature. The emissivity is between 0 and 1. explosive spalling The violent fracturing of a porous material, especially when moisture is liberated within the pore structure and thermally expands at a rate faster than it can migrate to the surface. extensive property A material property whose value is proportional to the quantity of the material. gray body An object with an emissivity (absorptivity) less than 1 at all wavelengths; it emits (absorbs) a proportionately lower fraction of the maximum possible radiant energy. gray body approximation The treatment of an object as having a constant emissivity (absorptivity) at all wavelengths, whether or not this is the case. heat transfer The flow of energy from matter at a higher temperature to matter at a lower temperature. incapacitation The inability to effect one’s own escape or to progress to a place of refuge, making it unlikely that one would survive without assistance. intensive property A material property whose value is independent of the quantity of the material. optically thick A semitransparent or opaque, hot radiating region (such as flame or smoke), for which the intensity of the emerging radiation is independent of the thickness of the region. optically thin A semitransparent, hot radiating region (such as flame or smoke), for which the intensity of the emerging radiation is dependent on the thickness of the region. radiant flux The radiant power emitted from or incident on a surface. radiation The transfer of heat in the form of electromagnetic energy.

Stefan-Boltzmann law An equation that specifies the intensity of radiation emitted by a black or gray body, in terms of its absolute temperature.

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thermal conductivity The characterization of a material’s propensity to conduct heat, typically represented using Fourier’s law of heat conduction. thermally thick material A material for which, during a heat transfer process at one surface, all of the other surfaces remain at ambient temperature. thermally thin material A material for which, during a heat transfer process at one surface, the temperature at one or more other surfaces exceeds the ambient temperature.

Challenging Questions 1.

Which of the following material properties are intensive and which are extensive: density, thermal conductivity, surface emissivity, weight, heat capacity, thermal thickness, temperature, melting point, vapor pressure.

2.

Calculate the rate at which heat will flow through a square meter of a 5-cm-thick concrete wall, if one side of the wall is at 700°C and the other side is at 20°C.

3.

The walls and ceiling of a corridor are covered with glazed ceramic tile (emissivity = 0.6). The effluent from a nearby fire has heated the corridor surfaces to 400°C. Calculate the radiative flux incident on a person walking down this corridor. What is the likely impact of this thermal radiation on the person? If the air temperature is also 400°C, what is the impact of the convective heat on the person? Estimate the peak wavelength of the emitted energy.

4.

An orange flame of a fire is emitting thermal radiation. Which species within the flame are responsible for this radiation?

5.

What percentage of the energy of a yellow flame is convected upward, and what percentage is radiated in all directions?

6. Give five reasons why the thermal radiation from a flame is of importance in fire safety.

7. If a material with a melting point of 200 °C were substituted for a steel column or a gypsum board wall, what might be the differences in endurance when exposed to the heat from a fire?

References 1. Drysdale, D. D. (2008). Chemistry and Physics of Fire. In: Fire Protection Handbook, 20th ed., A. E. Cote, ed. Quincy, MA: National Fire Protection Association, Chapter 3.

2.

Rockett, J. A., and J. A. Milke. (2008). Conduction of Heat in Solids. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 1–2.

3. Atreya, A. (2008). Convective Heat Transfer. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 1–3.

4. Tien, C. L., K. Y. Lee, and A. J. Stretton. (2008). Radiation Heat Transfer. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 1–4.

5. DiNenno, P. J. (2008). SFPE Handbook of Fire Protection Engineering, 4th ed. Quincy, MA: National Fire Protection Association, Appendix B. 6. Perry, R. W., and D. W. Green, eds. (1997). Perry’s Chemical Engineers’ Handbook, 7th ed. New York, NY: McGrawHill, Section 5.

7. Fishburne, E. S., and H. S. Pergament. (1979). “The Dynamics and Radiant Intensity of Large Hydrogen Flames.”

Proceedings of the Combustion Institute 7: 1063–1073.

8. Markstein, G. H. (1985). “Relationship Between Smoke Point and Radiant Emission from Buoyant Turbulent and Laminar Diffusion Flames.” Proceedings of the Combustion Institute 20: 1055–1061.

9. Tewarson, A. (2008). Generation of Heat and Chemical Compounds of Fires. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 3–4.

10.

ISO 13571:2012. (2012). Life-Threatening Components of Fire: Guidelines for the Estimation of Time to Compromised Tenability in Fires. Geneva, Switzerland: International Standards Organization.

11.

ASTM E119-12. (2012). Standard Test Methods for Fire Tests of Building Construction and Materials. West Conshohocken, PA: ASTM International.

12. ISO 834, Parts 1 through 12. (Various dates). Fire Resistance Tests. Geneva, Switzerland: International Standards Organization.

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Incapacitation is defined as the inability to effect one’s own escape or to progress to a place of refuge, making it unlikely that a person would survive without assistance. (See the Smoke and Heat Hazards chapter.)

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CHAPTER 6

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Combustion, Fire, and Flammability OBJECTIVES After studying this chapter, you should be able to: • Describe how the U.S. fire incidence database enables development of a national profile of fires and fire losses.

• Define the process of combustion.

• Explain flammability, in terms of both fire properties and practical application.

• Explain the nonflaming and flaming stages of fire.

• Discuss the fire tetrahedron and explain how it is a focus for a unified view of fire initiation, growth, and termination.

• Discuss the terms fire consequences, hazard, and risk.

Introduction Understanding fire types and the characteristics of each type is fundamental in improving fire safety. The United States has a unique capability to track the frequency, severity, and nature of fires. The National Fire Incident Reporting System (NFIRS) is the world’s largest national database of fire incident information [1]. Each year, approximately 23,000 fire departments in 50 states and the District of Columbia report information on fires in their jurisdictions. The input data to NFIRS reflect 75 percent of all reported fires that occur annually. The U.S. Fire Administration, the U.S. Consumer Product Safety Commission, and the National Fire Protection Association publish complementary analyses of the NFIRS data. These analyses enable the fire safety community to quantify the largest contributors to the United States’ fire problem, identify emerging threats to fire safety, and estimate the effectiveness of new standards and code provisions.

To illustrate the power of this database, consider the analysis of fires involving residential upholstered furniture and beds as the first item ignited (Table 6-1). The two shaded rows in Table 6-1 show that, while these fires represented only 5 percent of all residential fires, they led to one-third of all fire deaths. A subsequent analysis by John Hall of NFPA [2] estimated the fire losses for fires in which residential upholstered furniture was not the first item ignited, but was the principal amplifier of the hazard resulting from the initial ignition of another item. These additional contributions to fire loss are reflected in the upper “Upholstered Furniture” row. (A similar analysis has not been performed for mattresses and bedding.) The combined data show that furniture and bed fires are seven times more deadly and three times more likely to result in an injury than the average household fire.

Figure 6-1 shows that NFIRS compiles information that allows identification of the prevalent ignition sources.

Cigarette ignition of upholstered furniture is the largest single cause of fire deaths [3]. Figure 6-2 demonstrates yet another dimension of the data—namely, that nearly two-thirds of the U.S. fire deaths that began with upholstered furniture resulted from fires that had extended beyond the room of fire origin. These fires are interpreted as having passed the point of room flashover.

The knowledge gained from these analyses enables fire scientists to examine the most relevant fire phenomena, the fire service to align its response capability and operating procedures with the most important fires, and fire codes and standards developers and regulators to guide the marketplace toward effective solutions to fire problems. Supporting all these members of the fire safety community is a common understanding of fire and its consequences. This chapter begins the presentation of that understanding.

Table 6-1 Annual U.S. Residential Fire Losses, 2005–2009, by First Item Ignited (Plus Direct Involvement of Upholstered Furniture)

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Figure 6-1 Ignition sources in home structure fires that began with upholstered furniture, 2005–2009.

Reprinted with permission from SFPE Handbook of Fire Protection Engineering, 4th edition; Society of Fire Protection Engineers, Boston, p. vii, Copyright © 2008, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and official position of the NFPA or the referenced subject, which is represented only by the standard in its entirety.

Combustion Combustion is an exothermic chemical reaction between a fuel and an oxidizer resulting in the generation of substantive heat and often light. Interestingly, dictionaries often use the same words to define fire.

“Fire” generally conveys a connotation of being potentially uncontained, unwanted, and destructive— which is the context for this text. (There are some cases of positioned, intentional, and beneficial fires, such as those set to clear the rubble from last year’s crops.)

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Figure 6-2 Home structure fires that began with upholstered furniture by extent of flame damage, 2005–2009.

Reprinted with permission from SFPE Handbook of Fire Protection Engineering, 4th edition; Society of Fire Protection Engineers, Boston, p. 5, Copyright © 2008, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and official position of the NFPA or the referenced subject, which is represented only by the standard in its entirety.

To understand the content of this and the following chapters, it is important to understand the definition of combustion. Each of the following words or phrases is central to differentiating combustion in general, and fire in particular, from other processes.

• • •

•

•

•

Chemical reaction. Combustion involves chemical change: the arrangements of atoms in the combustion products are different from the arrangements in the reactants. Thus, while a liquid material may be vaporized during combustion, the phase change in itself is not combustion.

Exothermic. The overall combustion reaction gives off heat. This heat can raise the temperature of additional reactants, thereby continuing the combustion process; expand gases that drive turbines; or burn skin.

Fuel. Combustible materials can be gases (e.g., methane), liquids (e.g., gasoline), or solids (e.g., wood). Noncombustible materials, such as helium, water, and stone, do not satisfy this definition.

The nature, intensity, and duration of the combustion are in part determined by the fuel chemistry and supply.

Oxidizer. The most common oxidizer in combustion is the oxygen present in air, and this is especially true for fires. As is true for the fuel, the nature, intensity, and duration of the combustion are in part determined by the oxygen supply. A restricted supply of air or the depletion of oxygen in the air by the combustion, referred to as vitiation, can reduce the burning rate and alter the nature of the combustion products. Combustion does not necessarily require oxygen molecules or even oxygen atoms. An example of the former is the use of potassium perchlorate (KClO4) in fireworks and munitions; an example of the latter is the reaction of CF3Br (halon 1301, an effective extinguishant for hydrocarbon fires) with magnesium. Other chemicals self-combust—these are presented in the chapter on solid combustibles.

Substantive heat. Combustion reactions are a subset of exothermic oxidation reactions. Some exothermic oxidation reactions are extremely slow. For example, rusting never increases the temperature of the metal more than a degree or so above that of the surroundings and may go unnoticed for months. By contrast, a combustion reaction generates heat faster than it is dissipated, causing a substantial temperature rise (at least tens of degrees Celsius, and often 1000 °C or more).

Light. When the combustion-generated temperature is high enough, visible light is emitted from the combustion reaction zone. This can be seen as a glowing solid, such as a fireplace ember, or in the

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gas phase, such as a flame.

As can be gathered from these definitions, the initiation and continuance of combustion or a fire require four components, as depicted in the fire tetrahedran (Figure 6-3): 1.

2.

3.

4.

Fuel Oxidant Elevated temperature (heat) Chemical chain reaction

The balance among these components is critical. Too small a concentration of fuel may not be sufficient to propagate combustion, even if plenty of air is available. Similarly, fuel and air may not react if the temperature is too low.

Figure 6-3 The fire tetrahedron.

The following is a brief presentation of combustion as it pertains to fires. For more detailed information, refer to references such as [5], [6], and [7].

Flaming and Nonflaming Combustion ISO 19706 defines a set of phenomenological stages of a fire [8]: 1. Nonflaming a. Self-sustained (smoldering combustion; also called glowing combustion) b. Oxidative pyrolysis from externally applied thermal radiation or conduction c. Anaerobic (oxidizer-free) pyrolysis from externally applied thermal radiation or conduction 2. Well ventilated flaming free burning 3. Underventilated flaming a. Small, localized fire, generally in a poorly ventilated compartment b. Post flashover fire The existence, sequencing, and duration of these phenomenological stages can vary from fire to fire. The roles of these stages are best explained using examples.

Figure 6-4 depicts the timeline for two hypothetical fires. The curve in blue is for a fire occurring in a typical residential bedroom with the door open and the windows closed. The curves in red are for a fire in the same room with the door initially closed. The following sections of this chapter provide a general

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description of the stages of these fires. More detailed information on principles of fire phenomena is presented in the subsequent chapters.

Figure 6-4 Rate of heat release from two bedroom fires under different ventilation conditions. Blue curve: Door open and windows initially closed. Red curve: Door and windows initially closed. Dashed red curve: Door and windows remain closed.

A. Smoldering ignition of the bed by a cigarette; some oxidative pyrolysis B. Smoldering transition to well ventilated flaming; some oxidative pyrolysis C. Room flashover; ventilation-limited flaming; extensive pyrolysis D. Window breaks from the heat; burning no longer ventilation-limited E. Maximum mass burning rate; extensive pyrolysis F. Fuel begins to run out; extensive pyrolysis G. Flames out; rubble smoldering H. Door is opened

Fire Initiation We are surrounded by combustible items (e.g., trees, beds, clothes, cars, and even our own bodies) that are in contact with the oxygen in air. Why do these fuels not ignite and burn? How can reactantsâ&#x20AC;&#x201D;for which we can write a balanced, exothermic chemical equationâ&#x20AC;&#x201D;coexist without consuming each other?

The answers to these questions lie in chemical kinetics, as introduced in the Physical and Chemical Change chapter. At room temperature, very few collisions between fuel molecules and oxygen molecules have enough energy to break chemical bonds. At the macroscopic level, this means that the reaction rates of common combustibles with oxygen are extremely slow at room temperature. However, these reaction rates increase sharply with increasing temperature. If a sufficiently high temperature is created in even a small region where both the combustible material and oxygen are present, ignition will occur. The oxidation process will accelerate in this region, generating its own heat and supply of free atoms and free radicals. The heat will then spread to adjacent areas, and the combustion will be sustainedâ&#x20AC;&#x201D;that is, there will be a fire.

Depending on the ignition source, a fire can begin as smoldering combustion, perhaps progressing later to flaming, or the fire can begin directly as flaming.

Smoldering is the most common initial stage of combustion in fires that lead to injury or death (Figure 6-1). Smoking materials (cigarettes), hot embers and ashes, space heaters, and overheated electrical equipment are potential ignition sources for smoldering. Once ignited, upholstered furniture containing cotton fabrics and cotton or polyurethane foam padding can smolder for an hour or more. A large pile of wood chips, sawdust, or coal can smolder for weeks or even months. Peat fields have smoldered for decades.

Smoldering generally is limited to porous materials that can form a carbonaceous char when heated.

The oxygen in the air slowly diffuses into the pores of the material, where it reacts directly with the solid

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carbon and generates heat. These porous materials are poor conductors of heat (i.e., good thermal insulators); therefore, even though the combustion reaction occurs slowly, enough heat is retained in the reaction zone to maintain the elevated temperature needed to sustain the reaction. The reaction zone within the material can be glowing red, but typically is not readily visible from the outside.

The reaction zone in a smoldering mattress might spread a foot or more from the ignition point Figure 6-5 or might even consume the entire mattress. At some point, the mattress might suddenly burst into flames. This change in the nature of the combustion typically results from an increase in the temperature of the smoldering zone, often a result of an increase in air flow across the furniture. The mass rate of burning (and the rate of heat release) during flaming combustion is many times higher than during smoldering combustion.

During the smoldering process, adjacent material is also being pyrolyzed by the heat being generated or by the heat from the ignition source. Pyrolysis is different from smoldering in that pyrolysis stops when the heat source is removed, while smoldering generates sufficient heat to continue without external heat input. The combustion products from smoldering and pyrolysis also are different. Pyrolysis occurring due to, e.g., a space heater “cooking” a nearby chair, is oxidative pyrolysis, since there is ample surrounding air. Anaerobic pyrolysis is likely when overheated household power cable maintains contact with wood in the stud space behind gypsum wallboard.

A flame is the result of rapid gaseous oxidation reactions. From Equation 3-2 in the Physical and Chemical Change chapter, the temperature rise for a given heat generation is ΔT = ΔH/mcp. For typical combustion reactions, ΔH is quite large and both m (the mass of air being heated) and cp (the specific heat of air, shown in the Physical and Chemical Change chapter) are small. Thus, the temperature rise is large. The yellow-orange glow of the soot in the flame is characteristic of temperatures near 1500 K.

Figure 6-5 Firefighters attending to a smoldering mattress during a training session.

Courtesy of the Upper Allen Fire Department.

Flames from a condensed (solid or liquid) fuel need gaseous fuel to continue burning. When a condensed fuel, such as a candle or a pool of gasoline, burns, a portion of the heat of the gaseous flame is transferred to the fuel, causing it to gasify. When this gasification occurs without chemical decomposition of the molecules, it is simple evaporation. If chemical decomposition occurs, which is typical of solid fuels, the process is pyrolysis.

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Fire Spread The spread mechanism of a fire depends heavily on whether the combustibles are gaseous, liquid, or solid. In the case of a flammable gas, the spread is determined by the extent of mixing of the gas with air prior to combustion as well as by the degree of motion and turbulence of the gas. For a liquid, the spread of combustion depends on whether the combustion occurs in a still pool of liquid, a flowing liquid, a spray, a thin film, or a foam.

A solid might be in the form of a powder, a thin sheet (e.g., paper), or a thick solid object. The speed of fire spread over a solid can be as fast as several meters per second or as slow as a fraction of a millimeter per second, depending on the conditions. Combustion spreads much more rapidly over thin solids and solids with high surface areas. The spread rate on a vertical solid surface is much faster in the upward direction than in the horizontal or downward directions. The fire spread rate over a horizontal surface depends on whether air currents are moving toward or away from the combustion zone.

In most cases of fire spread, the basic mechanism is the same. A portion of the heat produced by a burning item is transferred, by radiation, conduction, or convection, to an unburned part of the item or to a nearby combustible that is not yet burning. This cool fuel is heated to the temperature at which its pyrolysis is fast enough that the concentration of combustible vapor above the fuel surface exceeds its lean flammability limit. The flames then extend to this region and begin to pyrolyze the next area of cool fuel. Fire also can spread by the melting and dripping of burning material, or by airborne firebrands.

Fire Ventilation Flaming combustion generates the large release of heat that can lead to room flashover, with the degree of ventilation often determining whether flashover is reached or not. When a flaming fire is small and is surrounded by fresh air, the fire can entrain all the oxygen it needs. The fire is said to be well ventilated.

As the fire grows, it can reach a point at which it is consuming all the air that can enter the room through, e.g., an open door. At this point, the fire cannot burn any faster; it is ventilation-limited. If the fire is burning in a closed room, the ambient air becomes oxygen-depleted, and this fire also becomes ventilation limited.

During the course of a fire, a ventilation-limited condition can change suddenly to a more nearly ventilated condition. This can occur when the fire breaks a window, an occupant opens a door, or firefighters vent a roof. At this time, the heat release rate of the fire rises quickly to a level determined by the size and location of the new opening.

Fire Termination The fire tetrahedron (Figure 6-3) shows that combustion requires supplies of fuel and oxidant, a high temperature, and reactions that proceed fast enough at this high temperature to generate heat as fast as it is dissipated, so that the reaction zone will not cool down. Any action that sufficiently upsets this balance will extinguish the fire. In practice, most firefighting approaches affect more than one component of the fire tetrahedron. A more detailed presentation of fire extinguishment appears in the Fire Fighting Chemicals chapter.

Adding a coolant absorbs heat from and reduces the temperature of the combusting system; that is, the coolant serves as a heat sink. The coolant can be any material with a relatively high heat capacity. It is not necessary for the coolant to absorb heat as fast as the heat is being generated, because the reaction zone in a fire is already losing some heat to the cooler surroundings. In some cases, only modest additional heat dissipation is needed to tip the balance toward extinguishment.

Extinguishment can be accomplished by cooling either the gaseous combustion zone or the solid or liquid combustible. In the former case, the cooling is accomplished by introducing a gas or liquid with a high heat capacity into the flame zone. This mechanism is how carbon dioxide extinguishers work, in addition to displacing or diluting the supply of oxygen. In the latter case, the coolant is applied to the fuel surface, preventing the production of combustible vapors. This is what you accomplish when you douse a campfire with water.

As a simple demonstration of extinguishment by cooling, support a thick, 20-cm-long vertical strip of cardboard from the top and ignite the bottom with a match Figure 6-6. The flame will attach to the bottom of the paper and spread rapidly to the top of the strip. Then repeat the experiment, but first soak the top half of the cardboard with water. When the flame reaches the wet section, it is extinguished. The water acts as a heat sink and upsets the balance between heat production and heat loss. The flame cannot

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heat the cardboard to its pyrolysis temperature in the available time because of heat absorption by the water.

Extinguishment also can be accomplished by introducing a barrier between the combustible surface and the flame, the source of heat. For example, applying a layer of aqueous foam to a fuel spill cools the surface, shields the surface from the flame radiation, and blocks any gasified fuel from replenishing the flame Figure 6-7. In addition, the potential fuel may be separated from the fire by placing a fire blanket over the combustible item. Some fire-retardant additives, when heated, expand to form an insulating layer over the material they are protecting.

Figure 6-6 Demonstration of fire extinguishment by absorption of heat.

Another means of extinguishing a fire is to reduce the availability of oxygen by closing the openings to a burning compartment. This practice constricts the inflow of air, while diluting the existing air with noncombustible combustion products, such as carbon dioxide. Figure 6-8 shows a simple experiment that demonstrates the principle of this approach. In the left drawing, the glass beaker is held above the fuel surface. Fresh air is entrained into the base of the flame, and the candle continues to burn. In the right drawing, the beaker has been lowered. Less air is entrained into the base of the flame, and the atmosphere surrounding the flame is depleted in oxygen and rich in carbon dioxide from the combustion, causing the flame to extinguish. When extinguishing a fire by eliminating any ventilation, the airtight integrity of the compartment needs to be maintained until the fire is completely out and all hot surfaces have cooled below ignition temperatures. Otherwise, opening a door will introduce fresh air into a hot, fuel-rich environment. The ignited flames may then expand violently out the doorway, harming any who are nearbyâ&#x20AC;&#x201D;the dangerous condition known as backdraft. This is graphically shown in Figure 6-9, where two photographs, taken only seconds after the compartment window was opened, show the rapid and extreme hazard of a backdraft.

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Figure 6-7 Fire extinguishment with foam. © Bortel Pavel/ShutterStock, Inc.

Figure 6-8 Extinguishment of a wax candle flame by surrounding it with its own combustion products. A: Freely burning candle; B: Extinguished candle.

Note The same approaches to interference with the fire tetrahedron can control a fire, keeping it at a manageable level until first responders arrive on the scene. One of the functions of an automatic sprinkler system is to contain fire spread when extinguishment is not possible.

Texture: © Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

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Figure 6-9 Demonstration of a backdraft. Š Jones & Bartlett Learning. Photographed by Glen E. Ellman.

The chemical reactions in flames are sustained by free radicals. Some bromine-, chlorine- and phosphorus-containing chemicals reduce the concentration of free radicals when added to flames and slow down the combustion reactions. Examples include halon fire extinguishants and ammonium phosphate, which is dropped onto forest fires.

These approaches can also prevent ignition. A fuse or circuit breaker, recognizing that too much current is being drawn, can prevent an electrical ignition by tripping before any fuel is hot enough to ignite. Some gaseous fire suppressants inert a space by increasing the heat capacity of the air or by chemically interfering with the ignition process before combustion can be sustained (addressed in the Fire Fighting Chemicals chapter).

In many real-world fires, the hazard continues even after the flames have been extinguished. Although the combustion of liquid and gaseous fuels ends very soon after the flames have been quenched, some solid fuel fires leave a hot, porous rubble that can continue to smolder for days. This smoldering is fundamentally similar to the smoldering that occurs in the earliest stage of a fire. However, the mass of smoldering material is considerably larger.

Two Examples of Room Fires For decades, fire scientists have conducted real-scale fires to learn how fires ignite and progress to the point where lives and the integrity of the building are threatened. Figure 6-10 shows a sequence of frames from such a fire test. The furnishings in the family room are a sofa and a loveseat flanking an end table with a lamp. There are draperies behind the sofa.

The fire begins in the corner of the sofa, perhaps as the result of a cigarette that fell into the crack between the seat, arm, and back cushioning. The initial smoldering transitions to flaming, slow at first but growing. At about 60 seconds, the flame is as high as the back of the sofa, and wispy black smoke is rising toward the ceiling. By 1:30, a black smoke layer has spread across the entire room. The bottom of the layer is about 1.5 m from the floor. At 2:50, in the upper layer, flames can barely be seen, but are spreading across the ceiling. This is the phenomenon called flameover or rollover. The air in the lower part of the room (away from the fire itself) is still relatively cool and clear. However, just after 3 minutes, the upper layer is now so hot that the radiant energy is intense enough to ignite the loveseat in a location that is away from the burning sofa. This is the phenomenon called flashover, the point at which all the combustibles in the room are aflame. The pyrolysis rate of the sofas is very high, and there is not enough oxygen in the room to burn all the gaseous fuel that is being generated.

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Figure 6-10 Scenes from a video of a furniture fire in a family room.

Courtesy of National Institute of Standards and Technology.

During the early flaming, the bottom of the upper layer descends below the top of the door frame.

Some of the smoke flows out of the room through the upper part of the doorway, while fresh air is drawn in through the lower part. As the burning accelerates, these two flows increase dramatically, and the smoke from the upper layer is driven into other parts of the building. This threatens the escape capability of people not in the initial fire room. At 3:14, the unburned gases jetting through the doorway are hot enough that they react with the oxygen outside. The turbulent flames surging through the doorway can spread the fire to adjacent compartments. At this time, approximately 3 minutes after the first flames became evident, the building and all its occupants are at risk.

The growth rate of this fire and the time to flashover may be surprising, but are not at all unusual. In fact, some fires reach serious proportions much faster. Figure 6-11 shows images of a dry Christmas tree fire in a small living room. The tree is ignited near the floor, perhaps by an overheated string of decorative lights. The black smoke layer is rolling across the ceiling and down the walls only 8 seconds after ignition.

One of the chairs has ignited by 17 seconds, and the room reaches flash-over in 26 seconds. By 40 seconds, even the flooring is burning.

If the door had been closed in either of these two fires, the intense thermal radiation from the upper layer would have pyrolyzed material at the same time that the burning depleted the oxygen in the room.

Shortly after opening the door, the emerging flames due to the backdraft would look similar to the final frame in Figure 6-10.

Flammability 82

In the broadest sense, we as a community use the term flammability to connote how vigorously something burns. The term combustibility is also used in some cases to describe the flammability of certain building construction materials. As we seek a more precise definition of flammability, however, the degree of agreement diminishes. Because the next three chapters treat the fire characteristics of gaseous, liquid, and solid combustibles, here are some thoughts for consideration as you read about fire properties and their meaning in the context of flammability. Rest assured that there is no simplistic resolution of the disagreements about definitions; however, people who will be practitioners in fire safety need to understand the phenomena they will face.

A first approach to defining flammability derives from what our eyes tell us. Does something ignite?

Does it keep burning? Water does not ignite, so it is not flammable. Gasoline is designed to burn in engines, so it is flammable. Hold a match to a 2 Ă&#x2014; 4 stud and it will not ignite, yet we know wood burns because we have seen houses destroyed by fire.

This suggests a second approach to defining flammability, one that lies in the measurement of fire behavior. Classically, an apparatus is constructed that appears to replicate a fire situation that has caused considerable harm. An ignition source of a chosen size is placed in contact with a specimen of a chosen size for a chosen duration. A quantity, such as the time to ignition or self-extinguishment, the rate of flame spread, or the opaqueness of smoke, is measured. Based on this value, the product from which the test specimen was cut is assigned a flammability rating. More recently, apparatus have been designed to quantify material fire properties, such as the rate of heat release or the minimum radiant flux necessary to spread flames. Reference [9] compiles nearly all of the standard fire tests used in North America.

When one of these tests is accepted, and the use of the test result is incorporated into practice, the test output becomes the definition of flammability. Commercial materials and finished products are then designed to the test, and product specifications are set according to the test.

As can be seen in Reference [9], multiple test methods may exist for the same property (e.g., ignition delay time, rate of flame spread). These do not always give the same result for a given product, nor do they always rank products in the same order. This text is intended to provide a sufficient knowledge of the scientific principles underlying fire that the readers can understand the concepts and metrics of flammability and recognize those actions that can improve a nationâ&#x20AC;&#x2122;s fire profile and those that do not.

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Figure 6-11 Frames from a video of a Christmas tree fire in a family room.

Courtesy of National Institute of Standards and Technology.

Fire Consequences, Hazard, Risk, and Flashover Three definitions regarding the outcomes of fires are used throughout this text (and in the fire literature as a whole): •

A fire hazard is a condition with the potential to create harm under a specified set of conditions called a fire scenario. An example of a fire scenario is an intoxicated person, alone in an apartment, smoking in bed. The hazard is that he could drop the lit cigarette onto the bed, where it ignites the bedclothes and mattress.

• A fire consequence is a fire hazard quantified as to its severity. In this example, the consequence of the fire could be the smoker’s death from inhalation of smoke from a smoldering pillow.

• A fire risk is the combination of consequences, each multiplied by the likelihood (probability) of each scenario occurring and the probability that the scenario will result in each consequence. As

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evidenced in the U.S. fire loss statistics, the risk of fire death from smoking in bed is greater than the risk of death from smoking outdoors.

Note The following data put the concepts of fire risk and fire consequences into perspective. With approximately 400,000 residential fires reported each year and about 100 million households in the United States, it is likely that each of us will see and hear fire trucks in our neighborhood in our lifetime. These fires are consequential to both civilians and fire fighters: someone in the United States dies or is injured in a fire every 5 minutes [10].

Texture: © Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

The consequences of a fire are sharply increased if the fire in a room proceeds to flashover. In physical terms, flashover is the point at which the thermal radiation from a fire and the hot smoke layer becomes sufficiently intense (in excess of approximately 20 kW/m²) that nearly all combustible surfaces in the compartment ignite simultaneously. This point occurs when the temperature of the hot smoke layer under the ceiling approaches 1100 °F (600 °C).

Before a room reaches flashover, the fire hazard is mostly limited to the people within the room and to the room contents, because the fire is localized and little smoke or heat leaves the room. (There might be sufficient smoke flow through a doorway to activate a smoke alarm that is properly placed outside the room.) After flashover, the flow of heat and toxic gases into other spaces in the building is greatly accelerated. In the United States, most fire deaths occur outside the room of fire origin from fires that have proceeded past flashover.

A principal goal of fire safety science, technology, and engineering practice is to reduce the likelihood of flashover. This is equivalent to managing the heat release rate that might occur in a room. The minimum heat release rate for flashover increases with the size of the room and depends on the ventilation in the room. If too little ventilation is available, the fire is starved for air and goes out or the burning generates too little heat for flashover. If the ventilation is excessive, the excess air flow dilutes and cools the smoke, so a larger rate of heat release is needed to reach the critical temperature condition for flashover. The materials of construction and thickness of the ceiling and upper walls are other important factors in determining whether flashover will occur and, if so, how soon.

Reference [2] has compiled quantitative relationships that can be used to calculate the critical fire size for flashover. A point of reference is that a heat release rate of 1 MW will bring an ordinary residential bedroom to flashover, if this intensity is sustained long enough for the extensive radiative ignition to occur, which can be as little as 1 minute. As seen in Figure 6-10, an upholstered sofa can bring a small living room to flashover in approximately 3 minutes from the time of ignition.

WRAP-UP Chapter Summary • Analysis of the data compiled in the National Fire Incident Reporting System database can identify the relative frequency of various types of fires. The knowledge gained from such analyses enables fire scientists, the fire service, fire codes and standards developers, and regulators to provide guidance geared toward reducing losses from fires.

• Combustion is an exothermic chemical reaction between a fuel and an oxidizer resulting in the generation of substantive heat and often light. Interestingly, dictionaries often use the same definition for fire. Fire is a form of uncontained, unwanted, and destructive combustion.

• The flammability of a material or product can be defined by whether it ignites and burns, although flammability is more commonly defined in terms of a measured property in a fire test.

• A fire can proceed through both nonflaming and flaming stages. The nonflaming stages consist of smoldering and pyrolysis (with or without the participation of oxygen); the flaming stages include wellventilated, under-ventilated, and post-flashover fire.

• The initiation and continuance of a fire require the four components that make up the fire tetrahedron: fuel, oxidant, heat, and chemical chain reaction.

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• The reactions that initiate a fire proceed very slowly unless the fuel is heated to hundreds of degrees Celsius. Common ignition sources include cigarettes, matches, and sparks; friction; overheated electrical circuits; welding operations; space heaters; and lightning.

• For a fire to spread, some of the enthalpy from the ignition region must heat unburned material to a temperature at which it can burn. Fire also can spread by the melting and dripping of burning material or by airborne firebrands.

• A fire can be extinguished by cooling, reducing the oxygen supply, separating the fuel and the oxidizer, and decreasing the concentration of the flame-propagating free radicals.

• Fire hazard is the potential for harm from a particular fire scenario. A fire consequence is a specific quantified hazard. Fire risk combines the severity of consequences and the probability that the consequences will occur. There is a sharp increase in the consequences and risk from a fire in a room if it proceeds to flashover.

Key Terms backdraft Intense flames emanating from a just opened doorway that introduced fresh air to a fire that had been oxygen starved. fire consequence A specific, quantified fire hazard. fire hazard A condition with the potential to create harmful consequences under a specified fire scenario. fire risk The undesired consequences of a fire multiplied by the likelihood of their happening. fire scenario A specified set of fire conditions, including details of the fire site and its condition, the combustible items, the number and characteristics of the occupants, and anything else that might affect the outcome of the fire. flashover The often-sudden transition from local burning to almost simultaneous ignition of (nearly) all of the exposed combustibles in a confined area. glowing A descriptor of smoldering combustion when it is accompanied by visible thermal radiation. ignition The onset of combustion. pyrolysis The anaerobic or oxidative decomposition of a gas, liquid, or solid into other molecules when heated. rollover (flameover) The stage of a structure fire when fire gases in a room or other enclosed area ignite. smoldering (nonflaming combustion) The slow, low-temperature, flameless combustion of a solid. vitiation The depletion of oxygen and incorporation of the resulting combustion products in air by a fire.

Challenging Questions 1. When is oxidation not combustion? When is combustion not oxidation?

2. Give three examples of combustion other than fire.

3. Which kinds of materials can undergo smoldering combustion?

4. Name six ways in which a fire can originate.

5. What is the basic mechanism of fire spread?

6. What are the four fundamental ways of extinguishing a fire?

7. Perform the experiment shown in Figure 6-6. A strip of the cardboard from the back of an 8½ in. × 11 in. notepad is a good test subject, and ordinary kitchen tongs can be used to hold the cardboard safely. What is the general concept demonstrated by this test? (Note: Conduct any experiments with fire outside and away from adjacent structures and materials that could ignite. Wear leather gloves and have a pail of water handy.)

References 1. National Fire Incident Reporting System website. http://www.usfa.fema.gov/fireservice/nfirs 2. Pitts, W. M. (2013). Summary and Conclusions of a Workshop on Quantifying the Contribution of Flaming Residential Upholstered Furniture to Fire Losses in the United States (NIST Technical Note 1757 Revised). Gaithersburg, MD: National Institute of Standards and Technology.

3.

Ahrens, M. (2011). Home Fires That Began with Upholstered Furniture. Quincy, MA: National Fire Protection Association.

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4. Evarts, B. (2011). Home Structure Fires That Began with Mattresses and Bedding. Quincy, MA: National Fire Protection Association.

5. Drysdale, D. (2011). An Introduction to Fire Dynamics, 3rd ed. New York, NY: John Wiley.

6. DiNenno, P. J., ed. (2008). SFPE Handbook of Fire Protection Engineering, 4th ed. Quincy, MA: National Fire Protection Association.

7. Glassman, I. (1996). Combustion, 3rd ed. New York, NY: Academic Press.

8. ISO 19706: Guidelines for Assessing the Fire Threat to People. (2011). Geneva, Switzerland: International Standards Organization.

9.

ASTM Fire Standards and Related Materials, 7th ed. (2007). West Conshohocken, PA: ASTM International. www.ASTM.org 10. Karter, M. J., Jr. (2011). Fire Loss in the United States during 2010. Quincy, MA: National Fire Protection Association.

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CHAPTER 7

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Fire Characteristics: Gaseous Combustibles OBJECTIVES After studying this chapter, you should be able to: • • • • • • •

Describe the categorization of flames.

Characterize laminar and turbulent flames.

Define deflagration and detonation, and explain the difference between the two.

Discuss flammability limits and burning velocity, as well as their relationship to fire hazard.

Understand the difference between piloted ignition and autoignition.

Explain the potential hazard from a gas leak.

Explain the importance of chain branching in combustion chemistry.

Introduction One evening, as one of the authors was approaching his house after a day at the fire research laboratory, he noticed an apparently vacant lot on which a house had stood just that morning. The neighbors reported that a crew doing some renovations to that house had nicked the natural gas line. Fortunately, they then went to lunch—which allowed them to avoid the methane explosion and fire that reduced the entire house to rubble.

Figure 7-1 The Hindenburg disaster.

Courtesy of the US Navy.

This is a cautionary story. Gaseous fuels serve us well in a range of applications. Some of us cook indoors with natural gas (methane) and outdoors with propane. Some individuals weld with acetylene. However, the oxidation of these fuels generates a lot of heat, which in turn has the potential for inflicting serious harm. This relationship explains why blimps are now filled with nonflammable helium rather than potentially explosive hydrogen, even though hydrogen is the lighter, and thus more buoyant, of the two gases (Figure 7-1).

The direct fuel in nearly all flaming fires is in the gas phase. This chapter presents a general characterization of gas phase flames, regardless of the original phase of the fuel. It also includes special considerations for cases in which a gas is the original combustible. Later chapters will address how liquid and solid fuels gasify and how fire control can be achieved by attacking the condensed phase fuel.

Categorization of Flames 89

Flame types fall into the following categories: • • • •

Premixed flames or diffusion flames Laminar flames or turbulent flames Stationary flames or propagating flames Subsonic flames (deflagrations) or supersonic flames (detonations)

Not all of the 16 possible combinations of these categories are common. Diffusion flames, for example, are rarely supersonic. Some of the flame types are more important to fire safety. For example, incipient residential fires are generally subsonic, laminar diffusion flames. Most fires are subsonic, turbulent diffusion flames at the time that they are particularly hazardous. Fires are stationary when the combustibles are already fully enveloped in flames. Fires are propagating when flames spread across a combustible item or from one item to another. These categories are discussed in the following sections.

Premixed versus Diffusion Flames In a premixed flame, as the name implies, the fuel and the oxidizer (air) are uniformly mixed prior to ignition. An example would be if a gas leak occurred in a house and many minutes passed before someone came home and struck a match to light a cigarette or candle.

Note Natural gas is predominantly methane (CH 4), plus as much as 20 percent of other gases, mostly small hydrocarbons. Its exact composition depends on the specific source. In the examples in this text, natural gas is assumed to be composed solely of methane. Methane is colorless and odorless.

A small amount of a mercaptan (an acrid sulfur-containing compound) is added to natural gas that is delivered to homes and businesses to alert people to a gas leak and the hazard it presents.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

Imagine that such a gas leak has occurred. The room contains 9.5 percent methane by volume and 90.5 percent air, and the gases are thoroughly mixed. Because air contains 21 percent oxygen by volume, and because 21 percent of 90.5 is 19, the compartment contains 19 percent oxygen by volume. Recall from the Physical and Chemical Change chapter that the volume percent of a gas is the same as the mole percent. Therefore, the ratio of the moles of oxygen in the compartment to the moles of methane is 19/9.5, or 2. This is a stoichiometric mixture, according to the balanced equation CH4 + 2 O2 → 2 H2 O. If ignition of this mixture were to occur in the center of the room, perhaps due to a spark from an electric heater, then a small blue spherical flame would form around the spark and spread radially outward at a burning velocity of approximately 3 m/s (10 ft/s). If no heat losses occurred, the peak temperature in the flame would be 2230 K. Similar behavior would result if the methane percentage in air were somewhat lower or somewhat higher than 9.5 percent, except that the flame would propagate more slowly and the peak temperature would be lower. For fuel-rich mixtures (i.e., greater than 9.5 percent methane in air), there would be insufficient oxygen to completely oxidize the CH4 to CO2 and H2O, so the products would include CO, H2, and—for very rich mixtures—some solid, carbonaceous particles (soot).

A leak of propane or liquefied petroleum gas (LPG) from its storage container could give rise to the same hazard. LPG is either mostly propane, mostly butane, or a mixture of the two. Although propane and LPG are stored as liquids, propane has a boiling point of –44 °F (–42 °C), and butane has a boiling point of +30 °F (–1 °C). Thus the ullage1 in the storage container is occupied by propane or butane at a pressure greater than 1 atm, and any leakage will be gaseous.

Some other gases that might exhibit similar behavior are discussed at the end of this chapter.

Moreover, as will be seen later, a stationary flame over a liquid or solid fuel has an important premixed zone. Combustion scientists have developed techniques for stabilizing a premixed flame so that it burns in a fixed (stationary) position.

Figure 7-2 shows three burner arrangements that produce stationary premixed flames. Such burner setups permit combustion scientists to measure flame properties accurately, as it is easier to measure

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these properties under conditions that do not vary with time. The distributions of temperature and chemical species within the flame became the basis for understanding the detailed sequences of chemical reactions and the ways they vary for different fuels. Measurement of the gas flows at which the flame stabilizes is a good way to measure the burning velocity of the flame. Because the gases are already mixed, the burning velocity is determined by the overall chemical reaction rate of the fuel and the oxidizer.

The diffusion flame stands in contrast to the premixed flame. As the name implies, the fuel and oxidizer gases in a diffusion flame initially exist as separate volumes. When these two volumes come together, each of the gases slowly diffuses into the other.

Figure 7-2 Techniques for stabilizing premixed flames. The arrows represent the flow of a premixed fuelâ&#x20AC;&#x201C;air mixture.

To see how a diffusion flame works, let us return to the example of a slow gas leak into a house. The gas emerging from the opening in the pipe is 100 percent methane. The gas elsewhere in the room is 100 percent air. If the ignition source is located at the gas leak, no oxygen is available to react with the methane. If the ignition source is located at the far side of the room, there is no fuel to combust.

However, if the ignition source is located near the interface between the methane and the air, the fuel and the oxidizer will react, with the combustion then spreading rapidly over the interface. The flame will be sustained at the rate that the gases replenish this thin interface, and the burning rate will be determined by the diffusion rate of the gases, which is slower than the reaction chemistry. Because the early stages of the chemistry deplete the oxygen in the interface, diffusion flames are characterized by some degree of incomplete combustion and, therefore, by the formation of soot. The flame temperature is lower than that of a premixed flame, partly because the incomplete combustion releases less heat and partly because the soot is a black body that efficiently radiates heat from the flame. The temperature at the heart of the flame is on the order of 1500 K, and the soot incandescence at this temperature appears yellow-orange to the human eye.

Because fires are predominantly diffusion flames, researchers have developed laboratory burners that can elucidate the events in this type of combustion. Figure 7-3 shows four arrangements for creating stationary diffusion flames. Note that the drawing labeled â&#x20AC;&#x153;candle-like flameâ&#x20AC;? closely resembles a Bunsen burner. In fact, if the air inlet to the Bunsen burner is closed, the Bunsen flame becomes a diffusion flame.

Laminar versus Turbulent Flames In a laminar flame, the flow streamlines are smooth, and fluctuations in the velocity components are negligible. By contrast, a turbulent flame is characterized by significant local fluctuations in the velocity and temperature profiles Figure 7-4. The path of any particle in a turbulent flame is erratic, with many changes of direction, rather than the straight or gently curving line characteristic of a laminar flame. As presented in the Flow of Fluids chapter, turbulent flows are characterized by high values of the Reynolds number, whereas laminar flows are characterized by low Reynolds number values. As a general rule, a diffusion flame taller than 0.3 m (1 ft) will be turbulent, while a diffusion flame shorter than 0.1 m (4 in.) will be laminar, unless a high-velocity jet is involved.

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Figure 7-3 Burners for stabilizing gaseous diffusion flames.

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Figure 7-4 A laminar flame (a) and a turbulent flame (b). Š irin-k/ShutterStock, Inc. (a) and Š Tigergallery/ShutterStock, Inc. (b).

The presence of turbulence in a flame enhances heat transfer and mixing and affects the rate and products of the reaction chemistry. Accordingly, rates of combustion and rates of convective heat loss to walls are considerably higher in turbulent flames than in laminar flames. These phenomena make it difficult to predict the behavior of large-scale fires, where the dimensions are on the order of meters, based on small-scale (bench-top) fire tests, where the dimensions are a few tens of centimeters or fewer.

Ignition of Gases The first step of the ignition process is the generation of species that are so highly reactive that they can destabilize a fuel molecule. This can be accomplished by a spark or a flame (both called piloted

93

ignition) or by raising the temperature by the infusion of a large amount of energy or enthalpy (nonpiloted ignition, thermal ignition, autoignition).

When a spark jumps a gap between a cathode and an anode, it ionizes the molecules in the gap, forming positive ions, free electrons, and fragments of the fuel molecules. This process is not characterized by a given temperature; in other words, the system is not at thermal equilibrium. As such, the minimum electrical energy for ignition is not particularly sensitive to the temperature of the surroundings. Instead, it depends on the energy of the spark, the gaseous fuel, and the equivalence ratio of the fuel–air mixture in the gap.

Experiments show that ignition of a nearstoichiometric mixture of a combustible gas and air requires much less spark energy than a lean mixture with substantial excess air or a rich mixture with substantial excess combustible. A lean mixture contains fewer fuel molecules for the reactive species to attack and more inert molecules that cool the energetic species and decrease the subsequent reaction rates. In a rich mixture, fewer oxygenated species are available to attack the fuel molecules, and the excess fuel molecules cool the energetic species and decrease the subsequent reaction rates. Thus a nearstoichiometric mixture demonstrates the greatest sensitivity to ignition; accordingly, combustion scientists use such a mixture to characterize an ease of ignition for a fuel. Table 7-1 shows values of minimum ignition energies for a series of combustible gases and vapors mixed either with air or with pure oxygen. In both cases, the mixture compositions are near-stoichiometric.

Table 7-1 shows the following: 1. Fuel-oxygen mixtures ignite with weak sparks far more easily than fuel-air mixtures. This is due to the absence of nitrogen as a heat sink and diluent in the latter mixtures.

2. Saturated hydrocarbons (CnH2n + 2) all ignite similarly in air, due to the similarity of the C—H bond strength in these molecules.

3. Progressive unsaturation (double or triple bonds) in a hydrocarbon molecule favors much easier ignition, as in the sequence C2H6, C2H4, C2H2.

4. Certain gases, such as carbon disulfide, hydrogen, and acetylene, can be ignited with sparks less than one-tenth as strong as those required to ignite alkanes.

The minimum ignition energy that can ignite most fuel vapor–air mixtures is approximately inversely proportional to the square of the absolute pressure. Thus an extremely low-energy spark could ignite a combustible gas mixture at high pressure.

A second way to ignite a gas mixture is with an existing flame, such as a match. This piloted ignition introduces free radicals (O atoms, H atoms, and OH radicals) at local concentrations that are already sufficient to sustain the igniting flame.

A gas/vapor–air mixture also can be ignited, in the absence of a flame or spark, by raising its temperature sufficiently, generally by contact with a hot solid surface. The minimum surface temperature at which thermal ignition occurs can be measured for any combustible mixture, but unfortunately the result depends on the size, shape, orientation, and nature of the surface as well as the state of motion of the gas. For example, Reference [2] reports that 17 different investigations of the ignition temperature of hydrogen–air mixtures gave results varying from a low of 770 °F (410 °C) to a high of 1700 °F (930 °C); Reference [3] reports ethyl ether–air ignition temperatures from three sources as 366 °F (186 °C), 650 °F (343 °C), and 915 °F (491 °C). Reference [4] says that for short (less than 0.1 second) fuel–surface contact times, autoignition on metal surfaces occurred at temperatures 570 °F (300 °C) higher than test results for longer contact times. Accordingly, any value reported for an autoignition temperature is valid for only one set of conditions. If any one experimental technique is used consistently, however, relative ignition temperatures for a series of gases can be obtained. With this caution in mind, Table 7-2 lists some thermal ignition temperatures of gases in air. Diethyl ether is included because of its remarkably low ignition temperature. Note that carbon disulfide vapor, with an even lower ignition temperature of 194 °F (90 °C), can be ignited by a steam pipe at 212 °F (100 °C).

Table 7-1 Minimum Spark Ignition Energies of Near-stoichiometric Mixtures of Gases and Vapors in Air or Oxygen at 25 °C and 101 kPa (1 atm) [1] Minimum Ignition Energy (µJ) Combustible

Air

Oxygen

300

3

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methane, CH4 propane, C3H8

260

2

n-hexane, C6H14

290

6

ethane, C2H6

260

2

ethylene, C2H4

70

1

acetylene, C2H2

17

0.2

carbon disulfide, CS2

15

—

hydrogen, H2

17

1.2

Reproduced from: Kuchta, J.M., “Investigation of Fire and Explosion Accidents in the Chemical, Mining, and FuelRelated Industries—A Manual,” p. 33, Bulletin 680, U.S. Bureau of Mines, Washington, D.C., 1985.

Table 7-2 Thermal Ignition Temperatures of Selected Gases and Vapors in Air [5] Gas or Vapor

Thermal Ignition Temperature (°C)

methane, CH4

540

propane, C3H8

450

n-hexane, C6H14

225

n-octane, C8H18

220

ethane, C2H6

515

ethylene, C2H4

490

acetylene, C2H2

305

carbon disulfide, CS2

90

diethyl ether, C2H5OC2H5

160

hydrogen, H2

400

The same group of gases demonstrates both differences and similarities in their ease of ignition indicators for thermal ignition and spark ignition: 1. The thermal ignition data show that carbon disulfide is far easier to ignite than hydrogen, while the spark ignition data show that almost the same ignition energies are required for this pair.

2. The thermal ignition data show a progressive decrease of ignition temperature with increasing molecular weight for the saturated hydrocarbons (alkanes), while the spark ignition data show no such trend.

3. Acetylene is easier to ignite than ethane in both modes.

4. Ignition of a fuel-air mixture can be accomplished with a relatively weak spark or with a moderately hot surface.

Flammability Limits and Propagation Rates of Premixed Flames Flammability Limits Earlier in this chapter, we explained that the easiest to ignite and most rapidly burning fuel–air mixtures are stoichiometric in composition, and that moving to the rich or lean side of the stoichiometric point results in mixtures that are more difficult to ignite and slower to burn. Intuitively, it might be expected that a mixture that is too rich or too lean will be nonflammable, and that is so.

Note Flammable means that something is capable of catching on fire. However, some confusion has arisen regarding the words nonflammable and inflammable. Unfortunately, the prefix in- has two opposite meanings. It can mean “not,” so inaccurate is the opposite of accurate. It can also mean

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“easily” or “very.” This ambiguity is unacceptable from a fire safety standpoint. Consequently, the fire protection profession strongly urges that the words flammable and nonflammable be used as appropriate, and that the words inflammable and inflammability never be used.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

Suppose we have a stoichiometric mixture of methane and pure oxygen, at 77 °F (25 °C). If ignited, this mixture will burn vigorously, with a flame temperature of 5070 °F (2800 °C). If we had a stoichiometric mixture of methane and air (9.5 percent CH4, 19 percent O2, and 71.5 percent N2, all by volume), it would also burn, albeit not quite as vigorously, and the flame temperature would be 3560 °F (1960 °C). The flame reactions would proceed more slowly because of the lower temperature. This lower temperature arises because of the presence of inert nitrogen, which absorbs some of the heat released by the CH4 + 2 O2 reaction.

If still more nitrogen were added to the methane–air mixture, it would approach a point where the mixture would no longer be flammable, known as the lean flammability limit, or simply the lean limit.

A mixture of borderline flammability—for example, 6.2 percent CH4, 12.4 percent O2, and 81.4 percent N2—would burn with a flame temperature of approximately 2200 °F (1200 °C). The hydrocarbon oxidation reactions in a flame do not occur rapidly enough at temperatures less than 2200 °F (1200 °C) to overcome either the heat losses from the flame to the surroundings or the rate of loss of free atoms and radicals in the flame by recombination. In such a situation, adding just a little more nitrogen suffices to quench the flame or keep it from igniting. Another way of looking at the lean flammability limit is that highly diluted flames propagate so slowly that free convective motion of the flame becomes larger than the propagation speed, thereby disrupting the flame structure.

A different phenomenon occurs when more methane is added to a stoichiometric mixture, creating a rich mixture. Not enough oxygen is available to react fully with all the methane, and some of the carbon in the methane is only partially oxidized, forming CO. The final reaction step that oxidizes CO to CO2 is very exothermic. Thus the partial oxidation of the rich mixture generates less heat and does not raise the temperature of the system as high as a stoichiometric mixture does. A factor contributing to the lesser temperature rise is the heat adsorption by the additional fuel.

Figure 7-5 shows the flammability limits of methane–air mixtures, as well as the effect of introducing additional nitrogen. First, look at the ordinate (“methane/volume %”). The methane–air flammability range goes from a lean limit of 5 percent to a rich limit of 15 percent. The stoichiometric composition of 9.5 percent is approximately in the middle.

Next, note the large amount of space in Figure 7-5 that is marked nonflammable. If more than about 35 percent by volume nitrogen is added to a stoichiometric methane–air mixture, the mixture crosses a boundary and enters into the region of nonflammable mixtures, regardless of the percent by volume of methane or oxygen. Furthermore, Figure 7-5 shows that somewhat smaller percentages of nitrogen could be added to lean or rich mixtures to render them nonflammable. As will be seen in the Fire Fighting Chemicals chapter, fires can be suppressed by surrounding them with inert gases. This dilution moves the fuel–air mixture down and to the right in Figure 7-5.

A plot like Figure 7-5 can be constructed for any flammable gas or vapor, and measurements of flammability limits have been determined for hundreds of substances. Reference [6] includes an extensive compilation of flammability limits, which also is reproduced in Reference [5]. Table 7-3 presents some selected values from this compilation. These flammability limits were measured at 77 °F (25 °C). At higher temperatures, the flammability limits are wider. At pressures greater than approximately 50 kPa, the flammability limits are effectively independent of pressure. In contrast, at much lower pressures, the flammability limits approach each other, meaning that it is difficult to establish a flammable mixture. At pressures less than 4 kPa, no propane–air mixture is flammable.

As is evident in Table 7-3, some substances (hydrogen, carbon disulfide, and acetylene) have extremely wide limits of flammability and, therefore, are especially hazardous. For example, acetylene —even in the absence of oxygen—can burn with a flame that produces hot carbon and hydrogen as products.

The upper flammability limit dramatically increases with oxygen enrichment of the atmosphere, but the lower flammability limit hardly budges because an excess of oxygen is present at this limit anyway.

In general, the flammability limits are much wider when fuel vapors are mixed with oxygen rather than with air. For example, the lower and upper limits for methane in air are 5 percent and 15 percent,

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respectively, by volume, while the lower and upper limits of methane in oxygen are 5 percent and 61 percent, respectively [7].

Figure 7-5 Limits of flammability of various methane–air–nitrogen mixtures at 25 °C and 101 kPa (1 atm) [5].

Table 7-3 Limits of Flammability of Gases and Vapors in Air at 25 °C and 101 kPa [7] Flammability Limits (Volume %) Material

Lower (Lean) Limit

Upper (Rich) Limit

acetone

2.6

13

acetylene

2.5

100

ammonia

15

28

carbon disulfide

1.3

50

ethane

3.0

12.4

ethanol (ethyl alcohol)

3.3

*

ethanol

3.3

36

ethyl ether

2.7

36

97

n-hexane

1.2

7.4

hydrogen

4.0

75

methane

5.0

15

methanol (methyl alcohol)

6.7

*

propane

2.1

9.5

* The vapor pressure of these liquids at 25 °C is insufficient to reach the upper limit concentration.

Reproduced from: Zabetakis, M.G., “Flammability Characteristics of Combustible Gases and Vapors,” Bulletin 627, U.S. Bureau of Mines, Washington, D.C., 1965, p. 29.

Burning Velocity The rate at which a flame moves through a combustible gas mixture can have practical implications for fire safety. For example, suppose such a mixture forms and ignites within a compartment that is initially at approximately 300 K and contains only small openings (e.g., the cutout below a door or an airconditioning vent to another compartment). The flame temperature may be six or seven times the ambient temperature, meaning that the temperature of the gas mixture will rise significantly. According to the ideal gas law, the pressure will rise proportionately.

If the mixture burns slowly, the hot gases have enough time to cool through contact with the walls.

The gas will flow outward through the small openings, equilibrating the room pressure with that of the outside world. Little fresh air will enter the room, and the combustion chemistry will not proceed to the formation of CO2 and H2O. Instead, the products of incomplete combustion will include CO and other toxic compounds. The air will also be laden with unburned fuel. As noted in the previous chapter, this situation could lead to a backdraft.

Conversely, if the burning velocity is very high, the temperature and pressure will rise sharply, faster than the heat loss to the walls and flow through the small openings can abate. The force on windows will exceed the strength of the glass, so the windows will blow out. The large inflow of fresh air will enable the fire to sustain and perhaps intensify.

Given such a scenario, it is important to examine the concept of burning velocity more closely. This quantity is defined as the rate at which a planar flame moves through a stationary, quiescent flammable mixture of infinite extent. Combustion scientists now understand in considerable detail the relationship between the burning velocity of a mixture and the rates of the chemical oxidation reactions occurring in its flame. Theoretically, the burning velocity is approximately proportional to (q″·κ)1/2, where q″ is the mean rate of heat generation in the flame (per unit volume of reaction zone) and κis the thermal conductivity of the gas mixture. References [8, 9], and [10] provide additional information.

This is obviously an idealized rendition. The measurement of true burning velocities would require a very large apparatus, and the mixture within that volume would need to be motion free and stay mixed.

These and other practical constraints make it difficult to make absolute measurements. Furthermore, in a real combustion situation, whether inside an internal combustion engine or a fire room, these conditions are unlikely to hold.

It is thus more instructive to grasp the magnitude of burning velocities and to recognize how they might be affected by various burning conditions. Figure 7-6 depicts curves representing burning velocity measurements for several gases as a function of the fuel–air equivalence ratio ϕ. Recall that ϕ = 1 for a stoichiometric mixture, ϕ < 1 for a lean mixture, and ϕ > 1 for a rich mixture. At the ends of the curves, a flame could not be sustained. Reference [10] also lists burning velocities of about 100 compounds.

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Figure 7-6 Laminar burning velocities of some combustibles in air at 25 °C and 101 kPa (1 atm) [9].

The first observation from Figure 7-6 is that the magnitude of the burning velocity is similar for most of the hydrocarbons. The exception is acetylene, whose higher burning velocity might be a safety concern in areas where welding or metal cutting is performed. The burning velocity of hydrogen is far higher, which is a consideration in the design of fueling stations for hydrogen-powered vehicles. The next observation is that, for the hydrocarbon fuels, the burning velocity peaks a little to the rich side of stoichiometric, and the falloff to either side is measurable, but not large. The burning velocity curve for hydrogen is quite different. The curve peaks at nearly Ď&#x2022; = 2, and there is a sharp falloff toward a stoichiometric mixture.

A flame can actually move considerably faster than these measured values of the burning velocity, for multiple reasons: â&#x20AC;˘ The surfaces in the small laboratory apparatus used to make these measurements slow the flame movement.

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• The real-world flame raises the temperature of the gas from 300 K to perhaps 2100 K, causing a sevenfold expansion of the hot portion of the gas mixture, according to the ideal gas law. The expansion causes motion of the gas and carries the flame toward the unburned gas.

• The burning velocity is based on laminar flame propagation. A turbulent premixed flame will propagate several times faster than a laminar one.

If the combustible mixture is at an elevated temperature before ignition, the burning velocity will be greater. If the oxidant consists of pure oxygen instead of air, the burning velocity will be 5 to 10 times as great. The burning velocity also will depend somewhat on the ambient pressure.

Explosions, Deflagrations, and Detonations To the extent that these three terms have entered common usage, they have acquired varied and subjective meanings to many people. The technical definitions of these three terms are as follows: •

An explosion is the rapid release of energy and increase in pressure when a premixed flame propagates in a confined space.

• A deflagration is subsonic combustion, so nearly all unwanted fires are deflagrations.

• A detonation is supersonic combustion.

A premixed flame may burn as either a deflagration or a detonation. Diffusion flames are typically deflagrations.

The descriptions presented so far have involved deflagrations. A deflagration propagates at subsonic velocity, less than 340 m/s. The speed with which a deflagration moves (i.e., the burning velocity) depends on the chemically controlled rate of heat release in the flame. If the deflagration occurs in a confined space, the pressure rise is relatively uniform on all the surfaces and is sustained until the environment cools or the walls are breached.

A detonation propagates at supersonic velocity, relative to the unburned gas. Its velocity is independent of the rates of the heat-generating chemical reactions. A detonation is a shock wave that, by compression, heats a reactive gas mixture to a high enough temperature to release the heat of combustion. It is possible to calculate the velocity of a detonation from knowledge of the heat of combustion and the physical properties of the mixture.

The damage caused by a detonation usually results from the very high pressure generated in such an event. For example, a detonating methane–oxygen mixture, originally at 1 atm, generates a pressure of more than 30 atm and moves at a velocity of 2500 m/s. (Detonations of liquid or solid explosives produce pressures of tens of thousands of atmospheres.) This pressure rise is sharp and short-lived, and typically moves in the direction of the combustion propagation.

In general, mixtures of combustible gases with oxygen are much more likely to detonate than mixtures with air. However, mixtures with air can detonate under suitably confined conditions or when a powerful enough initiating event occurs. For example, ignition of a gas–air mixture at the closed end of a long tube can produce an accelerating flame, becoming turbulent, with a build-up of pressure and a transition to detonation. Further information is given in Reference [11].

Chemical Mechanisms of Combustion of Gases Elementary Chemistry Scientists have been probing the oxidation of fuels almost since Priestley discovered oxygen, circa 1776. Today, much is known about the chemical reactions occurring in both premixed flames and diffusion flames—but much remains to be learned.

This section presents two of the basic cases of fuel oxidation chemistry. The first involves the simplest fuel, hydrogen. The second demonstrates the substantially increased makeup of the mechanism for the smallest carbon-containing fuel, methane.

Hydrogen Oxidation The sequence of reactions in H2–O2 flames, both premixed and diffusion, is understood thoroughly.

This reaction does not occur by H2 and O2 molecules colliding with each other, as that would involve

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the unlikely meeting of three molecules (two H2 and one O2) in just the right orientation, along with the simultaneous breaking of two hydrogen–hydrogen bonds and one oxygen–oxygen bond, all of which are very strong. Instead, hydrogen oxidation occurs by a chain reaction in which the critical steps involve two-species interactions, featuring the free atoms and radicals H, O, and OH. The most critical reaction is H + O2 → OH + O In this reaction, a single H atom (produced by the ignition source) reacts with a stable molecule, O2, producing two highly reactive species, OH and O. This step is referred to as a chain branching reaction because it increases the number of reactive species.

The OH (hydroxyl radical) reacts very rapidly with H2: OH + H2 → H2O + H The OH is consumed in the formation of a stable water molecule, but it produces an H atom that can continue the chain reaction. Because this step keeps the overall reaction sequence going but does not increase the number of highly reactive species, it is called a chain propagation reaction.

Meanwhile, the O atom produced in the first reaction can react very rapidly with H2 to form two additional chain carriers: O + H2 → OH + H Thus each H atom, when introduced into an H2–O2 mixture, will be transformed by a sequence of rapid reactions (requiring a fraction of a millisecond) to form two molecules of H2O and three new H atoms and a lot of enthalpy Figure 7-7. It is the contribution of this branching sequence that can lead to an H2–O2 explosion. (If the sequence takes 1 ms, then in 100 ms, each H atom will generate approximately 1050 H atoms—a very large number and a very large amplification of the overall reaction rate.)

The reaction sequence continues until one or both of the reactants are consumed. Then, in chain termination reactions, the remaining H, O, and OH species recombine when they reach a surface or according to the gas-phase reactions H + O → OH and H + OH → H2O. The entire sequence of reactions in H2–O2 flames involves only about 10 chemical species and 10 reactions. This small number is unique in the world of fire and combustion. The chemistry is the same for premixed and diffusion flames due to the simplicity of the mechanism and the very high diffusion rate of H atoms.

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Figure 7-7 Chain reaction mechanism in the hydrogen–oxygen flame.

Premixed Methane–Oxygen Flame Chemistry The burning of methane, the simplest of the hydrocarbon fuels, still involves 53 chemical species and 325 reactions [12]. As in H2–O2 flames, this burning process involves chain initiation steps, chain propagation steps, chain branching steps, and chain termination steps. Once again (and in nearly all combustion reactions in air), the chain branching reaction step, H + O2 → OH + O, is important.

Figure 7-8 shows some of the most important steps in the methane–oxygen premixed flame. (In a methane–air flame, nitrogen can be considered to be inert, unless the analysis focuses on the formation of traces of nitrogen oxides. These products are important mainly for addressing air pollution and smoke toxicity.)

As Figure 7-8 shows, the reaction proceeds through several paths, and the relatively stable intermediate CO must form before the formation of CO2. If the flame gases cool before the CO is completely converted to CO2 via oxidation by OH, then CO will appear in the products, even if an excess of oxygen is available in the environment.

Combustion of Larger Hydrocarbon Fuels As the number of carbon atoms in the fuel increases, so do the numbers of chemical species and reactions that constitute the overall flame mechanism. For isooctane, the combustion mechanism includes nearly 900 species and 3600 reactions. Fortunately, computational techniques are available to identify which of these dominate this combustion process. This enables the use of a much simpler mechanism.

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Figure 7-8 Some reaction steps in the methane–oxygen premixed flame (in addition to the H, O, OH, H2, and O2 reactions of Figure 7-7).

Specific Hazardous Gases The following sections present information about some particularly hazardous gases. Additional information can be found in Reference [13] at the end of this chapter.

Hydrogen (H2) From a fire viewpoint, hydrogen is among the most dangerous gases. It is odorless and burns with a flame that is almost invisible (except in a darkened room). As can be seen from the y-axis of Figure 79, hydrogen’s flammability limits are unusually wide. Mixtures of this gas with air are ignited very easily, such as with a low-energy spark. Moreover, hydrogen’s burning velocity is higher than that of any other combustible, so a hydrogen–air mixture in a suitably confined space can detonate.

Fortunately, flammable levels of hydrogen are generated in a few, well-known circumstances.

Hydrogen can be released from hydrogen generation facilities, leaky compressed gas containers, and storage batteries during charging—all considerations that arise with the proliferation of hydrogen-fueled passenger vehicles. Hydrogen is also generated when acids attack metals. Both sodium and potassium, for example, react violently with water to form hydrogen.

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Figure 7-9 Limits of flammability of hydrogen in air (downward propagation of flame), including the effects of added inert gases.

In addressing possible fire hazards of hydrogen, it is important to recognize that this gas is much lighter than air. Thus, hydrogen rises in the atmosphere and can concentrate in an upper area of a compartment. As a result, a locally ignitable hydrogenâ&#x20AC;&#x201C;air mixture may be present even if the average concentration in the compartment is below the lean flammability limit.

Also shown in Figure 7-9 is the effect of adding an inert gas to a hydrogenâ&#x20AC;&#x201C;air mixture. This addition has very little effect on the lean limit until the inert gas has displaced half of the original atmosphere.

Therefore, the main fire safety effect of flushing or ventilating a hydrogen-containing compartment is to promote mixing and prevent localized build-up.

Acetylene (C2H2) Acetylene is an extremely reactive, flammable gas. In its pure state, it cannot be stored at high pressure without the possibility of a highly exothermic dissociation into carbon and hydrogen. For this reason, acetylene is stored in cylinders that are first filled with a very porous massâ&#x20AC;&#x201D;for example, cement, charcoal, or diatomaceous earth. The small pores limit the volume of gas in any particular space.

Before putting the acetylene into the cylinder, the anhydrous filler mass, containing about 80 percent void space, is soaked with acetone or dimethylformamide. Acetylene gas readily dissolves in these liquids, so acetylene is effectively stored as a liquid. When the storage cylinder is opened, the acetylene flows out as a gas, leaving the solvent behind. Acetone dissolves 25 times its own volume of acetylene for each 14.7 psig (1 bar) pressure. An ordinary welding-type acetylene cylinder contains 5.5 gal (18.5 L) of acetone (43 lb, 19 kg) and about 20 lb (9 kg) of acetylene.

The flammability limits for acetylene gas are very wide, extending from 2.5 percent to 81 percent by volume. Under certain conditions, acetylene will dissociate at gas concentrations from 81 percent to 100 percent by volume, releasing heat in the process.

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Because of the reactivity and unconventional storage method of acetylene gas, all acetylene tanks in North America must contain fusible plugs that open at about 212 °F (100 °C). Should this event occur near an ignition source, a flaming torch of burning gas will extend some distance—10 ft to 12 ft (3 m to 3.5 m)—from the opened vent. This flame need not be extinguished unless it endangers people or combustibles nearby. After a short time, the torch will die down and the cylinder will cool sufficiently that it can be moved to a safe place where it can continue to vent. Some possibility exists that the flame could propagate back into the cylinder or tank, at which time the tank will heat up and must be cooled with water sprays. Danger of explosion of the tank exists only if it becomes heated to a glowing red color.

Acetylene–air flames propagate very rapidly, and an acetylene–air mixture in a suitably confined space is capable of detonating. Acetylene in contact with copper forms copper acetylide, an extremely unstable solid that can explode; therefore, copper and copper alloys are not used in the storage or delivery of this gas.

Methane (CH4) Methane is an odorless gas, somewhat lighter than air. It is the main constituent of natural gas, to which odorants (sulfur compounds) generally are added as an aid to leak detection. Methane also is found in coal mines, and is a major cause of coal mine explosions. Methane’s flammability characteristics are similar to those of other saturated hydrocarbon gases (ethane, propane, n-butane, isobutane), all of which are odorless (see Figures 7-4 and 7-5).

Ethylene (C2H4) Ethylene, also called ethene, is used widely as an industrial gas. It has a faint sweet odor. Its mixtures with air have wider flammability limits, are easier to ignite, and propagate flame more rapidly than saturated hydrocarbon gases. Ethylene flames are more luminous (sooty). Mixtures of ethylene with air can detonate, albeit not as readily as acetylene or hydrogen.

Ammonia (NH3) Ammonia is used widely as a commercial refrigerant and a fertilizer. Some of its mixtures with air are flammable—a fact that is not commonly known. An ammonia leak is easily detectable because of its sharp pungent odor. Ammonia–air mixtures are more difficult to ignite and burn more slowly than saturated hydrocarbon–air mixtures

Figure 7-10 Anhydrous ammonia vapor escaping from a pressurized container.

WRAP-UP Chapter Summary • The molecules that combust in a flaming fire are nearly always in the gas phase.

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• • • •

•

•

Flames can be characterized as premixed or diffusion, laminar or turbulent, and stationary or propagating. A deflagration is a subsonic flame; a detonation is a supersonic flame.

A mixture of fuel and air is flammable if the fuel concentration is greater than the lean flammability limit and less than the rich flammability limit.

A premixed flame has a higher burning velocity than a diffusion flame of the same fuel. The burning rate of the former is determined by the flame chemistry, while the burning rate of the latter is determined by the diffusion of fuel and air into each other.

Piloted ignition is the ignition of a flammable mixture by a source that provides the high temperature and/or a source of the free radicals needed to initiate the flame chemistry. Autoignition, also known as thermal ignition, occurs when the temperature of the fuel–air mixture is high enough for the collision energy of the molecules to break the molecules’ bonds.

The set of reactions for the combustion of fuel molecules includes chain initiation steps, chain propagating steps, chain branching steps, and chain termination steps. The initiation steps create the free radicals that propagate the flame; the termination steps remove the free radicals, converting them into stable species. The chain branching steps multiply the number of active species in the flame, thus determining the number of simultaneous chain propagating steps. The key chain branching reaction step is H + O2 →OH + O. A leak of a flammable gas can pose a serious fire hazard. Near the leak, the atmosphere is fuel rich.

Far away from the leak, the atmosphere is fuel lean. If an ignition source is active in the region where the mixture is within the flammable limits, an explosion is possible.

Key Terms autoignition (nonpiloted ignition, thermal ignition) Ignition resulting from heating a fuel without the presence of a flame or spark. burning velocity The speed with which a flame moves through unburned gas. chain branching reaction A chemical reaction in which there is a net increase in the number of free atoms or free radicals. chain propagation reaction A chemical reaction in which the number of free atoms or free radicals is unchanged. chain termination reaction A chemical reaction in which the number of free atoms or free radicals is decreased. deflagration Combustion propagating through a gas or an explosive material at a subsonic velocity, driven by the transfer of heat. detonation Combustion of a gas or explosive material propagating at a supersonic velocity and driving a shock front directly in front of it. diffusion flame A flame whose propagation is governed by the interdiffusion of the fuel and oxidizer. ignition The starting of combustion. laminar flame A flame in which the flow streamlines are smooth and fluctuations in the velocity components are negligible. lean flammability limit (lean limit) The lowest volume percent of a gas or vapor in air capable of ignition. piloted ignition Ignition that results from the presence of a flame or spark. premixed flame A flame in which the fuel and oxidizer are mixed prior to ignition. propagating flame A flame that is moving to an adjacent region containing oxygen and fuel that has not yet ignited. rich flammability limit (rich limit) The highest volume percent of a gas or vapor in air capable of ignition. stationary flame A flame whose location is not changing over time. turbulent flame A flame in which the fluid movement and the temperature are characterized by irregular fluctuations.

Challenging Questions 1. List the eight types of deflagrations and give an example of each.

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2.

Why is a premixed flame more hazardous than a diffusion flame? Why is a diffusion flame more hazardous than a premixed flame?

3. Why is turbulence important in combustion?

4. What is the difference between a deflagration and detonation?

5. What is a chain branching reaction? Why are these reactions important?

6.

Which is more easily ignited: a hexane vapor–air mixture or a hydrogen–air mixture? Does it depend on whether a spark or a hot surface is the ignition source?

7. Why are combustible gas–air mixtures flammable in certain proportions and not flammable in other proportions?

8. How would you use the word “inflammable?”

9. In tests of the fire resistance of a wall, floor, or ceiling, one side of the partition is subjected to increasing temperature. One rating criterion is the time at which the temperature on the cold side of the partition exceeds 250 °C. The intent is to reduce the likelihood of ignition of combustibles in a compartment adjacent to the one in which the fire has started. Would such an ignition be piloted or thermal? Is such an ignition likely at this temperature?

10.

What is the burning velocity of a gas mixture? Under which circumstances can a flame move through a mixture faster than its burning velocity?

References 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

1

Kuchta, J. M. (1985). Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries: A Manual (Bulletin 680). Washington, DC: U.S. Bureau of Mines, p. 33.

Jost, W. (1946). Explosion and Combustion Processes in Gases. New York, NY: McGraw-Hill.

Mullins, B. P. (1955). Spontaneous Ignition of Liquid Fuels. London, UK: Butterworth.

Smyth, K. C., and N. P. Bryner. (1997). “Short-Duration Autoignition Temperature Measurements for Hydrocarbon Fuels near Heated Metal Surfaces.” Combustion Science and Technology 126: 225–253.

Zabetakis, M. G. (1965). Flammability Characteristics of Combustible Gases and Vapors (Bulletin 627). Washington, DC: U.S. Bureau of Mines, p. 29.

Zabetakis, M. G. (1965). Flammability Characteristics of Combustible Gases and Vapors (Bulletin 627). Washington, DC: U.S. Bureau of Mines, pp. 114–115.

Zabetakis, M. G. (1965). Flammability Characteristics of Combustible Gases and Vapors (Bulletin 627). Washington, DC: U.S Bureau of Mines.

Glassman, I. (1996). Combustion, 3rd ed. New York, NY: Academic Press, 1996.

Drysdale, D. (2011). An Introduction to Fire Dynamics, 3rd, ed., New York, NY: J. Wiley.

Strehlow, R. A. (1984). Combustion Fundamentals. New York, NY: McGraw-Hill.

Zalosh, R. (2008). Explosion Protection. In: SFPE Handbook of Fire Protection Engineering, 4th ed., P. J. DiNenno, ed. Quincy, MA: National Fire Protection Association, Chapter 3–15.

GRI-MECH 3.0. http://www.me.berkeley.edu/gri_mech/ Lemoff, T. C. Gases. In: Fire Protection Handbook, 20th ed., A. E. Cote, ed. Quincy, MA: National Fire Protection Association, pp. 6–165.

The ullage in a container is the space that is unfilled by a liquid or solid.

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CHAPTER 8

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Fire Characteristics: Liquid Combustibles OBJECTIVES After studying this chapter, you should be able to: • Describe the flash point, fire point, and autoignition temperature of a flammable liquid.

• List the three classes of flammable liquids, based on flash point and potential ambient temperatures.

• Define the linear burning rate of a pool of liquid and explain why it varies with the diameter of the pool.

• Describe the physical considerations that affect the rate of flame spread of flammable liquids.

• Explain boilover.

• Explain a boiling liquid/expanding vapor explosion (BLEVE).

Introduction How many movies have you seen in which a lit cigarette ignites a gasoline spill? In reality, this is not a very likely occurrence. A cigarette is a small ignition source, and the spill is a large heat sink. Most likely, the gasoline will cool and quench the cigarette before the cigarette heats the gasoline—but that rather simple outcome would not serve the filmmaker’s intent to have the dramatic effect of flames destroying a car, a house, or a service station.

Liquids do ignite and burn, sometimes quite violently. The chemistry of a burning liquid is the chemistry of its vapor.

If a compartment contains an ignitable volume fraction of n-hexane, the chemistry does not “know” whether the fuel entered the compartment as a gas or whether a small amount of spilled liquid vaporized. Thus, the combustion chemistry of burning gases discussed in the previous chapter provides a sufficient basis for understanding the chemistry of burning liquids. However, physical considerations also affect the ignitability and rate of flame spread of flammable liquids as well as the tactics used to limit these hazards. These are the topics covered in this chapter.

Ignition of Liquids: Flash Point, Fire Point, and Autoignition Temperature It is the vapor of a liquid that burns; therefore, the principal property of a flammable liquid that affects its susceptibility to ignition is the ease with which the molecules vaporize to form a gaseous fuel–air mixture that is within the liquid’s flammability limits.

Placing a match just above a pool of a flammable liquid will not lead to ignition unless the vapor concentration exceeds the lower flammable limit of that vapor in air. In the Physical and Chemical Change chapter, we saw that at 32 °F (0 °C), the vapor pressure of methanol is 3.96 kPa (3.92 atm).

The total pressure is 101 kPa (1 atm), so the volume percent, or mole percent, of methanol vapor in the air just above the liquid surface is 100 · 3.96/101 = 3.9 percent by volume. In the Fire Characteristics: Gaseous Combustibles chapter, we saw that the lower limit of flammability of methanol vapor in air is 6.7 percent by volume. Therefore, methanol should not be, and is not, flammable at 32 °F (0 °C).

The vapor pressures of liquids, however, increase sharply with increasing temperature. If a match flame were held next to a small enough quantity of methanol liquid for a long enough time (and if the match did not burn out), the liquid would be heated to 54 °F (12 °C). At this temperature, the vapor pressure of methanol is 7.17 kPa, and the percent by volume is 7.1 percent. This exceeds the lean flammability limit, so the liquid would ignite.

One of the standard tests for ignition of liquids, ASTM D92 [1], replicates this behavior. A small, open cup of cold methanol is heated gradually from below. A small flame from a tiny burner is passed across the liquid surface every 10 seconds. When the liquid reaches about 54 °F (12 °C), a flame moves rapidly across the surface, consuming the methanol vapor above the surface. After a fraction of a second, no further combustion would occur, because the combustible vapor has been consumed, and the heat transfer from the small flame is too little to overcome the evaporative cooling of the liquid surface and sustain the vaporization. By the time that additional vaporization can restore the original

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vapor concentration, the burner has been removed, and an ignition source is no longer present. Ten seconds later, the burner is passed over the liquid again, and the sequence repeats itself. The minimum temperature at which this behavior occurs (54 °F, 12 °C for methanol) is called the flash point of the liquid.

When the methanol is heated further (usually 10 °F to 30 °F, or 5 °C to 15 °C higher than the flash point), and the ignition flame is applied from time to time, combustion is sustained after removal of the ignition source. At this temperature, called the fire point of the liquid, the liquid temperature is high enough to maintain a supply of vapor as fast as it is consumed by the flame.

Multiple tests for flash point and fire point temperatures have been developed, which yield some variation in the measured values. These discrepancies arise because the thermal and flow environment above the liquid depends on four properties: • • • •

The intensity and size of the ignition source The length of time for which the ignition source is held over the liquid The rate of heating of the liquid The degree of air movement over the liquid

Nevertheless, the measured values, especially the flash point, are widely used as guides to the safe handling of liquids. The flash point, being lower than the fire point, is a more conservative value to use.

Liquids can be divided into classes (which are divided further into subclasses) based on their flash points [2]: 1. Class I: Liquids with flash points below 100 °F (38 °C) an indoor temperature that could be reached sometime during the year.

2. Class II: Liquids with flash points at or above 100 °F (38 °C) but below 140 °F (60 °C), a temperature that could be reached with only a modest degree of heating.

3. Class III: Liquids with flash points at or above 140 °F (60 °C), a temperature that would require considerable heating.

Table 8-1 Flash Points of Some Common Liquids Flash Point* °C

°F

gasoline

–45.5

–50

ethyl ether (anesthetic)

–28.9

–20

n-hexane

–3.9

25

JP-4 (jet aviation fuel)

–18

0

acetone

–17.8

0

toluene

4.4

40

methanol

12.2

54

ethanol

12.8

55

35

95

>38

>100

40 to 55

104 to 131

Jet A (jet aviation fuel)

47

117

kerosene

37.8

100

>54

>130

66

151

171

340

243

469

Class I Liquids

††

†

turpentine

Class II Liquids ††

No. 2 fuel oil (domestic) †

diesel fuel

††

††

No. 5 fuel oil

Class III Liquids ††

JP-5 (aviation jet fuel)

††

SAE No. 10 lube oil

††

tricresyl phosphate

* Data from Reference [3], except as noted.

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† Data from Reference [2]. This value was obtained using a closed-up method, which typically gives flash point values that are 5 °C to 10 °C lower than open-cup values. †† Data from Reference [4]. These data were obtained using a closed-up method, which typically gives flash point values 5 °C to 10 °C lower than open-cup values.

Table 8-1 lists flash points for some common liquids. Notice the wide range, from –50 °F to 469 °F (–45.5 °C to +243 °C). Note also that these values are meaningful only for a bulk liquid. If a liquid with even a high flash point is formulated as a spray or a foam, is released with air present, and comes into contact with even a small ignition flame, the tiny amount of liquid in contact will immediately be heated to a temperature higher than its flash point and will start burning. The combustion enthalpy released will vaporize the surrounding spray or foam, and the fire will propagate (spread).

As an example of the fire potential of liquids in these three classes, consider a liquid spill on a summer day when the ground has been heated by the sun to 95 °F (35 °C). If the spill consisted of nhexane, a Class I liquid, there would be a race between the wind dispersing this volatile chemical and the introduction of an ignition source. On a still day, the vapor would ignite; on a very windy day, it probably would not. If the spill consisted of kerosene, a Class II liquid, a fire hazard would exist only if the liquid was exposed to an additional heat source capable of raising the temperature of some part of the liquid by at least 25 °F (14 °C). JP-5, a Class III liquid, was designed to burn well in jet engines, but poses a significantly lower ignition risk during handling and storage than JP-4, a Class I liquid.

Fire points and flash points depend heavily on pressure. The flash point is the temperature at which the vapor pressure of the liquid equals the lower flammability limit. If the atmospheric pressure were 50 kPa instead of 100 kPa, the vapor pressure of the liquid need be only one-half as great to achieve the lower flammable limit. Therefore, flash points and fire points are lower than normal at pressures below atmospheric pressure and higher than normal at pressures above atmospheric pressure. This variation is very important in assessing the flammability in, for example, the vapor space of an aircraft fuel tank. It should also be considered in cities at higher elevations, such as Denver and Albuquerque, where the average atmospheric pressure is approximately 80 kPa (0.8 atm).

Autoignition, in which a vapor–air mixture is ignited strictly by heating, was discussed in the Fire Characteristics: Gaseous Combustibles chapter. Autoignition temperatures are typically hundreds of kelvins higher than flash points or fire points.

Burning Rates of Liquid Pools Once a pool of a flammable liquid is ignited, the flames generally spread to cover the full surface area of the pool. The liquid will then burn at a more or less steady rate until the liquid is consumed. (Some very-low-volatility, high-fire-point fluids burn locally, perhaps with small, irregularly moving flames. There is some discussion of these later in this chapter.)

The rate of burning of a liquid pool is often expressed as a linear burning rate (in mm/s)—that is, the rate at which the surface of the pool recedes. The following discussion assumes that the liquid is sufficiently deep that steady burning can be established. Treatment of shallower pools is beyond the scope of this book.

The linear burning rate is readily converted to a linear mass burning rate (in kg/m²-s) by multiplying the linear burning rate by the density of the liquid (in kg/m³) and dividing the product by 1000 (to yield mm/m). Then, obtaining the heat of combustion of the liquid (J/g) from a handbook, the rate of heat release per unit area can be calculated, assuming complete combustion. The total heat release rate for the pool is the rate of heat release per unit area multiplied by the surface area of the pool.

To illustrate the magnitude of these rates, a pool of gasoline 3.3 ft (1 m) in diameter and 25 mm (1 in.) deep will be consumed in approximately 4 minutes. The average linear burning rate is 25 mm/240 s or about 0.1 mm/s. Gasoline (and many hydrocarbons of similar molecular weight) has a density of approximately 700 kg/m3 and a heat of combustion of approximately 45 MJ/kg. The surface area of the pool is πd2/4 =0.79 m2. Using these values, the mass burning rate of the pool can be calculated as about 0.06 kg/s. The average rate of heat release for this pool fire would be about 2.5 MJ/s or 2500 kW. The linear burning rate of a pool of liquid depends not only on the nature of the liquid but also on the diameter of the pool. Figure 8-1 shows the linear burning rate of n-hexane as a function of pool diameter from about 0.2 m to 2.5 m.

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Figure 8-1 Effect of pool diameter on the linear burning rate of n-hexane [4].

Note The burning rate of a pool smaller than 0.2 m in diameter is of less interest in fire protection, as its hazard would be limited to its role as an ignition source. For fires of such small diameter, the flames heat the lip of the container, and the hot container heats the adjacent liquid, increasing the local evaporation rate and, therefore, the overall burning rate. As the diameter increases, the central area of the pool becomes larger relative to the area of the ring near the lip, the edge effect becomes less important, and the linear burning rate decreases.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

For pools larger than 0.2 m in diameter, the burning rate increases with increasing pool diameter, reaching a limit at a diameter of approximately 3 m. The reason for this increase becomes apparent when we consider the three factors that control the burning rate of a liquid pool.

First, little of the flame radiation is needed to vaporize enough liquid fuel to sustain the fire. Each gram of n-hexane that burns releases 44,860 J. The rate of burning of the n-hexane is controlled by its rate of vaporization. To vaporize n-hexane, the latent heat of vaporization—that is, 371 J/g—must be supplied. Therefore, a little less than 1 percent of the combustion energy must return to the n-hexane surface, through the rising vapors, to maintain the vapor supply to the flame. (The bulk of the heat is convected upward with the combustion products and radiated sideways and upward.)

Second, the flame over a pool that is less than 0.2 m in diameter is a laminar diffusion flame. Its combustion is relatively complete, so little soot forms. The flame is optically thin, the emissivity of the flame is low, and the radiant intensity to the fuel surface is low. This small amount of radiation, combined with some conduction from the lip, suffices to vaporize enough liquid to keep the flame burning at a low level.

Third, the flame radiation to the surface increases with increasing pool diameter, up to a limit. At these larger diameters, the flames evolve from laminar to increasingly turbulent in nature. As this happens, more soot forms, and the optical thickness of the flame increases. Figure 8-2 shows how the thermal radiation intensity increases with increasing sootiness.

The lowest curve in Figure 8-2 simulates an optically thin flame. As more soot is “added” to the flame, the radiant intensity increases. The “average” soot curve reaches a limiting value at about 2 m physical thickness. At this point, the flame is optically thick, and a further increase in the physical

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thickness will not increase the radiant output. The same is true for adding more soot (as in the “high soot” flame, which has eight times the soot of the “very low soot” flame)—this extra soot simply ensures that the “high soot” flame reaches the radiative limit at a smaller flame diameter.

Linear burning rates have been measured for other liquids [4] and have been extrapolated to limiting values for large pool sizes. For these pools, the flames are optically thick, so the energy radiated to the surface is constant. Thus one might expect that the linear burning rate would depend on how easy it is to vaporize the liquid. Figure 8-3 demonstrates this relationship. For these five liquids, the linear burning rate correlates with the reciprocal of the latent heat of vaporization per unit volume (the product of the latent heat of vaporization per gram and the density). The slope of the correlation line is about 3 J/s•mm² (30 kW/m²). Thus the radiation from optically thick flames of any of the five combustibles imposes an average heat flux of approximately 30 kW/m² on the liquid surface, regardless of the chemical nature of the combustible.

Figure 8-2 Calculated radiative intensity coming from a hot, semi-transparent, sooty gas of various thicknesses, for three soot levels.

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Figure 8-3 Burning rates of large liquid pools versus inverse of product of latent heat of vaporization (L) and density (p).

Flame Spread Rates over Liquid Surfaces The previous section dealt with burning rates of burning liquid pools, where the flame covers the entire surface. In these scenarios, the spread over the surface is fast compared to the surface regression rate.

This section addresses situations in which the rate of spread of the flame over the surface, after a local region of the surface has been ignited, is important.

Figure 8-4 shows data for the flame spread rate over the surface of a liquid, n-butanol (C4H9OH).

(Note the logarithmic scale on the vertical axis.) This compound has a flash point of 110 °F (43 °C), as measured by the open-cup method described earlier. Above this temperature, the flame spread rate is 2 m/s (6.5 ft/s) and is independent of the liquid temperature. Below 110 °F (43 °C), the flame spread rate heavily depends on the liquid temperature. Indeed, at 68 °F (20 °C), the flame spread rate is only 1/100 of the value at 122 °F (50 °C).

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Figure 8-4 Rate of flame spread over the surface of n-butanol [5].

This behavior is typical of flammable fluids. A flame will spread, albeit very slowly, over a liquid whose temperature is well below its flash point. To start the burning, the liquid must be heated locally to above its flash point. Then, if the radiative heat transfer from the flame is sufficient to heat the adjacent cold liquid (and induce convection currents in it), the flame will spread.

For a fluid whose temperature is well above its flash point, a combustible vapor concentration in excess of the lower flammability limit exists over the surface before the arrival of the flame, and the flame rapidly covers the entire surface. If the liquid is too warm, the vapor concentration just over the surface will exceed the upper flammability limit. In that case, air circulation over the liquid pool will decrease the combustible concentration with increasing height above the surface, and somewhere there will be a zone containing a near-stoichiometric mixture.

The direction and velocity of any air flow can have a large impact on flame spread over a liquid pool.

A (co-flow) breeze in the same direction as the flame spread will increase the spread downwind. A (counterflow) breeze opposite to the flame spread direction will decrease and perhaps even halt the flame spread.

Hazards of Liquid Fuel Fires With this understanding of the characteristics of fires of liquid fuels, it is helpful to identify five categories of liquid fires that constitute nearly all the encountered configurations.

1. A pool of liquid, such as an open tank or the result of a spill. If the temperature of the liquid exceeds its fire point, it can be ignited and will sustain flaming. Given the proper equipment, fires on stagnant liquids are routinely extinguished using techniques such as those discussed in the Fire Fighting Chemicals chapter.

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A serious outcome can result from boilover from a pool fire. The key condition for boilover is the presence of two immiscible layers: • An upper layer of a low density combustible liquid • A lower layer of a higher-density fluid whose boiling point can be exceeded due to heat transfer from the upper fluid layer Although this phenomenon has been recognized for more than a century, boilover was not understood fully until about 25 years ago [6].

The classic case of boilover begins with a burning pool of hydrocarbon fuel. This liquid source could be as small as a deep-fat fryer or as large as a petroleum storage tank at an oil refinery.

Hydrocarbon fuels consist of a mixture of highly volatile and slightly volatile components. A heated sample would start to burn at a fuel temperature below 212 °F (100 °C). As the fire burns in the open container, the more volatile components are driven out of the topmost few millimeters of liquid, and the temperature of this “slice” rises to approximately 572 °F (300 °C). Heat is conducted downward through the liquid to the next few millimeters, causing gasification of volatile components in that slice. The volatiles form bubbles below the surface. The motion of these bubbles greatly accelerates the mixing of the hotter upper fuel and the cooler lower fuel.

Enhanced by this bubble-induced mixing, a hot zone spreads downward through the fuel.

Seeing the burning fuel, a nearby person applies water to quench the flames. The water, being of higher density than the hydrocarbon fuel, rapidly sinks to the bottom of the container. Now there are two layers that match the description given earlier: fuel on top and water on the bottom.

As the water sinks, it encounters hot oil at temperatures well above 212 °F (100 °C), and the subsurface water starts to boil vigorously. The volume of 1 kg of hot water vapor is more than 1000 times the volume of 1 kg of liquid water. Under the best of circumstances, the expanding water pushes up on the burning oil, which then overflows the container and spreads the fire. More seriously, the rapidly expanding water can propel blobs of the hot, burning oil out of the container.

These airborne blobs have more surface area than the original pool surface, so the rate of oil combustion increases, heating the air surrounding the blobs and further accelerating their dispersion. The outcome is a rapid expansion of the fire outside the pan or tank. The consequences for nearby people or combustibles can be disastrous.

2. A flowing liquid, such as from an overflowing or rapidly leaking tank. As in the first category, if the temperature of the liquid exceeds its fire point, the material can be ignited. Flowing liquid fires are very difficult, and sometimes impossible, to extinguish as long as the liquid continues to flow and the flames continue to move.

3. A spray from a small orifice at high pressure (e.g., from a leaking hydraulic fluid line). For fires involving liquids in the form of sprays, the fire point is not a relevant measure of flammability. A pool of domestic (No. 2) fuel oil, a Class II fuel, at 68 °F (20 °C) cannot be ignited with a match (unless a wick is present). However, when the same oil in the form of a spray or foam comes into contact with even a very small ignition flame, the tiny amount of liquid in contact will immediately be heated to above its flash point and will start burning. The combustion energy released will vaporize the surrounding spray or foam, and the fire will propagate (spread). The concept of flammability limits still applies, and the values for a given chemical are similar to those for a vapor–air mixture of the same chemical. Because the flames are spatially linked to the orifice, these stationary fires can be attacked by relieving the fluid pressure and thereby turning a spray into a flow of potentially lower hazard, by inerting the environment, or by applying a gas-phase active fire suppressant.

4. A thin liquid layer drawn up by capillarity (wicking action) over the surface of a porous medium, such as a fabric or paper. A wick can consist of any nonmelting porous material that the liquid is capable of wetting and that is in contact with the pool of liquid. The liquid is drawn up the wick by surface tension (capillarity), and the wick becomes covered with a thin film of the liquid. (As an example, immerse one corner of a handkerchief in a glass of water and observe what happens.)

When an ignition source is applied to the wick (such as a match to a candlewick), the thin film of liquid is heated rapidly to above its fire point and it ignites. As it burns, additional liquid is drawn up the wick and feeds the fire. Such a fire is in itself quite small. However, if the flame from this small fire comes in contact with a large liquid pool or a combustible solid, its heat could eventually

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warm the fuel immediately adjacent to it so that the fire would spread from the wick to a portion of the adjacent fuel, ultimately growing into a much larger fire.

5. A confined liquid in a pressure vessel, heated by an external fire. Liquefied gases, such as propane, are stored in tanks that are designed to withstand high pressures from within. The compression of a gas to form a liquid enables storage of approximately 1000 times the mass of the chemical in the tank volume. At atmospheric pressure, propane boils at –44 °F (–42 °C); its vapor pressure is 101 kPa (1 atm) at this temperature. At 77 °F (25 °C), its vapor pressure is 960 kPa (9.6 atm); and on a hot day at 100 °F (38 °C), its vapor pressure is 1320 kPa (13 atm). Thus the tank is designed to withstand at least this pressure. It is fitted with a pressure relief valve, which opens if the liquid overheats and generates an excessive pressure. A typical setting for the pressure relief valve on a propane tank is about 2000 kPa. If a fire should be burning near the outside of the tank, the temperature of the tank will rise due to the radiative and convective heat transfer from the flames. The temperature, and thus the pressure, of the propane inside this container will rise due to conduction through the tank wall.

The liquid propane, which was initially above its boiling point, now boils vigorously.

As the wall of the steel tank gets hot, its tensile strength diminishes. (At about 930 °F (500 °C), the yield point of steel is approximately half of its normal value.) The combination of increased internal pressure and weakened steel ruptures the tank. The liquid contents, which were boiling at perhaps 10 atm, are now suddenly at 1 atm. A sudden expansion of the vapor occurs, resulting in an enormous eruption of vapor and liquid aerosol called a boiling liquid/expanding vapor explosion (BLEVE; pronounced “blev-ee”).

Note As can be seen from the name, a BLEVE is a change of state from a liquid to a gas, leading to a major pressure increase and (catastrophic) failure of the container. The use of the word “explosion” is actually misleading, because an explosion involves chain branching chemistry. The liquid whose expansion leads to a BLEVE need not be flammable [7]. For example, BLEVEs of water and liquid nitrogen are possible.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

The potential for loss of life and property can be increased significantly if the expelled fluid is ignited by contact with the external fire. This can lead to a burning cloud that can extend as much as hundreds of meters in diameter and propel pieces of the container as far as 1 kilometer. An example is shown in

Figure 8-5 The fireball formed from an ignited BLEVE. The small, dark object above the plume is a helicopter in the background. © Ivan Cholakov/ShutterStock, Inc.

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WRAP-UP Chapter Summary •

• •

•

• •

•

Three temperatures characterize the ability to ignite a liquid. The flash point is the temperature at which piloted ignition occurs, but is not sustained. The flash point is the temperature at which the vapor pressure of the liquid equals the lower flammability limit. At the slightly higher fire point, the flame is sustained. The autoignition temperature is hundreds of kelvins higher and applies to unpiloted ignition.

Class I liquids have flash points below 100 °F (38 °C), Class II liquids have flash points between 100 °F and 140 °F (38 °C and 60 °C), and Class III liquids have flash points at or above 140 °F (60 °C).

For a liquid pool of diameter less than about 0.2 m, the linear burning rate (surface regression rate) decreases as the pool diameter increases. In this pool-size range, fuel vaporization is affected by the container edges, which are heated by flame radiation. For pool diameters greater than 0.2 m, the burning rate rises with increasing diameter. The flames become turbulent and sootier, so flame radiation increases and vaporizes the fuel faster. A limit to the burning rate is reached at pool diameters near 2 m, where the flames are optically thick and the flame radiation to the fuel surface is no longer increasing.

If the liquid is at or above its flash point, the flame spread rate is fast, and the entire pool is engulfed within seconds. In such a case, the liquid is already evaporating sufficiently to reach the lower flammability limit in the vapor phase over the fuel surface. As the liquid temperature decreases, flame radiation must both heat the liquid to the flash point temperature and supply the heat of vaporization.

As a result, the flame spread rate decreases sharply.

Liquid fuel fires generally fit into five categories: (1) a liquid pool, (2) a flowing liquid, (3) a spray from a small orifice at high pressure, (4) a thin liquid layer drawn up by wicking action over the surface of a porous medium, and (5) a liquid confined in a pressure vessel that is heated by an external fire.

Two consequences of fires are particularly hazardous: boilover and BLEVE. A boilover is a special case of a liquid pool, which can occur in an open container with flames over the upper of two immiscible liquid layers. If the lower layer, which has a higher density, has a lower boiling point, it can expand violently when its temperature reaches its boiling point.

A BLEVE (a liquid confined in a pressure vessel that is heated by an external fire) is also a result of the sudden expansion of a liquid when it vaporizes to a gaseous state. When this expansion occurs because the wall of the pressure vessel is weakened due to heating by the external fire, the vessel wall ruptures catastrophically. If the fluid is flammable and is ignited by the external fire, the flames are accelerated, and the potential damage is multiplied.

Key Terms boiling liquid/expanding vapor explosion (BLEVE) A violent pressure release that occurs when a closed container of liquefied gas is heated externally, resulting in vaporization of the liquid, creating an internal pressure that exceeds the strength of the container material. boilover The rapid overflow or expulsion of burning liquid fuel from an open container when water, located below the fuel surface, boils and expands. fire point The minimum temperature to which a liquid must be heated, in a standardized apparatus, so that sustained combustion results when a small pilot flame is applied, as long as the liquid is at normal atmospheric pressure. flash point The minimum temperature to which a liquid must be heated, in a standardized apparatus, so that a transient flame moves over the liquid when a small pilot flame is applied. linear burning rate The rate at which the surface of a liquid pool recedes as it burns.

Challenging Questions 1. From experiments, we know that the flammability limits of benzene are 1 percent by volume and 8 percent by volume, and that benzene’s temperature (°C) at a given vapor pressure (kPa) is approximated by T = [1200/(5.8 – log Pvap)] – 220. Calculate the flash point.

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2. Under which conditions can a liquid burn if its ambient temperature is below its flash point?

3. For a chemical storage warehouse in a climate where building temperatures have never exceeded 90 °F, but where power outages are frequent, should safety precautions be taken for Class I liquids? Why? What if only Class II liquids were stored in the warehouse?

4. n-Hexane is being stored in a cylindrical container (1 m in diameter, 1 m tall) that is located inside the same chemical storage warehouse described in Problem 3. The owner is concerned that, in the event of a leak in the container, the fluid might flow across the floor to the electrical box. In such a case, a local flammable vapor–air mixture might arise and be ignited by the electrical box. The warehouse owner builds a dam around the container, with the walls being deep enough to hold the entire contents. The dam is 3 m in diameter and 12 cm high, and the container stands above the liquid level.

A. What is the maximum linear burning rate, mass burning rate, and rate of heat release should a major leak occur?

B. A calculation indicates that a heat release rate of 10 MW will cause serious damage to the warehouse. So, instead of one large dam, the owner constructs 10 small dams, each separated from the other. If all 10 pools should ignite in the event of a major leak, what is the maximum linear burning rate, mass burning rate, and rate of heat release?

5. How does the rate of flame spread over the surface of a liquid depend on the flash point?

6. Under which conditions could boilover occur?

7. Under which conditions could a BLEVE occur?

References 1.

2.

3.

4.

5.

6.

7.

ASTM D92-11: Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester. (2011). West Conshohocken, PA: ASTM International.

Slye, O. M., Jr. (2008). Flammable and Combustible Liquids. In: Fire Protection Handbook, 20th ed., A. E. Cote, ed.

Quincy, MA: National Fire Protection Association, Chapter 6.12.

Rosales, K., and J. M. Stoltzfus. (2008). Oxygen-Enriched Atmospheres. In: Fire Protection Handbook, 20th ed., A. E. Cote, ed. Quincy, MA: National Fire Protection Association, Chapter 9.17.

Kuchta, J. M. (1985). Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries: A Manual (Bulletin 680). Washington, DC: U.S Bureau of Mines.

Burgoyne, J. H., and A. F. Roberts. (1968). “The Spread of Flame across a Liquid Surface.” Proceedings of the Royal Society, London, A308: 39–79.

Hasegawa, K. (1989). Experimental Study on the Mechanism of Hot Zone Formation in Open-Tank Fires. In: Fire Safety Science: Proceedings of the Second International Symposium. New York, NY: Hemisphere, pp. 221–230.

Peterson, D. F. (2001, April 1). “BLEVE: Facts, Risk Factors, and Fallacies.” Fire Engineering.

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CHAPTER 9

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Fire Characteristics: Solid Combustibles OBJECTIVES After studying this chapter, you should be able to: • • • • • • •

List the three significant differences between the burning of a solid fuel and the burning of gaseous and liquid fuels.

Describe the thermal and chemical processes that result in the ignition and burning of a solid.

Describe how char formation and melting occur and how they affect the burning rate.

List the types of combustible solids.

Describe the types of polymers and explain how they gasify.

Describe at least four classes of mechanisms by which fire retardant additives act to modify the ignition and burning of solids.

Discuss the use of calorimetry to measure the heat-release rates of materials and products.

Introduction Unexpectedly vigorous burning of solid combustibles has been at the core of some of the pivotal fires in our lifetimes.

These include the 1944 Hartford circus fire, which involved a tent that was waterproofed with paraffin wax; the 1967 Apollo 1 fire, where the capsule environment consisted of 100 percent oxygen; the 1986 Dupont Plaza Hotel fire, which was fed by stacked unused furniture; The Station and Kiss nightclub fires (2003 and 2013, respectively), both of which were fed by foam insulation on the walls and ceilings; and the 2007 Sofa Super Store fire. These fires, and millions of less spectacular fires in the United States, underscore the importance of understanding solid fuels, the means by which they burn, and the hazards these fires present.

Fire Stages and Metrics Solids versus Gases and Liquids The presentation of fire characteristics in this text began with the simplest case, gaseous fuels. Given that flaming combustion is a gas-phase process, a fuel that starts out as a gas needs only a pathway to ignition. Furthermore, the fuel composition remains identical to the composition of the initial gas mixture throughout the fire, simplifying the combustion chemistry. A gas-phase flame spreads by the chemical action of the chain that propagates the necessary atoms and free radicals. Liquid fuels have a similar degree of simplicity in their gas-phase fuel chemistry, which most commonly involves the vapor from the liquid. As with a gaseous fuel, there is often no change in the fuel chemistry over time. The energetics of vaporization and, for liquid mixtures, the preferential evaporation and burning of the lighter component(s), however, require consideration, as they affect the rates of burning and surface flame spread.

The burning of a solid fuel has three significant and consequential differences from the burning of gaseous and liquid fuels: •

Significant chemical change generally occurs within the solid during burning. This change results in (1) the fuel becoming non-uniform and (2) this lack of uniformity varying with the extent of the burning and, therefore, over time.

• The emitted volatiles may not have the same chemistry as the virgin solid.

• The heat transfer to, from, and within the solid requires consideration of both the changes in the fuel surface and the chemical changes that have occurred below the surface.

Materials and Products At this point, two important terms must be clarified: • A material is a single substance. The simplest material is made of a single chemical component, such as a sheet of a pure plastic. Some materials, such as particleboard, are mixtures of

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chemicals—in this case, ground wood and a binder. Still other materials, such as a fiber-reinforced composite, are nonhomogeneous.

• A product is (or is similar to) an item that is available commercially, and is alternatively referred to as a commercial product or a finished product. Examples include an electric cable, an upholstered chair, and a carpet. Such items are composed of one or more materials and are typically not chemically homogeneous.

Real combustible items are products. Large-scale tests are used to measure the burning behavior of products, whereas the fire properties of materials are often determined in bench-scale tests. Some bench-scale tests have been used to characterize small mock-ups of products. The relationship between the fire properties of a product and the fire properties of its component materials remains a subject of research. The use of reduced-scale mock-ups of large combustibles (e.g., an upholstered chair) to characterize ease of ignition has achieved some success, but the use of mock-ups to characterize mass burning rate and flame spread rate requires care in design of the mock-up and interpretation of the results.

Pyrolysis The involvement of a solid fuel in a fire generally begins with radiative or conductive heat decomposing the solid into sufficiently small fragments that the fragments are able to escape the solid surface and become a gas-phase fuel. Radiant heat, for example, might come from a nearby space heater or the flames from an already burning item; conductive heat might come from an overheated electrical component. Convection can contribute to these modes of fire evolution, but generally convective flow temperatures are not hot enough to volatilize a solid. An important exception to this statement is the hot upper layer in a room near or post flashover.

The decomposition process for a solid fuel is called pyrolysis. If no oxygen is present, the process is termed anaerobic pyrolysis. Most commonly, pyrolysis occurs in air, in which case it is called oxidative pyrolysis. Anaerobic pyrolysis is endothermic; that is, heat must be supplied from somewhere for the decomposition reactions to occur. Oxidative pyrolysis is usually endothermic or thermally neutral.

Pyrolysis typically stops when the heat source is removed or turned off.

As discussed in the Physical and Chemical Change chapter, pyrolyzing a solid requires raising the material’s temperature to the point where chemical bonds begin breaking, overcoming any phase change that might occur during this heating process, and releasing volatile compounds or molecular fragments.

The heat input required to accomplish this feat is the heat of gasification of a material (which has units of kJ/g). It is an important measure of the ease of ignition of a solid and the flammability of a solid, once ignited. If the chemistry of the remaining fuel changes over time, so will the heat of gasification.

If it is important to understand (or reconstruct) a particular segment of a fire (perhaps the spread of a fire from one combustible item to another), it is necessary to know the heat of gasification for a product during that time interval. If the task is to determine the threat posed by a large fire to the structural integrity of the building, it may be sufficient to use a heat of gasification averaged over the burn life of the combustible.

The minimum condition for igniting a solid is the heating of its exterior surface to a high enough temperature that the pyrolysis gases are produced rapidly enough to exceed their lower flammability limit in the space above the surface. Unlike the vapor from a pure liquid, the pyrolyzate is commonly a mixture of many decomposition products. Its composition depends on the chemistry of the solid fuel, the rate of pyrolysis, and the availability of oxygen. As a result, no tables of flammability limits for solid fuels can be developed. Instead, the gasification rate for ignition is experimentally determined for a particular heating scenario; as indicated in the next paragraphs, it is not a unique property of the fuel in the same way that a heat of vaporization is unique to a liquid.

If the solid is being heated by conduction, the heat generally is supplied at a location away from the fuel’s outer surface. For example, an overloaded electrical conductor heats the wire insulation from the inside, with the heat then being transferred to the inside of the cable jacket. The surface temperature, then, may be the lowest temperature in the solid. In this case, the apparent heat of gasification will be higher than for the case where radiant heat is applied to the exposed surface of the cable jacket.

If the solid is being heated by radiation, and if the radiation is entirely absorbed by the top surface of the solid (i.e., the solid is optically thick), the top layer of the solid will decompose first, followed by further decomposition in depth. Some solids, however, are somewhat transparent to infrared radiation. An example is an acrylic window panel. In this case, heating also takes place below the surface. When

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decomposition occurs below the surface, subsurface bubbles of pyrolyzate form, pass through the hotter (lower-density) outer material, reach the surface, and burst, spurting volatiles into the air. This process can be a more efficient mode of gasification than surface heating.

Whether the incident heat involves radiation or conduction, an increase in the heating rate is likely to increase the pyrolysis rate. Conductive heat is important in some ignition modes, but is rarely the principal contributor to fire spread and burning intensity. Chemical kinetic principles, as discussed in the Physical and Chemical Change chapter, reveal that the pyrolysis reaction rate increases rapidly with increasing temperature. Under normal indoor ventilation conditions, gasification rates on the order of just a few grams per square meter per second are needed to achieve an ignitable mixture in air.

Ignition to Flaming Combustion For most organic solids, a temperature between 520 °F and 750 °F (270 °C and 400 °C) is necessary for piloted ignition. As with gaseous and liquid fuels, unpiloted ignition or autoignition is possible if the surface reaches a sufficiently high temperature. For example, when wood is heated radiatively, piloted ignition (initiated by a flame maintained near the surface) occurs when the wood surface reaches a temperature of 570 °F to 750 °F (300 °C to 400 °C), while the same surface must be heated to about 1100 °F (600 °C) to induce autoignition.

The minimum radiative flux that must impinge on a solid to make it ignitable by a pilot flame has been measured for many materials [1]. These values range from 10 kW/m² to 40 kW/m² depending on the nature of the material, including its chemical constituents, reflectivity, size, and orientation with respect to the radiative source. For fluxes in excess of the minimum value, the time to ignition decreases as the flux increases (Figure 9-1).

In most serious fires, more than one combustible product is involved. The first product might be ignited by, for example, a candle. The flames from this product grow and generate far more heat than the original ignition source provided. As a result, there are two distinct modes for the ignition of a second combustible product: •

The first mode is piloted ignition. If the second product is close to the already burning item, the radiation from the flames pyrolyzes the surface material(s) of the second item. The combination of pyrolysis gases from the two items creates a flammable fuel–air mixture in the space between the two products. The flames extend along this flammable mixture and become attached to the second item.

• If the second product is farther away from the burning item, it can still become involved by unpiloted radiative ignition. As in the first mode, the thermal radiation from the flames from the burning item pyrolyzes the surface material(s) of the second item. Continuing irradiance leads to increasingly higher surface temperatures, and the pyrolyzate autoignites. This process can be enhanced by the pyrolyzate from the second item absorbing some of the flame radiation from the first item. This heat absorption adds to the temperature rise of the pyrolyzate and shortens the time required to generate a sufficient concentration of flame-propagating free radicals.

During the piloted and unpiloted ignition of solid fuels, potential surface heat losses can affect whether and when ignition will occur. These losses are analogous to the difference between the flash point and the fire point for liquid fuels: 1. The surface can lose heat by conduction into the interior of the solid. Such a loss will occur if the solid is heated quickly at the surface. If the solid starts to flame, but the heat source is removed, the high temperature at the surface is dissipated by conduction into the depth of the solid. The heat feedback from the flame is insufficient to maintain the surface temperature, and the flame goes out.

By contrast, if a thick solid is heated very gradually or from within, when its surface reaches the ignition temperature, its interior is already quite hot and will not drain heat very fast from the surface. Only a little heat feedback from the flame is needed to sustain a flammable concentration of pyrolyzate. Thus, the preignition heating rate and mode are important in achieving sustained ignition of thick solids.

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Figure 9-1 Effect of radiative flux intensity on time to achieve piloted ignition [1].

Data from: Babrauskas, V., Ignition Handbook, Fire Science Publishers, Issaquah, WA, 2003.

2. The surface can lose heat by radiating it away to cooler surroundings. This is best demonstrated by an example. If you place a small burner between two large pieces of wood whose surfaces are facing each other (Figure 9-2), the surface will ignite when the temperature of the two wood surfaces approaches 750 째F (400 째C). The surfaces, which quickly become charred and black, radiate energy to each other with an intensity characteristic of a 750 째F (400 째C) black body. When the burner is removed, this reciprocal radiation/absorption continues. There is no net radiative heat loss from either surface, and the flaming can continue.

If the same test is repeated with a single piece of wood, as in the second panel of Figure 9-2, either the surface will not ignite or it will take a larger burner to cause the ignition. In this scenario, there is no incident thermal radiation from a facing surface, as in the two-piece case. When the burner is removed, the heat continues to be radiated to the surroundings, and the surface cools.

Soon the surface is not hot enough to generate a flammable concentration of pyrolyzate.

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Ignition to Nonflaming Combustion As presented in the Combustion Fire and Flammability chapter, nonflaming combustion, also called smoldering or glowing combustion, can occur in a material with the following properties: •

The material is initially porous—that is, it has a large interior surface area as well as numerous internal “tunnels” that enable the diffusion of oxygen to those surfaces.

• The material’s interior surfaces support exothermic reaction with oxygen, producing or maintaining a self-supporting, porous, carbonaceous char.

• The material is a good insulator—that is, heat generated by the surface reaction accumulates within the material and is not efficiently lost to the surroundings.

Figure 9-2 Effect of radiative enhancement on sustained ignition. The flames in the left panel are sustained when the burner is removed; the flames in the right panel are not.

Because of the importance of oxygen diffusion, we might expect the smoldering rate to be dependent on the ambient oxygen volume percentage, and smoldering combustion does occur more readily in oxygen-enriched air than in normal air. Data from Reference [2] show a fourfold increase in the smoldering rate of cellulose rods when the oxygen was increased from 21 percent by volume to 96 percent by volume.

Smoldering can be started by a nonflaming ignition source such as a lit cigarette dropped onto an easy chair or an overheated electrical cable passing through a wood stud. When such an ignition source is applied to the material, the local temperature increases by as much as hundreds of kelvins above room temperature, and the reactivity of oxygen with the material surface begins at that temperature. If the smoldering is to self-sustain or progress, the rate at which heat is generated must exceed the rate at which heat is diffused away. When the ignition source is located in the interior of the material, significant insulation surrounds the hot spot and the early reaction may proceed at a fairly slow rate. In contrast, if the ignition occurs at the surface of the material, a higher heat-loss rate must be overcome, and the initial smoldering reaction must be faster.

The thermal process of spontaneous ignition followed by self-heating is similar, albeit with one major difference: the starting temperature. The classic example is the self-ignition of a haystack [3]. Very little happens unless the moisture content of the hay is greater than approximately 25 percent, which could

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result from rain or relative humidity of the air near 90 percent. Under humid conditions, aerobic fungi and bacteria grow in the hay at normal outdoor temperatures, generating heat biologically. If the haystack is thermally thick (a few meters across) and heat losses are small, then the biologically generated heat will increase the interior temperature of the haystack to about 165 °F (75 °C), perhaps taking several days or weeks. Above this temperature, the organisms are no longer active. At 165 °F (75 °C), however, the rate at which oxygen reacts with the decomposition products of the hay (which were formed during the biological heating) is significant. This chemical heat raises the temperature of the haystack’s interior. As the temperature continues to rise, however slowly, the temperature-dependent oxidation rate accelerates, resulting in thermal runaway. After several more weeks, the temperature could rise to the point where flaming ignition occurs spontaneously. This outcome could also happen if the pile were disturbed (e.g., with a pitchfork), bringing fresh air in contact with the glowing region in the interior or if the glowing zone progressed from the interior to the surface.

The key factor in this scenario is that the initial rate of heat generation can be very slow if the insulation is effective at trapping the heat (Figure 9-3). The slope of the green curve in Figure 9-3 depicting the rate of heat generation is initially small but increases progressively with increasing temperature, as is characteristic of chemical reactions. The two heat-loss curves, for a faster cooling rate (red) and a slower cooling rate (blue), are approximately linear (constant slope) because the rate of convective cooling is directly proportional to the difference between the temperature of the warm object and the temperature of the surroundings.

At the initial temperature, the rate of heat generation is positive, but the rate of cooling is zero because the object is initially at the same temperature as the surroundings. As a result, the temperature of the material must rise, and must continue to rise until the cooling curve intersects the heating curve.

This intersection point, marked by a black dot in Figure 9-3, corresponds to a balance of heating and cooling, such that no further temperature increase occurs. At the rapid heat-loss rate, the green and red curves intersect, and the heating ceases; that is, the green curve would stop at this temperature. The slower, blue heat-loss curve never intersects the heat-generation curve, and the material proceeds toward sustained burning, as shown in Figure 9-3.

Few materials burst into flame spontaneously. In fact, most common solid materials react so slowly with oxygen at normal temperatures that the self-heating, if measurable at all, usually amounts to a temperature increase of no more than one or two kelvins. Table A-10 in the Fire Protection Handbook [4], however, lists 75 substances that are capable of hazardous spontaneous heating. Among the most dangerous are rags or other fibrous materials in contact with corn oil, fish oils (e.g., cod liver oil), linseed oil, pine oil, soybean oil, tung oil, or any unsaturated oil. Such oils are reactive with oxygen at room temperature. In addition, the rags or fibrous material provide an extensive surface area for the oil–oxygen reaction to take place, and they confine the heat, permitting the temperature to rise. By contrast, saturated oils, such as petroleum-derived oils (common lubricating oil or heating oil) do not cause spontaneous heating. Common materials that are prone to spontaneous ignition, if stored in bulk, include charcoal briquettes, low-grade coal, and some types of animal feed.

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Figure 9-3 Relative heat-generation and heat-loss rates, which may sometimes lead to runaway heating and self-ignition.

Char Formation and Melting After a solid has been ignited and the flame has begun to spread across its surface, two distinct categories of burning behavior are apparent. One class of materials, including woods and certain plastics, burns with the formation of a growing surface char layer. The other class of materials, which includes many of the more common plastics (polyethylenes, polystyrenes, and acrylics1), burns with either no char or a small amount of surface char that blackens the fuel surface but never builds up to a thick layer. The importance of char formation is seen in Figure 9-4, which illustrates heat-release rate versus time for a char-forming material (particleboard) and a non-char-forming material (polymethylmethacrylate [PMMA]).

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Figure 9-4 Heat release rates versus time for particleboard and acrylic (polymethylmethacrylate) samples under imposed radiative heat fluxes of 25 kW/m² and 50 kW/m².

Note In terms of its chemical and physical behavior during burning, PMMA is one of the simplest materials. First, during pyrolysis, nearly all of the mass that is volatilized is in the form of methylmethacrylate (MMA). Second, until it reaches its melting point, PMMA does not melt or drip; it sublimes. For these reasons, PMMA is one of the most convenient materials for developing simple combustion models. Very few solid materials exhibit such ideal behavior, so combustion modeling for realistic materials constitutes to be an active field of research.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

The char formed when the particleboard is heated has a structure similar to that of graphite (pencil lead), a very stable form of carbon. In graphite, the carbon atoms are connected in adjacent six-

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membered rings (polycyclic structure), forming the orderly structure shown in Figure 9-5. In a firegenerated char, irregularities are present in the array of carbon atoms, and one hydrogen atom is typically bonded to one out of every five or six carbon atoms.

Most of the readily pyrolyzed and flammable material is gone at the point that char has formed. The char is brittle, with a very porous, cellular structure, with thin walls and a large fraction of open space.

(Think of a household sponge that is rigid, rather than flexible.)

The char is not a good conductor of heat and protects the subsurface material from the heat of the flames. As the char becomes thicker, it progressively slows the rate of conductive heat transfer from the flames above the surface to the virgin material below the surface. This insulation factor reduces the endothermic pyrolysis of this material to form combustible gases and, therefore, slows the rate of burning. This can be seen as the decreasing heat-release rates for the particleboard specimens in Figure 9-4. (The final small increase in the curves near the time of flameout is artificial. The test specimens were mounted on an insulated sheet, which became heated during the test. The last few millimeters of the particleboard specimen were heated both from the front and from the back, so they burned out more quickly.)

The particleboard curves have been smoothed. In a test, as the char gets thicker, it develops cracks and fissures that can provide narrow pathways to the underlying material, releasing brief spurts of flammable vapor before the pathways char over. This mirrors the charring in an actual fire.

The rate of char formation of woods has been reported to be proportional to the radiant heat flux impinging on the surface [5]. For a typical radiant heat flux of 30 kW/m², which might exist just under a flame, the average charring rate would be approximately 0.025 in./min (0.01 mm/s).

Noncharring combustibles generally melt while burning, so there is no insulating layer to provide thermal protection for the subsurface material. In some cases, the melt is very viscous, and little flowing occurs. In other cases (e.g., with some polyethylenes, polypropylenes, and polystyrenes), the melt has a watery consistency. Such materials tend to burn at a high rate throughout the burning period until the fuel is consumed, as exemplified by the acrylic samples in Figure 9-4. This high burning rate can be enhanced if burning drops of molten plastic fall or flow downward, providing a means of spreading the fire. Noncharring combustibles are generally more hazardous than charring combustibles.

Figure 9-5 Ball and stick portrayal of the structure of graphite. The black balls are all carbon atoms.

Mass Burning and Flame Spread Mass Burning Rate Once a solid combustible is ignited, its contribution to a fire’s intensity is determined by three related parameters: • The rate at which a unit area of the burning surface is consumed • The rate at which flames spread over the fuel surface, increasing the burning surface area • The combustible’s ability to ignite other combustible items in the vicinity

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The first of these, the rate of mass consumption for a defined segment of a burning combustible, is the subject of this section.

The rate of burning of a material or product is expressed as a mass burning rate (g/m2•s) or heatrelease rate (kW/m2). If the incident radiative flux to the specimen is increased, both charring and noncharring materials burn more rapidly.

When a material is burning steadily, there is a heat balance at the surface: The net heat to the surface just balances the heat needed to keep supplying fuel to the flame. The net heat to the surface is the heat flux from the flame to the surface minus the rate at which heat is lost by reradiation from the hot surface to the cold surroundings, with both terms expressed in kW/m2 (which is equivalent to kJ/m2•s).

The rate of heat absorption per unit surface area (kJ/m2•s) required to sustain a flow of combustible pyrolyzate is the product of the mass rate of gasification per unit surface area (g/m2•s) and the heat of gasification (kJ/g).

Early in the combustion of the first item burning in a compartment, the heat input is derived solely from its own flames. As the burning surface increases, the radiative feedback to the surface at the edge of the flames is approximately 30 kW/m2. As the fire reaches approximately 250 kW in intensity in a room of normal residential dimensions, the hot fire gases in the upper layer of the room and the flame-heated walls reach temperatures where their black body radiation to the product surface is appreciable.

Recalling Equation 5-5, and assuming that the soot is radiating as a black body (emissivity = 1), the heat flux per unit surface area (kW/m2) is given by Equation 9-1:

For an upper layer that has reached 500K, the radiant flux to the burning product will be approximately 3.5 kW/m2, which is small compared to the flame radiation to the surface. At 800 K, a temperature indicative of imminent flashover, the radiant flux will be approximately 23 kW/m2. Post flashover, the upper-layer temperature can reach approximately 1100K, which emits a radiant flux of 83kW/m2.

Table 9-1 presents some values for heats of gasification of a number of combustible solids. Three observations are worth noting: 1. The range of the values is quite wide, from 1.19 kJ/g to 3.74 kJ/g.

2. The chemical composition within a generic type of material may vary considerably, so it comes as no surprise that the data show variation in heats of gasification of samples in each of the rows where there are multiple samples from different sources.

3. If these heats of gasification (1.19 kJ/g to 3.74 kJ/g) are compared with the heats of combustion of the same materials (15 kJ/g to 44 kJ/g), it is apparent that only a small portion of the heat released by burning must return to the pyrolyzing solid to maintain a continuing supply of combustible vapor to the flame.

Table 9-1 Heat of Gasification for Selected Solids [6] Material Type

Heat of Gasification (kJ/g)

Number of Materials Tested

acrylonitrile-butadiene-styrene (ABS)

3.23

1

corrugated paper

2.21

1

Douglas fir

1.82

1

nylon 6/61

2.35

1

phenolic plastic

1.64

1

polyesters (PETs) with glass fibers

1.39 to 1.75

2

polyethylenes (PEs)

1.75 to 2.32

2

polyisocyanurate (PIC) foams

1.52 to 3.74

2

polymethylmethacrylate (PMMA)

1.63

1

polyoxymethylene (POM)

2.43

1

polystyrene (PS), granular

1.70

1

polystyrene foams

1.31 to 1.94

5

polyurethane (PU) foams

1.19 to 2.05

7

polyvinylchloride (PVC), rigid

2.47

1

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Reproduced from: Tewarson, A., “Generation of Heat and Chemical Compounds of Fires,” Chapter 3–4, SFPE Handbook of Fire Protection Engineering, 4th Edition, DiNenno, P.J. et al, eds., National Fire Protection Association, Quincy, MA, 2008.

Table 9-2 Heat Balance for a Horizontal PMMA Slab Burning in Air [7] Component

Value

Conditions Flame base

0.305 m × 0.311 m

Mass-burning rate

1.00 g/s

Surface temperature

725 °F; 385 °C

Heat of gasification

1.61 kJ/g

Heat of combustion

24.9 kJ/g

Combustion efficiency

85 %

Heat-release rate

21.1 kW

Radiative fraction

0.34

Heat flux from flame to surface By radiation

1.91 kW (73 %)

By convection

0.72 kW (27 %)

Total

2.63 kW

Heat absorbed by gasification

1.61 kW (61 %)

Surface re-radiation loss

1.02 kW (39 %)

Total

2.63 kW

Reproduced from: deRis, J., “Fire Radiation-A Review,” Proceedings of the Combustion Institute 17, 1003–1016, (1979).

Copyright Elsevier.

Table 9-2 shows a detailed energy balance for a 30.5 cm by 31.1 cm (approximately 1 ft by 1 ft) slab of PMMA burning in a horizontal configuration. Approximately three-fourths of the heat transfer from the flame to the surface occurs by flame radiation, and the remaining one-fourth involves convection from the flame gases just above the surface. Almost half of the total heat transfer to the surface is re-radiated from the surface. Thus radiation is important for a fire of this size.

If the test piece of PMMA had been larger, then, because of a thicker flame, the flame radiation flux per unit area would have been even greater, and the burning rate per unit area would have been larger.

Figure 9-6 illustrates this effect by showing burning rates for horizontal PMMA slabs of various sizes.

Given the data in Figure 9-8, what would happen to the burning rate per unit area if a very large slab of PMMA (e.g., 16 ft (5 m) square), were burned? The temperature in this very large flame would be about the same as in a smaller one, because, for a heat source at a given temperature, there is an upper limit to the radiant intensity it can generate. Therefore, the rising curve in Figure 9-6 would level off at some burning rate. This limit for PMMA has not been measured, but it has been estimated to be between 30 g/m² s and 60 g/m² s.

Even for a piece of PMMA within the size range shown in Figure 9-6, the mass burning rate can depend on other factors. For example, radiative feedback from hot walls and ceiling or from another flame would increase the burning rate. Reduced oxygen in the air entering the flame due to dilution with combustion products would reduce the burning rate.

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Figure 9-6 Mass burning rates of square, horizontal PMMA slabs of various sizes [8].

As expected, a material other than PMMA would likely burn at a different rate. For instance, a 0.305 m by 0.305 m slab of a POM would release heat at less than one-half the rate of PMMA, while the same size slab of a PS would release heat 50 percent faster than PMMA and 3.3 times as fast as POM [7].

These differences reflect differences in the thermal decomposition mechanism of the material, leading to differences in flame radiation, in surface re-radiation, and in heat of gasification.

Flame Spread Rate Generally, a fire originates at a discrete location on the surface of a combustible product, and spreads from there. For residential fires, this initial ignition area is often small because the most common ignition sources are cigarettes, faulty or improperly located electric devices, and heating apparatus. Once the flame has progressed a short distance from the source, its rate of flame spread becomes independent of the ignition source, but is dependent on four other variables: 1.

2.

3.

4.

Orientation of spread Degree of radiative preheating The magnitude and direction of any external air flow Thermal thickness of the solid

The rate of fire spread in a horizontal or downward direction over the surface of a thermally thick solid is generally very slow (“creeping flame spread”), on the order of a few hundredths of a millimeter per second (0.1 in./min), unless the surface has been preheated. For example, if PMMA is preheated for 1 minute with a radiative flux of 20 kW/m², the downward spread rate increases from 0.05 mm/s to 0.5 mm/s—that is, it increases by a factor of 10. More prolonged or more intense preheating can produce further acceleration, perhaps by at least another factor of 10. If the burning material melts and drips, this presents an alternative means for spreading flames downward. In such cases, spread rates can be much faster.

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Flame spread in an upward direction is far more rapid than horizontal or downward spread.

Furthermore, the flame accelerates as it spreads upward. The reasons for this difference in behavior are as follows.

The hot gases in a flame rise though natural buoyancy. As they do, fresh air is drawn into the base of the flame. For downward or horizontal spread, the direction of this air flow pushes the flame away from the unburned material, as shown in the left and center panels of Figure 9-7. Thus, very little of the flame radiation impinges on the adjacent fuel surface, providing little heat for decomposition and gasification.

Even if the surface is heated to some degree by the flame radiation, it is simultaneously cooled by the approaching air.

Conversely, with upward spread, the flame is in close contact with the not-yet-ignited portion of the combustible and can preheat it efficiently by both convection and radiation, so rapid upward spread can be expected. Furthermore, as the flame spreads upward, it becomes taller. Its greater length and thickness promote radiative heat transfer, and its greater length and higher gas velocity promote convective heat transfer. Thus, the upward spread rate increases progressively. On high walls, flame spread speeds of several meters per second are possible.

Figure 9-7 Downward, horizontal, and upward flame spread and direction of buoyancy-induced airflow at the base of the flame. The heavy line on the surface denotes the burning area.

A powerful factor enhancing the flame spread rate is the presence of an additional air flow in the direction that the flame is already moving. This flow could be due to cross-ventilation from open windows or could result from a mechanical ventilation system. This co-flowing air extends the flame, increasing the fuel surface area subject to the flameâ&#x20AC;&#x2122;s thermal radiation and thus boosting the generation rate of pyrolysis gases. The air flow also provides the additional oxygen needed to combust these gases.

An external air flow counter to the direction of flame spread has an opposite effect. It pushes the flames back toward fuel that is already burning or has already been consumed. This decreases the fuel surface area subject to flame irradiation, slowing the spread rate. However, the increased supply of oxygen can increase the mass burning rate of the fuel surface that is already burning.

The fourth factor affecting flame spread, in addition to orientation, radiative preheating, and air flow, is the thermal thickness of the solid. Downward spread experiments with thermally thin cardboard and fabric samples have shown that the rate of spread is inversely proportional to the physical thickness. This behavior can be predicted from the concept that, for the flame to spread, it must heat the adjacent material to the ignition temperature. Conductive heat transfer through the thickness of thin specimens is fast because the heat does not have far to travel. Thus, if the material is twice as thick, it will take twice as long to be heated, and the flame will advance half as quickly. This phenomenon explains why kindling wood is used to start a campfire. As a consequence of this principle, flames can spread very rapidly over a material composed of thin elements, such as a pile of wood shavings or tissue paper.

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Effects of Uncommon Fire Environments In special environments, such as in the vicinity of pure oxygen being used for medical purposes, the percentage of oxygen by volume exceeds the percentage in normal air. This higher percentage of oxygen greatly increases the fire hazard. Although the heat capacities of oxygen and nitrogen are comparable, the increased percentage of oxygen present makes the combustion reactions proceed at a faster rate, in turn generating heat at a faster rate. The combination of faster chemistry and no change in thermal mass leads to a higher flame temperature. The higher flame temperature has several consequences: •

The rate of heat transfer from the flame to the surroundings is greater, increasing both the flame spread rate and the potential for igniting nearby combustibles.

• The flame is less easily quenched by still cold, adjacent fuel surfaces, so the flame can spread more rapidly.

• The process of soot formation in a turbulent diffusion flame is enhanced by higher temperature, so the flame becomes sootier and emits more radiation.

• Certain solids that will not burn in normal air, unless preheated, will burn in oxygen-enriched air.

Figure 9-8 shows how the rate of horizontal flame spread increases with increasing oxygen percentage for charring and noncharring materials. The magnitude of the change in the rate of flame spread depends on the thickness and orientation of the specimen. The burning rate per unit area also increases at higher oxygen concentrations, as shown in Figure 9-9.

In some situations, the air pressure is not the 101 kPa found at sea level. The cabin of a commercial aircraft flying at high altitude, for example, is normally pressurized to an absolute pressure of approximately 75 kPa (0.75 atm), and the pressure in Denver, the “mile-high city,” is similarly lower than 1 atm. Hyperbaric chambers for medical treatment can operate at pressures of as much as 300 kPa, sometimes at atmospheres containing 100 percent oxygen.

If the volume percentage of oxygen in the environment remains 21 percent, then the flame temperature will be nearly the same for atmospheric-and non-atmospheric-pressure fires. Thus, the differences in fire behavior are not as profound as when the percentage of oxygen changes.

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Figure 9-8 Effect of oxygen percentage in the atmosphere on horizontal flame spread over surfaces [9, 11].

Data from: Lastrina, F.A., Magee, R.S., and McAlevy, F.R., “Flame Spread Over Fuel Beds: Solid Phase Energy Considerations,” Proceedings of the Combustion Institute 13, 935–948, (1971).

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Figure 9-9 Effect of oxygen percentage on the burning rate on small, thin, horizontal samples of plastics and heptane [6].

Reproduced from: Tewarson, A., “Generation of Heat and Chemical Compounds of Fires,” Chapter 3–4, SFPE Handbook of Fire Protection Engineering, 4th Edition, DiNenno, P.J. et al, eds., National Fire Protection Association, Quincy, MA, 2008.

The rate of flame spread over solid surfaces increases with increasing pressure. For instance, the rates of horizontal spread over thick slabs of PMMA and polystyrene vary with the 0.82 power and 0.76 power of pressure, respectively [9]. Other fire properties, such as ignition temperature, tendency to produce smoke, flame radiation, and toxic gas concentrations, also vary somewhat with pressure.

Most atmospheres that contain enough oxygen to be breathable will support combustion.

Nevertheless, it is fascinating to note that a mixture of 10 percent oxygen and 90 percent nitrogen at 2 atmospheres absolute pressure is easily breathable, but will not support combustion of most materials.

The advent of travel though outer space has introduced the dynamic of reduced gravity into fire safety considerations related to the astronauts’ environment. A fire in a vehicle orbiting Earth or continuing to further planets would behave differently from an ordinary fire on Earth. There is a near absence of gravitational force in these vehicles. (The term microgravity is used instead of zero gravity because of very small residual gravitational effects.) Thus, the fire effluent would not be buoyant and would not rise from the combustion zone to make room for the fresh air needed to continue the oxidation. However, experiments in the space shuttle have shown that flames in this environment do not smother themselves, but rather continue to burn by a diffusion process. A wax candle, for example, has continued to burn for 45 minutes. It takes less energy to ignite a solid in low gravity because of the reduced convective cooling of the heated surface by the surrounding cold atmosphere. Flame spread rates across surfaces and burning rates are somewhat slower, but the forced convection imposed by spacecraft ventilation systems can accelerate burning rate and flame spread. Flames are generally cooler, are somewhat sootier, and have different shapes from flames in Earth’s gravity. The burning of some materials in low gravity might result in hot globules of the material drifting in all directions, possibly starting other fires [10].

Test Methods for Measuring Ignition, Burning Rate, and Flame Spread Rate From the preceding discussion, it should be clear that ignition and heat release and flame spread rates are sensitive to the magnitude of the incident thermal radiation. Accordingly, a test method intended to provide meaningful, and not misleading, information about a material or product must simulate the radiation correctly.

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Figure 9-10 shows how the heat-release rate of a slab of material can react to the imposed radiative flux. Note the difference in magnitude of the heat release rates for each material as the external flux increases by 10 kW/m² or 20 kW/m². In the case of the wood specimens, a single thick slab will not even continue to burn except with an external radiative flux.

Thermal radiation may come from four sources: the flames from the burning product, the flames from other burning objects, the hot smoke (generally in the upper layer of the room), and the hot ceiling and upper walls. Although the specimens in small-scale test methods are typically of the order of 0.1 meter in dimension, the first of these sources might be replicated. However, the other three sources are not directly related to the test specimen; their values vary depending on the fire scenario.

Several apparatus have been developed that impose a variable radiant flux on a specimen [12]. In these apparatus, the thermal radiation comes from electric heating elements or gas-fired panels. Much effort has been expended on relating the results of these laboratory tests to the results from burning of large combustibles, with some success.

One such apparatus is the basis for ASTM E 1354, Standard Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter (Figure 9-11). In this apparatus, a uniform radiant flux is imposed on a specimen up to 10 cm × 10 cm in area and 5 cm thick. Because the thermal radiation source is shaped like a truncated cone, the apparatus is colloquially called the “cone calorimeter.” The specimen is weighed continuously during a test. The rate of heat release is measured using oxygen consumption calorimetry, whose underlying principle is presented later in this chapter. Dividing the rate of heat release by the rate of mass loss and comparing this number with the heat of complete combustion of the specimen provides an indicator of the combustion efficiency —that is, what fraction of the theoretical heat release is actually generated. Furthermore, by testing specimens at different values of the irradiance from the cone heater, one can determine ignition delay times and the critical radiant flux for ignition.

For decades, the Oxygen Index Test [14] has been used as an indicator of changes in flammability as the formulation of a material is changed. In this test, a mixture of oxygen and nitrogen flows upward over a vertical, pencil-sized sample. The initial volume percent of oxygen is sufficiently high that the specimen can be ignited at the top. The volume percent of oxygen is then reduced until the flame goes out. The percentage of oxygen by volume at which the flame is extinguished is called the limiting oxygen index (LOI). A high LOI value indicates a material that is more difficult to ignite and/or burns less vigorously, if at all. The results of the Oxygen Index Test do not correlate consistently with ignition delay time, flame spread rate, or heat release rate. Furthermore, under the conditions of the Oxygen Index Test, some known-combustible materials (e.g., red oak) do not burn in air. (If the temperature of test specimens were increased by heating the air flow or with radiant heaters, the measured LOI values would decrease dramatically.) Nonetheless, LOI values, which are available for many materials [4], are frequently used to estimate relative flammability.2

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Figure 9-10 Heat release rates of solids as influenced by externally imposed radiation [5, 13]. The magnitude of the heat release rate per unit area depends on the size and orientation of the sample.

Data from: Lastrina, F.A., Magee, R.S., and McAlevy, F.R., “Flame Spread Over Fuel Beds: Solid Phase Energy Considerations,” Proceedings of the Combustion Institute 13: 935–948, (1971).

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Figure 9-11 Schematic of the cone calorimeter.

Combustible Solids Cellulosic and Other Natural Materials Until the middle of the 20th century, nearly all combustibles in residences, offices, and other structures were made of natural materials. Mattresses and upholstered chairs were stuffed with cotton and horsehair and covered with cotton, wool, and leather; rugs were made of cotton and wool; draperies were woven from cotton, wool, and silk. These materials (with the exception of horsehair) remain popular today, although synthetic materials are now more common in soft goods. Woods and wood products continue to be prevalent as the hard materials in furniture, bookcases, and similar items.

From a fire viewpoint, the most important of these natural materials are cellulosicsâ&#x20AC;&#x201D;that is, materials whose molecular chemistry is based on cellulose. Cellulose, which can be isolated from cotton or chemically synthesized, is a condensation polymer of glucose, C6H12O6, a form of sugar. The chemical structures of glucose and cellulose are shown in Figure 9-12.

Figure 9-12 The structures of glucose and cellulose.

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A polymer is a long string of repeated chemical building blocks. A condensation polymer is formed by eliminating a small molecule—in this case, water (H2O)—each time another link is added to the polymer chain. As an example, consider this sequence for the formation of cellulose from glucose:

where n generally exceeds 20,000 C6H10O5 units in each molecule of cellulose. If cellulose is boiled in an acidic solution, it decomposes, the water is restored, and glucose is a product. Cotton consists of more than 90 percent cellulose.

Woods contain only 40 percent to 50 percent cellulose. The process of making paper from wood removes much of the noncellulosic material; as a consequence, paper has a fairly high cellulose content.

The exact percentage depends on the type of paper.

Many types of wood exist, and they exhibit substantial variations in their physical properties. For example, the densities of ponderosa pine (a softwood) and white oak (a hardwood) are 0.42 g/m³ and 0.73 g/m³, respectively. The chemical composition of dry woods also varies, depending on the type of wood and the growing conditions: • • • • •

40 to 50 percent cellulose 18 to 35 percent lignin 10 to 30 percent hemicelluloses 5 to 20 percent “extractives” (e.g., oils, tars, gums) 0.2 to 1 percent minerals

Woods are somewhat porous in structure and can absorb large amounts of moisture. After long contact with dry air at 68 °F (20 °C) and 20 percent relative humidity, a typical wood has a moisture content of about 5 percent. In extreme conditions, this moisture content can increase to as much as 25 percent. The fire behavior of a wood, especially its ease of ignition, is influenced by the moisture content.

The heat absorbed by a wood product in an incipient fire first eliminates the absorbed water, a process that occurs at or below the boiling point of water (i.e., 212 °F; 100 °C). The temperature of the wood stalls in this low temperature range and does not resume its rise until the water is gone. Consequently, it takes a lot of heat to raise the temperature of the wood to approximately 480 °F (250 °C), at which point cellulose gasification begins, with ignition then following. The same process slows the rate of flame spread and the mass burning rate of a moist wood or wood product. Conversely, drier wood has less water to eliminate and burns faster. This is why wildland fires grow so rapidly when there has been little rain and when they are spurred by hot, dry winds.

Note Lignin, a cross-linked polymer (explained later in this chapter), acts to bind the cellulose fibers together and add strength. (When brown wrapping paper is made from wood, not all the lignin is removed, providing for a stronger paper.

Much more of the lignin is removed in making tissue paper.) When heated, lignin decomposes in a manner different from cellulose. Cellulose starts to decompose at about 480 °F (250 °C), and is mostly decomposed at 700 °F (370 °C), leaving a small amount of char behind. Lignin starts to decompose at temperatures below 570 °F (300 °C), but even after prolonged heating at high temperatures in an inert atmosphere, roughly half of the original mass remains as char.

Hemicelluloses are polymers of glucose and other sugars, and have a much lower molecular weight than cellulose. One of these molecules can contain several hundred sugar units instead of tens of thousands of such units.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

Looking at the structure of cellulose in Figure 9-12 and adding the knowledge that wood products contain other polymeric components, we might expect that the gasification of woods would be a complex process. Indeed, the hundreds of pyrolysis products of woods include carbon dioxide, water, the

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pleasantly odorous gases from a fireplace, toxic and irritant gases such as carbon monoxide and acrolein (covered in the Smoke and Heat Hazards chapter), the creosote deposited in chimneys, and the charred remains often seen on the fire ground. The relative proportions of the various species depend on the type of wood, the heating conditions, and the availability of oxygen.

Figure 9-13 shows the gasification of a wood when this material is heated slowly in an inert (oxygenfree) atmosphere. This gasification process occurs over a fairly wide range of temperatures, leaving a substantial char residue (approximately 25 percent of the original mass).

The net heat of combustion of dry woods, approximately 18 kJ/g, reflects both the heat of combustion of the volatiles produced during pyrolysis (approximately 14 kJ/g of volatiles) and the heat of combustion of the residual char (approximately 34 kJ/g of char). In an actual fire, the char usually burns at a later time than the volatiles, if it burns at all.

Other natural materials in general use include wool, leather, and silkâ&#x20AC;&#x201D;all animal products with high protein content. The proteins are polymeric molecules with the monomer units (amino acids, An) connected by peptide linkages:

Upon thermal decomposition of these materials, the gases evolved include ammonia (NH3), amines (e.g., CH3NH2), and some hydrogen cyanide (HCN). Wool also contains sulfur, so its pyrolysis generates sulfur-containing compounds. Generally, these animal-based materials are appreciably less flammable than cellulosic materials, but they do burn under high heat fluxes and produce nitrogen-containing irritant and toxic gases and, in the case of wool, sulfur-containing gases, in addition to the usual toxic species found in cellulosic fires.

Figure 9-13 Gasification of wood during slow heating [15].

Data from: Kirk-Othmer Encyclopedia of Chemical Technology, 5th edition, Kroschwitz, J.I., ed., J. Wiley, New York, 2007.

Synthetic Polymeric Materials Definitions Following World War II, many products made of human-made polymers entered the marketplace. These materials are not limited to those chemistries that have evolved in nature; and they exhibit diverse

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physical, visual, and chemical properties. Today, synthetic polymeric materials dominate such product lines as upholstered furniture, paint, carpet, appliance housings, and clothing.

As noted in the previous section, a polymer is a long strand of repeated chemical building blocks. The building block is called a monomer. A molecule consisting of a few monomers is called an oligomer.

More specifically, a molecule consisting of two monomers is a dimer, three monomers combine to form a trimer, and so on.

Polymers can be sorted in different ways: •

Addition and condensation polymers. This classification is based on the nature of the chemical synthesis. These polymers are the subject of the following sections.

• Linear, branched, or cross-linked polymers. This classification is based on the three geometric configurations of the assembled monomer units.

• Homopolymer or copolymer. In a homopolymer, all of the building blocks are identical. A copolymer consists of two or more different monomers.

• Thermoplastic, thermoset, and elastomeric polymers. This classification is based on polymers’ use properties. Thermoplastic polymers can be melted and remolded. Thermoset polymers are cured irreversibly once they are in their final shape. Elastomeric polymers, also called elastomers or rubbers, realign their strands when stressed.

Addition Polymers The building of an addition polymer involves the addition of successive monomeric units without elimination of any molecule or fragment. The earliest addition polymer, a polyethylene, was first synthesized in 1898, with a practical synthesis developed in 1933. Both developments were accito and twisted around each other, held together by weak attractive forces. The melting of polyethylene, a thermoplastic vinyl polymer, is the result of the absorbed heat overcoming these attractive forces.

The monomer of an addition polymer has a carbon–carbon double bond. Ethylene (C2H4) is the simplest of these monomers. Its polymer chain comprises a string of pairs of carbon atoms, each singly bonded to its neighboring carbon atoms and to two hydrogen atoms. (See the Physical and Chemical Change chapter.) After the synthesis of the polymer, the carbon atoms in the pairs are indistinguishable; hence, this polymer was originally called polymethylene. Polyethylenes are used in such diverse products as plumbing piping, bubble wrap, and plastic bottles. The harder polyethylenes can be used in machine parts and artificial joints.

Polyethylene is the simplest of an important family of additional polymers called vinyl polymers, so named because of the presence of the H2 CH fragment, called the vinyl radical. The carbon on the right has only three bonds to it. Binding an atom or molecular fragment to this site enables the formation of a variety of vinyl monomers. Figure 9-14 shows a number of these substituted vinyl monomers and indicates the commercially important polymers formed from them.

Vinyl monomers can form linear polymers that consist of long chains of carbon atoms (the backbone) with hydrogen atoms and pendant groups attached. The term “linear” means that the chains do not contain any branches. However, the carbon atoms do not actually lie on a straight line; the chains are twisted and coiled. A piece of polyethylene consists of a very large number of polymer strands, lying next to and twisted around each other, held together by weak attractive forces. The melting of polyethylene, a thermoplastic vinyl polymer, is the result of the absorbed heat overcoming these attractive forces.

The adjacent strands can be tied together chemically to form a three-dimensional network and thereby reduce the flexibility, increase the strength or hardness, or increase the melting point of the polymer. This process is called cross-linking. For vinyl polymers, cross-linking is accomplished by adding a small percentage of a monomer containing two double bonds per molecule (a cross-linking agent) to the principal monomer. For example, if 1 percent of divinylbenzene monomer, CH2 CH— (CH6H4)—CH CH2 is added to styrene (C6H5—CH CH2) monomer, then linkages will form during polymerization that tie together nearby polystyrene chains at various random points. Similarly, the “B” in the well-known ABS copolymer is butadiene, CH2 CH—CH CH2, which acts to cross-link the otherwise linear chains of the mixture of the two vinyl monomers acrylonitrile (“A”) and styrene (“S”). ABS is used in waste pipes, automotive trim components, medical devices, enclosures for electrical and electronic assemblies, safety helmets, canoes, luggage, and small appliances housings.

Cross-linking has an important stabilizing effect on the fire behavior of a polymer. A linear polymer tends to decompose by thermal fracturing of the backbone—that is, by breaking one of the carbon– carbon bonds. The smaller fragments, generally monomers or oligomers containing 2 to 12 carbon

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atoms, gasify. When their concentration in air exceeds the lower flammability limit, ignition or flame spread can occur. All of the carbonâ&#x20AC;&#x201C; carbon bonds in the backbone of a vinyl polymer are approximately the same strength. Eventually, the backbone becomes nearly completely fragmented into gasified species, leaving little or no residue. By contrast, a cross-linked polymer can be visualized as a ladder.

Even after breaking a rung or an upright, connections continue to hold the ladder together. For a fragment of a cross-linked polymer to break off and gasify, multiple carbonâ&#x20AC;&#x201C; carbon bonds must be fractured, which in turn requires a higher temperature. At the higher temperature, other bonds can be broken or rearranged, providing an opportunity for the formation of the graphitic carbon structure of a charred residue. The carbon in the char is not readily gasified, so the mass of combustible volatiles is reduced.

Figure 9-14 Some vinyl monomers capable of forming vinyl polymers by linear addition.

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Figures 9-15 and 9-16 show some other monomers that are not built on a vinyl structure, but that also serve as precursors to addition polymers.

The pyrolysis of specific addition polymers, described generically earlier in this chapter, differs depending on the polymer. Three basic modes of decomposition are possible: 1. Unzipping. The backbone breakage preferentially occurs at the linkage to the end monomer unit.

The composition of the pyrolyzate is thus dominated by monomer. Examples of polymers that unzip include PMMA, POM, and PTFE. 2. Random scission. The backbone breaks at random locations, and the smaller oligomers vaporize.

The pyrolysis of polyethylene and polypropylene follows this pattern.

3. Elimination. Polyvinyl chloride (PVC) pyrolyzes in a manner different from either unzipping or random scission. At approximately 482 °F (250 °C), the HCl molecule splits out from the chain and gasifies. The remaining backbone, with unbound sites at nearly all carbon atoms, rearranges to form a stable graphitic char. Very little of the carbon-containing mass undergoes gasification, so pure PVC is very difficult to ignite or spread flames. Unfortunately, HCl is toxic and corrosive.

Note Pure PVC is a rigid material, and substantial proportions of plasticizers are often added to it to impart flexibility. The resulting plasticized PVC can be used as electric cable insulation or vinyl furniture covering. In general, the plasticizers volatilize or decompose, which increases the flammability of the commercial PVC product. However, the released HCl is a significant flame inhibitor, so even plasticized PVC products are difficult to burn.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

A few other addition polymers merit mention.

•

Modacrylics are copolymers of acrylonitrile and either vinyl chloride or vinylidene chloride (CH2Cl2). The introduction of the chlorine atoms improves the fire performance relative to ordinary acrylic polymers. Uses of such polymers include synthetic fur, rugs and carpets, work clothing, and wigs.

• Totally fluorinated polymers, such as the polytetrafluoroethylenes, are almost nonflammable. (They do burn in pure oxygen.) Partial fluorination of a polymer does not improve fire performance as much as chlorination or bromination. The incorporation of bromine in a polymer is more effective against fire than chlorination.

• Polyurethanes are used widely both in solid form and as foam, with the foam being either flexible or rigid. These materials’ burning properties vary widely, depending on their physical and chemical properties.

• Isocyanurate cross-linked rigid foams, used for insulation, are less flammable than flexible foams.

Condensation Polymers Although some condensation polymers were processed in the 19th century, the first commercially successful synthesized condensation polymers date to the 1930s. Neoprene and nylon originally were brand names, but are now generic names for families of polymers (and, therefore, are not capitalized).

The first polyesters date to the same decade. Neoprenes are used as cell phone sleeves, knee braces, electrical insulation, and automotive belts. Nylons appear in carpets, clothing, tents, automotive cams and bearings, and stockings (the original use). Polyesters are used in clothing, upholstery fabrics, rope, and pillow stuffing.

As described earlier in this chapter, a condensation polymer is formed by eliminating a small molecule each time another link is added to the polymer chain. Figure 9-17 shows some examples of condensation polymers. Each monomer must contain two functional groups for a chain to be constructed.

These polymers involve combinations of organic acids (—COOH) with alcohols (—OH), or organic acids with amines (—NH2), with subsequent elimination of H2O. Cellulose, discussed previously, is a condensation polymer. Note that aramids (commercially Nomex and Kevlar) have greater thermal stability than nylons because of the aromatic carbon rings in the polymer chain. Protective gear for fire fighters, flight crews, and racecar drivers contains aramids.

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Figure 9-15 Some nonvinyl monomers capable of forming addition polymers.

Data from: Kirk-Othmer Encyclopedia of Chemical Technology, 5th edition, Kroschwitz, J.I., ed., J. Wiley, New York, 2007.

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Figure 9-16 Formation of polyurethanes and polyisocyanurates (addition polymers). R and R’ denote various organic radicals. Partial substitution of a trifunctional isocyanurate permits cross-linking.

The polymers presented so far have been thermoplastics, but an array of thermoset polymers is also available. These extensively crosslinked materials do not melt, but char when heated. Urea (NH2CONH2)–formaldehyde (HCHO) polymers are used as decorative laminates, as the binder in particleboard, and as additives to impart wrinkle resistance to fabrics. Phenol (C6H5OH)–formaldehyde polymers, also called phenolics, are used in circuit boards and hard molded products, such as countertops. Melamine [C3N3(NH2)3]–formaldehyde polymers, also called melamines, are used as decorative laminates and kitchen utensils.

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Figure 9-17 Reactions leading to condensation polymers.

Many additional polymers exist. Information about specific polymers can be found in the Kirk-Othmer Encyclopedia of Chemical Technology [15].

Commercial plastics are not pure polymers; they typically contain substantial proportions of additives to obtain the desired mechanical, thermal, electrical, and visual properties, or inexpensive fillers to reduce their cost. Owing to these additives and fillers, the properties and combustion behavior of a commercial plastic may not always match those of the polymer from which it is made. Also, polymers can be made into fibers and fabrics, elastomers (rubbery materials), foams, (flexible or rigid), coatings, films, or solid plastics; each of these forms has different fire properties.

Fire Retardants Certain properties of solid materials characterize each material’s contribution to fire hazard, including the material’s ease of ignition to both flaming and smoldering combustion, mass burning rate, flame spread rate, soot production, and persistence of glowing combustion following the extinguishment of flames.

Some materials and products have been identified as significant contributors to numerous or particularly disastrous fires, and tests have been developed to identify and discourage/prohibit the use of these materials.

Some materials inherently do well in these tests. Many otherwise desirable polymeric materials, both natural and synthetic, do not. So that they can pass the tests, the latter materials are modified by the addition of chemicals called fire retardants. The resulting material might be explicitly designated as “fire retardant,” although many products do not bear such labels. Note that no fire safety regulations actually require the use of fire retardant (FR) chemicals.

Instead, these chemicals are added to materials to expand consumer choice by enabling a wider range of materials to be used for a particular product. This demand has led to the worldwide production of FR chemicals with a value in excess of $4 billion (7 billion kg) annually. Approximately half of that

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volume consists of halogen (bromine and chlorine) compounds, with the rest being mainly alumina trihydrate, antimony oxide, phosphorus-containing compounds, and boron-containing compounds.

Most fire retardants provide resistance to small ignition sources such as a cigarette, match, or spark.

For carpeting, the ignition source is an incendiary tablet; hence the relevant test is called the “pill test.”

For expansive applications such as wall coverings or mattresses, testing for rates of heat release, flame spread, or smoke production involves a large flame as the ignition source, providing ample opportunity for ignition.

People generally choose products for properties other than low flammability—for example, flexibility, durability, strength, electrical and thermal insulation, color variety and pattern, resilience, and texture.

Large amounts of additives can compromise these properties and add to the purchase price; thus FR chemicals are typically added to a material at or near the minimal level required to pass the fire test and enter the marketplace. Under more severe fire conditions (e.g., high radiant flux or elevated oxygen concentration), then, the fire retardant effectiveness may be significantly lessened or even negated.

All but a few fire tests are performed with new materials or products. Should a product’s formulation change with use or age, the fire performance would also change. For example, if a FR additive was not bound to the host material, it could migrate over time. Some segments of the product would then become richer in the additive, while other segments would contain less than the original concentration and might not show the degree of fire protection expected from the original test performance.

FR additives act to modify the ignition and burning of solids through at least four known mechanisms.

Within these classes, the chemical details are not always completely understood. In some cases, a fire retardant or mixture of fire retardants has been found to act through more than one of these mechanisms. Thus the formulation of a FR “cocktail” for a particular polymer and a particular application is often empirically developed. The four mechanism classes are as follows:

Note Fire retardant chemicals are currently the subject of extensive public discussion. Some critics claim that these additives are harmful to people and the environment worldwide and are ineffective in reducing product flammability, and thus are unnecessary. Others contend that fire retardants have saved lives and reduced fire losses, that the evidence for harm is limited to a very few commercial chemicals, and that the potential harm is outweighed by the fire loss prevention. To date, no systematic, scientific appraisal of the risks and benefits of all the various types of FR additives has been published. However, for a few FR chemicals, sufficient evidence has been gathered to ban them from sale or to withdraw them from the marketplace.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

1. Char formation (residue enhancement). The additive promotes the formation of large, stable molecules within the solid, resulting in a carbonaceous char or a tarry substance. This promotion occurs faster than the breakdown of the polymer into small, volatile fragments that can ignite.

Some metal-containing and phosphorus-containing compounds operate this way. The more effective this type of FR additive is, the smaller the mass of carbon oxidized and the lower the heat release and heat release rate.

2. Gas-phase flame inhibition. The additive releases gases that slow or extinguish the gaseous combustion reactions by dilution, cooling, and/or chemical interference with chain branching or propagation reactions. Such additives often involve halogen, metal, or phosphorus atoms. Slowing or termination of the combustion chemistry can lead to enhanced escape of the products of incomplete combustion, such as soot or carbon monoxide. Should the flames be extinguished, the decrease in the burned mass outweighs the modest increase in incomplete combustion products.

3. Solid-phase heat absorption. The additive decomposes endothermically, absorbing heat that otherwise would have been available to decompose the host polymer. Hydrated alumina (alumina trihydrate, Al2O3 · 3H2O) and limestone (CaCO3) operate in this manner. When heated, the compound decomposes with absorption of heat and release of an inert gas (H2O or CO2, respectively, in the two examples here), which cools and dilutes the flame:

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The released gas also provides some flame retardancy by diluting the ambient oxygen.

4. Barrier formation. The additive forms a glaze or a foam over the fuel surface, which to some degree isolates the subsurface material from the flame above. Char formation also provides a barrier. Some phosphorus-containing compounds act by this mechanism.

As discussed in the Fire Fighting Chemicals chapter, externally applied fire suppressants work by similar mechanisms.

The following are some examples of the application of FR additives: •

Woods are impregnated with inorganic salts to promote char formation. The positive ions of the most effective salts are ammonium (NH4+), sodium (Na+), potassium (K+), and zinc (Zn+2), while the most effective negative ions are phosphate (PO4)–3, borate (BO3)–3, silicate (SiO4)–4, sulfate

• •

• •

(SO4)–2, and sulfamate (NH2SO3)–. Woods are also impregnated with organic compounds containing phosphorus, boron, halogens, or nitrogen (usually as NH2 compounds).

Cellulosic materials are filled with borax and/or boric acid to reduce the tendency to smolder or flame, respectively.

Rigid solid materials are coated with fire retardant paints, including intumescent paints, which expand into a foam when heated. The expanded foam shields the host materials from flame radiation and slows the transport of pyrolyzate to the flame. These FR coatings, however, often do not survive long exposure to wet or very humid atmospheres. They are also subject to abrasion.

Synthetic polymers are copolymerized with small proportions of bromine-containing monomers.

Vinyl polymers achieve fire retardant characteristics through a combination of a halogenated organic compound and antimony oxide (Sb2O3). The current understanding is that the effectiveness of this combination results from formation of a volatile antimony halide or antimony oxyhalide (e.g., SbOCl), which inhibits the gaseous combustion reactions.

References [16] and [17] at the end of this chapter provide further information on the chemistry and uses of fire retardants.

In some applications, the required fire performance cannot be achieved with levels of fire retardant additives that do not interfere with other required properties of the material or product. In those cases, a product may be wrapped with a barrier material, also called a fire-blocking layer. Most often, these materials slow or prevent the passage of pyrolyzed material to fresh air and/or absorb the flame radiation without passing it to the vulnerable materials within.

Composite Materials and Furnishings Most commonly encountered combustible items are composite in nature, rather than consisting of a single material. For example, an upholstered chair might consist of a wood frame, a polyurethane foam pad, a layer of a polyester fiber, and a wool cover fabric. A rug might consist of a nylon pile, jute padding, and a (nonslip) rubber backing. The copper conductors in an electric cable might be wrapped in a nylon insulation layer, and the cable protected with an outer sheath of plasticized polyvinyl chloride.

Furthermore, the geometry of these products differs sharply from the simple specimens that are of necessity tested in most small-scale flammability apparatus. Surfaces are not always flat, some products are typically covered with other products (e.g., mattresses with bedclothes), and there may be joints where (like or unlike) materials meet. Some products (e.g., wall insulation) would not immediately be exposed to a fire in a room.

It is not yet possible to determine the fire behavior of composite products from knowledge of their constituent materials. To the degree that it is practical, their burning characteristics are determined by conducting full-scale fire tests with the actual products and realistically simulating an ignition source and radiative environment.

No systematic measurements have been completed regarding the ease of flaming ignition (minimum heat or duration of applied heat) of full-scale combustibles such as wall linings, upholstered furniture, and mattresses. Fortunately, ignition studies with a small representative portion of an item can sometimes be conducted [18]. Figure 9-18 shows how a small mock-up (segments of fabric over padding) is used to determine whether smoldering by a lit cigarette will ignite an upholstery fabric [19].

Historically, it was not possible to estimate the rate of heat release of a complex item such as a piece of upholstered furniture, including the period of fire growth, the period of near-maximum heat release,

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and the burnout period, from small-scale tests. Instead, researchers built large (up to room-scale and beyond) calorimeters to measure the burning rate versus time of individual furnishing items, wall coverings, and other room contents. In these calorimeters, the combustible item is placed on a load cell (to measure the specimen mass), which is located underneath a collection hood connected to an exhaust duct. The exhaust clears the combustion products from the laboratory and, as will be discussed shortly, provides a location for certain needed measurements.

Figure 9-18 Mock-up design for cigarette ignition resistance testing of fabrics for use in upholstered chairs.

Reproduced from: NFPA 260, Standard Methods of Test and Classification System for Cigarette Ignition Resistance of Components of Upholstered Furniture, National Fire Protection Association, Quincy, MA, 2013.

The earliest versions of these calorimeters estimated the heat release rate by weighing the combustible item during the test and multiplying the mass loss rate by the heat of combustion of the item.

This technique had several serious drawbacks. First, obtaining the heat of combustion for an item such as a chair is not straightforward. To do so, the tester needs to separate the chair into its component materials, weigh each material, determine its individual heat of combustion, and then calculate a weighted sum of all of these data. Second, for a product that consists of an assembly of materials, the individual materials burn at different rates during a test. Because each material also has a different heat of combustion, the instantaneous mass loss cannot be correlated with the heat-release rate at that time.

Third, any unburned residue almost certainly has a chemistry, and thus a heat of combustion, that is different from that of the original product. Fourth, the mass loss rate gives no information about the portion of the combustible item that vaporizes but fails to burn completely. Finally, if the combustible includes a chemical element that burns to form a solid oxide (e.g., aluminum, boron, silicon), then a mass gain rather than a mass loss might result, which would confuse the interpretation of the data.

The next stage in the evolution of large-scale calorimeters involved measurement of the volumetric flow and temperature rise (relative to ambient temperature) of the gas in the exhaust duct, followed by calculation of the convective component of the heat-release rate. Simultaneously, measurements were made of the radiative component of the heat-release rate, with one or more radiometers viewing the burning object from the side. This procedure required the assumption that the radiation was isotropicâ&#x20AC;&#x201D; that is, it was emitted equally in all directions. Some of the convective heat was lost to the surroundings.

Moreover, for tests conducted in a room, the walls and ceiling of the test room could become sufficiently

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hot that their radiation could not be discounted. A time constant problem also became apparent: the radiative heat reached the radiometers nearly instantaneously, but the temperature was measured far enough into the exhaust duct that the exhaust gases were well mixed. As a consequence, there was a time delay relative to the radiation measurement. This time delay varied, because the gas velocity in the duct changed with temperature.

These uncertainties in the heat release rate determinations were greatly reduced with the advent of oxygen consumption calorimetry. This technique is based on the underlying principle that for almost all common combustibles, and for mixtures of combustibles, approximately 13.1 kJ of heat is released per gram of oxygen consumed [20]. This value varies only approximately Âą5 percent for different materials, and the rule holds even if incomplete combustion occurs [21]. Thus the basic required measurements in oxygen consumption calorimetry comprise continuous gas flow and temperature in the duct and the oxygen concentration in that flow. Figure 9-19 shows such an example of such a calorimeter. These devices now exist in fire research and test laboratories around the world. Depending on the design, they can measure fires with heat release rates up to 20 MW.

Figure 9-19 Example of a large fire calorimeter.

The advent of such calorimeters has made possible the regulation of furnishings based on their heat release rates. California requires that upholstered furniture used in high-risk occupancies have a peak heat release rate of less than 80 kW [22]. The U.S. Consumer Product Safety Commission requires that all mattress/foundation sets have peak heat release rates less than 200 kW during the first 30 minutes of burning [23].

Table 9-3 shows peak rates of heat release for a variety of full-size objects burned under calorimeters like those described previously, as well as some indication of the duration of intense burning. Reference [24] provides additional data on heat release rates.

It is still not possible to predict the ignitability and burning behavior of all combustibles from knowledge of their chemistry or using data from small-scale tests, with one exception: researchers can estimate heat-release rates for upholstered furniture. Due to the importance of these furnishings to fire hazard and risk, extensive research has led to empirical methods for estimating the peak heat-release rate, the time to this peak, and the total heat released. The source of the burning data for these estimations is the cone calorimeter [25].

Acidâ&#x20AC;&#x201C;Base Pairs 151

Alkaline substances, also known as caustics or bases, will react with any of the numerous organic and inorganic acids, or even with water, releasing substantial amounts of heat, which could cause other nearby materials to ignite. The most common alkalis are sodium hydroxide (lye) and calcium oxide (lime or quicklime). When wet, these alkalis can generate hydrogen when in contact with aluminum or galvanized steel (zinc). The carbides of lithium, sodium, potassium, calcium, and barium react with water to form acetylene, a highly flammable compound.

Table 9-3 Typical Heat-Release Rates for Various Combustible Items Item

Peak Rate of Heat Release (MW)

Mattress/foundation (U.S. twin, pre-2006)

> 2*

Approximate Time of Intense Burning (s)

Mattress/foundation (U.S. twin, compliant with 16 CFR â&#x2030;&#x2C6; 0.1 (estimated) 1633) Cathode ray tube television, FR plastic cabinet (U.S.)

0.2

300

Upholstered chair (PU foam, polyolefin fabric)

2.0

100

Upholstered chair (California TB133 compliant)

< 0.08

Stack of six steel-frame, polypropylene chairs

1.9

300

Wooden dresser

1.8

200

Curtains (two, 2.13 m Ă&#x2014; 1.25 m)

0.13 to 1.2

Automobile

1.2 to 8.3

1000

School bus

29 to 34

5000

Source: Reprinted with permission from NFPA 260, Standard Methods of Test and Classification System for Cigarette Ignition Resistance of Components of Upholstered Furniture, National Fire Protection Association, Quincy, MA, 2013.

Metals The combustion of metals remains rare in residential, office, or vehicle fires. However, extremely hazardous fires involving metals can occur in some commercial, warfare, and pyrotechnic settings.

Contributing factors to metal-based fires include a large metal surface area and potent ignition sources.

The oxidation of a metal is a highly exothermic process. The heats of combustion approximate the values for many synthetic polymers. Thus, in a thermodynamically controlled world, metals would make significant contributions to fires and fire losses. In the real world, however, certain processes reduce these contributions.

The thermal conductivity of metals is very high, about 1000 times that of the most common organic polymers. Thus, it is almost impossible to heat the surface of a massive piece of metal to a high temperature without simultaneously heating the entire mass to nearly the same temperature.

Accordingly, massive pieces of metal rarely burn, except after being heated at high intensity or for a long period.

However, if a metal were in the form of a fine powder and suspended in air, even a small flame or spark could heat the nearby particles, and the dissipation of heat away from the surface would be minimal, as would conduction to other particles. Indeed, most metal powders are combustible. In certain cases, the metal need not be as fine as a powder to burn, but rather could take the form of chips or shavings, as from machining.

Metals can be divided into two categories: those that burn on their surface and those that burn in the vapor phase. Table 9-4 presents examples of both types.

The first four elements listed in Table 9-4 are believed to burn on the surface. All have extremely high melting points (TMP) and boiling points (TBP)â&#x20AC;&#x201D;higher than their flame temperatures, in fact. The flame temperature of a metal is limited by the boiling point of its oxide because some of the heat of metal oxidation is spent on vaporizing the oxide rather than raising the temperature of the entire combustion system. In fact, because the oxides have very high heats of vaporization, the flames generate only enough heat for partial vaporization of the oxides.

Table 9-4 Metals That Burn on Their Surface and Metals That Burn in the Vapor Phase [27]

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* Boron and silicon are actually metalloids, with properties between those of metals and nonmetals.

Republished with permission of Taylor and Francis Group LLC Books, from Handbook of Chemistry and Physics, 92th ed., Haynes, W.M., ed., Section 4, CRC Press, Boca Raton, FL, 2011; permission conveyed through Copyright Clearance Center, Inc. The Handbook is updated annually. For further information, see http://www.hbcpnetbase.com.

The last six metals listed in Table 9-4 vaporize while burning. As a general rule, materials that burn on the surface burn more slowly than materials that can vaporize first because the vapors can more readily contact the surrounding oxygen in the air. The surfaces can also become coated with the metal oxide, reducing oxygen contact with the elemental metal.

Another general characteristic of metals—especially hot metals—is that most of them can react rapidly and exothermically with water to form hydrogen. For example, 2Al + 3H2Oliq → Al2O3 + 3H2 + 819 kJ Consequently, if a partially wet metal is burning, fire fighters must deal with a hydrogen fire as well as a metal fire. Also, water under the surface of hot molten metal will turn rapidly into steam and erupt, throwing molten metal around.

Some details about the combustion of specific metals follow [28]. Information regarding hazardous metals and a wide variety of other materials can be found in Reference [29].

Magnesium in the form of powder, ribbons, or shavings can ignite under some conditions at about 932 °F (500 °C). A massive piece of magnesium must be heated to its melting point (1202 °F; 650 °C) for ignition to occur, as has happened in military combat operations. Some magnesium alloys have lower ignition temperatures. Magnesium chips wet with animal or vegetable oils have been known to ignite spontaneously. Molten magnesium in contact with iron oxide (rusted iron) produces a highly energetic thermite reaction3: 3 Mg + Fe2O3 → 3 MgO + 2 Fe + 971 kJ When finely divided, magnesium burns in an atmosphere of any of the following pure gases, to yield the products shown: steam

MgO + H2

carbon dioxide

MgO + CO

nitrogen

Mg3N2

halon 1301 (CF3Br)

MgF2, MgFBr, MgBr2, C (This reaction releases more heat than magnesium powder burning in oxygen.)

Magnesium powder mixed with polytetrafluoroethylene, (C2F4)n, even under an inert gas such as helium or argon, will burn to form MgF2 and carbon.

Clearly, the choice of suppressants when fighting a magnesium fire is severely limited. Extinguishing agents are discussed in the Fire Fighting Chemicals chapter.

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Aluminum has a much higher boiling point than magnesium, so it cannot be ignited as readily.

However, aluminum powder, flakes, very fine chips, and shavings can be ignited and, once ignited, will burn like magnesium chips or powder. Aluminum powder is a major ingredient in solid propellant rocket fuels, and it burns rapidly in rockets to give a very high exhaust temperature. Aluminum powder also reacts violently with halogenated liquids, including common solvents (e.g., trichloroethylene, dry cleaning fluid) and fire suppressants.

Aluminum reacts with iron oxide in a thermite reaction producing, for example, temperatures high enough to melt steel. The relatively low melting point of aluminum favors melting in a fire; consequently, fire investigators use it as a temperature marker. Aluminum will not burn in nitrogen.

Iron and steel generally do not burn in air, but can burn in pure oxygen. Fine steel wool or steel dust in air can be ignited with a torch. Pure iron powder, when exposed to air for the first time after manufacture, can ignite spontaneously.

Massive pieces of titanium generally cannot be ignited. In spite of this element’s high melting point and high boiling point, fine turnings and thin chips of titanium can be ignited with a match. Once ignited, vigorous burning results. Titanium dust clouds can ignite in air when heated. Like magnesium, titanium dust will burn in pure carbon dioxide or pure nitrogen.

The alkali metals—lithium, sodium, and potassium—have some unusual fire properties. These metals have low melting points (366 °F, 208 °F, and 144 °F, [86 °C, 98 °C, and 62 °C], respectively). Sodium and potassium, on contact with water at room temperature, generate hydrogen exothermically and burst into flame spontaneously. Lithium reacts more slowly with water, without bursting into flame. These alkali metals can be ignited by heating in dry air. Once ignited, they burn vigorously, producing dense white clouds of metal oxide particles. Lithium differs from sodium and potassium in that it can burn in pure nitrogen. Alkali metals often are stored under kerosene or oil to isolate them from air and moisture. Alkali metals react violently with halogenated hydrocarbons or sulfuric acid.

NaK is a sodium–potassium alloy with a very low melting point; it is a liquid in the vicinity of room temperature. NaK is used as a heat-transfer fluid. Its fire properties resemble those of sodium and potassium, but its reactions are more vigorous. Reference [27] discusses the fire properties of zirconium, calcium, zinc, uranium, and plutonium.

Exothermic Materials An exothermic material comprises either a pure substance or a mixture of substances that can undergo chemical reactions that liberate heat without requiring oxygen from the air. Such a material poses a fire or explosion hazard. Moreover, if such a material becomes involved in a fire, it usually will cause easier ignition, faster fire growth, higher flame temperatures, and more difficult extinguishment. In some cases, it enables propagating combustion even when the oxygen fraction is too low for other combustibles to burn.

Numerous such materials can be found among industrial chemicals, in multiple forms—gases, liquids, and solids.4 The strategy for fighting a fire involving an exothermic material is specific to the material and depends on the quantity involved, likelihood of explosion, toxicity, solubility in water, and compatibility with extinguishing agents, among other factors. References [30] and [31] provide guidelines for dealing with many exothermic materials.

Exothermic compounds release heat through four processes: 1.

Thermal instability. These materials decompose into smaller molecules while releasing heat.

Generally, the number of moles of gas released is larger than the number of moles of the source compound. Combined with the heat release, this volumetric increase can result in a severe pressure wave. Examples of materials that exhibit thermal instability include ozone (O3), nitrous oxide (N2O), acetylene (C2H2), hydrogen peroxide (H2O2), hydrazine (N2H4), ethylene oxide (C2H4O), lead azide (Pb(N3)2), diborane (B2H6), and methyl hydrazine (N2H3CH3).

In addition, ethers have the capability of forming peroxides after storage for a month or more.

Isopropyl ether (C3H7—O—C3H7) is more susceptible to peroxide formation than other ethers; detonation can occur when this peroxide is heated. Many other peroxides, such as the widely used benzoyl peroxide, (C6H5)—(C O)—O—O—(C O)—(C6H5), can undergo rapid exothermic decomposition.

2. Self-polymerization. When organic monomers combine to form polymers, the polymerization process releases heat. In controlled polymerization, enthalpy decreases as heat is released,

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generally by the circulation of a coolant. In uncontrolled polymerization, heat is not removed, and excessive temperatures result. (Examples of polymeric reactions were given earlier in this chapter.)

3. Intramolecular oxidation–reduction. Some molecules contain a group of atoms with oxidizing power, such as a nitro group (—NO2) or a peroxide group (—O—O—), and a group of carbon and hydrogen atoms that can be oxidized. Such molecules often are used as explosives or rocket propellants. Figure 9-20 shows three examples and the products of their complete combustion.

Like the thermally unstable compounds, each of these molecules reacts to form additional moles of gas as well as heat.

4. Oxidizing agent in contact with a reducing agent. Unlike the previously mentioned compounds, these oxidizing agents (e.g., potassium nitrate, sodium chlorate, and fluorine) are not capable of exothermic decomposition. They are mixed with an oxidizable material (reducing agent), which can consist of almost any organic material or metal. These combinations are used as explosives, pyrotechnics, and solid propellants for rockets. Examples include: • Black powder: KNO3 + 3/2 C + 1/2 S → CO2, CO, N2, K2S, K2SO4, K2CO3, S • Explosive: n NH4NO3+ 1/3 (CH2)n(fuel oil) → n [N2 + 7/3 H2O + 1/3 CO2] • Pyrotechnic delay mixture: BaCrO4 + B → BaO + 1/2 B2O2 + 1/2 Cr2O3

• Solid propellant: n(NH4ClO4 + Al) + 1.4 (CH2)n → N2, H2, H2O, HCl, Al2O3, CO, CO2 Such mixtures are prepared with extreme care and presumably are stored in bunkers or isolated places.

A more commonly encountered problem relates to oxidizers that have not been deliberately mixed with anything, but might become mixed by accident, such as in the course of a fire. Any of these oxidizers, when it comes in contact with common materials such as paper, cotton, wood, plastics, hydrocarbons, alcohols, vegetable and animal oils and fats, sulfur, and metals, can react exothermically even in the absence of air, although in some cases an elevated temperature would be needed to initiate reaction. The more common classes of oxidizers include the following: • • • • • • • • • •

Nitric acid and its salts (nitrates) Perchloric acid and its salts (perchlorates) Chromic acid and its salts (chromates and dichromates) Permanganic acid and its salts (permanganates) Fluorine, chlorine, bromine, and iodine (in order of decreasing reactivity) Inorganic peroxides Calcium hypochlorite Potassium persulfate Manganese dioxide Chlorate, bromate, and iodate salts

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Figure 9-20 Examples of molecules that undergo internal oxidation–reduction reactions.

WRAP-UP Chapter Summary •

The pyrolysis of a solid to form gaseous fragments can be characterized by a heat of gasification.

During pyrolysis, chemical change occurs within the solid, and the chemistry of the volatiles is rarely the same as the chemistry of the solid.

• The ignition of the pyrolyzate from a solid is similar to the ignition of gases and vapors, except that heat losses from the fuel surface to the interior and to the surroundings can delay or prevent the ignition.

• For a fuel to smolder or self-heat, it must be porous, its interior surfaces must react exothermically reaction with oxygen, and the material must be a good insulator.

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• • • • •

• • • • •

Surface char formation slows burning by insulating the subsurface material from the flame radiation and by sequestering carbon.

For solids that melt during burning, dripping or running of the liquid increases the fire spread rate due to the increase in burning surface. The flaming liquid can also ignite other combustibles on contact.

The burning rate depends on the radiant flux to the surface. Early in a fire, this flux comes from the local flames; later, most of the flux is derived from the hot surroundings.

The flame spread rate depends on whether the spread direction is upward (fastest), lateral (slow), or downward (slow, except when dripping occurs); the degree of radiative preheating; the thermal thickness of the solid; the ambient oxygen percentage; and the direction of any air flow.

Nearly all common solid combustibles are polymers, which consist of strings of identical building blocks. Polymers are classified based on whether they are formed by addition or by condensation; synthesized by nature or by humans; linear or cross-linked; composed of a single building block or multiple building blocks; or thermoplastic, thermoset, or elastomeric.

Many formulations of each of the families of plastics exist, as do many different types of woods. The burning properties of these solid combustibles can vary considerably.

Fire retardants are added to combustible materials to help them meet flammability criteria. These additives work by promoting char formation, inhibiting the flame chemistry, absorbing heat in the solid, and/or forming a barrier between the flame and the fuel surface.

Most common combustible items consist of multiple materials. We cannot yet quantify the fire behavior of a composite from its constituent materials. Instead, the fire performance is measured by burning the entire item or estimated from tests of small specimens of the materials or a representative composite.

Oxygen consumption calorimetry is a tool for determining the rate of heat release from a product (or specimens from a product).

Exothermic materials and finely divided metals pose unusual and potentially severe fire hazards, but are found almost exclusively in nonresidential settings.

Key Terms addition polymer A polymer formed without the loss of any atom or molecule. fire barrier material A protective layer that prevents or severely retards the contribution to a fire from any subsurface material(s). cellulosic material A material composed entirely or mostly of a polymer of glucose. composite A product consisting of more than one material, or a material consisting of multiple solids that remain physically distinct. condensation polymer A polymer that splits out small molecules, usually water, during its formation. copolymer A polymer made of two or more different monomers. cross-linked polymer A polymer in which the long chains are bonded to one another at intermediate points. exothermic material A material that can undergo chemical reaction that releases heat without an additional oxidizer, such as oxygen from the air. fire retardant A chemical additive that slows the ignition and/or burning rate of a material. homopolymer A polymer that contains only one type of repeat unit. limiting oxygen index (LOI) The minimum volume percent of oxygen that will support flaming of a material, as measured in a standard apparatus. monomer A small molecule that can combine with other molecules of the same kind (or different kinds) to form a repeating chain molecule, or polymer. oligomer A molecule consisting of a few monomer units. oxygen consumption calorimetry The determination of heat release rate in combustion using measurement of the depletion of oxygen from the incoming air. polymer A large molecule consisting of very many repeated units, called monomers. thermal runaway Self-heating which rapidly accelerates to high temperatures. thermoplastic polymer A polymer that softens upon heating and returns to its original state upon cooling. thermoset polymer A polymer that, upon heating, undergoes irreversible change. vinyl polymer A polymer synthesized from monomers that contain a carbon-carbon double bond.

Challenging Questions 157

1. What is the difference between a material and a product?

2. Why does it generally take more enthalpy to ignite a solid than a gas or liquid?

3. What is the difference between spontaneous ignition and piloted ignition of a solid?

4.

If a pilot flame is present, how intense must a thermal radiative flux be to ignite wood? How does this flux compare with the intensity of sunlight (approximately 0.7 kW/m² at noon in the tropics)?

5. Why will a single wooden log, if ignited, soon self-extinguish, while a group of logs near each other continues burning?

6. How does the formation of a char layer affect the burning of a solid?

7. Why is upward flame spread over a vertical surface more rapid than downward flame spread?

8. Why do flames spread more rapidly on thin materials?

9. The acrylic top of a table is burning, and the flame has spread to the edges. Of the heat transferred from the flame to the table surface (to supply the heat of gasification), which fraction is radiative and what which is convective?

10. What are the upper and lower extremes of the moisture content of wood?

11. What do woods, papers, and cottons have in common?

12. What is the difference between a linear polymer and a cross-linked polymer?

13. What is the Oxygen Index Test? Why are the results of this test of limited use?

14.

What is the cone calorimeter? How does it capture the effect of a fire environment on a small test specimen?

15.

What are the four ways in which fire retardants can act? Which type(s) of fire retardant would be effective for a material exposed to persistent radiative heat of high intensity, such as from a nearby fire?

16. Under which conditions can the presence of fire retardants cause problems?

17. The heat of combustion of aluminum is similar to (actually a little lower than) the heat of combustion of common woods. Why is an aluminum frying pan less of a fire hazard than the overhead wooden kitchen cabinets if the food in the pan ignites?

18. Why is it not advisable to apply water to burning metals?

19.

The tendency of oily rags to ignite spontaneously depends on the type of oil. Why are many vegetable oils and fish oils far more dangerous than mineral oils or lubricating oils?

20.

Why can certain liquids and gases, such as ethylene oxide or acetylene, burn in the complete absence of oxygen?

21. Is it true that any artificial atmosphere that will support life will also support combustion of common materials?

References 1. Babrauskas, V. (2003). Ignition Handbook, Issaquah, WA: Fire Science Publishers,.

2. Moussa, N. A., T. Y. Toong, and S. Backer. (1973). “An Experimental Investigation of Flame-Spreading Mechanisms Over Textile Materials.” Combustion Science & Technology 8: 165–175.

3. Bowes, P. C. (1984). Self-Heating: Evaluating and Controlling the Hazards. New York, NY: Elsevier.

4. Cote, A. E., ed. (2008). Tables and Charts: Chapter 6.17. In: Fire Protection Handbook, 20th ed. Quincy, MA: National Fire Protection Association.

5. Butler, C. P. (1971). Notes on Charring Rates in Wood. Fire Research Note 896. Borehamwood, Herefordshire, UK: British Fire Research Section.

6. Tewarson, A. (2008). Generation of Heat and Chemical Compounds of Fires. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

7. de Ris, J. (1979). Fire Radiation: A Review.” Proceedings of the Combustion Institute 17: 1003–1016.

8. Unpublished data. Norwood, MA: FM Global.

9. McAlevy, R. F., and R. S. Magee. (1969). “The Mechanism of Flame Spreading over the Surface of Igniting Condensed Phase Materials.” Proceedings of the Combustion Institute 12: 215–227.

10. Proceedings of International Microgravity Combustion Workshops. Washington, DC: National Aeronautics and Space Administration.

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Lastrina, F. A., R. S. Magee, and F. R. McAlevy. (1971). “Flame Spread over Fuel Beds: Solid Phase Energy Considerations.” Proceedings of the Combustion Institute 13: 935–948 ASTM Fire Standards and Related Materials, 7th ed. (2007). West Conshohocken, PA: ASTM International.

Magee, R. S., and R. D. Reitz (1975). “Extinguishment of Radiation-Augmented Plastic Fires by Water Sprays.”

Proceedings of the Combustion Institute 15: 337–347.

ASTM D 2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-like Combustion of Plastics (Oxygen Index). (1991). West Conshohocken, PA: ASTM International.

Kroschwitz, J. I., ed. (2007). Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed. New York, NY: John Wiley.

Lyons, J. W. (1970). The Chemistry and Uses of Fire Retardants. New York, NY: Wiley-Interscience.

Wilkie, C. A., and A. B. Morgan, eds. (2009). Fire Retardancy of Polymeric Materials. Boca Raton, FL: CRC Press.

Gann, R. G., R. H. Harris, Jr., J. F. Krasny, R. S. Levine, H. Mitler, and T. J. Ohlemiller. (1988). Effect of Cigarette Characteristics on the Ignition of Soft Furnishings. NBS Technical Note 1241. Gaithersburg, MD: National Bureau of Standards.

NFPA 260: Standard Methods of Test and Classification System for Cigarette Ignition Resistance of Components of Upholstered Furniture. (2013). Quincy, MA: National Fire Protection Association.

Huggett, C. (1980). “Estimation of the Rate of Heat Release by Means of Oxygen Consumption Measurements.” Fire and Materials 4: 61–65.

Krause, R. F., and R. G. Gann. (1980). “Rate of Heat Release Measurements Using Oxygen Consumption.” Journal of Fire and Flammability 11: 117–130.

Flammability Test Procedure for Seating Furniture for Use in Public Occupancies. Technical Bulletin 133. (1991). State of California, Bureau of Home Furnishings and Thermal Insulation.

Standard for the Flammability (Open Flame) of Mattress Sets. (2006). 16 CFR Part 1633, U.S. Consumer Product Safety Commission.

Babrauskas, V. (2008). Heat Release Rates. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Babrauskas, V. (2008). Upholstered Furniture and Mattresses. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed.

Quincy, MA: National Fire Protection Association.

Ohlemiller, T. J., and R. G. Gann. (2002). Estimating Reduced Fire Risk Resulting from an Improved Mattress Flammability Standard. Technical Note 1446. Gaithersburg, MD: National Institute of Standards and Technology.

Haynes, W. M., ed. (2011). Handbook of Chemistry and Physics, 92th ed. Boca Raton, FL: CRC Press, Section 4. The Handbook is updated annually. For further information, see http://www.hbcpnetbase.com/.

Christman, T. (2008). Metals. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Baker, C. J. (2006). Fire Fighter’s Handbook of Hazardous Materials, 7th ed. Sudbury, MA: Jones and Bartlett.

Fire Protection Guide to Hazardous Materials, 11th ed. (2010). Quincy, MA: National Fire Protection Association.

Lewis, R. J. (2012). Sax’s Dangerous Properties of Industrial Materials. New York, NY: Wiley.

1

There are many formulations in each of these families of plastics, as well as many different types of woods. The burning properties of these formulations can vary considerably. For this reason, it is important to obtain flammability data for the specific formulation of interest. Estimating the properties of one formulation from data for other formulations requires knowledge of the degradation mechanisms of the formulations and is beyond the scope of this book.

2

In this, and other, tables of LOI data, many of the entries consist of a single number for an entire class of polymers, such as polyethylenes. There are many formulations of polyethylene, so the compiled values should be used with caution.

3

In a thermite reaction, a metal and an oxide of a different metal react exothermically, with the oxygen “changing partners.”

4

All of these forms are discussed here, because the principles are common to all.

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CHAPTER 10

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Combustion Products OBJECTIVES After studying this chapter, you should be able to: • Describe the two main types of smoke aerosols and explain why they are important in fires.

• Explain how soot forms.

• Describe the two principal methods for quantifying the aerosol content of smoke produced in an experimental fire.

• Describe the smoke-point height method for estimating the relative sooting tendency of a gaseous fuel.

• List some relationships between fuel chemistry and sooting tendency.

• Estimate the mass of burned fuel that can lead to loss of visibility due to smoke obscuration.

• List the principal combustion products formed in fires.

• Explain the principles of operation for ionization smoke alarms and photoelectric smoke alarms, and identify the differences in what they detect.

Introduction “Where there’s smoke, there’s fire” is one of the most common of sayings, generally meaning that if it looks like something is wrong, something probably is wrong. This cliché has been used to imply political antics, marital infidelity, athletes’ use of banned substances, and (more prosaically) fire detection. When an idiom reaches such widespread use, it indicates that generations of speakers were able to assume that the audience recognized as truth that a fire emitted a visible signature.

The National Fire Protection Association (NFPA) defines smoke as follows: The airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, together with the quantity of air that is entrained or otherwise mixed into the mass.

Some in the fire professions do not include the gaseous component in their definition of smoke. This text uses the NFPA definition and the synonymous term fire effluent, which is used by the International Standards Organization (ISO), another organization that prepares standards.

Smoke components are characterized by the quantities generated and the character (chemical or physical) of the components. The first of these descriptors, the quantity generated, is denoted by the yield, the mass of smoke or a component of the smoke per mass of fuel combusted or pyrolyzed. The type and quantity of smoke produced in any fire depend both on the materials and products that are burning and on the burning conditions.

The Combustion, Fire and Flammability chapter discussed smoldering combustion. The Fire Characteristics: Gaseous Combustibles chapter presented the fundamentals of the mixing of gasified fuel and air. The Fire Characteristics: Liquid Combustibles and Fire Characteristics: Solid Combustibles chapters discussed the chemical nature of liquid and solid fuels, respectively, and the generation of the gaseous molecules that react with air in flaming combustion. Following a section on smoke aerosol formation, this chapter examines the measurement of smoke components and the use of these data to detect incipient fires. It concludes with a look at the formation and measurement of combustion-generated gases.

Smoke Aerosols General Nature Smoke aerosols are important in fires for four reasons: 1. Radiative heat transfer rate from the soot in flames is a principal determinant of the gasification of fuel and, therefore, the combustion rate of an item. Typically, at least 80 percent of the thermal radiation from a luminous (sooty) flame comes from the soot and 10 to 20 percent comes from the hot CO2 and H2O molecules.

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2. Radiative heat transfer from the flames and from the hot upper layer in the fire room is often responsible for item-to-item flame spread (i.e., the ignition of combustibles that are not necessarily adjacent to the already burning item).

3. Outside the flames, the aerosols determine the visual obscuration from fires.

4. These aerosols are currently the principal fire signature detected by residential smoke alarms.

There are two main types of smoke aerosols 1. Carbonaceous solid particles (soot), which generate the incandescent orange-yellow glow within a flame and are seen as black smoke emanating from the flame.

2. Liquid droplets (aerosol mist), which form as some gas molecules cool and condense. They are seen as light-colored smoke.

Figure 10-1 The two basic kinds of smoke emanating from a fire. Š Kratka Photography/ShutterStock, Inc.

Soot Formation Carbon is the main constituent of soot particles, and hydrogen is a secondary constituent; in other words, carbon atoms significantly outnumber hydrogen atoms in a soot particle. Depending on the fuel, soot may also include some mineral matter and a small number of other atoms, such as nitrogen and oxygen.

Soot formation is a result of underventilated burning and, therefore, is characteristic of diffusion flames and fuel-rich premixed flames. Soot forms on the fuel side of the fuelâ&#x20AC;&#x201C;air interface. The high temperature causes thermal fragmentation (pyrolysis) of the fuel molecules. The near-absence of oxygen prevents significant oxidation of the fuel fragments. Instead, some of these fragments collide with each other and merge to form larger and larger molecules and molecular radicals. Within the flame, most of the soot particles are roughly spherical and have diameters ranging from 0.01 to 1 Âľm.

While the flames are still small, the particles emerging from the flames are fairly dilute. They do not collide much with each other, so they have few opportunities to stick together and form larger particles.

In larger, more turbulent fires, many more soot particles form. When they leave the flame, they encounter and stick to other particles, forming aggregates as much as thousands of times larger, a process called coagulation. In Figure 10-2, a coagulated aerosol particle is clearly seen to be composed of numerous fine particles. These aerosols are typically visible.

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Figure 10-2 Electron micrograph of a soot aggregate. The diameter of the fine particles is approximately 0.03 Âľm, and the overall size of the agglomerate is approximately 6 Âľm [1].

Reproduced from: National Institute of Standards and Technology.

The chemical sequence that builds the in-flame soot particles involves many steps. These steps all take place in the small fraction of a second during which the fuel molecules are present in the thin reaction zone of a flame. When methane is the fuel, the sequence begins with steps like those shown in Figure 10-3.

This series of reactions reflects the fact that on the fuel side of a diffusion flame, stable acetylene molecules form. These molecules readily add to hydrocarbon radicals. Hydrogen atoms are also present in high concentration; each of these atoms extracts another H atom from a hydrocarbon to form H2. Within the hydrocarbon species, these reactions collectively result in an increase in the number of carbon atoms, an increase in the degree of unsaturation, and a decrease in the ratio of hydrogen atoms to carbon atoms.

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Figure 10-3 Some reactions leading to the formation of soot from methane.

When a molecule become large enough (usually six carbon atoms), it “realizes” that it can move to a more stable enthalpy state by forming a closed ring. The two end carbon atoms bind to each other, forming a cyclic compound. The most stable state of such a cyclic compound is the one with alternating double and single bonds—that is, an aromatic ring. Further additions to the aromatic ring lead to the formation of the polycyclic aromatic hydrocarbons (PCAHs) that grow to become soot particles. Some of these PCAHs, such as benzo(a)pyrene, are carcinogenic. Figure 10-4 shows some of the polycyclic aromatic hydrocarbons found in sooting flames. For the molecules in this figure, notice how small the H/C ratio is (0.5 for coronene) compared to the value of approximately 2 for most alkanes.

If, instead of methane, the fuel had consisted of a compound with more than one carbon atom, the sequence of reactions building up to soot would be shorter. Propane, C3H8, for example, would be expected to burn with a sootier flame than methane—and this is indeed the case. Also, because carbon

164

compounds with a double bond (CnH2n), two double bonds (Cn H2n–2), or a triple bond (also CnH2n–2) are closer in H/C ratio to soot than are saturated hydrocarbons (Cn H2n+2), they are expected to give sootier flames—and this also is the case. Finally, because soot has an aromatic structure, if the original fuel has aromatic character, such as benzene (C6 H6) or toluene (C6 H5 CH3), this would be expected to cause an extremely sooty flame, and it does.

Figure 10-4 Four soot precursor molecules: polycyclic aromatic hydrocarbons.

For turbulent burning polymeric materials, an alternative pathway to soot formation exists. In this pathway, the flame radiation pyrolyzes the fuel to gasify oligomeric fragments—that is, fragments containing a few monomer units of the polymer. These large molecules are drawn into the turbulent flame, where they sidestep the degradation to very small (C1 or C2) fragments and aggregate to form larger molecules. When the turbulent structure sweeps them into the hot realms of the flame, they eliminate hydrogen molecules and form the aromatic ring structures characteristic of soot.

A factor contributing to the emission of soot from diffusion flames is the cooler temperature of sooting flames relative to nonsooting flames. In a clean flame (e.g., a stoichiometric premixed flame), all the heat release goes toward increasing the temperature of the fuel, air, and combustion products. In a sooting flame, much of that heat is radiated out of the flame zone, causing the flame to burn at a lower temperature. Because of this lower temperature, the oxidation reactions in a diffusion flame can cease before completion, such that the products of the reactions include substantial carbon monoxide, unconsumed soot, and other partially oxidized molecules.

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A simple way to measure the relative tendency of a gaseous fuel to generate soot has been devised. Using a Bunsen burner (Figure 7-2) with the air vent closed, ignite the gaseous fuel. The result will be a conical diffusion flame that glows yellow from the soot within the flame envelope. Adjust the fuel flow, noting that a higher flow leads to a taller flame. Find the smallest fuel flow at which black soot just begins to emerge from the flame tip. The height of this luminous flame is the smoke-point height.

The shorter the smoke-point height, the greater the tendency of the fuel to soot.

The explanation for this behavior is as follows. In such a diffusion flame, the fuel is consumed as it moves upward from the burner. Burning soot is found near the top of the flame. When the fuel flow is very low, the height of the flame is determined by the distance it takes for enough oxygen to diffuse to the burning zone and consume the soot. As the fuel flow increases, the diffusion of air into the flame cannot keep up with the supply of soot, and some of the soot escapes. A fuel that has a high soot yield overwhelms the oxygen diffusion at a low fuel feed rate. A lightly sooting fuel needs less oxygen to burn all the soot.

Table 10-1 shows the smoke-point heights for a series of gases and vapors. The large differences caused by chemical variations are consistent with the expectations expressed earlier. Saturated hydrocarbon fuels have large smoke-point heights, indicating a low tendency to soot. In fact, the methane flame is so tall that it becomes turbulent, and s smoke-point height cannot be determined.

Although smoke-point heights are measured in laminar flames, Figure 10-5 shows that they are closely correlated with the radiative fraction for turbulent flames for the same combustibles.

Table 10-1 Laminar Smoke-Point Heights for Various Combustible Gases and Vapors Combustible

Smoke-Point Height (cm)

ethane, C2H6

24.3

propane, C3H8

16.2

n-butane, n-C4H10

16.0

ethylene, C2H4

10.6

propylene, C3H6

2.9

isobutylene, i-C4H8

1.9

acetylene, C2H2

1.9

1,3-butadiene, 1,3-C4H6

1.5

benzene (vapor), C6H6

0.8*

toluene (vapor), C6H5CH3

0.6*

naphthalene (vapor), C10H8

0.4*

Source: Data from Reference [2] except data marked (*), which are from Reference [3].

It would be useful to have an equally simple technique for obtaining the relative tendency for solid fuels to form soot. Unfortunately, this method is not readily adaptable to nongaseous fuels. At present, the most straightforward approach is to burn a specimen in the cone calorimeter. (See the Fire Characteristics: Solid Combustibles chapter.)

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Figure 10-5 Radiative fraction for turbulent fuel-jet flames of various hydrocarbon fuels versus smoke-point laminar flame height [4].

Reproduced from: Markstein, G.H., “Relationship Between Smoke Point and Radiant Emission from Buoyant Turbulent and Laminar Diffusion Flames,” Proceedings of the Combustion Institute 20, 1055–1061 (1985). Copyright Elsevier.

Aerosol Mist Formation When the smoke leaves the flame vicinity, it cools rapidly from temperatures greater than 2200 °F (1500 K) to temperatures near 80 °F (300 K). This cooling leads to ordinary condensation of molecular vapors to form liquid droplets, such as water droplets. These droplets can have diameters on the order of 1 µm to 10 µm. In addition, the vapors can condense onto the surface of the coagulated soot particles as they leave the flame vicinity. Both processes produce a moist, sometimes oily aerosol with an H/C ratio that is much larger than that of soot particles.

Measurement of Aerosol Yields Two principal methods are employed to quantify the aerosol content of smoke produced in an experimental fire: 1. Collect and weigh the aerosol (gravimetry) and then calculate the ratio to the mass loss of the combustible.

2. Measure the attenuation of a beam of light passing through the smoke and normalize the result.

The two methods roughly correlate with each other, but they are not simply proportional Figure 10-6.

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In the gravimetric approach, one end of a tube is placed at the location for which smoke data are desired. Such locations might include the exhaust duct of a small-scale apparatus, in the doorway separating a fire room from an adjacent space, or at a site near the fire room where a fire sensor might be placed. A pump attached to the other end of the tube draws air laden with combustion products through a filter. The filter collects the solid and liquid components, while allowing the gaseous components to pass through. A total mass of aerosols is obtained by weighing the loaded filter and subtracting the mass of the filter prior to the test. If the collection system is at room temperature, then some of the combustion-generated water vapor will condense on the filter. If the filter is heated to avoid this condensation, liquid aerosols might also be evaporated. Most often, the filter temperature is selected to collect just the soot and tar-like aerosols.

If the combustion products are well mixed within the air flow, such as in the exhaust duct from a fire test, then the total generated soot mass, MT,soot, is calculated as:

where Mcoll,soot is the collected mass, is the total volume flow in the duct, and flow in the sampling probe. The soot yield, Ysoot, is

is the volume

where Mlost is the mass loss of the combustible item(s).

The aerosol may be collected over the entire fire test, thereby obtaining a total soot yield.

Alternatively, one can collect multiple aerosol samples, each over a separate time interval, and then divide each aerosol mass by the specimen mass loss during the same time interval. This method would identify soot yields for different stages of the burning.

In addition to weighing the collected soot, scientists have used electron microscopy to obtain pictures and dimensions of the soot. As seen in Figure 10-2, hundreds or even thousands of the soot spheres can form an aggregate that is not at all spherical. The size of this aggregate depends on the combustion conditions, the chemical nature of the combustible, and the changes occurring as the smoke ages [1]. To the extent that the individual particles are tarry and somewhat fluid, the aggregate could look like an irregular ball. Such aggregates can have mean diameters as large as millimeters.

The most common optical method for measuring smoke is based on the scattering of light. This type of method also provides information about visibility through the smoke. A beam of light is directed across a gap, with a photometer (light detector) at the other end of the space. Before the fire test begins, the intensity of the light beam at the photometer is I0. When smoke aerosol is present in the gap, the photons in the light beam can do one of three things: pass between the particles and reach the photometer, be scattered off the particles and miss the detector, or be absorbed by the particles and not reach the detector. The light reaching the detector has a lower intensity, I. The relationship between I and I0 is given by Bouguerâ&#x20AC;&#x2122;s law, also known as the Beer-Lambert law:

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Figure 10-6 Comparison of smoke measurements by using the optical technique at a wavelength of 0.633 µm and by weighing the collected smoke [5].

The value D is called the optical density per meter or just the optical density, and x is the path length (m) occupied by the smoke. If the smoke aerosol is dilute, then a photon will have only one (or no) encounter with an aerosol particle. In this case, D can be separated into a term that is a property of the aerosol and a term that represents the concentration of particles:

where Dm is the mass optical density (m2/g) and mv is the mass of particles per unit volume (g/m3).

Dm depends on the nature of the combustible and the way it is burned, as well as the wavelength of the light used. For smoke from flaming fires, Dm is 3.8 m2/g ± 13 percent for a wide range of fuels and for the red light (wavelength = 0.633 µm) of a helium–neon laser. Experimentally, to obtain the mass density of the aerosol, we use the logarithmic form of Equation 10-3:

Smoke aerosol can also be measured optically by collecting the light that scatters away from the forward direction. As discussed later in this chapter, smoke alarms operate based on this principle.

Quantity of Smoke Particles Produced Table 10-2 shows some measurements of the smoke particles produced from selected gaseous, liquid, and solid combustibles burned under laboratory conditions. Both gravimetric and optical data are included. These data should be used only to visualize the potential ranges of values, because different specimens with the same generic name (e.g., polystyrene) might have had different chemical formulas.

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Furthermore, not all the tests were performed using the same type of test apparatus, and such properties as ventilation and specimen size may differ among the various test methods. References [1] and [6] provide additional smoke particle measurement data.

The data in Table 10-2 suggest some principles that are supported by wider sets of data than those shown here. â&#x20AC;˘

Flaming fuels containing oxygen tend to generate less smoke than oxygen-free fuels. Thus methanol and polyoxymethylene burn with practically no particulate production. PMMA has a low smoke yield. Woods, under flaming combustion conditions, produce approximately only 1 percent of smoke particles relative to their weight loss. (Under pyrolyzing conditions, their production of smoke particles increases by an order of magnitude.) â&#x20AC;˘ Aromatic materials, such as polystyrenes, can produce very high smoke yields. The aromatic building blocks for soot formation are already present in these fuels and do not need to be formed by a complex chemical build-up, as described earlier in this chapter. â&#x20AC;˘ Molecules containing halogen atoms (X = F, Cl, or Br) tend to burn with high smoke production, because HX splits out of the decomposing molecule, leaving anunsaturated molecule that is transformed readily into a cyclic hydrocarbon and then to soot. (See the Fire Characteristics: Gaseous Combustibles chapter.) HX is also a flame inhibitor, whose presence in the flame favors incomplete combustion.

As a general rule of thumb, the smoke yield rises significantly as the fire proceeds from well ventilated to underventilated conditions. Measurements of smoke yields from underventilated flaming in a small-scale device indicate that yields approximately double with well ventilated flaming of the same materials. The smoke yield from a wood crib, whose interior is underventilated, increases into the same range as for plastics.

Much discussion has focused on the trade-off between reduced combustibility and increased smoke yield when fire retardants are used in materials. These additives are often incorporated into materials to reduce the susceptibility of a material or product to small ignition sources. However, smoke yield measurements require that the test specimen burn and burn well enough to collect a smoke specimen.

When this type of treated material is forced to burn, the additive may be overwhelmed to such an extent that the fire retardant additive has little or no effect on the burning performance. In contrast, if the purpose of the fire retardant additive(s) is to reduce the mass burning rate or flame spread rate of a material, then the completeness of combustion in testing will likely be affected. Even so, if the consumed fuel mass is significantly reduced, then the overall mass of smoke is also reduced, and the smoke hazard is lessened. (See the Smoke and Heat Hazards chapter.)

Table 10-2 Soot Values from Small-Scale Burning of Fuels under Well-Ventilated Conditions

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Sources: Soot yield data from Reference [7], except as noted. Dm data from Reference [1], except as noted. a

Data from Reference [1].

b

Data from Reference [7].

c Data from Reference [8].

Researchers have had some success in finding correlations among the smoke yields from liquid fuels for pools of different diameters. To date, these efforts have not succeeded in relating the smoke yield data from small-scale tests to the yields from room-scale tests of finished products, such as chairs and wall coverings. The difficulties arise for the following reasons: •

In a realistic fire in a compartment, flaming combustion and pyrolysis often occur at the same time.

• Finished products often consist of multiple materials, and different parts of the product may burn at different times.

• The restricted ventilation and partial recirculation of combustion products to the flame in a compartment fire are difficult to simulate in a bench-scale test.

• Comparison of data from different laboratories (or even different test series from the same laboratory) often involves different materials, despite the same generic name.

Visibility through Smoke 171

When light encounters a particle whose diameter is comparable to the wavelength of the light, the light will be scattered at various angles from its original direction, including back-scattering toward the light source. Visible light has wavelengths ranging from 0.4 µm (blue light) to 0.8 µm (red light), and a large mass fraction of the fire smoke aerosols have diameters between 0.1 and 1 µm. This scattering effect causes objects viewed through smoke to appear blurry or indistinguishable Figure 10-7.

Figure 10-7 The light scattering effect of smoke, which reduces the ability to see people, obstacles, and signs.

Courtesy of Cpl. K. A. Thompson/U.S. Marines.

Visibility through smoke is often expressed with reference to the distance over which an exit sign can be seen.1 A sign that emits light is easier to see than one that must be viewed in reflected light.

Reference [1] provides the basis for equations for relating the distance, l (m), over which each of the two types of signs can be seen to the mass optical density, Dm (m2/g), of the smoke:

for a light-reflecting sign, and

for a light-emitting sign. In these formulas, m/V is the mass loss (g) of the combustible divided by the volume (m3) that contains a uniform mixture of the smoke. This type of calculation is based on the concept of minimum detectable contrast—that is, the minimum visible brightness difference between an object and a background—which is approximately 0.02 [9].

When we insert into Equation 10-7 an average Dm value of 0.2 m2/g (Figure 10-6) and a distance between the observer and the light-reflecting exit sign of 10 m, we obtain a mass density of smoke aerosol (m/V) of approximately 0.65 g/m3. At this mass density of smoke, people 10 m away would begin to have difficulty seeing a reflecting exit sign. For a light-emitting sign, this limiting mass density of smoke aerosol would be approximately 1.7 g/m3. To put this in perspective, if the floor area of a room were 14 m square and if the smoke layer had descended 1 m from the ceiling, the volume filled by the smoke would be 200 m3. If the yield of smoke aerosol were 0.1 g/g, and if all of the smoke aerosol stayed suspended in the air, this visibility threshold would be reached for light-reflecting signs when the

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fire had consumed approximately 1.3 kg (3 lb) of fuel. The fuel consumed to meet this visibility criterion would be smaller if the fire had become under-ventilated, because the smoke yield would be higher.

These formulas are useful for scoping the magnitude of allowable fire size and the design of fire intervention strategies. Nevertheless, they remain only approximations because of the following factors: • The variability of human vision.

• The effect of the varying background illumination level. Visibility of light-emitting exit signs in a smoke-filled room improves significantly when the ambient lighting is at a fairly low level, because higher levels of ambient light obscure signs due to light scattering.

• The frequent presence of eye irritants, such as acrolein or hydrogen chloride, in the smoke.

An important consideration for smoke obscuration is the level at which it seriously affects the ability of people to orient themselves and constructively identify a path to safety. At this level, a person cannot see even nearby objects, such as a hand in front of one’s own face, at a distance of only 0.5 m.

Repeating the previous calculation, but with this shorter perception distance, we find that a person can begin to lose perception of an object 0.5 m away at a smoke aerosol concentration of 13 g/m3. At this point, the fire would have consumed about 26 kg (60 lb) of fuel.

The first of these appraisals of two different smoke hazards is more conservative. The onset of the difficulty of sign perception occurs at a lower smoke density (0.65g/m3) than does the inability to orient oneself in one’s immediate vicinity (13 g/m3). Limiting a fire size or designing a smoke dispersion system to the lower smoke density while calculating building evacuation time using the higher smoke density allows for departures from the assumptions behind this simple equation—for example, pockets of smoke that are blacker than the average.

As in all such calculations, the calculated values include some level of uncertainty. Here, the uncertainty is plus or minus at least a factor of two, reflecting the following considerations: • • • •

Variation among measurements of smoke-yield data for a given material Differences in smoke yields for different materials Changes in the optical properties of the smoke as it cools and coagulates The extrapolation of the experimental findings to short distances has not been validated

When the fire involves a single material for which the aerosol yield has been measured under combustion conditions germane to that fire, this uncertainty is significantly smaller.

Seeing through smoke from even a small fire can be challenging. This difficulty can be overcome by making use of the principle that any object whose temperature is between ordinary room temperature and the temperatures encountered in fires both emits infrared radiation and reflects visible light (see the Heat Transfer chapter). Infrared radiation has a longer wavelength than visible light, including wavelengths that are large compared with the diameter of smoke particles. Hence, much less scattering occurs. Infrared-sensitive video cameras, which operate at wavelengths as long as 14 µm, are now available. When using these devices, emergency responders have increased ability to see through smoke, enabling them to locate flames, doorways, occupants needing rescue assistance, and fellow fire fighters.

Gaseous Combustion Products CO2 and H2O As discussed in the Physical and Chemical Change chapter, complete combustion of organic fuels results in all the carbon atoms oxidizing to form CO2 and all the hydrogen atoms reacting to form H2O. Complete combustion is rare in the real world. However, even if the flaming is less than complete, these two compounds account for most of the C and H atoms.

CO Almost all flaming fires of significant size are turbulent diffusion flames, so some of the CO will escape the flames before being oxidized fully to CO2. When most carbon-containing combustibles burn in a compartment with more than the stoichiometric requirement of air available (i.e., for a fire with a rate of heat release less than 300 kW in a small- to medium-size room), the CO yield is generally low. The ratio

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of the concentrations of CO and CO2 (denoted as [CO]/[CO2]) is typically less than 0.05 [10]. For underventilated fires, including post-flashover fires, where less air is available than the amount needed for complete combustion, [CO]/[CO2] is typically in the range 0.1 to 0.4, and the CO content of the smoke can be as high as 5 or 6 percent by volume. Even with a more adequate air supply, fuels burning with a very sooty, highly luminous flame (e.g., benzene, polystyrene, or polyvinyl chloride) can generate 2 percent by volume (20,000 µL/L) of CO in the smoke.

Enhanced volume fractions of CO may potentially be formed in some conditions. Near or after flashover, the upper gas layer in a fire compartment is highly deficient in oxygen and is hot enough to pyrolyze the lining materials on the upper walls and ceiling. If the lining materials consist of wood or another cellulosic material that contains its own oxygen, the pyrolysis will generate CO levels of as much as 14 percent by volume [11]. Note that the upper layer of the fire room may not be uniform in composition, and prediction of the specific contribution of pyrolysis-enhanced CO generation is beyond current capability Experiments and combustion modeling have shown that the post-flashover CO level depends on the following factors [11, 12, 13]: • The global equivalence ratio in the upper part of the compartment, which is the overall ratio of combustible vapor to air, relative to the stoichiometric ratio • The temperature of the hot layer • Any mixing of air into the hot layer • The surface area of any wood or cellulosic material in contact with the hot layer A default value for the yield of CO in a post-flashover fire is approximately 0.2 g CO per gram of consumed fuel [14].

In smoldering fires, the oxidation of the carbon-containing fuel takes place at the fuel surface.

Because no flames are present, the oxidation zone is very thin and cool relative to the flame temperatures. Little OH is present to oxidize the CO to CO2, so the [CO] to [CO2] concentration ratio can be as high as 1.

Partially Oxidized Organic Molecules Moderately ventilated and underventilated fires can generate as many as hundreds of organic molecules that are small enough to be volatilized. Some contribute to the odors that we associate with a fire; some of the larger molecules are carcinogenic. A few of the smaller molecules are toxic or irritating, as will be discussed in the Fire and Smoke Hazards chapter. Those organic irritant gases of most importance are two aldehydes: acrolein (H2C CH—HC O) and formaldehyde (H2C O).

Hydrogen Halides When fluorine, bromine, or chlorine atoms are present in a burning item, the corresponding halogen acid, HF, HBr, or HCl, respectively, is present in the smoke. Halogen atoms can be found in the polymer structure of the fuel (e.g., polytetrafluoroethylene, polyvinyl chloride) or in an additive (e.g., a flame retardant).

The halogen acids are formed in two ways. First, they may be generated during the pyrolysis stage.

The carbon–chlorine and carbon–bromine bonds are weaker than the carbon–hydrogen bonds, so these halogen atoms frequently are released ahead of the organic pyrolyzate. Sometimes they take a hydrogen atom with them, as in PVC decomposition. Otherwise, in the flame zone, they will find plenty of hydrogen atoms with which to bond. Second, hydrogen atoms attacking halogen-containing fragments can extract the halogen atoms, forming the halogen acid. As will be seen in the Firefighting Chemicals chapter, HCl and HBr are very efficient at weakening or quenching flames.

If the only component present in a burning item is a chlorine- or bromine-containing polymer or fire retardant, nearly all of the Cl or Br atoms will be found as HCl or HBr. However, many products often include filler materials, often added for their cost or physical properties. Calcium carbonate (CaCO3), for example, is added to vinyl (PVC) floor tile and electrical cable jacketing. As the HCl is being released from the PVC polymer, it encounters the CaCO3 and reacts rapidly, with the overall reaction being

CaCO3 + 2 HCl → CaCl2 + H2O + CO2 174

The result is that some of the chlorine atoms and HCl molecules never make it to the gas phase.

Some surfaces effectively remove HCl and HBr from smoke. Thus, as the smoke flows around the fire room and then into other parts of a building, the HCl or HBr concentration might be reduced by means other than dilution with fresh air.

The carbon–fluorine bond is the strongest single bond in organic chemistry, and fluoropolymers are thermally very stable. As the fluoropolymer pyrolyzes, the F atoms are not released until the polymer has undergone considerable breakdown. The hydrogen–fluorine bond is also very strong, so once HF is formed, it plays no further significant chemical role in the flame.

All three of these halogen acids readily dissolve in water. As indicated earlier, fires generate a large mass of water vapor. As the smoke cools, the vapor condenses to form water droplets. If halogen acids are present in the gas phase, they will dissolve in the water mist, creating a local version of acid rain.

HCN Hydrogen cyanide (HCN) can be formed during the pyrolysis and subsequent reaction of nitrogencontaining polymers. Unlike CO and the halogen acids, HCN is a trace product, representing only a small fraction of the possible nitrogen. The fraction of fuel nitrogen emerging as HCN depends on the nitrogen bonding environment within the polymer. For instance, ABS and modacrylic polymers contain acrylonitrile (H2C–CH–C≡N). The nitrile group (–C≡N) is also sometimes called a cyano group; it is chemically the same as the CN in HCN. Such polymers, therefore, tend to form HCN. Thus, one expects that such polymers would be more prone to HCN formation than, for example, a polymer in which the nitrogen is singly bonded to a carbon atom. In fires, little if any HCN is formed from fixation of nitrogen in the air. The N≡N triple bond is too stable to be broken at pyrolysis temperatures.

Nitrogen Oxides Nitric oxide (NO) and nitrogen dioxide (NO2) can form as the result of the oxidation of the nitrogen in the fuel or, at flame temperatures, the oxidation of nitrogen in the air. These are also trace species. NO2 can dissolve in water to form nitrous acid, HNO2, and then oxidize to form nitric acid, HNO3.

The nitrogen oxides form in the air-rich flame zones. HCN forms in regions where the oxygen is highly depleted. When HCN is transported to a hot, air-rich zone, it is oxidized to NO and NO2.

Other Combustion Gases To the extent that other types of atoms are present in the burning item(s), gases containing these atoms may be generated during combustion. For example, the presence of sulfur in wool or certain sulfurcontaining polymers can lead to the generation of H2S and SO2 in a fire.

Smoke Alarms Room fire tests have shown that flashover can occur within 4 minutes after flaming ignition of an upholstered chair or a mattress whose manufacture predated today’s low heat-release rate regulations.

Smoldering fires take longer to develop, but can transition unpredictably to flaming; thus, prompt notification of a building’s occupants of the presence of any type of fire is essential. Fortunately, residential smoke alarms have been widely available for more than three decades and are now installed in more than 90 percent of residences.

The response time of a smoke alarm is a function of the speed with which the smoke reaches it, the number and size of the particles generated by the fire, the sensitivity of the detection mode of the device, and the sensitivity setting. The proper placement of smoke alarms takes advantage of the buoyant flow of combustion products away from the location of the fire. The units should be located high on a wall or on a ceiling, where the natural flow of the fire effluent reaches them before being fully diluted with room air. The buoyancy from a smoldering source is quite small, so the transport to the smoke alarm site can be slow. Flaming fires generate stronger upward currents, so the smoke is likely to reach the alarm site sooner. The direction of any forced ventilation in the room will affect the transport of the smoke to the unit. This can be especially important if the smoke alarm is in an adjacent room and the smoke has to pass a door soffit to reach it.

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Nearly all currently installed residential smoke alarms react to the liquid and solid aerosols in the fire-generated flow. Some of that flow enters a chamber inside the alarm housing, where the aerosols are sensed in one of two manners.

In an ionization smoke alarm, a voltage is imposed across the sensing chamber Figure 10-8. A tiny quantity of a radioactive element, 241Am (americium-241) in the smoke alarm emits a steady number of alpha particles, which are the same as helium nuclei. The alpha particles ionize some of the nitrogen and oxygen molecules in the chamber; the positive ions flow to the cathode and the free electrons flow to the anode, creating a current flow that is measured continuously. When aerosols enter the chamber, some of the ions and electrons attach themselves to the particles or droplets. The velocity of a charged species in an electric field varies inversely with the species mass. The mass of an aerosol droplet or particle is far greater than that of a nitrogen or oxygen molecule, so the charged aerosol moves more slowly across the chamber. The result is a decrease in the current flow between the electrodes. When a preselected current decrease is sensed, the unit goes into alarm.

On one side of the chamber in a photoelectric smoke alarm is a light-emitting diode (LED), which sends a beam of light across the chamber Figure 10-9. Also facing the chamber is a photodetector that is directed at a right angle to the direction of the light beam. When the air in the chamber is clear, no light reaches the photodetector. When the aerosol enters the chamber, it scatters some of the light in the direction of the photodetector. The incident light causes the detector surface to generate an electrical current. When a preselected minimum current is sensed, the unit goes into alarm.

Both types of detectors can be designed to be exceedingly sensitive. However, to avoid frequent nuisance alarms caused by dust, cigarette smoke, cooking fumes, and consumer aerosols such as hairsprays or furniture polish, the detectors must respond only to fairly high concentrations of particles.

Photoelectric detectors are designed to respond to a smoke obscuration level of approximately 13 percent per meter for gray (cellulosic) smoke; ionization detectors are designed to respond to a level about half that. If both types of detectors are adjusted to have the same sensitivity for a certain size of particle, then the ionization detector will be more sensitive to particles smaller than this size, while the optical detector will be more sensitive to particles larger than this size.

Figure 10-8 Schematic of an ionization smoke alarm.

Figure 10-9 Schematic of a photoelectric smoke alarm.

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For wood or paper, flaming fires produce small particles (fine soot), while smoldering fires produce larger particles (aerosol droplets). Therefore, an ionization detector should respond sooner for a flaming cellulosic fire, and an optical detector should respond sooner for a smoldering fire. For plastics fires, no such simple rules apply, as this class includes a wide diversity of synthetic materials with varying smoke formation chemistry.

Flaming fires have higher mass burning rates, generating a more powerful buoyant plume and generating smoke at a higher rate than do smoldering fires. Therefore, smoke from a flaming fire will reach a smoke alarm sooner than the smoke from a smoldering fire. However, flaming fires grow to lifeand property-threatening size much faster than do smoldering fires. Thus alarm for a flaming fire needs to occur sooner than one for a smoldering fire.

No matter which type of smoke alarm is used, smoke can be detected well before the concentration at the detector has reached levels where visibility is seriously impaired or short-term toxic gas concentrations have been reached.

Some smoke alarms detect carbon monoxide (CO) rather than aerosols. These devices are primarily designed to protect against faulty heating equipment, cars left running in a garage, and similar scenarios. There are also two principal types of CO sensors: electrochemical and semiconductor electrical resistance.

In an electrochemical CO sensor, two electrodes are immersed in an electrolye, often a sulfuric acid solution. CO is oxidized to CO2 at one electrode. The resulting current flowing between the electrodes is proportional to the volume fraction of CO in the atmosphere. The detector can be constructed to alarm at a predetermined CO level or can quantify the amount of carbon monoxide that is present, perhaps using a digital readout.

The sensor in a semiconductor electrical resistance detector is made from thin wires of semiconducting tin dioxide that rest on an insulating ceramic base. The sensing element is heated to approximately 750 °F (400 °C). At this temperature, oxygen adsorbs on the surface of the tin dioxide and increases the wire’s resistance, while adsorbed CO reduces the resistance. An integrated circuit monitors the sensor and sounds the alarm at a preselected reduction in resistance.

Both types of detectors have lifetimes of five years or greater. The electrochemical cell has the advantage in that it’s output is proportional to CO concentration and that it requires minimal power, as it is operated at room temperature. Most CO alarms in the U.S. and Europe operate on this principle. The semiconductor alarms are more common in Asia, but are giving way to electrochemical units.

Combination smoke alarms have been available for several years. These devices house more than one type of sensor in the same housing.

Of interest is a new generation of smoke alarms that contain multiple sensors along with pattern recognition intelligence. For example, if a unit contains both photoelectric and carbon monoxide sensors, it might react to a smoldering fire, as that source generates both products. It might not react to condensed steam from a shower, which contains water aerosol but no CO, or to a passing cigarette smoker, as the aerosol and CO signals would quickly rise and fall, while a fire signal might only rise over the same time period.

WRAP-UP Chapter Summary •

Smoke, or fire effluent, consists of aerosols (soot particles and liquid droplets) and gases. Each of the smoke components, as well as the smoke itself, is characterized by its yield—that is, its mass per unit mass of fuel consumed.

• Soot consists of mostly carbon and results from underventilated burning. Aerosol droplets result from condensation of gases that cool as they leave the vicinity of the flames. The yields can be determined by collecting and weighing the condensed matter or by measuring the scattering of a light beam. Soot yields typically range from 0.01 to 0.2 g/g.

• For a gaseous fuel, the relative tendency to form soot can be determined using the smoke-point height method. For any type of fuel, sooting is promoted by a fuel that contains halogen atoms and some aromatic bonding; it is reduced by oxygen in the chemical structure of the fuel.

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• Radiative transfer from the soot from flames and from the hot upper layer in the fire room determines the rate at which a fuel burns and the likelihood of ignition of additional combustibles that are not adjacent to the already burning item(s).

• Both soot and aerosols reduce the ability to see in a fire. The two common metrics for smoke obscuration are the distance over which an exit sign can be seen and the level at which people cannot orient themselves and constructively identify a path to safety.

• Soot and aerosols are the principal fire signature detected by current residential smoke alarms.

Ionization smoke alarms measure the decrease in the transport of charged particles due to the presence of the soot and aerosols. Photoelectric smoke alarms measure the light scattered by the soot and aerosols.

• The principal gaseous products from fires are H2O and CO2. Incomplete combustion generates CO and a mix of partially oxidized organic molecules. Nitrogen-containing fuels generate HCN, NO, and NO2. Halogen-containing fuels generate hydrogen halides.

Key Terms aerosol mist Fine solid particles or liquid droplets dispersed in air or another gas.

Bouguer’s law (Beer-Lambert law) A mathematical relationship describing the decrease of intensity of a beam of light as it passes through a semitransparent medium, such as smoke. coagulation The adhesion of particles to form larger particles, generally due to forces weaker than chemical bonding. electrochemical CO sensor A device that measures the volume fraction of carbon monoxide in the environment by the current the gas induces in a galvanic cell. extinction coefficient A parameter, obtained using Bouguer’s law, that defines how strongly a substance absorbs or scatters light at a given wavelength or over a given wavelength range. fire effluent Smoke from a fire. global equivalence ratio In a volume or flow, the overall ratio of fuel vapor to air, relative to the stoichiometric ratio for the same fuel. ionization smoke alarm A device that detects fire-generated aerosols by their transport of electric charge. optical density The logarithmic ratio of the radiation incident on a material to the radiation transmitted through the material. photoelectric smoke alarm A device that detects fire-generated aerosols by their scattering of light. semiconductor electrical resistance CO detector A device that detects carbon monoxide by the change in electrical resistance resulting from the gas being adsorbed on a semiconductor. smoke The airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, together with the quantity of air that is entrained or otherwise mixed into the mass. smoke-point height For a combustible gas or vapor, the height of the shortest laminar diffusion flame that will just release black smoke from its tip. soot Carbonaceous particles resulting from the incomplete combustion of organic fuels. yield The mass of smoke or a component of the smoke per mass of fuel combusted or pyrolyzed.

Challenging Questions 1.

Smoke is sometimes black and sometimes white in appearance. Explain the reason for this difference.

2. Why is an exit sign in a smoke-filled room less visible when the ceiling light is more intense?

3. Of what use in firefighting is an infrared video camera?

4. What is the relationship between the particle size of smoke and the sensitivity of a smoke alarm?

5. Which factors determine the response time of a smoke alarm?

6. Why are fire flames usually yellow or orange?

7. Two chairs burn at the same mass burning rate. Chair A emits twice as much smoke mass as chair B, and the length of the burn room is half that of the room in which chair B is burning. A person is

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standing in the middle of each room. For which chair is sign visibility compromised first?

8. In which fire stages (see the Fire and Flammability chapter) would you expect to find emissions of HCl? of HCN? of large quantities of CO?

References 1. Mulholland, G. W. (2008). Smoke Production and Properties. In: The SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

2.

Schug, K. P., Y. Manheimer-Timnat, P. Yaccarino, and I. Glassman. (1980). “Sooting Behavior or Gaseous Hydrocarbon Diffusion Flames and the Influence of Additives.” Combustion Science and Technology 22: 235.

3. Hunt, R. A. (1953). “Relation of Smoke Point to Molecular Structure.” Industrial & Engineering Chemistry 45: 602–606.

4. Markstein, G. H. (1985). “Relationship between Smoke Point and Radiant Emission from Buoyant Turbulent and Laminar Diffusion Flames.” Proceedings of the Combustion Institute 20: 1055–1061.

5. Tewarson, A. (1988). Generation of Heat and Chemical Compounds in Fires. In: The SFPE Handbook of Fire Protection Engineering, 1st ed. Quincy, MA: National Fire Protection Association.

6. Tewarson, A. (2008). Generation of Heat and Gaseous, Liquid, and Solid Products in Fires. In: The SFPE Handbook of Fire Protection Engineering, 4th ed. Quincy, MA: National Fire Protection Association.

7. Mulholland, G. W., V. Henzel, V., and V. Babrauskas. (1989). The Effect of Scale on Smoke Emission. In: Fire Safety Science: Proceedings of the Second International Symposium. New York, NY: Hemisphere Publishing, 347–387.

8. Drysdale, D. (2011). An Introduction to Fire Dynamics, 3rd ed. New York, NY: John Wiley, 360.

9. Friedlander, S. (1977). Smoke, Dust, and Haze. New York, NY: John Wiley, 143.

10. ISO 19706: Guidelines for Assessing the Fire Threat to People. (2011). Geneva, Switzerland: International Standards Organization.

11. Pitts, W. M. (1995). “The Global Equivalence Ratio Concept and the Formation Mechanisms of Carbon Monoxide in Enclosure Fires.” Progress in Energy and Combustion Science 31: 197–237.

12. Gottuk, D. T., R. J. Roby, and C. L. Beyler. (1995). “The Role of Temperature of Carbon Monoxide Production in Compartment Fires.” Fire Safety Journal 24: 315–331.

13. Pitts, W. M. (1997). An Algorithm for Estimating Carbon Monoxide Formation in Enclosure Fires. In: Proceedings of the Fifth International Symposium. International Association for Fire Safety Science: 535–546. (Available from Society of Fire Protection Engineers, 7315 Wisconsin Ave, Bethesda, MD 20814.)

14. NFPA 269: Standard Test Method for Developing Toxic Potency Data for Use in Fire Hazard Modeling. (2012).

Quincy, MA: National Fire Protection Association.

1

In the United States, exit signs are typically located at eye level or higher. This puts them within the highest concentration of smoke. In Japan, the exit signs are placed near the floor, making them easier to see during a fire.

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CHAPTER 11

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Smoke and Heat Hazards OBJECTIVES After studying this chapter, you should be able to: • List the hazards to people and property from a fire.

• Explain the following types of harm from a fire: acute effects, postexposure effects, and chronic effects.

• List the most important toxic gases in smoke.

• Explain the differences between narcotic gases and irritant gases.

• Explain the concept of fractional effective dose.

• Explain the underlying principle of Haber’s rule.

• Explain the concept of limiting hazard and its role in fire protection.

Introduction Shortly after 7:00 A.M. on November 21, 1980, a small fire broke out in one of the restaurants on the casino floor of the 26-story MGM Grand Hotel in Las Vegas. By the time the fire was under control, around 10:00 A.M., 85 people had died and more than 700 were injured. Although the fire never extended above the casino level, victims were located as high as the 25th floor. The smoke had spread with minimal dilution up the elevator shafts and other vertical pathways, exposing occupants of the upper floors. Seventy-five of the fatalities were from smoke inhalation.

This outcome is indicative of the profile of causes of fire deaths in the United States. From 1970 to 1985, 41 percent of the fire deaths in structures were caused by smoke inhalation, and another 46 percent were caused by a combination of smoke inhalation and burns. Eighty percent of the fatalities resulted from fires that spread beyond the room of origin, and the majority of the victims of those post-flashover fatal fires were outside the area of origin when the fire began. Nearly all of these remote victims of post-flashover fires died from smoke inhalation—either smoke inhalation only (54 percent) or smoke inhalation with burns (36 percent)—as opposed to burns only (2 percent) or other causes (8 percent) [1].

These data are consistent with two key points made in the preceding chapters: (1) post-flashover fires are underventilated, and (2) underventilated fires serve as the source of products of incomplete combustion. This section relates the products of incomplete combustion to the toxic potency of the fire-generated atmosphere.

Hazards of Smoke Exposure If an occupant of a building is confronted with a fire, multiple levels of harm might result from exposure to the smoke [2]. At the highest level of exposure, the occupant might perish while still in the building.

Somewhat lower exposures might have incapacitating effects that hinder escape capability and make it unlikely that a person would survive without assistance. Sub-incapacitating effects include impaired mobility and other physical capabilities, reduced clarity of thinking, and detrimental behavior. Any of these sublethal effects might lead to injury or death due to lengthened immersion in the smoke.

These acute effects are experienced at the time of the fire and result from smoke exposure in a single fire. Additional effects might be experienced well after leaving the fire scene. For building occupants, postexposure effects can result from a single smoke exposure; for fire fighters, chronic effects can result from accumulated damage from multiple exposures.

Both forensic investigations and laboratory studies have enhanced our knowledge of the smoke components that lead to these casualties. The most thorough forensic program was based on autopsies of all fire victims in the State of Maryland over several years in the 1970s [3]. Of the 530 victims, approximately 60 percent had more than half of the hemoglobin in their blood converted to carboxyhemoglobin (COHb) as a consequence of having inhaled CO shortly before dying. CO binds to hemoglobin approximately 200 times more effectively than does O2, so its presence reduces the blood’s capacity for transporting oxygen throughout the body. In experiments in which test animals were exposed to carbon monoxide, conversion of more than about half the hemoglobin to COHb caused incapacitation and usually death. An additional 20 percent of the Maryland fire victims had a somewhat

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lower COHb level, but also had preexisting cardiovascular disease, suggesting that they were vulnerable to lower exposures of carbon monoxide than healthy people. Thus 80 percent of the victims could have been killed by carbon monoxide; of the remaining 20 percent, 11 percent died from burns, and the cause of death was not established for 9 percent.

Hydrogen cyanide (HCN) can be generated from burning materials that contain nitrogen in the chemical structure. High levels of blood cyanide were found in the blood of a number of victims in the Maryland study, and HCN inhalation was suspected to be a cause or contributor to death in those cases. Autopsies performed in subsequent investigations of fires in Pennsylvania [4] and Puerto Rico [5] found that hydrogen cyanide inhalation was the primary cause of death for some of the 136 fire victims.

There are no standard procedures for quantifying the presence of other inhaled toxic gases in the deceased. Hence, there are no data on these smoke components from these or any other fire victim studies.

Broader knowledge of the smoke components that contribute to fire deaths has been gathered through laboratory studies that have identified the gases generated during the flaming, pyrolysis, or smoldering of materials and products. These results enabled construction of extensive lists of potentially harmful gases, although two important limitations apply when using these lists to assess the toxic potency of a fire environment: •

The completeness of the lists is unknown, as different chemical analysis techniques were used for different groups of chemicals, and the chemical methods were chosen based on expectations of the nature of the combustion products. As a consequence, an unexpected, highly toxic chemical might not have been found.

• These studies did not provide a basis for quantifying the toxicological effects of any one of the identified gases or of combinations of those gases. A mixture of components might be much more or less toxic than the summed potency of the individual gases.

This line of research was complemented by bioassays, in which laboratory animals were exposed to single gases, combinations of gases, or smoke from both pyrolysis and combustion, providing extensive quantitative toxicological data [6, 7]. In these test series, the animals were exposed for different time intervals to different concentration levels. The data were reduced to determine the EC50, the concentration (C) that led to an effect (E) in 50 percent of the test subjects. In nearly all the studies, the effect was lethality (LC50) or one of several measures of incapacitation (IC50). Ideally, the animal species were selected because of the similarity of their toxicological response to humans’ response.

However, the high cost of such testing using monkeys, baboons, or dogs led to many of the experiments being performed using carefully bred rodents, in order to reduce variability among the test subjects. In any case, the quantitative extrapolation of the data to humans introduced some degree of uncertainty. No new animal smoke exposure studies have been reported for more than two decades.

Based on the various research studies, we can identify some general precepts that are fundamental to fire toxicology [6]: 1. The toxic gases can be grouped into three classes: a. Asphyxiants (or narcotics). Upon inhalation, asphyxiant gases, also called narcotic gases, deprive the body of oxygen, either by interfering with oxygen’s transport to cells or by reducing the cells’ ability to use the oxygen. The two most important asphyxiants are CO and HCN. b. Irritants. Sensory irritant gases (or, simply, irritant gases) that affect the senses (e.g., eye irritation) and upper respiratory tract tend to influence physiology and behavior during exposure. Those that affect the lower pulmonary tract (lungs) tend to have later effects. The most important irritant gases are the halogen acids and some partially oxidized organic gases, such as acrolein and formaldehyde. Some sulfur- and phosphorus-containing compounds may contribute if the combustibles contain these atoms. c. Others. In a few instances, the smoke from a small material specimen generated extremely toxic smoke or an unusual toxic effect on laboratory animals. In each case, the offensive combustion product was identified by specialized chemical analysis. There was no commonality of chemical structure of the materials or the toxicants.

2. Within each of the first two groups, the effects of the gases can be combined.

3. The asphyxiants’ effects are dose related; that is, the effect results from the concentration of the inhaled gases and the time interval over which they are inhaled. A person moving through a

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smoky building accumulates a fractional effective dose (FED) during each time increment, such as 0.25 minute [8]. These values are summed, and when the FED reaches 1, the average person experiences the effect (e.g., death, incapacitation). Haber’s rule is often used to relate these two components: the time and intensity of exposure. It states that, for a given toxicological effect, the product of the volume fraction and the exposure time is a constant. While this tool gives a fair estimation for individual asphyxiants, it is less accurate for combinations of such gases.

4. The sensory effects of the irritant gases are concentration related; that is, they occur very quickly.

As a person experiences successively higher concentrations of an irritant gas, the fractional effective concentration (FEC) increases, with the average person experiencing the effect at FEC = 1.

5. Despite the large number of combustion-generated, potentially toxic chemicals, the deaths of rats exposed to fire smoke can be estimated from the contributions of only a few (N) gases. In the resulting N-gas model, N has been found experimentally to be no larger than about 7 for a wide variety of materials [7]. The N-gas model is also applied to lethal and incapacitating effects of smoke on people.

6. The smoke exposure that is lethal is approximately two to three times the incapacitating exposure [9, 10].

There are thresholds of both exposure time and concentration below which no noticeable effect occurs.

Experts have compiled the following equations for the incapacitation due to inhalation of gases [11]: • Narcotic gases:

where [CO] is the average volume fraction (µL/L) of CO during the time increment, Δ t; [HCN] is the average volume fraction (µL/L) of HCN during the time increment, Δ t; and Δt is the time increment (min). At each time increment, the two terms on the right-hand side of Equation 11-1 are multiplied by a frequency factor, vCO2, to allow for the increased breathing rate (and thus the increased uptake of the narcotic gases) caused by the presence of CO2:

where [CO2] is the average volume percent of CO2 during the time increment.

• Irritant gases:

where [Z] is the volume fraction of an irritant gas, Z, (µL/L), and FZ is the volume fraction of that gas (µL/L) expected to lead to an FEC value of unity. The best-judgment approximate values for FZ for some irritant gases are provided here: HCl or HBr: 1000 µL/L HF: 500 µL/L NO2 or formaldehyde: 250 µL/L SO2: 150 µL/L Acrolein: 30 µL/L For example, a person exposed to 1000 µL/L of a mixture of HBr in air would immediately experience severe eye irritation and throat constriction that would render the person unable to effect his or her own escape. The same outcome would occur with a mixture of 125 µL/L of NO2 and 250 µL/L of HF. The following sections provide additional information for specific toxicants.

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Toxicity of Prominent Fire Gases Carbon Monoxide Carbon monoxide (CO) is a narcotic toxicant common to nearly all fire smoke. It often makes the major contribution to an incapacitating or lethal exposure. As with all smoke, the concentration in the upper layer of the room is particularly important because this gas mixture emanates from the room as the temperature rises and the gas expands. As noted earlier, most fire deaths occur outside the room of fire origin, and people outside the fire room may be exposed to this flow, albeit perhaps in a diluted version.

Table 11-1 shows the symptoms that develop in a healthy person as the percentage of COHb increases. People who are more susceptible will experience symptoms at lower COHb levels; people who are less susceptible will experience symptoms at higher COHb levels.

Equation 11-1 relates the CO volume fraction in the inhaled air to the exposure time that would lead to incapacitation of the average person. Other equations [6] relate the percent COHb in the blood to the volume fraction of the inhaled air and personal variables, such as a person’s breathing rate, body mass, and activity level. Figure 11-1 shows the exposure times required to incapacitate a 154 lb (70 kg) person performing different levels of activity [6]. CO appears to follow Haber’s rule over most of the volume fraction and time ranges, although asymptotes are visible at the extremes of curves B and C in Figure 11-1. Low volume fractions of CO (less than a few hundred µL/L) apparently would not be incapacitating for common exposure times to fire smoke. At the very high CO volume percentages, exposures of less than two minutes would result in lethal COHb levels.

Table 11-1 Symptoms as the Percentage of Hemoglobin Converted to Carboxyhemoglobin (COHb) Increases in the Human Bloodstream [12] Percent by Volume of Carboxyhemoglobin

Symptoms

10

Judgment inefficiencies

10–20

Slight headache

20–30

Headache, fatigue, dizziness

30–40

Severe headache, weakness, dizziness, confusion, vision dimness, nausea, vomiting and collapse

40–50

Death for some

50–60

Coma, death for most

60–80

Death for all within a few hours

80–90

Death within an hour

>90

Death within a few minutes

Data from: ISO 27638-11, “Analysis of Blood for Asphyxiant Toxicants – Carbon Monoxide and Hydrogen Cyanide,” International Standards Organization, Geneva, 2011.

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Figure 11-1 Relationships between the volume percent of CO and the time to incapacitation by carbon monoxide for a 70 kg (154 lb) human at different levels of activity [6].

Curve A: Person at rest, 40 percent COHb, breathing rate of 8.5 L/min Curve B: Light work (e.g., walking at 6.4 km/h, 4 mph), 30 percent COHb, breathing rate of 50 L/min Curve C: Heavy work (e.g., slow running at 8.5 km/h, 5.3 mph, or walking up a 17 percent incline at 5.6 km/h, 3.5 mph) Reproduced from: Purser, D. A. (2008). Assessment of Hazards to Occupants from Smoke, Toxic Gases, and Heat. In: The SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Carbon Dioxide Carbon dioxide (CO2) is generated in all fires involving organic materials. Because most of the oxygen depleted from the air during a fire is bound into CO2, an estimate of the volume fraction of CO2 in the fire effluent can be obtained by subtracting the volume fraction of oxygen in the smoke from 0.21. For severely underventilated burning or for fuels that contain large amounts of oxygen (i.e., significant numbers of oxygen atoms), this estimate can be adjusted using the balanced chemical reaction equation.

Up to concentrations of 5 percent by volume, CO2 is not toxic [6]. Instead, it mainly increases the respiration rate (Equation 11-2) and dilutes the toxicants and oxygen in the air. As the CO2 concentration approaches 10 percent, however, a person would experience dizziness and loss of consciousness.

As described later in this chapter, in a residential fire room, the temperature and heat exposure would reach untenable levels before the CO2 concentration reaches 5 percent. Post flashover, the CO2 concentration in the smoke flowing from the fire room could exceed 10 percent by volume; however, this flow is also untenably hot. By the time that the smoke temperature has cooled below 100 °C, the

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CO2 concentration will generally be less than 5 percent due to dilution of the effluent by air outside the fire room.

Hydrogen Cyanide The lethal toxic potency of hydrogen cyanide (HCN) is approximately 20 times that of CO. Fortunately, the maximum measured HCN volume fractions in most smoke are lower than CO volume fractions by at least the same factor. Like CO, HCN affects the amount of oxygen made available to cells in the body and especially the brain. However, its mechanism in doing so differs from that used by CO. HCN does not interfere with the hemoglobin transport of the oxygen. Rather, after entering the bloodstream through the lungs, HCN combines with the enzymes in the cells and inactivates them, so the cells can no longer accept oxygen. Table 11-2 summarizes the effects of acute exposure to HCN. Equation 11-1 is a best fit to data for the exposure of monkeys to various concentrations of HCN in air. This equation is deemed reasonably appropriate for simulating the susceptibility of people performing light activity, such as a brisk walk. Note that the exponent of 2.36 means that HCN does not follow Haber’s rule [6]. Equation 11-1 indicates that the average person would be incapacitated from a 10-min exposure to 130 µL/L of HCN in air and from a 3-min exposure to 220 µL/L of HCN in air. The data reveal that a threshold effect occurs: exposure to HCN concentrations below 80 µL/L for up to an hour results in only minor hyperventilation.

Hydrogen Chloride and Hydrogen Bromide The toxic potencies of hydrogen chloride (HCl) and hydrogen bromide (HBr) are approximately equal.

Researchers have published far more data for HCl, so the following discussion focuses on these data.

When laboratory animals, or people, are exposed to HCl at very low volume fractions, an immediate irritant reaction occurs in the sensitive areas of the eyes, nose, and throat. At higher volume fractions, the irritation becomes painful, impairs escape, and eventually becomes incapacitating. For high doses (volume fraction multiplied by duration), an insult to the lower respiratory tract can result in long-term harm and even death. This lower respiratory tract damage can be enhanced by HCl adsorbed on small soot particles or dissolved in small liquid droplets in smoke. Upon inhalation, these submicrometer aerosols bypass the body’s defenses in the upper respiratory tract and reach the lungs unabated. The magnitude of this enhancement has not been measured.

Table 11-2 Symptoms as the Concentration of Blood Cyanide Increases in the Human Bloodstream [12] Blood Cyanide (µg/mL)

Symptoms

0.5–1.0

Conscious, but flushed skin, rapid pulse, and headache

1.0–2.5

Stuporous, but responsive to stimuli, excessively rapid breathing and heart rate

≥ 2.5

Comatose, blue skin tone, gasping for breath, death

Data from: ISO 16312-1, “Guidance for Assessing the Validity of Physical Fire Models for Obtaining Fire Effluent Toxicity Data for Fire Hazard and Risk Assessment. Part 1: Criteria,” International Standards Oragnization, Geneva, 2011.

The data are inconsistent regarding the volume fraction of HCl that results in almost immediate incapacitation of the average person. The consensus among experts is that this level approximates 1000 µL/L [10]. Noticeable impairment of escape is expected at approximately one-fifth this volume fraction [6].

Nitrogen Oxides Oxidation of the nitrogen in the burning products leads to the formation of nitrogen dioxide (NO2) and nitric oxide (NO). NO2 is the favored combustion product, and its toxic potency is approximately five times that of NO. NO2 is a strong irritant with an estimated incapacitating volume fraction of 250 µL/L [11].

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Organic Irritants Among the small, partially oxidized organic molecules that act as strong irritants (formaldehyde, acetaldehyde, and acrolein), acrolein (CH2 CH—CHO) generates notable concern, because the toxicological literature indicates that exposure to volume fractions as low as 10 µL/L could be lethal to people. However, baboons exposed to 500 µL/L of this gas for 5 min were able to perform a task and survived afterward [13]. Equation 11-2 indicates an incapacitating exposure of 30 µL/L of acrolein, but in light of the baboon experiments, this may well be excessively conservative. This discordance may be unimportant, because toxicologically significant concentrations of acrolein have not been quantified in the smoke from burning or smoldering materials.

Other Toxic Species Many other gases occasionally found in smoke are believed to be harmful. These gases, which rarely appear in significant concentrations except when special combustibles are involved in the fire, include ammonia (NH3), sulfur dioxide (SO2), hydrogen sulfide (H2S), hydrogen fluoride (HF), isocyanates, and phosphorus compounds. A very few compounds whose toxic potency is far higher than that of the compounds discussed here are labeled as “supertoxicants.” Reference [6] offers additional information on these and other gases.

Oxygen Deficiency Even if only carbon dioxide, water vapor, and nitrogen were present in the air inhaled by a fire victim, the oxygen concentration would be reduced below the normal 21 percent by volume. As the percentage drops toward 14 percent, the effects of oxygen deficiency appear, including lethargy, impairment of coordination, and nausea. Judgment, memory, and work capacity decline as the oxygen concentration drops toward 10 percent. All these effects are heightened when the change in oxygen concentration occurs suddenly—the typical case in a fire. In an environment containing less than 10 percent oxygen, people rapidly lose consciousness and will die if not revived quickly.

As implied in the discussion of CO2, fire fighters and building occupants who are enveloped by the undiluted smoke flow from a flaming fire might be exposed to oxygen concentrations in the lethal range, but this flow is also untenably hot and rich in toxic gases. By the time that dilution has cooled the smoke to a temperature less than 50 °C, the O2 concentration will generally be greater than 15 percent. In some instances, a reduced O2 level is not accompanied by a highly elevated temperature. A person who is asleep in a bed or on an upholstered chair that is smoldering might inhale smoke that is only warm, but contains significantly diminished oxygen compared to normal air.

Inasmuch as two of the principal toxicants in smoke, CO and HCN, act to deprive the brain of oxygen, their effects would be enhanced if the oxygen level in the air fell significantly below 21 percent [4]. These effects can be reversed by immediately administering oxygen to persons rescued from fires (Figure 11-2).

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Figure 11-2 Fire fighter administering oxygen to a fire victim. © Patti McConville/Alamy Images.

Smoke Toxic Potency Measurement There is insufficient time and laboratory capacity to burn all commercial products under the multiple possible fire conditions and quantify the toxic potency of the smoke. Therefore, over the years, a number of test methods had been developed as means to obtain such information more quickly and less expensively [14]. The apparatus in these methods, also known as physical fire models, differ markedly in their combustion conditions, test specimen size and configuration, and products or materials tested. They also differ in their gas sampling and measurement methods and in how (and whether) laboratory animals are exposed to the smoke.

These test methods were intended for use as stand-alone indicators of the acceptability of a material. A low LC50 value or a high concentration of one or more toxic gases would make a material unacceptable for use. However, the results obtained from different test procedures do not always agree.

For example, one test procedure showed the LC50 for red oak to be 3.4 times lower than that for polystyrene, while the polystyrene LC50 was deemed 1.7 times lower than the value for red oak by another test procedure [15].

It is now recognized that a fire hazard or risk assessment is the most technically sound basis for specification of a product’s acceptability. In addition to toxic potency values for the potential combustibles, this assessment takes the following information into account: • • • • • • •

Mass and burning properties of the combustibles Room/building properties, including dimensions and locations of doors and windows Nature of the building (e.g., apartment, hospital) Nature of the occupants, such as age and mobility Locations of the occupants Types of potential fires Outcomes to be avoided (e.g., deaths, serious injuries, loss of building function)

A standard that identifies the principles for describing and evaluating toxic potency measurement methods, including those that do not involve exposing test animals has been established [16]. These

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principles are classified into seven categories: 1. The apparatus replicating one or more fire stages: well-ventilated burning, vitiated burning, postflashover burning, or smoldering 2. The test specimen being representative of the product being evaluated 3. If test animals are to be exposed to the smoke, statement of the choice of physiological effect to be measured and documentation of the relationship between the animals’ response and the anticipated human response 4. Analysis of the combustion or pyrolysis products that are known to be generally produced and those that might be specific to the product being investigated 5. Measurement of the mass loss of the test specimen 6. Determination of the accuracy of the toxic species yields and the resulting estimate of smoke toxic potency, properly by comparison with a set of real-scale tests of the same products 7. Determination and documentation of the within-laboratory repeatability and the inter-laboratory reproducibility of the test results A companion document applies these principles to a dozen currently cited methods [17]. Most of the apparatus have been used to test materials, rather than products.

Since there is a very large number of different products and materials in our residences, and since a single fire usually involves multiple products, it would be a useful simplification if nearly all combustibles could be represented by a single LC50 or IC50 value. Products whose smoke is extremely toxic could be treated as exceptions. Table 11-3 summarizes the results of a 2004 compilation of published toxic potency data. At that time, these were all the published results for animal exposure tests that generated an EC50 and for which the combustion conditions in the apparatus could be related to a single fire stage [10]. Only animal data were used, as these were the only tests in which it could be assumed that the effects of all the toxicants were included; all the test animals were rats or mice, and their exposure to the smoke was for nominally 30 min. The materials and products included materials/products whose chemical structures were aromatic and nonaromatic, and some that contained atoms other than C, H, and O. The numbers in parentheses indicate the number of materials and products for which data had been reported, followed by the number of specimens whose test results were at least 30 percent above the average value and the number that were at least 30 percent below the average value. (The estimated uncertainty in the EC50 for an individual material was estimated to be ±30 percent.)

Examination of these results reveals the following points: • The mean EC50 values are not very sensitive to the mode of combustion.

• The dose that leads to incapacitation is approximately one-third to one-half the lethal dose.

• For many specimens, their EC50 values lie outside the experimental range of uncertainty. (The

individual values ranged from as low as 1 g/m3 to as high as 100 g/m3.) Recalling that low EC50 values represent smoke of higher toxic potency, the data indicate that the smoke toxic potency for some materials can be more than 10 times that of the average material. If a significant mass of one of these materials became involved in a fire, it would be prudent to use caution in assessing the tenability using an average EC50 value.

Some engineering calculations have assumed that CO was the only toxicant and based the survivability time on this premise. The average LC50 values from Table 11-3 are comparable to the value for a post-flashover atmosphere in which the only harmful gases are CO and CO2. Thus, the large numbers of materials with lower LC50 values reflect materials containing other toxicants and/or unusually larger amounts of CO. The CO-only assumption, then, is a practical starting point but has clear limitations.

Table 11-3 Mean LC50 and IC50 Values (g/m3) [10] Fire Stage

LC50

IC50

Well-ventilated flaming

30.4 (101; 22, 15)

11.2 (51; 18, 26)

Underventilated flaming

25.8 (10; 4, 0)

—

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Oxidative pyrolysis

27.8 (87; 18, 19)

11.5 (53; 14, 18)

Data from: ISO 13571, “Life-threatening Components of Fire – Guidelines for the Estimation of Time to Compromised Tenability in Fires,” International Standards Organization, Geneva, (2012).

ISO 13571 provides an alternative to the CO-only hypothesis for engineering calculations in which the fuel chemistry is unknown [11]. In these cases, a “generic” LCt50 value may be used—that is, 900

g/m−3·min for well-ventilated, pre-flashover fires and 450 g/m−3·min for vitiated, post-flashover fires. For incapacitation, the ICt50 values would be 450 g/m3·min for well-ventilated, pre-flashover fires and 220

g/m3·min for vitiated, post-flashover fires. The data in Table 11-3 indicate that this simplification has limitations similar to those for the CO-only assumption.

These values (and all those mentioned previously with a subscript “50”) correspond to conditions that would affect the average animal or person. Of course, some people may be more susceptible or less susceptible to the airborne toxicants. Designing a facility using these average values would put half the occupants at risk in the event of a fire. It is common practice to apply a safety factor in such situations. For smoke hazard calculations, one might use an ICt50 value that is one-third of the generic value; however, a more careful rationale is warranted. The factor of 3 might be too restrictive (i.e., it might rule out some otherwise desirable products) in a building in which the occupants are predominantly young, healthy adults. Conversely, the same factor of 3 might be insufficient for a building in which the occupants are expected to be more susceptible to airborne contaminants, such as a preschool or an assisted-living facility.

Thus far, the discussion has focused on the acute effects of fire smoke—that is, the effects that happen while a person is still in the burning building or shortly upon escape or rescue. Longer term effects, both on people and on the environment, are also possible. One such threat, mentioned earlier, is pulmonary damage from a high exposure to irritant gases.

Some combustibles either are carcinogenic (cancer-causing) compounds themselves or can generate carcinogenic compounds under fire conditions. These compounds can be transported (as part of the smoke) to remote parts of the building and to the outdoors. Indoors, their presence can lead to an extensive decontamination process of the entire building, which could delay reoccupation of the building for years. Outdoors, their effect can be widely spread if they enter the food chain.

As an example, consider the polychlorinated biphenyls (PCBs) formerly used as electrical insulating fluids. Figure 11-3 shows the structure of a PCB, which contains five chlorine atoms per molecule. The number and locations of chlorine atoms in the molecules can vary.

PCBs have been found to be toxic, and their manufacture was banned in 1979. Nevertheless, they may persist in older electronics, lighting fixtures, electric power transformers, and capacitor banks. Of even greater concern than the toxicity of PCBs is their possible thermal transformation, in part, to the much more toxic chemical class known as chlorinated dioxins, an example of which is also shown in Figure 11-3.

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Figure 11-3 Structure of a pentachlorobiphenyl and a chlorinated dioxin.

Nonthermal Smoke Damage Fire smoke is not only dangerous to people; it can also impair the functioning of critical equipment. This type of damage can be caused by a relatively small, localized fire, whose smoke spreads to distant points in the building.

This kind of damage from fire smoke often leads to effects that are chemical in nature. The active chemicals include organic acids (e.g., formic acid, acetic acid) and inorganic acids (e.g., HCl, HNO3) generated from the building materials and contents. HCl can penetrate porous structural concrete and attack its steel reinforcing rods. The subsequent slow corrosion can lead to structural weakening at a later time. Extinguishing agents, which may not be corrosive themselves, can generate corrosive byproducts (discussed in the Fire fighting Chemicals chapter). For example, when applied to a fire, the halogenated fire suppressants partially decompose to form HCl, HBr, and/or HF. Some dry-powder agents contain corrosive salts. Water can cause damage in some cases (e.g., to books or documents), although vacuum-drying procedures have been developed to salvage most items. Smoke moving through a building will be absorbed by many textiles and will impart an odor to these materials.

Sometimes these odors can be removed by washing or dry cleaning; however, the chemistry associated with residual odors is not well understood.

Perhaps the most important category of nonthermal smoke damage is to electronic equipment, such as computers, telephone switchgear, manufacturing plant control rooms, and the electronic controls on everything from smoke alarms to microwave ovens. The most obvious type of damage is corrosion—the slow oxidation of metal exposed to air—which can be accelerated by a substance in the smoke, often an acid. For example, HCl in smoke will attack most metals to form the metal chloride, which will then promote (catalyze) further attack of the metal. The presence of moisture or high relative humidity (greater than 40 percent) generally is necessary for rapid corrosion to occur. A careful examination of the surface of a metal exposed to air under normal (nonfire) conditions often will show a chloride deposit of as much as 10 mg/m². This amount usually is not harmful. However, after exposure to smoke from a fire involving polyvinyl chloride, surface contamination of as much as thousands of mg/m² has been found, which can lead to significant damage.

Electronic equipment also can be rendered non-operative because of a short caused by conductive soot particles, which can bridge a gap between conductors in the circuit. Smoke aerosol deposits on electric contacts (as in connectors or relays, where protective plastic coatings are not feasible) can cause the contacts to stick.

Procedures for removing smoke contamination from electronic equipment involve such means as detergents, solvents, neutralizing agents, ultrasonic vibrations, and clean air jets. These procedures are

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largely empirical rather than scientifically based, and they are not always effective; indeed, sometimes they may provide only a temporary fix.

Thermal Damage The potential for heat from a fire to ignite and pyrolyze products and to burn skin was discussed in the Heat Transfer and Combustion, Fire, and Flammability chapters.

In a large fire, sufficient heat can be generated to trigger the structure’s collapse. This is what happened to World Trade Center Building 7 on September 11, 2001 [18]. A presentation on structural weakening and collapse is beyond the scope of this text. References [19] and [20] provide thorough introductions to this subject, and the Executive Summary in Reference [18] provides a rendering of an unexpected outcome that changed people’s perception of the vulnerability of tall buildings to fire.

Although this collapse of a steel structure (World Trade Center Building 7) solely due to fire was unprecedented, structural damage to and collapse of wooden structures does sometimes happen following a severe fire. This outcome results from thermal weakening and burning of, for example, the floor joists. Reference [21] documents the performance of several wooden floor assemblies during a severe fire.

The Limiting Hazard Concept When designing or repurposing a building is undertaken, the building code specifies fire safety requirements that must be met, to which the owner adds functional and economic objectives and perhaps additional safety enhancements. It is then prudent to conduct a (semi-)quantitative assessment of the expected performance of the building in an emergency to highlight the potential vulnerabilities. In addressing these issues, it makes sense to focus on the most important shortcoming.

This chapter and the preceding chapters have addressed the multiple threats to people in a building subjected to a fire: incapacitation from narcotic gases, irritant gases, and skin burns; reduced visibility, affecting the efficiency of actions to escape or find a place of refuge; and structural collapse. As the fire progresses, each of these threats grows, with the growth rate being different in different locations in the building and being experienced differently by each person. The effect on a person depends on such factors as his or her location, physical condition, and means (or possibility) of moving. At some point in time, one of these threats reaches a level at which the person’s life is in peril. This limiting hazard is the hazard that is most urgently considered in a revised fire protection plan.

Assessment of fire hazards and risks can be performed using a series of calculations or a computational model of a fire in a building. (See the chapter on Computational Modeling of Fires.) In doing this assessment, it is a good idea to let the model continue to run to determine whether other threats reach perilous levels almost concurrently with the limiting hazard. For example, when a room reaches flashover, the toxicological, thermal, and visual threats to people in the adjacent room all increase quickly. Preventing flashover—for example, by choice and extent of furnishings or installing automatic sprinklers—might be more effective as a means to address these threats than locating the exit signs near floor level, which might be a reasonable approach if the limiting hazard were simply visibility through smoke. One could also construct lists and perform limiting hazard analyses for other undesirable outcomes of a fire, such as an interrupted operation (e.g., on a stock exchange trading floor), extended or permanent loss of use of the building (e.g., a home), and loss of sensitive equipment (e.g., a computer).

WRAP-UP Chapter Summary • Smoke inhalation is a factor in most fire deaths in the United States, especially those that result from post-flashover fires.

• A single exposure to fire smoke can have both acute effects (e.g., death, incapacitation, impaired mobility, and reduced clarity of thinking) and postexposure effects (e.g., lung damage). Fire fighters can experience chronic effects from multiple exposures to smoke.

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• Most of the toxic gases fall into two classes.

• Asphyxiant (narcotic) gases, mainly CO and HCN, deprive the body of oxygen. The effects are dose related—that is, a function of the volume fraction of the gas and the duration of exposure.

Oxygen depletion and carbon dioxide increase the effect.

• Irritant gases can have acute, instantaneous effects on the senses and upper respiratory tract as well as long-term effects on the lungs. Prominent irritant gases include the halogen acids, nitrogen oxides, formaldehyde, and acrolein.

• The fractional effective dose concept enables combining the effects of the individual toxic gases and estimating the intensity and length of exposure to smoke that would result in incapacitation or death.

• Experiments with laboratory animals have shown that the toxic potency of smoke generally can be estimated from the contributions of fewer than 10 toxic gases.

• To a first approximation, one can use an average toxic potency value in engineering calculations.

However, the toxic potencies of smoke from a large number of materials are significantly worse than the average value.

• Smoke can cause nonthermal damage to electronic circuitry contacts and building structural components.

• The heat from a fire can cause skin burns and can weaken a structure to the point of collapse.

• Using the full range of knowledge to identify the limiting hazard(s) in the event of a fire can help developers and owners of structures implement the most effective fire protection technology.

Key Terms acute toxic effect(s) The effect(s) on a person during exposure to smoke from a single fire. asphyxiant gas (narcotic gas) A gas whose inhalation can cause an adverse physiological effect due to lack of oxygen. chronic toxic effect(s) The accumulated damage from exposure to smoke in multiple fires. fractional effective concentration (FEC) The sum of the volume fraction of each irritant gas divided by its concentration that causes a harmful effect in the average person. fractional effective dose (FED) The accumulated product of the volume fractions of narcotic gases and the time interval over which they are inhaled divided by the volume fraction-time product that causes incapacitation, death, or any other harmful effect in the average person.

Haber’s rule The empirical finding that, for a particular gas and toxicological effect, the product of the volume fraction and the exposure time is a constant. irritant gas A gas that causes a physiological effect by affecting the eyes and/or upper respiratory tract. limiting hazard The fire threat that first reaches a level at which a person’s life is in peril.

N-gas model The empirical finding that death or incapacitation from smoke inhalation can be attributed to just a few of the numerous components of the smoke. physical fire model An apparatus, including the operating procedure, test specimen configuration, and combustion environment, that is intended to represent a certain stage of a fire. postexposure toxic effect(s) The delayed effect(s) on a person attributable to exposure to smoke from a single fire.

Challenging Questions 1.

What are the most important toxic gases in smoke? Which are narcotic gases and which are irritant gases?

2. What is Haber’s rule?

3. In a closed room containing 40 m³ of air and an electric fan to promote uniform mixing, how many kilograms of a typical combustible would have to burn to create an atmosphere that might incapacitate people in 30 minutes? Assuming Haber’s rule holds, how many burned kilograms of fuel might cause incapacitation in 10 minutes?

4. Explain why administering oxygen to fire survivors is so beneficial.

5. While escaping from a building in which a fire is burning, 40 people pass through a smoky room in which the CO volume fraction is 1000 µL/L, the HCN volume fraction is 50 µL/L, and the CO2 volume fraction is 0.02. Despite some visible smoke in the room, they cross the room in about 10

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seconds. Calculate the fractional effective dose of the narcotic gases each person has accumulated and estimate whether all of them will make it to the exit door.

6. In the same situation described in Problem 5, the HCl volume fraction was 200 µL/L and the SO2 volume fraction was 15 µL/L. Calculate the fractional effective concentration and estimate whether this might keep people from reaching the exit door. Which of the two effects do you think is the limiting hazard and why?

References 1. Hall, J. R. Jr., and B. Harwood, (1989). “The National Estimates Approach to Fire Statistics.” Fire Technology 25: 99– 113.

2. Gann, R. G., and N. P. Bryner. (2008). Combustion Products and Their Effects on Life Safety. In: The SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

3. Birky, M., B. M. Halpin, Y. H. Caplan, R. S. Fisher, J. M. McAllister, and A. M. Dixon. (1979). “Fire Fatality Study.” Fire and Materials 3: 211–217.

4. Esposito, F. M., and Y. Alarie. (1988). “Inhalation Toxicology of Carbon Monoxide and Hydrogen Cyanide Gases Released during the Thermal Decomposition of Polymers.” Journal of Fire Sciences 6: 195–242.

5. Levin, B. C., et al. (1990). “Analysis of Carboxyhemoglobin and Cyanide in Blood from Victims of the Dupont Plaza Hotel Fire in Puerto Rico.” Journal of Forensic Sciences 35: 151–168.

6. Purser, D. A. (2008). Assessment of Hazards to Occupants from Smoke, Toxic Gases, and Heat. In: The SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

7. Babrauskas, V., R. H. Harris, Jr., E. Braun, B. C. Levin, M. Paabo, and R. G. Gann. (1991). The Role of Bench-Scale Test Data in Assessing Real-Scale Fire Toxicity. Technical Note 1284. National Institute of Standards and Technology.

8.

Hartzell, G. E., D. N. Priest, and W. G. Switzer. (1985). “Modeling of Toxicological Effects of Fire Gases: II. Mathematical Modeling of Intoxication of Rats by Carbon Monoxide and Hydrogen Cyanide.” Journal of Fire Sciences 3: 115–128.

9. Kaplan, H. L., and G. E. Hartzell. (1984). “Modeling of Toxicological Effects of Fire Gases: Incapacitating Effects of Fire Gases.” Journal of Fire Sciences 2: 286–305.

10. Neviaser, J. L., and R. G. Gann. (2004). “Toxic Potency Values for Smoke from Products and Materials.” Fire Technology 40: 177–200.

11. ISO 13571: Life-Threatening Components of Fire: Guidelines for the Estimation of Time to Compromised Tenability in Fires. (2012). Geneva, Switzerland: International Standards Organization.

12.

ISO 27638-11: Analysis of Blood for Asphyxiant Toxicants: Carbon Monoxide and Hydrogen Cyanide. (2011).

Geneva, Switzerland: International Standards Organization.

13. Kaplan, H. L., A. F. Grand, W. G. Switzer, D. S. Mitchell, W. R. Rogers, and G. E. Hartzell. (1985). “Effects of Combustion Gases on Escape Performance of the Baboon and the Rat.” Journal of Fire Sciences 3: 228–244.

14. Kaplan, H. L., A. G. Grand, and G. E. Hartzell. (1983). Combustion Toxicology, Principles and Methods. Lancaster, PA: Technomic Publishing.

15. Clarke, F. B. (1983, September). “Toxicity of Combustion Products: Current Knowledge.” Fire Journal 84–108.

16. ISO 16312-1: Guidance for Assessing the Validity of Physical Fire Models for Obtaining Fire Effluent Toxicity Data for Fire Hazard and Risk Assessment. Part 1: Criteria. (2011). Geneva, Switzerland: International Standards Organization.

17. ISO/TR16312-2: Guidance for Assessing the Validity of Physical Fire Models for Obtaining Fire Effluent Toxicity Data for Fire Hazard and Risk Assessment. Part 2: Evaluation of Individual Physical Fire Models. (2011). Geneva, Switzerland: International Standards Organization.

18.

McAllister, T. P., et al. (2008). Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7, Federal Building and Fire Safety Investigation of the World Trade Center Disaster. NIST NCSTAR 1-9.

Gaithersburg, MD: National Institute of Standards and Technology.

19. Franssen, J. M., and N. Iwankiw. (2008). Structural Fire Engineering of Building Assemblies and Frames. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

20. Cote, A. E., ed. (2008). Fire Protection Handbook, 20th ed. Quincy, MA: National Fire Protection Association, Section 12.

21. Su, J. Z., N. Bénichou, A. C. Bwalya, G. D. Lougheed, B. C. Taber, P. Leroux, et al. (2008). Fire Performance of Houses: Phase I. Study of Unprotected Floor Assemblies in Basement Fire Scenarios: Summary Report. RR-252.

Ottawa, ON: National Research Council Canada.

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CHAPTER 12

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Movement of Fire Gases OBJECTIVES After studying this chapter, you should be able to: • • • • •

Describe the three zones of the plume of a fire burning in the open and calculate the air entrainment into the flame and the height of the luminous flame.

List three reasons why the nature of the ceiling jet is important.

Calculate the mass outflow from a room in which a steady-state fire is burning.

Estimate the minimum rate of heat release that leads a room to flashover.

List nine reasons why calculating the smoke flow through most buildings requires a computational fire model.

Introduction A 2007 fire in a furniture store began outside an enclosed loading dock area and spread into the retail showroom.

During the early stages of the fire, its spread was slowed by the limited supply of fresh air. This underventilation led to the generation of a large mass of pyrolyzed and only partially oxidized effluent. The combustible gases flowed above the suspended ceiling of the main retail showroom and into the showroom itself, forming a hot smoke layer below the suspended ceiling. The fire at the back of the main showroom and the gas mixture below the suspended ceiling were both still rich with fuel. When the front windows were broken, the inflow of additional air rapidly increased the heat release rate of the fire and added air to the hot upper layer, enabling the ignition of the unburned fuel/air mixture. The fire swept quickly from the rear to the front of the main showroom, trapping nine fire fighters [1].

The description of each stage of this fire involves the word “flow” or a synonym of it. This is not uncommon. The flows to and from the fire determine the magnitude and direction of fire growth, and the flow from a fire transports toxic gases, aerosols, and heat to locations where they can be detected or do harm.

This chapter applies the fluid flow, heat generation, and chemical concepts developed previously to the movement of the gases by a fire. The treatment begins with the local movement within the fire plume and progresses to movement throughout a building.

Structure of a Fire Plume in the Open The fuel flow into a fire comes from gasification of the liquid or solid that is burning. When the molecular fragments leave the fuel surface, they have almost no momentum. They rise into the air above strictly due to buoyancy (see the Flow of Fluids chapter), entraining (drawing in) cool air along the perimeter as they burn. This air entrainment is distinctly more than the amount of air needed for combustion. In turn, even with exothermic reactions occurring wherever the gasified fuel and air mix within the flammability limits, the overall temperature of the fire plume begins to decrease with increasing height.

At some height, the plume temperature (and thus the gas density) essentially matches the temperature of the surrounding air. The buoyant force, which depends on a temperature difference, drops to zero; and the smoke spreads outward rather than being directed upward.

The overall fire plume structure can be regarded as having the three zones Figure 12-1 [2]: 1. An always luminous flame zone. Because the flame temperature exceeds the temperature of the fuel surface, the gases accelerate upward.

2. An intermittently flaming zone. A movie of zone 2 would show fluctuating orange flames and transparent gases, with the lower part of the zone being orange more often, and the top of the zone being defined as the location where no orange is seen. The gas temperature, while fluctuating, has an average value that remains constant from the top to the bottom of this zone.

Thus the upward velocity is essentially constant.

3. A buoyant plume. The plume is nonluminous. The temperature and the buoyant velocity decrease with height.

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The height of the luminous flame, h (m), is the sum of the height of zone 1 and a fraction of the height of zone 2. The fraction of the zone 2 height that is included in h is the distance from the bottom of the zone to the point at which the frames would show a plume that is orange half the time and transparent half the time. The luminous flame height (m) has been shown experimentally to fit the following Equation 12-1:

Figure 12-1 Three zones of a buoyant diffusion flame. Š Robert Rathe, www.robertrathe.com

where d is the diameter of the base of the fire (m) and Qc is the convective heat release of the flame (kW) [3]. For ordinary combustible products, the convective fraction of the total heat release is typically in the range of 0.6 to 0.7 for nonaromatic fuels and in the range of 0.3 to 0.5 for aromatic fuels [4]. The remaining fraction is radiative.

Some solutions to this equation are plotted in Figure 12-2. The ratio h/d can vary at least from 1 to 44. When conditions are such that h/d is less than 1, the flame breaks up into a number of small flamelets.

The turbulence intensity in zones 2 and 3 is quite high. The velocity fluctuations at the center can be on the order of 30 percent of the average velocity; the temperature fluctuations can be even greater.

These fluctuations and the eventual decrease in plume temperature reflect the rate of air entrainment into the plume. Precise calculation of the rate of entrainment into a fire plume is not of practical value, because small ambient disturbances in the air near the plume can have substantial effects on the entrainment rate. A rough approximation of the air entrainment rate mâ&#x20AC;˛ (kg/s) for a turbulent plume of height z (m) and plume surface area A (m2) is given by

In most plumes, combustion occurs only in zones 1 and 2. At the top of zone 2, the mass of entrained air is roughly an order of magnitude greater than the mass needed for complete combustion.

In zone 3, the combustion has ceased and the height is large compared to the width of the base of the plume. The average midline temperature (relative to the ambient temperature) decreases at a rate

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inversely proportional to the 5/3 power of the height. The average midline velocity decreases more slowly, at a rate inversely proportional to the 1/3 power of the height. The diameter of the plume increases at a rate directly proportional to the height [2].

Fire Plume under a Ceiling For a fire in a building, the plume will impinge on the ceiling, unless the fire is very small or the ceiling is very high. When this happens, the hot gases make a 90-degree turn and spread out radially under the ceiling, forming a ceiling jet Figure 12-3. This ceiling jet is important for at least two reasons: â&#x20AC;˘

Devices to detect the fire, including automatic sprinklers and residential smoke alarms, are generally mounted at heights just below the ceiling, and knowledge of the time of arrival and properties of the ceiling jet are crucial for predicting the point of actuation for a detection device.

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Figure 12-2 Calculated flame height of turbulent diffusion flames versus the convective heat-release rate for two fire sizes.

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Figure 12-3 A turbulent ceiling jet under an unconfined ceiling with walls remote from the fire. “X” marks a distance along a ceiling jet radius that is equal to the distance between the base of the fire and the height of the ceiling.

•

The downward thermal radiation from the ceiling jet, and, a little later, from the hot ceiling itself, affects the rate of fire spread. It is also a major factor in preheating and igniting combustible items not yet involved in the fire.

If the fire is burning at steady state and if the fire centerline is far from the nearest wall (e.g., the fire is near the center of a large room), the maxima of ceiling jet velocity and temperature exist at a distance below the ceiling equal to about 1 percent of the distance from the base of the fire to the ceiling. The radial velocity of the jet progressively decreases as it moves farther away from the fire centerline.

Note The decrease in the radial velocity of the ceiling jet occurs for three reasons: 1. The leading edge of the flow is a circle of increasing circumference, while the mass flow is unchanged.

2. The ceiling jet is generally turbulent, and mixing occurs between the jet and the air below. This entrainment slows down the jet and reduces its temperature.

3. The jet transfers heat to the ceiling, which reduces the temperature of the jet. According to the ideal gas law, the volume of the jet decreases proportionately. The mass flow is unchanged, so the velocity decreases.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

Formulas have been developed for calculating the temperature and velocity distribution in such a ceiling jet [5]. For example, focus on the location marked by “X” in Figure 12-3. At this location, the calculated maximum velocity in the jet will have dropped to half the value near the fire centerline. The difference between the jet temperature and the ambient temperature will have dropped to approximately 40 percent of the value near the fire centerline. If the walls are much farther away than the fire-to-ceiling distance, the temperature and velocity of the ceiling jet will decay to negligibly low values before the jet encounters the nearest wall.

Filling of a Fire Compartment by Smoke If a fire is ignited in a compartment without openings, one of two things will happen: •

The release of heat causes an increase in the pressure and temperature of the gases in the compartment, according to the ideal gas law. Ordinary construction materials can withstand a substantial increase in pressure if the pressure is applied evenly and gradually. However,

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windows can break in a fire because of stresses created when the viewable area of the glass is heated and expands more than the area shaded by the frame. In case of a prolonged fire, gypsum wallboard panels can crack, but typically large gaps do not open up until the cooling phase after the fire. â&#x20AC;˘ If no rupture occurs, the oxygen in the compartment becomes depleted to the point that the combustion ceases.

In either case, as long as the compartment does not contain an opening, a hot layer forms near the ceiling. This upper layer becomes deeper as the burning continues. As the bottom of the smoke layer approaches the flames, the luminous flame height decreases, because the flame is attempting to extend into a region characterized by severe oxygen depletion. Meanwhile, the fire has set up a convective flow pattern, with the gas rising above the fire, traveling along the ceiling and down along the walls, and finally being re-entrained into the flames. This pattern produces two results. First, the upper and lower layers in the compartment are mixed, so the environment becomes more uniform throughout the compartment. Second, the fire entrains air that is increasingly depleted of oxygen (vitiated), and the burning rate decreases accordingly.

Smoke Flow from a Compartment with an Opening A more common case is that of a compartment with an opening, either by design or due to a window rupture. Figure 12-4 depicts a fire in a compartment with an open doorway. The ceiling jet has reached the walls, and a hot, smoky gas layer has formed near the ceiling. Continued burning has increased the (top-to-bottom) thickness of the layer until it extends below the top of the door opening; in addition, the hot, smoke-laden gases are flowing into the next compartment. The interface between the hot upper layer and the cool lower layer composes a somewhat wrinkled horizontal plane, called the neutral plane. (No such plane would form if the burning item were close to the doorway.)

Knowledge about this hot upper layer is crucial to assessment of life safety in a building fire. The layer thickness, temperature, and optical density all affect the intensity of downward thermal radiation incident on people and combustibles in the lower part of the compartment. The rate of outflow of the hot layer influences life safety in and fire spread to the adjacent space(s).

The layer temperature and optical density are determined by the heat release from the fire, the heat losses to the ceiling and walls, the fraction of the burned fuel converted to soot, and the volume of air into which the heat and soot are dispersed. Continuing with the compartment geometry depicted in Figure 12-4, assume that the burn rate has reached a steady state. Air enters the compartment through the lower part of the doorway, is entrained into the fire plume, and buoyantly flows upward into the hot gas layer. Hot gas escapes from the compartment through the upper part of the doorway.

Figure 12-4 Smoke layer from a fire in a compartment with an open doorway.

These inward and outward doorway flows are driven by pressure differences. From the Flow of Fluids chapter, recall that the difference in pressure between the top and bottom of a column of gas of height h is equal to gĎ h, where Ď is the gas density. The gas in the hot layer has a substantially lower density than the air in the lower part of the room or the air outside. Figure 12-5 shows the resulting

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pressure variations with height. In the doorway, there is a height at which the inside and outside pressures are the same; it corresponds to the intersection of the neutral plane with the doorway opening. Above this height, the pressure is higher inside, causing an outflow. Below the neutral point, the reverse is true, and an inflow occurs.

The driver for this two-directional air flow at the doorway is the rate of air entrainment into the fire plume. This rate is proportional to the burning rate of the fire and the vertical distance between the base of the plume (the top surface of whatever is burning) and the neutral plane (the bottom of the hot layer).

The greater the entrainment rate, the lower the bottom of the hot layer. The mass rate of outflow is slightly greater than the mass rate of air inflow because of the added mass of the gasified combustible(s).

The â&#x20AC;&#x153;dragâ&#x20AC;? on this outflow is the heat loss from the hot layer to the ceiling and the upper portion of the walls. This heat loss depends on the factors discussed in the Heat Transfer chapter, including the thermal inertia of the walls, the heat capacity of the upper-layer gas (air plus combustion products), and the degree of turbulence in the upper layer.

Knowing all these input values, fire scientists routinely use computational fire models (discussed in the Computational Modeling of Fires chapter) to calculate the height of the neutral plane and the flows in and out of the compartment. The pertinent equations were developed from and confirmed by numerous laboratory experiments.

In such an experiment, the researcher measures the top-to-bottom temperature and pressure profiles in the doorway. The height at which the pressure differential changes from higher in the room to lower in the room locates the plane separating the inflow and outflow. The temperatures enable calculation of the gas density in each flow. An estimate of each mass flow is obtained using the square root of the average pressure differential, the gas density, the ideal gas law, and the partial area of the doorway through which that flow passes. The (unmeasured) degree of heat loss is estimated from the measured temperatures, which are lower than they would be if no heat losses occurred.

Figure 12-5 Pressure gradients at the doorway, caused by the relative densities of cold air and hot gases.

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Early in the fire, the smoke mainly fills the upper layer of the room. The flow of smoke from the room might suffice to activate a smoke alarm in the next compartment, but does not put people or property in that room at risk. This situation changes when the bottom of the upper layer becomes lower than the door soffit. Smoke begins to billow out the doorway. If the fire does not run out of fuel, the fire grows quickly and the room rapidly reaches a flashover condition, perhaps within tens of seconds. At this time, the burning intensity is at a steady state.

Based on observations from numerous experiments, fire engineers have arrived at an approximate, two-stage portrayal of the smoke outflow from the room: no outflow before flashover, steady outflow after flashover. From these experiments, they also evolved a simple equation for estimating the steady inflow or outflow of hot gases through a door or window in the fire room. If the opening is of height H (m) and area A (m2), then the mass flow out of the opening, (kg/s), is given by

The value of C is generally taken as 0.5 kg/s·m5/2 [6].

Using this relationship, several engineers have developed equations for the minimum heat release rate that leads to flashover. These equations differ slightly due to differences in the assumptions regarding the thermal physics of the combustion and the heat losses. For reasonable compartment sizes and openings, they agree within approximately ±20 percent. The simplest such equation is

where is given in units of kW. For a compartment with a common doorway size (0.6 m × 2.0 m), is approximately 1.3 MW. It is a common practice to use a mnemonic of 1 MW to characterize the approximate heat-release rate that threatens a typical residential room reaching flashover.

Calculation of the flows requires one of the computational fire models discussed in the Fire and Smoke Hazards chapter in the following cases: • The fire is not at or near a steady state.

• The combustible is located near the opening (and thus the boundary between the layers is not flat).

• The calculation focuses on a hypothetical fire rather than one that has been measured.

Smoke Movement in Buildings If a building consists of just a series of compartments, all on the same level and all connected by open doorways, you can estimate the flows using initially zero. As the smoke moves down the corridor, it becomes diluted by the air in the corridor and is cooled both by that air and by heat loss to the ceiling and walls. At some distance from the fire room, the smoke flow essentially reaches the ambient temperature, and no demarcation between a hot layer and a cool layer is apparent. Only after a large volume of smoke has heated the corridor from the ceiling down to the height of the soffits will the uniform layer depth be realized.

1. At the beginning of the fire, some doors might be closed and some windows might be open; also, during the fire people might open or close doors or windows, or the fire might break a window.

The previously described calculation methods might still be applicable if the number of changes is small and one change does not affect another. (Interacting changes might, for example, lead to cross-ventilation in a compartment.) If the effect of each change may not be independent, it is prudent to use a computation fire model.

2. If the building contains long corridors, the assumption of a corridor filling uniformly from the top down is not realistic. As the smoke leaves the fire room, the height of the smoke layer is determined by the height of the neutral plane in that doorway. In the corridor, the depth of the smoke layer is initially zero. As the smoke moves down the corridor, it becomes diluted by the air in the corridor and is cooled both by that air and by heat loss to the ceiling and walls. At some distance from the fire room, the smoke flow essentially reaches the ambient temperature, and no demarcation between a hot layer and a cool layer is apparent. Only after a large volume of smoke has heated the corridor from the ceiling down to the height of the soffits will the uniform layer depth be realized.

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3. A multistory building contains stairwells, elevator and ventilation shafts, and possibly atria. These areas serve as both pathways for vertical smoke movement and repositories for large masses of smoke.

4. The building might have a sloping ceiling or a ceiling supported by beams, both of which would modify the behavior of the ceiling jet.

5. The presence of wind outside a building influences air movement within the building, if any doors or windows are open.

6. An operating heating, ventilation, or air-conditioning system has a profound effect on smoke movement in a building. Even if the system is shut down, hot gases might still move through the ducts due to buoyancy or expansion due to the fire’s heat release.

7. In a tall building, a stack effect might arise depending on the weather. The pressure difference between the top of the building and the bottom of the building is given by the equation ΔP = ρgh.

On a cold day in winter, the temperature within the building is higher than the temperature outside, so the ideal gas law says that the indoor density, ρi, is proportionately lower than the outdoor density, ρo. The height of the building and the gravitational constant are the same outdoors and indoors. Therefore, the pressure difference indoors, ΔPi, is lower than the pressure difference outdoors, ΔPo. As a result, air would leak into the building at the lower levels and leak out at the upper levels, prior to the occurrence of a fire. The resulting upward flow inside the building would help carry smoke upward. On a hot day in summer, this effect is reversed if the building is air conditioned. In such a case, the air density is higher indoors, and the pressure difference is accordingly higher indoors. As a result, air would leak out of the building at the lower levels and leak in at the upper levels. The hot, buoyant smoke would then be flowing against the downward flow from the stack effect.

8 The force of the flow from an activated fire suppression system would change the nature of the air patterns within the fire room.

9. The hazard potential of the smoke changes through processes other than dilution with fresh air.

Larger aerosol particles and droplets are removed from the upper layer, either by sticking to walls or by falling due to gravity. Hydrogen chloride and hydrogen bromide stick to some surfaces.

Lest this all seem overwhelming, the physics behind items 1 through 8 has been incorporated by experts into widely available computational fire models. For item 9, relatively few data are available regarding the kinetics of the generation and evolution of smoke aerosols and even fewer data on the loss of gases on realistic surfaces. Therefore, the yields of the smoke components serve as input data to the models, and the models simply transport and dilute the components as they flow throughout the building.

WRAP-UP Chapter Summary • The plume of a fire burning in the open can be depicted as containing three zones. The lowest zone is luminous, and the gases accelerate upward. The flames in the middle zone are intermittent; the average gas temperature and upward velocity are constant. The upper zone is a nonluminous buoyant plume, whose temperature and velocity decrease with height.

• When a fire plume hits a ceiling, it spreads horizontally. The ceiling jet activates automatic sprinklers and smoke alarms and radiates thermal energy onto combustibles, accelerating fire growth.

• In a fire in a compartment, the ceiling jet forms a hot, smoky upper layer, which becomes hotter, more optically thick, and deeper as the fire progresses.

• A two-way flow occurs through the doorway of a fire room. The inflow through the lower portion of the opening is driven by air entrainment into the flames. The mass outflow through the upper portion of the opening is slightly higher due to the added mass of the combustion products.

• Calculation of the smoke flow from large fires in most buildings requires the use of a computational fire model. The ventilation pattern can change during a fire, for example, and the geometry of building is rarely as simple as needed for hand calculations to be accurate.

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Key Terms air entrainment The drawing of air into a fire or fire plume due to the buoyant flow of the plume. ceiling jet The radially outward flow under a ceiling resulting when a fire plume impinges on a ceiling. luminous flame height The distance between the base of a flame and the point at which the plume is luminous half the time and transparent half the time. neutral plane The horizontal (slightly wrinkled) interface between the hot upper layer and the cool lower layer in a fire room or a room into which there has been flow of the hot fire effluent. stack effect The vertical flow of air in a building, driven by vertical pressure differences developed by thermal buoyancy due to a difference between the building interior temperature and the outdoor temperature.

Challenging Questions 1. List two reasons why the properties of the ceiling jet and the hot, smoky upper layer in a room are important. How do the ceiling jet and the upper layer differ?

2. A spill of n-hexane has resulted in a roughly circular pool fire that is effectively 1 meter in diameter.

Estimate the height of the luminous flame. Soon thereafter, more liquid spills, doubling the pool area. Estimate the new flame height.

3. Calculate the rate of air entrainment into the (initial) fire plume in Question 2 from the base to the flame tip.

4. A bedroom space heater placed too close to some draperies ignites them. The burning draperies fall onto the carpet and ignite it. The result is a fire that barely results in room flashover. In an identical bedroom, a candle ignites a bed. The fire burns at one-third the rate of the carpet fire and also barely leads to flashover. What is the ratio of the rates of heat release of the two fires?

5. Why might the movement of smoke from a fire in a tall building differ in summer and in winter?

References 1.

2.

3.

4.

5.

6.

Bryner, N. P., S. P. Fuss, B. W. Klein, and A. D. Putorti. (2011). Technical Study of the Sofa SuperStore Fireâ&#x20AC;&#x201D;South Carolina, June 18, 2007.â&#x20AC;? In: NIST SP-1118, Volume 1. Gaithersburg, MD: National Institute of Standards and Technology.

McCaffrey, B. J. (1979). Purely Buoyant Diffusion Flames: Some Experimental Results. NBSIR 79-1910.

Gaithersburg, MD: National Bureau of Standards (now the National Institute of Standards and Technology).

Heskestad, G. (2008). Fire Plumes, Flame Height, and Air Entrainment. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Tewarson, A. (2008). Generation of Heat and Chemical Compounds of Fires. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Alpert, R. L. (2008). Ceiling Jet Flows. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association Walton, W. D., and P. H. Thomas. (2008). Estimating Temperatures in Compartment Fires. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

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CHAPTER 13

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Fire Fighting Chemicals OBJECTIVES After studying this chapter, you should be able to: • Distinguish among fire extinguishment, fire control, and fire inerting.

• List the four classes of fires, as used in the United States.

• Describe the different ways in which water suppresses a fire, depending on its method of delivery and the geometry of the fire, and list the types of fires on which water should not be applied.

• Describe the roles of suppression-enhancing additives to water.

• List the types of nonaqueous fire suppressants.

• Understand why the use of halon fire extinguishants has been curtailed.

• Explain how powdered fire extinguishants are effective on a fire.

Introduction The end of the 20th century saw a dramatic change in fire suppression chemistry as the manufacture and use of fully halogenated fire suppressants was severely curtailed. The two most widely used of these compounds, halon 1301 and halon 1211, are chemically similar to the chlorofluorocarbons (CFCs) previously used in air conditioners, as aerosol propellants, and as degreasers. The bromine in these two halons is even more potent than the chlorine in the CFCs at destroying the protective ozone in Earth’s stratosphere. This loss reminds us that the ways we deliver fire safety are subject to external forces, and that a scientific basis for firefighting technologies is a prerequisite for timely deployment of new approaches.

Categories of Fire Suppressants The chemicals used to attack fires may be classified in a variety of ways. The Fire Characteristics: Solid Combustibles chapter discussed chemicals that are compounded into combustible materials and products, with the intention of making them more difficult to ignite and less prone to generating heat and combustion products and spreading flames. This chapter focuses on the chemicals used to respond to a fire.

Before discussing these chemicals and their uses, it will be helpful to understand the following terms: •

Flame or fire extinguishment. This term is the most straightforward. After extinguishment, no flames are visible, and the combustible item no longer generates heat or combustion products.

However, if the fuel remains hot or the ignition source has not been removed, reignition of the fire is possible.

• Fire control. This term is the opposite of “out of control.” It means that the fire is contained and the hazard is greatly reduced. Some residual flames or smoldering might persist, or the fuel might be sufficiently hot that it continues to generate combustion products. In such a case, either a second attack on the fire is expected to lead to extinguishment or the degree of containment is acceptable until the fire burns itself out. Fire control implies some degree of confidence that the fire will not flare up, but a change in circumstances (such as an unforeseen increase in wind velocity) could change this situation.

• Flame or fire suppression. Some consider this term to be synonymous with extinguishment; others consider it to be synonymous with control. The distinction between suppression and extinguishment can be important if the fire is in a concealed space and complete termination cannot be verified immediately. In this text, suppression and extinguishment are used interchangeably. The chemicals are synonymously referred to as extinguish-ants, extinguishing chemicals, extinguishing agents, suppressants, suppressant chemicals, and suppression agents.

Note that, when water is the agent of choice in an automatic sprinkler system, NFPA design standards do distinguish between control mode and suppression mode sprinklers.

• Fire inerting. Fire inerting is often a preemptive action. It means that, within the environment in which a fire might be of concern, the fire tetrahedron (discussed in the Fire and Flammability

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chapter) has been interrupted such that a fire cannot exist. Some of the same chemicals used for active attack on some fires are applied in other circumstances to prevent a fire from starting. In addition, some extinguishment tactics prevent reignition by inerting the environment.

The applied chemicals may also be organized in several different ways: •

• • • •

Type of fire on which they are effective. The following classification system is used in the United States: • Class A: Fires involving “ordinary combustible materials,” such as wood, paper, cloth, and certain plastics.

• Class B: Fires involving flammable liquids, gases, and greases. In addition, a number of the most common thermoplastics, including polyethylenes, polypropylenes, and polystyrenes, form a low-viscosity melt when they burn and have some of the characteristics of a flammable liquid rather than a solid. Fires involving these materials are formally categorized as Class A fires, but might well be attacked as if they were Class B. • Class C: Fires involving energized electrical equipment, where a shock hazard is present.

• Class D: Fires involving combustible metals.

Phase of the chemical: gaseous, liquid, or solid.

Mechanism of action: chemical interference with the flame propagation, thermal interaction with the fire, dilution of the fuel vapors and/or oxygen, or separation of the fuel and the flames.

Mode of application: directed at the fire or filling the volume in which the fire is burning. The former is sometimes called streaming; the latter is sometimes called total flooding.

Application system: fixed or mobile/portable.

Note Different classification systems are used elsewhere in the world. For example, in the British system, Class A fires involve carbon compounds that form glowing embers; Class B fires involve liquids or liquefiable solids, of which B1 materials are miscible with water and B2 materials are immiscible with water; Class C fires involve flammable gases or vapors; and Class D fires involve metals. Electrical fires in energized equipment constitute a fifth category.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

The following discussion of each of the agents or groups of agents includes how each fits into these subdivisions.

Aqueous Agents Water Water was certainly the earliest used fire suppressant. Three to seven million years ago, our ancestors observed fires being started by lightning and volcanic eruptions and recognized that rain put the fire out.

Eventually, they learned how they could use water for this purpose, built water storage containers for extinguishing fires on nonrainy days, and adapted transportation vehicles to carry water to fires more efficiently.

The first automatic sprinkler system was built in 1875 and was conceptually similar to many in use today. The water was stored behind a self-opening valve. The “switch” on the valve consisted of a piece of metal with a low melting point. When the hot air from the fire fused (melted) the metal link, the valve opened, releasing the water. Some of today’s systems use an alternative trigger for the water flow— namely, a small sealed glass bulb that has been partially filled with a liquid that expands as its temperature rises. The expanding liquid compresses the gas in the bulb ullage, raising the pressure to the point where the glass breaks, allowing the valve (defined as an orifice cap) to open.

The heat to fuse the link or break the bulb comes from the fire. Because hot gases rise, sprinklers are placed near the ceiling or high on a wall. The NFPA 13 series of standards prescribes the installation, the number of sprinklers and their spacing, and the water supply [1].

Sprinklers should actuate in a predictable manner and under conditions when they are most effective.

If the first sprinkler in a room actuates too soon (i.e., when the air temperature near the ceiling is too low), something other than a fire might potentially instigate water flow and the resulting water damage. If a fire occurs and too many sprinklers actuate because they are too sensitive, the water pressure feeding the

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sprinklers will drop, and the water flow and dispersion pattern will not deliver the designed performance.

If the sprinkler actuates too late (i.e., at too high a temperature), the fire may get out of control. Typical residential sprinklers are designed to actuate near the time when they sense an air temperature of approximately 135 °F to 170 °F (57 °C to 77 °C). For all of these reasons, the NFPA sprinkler design standards offer specific guidance to ensure a proper temperature rating is selected based on the environment in which the sprinkler will be located.

The sensitivity of a sprinkler is characterized by the response time index (RTI), which is given in units of m1/2s1/2 (ft1/2s1/2) [2]. The RTI is measured using the plunge test. Within this test apparatus, air flows at a known velocity and elevated temperature. A sprinkler is plunged into this flow, and the time to fuse the link or rupture the bulb is measured.

The heat transfer from the hot air to the sprinkler actuator has been experimentally determined to be approximately proportional to the square root of the flow velocity. The RTI for a particular sprinkler is given by Equation 13-1:

where Mt is the mass of the trigger (the fusible link or the glass bulb) (g), ct is the heat capacity of the trigger (J/g K), hc is the convective heat transfer coefficient from the air to the trigger (J/m2 K s), A is the

area of the trigger that is exposed to the hot air flow (m2), and v is the air velocity (m/s).

Note that the final units of the RTI are simply a square root of length multiplied by a square root of time. All the other units cancel out. Thus, the input terms can be expressed in any consistent set of units.

All common unit sets express time in seconds, but other length units are possible. The use of lengths in units other than meters changes the RTI value by a constant. For instance, if the dimensions are expressed in feet, the right hand side of Equation 13-1 will be 1.81 larger than if the dimensions were in meters. (There are 39.37 inches in a meter and 12.0 inches in a foot. (39.37/12.0)1/2 = 1.81.)

The preceding discussion gives an idea of how physical principles can be applied to engineer a water-based fire suppression system. Water has many desirable features for use on solid combustibles: • Water is readily available at low cost.

• It is nonflammable.

• It has a high heat of vaporization.

• Owing to its boiling point at 100 °C, water gasifies readily and well below the range of pyrolysis temperatures for most solid combustibles (250 °C to 450 °C).

• Upon vaporization, water expands greatly. One mole of water at 298 K occupies 0.018 L. The same 1 mole of steam at the boiling point of water (373 K) occupies 30 L. • Water is a readily transportable liquid at normal ambient temperatures.

• It is nontoxic and does not decompose to form toxic products.

• Used in moderation, water causes limited property damage. (Some, but not all, water-damaged items can be salvaged after a fire, whereas items directly affected by the fire cannot be salvaged.)

Nonetheless, water cannot be used without care to minimize its limitations. Pure water is a less desirable fire suppressant for Class B (liquid fuel) fires. Water has a higher density than many hydrocarbon fuels, such as gasoline. Thus, when the water reaches the fuel surface, it sinks, which can then cause the fuel to spread or even boil over the edge of its containment. (See the Fire Characteristics: Liquid Combustibles chapter.) Water must also be used with care in the vicinity of energized electrical equipment (Class C) fires. As an electrical conductor, its application creates the potential for circuit damage and electrical shock or electrocution. Water also reacts—sometimes explosively—with some metal vapors, so its use on Class D fires should be avoided.

The modes by which water controls and perhaps extinguishes a fire depend on the way it is delivered to the fire and the total mass applied per unit area of the fire. When an overwhelming mass of water is applied, such as by flooding a compartment, the water cools the fuel and isolates the fuel surface from the air above. Geometrically, this approach is effective only when the fire is on or near the floor. When a high-momentum hose stream is directed at a large fire, the bulk of the water reaches the fuel surface, substantially cooling the contacted fuel area and decreasing the rate of pyrolysis or evaporation. In addition, the hot surface vaporizes some of the water. The steam raises the heat capacity of the air, thereby lowering the gas temperature, and also dilutes the oxygen in the air that is feeding the flames.

The vaporization of the water absorbs enthalpy. These three effects slow the rate of combustion. This use of water is effective when fire fighters have a direct line of sight between the hose and the fuel. When

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fire fighters do not have direct access to the fire, a hose stream can help control the fire by cooling the surroundings, thereby reducing their potential for pyrolyzing and igniting.

The water discharged from an automatic sprinkler or a spray nozzle is in the form of droplets. The size distribution of these droplets has important implications. If the droplets are too small, they cannot penetrate to the seat of the fire because they will be carried upward by the fire plume or will evaporate too quickly. If they are too large, their surface-to-mass ratio will be small and, in turn, their (endothermic) vaporization rate will be small. In such a case, the water will be less effective at cooling the fire gases or the ceiling above the fire. It would appear that a stream of large droplets with a number of entrained small droplets would provide the best aspects of both size ranges, but such a distribution is not easy to achieve.

Figure 13-1 shows the droplet flow from an automatic sprinkler. Figure 13-2 shows a representative drop-size distribution from a commercial automatic sprinkler, indicating that the mean droplet diameter is typically in the millimeter range. The mean droplet diameter increases with the radial distance from the sprinkler axis.

The mean diameter of water droplets from a sprinkler or spray nozzle depends on three variables:

Figure 13-1 An pendent automatic sprinkler discharging water. Š Ocean/Corbis.

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Figure 13-2 Droplet diameter distributions at various radial distances, 3 m below a commercial 0.5-in. sprinkler operating at ΔP = 200 kPa (2 atm). The ordinate is the percent of the water mass in droplets with diameters smaller than d. [3].

• The design and size of the device—in particular, the orifice diameter, D • The pressure drop, ΔP, across the device (the water pressure) • The surface tension, σ, of the water The mean droplet diameter, dd, is approximated by the relationship Equation 13-2

where c is a constant that is determined experimentally.

The surface tension of the water has an effect in some applications where the water is not pure. On ships, seawater is normally used for fire control. The presence of even 10 percent sodium chloride raises the surface tension of water by only about 4 percent—a small change. However, the addition of a wetting agent to the water (e.g., to coat not-yet-burning fuel more effectively and thereby reduce the potential for fire spread) has a more profound effect. The addition of only 0.1 percent sodium octylsulfosuccinate lowers the surface tension by about a factor of 3.

Conventional sprinklers are most effective when the water reaches the fire directly and when the fire is located on or near the floor. The delivery rate required to control a fire can be expressed in units of volume of water delivered per unit floor area per unit time (m³/m²/min or gal/ft²/min) or as water depth accumulated per unit time (mm/min or in./min), in the same way that rainfall is measured. One gal/ft²/min is the same as 40.7 mm/min. The minimum application rate needed depends on many variables, including the nature of the combustible [2].

Water mist is a spray of very small droplets, typically with diameters on the order of hundreds of micrometers or less [4]. Fire fighters may direct a water mist at a fire using a hose with a special nozzle (fog nozzle). Alternatively, a fixed water mist system may flood the compartment with the droplets.

A fine water mist can extinguish a fire by any of four mechanisms: 1. The evaporating mist droplets remove heat, either at the surface of the combustible or within the flame.

2. The mist droplets evaporate in the hot environment, perhaps even before reaching the flame; this process generates steam, which acts as a diluent. To cool or dilute the flame to the point of extinguishment, a sufficiently high concentration of water mist must be achieved—approximately 15 percent water mist by weight in air. To do so, fire fighters can spray the mist directly at the flame.

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Fine water mist is also used as a total flooding agent; that is, the mist is sprayed into the fire compartment but not directly into the flame. In this case, some of the droplets settle to the floor in areas away from the fire. The settling rate depends on the droplet size. A droplet of 10 µm diameter would settle 0.3 m (1 ft) in about 100 s, while a 50 µm droplet would settle at this distance in about 4 s. Thus, to provide total flooding, the fraction of larger mist droplets must be kept small.

3. The mist droplets block radiative heat transfer from the flame to the combustible. Blockage of radiation by a mist will often reduce the intensity or spread rate of a fire, but will rarely suffice to produce extinguishment.

4. If delivered as a high-momentum stream, the mist droplets cool the fuel surface. Most often, the upward momentum of the pyrolysis products keeps the fine droplets aloft long enough to evaporate before reaching the surface.

In those tests in which water mists have extinguished fires, some combination of the first three mechanisms is believed to have played a role. However, it is difficult to quantify the relative contributions of the individual mechanisms.

Enhanced Water The properties of water for certain applications may be improved by using additives. For example, additives may alter the droplet size. These wetting agents also enable droplets to spread over a surface, thereby improving water’s ability to penetrate porous materials such as bales of cotton, stacked hay, or mattresses and increasing water’s efficiency for heat absorption.

For fire suppression in cold weather, adding a chemical—for example, an inorganic salt—that lowers the freezing point of the water can prove beneficial. Large additions of salts are necessary to extend the fluid state appreciably, and these mixtures are generally corrosive. For reference, seawater contains approximately 35 g of NaCl per liter of water and has a freezing point of 28 °F (−2 °C). The addition of 466 g of CaCl2 (80 percent pure, with sodium dichromate corrosion inhibitor) to 1 liter of water lowers the freezing point to −20 °F (−29 °C), a useful decrease. However, this inhibitor reduces but does not eliminate corrosion.

The freezing point of water can be lowered without introducing a corrosive substance through the addition of a miscible organic compound such as ethylene glycol [C2H4(OH)2] or glycerol [C3H5(OH)3]. A solution of 44 percent by volume of ethylene glycol (a freezing-point depressant in common automobile antifreeze) and 56 percent water has a freezing point of −20 °F (−29 °C). However, ethylene glycol has a boiling point of 191 °C and a flash point of 138 °C. After application to the fire, the water preferentially evaporates, so the residual liquid is more concentrated in ethylene glycol and could become flammable.

Glycerol has a boiling point of 554 °F (290 °C) and a flash point of 350 °F (177 °C), so it presents the same hazard. Freezing-point depressants cannot be used in sprinkler piping that is connected directly to a public water supply unless special check valves, known as backflow preventers, are installed.

In some recent fire events, the antifreeze solutions have not been mixed properly. The resulting discharge of the solution from the automatic sprinkler system actually made the fire situation worse.

The flow through a fire hose or sprinkler system is determined by the water pressure from the source (a fire department pumper, municipal water supply main, or storage tank), the inner diameter of the hose or piping, and the resistance to flow of the moving water at the inner surface. Friction-reducing additives enhance the slipperiness of the water, thereby increasing the discharge flow for a given water pressure and pipe diameter. Generally, smaller diameter pipe is preferred because it is less expensive and requires less space to install. Polyethylene oxide is a water-soluble linear polymer that, when added to water in amounts of approximately 0.02 percent by volume, can increase water delivery by 40 percent over pure water.

It is also possible to thicken water—that is, to increase its viscosity substantially, by use of an additive, such as sodium carboxymethylcellulose. When water is thickened and applied as a spray, it adheres more readily to a vertical surface of a combustible, rather than running off. This adherence, to both horizontal and vertical surfaces, increases the rate of extinguishment and reduces the danger of reignition. Also, when projected from a nozzle, a stream of water containing a thickening additive remains as a coherent jet longer (before breaking up) and projects farther.

The use of thickened water does present some problems, however. Its application is associated with poorer penetration of fuel masses, greater friction loss in hoses, inability to produce fine sprays, slippery floors, and more extensive clean-up effort after a fire. Accordingly, thickened water is not used frequently, except in fighting wildland fires, where slippery floors and post-fire clean-up are not issues.

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Aqueous Foams The broad array of aqueous foam agents constitutes a special class of enhanced water. These additives to water vary in their chemistry and the changes they effect in the mixtures. Nevertheless, these agents share some characteristics in common: each is added to water at levels of a few percent by volume, the mixture is aerated to produce a bubbly froth, and the resulting foam has greater effectiveness than straight water in extinguishing some types of fires [5].

The principal application of aqueous foam agents (Class B foams) is for fighting large flammable liquid fires, such as might occur in oil refineries, aircraft runways, and fuel storage areas. Typically, the flammable liquid has formed a pool, and the fire is two-dimensional (length and width). In other cases, foam agents (Class A foams) are used on three-dimensional solid fuel fires.

Many flammable organic fluids (e.g., gasoline) have densities lower than that of water and are immiscible in water. If straight water is applied to a fire involving such a fluid, the water sinks while the fuel continues burning on top of the water. If the burning liquid is an oil or fat whose temperature is substantially higher than the boiling point of water, then the water penetrates the hot oil, turns into steam below the surface, and causes an eruption of oil that accelerates the burning rate and might spread the fire. If the flammable liquid is water soluble, as are many alcohols, then addition of sufficient water will dilute the liquid to the point where it is no longer flammable. However, if the pool is deep, rather than a shallow spill, the fire might do significant damage before sufficient dilution can be achieved.

Typically, because it is highly aerated, an aqueous foam has a density much lower than water. It forms a continuous layer on top of the burning fluid, cooling the fluid surface to reduce fuel gasification, and separating the fuel from the air.

Certain fireground conditions can negate the benefits of aqueous foams: • High winds can blow the foam aside, exposing the flammable fluid to the air above.

• The heat from the fire can break down the cell structure of the foam and vaporize the water fraction of the mixture. The foam must be applied to a burning surface in sufficient volume and rate to compensate for this loss and to provide an additional amount to guarantee a residual foam layer over the extinguished portion of the burning liquid. Applying too little foam will put out only part of a fire, allowing the fire to build back to its original intensity.

• Chemical action of certain vapors or fluids can break down the additive or the foam structure.

• If the additive is mixed with water on demand (instead of being premixed in a storage tank), a mechanical device usually is needed for proportionally mixing the additive with the flowing water Figure 13-3 [5]. The reliability of such a proportioning device is never 100 percent, so the overall reliability of the protection system is reduced slightly below that achievable with the use of ordinary water.

In practice, firefighting foams are formulated for various modes of fire extinguishing action. Some foams are thick and viscous, sticking to surfaces and forming tough, heat-resistant blankets over horizontal or vertical burning surfaces. Other foams have low viscosity and surface tension, so they spread more rapidly. Some produce a vapor-sealing film of surface-active water solution on a liquid surface, and some are meant for use as large volumes of wet gas cells for inundating surfaces and filling cavities. The following categories of aqueous foams are currently in use [6]: •

Aqueous film-forming foam (AFFF). The surface-active chemical (surfactant) in an AFFF is a synthetic molecule that has a “head” and a “tail.” The head end is highly soluble in water, while the tail consists of a short fluorocarbon chain such as [CF3CF2CF2CF2CF2—], which is less soluble in water and more soluble in a liquid organic fuel. These molecules concentrate themselves in the interface between the fuel and the water, forming a stable barrier that cools the fuel surface and blocks fuel vaporization. This suppresses combustion and prevents reignition.

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Figure 13-3 The steps in air foam generation [5].

The film spreads over the surface of the fuel if the surface tension of the combustible liquid is greater than the surface tension of the AFFF solution by an amount in excess of the interfacial tension between the two liquids. Typically, surface tensions of hydrocarbon liquids and AFFF solutions are 24 dyne/cm and 16 dyne/cm, respectively, while the interfacial tension between the two liquids is between 1 dyne/cm and 6 dyne/cm, so the criterion is met. The low viscosity of the AFFF helps it re-cover any surface gaps resulting from, for example, falling debris and firefighting activity.

AFFFs have additional benefits. They can be used to protect flammable liquids that have not yet ignited. Also, because of the extremely low surface tension of the solution draining from AFFF, these foams are superior to water in penetrating a porous fuel mass. Moreover, because of the ease of creating AFFF-enhanced water mixtures, simple spray nozzles and sprinklers can be used to both generate and deliver the foam.

Some AFFF formulations, referred to as alcohol-resistant foaming agents, can suppress fires involving liquids that are miscible with water. Burning combustible liquids that require alcoholresistant foaming agents for extinguishment include alcohols, glycols, acetone, methyl ethyl ketone, isopropyl ether, acrylonitrile, ethyl acetate, amines, enamel thinners, and lacquer thinners. Even a small amount of these substances mixed with hydrocarbons (e.g., in gasohol) causes the rapid breakdown of ordinary firefighting foams.

• Protein foams. A mix of additives gives protein foams the properties that aid in fire-fighting. The foaming agent consists of a high-molecular-weight polymer of natural proteins. A metal salt is added to stabilize the bubble walls in the foam at elevated temperatures, and an organic solvent helps reduce viscosity at lower temperatures. The surface tension of protein foam solutions is much higher than the values for AFFFs (approximately 45 dyne/cm), so protein foam solutions cannot spread in a film-like manner over a hydrocarbon surface. If enough of the foam is applied, the fuel surface can be covered. Protein foams are biodegradable.

• Fluoroprotein foams. These foams contain an additional ingredient—a fluorinated surfactant— that increases their effectiveness in fire situations where some of the fuel can get on top of the foam. Fluoroprotein foams readily shed the fuel and form the surface layer typical of firefighting foams. These products are compatible with dry chemical fire suppressants (discussed later in this chapter) that break down regular protein foams.

• Medium- and high-expansion foams. Variations in the foam generation hardware can produce medium- and high-expansion foams. Conventional (low-expansion) foams have expansion ratios of 20 or less, meaning the final foamed volume is up to 20 times the volume of the starting solution.

Medium-expansion foams have ratios of 20 to 200, and high-expansion foams have ratios of more than 200. The foam concentrates used to achieve the higher degrees of expansion are chemically similar to those used in the foams described previously. When applied to a fire, these foams control or extinguish fires in much the way the other types of foams do. In addition, their light weight makes them more likely to stay on vertical surfaces.

Because the bubble shells of the higher-expansion foams contain very little material, these foams are delicate. The very lightweight bubbles are easily blown away by wind, so these foams are not well suited for use on outdoor fires. In addition, the combustion products can react with the

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foaming agents, and high flame temperatures can break down the cell structure. Consequently, these foams must be applied quickly and in sufficient quantity to extinguish the fire before the foam collapses.

These foams are particularly well suited for flooding confined spaces Figure 13-4. They can also control fires involving spilled liquefied natural gas, where the cold fuel freezes the water from the foam and stabilizes the foam layer.

Figure 13-4 High-expansion foam being applied in an aircraft hangar.

Courtesy of USACE Photo.

Nonaqueous Agents As noted previously, water and water-based fire suppressants are extremely powerful and versatile. In some circumstances, however, these agents may cause considerable damage—a factor that has prompted the development of alternative means of fire extinguishment. These circumstances include the following: • Fires in cluttered, enclosed spaces in which the location of the fire is not known and where line-ofsight application of the suppressant is not possible (e.g., electronics cabinets).

• The presence of irreplaceable objects (e.g., rare books) that are susceptible to water damage.

• Situations in which continued service interruption is problematic (e.g., an in-flight fire in an aircraft engine).

• Facilities in which the mass of a water storage and dispersion system is impractical (e.g., an aircraft engine).

• Fires involving materials that are not compatible with water.

• A number of metals can react exothermically with water to form hydrogen, which burns rapidly.

(See the Fire Characteristics: Solid Combustibles chapter.) Furthermore, violent steam explosions can result if water encounters molten metal.

• Certain inorganic chemicals react with water to form hazardous products. For example, alkali and alkaline earth carbides, of which the best known is calcium carbide, react with water to form acetylene, which is highly flammable. Lithium hydride, sodium hydride, and lithium aluminum hydride react with water to produce hydrogen. The peroxides of sodium, potassium, barium, and strontium react exothermically with water. Cyanide salts react with acidified water to form highly toxic HCN. Certain organic peroxides, used as polymerization catalysts in plastics manufacturing, are so unstable that they must be stored under refrigeration to avoid exothermic heating. If water at normal room temperature were to be applied to these materials, it would provide heat to the peroxide and promote its exothermic decomposition.

• Situations in which the runoff of contaminated water would cause groundwater pollution, such as fires involving pesticides or other toxic chemicals.

• Situations in which an electric shock hazard exists. (Projecting a water spray of discrete droplets is less hazardous than using a nozzle to project a solid stream. Even so, fire fighters standing in puddles of water who touch live electric equipment are obviously at risk.)

Over the past century and a half, a number of fire suppressants that are not based on water have emerged to meet these needs. These can be organized into three categories: inert gases; halogenated organic compounds, some of which can be used to inert a compartment; and powdered inorganic salts, also referred to as dry chemicals.

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Table 13-1 lists some liquids that might potentially be considered for use in fire suppression. A liquid’s fire suppression efficiency depends on it being a liquid (to reach the fire and to separate the fuel and air) and having a high heat of vaporization or heat capacity (to cool the fuel and the flames). Water, for example, is a liquid at ambient temperature and has a distinctively high heat of vaporization. The heat of vaporization values for all the other compounds listed in Table 13-1 are similar to each other and are about an order of magnitude lower; some are gases at ambient temperatures. The heat capacities of some of these compounds are somewhat higher than that of water, but they are toxic. Apparently the human body does not function well in the presence of some partially halogenated organic compounds, with some being toxic and others being anesthetics. The exception is C4F10, which has an extraordinarily high heat capacity; it is discussed further under “Inert Gases.”

Inert Gases A modest number of inert gases are not toxic and have been considered as total flooding fire suppressants or as inertants against fire initiation. These clean agents do not do any harm to the facility, nor do they leave any residue. Each of these gases, in sufficient quantity, prevents the combustion of anything except certain metals and certain unstable chemicals such as pyrotechnics, solid rocket propellants, and hydrazine. (See the Fire Characteristics: Solid Combustibles chapter.) The more efficient inert gases increase the heat capacity of the pyrolyzate–air mixture to the point where the flamegenerated heat cannot keep the temperature high enough to sustain combustion. The less efficient inert gases dilute the gas phase to the point where the mixture is below the lower flammability limit.

Table 13-1 Some Nonflammable Liquids That Could Be Considered as Fire Suppressants [7]

Despite their effectiveness, the inert gases (and to some extent all total flooding agents) are subject to some limitations in their use: •

If the enclosure in which the fire is burning is not airtight, the suppressant will be forced through any gaps. If the concentration falls below that needed for suppression before all embers or metals have cooled, the fire could reignite. Reignition is common with deep-seated fires, such as in upholstered furniture or in cartons of documents.

• It is not practical to flood a very large compartment, such as a warehouse or aircraft hangar, and flooding is not possible with a fire in the open.

• Inert gas available from a cylinder is limited in quantity, and the supply could be exhausted before complete flame extinguishment and quenching of any reignitions.

• Unlike the case for liquids, a gas cannot be projected very far from a nozzle.

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The apparatus most commonly used for measuring the effectiveness of a flooding agent is the cup burner Figure 13-5 [8, 9]. In this apparatus, a cup is filled from the bottom with a liquid fuel, generally nheptane (C7H16). The surface of the fuel is kept level with the lip of the cup. A cylindrical chimney surrounds the cup, and an upward laminar flow of air is established from the bottom of the chimney. The tester lights the fuel and then mixes an increasing flow of the suppressant into the air stream. The minimum volume percent of suppressant needed to extinguish the flame is identified as the test result.

Table 13-2 presents cup burner extinguishment volume percentages for a variety of inert gases. While used mainly for gaseous agents, this apparatus has been adapted to fine mists and powders.

Figure 13-5 Schematic of the cup burner apparatus.

The inert gases are organized into four groups: the noble gases, nitrogen, carbon dioxide, and the fully fluorinated organic molecules.

The noble gasesâ&#x20AC;&#x201D;helium, neon, argon, krypton, and xenonâ&#x20AC;&#x201D;have identical (20.8 J/mol K), very low molar heat capacities. It takes about 40 volume percent added to the air to inert the atmosphere or quench a flame. At these levels, asphyxiation becomes a concern. (See the Smoke and Heat Hazards chapter.) For this reason, IG 541 mixes inert gases with CO2, with the latter gas increasing respiration to create a tenable environment. Krypton and xenon are too expensive to be considered for fire suppression.

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Table 13-2 Minimum Extinguishing Percentages for Extinguishment of n-Heptane Flames in a Cup Burner by Inert Gases [10, 11] Suppressant

Percent by Mass

Percent by Volume

A

—

39.5

N2

31

33.6

CO2

32

23

IG 541 (52% N2, 40% A, 8% CO2)

—

31.7

CF4

37

16

C4F10

32

5.3

C6F12O

33

4.3

Nitrogen is only slightly more effective than the noble gases at inerting and flame quenching, requiring approximately a one-third volume fraction addition to the air. Nitrogen can be stored as a compressed gas, usually at pressure of approximately 14 MPa (140 atm). For space-efficiency reasons, nitrogen can be stored as a cryogenic liquid, where the volumetric compression is 1000 times that of ambient gas. The storage container, however, is quite heavy and bulky. Today, lightweight semi-permeable membranes through which the smaller nitrogen molecules in air pass preferentially relative to the larger oxygen molecules are available. The nitrogen-enriched component is used to inert unoccupied spaces such as aircraft fuel tanks. (The same technique is used to generate oxygen-enriched air for medical use.)

As with the noble gases, the required addition of nitrogen reduces the oxygen level to a point where exposed humans will suffer undesirable effects. Thus all personnel not equipped with self-contained breathing apparatus must be evacuated before flooding a compartment with these inert gases. Certain metals react exothermically with nitrogen (see the Fire Characteristics: Solid Combustibles chapter), so the only acceptable inert gases for these metals are helium and argon.

Carbon dioxide is the most commonly used inert gas. Table 13-3 identifies the minimum proportions of carbon dioxide gas, which, if added to air, form an atmosphere in which various vapors will not burn.

Although the table refers only to vapors, the data are also relevant for liquids or solids because they burn only by vaporizing or pyrolyzing. If steam were used as an inerting or extinguishing agent, the percentage by volume required would be intermediate between that required for carbon dioxide and for nitrogen.

On a volume basis, carbon dioxide is substantially more effective than nitrogen. However, a given volume of carbon dioxide is 1.57 times as heavy as nitrogen (44/28 molecular mass ratio), so the two gases have more nearly equal effectiveness on a mass basis. As noted in the Smoke and Heat Hazards chapter, inhalation of carbon dioxide is toxic at the concentrations required to extinguish a fire, and fatalities have occurred when CO2 suppression systems were accidentally activated in spaces where people were working and could not escape in time.

The storage of carbon dioxide is different from that of nitrogen or the other inert gases because CO2 can exist as a gas, liquid, or solid. However, as shown in Figure 13-6, CO2 can exist only as a gas or solid at normal atmospheric pressure.

At normal atmospheric pressure, the solid form of CO2, commonly known as dry ice, exists only below −110 °F (−79 °C), at which temperature it undergoes sublimation directly to the vapor, without melting.

However, CO2 liquid can exist at elevated pressures, as long as the temperature is above −70 °F (−57 °C) and the pressure is greater than 520 kPa (5.2 atm). This temperature and pressure condition is known as the triple point of carbon dioxide because solid, liquid, and vapor can coexist in this condition.

Table 13-3 Minimum Required Volume Ratios of Carbon Dioxide or Nitrogen to Air That Will Prevent Burning of Various Vapors at 25 °C

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Source: Values calculated from tabulations in Reference [12]. Additional data are available in Reference [13].

Liquid CO2 can be kept in a pressure vessel at any temperature between −57 °C and +31 °C (the critical temperature). Above the critical temperature, no liquid–gas interface will exist in the pressure vessel, so the fluid in the vessel will be a gas. A pressure vessel at 70 °F (21 °C) containing liquid carbon dioxide would be at a pressure of 5.8 MPa (58 atm), which is the vapor pressure of CO2 at that temperature. Comparably sized vessels hold about three times as much CO2 as nitrogen, which is why CO2 is more commonly used for fire extinguishment purposes.

Carbon dioxide is used conveniently as the agent in a hand-held extinguisher to combat a small fire that can be approached closely and is not deep seated. The high vapor pressure is used to expel liquid CO2 from the extinguisher cylinder. The cylinder contains an internal dip tube that reaches to the bottom, thereby ensuring that liquid rather than vapor is discharged Figure 13-7. As the liquid droplets emerge from a nozzle into the lower-pressure environment, instantaneous evaporation occurs, with evaporative cooling of the residual liquid in each droplet. This process causes solidification of the residual portion into dry ice particles at −110 °F (−79 °C). If the liquid were originally at a temperature of 70 °F (21 °C), approximately 75 percent of the discharged liquid would have evaporated and 25 percent would have been converted to dry ice particles.

Some of the dry ice particles might impinge on a combustible surface and have a cooling effect; however, because the heat of sublimation of carbon dioxide is only one-fourth the heat of vaporization of water and because only one-fourth of the CO2 discharged is converted to dry ice, the cooling effect on a hot surface is only 1/16 that produced by water discharged at an equal rate (on a mass basis).

The fully fluorinated hydrocarbons, or perfluorocarbons, have also been used to inert closed spaces against ignition [14]. The smallest of these molecules (CF4, C2F6, and C3F8) have boiling points of −198 °F, −108 °F, and −35 °F (−128 °C, −78 °C, and −37 °C), respectively. They are stored as compressed gases at high pressures. C4F10 has a boiling point of −2 °C; it is stored as a liquid with a substantial vapor pressure. The larger perfluorocarbons are liquids at room temperature and pressure. If their rapid discharge is required, the storage container must be pressurized by a smaller perfluorocarbon or nitrogen.

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Figure 13-6 Phase diagram of carbon dioxide.

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Figure 13-7 Schematic of a carbon dioxide fire extinguisher [15].

As indicated in Table 13-1, the heat capacities of C4F10 and thus the larger perfluorocarbons are very high. The concentrations needed for inerting a space are 10 percent by volume or less for C2F6, C3F8, and C4F10. These compounds are nontoxic, so they do not pose a threat to human tenability.

Unfortunately, upon their discharge, perfluorocarbons have extremely long lifetimes in Earth’s atmosphere. They also have high global warming potential (GWP) values. Consequently, the U.S. Environmental Protection Agency (EPA) restricts their use to applications where other agents are not technically feasible due to performance or safety requirements. (Such applications are discussed in the next section.)

A fully fluorinated organic molecule that has different properties is perfluoroethylisopropyl ketone, (C6F12O) which is used as a total flooding agent. This compound is stored as a liquid. It has a low heat of vaporization (Table 13-1), however; even though its boiling point is 120 °F (49 °C), its vapor pressure at ambient temperatures is about 40 kPa—well above the cup burner value for flame suppression (Table 13-2).

Active Halogenated Agents The active halogenated extinguishing agents are chemical derivatives of simple organic molecules such as methane (CH4) or ethane (CH3—CH3) in which some or all of the hydrogen atoms have been replaced by some combination of the halogen atoms: fluorine, chlorine, bromine, or iodine. Nearly all of these agents are liquids when stored in pressurized tanks at normal temperatures, but they exist as gases at atmospheric pressure and normal temperatures. The halogenated agents are classified as clean agents, although their combustion by-products (HCl, HF, and HBr) are corrosive. Inclusion of a bromine or iodine atom in such an agent provides the greatest fire suppression efficiency. Chlorine atoms rank next in effectiveness, while fluorine atoms contribute the least to fire suppression capabilities. The iodine compounds are generally toxic or unstable during long-term storage.

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Around 1950, the U.S. Army Corp of Engineers developed a name and nomenclature for this family of chemicals. It used the name halon as an abbreviation for halogenated hydrocarbon. Each of the specific compounds was then given a number that contained up to five digits. The first digit indicated the number of carbon atoms in the molecule. The succeeding digits identified the numbers of fluorine, chlorine, bromine, and iodine atoms. Terminal zeros were dropped. Any bonds that were not to halogen atoms were assumed to be to hydrogen atoms. Table 13-4 lists some of the halons.

Some years later, a nomenclature developed for similar compounds used as refrigerants came into use for fire suppressants. In this system, the name â&#x20AC;&#x153;halonâ&#x20AC;? is limited to the classic chlorine- and brominecontaining fire suppressants. All other halogenated compounds are designated by a prefix followed by a series of numbers. The prefix indicates the general nature of the compound. For some compounds, the first number indicates the number of carbon atoms minus 1. (If this number is zero, it is not included.)

The second number gives the number of hydrogen atoms plus one. The third number identifies the number of fluorine atoms. The systems vary for compounds with different atoms and chemical structures.

Table 13-4 gives some examples of the compounds of interest and their nomenclature.

Table 13-4 Examples of Fire Suppressant Chemicals and Their Designations

FC = fluorocarbon; FIC = fluoroiodocarbon; FK = fluoroketone; IG = inert gas.

By the 1970s, halon 1301, owing to its high vapor pressure, had become the agent of choice for total flooding applications. These uses included protection of computer rooms, aircraft engine nacelles, rare book collections, and many other spaces where a clean agent was desirable. Halon 1211, a liquid at ambient temperatures, was used for streaming applications, such as hand-held extinguishers and liquid spill fires. Halon 2402, despite its high boiling point and questions regarding its toxicity, was used both as a streaming agent and for total flooding, albeit infrequently in the United States.

Table 13-5 lists some physical properties of these three halons. They are all liquids at normal temperatures when stored in pressurized tanks. These agents are stored under a high pressure of nitrogen if the liquid must be expelled from the tank more rapidly than would happen under the vapor pressure of the halon alone. The use of nitrogen for pressurization is especially important for outdoor storage in the winter.

Rapid delivery of these compounds is important. As part of the flame interaction mechanism, they react with hydrogen atoms to form the halogen acids, HF, HBr and HCl, all of which are toxic and corrosive. The first bit of deployed agent to arrive at the flame is at a concentration below that needed for quenching, so some breakdown is inevitable. If the rest of the agent arrives and establishes a quenching concentration quickly, little further halogen acid generation will occur.

To minimize halogen acid production, all three of these chemicals are forced from their storage bottles at high rates. What emerges is a cold stream of vapor and (mostly) liquid. The cold streams of halon 1211 and halon 2402, with their high boiling points, remain intact until vaporized by the fire. A large fraction of the stream reaches the fire at once, quickly quenching the flames. Halon 1301 emerges as

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blobs of liquid that are dispersed in multiple directions. Soon after their discharge, these blobs â&#x20AC;&#x153;realizeâ&#x20AC;? that they are at an external temperature greater than their boiling point. The blobs burst, acting as virtual dispersion sources throughout the compartment. Thus, even when there is no direct line of sight between the storage bottle and the fire, halon 1301 rapidly establishes the design concentration throughout the compartment, keeping acid formation to a minimum.

Nonetheless, some degree of halon decomposition does occur during fire suppression, and very small concentrations of HF or HBr can corrode sensitive metal and electronic surfaces. While this decomposition increases the toxic potency of the local environment, suppressing the flames cuts short the generation of toxic products from the fuel combustion and can keep the environment tenable, at least for a moderate time. (See the Smoke and Heat Hazards chapter.)

Table 13-5 Physical Properties and Chemical Formulas of Three Halon Extinguishing Agents

The fire suppression effectiveness of these bromine-containing chemicals is impressive. The addition of approximately 3 to 4 percent by volume to the air enables halon 1211 to quench the flames produced by most organic fuels, and 6 to 7 percent by volume suffices to inert a compartment. The numbers for halon 1211 are only slightly higher. Thus, the storage volume and weight of these agents are both considerably less than the comparable values for the inert gases described earlier. Furthermore, at these low levels, the oxygen percentage in the compartment is barely reduced from 21 percent for most combustibles. By contrast, the required amount of carbon dioxide would reduce the oxygen level to 14 or 15 percent.

Because halon 1301 was recognized as a promising option for quenching or preventing flames in normally occupied areas, researchers investigated its toxicological effects on people. No significant adverse health effects were reported from the proper use of the agent as an extinguishant over the more than 30 years of its use [11]. Little effect on people was noted with exposure concentrations of less than 7 percent by volume in air. At concentrations greater than 7 percent, tingling of the extremities and dizziness was reported; at concentrations greater than 10 percent, pronounced dizziness and reduction of dexterity occurred. These effects disappeared quickly after removal from the exposure. Human exposure data have also been developed for halon 1211. As it was used only as a streaming agent, possible exposures generally resulted from being downwind of the discharge. The effects observed with halon 1301 exposure were also observed with for halon 1211, but at exposures of one-third to one-half the concentration of halon 1301. The permitted exposure times in fire situations reflect these findings.

The mechanism that leads to the high flame quenching efficiency of these compounds differs from the mechanism associated with inert gases. When an inert gas is added to a fuelâ&#x20AC;&#x201C;air mixture, extinguishment occurs when the flame temperature is reduced to less than 1500 K to 1600 K. The extinguishing temperature when halon 1301 is added to a flame is about 1800 K. Something interferes with the flame even at high temperatures, causing it to go out before heat capacity effects do.

As described in the Fire Characteristics: Gaseous Combustibles chapter, the speed of the chain reactions in flame propagation depends on the presence of high concentrations of H atoms, O atoms, and OH radicals. The concentrations of these species are substantially higher than the equilibrium concentrations at flame temperatures. The bromine and chlorine atoms decrease these superequilibrium concentrations to equilibrium levels by catalyzing their recombination to form more stable compounds, such as water:

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where R—H is an organic (fuel vapor) molecule, X is a halogen atom, and Y is an H atom, O atom, or OH radical [16]. At lower atom and radical concentrations, the reaction chemistry occurs too slowly to sustain the flame. Each bromine atom removes 7 to 10 of the active species before it leaves the reaction zone of the flame; chlorine atoms are one-third to one-half as effective as bromine atoms.

Other chemically related compounds had been considered for use in fire suppression. In general, the compounds containing a single bromine atom were similar in effectiveness. The second bromine atom in halon 2402 increases its flame quenching efficiency, but by less than a factor of 2. Eventually, low toxicity and the physical properties determined which halons were the superior choices.

The preceding information was presented to facilitate understanding how chemically active fire suppressants work and why they are so desirable. Unfortunately, the latter part of the 20th century brought realization of a side effect that has severely curtailed these agents’ utility.

In 1974, scientists discovered that fully halogenated hydrocarbons, which are relatively chemically inert and thus long-lived in the lowest layer of Earth’s atmosphere (the troposphere), drift into the stratosphere. The stratosphere is the second layer of Earth’s atmosphere. Its altitude depends on the latitude, but it stretches from approximately 8 km (5 mi) to 50 km (30 mi) above Earth’s surface. Within it resides a concentration of ozone (O3) molecules that absorb most of the sun’s ultraviolet rays, protecting life on Earth from severe radiation damage. Once in the stratosphere, the fully halogenated hydrocarbons catalyze destruction of the ozone molecules to ordinary oxygen (O2). This phenomenon has led to increased ultraviolet radiation at Earth’s surface, especially in the regions near the north and south poles.

In terms of mass, the primary ozone depletion noted by scientists was from the chlorofluorocarbons used as refrigerants, propellants (in aerosol cans), and solvents. However, bromine atoms are more potent than chlorine atoms at destroying ozone, so the firefighting halons were significant contributors to stratospheric harm.

Starting with the 1987 Montreal Protocol on Substances That Deplete Stratospheric Ozone and its subsequent amendments and the 1994 U.S. Clean Air Act, production of chemicals with high ozone depletion potential (ODP) was phased out in all but a few developing countries. This included the firefighting halons 1301, 1211, and 2402. Today, use of halon 1301 is allowed in just a few applications, such as in-flight protection of aircraft engines and cargo bays and protection of combat vehicles. Current stockpiles of halons and halons recycled from discontinued applications continue to be used, but substitution of alternatives is occurring wherever possible.

The replacement chemicals’ contributions to ozone depletion, and thus their ODP values, depend on their chemical reactivity in the stratosphere and the amount of time they reside in the troposphere before decomposing to less injurious compounds (atmospheric lifetime). The first generation of replacement chemicals consisted of mainly hydrochlorofluorocarbons (HCFCs). Replacing bromine atoms with chlorine atoms reduces the ozone depletion once the molecule reaches the stratosphere. Replacing one or more of the halogen atoms with a hydrogen atom creates sites for attack by OH radicals that serve as the “cleaners” of the troposphere, further reducing the ODP values. These chemicals, which have smaller but nonzero ODP values, are also scheduled for phase-out.

For fire suppression in applications where halon 1301 and halon 1211 were used, system designers have turned to organic compounds that lack bromine or chlorine as more efficient alternatives to inert gases and water mist. Achieving this efficiency with little or no chemical activity requires compounds with high heat capacities. The compound must be fairly volatile to be used for compartment flooding and well fluorinated to negate flammability and reduce toxicity.

As chemical manufacturers and fire protection specialists began responding to the ozone depletion constraint, another environmental consideration arose. The intense study of atmospheric chemistry revealed that some chemicals that remain stable in the troposphere absorb infrared radiation from Earth’s surface and heat Earth’s atmosphere by reradiation. Similar to the process of warming greenhouses, this greenhouse effect led to another constraint on the search for firefighting chemicals.

The parallel to ODP is the global warming potential (GWP), which assesses the effectiveness of a compound’s atmospheric heating relative to that of CO2. The GWP value is a function of infrared absorption and atmospheric lifetime.

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Most organic molecules absorb significant amounts of infrared radiation, and the rate of absorption tends to increase as the number of atoms (and thus the number of chemical bonds) increases. Thus, smaller molecules tend to have smaller infrared absorption—a factor that conflicts with the need for the high heat capacity characteristic of larger molecules. The search for successive generations of fire suppressants, therefore, focused on chemicals that reacted with the OH radicals in the troposphere, leading to decreased atmospheric lifetimes, while maintaining low toxicity and some molecular feature that engendered efficient fire extinguishment.

Today, the mass of halon 1301 and halon 1211 in use has decreased substantially. For instance, in locations where water damage does not pose a problem, automatic sprinklers have been installed. With the advent of high-power desktop computers, many computer rooms (formerly protected with halon 1301) have been converted to other uses. Surrogate chemicals are used for training in the use of halon 1211 systems.

A number of clean chemical compounds or mixtures of compounds have been introduced as replacements for total flooding. For normally occupied areas, agents include the inert gases and mixtures of them: HFC-23, HFC-227ea, HFC-125, HFC-134a, and C2F5COC3F7. Mixtures containing hydrochlorofluorocarbons (HCFCs) continue to be used in a few applications, but they are scheduled for phase-out. For normally unoccupied areas, additional alternatives include carbon dioxide, CF3I, HCFC22, and HCFC-124.

For general streaming applications, blends of HCFCs are available. Bromotrifluoropropene (BTP) is being tested for use in hand-held extinguishers.

Table 13-6 presents the formulations and properties of some of the currently used clean suppressants. The important thing to notice about these data is that none of the compounds matches the flame-quenching efficiency of halon 1301 or halon 1211. The cup burner values for the compounds that do not include bromine or iodine atoms generally increase with decreasing specific heat; that is, the larger molecules are more efficient at flame suppression than the small molecules.

Dry Chemical Agents Dry chemical powders, or dry powders, are used mainly to extinguish Class B and Class C fires. The powders can be widely dispersed, thereby flooding a compartment, or they can be applied in a stream, such as with a portable fire extinguisher.

The powders, which are 10 µm to 75 µm in size, are often stored in containers and projected by an inert gas. This can be nitrogen, although in some cases, the gas itself has a fire suppressant value, such as that of an HFC. The particles can also be generated by a combustion reaction, where the evolved heat drives the powder dispersion.

Table 13-6 Properties of Some Currently Used Clean Fire Suppressants [17]

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* Data from manufacturer’s product bulletin.

Multiple processes occur when a dry chemical powder is applied to a fire: • The finer particles vaporize upon reaching the high temperature of the flames, releasing gas-phase species that interrupt the flame chemistry.

• Reaction of flame-propagating species on the particle surface catalyzes their decrease from superequilibrium concentrations.

• The particles shield the fuel surface from the flame radiation, reducing the fuel gasification rate.

• When extensive powder is applied and reaches the fuel surface, it can smother the fire, either by reacting with the fuel or by forming an insulating blanket.

Except in the last process, the extinguishing efficiency of the powder depends on the particle surface area. The particles are generally not smooth and spherical, and all particles are not the same size.

However, studies of particles with different diameters indicate that the least material needed for flame extinguishment occurs when the peak of the size distribution is approximately 40 µm [18]. For a given mass of powder, smaller particles offer more surface area, speeding the rate of gasification. If the particles are too fine, they are convected from the fire in the same way that ultrafine water mist is.

Seven general-purpose dry chemical agents are available. Table 13-7 lists the chemical names, formulas, and popular or commercial names of the various dry chemical agents. In each case, the particles of powder are coated with a chemical, such as zinc stearate or a silicone, to prevent caking and promote free flowing.

In addition to these general-purpose agents, some powders have found use in special situations. For example, rock dust is commonly spread on the floor of a coal mine gallery to suppress potential fires and explosions. For liquefied natural gas (LNG) spills, a special agent developed from granules of cellular glass floats on top of the spill, reducing the burning rate.

It is difficult to compare the effectiveness of one dry chemical with another because a true head-tohead test of chemical differences requires the two agents to have identical particle size distributions.

Furthermore, in the classic flammability limit determinations for gaseous agents, the mixture of agent in air is uniform and stable. Particles, however, tend to settle—a tendency mitigated by agitating the mixture.

Researchers have obtained further information on dry chemical agents’ effectiveness by modifying the cup burner and other types of laboratory flame apparatus to enable testing of the efficiency of powders [19]. Such tests show that KHCO3 is approximately twice as effective as NaHCO3 on a mass basis, because the former agent decomposes at a lower temperature. Other tests indicate that sodium bicarbonate and sodium chloride have comparable effectiveness and are several times as effective (on a

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mass basis) as powders such as limestone or talc, which are chemically inert in a flame. In some conditions monoammonium phosphate is more effective than potassium bicarbonate, while under other conditions, it is claimed to be less effective [20]. Monnex has been claimed to be twice as effective as potassium bicarbonate because of the rapid thermal decomposition of the complex formed between urea and potassium bicarbonate. This process breaks up the particles in the flame so that they form very fine fragments, which then gasify rapidly.

Table 13-7 Dry Chemical Agents [19] Chemical Name

Formula

Popular Name(s)

sodium bicarbonate

NaHCO3

baking soda

sodium chloride

NaCl

common salt

potassium bicarbonate

KHCO3

“Purple K”

potassium chloride

KCl

“Super K”

potassium sulfate

K2SO4

“Karate Massiv”

monoammonium phosphate

(NH4)H2PO4

“ABC” or multipurpose

urea + potassium bicarbonate

NH2CONH2 + KHCO3

“Monnex”

Of the seven types of dry chemicals listed in Table 13-7, only one—monoammonium phosphate—is effective against deep-seated fires because of the glassy phosphoric acid coating that forms over the combustible surface. Nevertheless, dry chemical agents other than monoammonium phosphate remain popular because of corrosion-related concerns. Any chemical powder can produce some degree of corrosion or other damage, but monoammonium phosphate is acidic and corrodes metals more readily than other dry chemicals, which are neutral or mildly alkaline. Furthermore, corrosion by the other dry chemicals is stopped by a moderately dry atmosphere, whereas phosphoric acid has such a strong affinity for water that an exceedingly dry atmosphere would be needed to stop corrosion.

Application of any dry chemical agent on electrical fires is safe (from the viewpoint of electric shock) for fire fighters. However, these agents, especially monoammonium phosphate, can damage delicate electric equipment. Dry chemical agents are typically applied to relatively small flammable liquid fires. For the special case of kitchen fires involving hot fat, monoammonium phosphate is not recommended because of its acidic nature; a dry chemical such as potassium bicarbonate is preferred.

According to Lake [21], the ingredients used in dry chemical agents are nontoxic, but can cause minor skin irritation, temporary breathing difficulty, and reduced visibility.

Another set of powders, as well as gaseous and liquid agents, are used for extinguishing metal fires [20]: • Powders: graphite, NaCl, KCl, BaCl2, Na2CO3, copper, sand • Liquid: trimethylboroxine • Gases: argon, helium, nitrogen, boron trifluoride These fires are difficult to extinguish because of the very high temperatures involved and the correspondingly long cooling times required. The choice of agent depends on the type of metal, the size of the fire, and the circumstances. Halons react with some metals and, therefore, should not be used on metal fires.

The variety of reactive chemicals and materials is so great that it is not possible to present the best techniques of fire fighting in each case here. Further information can be obtained in Fire Protection Guide on Hazardous Materials [22]. The Chemical Referral Center (CHEMTREK) can provide assistance in obtaining nonemergency fire safety information on chemicals; its telephone number is 800-262-8200.

Special Considerations for Fire Extinguishment Extinguishment of Flowing Gas Flames Extinguishment of a fire involving a continuously flowing combustible gas can be very difficult. There is no pyrolyzing surface to cool or isolate, and the gas usually has some momentum that will push a suppressant away. A slow or unsuccessful attempt at extinguishment while the gas continues to flow introduces the possibility of spreading a combustible mixture throughout a building, with the resulting risk

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of an explosion. In some cases, it might be preferable to let the flame continue to burn locally rather than risk a more widespread and intense fire. Most likely, firefighting operations using several techniques would be pursued.

The best tactic is to shut off the flow of gas. If that is not possible, then firefighters can pursue alternative approaches depending on the specific type of fire.

If fire fighters can approach the source of the burning gas, they may apply to the base of the flame a suppressant that will be entrained into the fuel flow. They can project a dry powder or gaseous agent in the same general direction as the burning jet or plume. Fire fighters should take precautions to minimize the possibility of reignition once the agent application is completed, including removing or deenergizing any ignition sources before attacking the fire and cooling any hot surfaces during or soon after flame extinguishment.

Another approach is to interrupt the fresh air supply to the fire. If the compartment can be almost entirely closed, leaving an exhaust vent to prevent pressure buildup, then the combustion products will accumulate and dilute the oxygen to the point where extinguishment occurs. Fire fighters can also accomplish such dilution by flooding the entire compartment with an inert agent.

This type of approach can be extremely risky for multiple reasons. First, flooding the compartment is a slow process. Table 13-8 shows the suppressant volume needed to quench flames of some common gaseous fuels if the fire occurs in a 500 m3 building (e.g., a small industrial facility or a moderate-size house with all interior doors open) initially filled with fresh air. Also, it takes time for the suppressant to mix evenly throughout the enclosure. Second, it is extremely difficult to ensure that all ignition sources are passivated. If the compartment develops a perforation or if someone attempts to enter or leave it, venting of a fuel-rich compartment will occur. As explained in the Combustion Fire, and Flammability chapter, this can lead to a backdraft. Third, at these additive levels, the oxygen volume fraction will be at dangerously low levels for anyone in the building. The carbon dioxide volume fraction is well above the toxic level. (A false alarm or a small fire triggering an automatic inerting system could be equally hazardous to building occupants.)

In the case of a flame propagating through a duct, the propagation can be prevented from passing a given point by the presence of a flame arrestor. A flame arrestor typically creates an array of narrow passageways of high thermal conductivity through which the fuelâ&#x20AC;&#x201C;air mixture must flow. Examples include a wire screen, a metal honeycomb, and a porous metal plate. The metal extracts heat from the flame and quenches it, thereby preventing it from passing through. Properly designed flame arrestors are generally quite effective. However, if the combustion has built up substantial pressure on the flame side of the arrestor, hot gases can be projected through the openings so rapidly that they are not cooled sufficiently and can reignite the flame on the other side. If the flame continues to burn on either side of the arrestor, it will heat the metal sufficiently that the temperature difference between the metal and the flame is small. As described in the Heat Transfer chapter, heat extraction by convection and conduction is proportionate to the temperature difference. Thus the flame arrestor may become ineffective.

Table 13-8 Volume (m3) of Suppressants, Added to a 500 m3 Compartment, That Ensures Noncombustibility of Three Gaseous Fuels

Source: Data from Reference [13] unless noted. * Data from Reference [23].

Extinguishment of a Shallow Liquid Fuel Spill Fire

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A shallow spill fire will burn at high intensity for a short time, perhaps even less than a minute, by which time the liquid will be consumed or the spill will have spread to become so shallow that heat losses to the floor reduce the evaporation rate. Even so, costly local damage can result from the flames if left unattended, and the flames could potentially ignite other combustibles that might burn longer and more intensely. Sufficiently rapid suppression requires an automatic system for sensing the fire and dispersing the agent.

An automatic sprinkler system producing water spray is an inexpensive and widely used approach to general fire control. While effective for Class A combustibles, in some cases it can extinguish flammable liquid fires: â&#x20AC;˘ When the fire point of the liquid is close to or above the boiling point of water, application of a water spray to the shallow spill can rapidly cool the liquid below its fire point. â&#x20AC;˘ When the liquid is water soluble (e.g., methanol, ethanol, propanol, acetone, or ethylene glycol), water application can rapidly dilute a shallow spill, rendering it nonflammable. â&#x20AC;˘ When the liquid is denser than water (e.g., carbon disulfide, phenol, or chlorine-containing liquids), the water spray can form a layer over the surface.

Even if none of these conditions is met, a sprinkler spray, while unable to extinguish the fire, might be effective in limiting the fire damage by cooling and wetting down nearby solid combustibles, as well as the ceiling.

When the fire involves significant volumes of flammable liquids that do not have these properties, the potential for damage is higher. For example, if the liquid has a low fire point and is less dense than water, the water droplets will sink below the surface and turn into steam, causing eruption of flammable liquid into the flames and increasing the burning rate. If this situation can be tolerated for a short time, the shallow spill will become cooled and the fire will go out. If the application of sprinkler water to the spill does not result in extinguishment, the force of the spray will extend the burning area unless a dike has been installed to contain the spilled liquid. One of the foam formulations or a dry powder system will be more effective when the liquid fuels do not meet the criteria outlined here.

Extinguishment of a Deep Tank Liquid Fuel Fire A fire in a deep open tank differs from a shallow spill fire in that it could burn for hours or days if not extinguished. It also differs in that the depth of the liquid makes it difficult to cool or dilute. Ideally, carbon dioxide or a dry chemical could be applied in such a way that the entire flame would be extinguished in one application. (Extinguishing even 90 percent of the flame accomplishes nothing because the flames over the remaining 10 percent will spread to cover the surface as soon as the suppressant disperses.)

The various foams, discussed earlier in this chapter, are ideal agents for extinguishing such fires.

Rules exist for the required quantities and rates of application of foam needed, depending on the exposed surface area of the liquid [5]. As always, nearby hot metal must be cooled to prevent reignition.

It is extremely difficult to extinguish a flammable liquid that is burning while issuing from an opening or overflowing a tank and falling some distance to the floor. This type of three-dimensional fire (or a flammable liquid spray fire) usually is extinguished only by stopping the flow.

Ultrafast Extinguishment of Fires In some instances, a fire must be quenched instantly to avoid dire consequences. The fireball from a shell penetrating the crew compartment of a military ground vehicle, for example, can immobilize the crew if not quenched in less than 0.25 s. Similar speed is necessary to keep an aircraft from suffering fatal damage from a round penetrating an avionics bay and igniting a fuel cell.

This high-speed suppression is achieved by sensitive sensing of the fire followed by one of two types of fast-response flame suppression systems. The first type comprises a storage container of an efficient fire suppressant under high pressure. When an incipient fire is sensed, a small explosive charge (squib) is triggered, opening the valve and releasing the agent. Halon1301, with is rapid dispersion and high flame suppression efficiency, is still used for this purpose. The second type of system consists of a gas generator, much like those used in automobile air bags. The fuel in the generator is a solid propellant that generates high volumes of a gas, generally nitrogen. The pressure wave of the expanding gas blows out the flame. A hybrid version contains a cell of an active fire suppressant that is dispersed by the nitrogen Figure 13-8 [24].

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Figure 13-8 Schematic of a hybrid propellant-driven gas generator [24].

WRAP-UP Chapter Summary • Chemicals that are used to extinguish or control fires can be categorized by the types of fires on which they are effective, the chemical phase, the mechanism of action, the mode of application, and the application system.

• Water is widely used for fire suppression because it is readily available at low cost, is nonflammable, has a high heat of vaporization and gasifies readily, is nontoxic and does not decompose to form toxic products, and causes only limited property damage when used in moderation.

• Water delivery and suppression effectiveness can be enhanced with chemical additives.

• Water should not be used on electrical or metal fires and should be used with caution on liquid fuel fires. For these uses, alternative agents, both chemically inert and chemically active, are available.

• Powdered agents (dry chemicals) are used on Class B and Class C fires.

• Inert gases prevent or quench flames by increasing the heat capacity of the atmosphere and diluting the oxygen. Chemically active suppressants generally interfere with the chain propagating and chain branching chemistry in flames. The choice and uses of chemically active agents are limited by their environmental impacts.

Key Terms aqueous film-forming foam (AFFF) A low-viscosity, water-based foam that spreads rapidly across the surface of hydrocarbon fuels, cooling the fuel surface and blocking fuel vaporization. atmospheric lifetime The estimated average time a chemical compound remains in the troposphere before being removed by chemical or physical processes. clean agent A fire suppressant that does no harm to the protected facility and leaves no residue. critical temperature The temperature of a pure substance above which distinct liquid and vapor phases cannot coexist, regardless of the pressure. cup burner A laboratory apparatus used to measure the volume fraction of a gaseous fire suppressant needed to quench a small laminar diffusion flame. dry chemical powder Any of several powders used to extinguish fires. fire control The containment of the hazards of a fire. fire extinguishment The point at which flames are no longer visible and the combustible item no longer generates heat or combustion products. fire inerting The creation of an environment in which ignition cannot occur. fire suppression Fire extinguishment. flame arrestor A device installed in a pipe or duct to prevent the passage of flame. fluoroprotein foam A firefighting foam that contains a fluorinated surfactant.

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global warming potential (GWP) A measure of how much heat a substance traps in the Earthâ&#x20AC;&#x2122;s atmosphere over a designated time interval, relative to a similar mass of carbon dioxide. halon An organic compound containing one or more halogen atoms. medium- and high-expansion foam A firefighting foam with an expansion ratio between 20 and 200 (medium-expansion) and over 200 (high-expansion). ozone depletion potential (ODP) The ratio of the global degradation of the Earthâ&#x20AC;&#x2122;s ozone layer due to a given substance over the global loss of ozone due to a similar mass of CCl3F (trichlorofluoromethane). perfluorocarbons An organic compound consisting only of carbon and fluorine atoms. plunge test An apparatus and procedure for measuring the response time of an automatic fire sprinkler following its sudden immersion in a heated air flow. protein foam A firefighting foam based on a high-molecular-weight polymer of natural proteins. response time index (RTI) A measure of the sensitivity of an automatic fire sprinkler to actuation in a heated air flow. streaming fire suppressant A fire fighting chemical that is applied to a fire in the form of a directed flow. total flooding fire suppressant A gas or aerosol that quenches a fire by filling the entire volume in which a fire has occurred. triple point The unique temperature and pressure at which all three phases (gas, liquid, and solid) of a pure substance can coexist. water mist A dispersion of very small water droplets used as a fire suppressant.

Challenging Questions 1. What are the differences among fire extinguishment, fire control, and fire inerting?

2. What are the four classes of fires in the United States?

3. How does an automatic fire sprinkler work?

4. Is it better if an automatic sprinkler discharges small droplets or large droplets? Why? 5. List six instances where the use of water for fire suppression is not advisable.

6.

List five ways in which the properties of water can be modified by additives to improve firefighting performance.

7.

What is the best agent for fighting a gasoline spill fire? A gasolineâ&#x20AC;&#x201C;alcohol mixture (gasohol) spill fire?

8. When should a high-expansion foam be used instead of a regular foam?

9. A compartment containing a fire is flooded with an inert gas such as carbon dioxide, and the flame is extinguished. After 10 minutes, the inert atmosphere is dissipated by leaks. What could happen next?

10. Does a cylinder of carbon dioxide at room temperature contain a gas, a liquid, or a solid? When the valve is opened, what emerges?

11.

What are the two environmental phenomena that have led to the limited production and use of halons 1302 and 1211 and that guide the development of their replacements?

12. Write the halon designations for the following compounds: CF3 Br, C2F4BrCl, C2F5I, C4F10, and fully fluorinated benzene.

13.

Why was halon 1301 often used instead of carbon dioxide (which is far less costly) as a flooding agent for a computer room?

14.

What are the relative extinguishing effectiveness qualities of an inert powder such as limestone (CaCO3) and a chemically active powder such as potassium bicarbonate (KHCO3)?

15. What is the first course of action when confronting a house fire that is driven by a natural gas leak?

16.

Under which conditions will an automatic sprinkler or water spray be effective against a liquid spill fire?

17. What is the best way to attack a fire involving a flowing liquid?

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NFPA 13: Standard for the Installation of Sprinkler Systems and NFPA 13D and NFPA 13R: Automatic Sprinkler Systems for Residential Occupancies Handbook. (2013). Quincy, MA: National Fire Protection Association.

Fleming, R. P. (2008). Principles of Automatic Sprinkler System Performance. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

You, H. Z. (1983, May). Sprinkler Drop-Size Measurement. FMRC J.I. OG1E7.RA. Norwood, MA: Factory Mutual Research Corporation.

Mawhinney, J. R. (2008). Water Mist Fire Suppression Systems. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Scheffey, J. L. (2008). Foam Agents and AFFF System Design Considerations. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Scheffey, J. L. (2008). “Foam Extinguishing Agents and Systems. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Haynes, W. M., ed. (2011). Handbook of Chemistry and Physics, 92th ed. Boca Raton, FL: CRC Press. The Handbook is updated annually. For further information, see http://www.hbcpnetbase.com/.

Hirst, R., and K. Booth (1977). “Measurement of Flame Extinguishing Concentrations.” Fire Technology 13: 296–315.

ISO 14520-1:2006: Gaseous Fire-Extinguishing Systems: Physical Properties and System Design: Part 1: General Requirements. (2006). Geneva, Switzerland: International Standards Organization.

Hamins, A., et al. (1994). Flame Suppression Effectiveness. In: Evaluation of Alternative In-Flight Fire Suppressants for Full-Scale Testing in Simulated Aircraft Engine Nacelles. SP 861, Grosshandler, W., R. G. Gann, and W. M. Pitts, eds.

Gaithersburg, MD: National Institute of Standards and Technology.

DiNenno, P. J., and G. M. Taylor. (2008). Halon and Halon Replacement Agents and Systems. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Kuchta, J. M. (1985). Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries: A Manual. Bulletin 680. Washington, DC: U.S. Bureau of Mines.

Zabetakis, M.G. (1965). Flammability Characteristics of Combustible Gases and Vapors. Bulletin 627. Washington, DC: U.S. Bureau of Mines.

Huggett, C. (1973). “Habitable Atmospheres Which Do Not Support Combustion.” Combustion and Flame 20: 140–142.

Conroy, M. T. (2008). Fire Extinguisher Use and Maintenance. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed.

Quincy, MA: National Fire Protection Association.

Linteris, G. T. (2007). Flame Suppression Chemistry. In: Advanced Technology for Fire Suppression in Aircraft. Special Publication SP 1069, Gann, R. G., ed. Gaithersburg, MD: National Institute of Standards and Technology. Available at http://www.nist.gov/el/fire_research/finalreport.cfm.

DiNenno, P. J. (2008). Halon Replacement Clean Agent Total Flooding Systems. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Fleming, J. W., and R. S. Sheinson. (2007). Aerosol Properties. In: Advanced Technology for Fire Suppression in Aircraft. Special Publication SP 1069, Gann, R. G., ed. Gaithersburg, MD: National Institute of Standards and Technology. Available at http://www.nist.gov/el/fire_research/finalreport.cfm.

Yu, H-Z., and J. S. Newman. (2008). Theory of Fire Extinguishment. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Hertzberg, M., K. L. Cashdollar, and C. P. Lazzara. (1981). “The Limits of Flammability of Pulverized Coals and Other Dusts.” Proceedings of the Combustion Institute 18: 717–729.

Lake, J. D. (2008). Chemical Extinguishing Agents and Application Systems. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Fire Protection Guide to Hazardous Materials. (2010). Quincy, MA: National Fire Protection Association.

Kumar, R. K., H. Tamm, and W. C. Harrison. (1984). Intermediate-Scale Combustion Studies of Hydrogen–Air–Steam Mixtures. NP-2955. Palo Alto, CA: Electric Power Research Institute.

Grosshandler, W. L. (2007). Powder Panel and Propellant Discharge Technologies. In: Advanced Technology for Fire Suppression in Aircraft. Special Publication SP 1069, Gann, R. G., ed. Gaithersburg, MD: National Institute of Standards and Technology. Available at http://www.nist.gov/el/fire_research/finalreport.cfm.

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CHAPTER 14

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Computational Modeling of Fires OBJECTIVES After studying this chapter, you should be able to: • • • • • •

Explain the value in using computer fire modeling.

Describe the difference between a deterministic and a probabilistic fire model.

Describe the characteristics of both zone and field models.

Describe the difference between retrospective and prospective use of a fire model.

Explain model validation, model verification, and model accuracy.

Explain the limitations of computer fire models.

Introduction Perhaps the most expansive application of computer fire modeling to date has been the reconstruction of the fires and the thermal environments they created in the three tall buildings of the World Trade Center in New York City on September 11, 2001. The synchronization of thousands of photographs and videos of the disaster from news outlets and the general public led to a view of the progress of the fires from outside the buildings. Adapting the latest modeling techniques, scientists were able to generate animated simulations of the interior fires, which were moving simultaneously on multiple floors and which resulted in the exterior observations. All told, analysis of the many months of computer runs enabled fire scientists and structural engineers to understand how the three buildings came to collapse on that day [1, 2].

Not all fire problems require approaches that are so complex and time consuming. This chapter begins by examining the simplest form of computational models and then discusses the increasing sophistication of more versatile models.

Types of Models For decades, fire safety engineers have used algebraic equations to estimate a wide range of fire phenomena, such as ease of ignition, temperature rise in a room, and time to untenable conditions.

Some of these equations appeared in earlier chapters of this text. In the 1980s, a compilation of many of these equations provided engineers with a coherent resource for a wide range of fire safety assessments [3].

Each of these equations is the embodiment of a fire model. For example, in this text, an equation in the Heat Transfer chapter (Equation 5-5) represents (models) an object whose surface temperature is uniform and whose emissivity is constant across the entire surface. Also implicit in this model is the notion that the surface area does not vary during the time interval of interest. This simplified model has widespread use and, in fact, serves as a mainstay of heat transfer calculations, whether near a fire or in outer space. The model would require adaptation for use in calculating the heat radiated from a fireplace log, where the temperature and emissivity vary over the surface.

What should be evident from even this basic example is that sometimes all of the assumptions behind a model are appropriate to a chosen application and sometimes they are not. The creator/author is responsible for identifying the assumptions behind the equation, and the user must then apply the equation only where appropriate.

The next level of sophistication encompasses calculus-based calculations. These calculations were solvable using even the earliest computers. Computing power and understanding of fire have grown concurrently, with the former enabling incorporation of the latter into ever more sophisticated and accurate formulations. The resulting entities are now known as computational fire models. A compilation of these models in 1992 cited 62 fire models of various types [4]. By 2002, this number had grown to 139 [5]. An update in 2007 indicated that the number of models was no longer increasing dramatically, but that some of the models had been substantially updated [6]. Subsequent sections of this chapter identify the categories in which these models are grouped (zone models, field models, detector response models, egress models, fire endurance models, and miscellaneous models).

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The construction of a computer fire model involves a sequence of steps: 1. Selection of the phenomena to be modeled, such as fire growth over an upholstered chair following ignition by a space heater, the spread of smoke throughout the building, detection of the fire, and the response of occupants to the fire and the alarm.

2. Identification of the physics, chemistry, and perhaps human factors that will be included in the model.

3. Creation or adaptation of equations to represent the physics and chemistry.

4. Creation of computer code that represents and solves the equations.

5. Specification of appropriate input data for each of the variables in the computer code.

6. Verification that the computer code correctly represents the phenomena.

7. Validation of the accuracy of the predictions of the computer code.

Although some computer models were written by a single person, the most comprehensive models incorporate the efforts of multiple people over many years.

Two basic types of computer fire models are distinguished [7].

• A deterministic model predicts the outcome of an individual fire. The calculation relies on a set of equations, with user-selected input values for the variables. This chapter focuses on deterministic models.

• A probabilistic model calculates the likelihood of the outcome of a class of fires and incorporates the relative occurrence (or fraction) of different alternatives at each sequential stage of the fire. The outcome of the individual fires in the class is often obtained from running a deterministic model for each fire. The fractions can be derived from experience (i.e., data on past fires), engineering estimates, or trial-and-error. Later, this chapter provides an example of how one can use deterministic modeling to work a probabilistic problem.

As for the subjects of the fire models, the largest class calculates the distribution of temperatures, smoke aerosol, and toxic gases in the fire compartment and perhaps in a series of interconnected compartments (i.e., a building). The model user supplies such information as the compartment and building geometry, the ventilation conditions, the types and locations of the combustible materials present, the ignition site, and usually the burning rate of each ignited object (or at least how it would burn if in the open—that is, in a well-ventilated area, with no radiative feedback from the surroundings).

Very few of the “fire models” can actually model the fire.

One class of submodels deals with the time required to actuate smoke alarms or automatic sprinklers at various locations. The user specifies the component of the fire effluent being sensed, the geometry of the alarm module, and the sensitivity of the sensor within.

Another class of models calculates the time needed to evacuate buildings [8]. The user supplies information such as the nature and initial locations of the occupants, their willingness and ability to follow programmed rules for their movement within the building, and their susceptibility to heat and smoke. The combination of fire behavior, smoke movement, fire detection, and people models enables the model user to estimate the times at which people will clear the building or stay in a location that is safe until they can be rescued. Some egress models do not include the fire; instead, they simply model how people would evacuate a building in an emergency.

Yet another class of models deals with fire endurance. If a fire continues for a long enough time, the heat may cause failure of steel, reinforced concrete, or wooden structural components. Such a model calculates how long a thermally exposed structural component can survive before weakening or failing.

Researchers now recognize that a larger segment of the structure (i.e., an assembly of structural components) must be modeled to predict thermal vulnerability. Only the most sophisticated models have this capability at present [9].

Other models have been developed for specific fire problems. At least five models deal with the interaction of water spray from sprinklers with a fire; their objective is to predict whether and when fire control is achieved. Other models have been developed to predict such aspects as the rate of flame spread, breakage of windows, extension of fire plumes from windows, smoke flow in ventilation systems, and fires in mine shafts, among others.

Each of these models was derived from a set of physical principles. Some models have been subjected to verification, the determination of how well the computer code captures the intent of the calculation method. Some have been subjected to validation, a set of experiments that assesses the

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accuracy with which a model predicts the fires for which it is intended. Some models have been examined to assess their sensitivity to uncertainty in the values of the input data or to alternative simplifications of the physics and chemistry in the fire. Regrettably, these processes have not been performed for all models; moreover, when they are conducted, the uncertainty and accuracy are not always stated explicitly.

Users of Models In general, there are two types of users of computer models: prospective and retrospective. Prospective users are interested in products with reduced flammability, better design guidelines for fire-safe structures or vehicles, or approval of an architectural plan. Retrospective users are concerned with reconstruction of the details of actual fires.

The prospective user addresses the facility before a fire occurs, with all the possibilities of design, furnishing, occupancy, fire type, and so on. Performance-based design (PBD) of a building and evaluation of whether a proposed building code exception maintains equivalence to the fire safety provided by the code are ideal examples of this type of model application. This type of assessment might address the following questions: 1. In a large building, what gain in life safety is obtained by providing an extra stairwell or an automatic sprinkler system?

2. To what extent does replacing the furniture or wall coverings with more fire-resistant products change the losses from the fire?

3. How many fire alarms should be installed, which types, and where?

4. What should be the capacity of a fan for smoke removal?

5. Which modifications of an existing structure are needed, if any, to result in a high probability of very low casualties from a fire?

6. Could a computer fire model be a key element in a building code?

Prospective users also include students who are learning about the extent to which the outcome of a fire depends on the properties of the building, combustibles, and fire characteristics.

Retrospective users usually consist of litigants, arson investigators, code officials, and instructors.

They focus on a fire that has already occurred, and for which many specific pieces of information are available (subject to the degree of destruction of the facility), such as where the fire started, how it grew over time, and who was in the building. Reliable observers are not always available, especially during the early stages of a fire, so a computational model can help bridge the gap between the origin of the fire and later observations and deductions from a postfire study of the remains. The importance of fire models to litigators or arson investigators is obvious. In addition, when a building conformed to the building and fire codes but nevertheless suffered a major fire, the code officials and code developers have a clear interest in determining what went wrong. Instructors can use the reconstruction as a teaching tool, including cases where the consequences of a fire should have been severe, but were not.

The most common types of deterministic computer fire models used by both types of users are presented in the next two sections. These predict the distribution of the products of a fire within a compartment and throughout a building.

Zone Models The Zone Approximation Zone models begin with the premise that the smoke concentrations and temperatures generated by a fire can be sufficiently simulated by dividing each compartment as a small number of subvolumes, or zones. Within each zone, the temperature, smoke opacity, carbon monoxide concentration, and other factors are all uniform. For example, a two-zone model (the most common choice) includes a hot, smoky upper layer and a cool, relatively clear lower layer; these layers are separated by a horizontal plane.

Typically, a fire is represented by a point source of heat and smoke. A fire plume entrains air at a calculated rate, transporting it to the plane separating the two zones. The mass and average velocity of air entering or leaving each zone and compartment are calculated by solving equations for conservation of mass and enthalpy. With todayâ&#x20AC;&#x2122;s desktop computers, one-, two-, and even three-room simulations can take only a few minutes, enabling rapid examination of a variety of â&#x20AC;&#x153;what if?â&#x20AC;? possibilities.

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Some zone models have only one zone per compartment. Their uses include prediction of flows under forced ventilation and post-flashover room temperatures.

The first step in using the zone model is to input the prefire conditions (e.g., room dimensions, location of doors and windows, thermal properties of the walls and ceiling). This task is straightforward.

The second step, which is not quite so straightforward, is to input the fire itself into the model. In general, this information must be obtained from fire tests performed under a calorimeter. (See the Fire Characteristics: Solid Combustibles chapter.) Often, if fire test results for the specific combustible item of interest are not available, an approximation of the fire behavior (heat-release rate versus time) is made based on available fire test results from somewhat similar items [10].

Figure 14-1 depicts the actual heat-release rate for two types of upholstered chairs, each ignited with a pilot flame. The first chair (solid blue curve) burns to total destruction within about 7 minutes. The dashed blue curve for the second chair shows a large initial peak as the fabric and padding materials in the cushions flame vigorously and then become fully consumed. This combustion ignites the wooden frame, whose burning rate increases, giving rise to the second peak. This peak declines gradually as the wood surfaces char, decreasing the pyrolysis rate.

Using this curve as input in a zone model presents a challenge. Figure 14-1 shows two possible simplified versions of the actual heat release rate profiles. The red triangles in the first approximation have the same peak heights and peak times as the actual heat release rate curves. The bases of the triangles are adjusted so that the total area under the triangles matches the area under the actual curve and the center of each triangle is near the time at which half of the heat release in the actual peak has occurred. The second approximation (green rectangles) might be used in a calculation where the peak heights are less important than the correct value for the total heat released. The area under the rectangular curve is the same as the area under the actual heat release rate curve.

Sometimes, an object may burn more rapidly in a fire compartment than it would in the open. This difference results from the radiant energy feedback from the hot smoke layer and the ceiling to the object. In house-size compartments, this effect is observed at heat release rates greater than approximately 250 kW. Eventually, the object might burn more slowly than in the open, or stop burning altogether. This point occurs when the oxygen volume fraction in the â&#x20AC;&#x153;airâ&#x20AC;? drawn into the base of the fire has been reduced by dilution with combustion products. (Recall that oxygen is converted into CO2, H2O, and CO, and the resulting vitiated air can be entrained into the fire along with fresh air from any openings in the room.) Some zone models include a rendition of these effects.

At each stage of this process, the computer also calculates the composition of the hot layer. To facilitate this process, the model user must specify in advance which fraction of the combustible will be converted to soot, carbon monoxide, and other significant combustion products. These data are generally approximated from prior experiments. Such a calculation is reasonably accurate when the top of the flame remains below the hot layer; it is more difficult to extract accurate smoke yield data from laboratory experiments in which the flame extends into a hot, vitiated layer.

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Figure 14-1 Actual and simplified renditions of the heat-release rate for two pieces of upholstered furniture.

The computer can keep track of the oxygen volume fraction as the plume enters the hot layer, and can terminate combustion when a limit of that fraction is reached, dependent on temperature as well as oxygen concentration. The computer can also track the heat radiated by the soot particles in the hot layer as well as the heated portions of the ceiling and walls. Computer code can be included in the model that allows for the effects of this radiant energy on accelerated burning and ignition of noncontiguous flammable objects in the fire compartment.

The output from these zone models can be used to estimate the time at which a detector just outside the compartment will actuate, the degree of visibility through the hot layer in the second compartment, and the exposure to toxic combustion products that people might experience during evacuation. A zone model has less utility for scenarios involving long corridors and large rooms, especially at short times, where the uniformity of the upper layer and the horizontal nature of the neutral plane have not been established.

The Consolidated Model of Fire and Smoke Transport Zone Model The Consolidated Model of Fire and Smoke Transport (CFAST) [11] is the most widely used fire zone model. Its popularity derives from the following characteristics: • It is easy to download and use.

• A detailed Users’ Guide describes how to obtain the model, verify its correct installation, create input data in an appropriate form, and analyze of the output of a simulation [12].

• A Technical Reference Guide documents its assumptions, equations, capabilities, and limitations [13].

• The validity of the model has been assessed extensively; and sensitivities, accuracy, and limitations are provided.

• CFAST has been used to work many problems, and the lessons learned from these applications have been integrated into the model and the Guides.

• The model runs quickly on an ordinary personal computer.

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• It is accompanied by Smokeview, software that provides visual indicators of the progress of the smoke spread and temperature fields, also on a personal computer.

• Both software packages are free.

CFAST is a two-zone fire model used to calculate the layer height, distribution of smoke components, pressure, and temperature in a maximum of 30 compartments of a building as they evolve over time during a fire. The modeling equations incorporate the conservation of mass, the conservation of enthalpy, the ideal gas law, and relationships for density and internal energy. The user, helped by an on-screen interface, enters the dimensions of the compartments (which can range from approximately 1 m3 to 1000 m3), the locations and sizes of openings between compartments (doors) and to the outside of the building (windows and doors), mechanical ventilation properties, thermal properties of the walls, and properties of smoke alarms and suppression systems. One or more fires are specified in terms of their heat release and species generation rates, all as a function of time. Because the software tracks thermal radiation, the user can specify radiative ignition of other combustibles. Of most help to the user, default values and look-up values are available for most of these parameters. The outputs of the model consist of the variables that are needed for assessing the environment in a building subjected to a fire— for example, the temperatures of the upper and lower gas layers within each compartment, the ceiling/wall/floor temperatures within each compartment, the visible smoke and gas species concentrations within each layer, flows through openings, target temperatures (people and combustibles), and sprinkler activation times. The fire is terminated when the oxygen volume fraction decreases to less than a specified level.

Figure 14-2 shows frames from a Smoke view presentation of a CFAST simulation of a fire experiment. In this scenario, the floor plan is a long corridor with two rooms attached. A small anteroom separates each room and the corridor. The door to the left-hand room is closed; the doors from the corridor to the right-hand room and to the outside are open. The pattern in each doorway denotes the temperature and the flow velocity distribution. The markings in the middle of each room indicate the depth and temperature of the smoke layer.

• The first frame is at 5 s after ignition of a burner, denoted by the orange flames in the back of the right-hand room. The temperature in the upper layer of the burn room is still fairly low, and the neutral plane in the corridor is essentially at the ceiling.

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Figure 14-2 Frames from a Smokeview visualization of a CFAST simulation of a fire in a multiple-room laboratory test fire.

• The second frame is at 10 s after ignition. The upper-layer temperature in the burn room exceeds 212 °F (100 °C), and victims can no longer stand up safely. A hot layer has formed in the corridor with a temperature of 40 °C. Hot gas is flowing through the upper portion of the doorway from the burn room into the corridor because of the expansion of gases in the compartment caused by the heat released. Fresh air is flowing into the burn room.

• The third frame is at 60 s after ignition. The upper layer temperatures in the burn room and the corridor are approximately 390 °F (200 °C) and 176 °F (80 °C), respectively. Victims can no longer stand up safely in the corridor. The flow through the upper part of the exit doorway from the corridor has increased significantly, and noticeable fresh air flow into the corridor is occurring through the lower portion of the doorway.

• The fourth frame is at 250 s. The upper-layer temperatures in the burn room and the corridor continue to rise and are near 480 °F (250 °C) and 212 °F (100 °C), respectively. The flow from the corridor is hotter and more pronounced.

Because it is a two-zone model, CFAST does not treat stairwells or long corridors effectively. In frame 2 in Figure 14-2, the actual corridor smoke profile would be thicker near the exit from the fire room and thinner (or even nonexistent) at the other end.

This type of simulation has been used to re-create portions of the U.S. fire loss experience [14]. For example, investigators used it to calculate the annual number of fire deaths resulting from upholstered furniture fires in one- and two-family residences. To do so, they established a set of fire scenarios that encompassed the fires that resulted in a known number of deaths in the United States. The

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investigators then calculated the number of fire deaths for each scenario, weighted the scenarios according to their actual prevalence, and summed the numbers of fire deaths. Some components of the scenarios include the following: • Layout of the house. The Census Bureau reports the fractions of the housing market accounted for by single-story and two-story residences. The prevalence of upholstered furniture in various rooms was estimated as well.

• Type of furniture ignited. The furniture was grouped according to the type of cover fabric, padding, and cushioning material. Estimates were developed for the prevalence of each group, based on market data and experience.

• Ignition mode and behavior. The distribution of ignition sources and final extent of the fire was based on coding in the national fire reports. The fire behavior, including possible ignition of additional combustibles, was based on laboratory test data accumulated from the published literature.

• Time of day of the fire. Investigators gathered these data from national fire reports.

• Occupant sets. These data included the number of people in the building, their possible locations, their ages, and their condition at the time of the fire. Much of this information came from Census Bureau data.

• Behavioral patterns. These patterns were derived from EXITT, a software program that incorporated interview data from people who had survived residential fires [15].

• Survival criteria. These criteria were developed from sources similar to those described in the Heat Transfer and Smoke and Heat Hazards chapters.

Altogether, more than 28,000 computer runs were performed. This large number shows the advantage of using a fast-running zone model for such a task.

For the cases in which upholstered furniture was the first item ignited, the predicted number of fire deaths was essentially correct, but the distribution among values of the variables was less accurate.

For instance, too small a fraction of daytime fatalities was predicted. When upholstered furniture was not the first item ignited, the model overpredicted the number of fire deaths by about half. In both sets of cases, the errors were attributable to overpredictions in as few as 2 percent of the scenarios.

That the computed results were reasonably close to reality inspires confidence in this merging of deterministic modeling with expert judgment to set up the operation and select the proper input data.

Both zone modeling and the basis for expert judgment have advanced significantly over the succeeding two decades, and one might expect a superior outcome if this effort were repeated today.

Field Models Characteristics of Field Models Field models, also referred to as computational fluid dynamics (CFD) models, predict threedimensional (3-D) distributions of velocity, fire products, and temperature [16]. In their developmental years, a CFD model required a mainframe computer to obtain these distributions in a reasonable time.

Today, these programs run on a conventional personal computer, although researchers sometimes use ensembles of computers to reduce the computation time.

To obtain spatial resolution of temperatures, gas composition, and flow velocity, a CFD model divides a space in many small volumes or cells. For a residential room, these volumes might be several centimeters in dimension. Some models have the ability to decrease the cell size in areas where activity is happening on a smaller scale and to increase the cell size in realms characterized by changes on a larger scale. The time frame for computing the evolution in each cell can also vary. To take mass and enthalpy considerations into account, CFD models add equations for momentum and acceleration.

Chemical reactions are temperature sensitive, and the smaller cells (compared to the large size of a layer in a zone model) offer the potential to include more detailed pyrolysis and combustion chemistry.

Figure 14-3 is an example of a field model calculation of the flow induced in a compartment by a fire located on the floor near the left of the frame. The door soffit appears at the top of the frame. The orientation and length of an arrow indicate the direction and velocity, respectively, of the flow at that point. Notice the low-velocity inflow of air at the bottom of the doorway and the higher-velocity outflow of combustion products under the door soffit. The computer also calculates the temperature and gas composition at each point.

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Figure 14-4 provides another example of a field model computation, this time involving the World Trade Center fire. The airplane flew into the north face (the top of the figure) of the tower. As the combustibles were consumed, the fires then spread mainly to the east and somewhat to the west. The stripes surrounding the image represent the windows and what was seen in the photographs and videos at that time. A black stripe denotes a broken window, an orange stripe means that flames at that window were outside the building, and a yellow stripe indicates that a fire could be seen inside the building behind the window. Each floor was subdivided into 125,000 cells, which were each approximately 0.5 m on a side. A simulation of 105 min of real time took about 1 week for each floor.

Figure 14-3 Field model calculation of fire-induced flow in a compartment with an open door [7].

Reproduced with permission from NFPA’s Fire Protection Handbook®, 20th edition, Copyright © 2008, National Fire Protection Association. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

Figure 14-4 Simulated upper-layer temperatures on the 94th floor of World Trade Center Building 1, 15 minutes after aircraft impact [1].

Reproduced from: Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers, Gann, R.G., ed., NIST NCSTAR-1, National Institute of Standards and Technology, Gaithersburg, MD, 2005. Available at http://wtc.nist.gov.

The Fire Dynamics Simulator

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The most widely used CFD fire model is the Fire Dynamics Simulator (FDS). Currently in Version 5, it is available for free download at http://www.fire.nist.gov. Smokeview, the program for graphically visualizing the output of FDS (as well as CFAST), can be downloaded from the same website, along with extensive manuals and other user support material. FDS is the most widely used fire CFD model for many of the same reasons that CFAST is popular. (However, like other CFD models, it does not run as quickly as a zone model.)

The core function of FDS is the prediction of smoke and heat transport from a fire. The fire and the yields of combustion products serve as input to a simulation. However, the model computes the thermal radiation field and, therefore, can simulate fire growth and ignition of secondary combustibles if a pyrolysis model is supplied. It calculates the spatial extent of the flames and the generation of heat in each cell using single-step, two-step, or multiple-step chemistry. The user enters the positions and sizes of compartments, vents, and obstructions; the thermal properties of the surfaces (mostly from a look-up table); the grid size; the time interval for computation; and other data. The computations can be quite lengthy and require large amounts of computer memory. Thus, the user needs to weigh the desired complexity and precision of a simulation against possible hardware limitations.

The outputs of FDS include temperature profiles (as in Figure 14-4), velocity profiles, total heat release and heat-release rate, thermal radiation profiles, actuation times for smoke alarms/detectors and sprinklers, and more. The results can be portrayed graphically or via Smokeview.

Figure 14-5 shows four frames from a Smoke view rendition of an FDS computation of the smoke flow from a fire that started in the first-floor kitchen of a two-story house. The cooktop is at the lower left of the frames. â&#x20AC;˘ The first frame is at 5 s after the fire began. Most of the smoke is contained in the kitchen by the overhead cabinets, but some has begun to leak into the adjacent space. Note that the wispy smoke layer is not uniform in depth or in blackness (optical depth). â&#x20AC;˘ In the second frame, 10 s after ignition, the smoke flow has reached the far wall and, upon cooling, has begun to run down the wall. Some smoke appears in the doorway. â&#x20AC;˘ The third frame is at 30 s after ignition. The upper layer in the kitchen is hot enough that pyrolyzed fuel is combusting, and the flames have spread to the adjacent area. Smoke is just visible in the doorway on the second floor, and a smoke alarm might be actuated at this point.

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Figure 14-5 FDS simulation of the smoke flow from a fire starting in the kitchen of a two-story residence.

â&#x20AC;˘

The fourth frame shows that in 60 s, the fuel-rich upper layer has extended to the floor in the kitchen. Fresh air is found near the floor, so some flaming is occurring there. The second story is filled with smoke.

For the same simulation, Figure 14-6 and Figure 14-7 show the resulting gas temperatures and wall temperatures, respectively. The gas temperatures are consistent with the location of the smoke layer and the duration of burning. Even in the last frame, the fire has not been burning long. The walls, with their high thermal inertia, are just â&#x20AC;&#x153;catching upâ&#x20AC;? with the air temperature on the first floor. The smoke on the second floor remains relatively cool, as evidenced both from the air temperature profile and from the fact that the smoke is not concentrated in an upper layer.

Computational Modeling and the Limiting Hazard Concept As discussed in the Fire and Smoke Hazards chapter, a fire can pose a number of hazards. The realization of each of these hazards occurs at a particular time in the fire event. This time is determined by some combination of the behavior of the combustibles, the building, the occupants, and the emergency responders. The hazard that first compromises a success objective represents the limiting hazard.

Figure 14-6 Vertical slices of the air temperatures from the fire in Figure 14-5.

Courtesy of the National Institute of Standards and Technology.

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Computational fire models are advancing toward the capability of putting all hazards in context, but have not yet reached the point where they have integrated thermal, toxic, behavioral, and structural components. However, within the realm of smoke effects on people, researchers have sufficient knowledge to estimate which of the hazards is likely to cause harm first. With this information, a rational set of procedures and requirements can be constructed for mitigating actions prior to and during a fire.

For example, for a fire of concern, one could use a computational model to simulate the timedependent temperature, thermal radiation, and concentrations of toxic gases and visible aerosols along the possible egress routes. Using the equations in the Heat Transfer, Combustion Products, and Fire Smoke and Heat Hazards chapters, one could determine whether a person’s likelihood of successfully following a route to safety would be limited by smoke of ordinary toxic potency. If the answer were “no,” then a repeat calculation could identify the smoke potency necessary to affect a person before, for example, the smoke opacity became limiting. If that IC50 were higher than the values for any known products, then smoke toxic potency might not need to be used to screen the combustible items in the building. Conversely, if the answer were “yes,” then options for mitigating this hazard might include such approaches as limiting the smoke potency of the combustibles, speeding the evacuation process, containing the size of the fire, or venting the smoke.

Figure 14-7 Wall temperatures from the fire in Figure 14-5.

Values and Limitations of Models Researchers, developers, and investigators make extensive use of computational fire models for building design and reconstructions of fires. Zone models run quickly enough that multiple variations of a building design and multiple fire threats can be examined. For forensics, this run speed allows for addressing “what if?” questions. Field models offer additional realism that may be necessary for identifying and presenting the provision of an acceptable level of fire safety in an innovative building design, especially one featuring irregular geometry of the interior spaces. For reconstruction of largeloss fires or fires that barely averted large losses, the additional time investment may well be warranted.

However capable computational models might be, their use requires technical insight and judgment.

First, determining whether and how the model includes phenomena that are pertinent to a particular fire

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problem requires careful reading of the technical documentation. Second, literature on the range of fire conditions for which the model has been validated provides a check on the degree to which the computer code renders the phenomena properly. Third, the sources of input data only infrequently provide an indication of the uncertainty in the values. Knowing the sensitivity of the model to its inputs can reduce the potential for misleading simulations due to questionable data. Finally, for a user with a particular problem, the ultimate confidence builder is a determination that the model has reproduced the output data from a similar fire test to within the uncertainty of the experiment.

WRAP-UP Chapter Summary • A variety of computational models are used in fire safety analyses, ranging from single equations to models that require long run times on a computer. All of these models contain assumptions. The model developer must document these assumptions, the sensitivity of the predictions to uncertainties in the input data, and the expected accuracy of the predictions. The model user must understand the assumptions and apply the model appropriately.

• A deterministic model predicts the outcome of a specific fire. A probabilistic model calculates the likely outcome of a class of fires, with the outcome of the individual fires in the class generally being obtained by use of a deterministic model.

• Fire models are used to calculate the magnitudes and distributions of temperatures, smoke aerosol, and toxic gases; the response of fire detection devices; the time needed to evacuate buildings; and the fire endurance of a building.

• Users of computer models may be either prospective (interested in fires that have not yet occurred) or retrospective (concerned with reconstruction of the details of actual fires).

• Zone models divide each compartment in a structure into a small number of zones. Within each zone, each predicted property has a single value. Field models, also known as computational fluid dynamics (CFD) models, predict three-dimensional distributions of the fire and its products.

Key Terms accuracy The ability of a computation to match the actual value of the quantity being measured. computational fire model A fire model that requires the use of a computer for its execution.

Consolidated Model of Fire and Smoke Transport (CFAST) The most widely used zone fire model. deterministic model A mathematical model in which the outcome is determined by fixed physical and chemical relationships, with given input always producing the same output. field model [computational fluid dynamics (CFD) model] A computer fire model that predicts the three-dimensional distributions of velocity, fire products, radiant flux, and temperature.

Fire Dynamics Simulator (FDS) The most commonly used computational fluid dynamics fire model. fire model A representation of one or more fire phenomena. performance-based design (PBD) A methodology for the design of a facility based on meeting a set of functional objectives for the facility. probabilistic model A computer model that treats fire and fire hazard development as a series of states, each of which has a distribution of values. sensitivity The degree of effect on the output of a computation that results from uncertainty in the input data or from alternative simplifications of the physics and chemistry of the fire.

Smokeview Software used to visualize the output from CFAST and FDS. validation The process of determining that a computer model, within its domain of applicability, possesses a satisfactory range of accuracy consistent with the intended application of the model. verification The process of confirming that physical phenomena in a model are correctly implemented. zone model A computational fire model based on the premise that the smoke concentrations the and temperatures generated by a fire can be sufficiently simulated by dividing each compartment as a small number of homogeneous subvolumes.

Challenging Questions 246

1.

What are the differences between zone models and field models? Why not use a zone model all the time? Why not use a CFD model all the time?

2.

A fire investigator has determined that the primary fuel in a house fire was an upholstered chair and is trying to determine near which wall of the (ground-floor) living room the chair was located.

The two pieces of evidence to be matched are the time at which the smoke alarm activated (as recorded by a commercial monitoring firm) and the fire fighters’ observations when they arrived.

How many zone model calculations are needed to resolve this? How many CFD model calculations are needed?

3.

The equation Δ H = m cp ΔT is an algebraic model used to predict the heat needed to cause a

chosen temperature rise in a material during a fire, and the equation Q = 56.7 · 10−12ε T4 is an algebraic model used to calculate the radiation from a hot ceiling onto a combustible item. What are some of the assumptions in each model?

4. Describe the difference between a deterministic and a probabilistic fire model.

5. How do model verification and model validation differ?

6. Give at least two examples each of prospective and retrospective applications of fire models.

7. How might a computer fire model be used as part of a building or fire code?

References 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

Gann, R. G., ed. (2005). Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers. NIST NCSTAR1. Gaithersburg, MD: National Institute of Standards and Technology. Available at http://wtc.nist.gov.

Gann, R. G. ed. (2008). Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team on the Collaps of World Trade Center Building 7. NIST NCSTAR-1A. Gaithersburg, MD: National Institute of Standards and Technology. Available at http://wtc.nist.gov.

Nelson, H. E. (1986). FIREFORM: A Computerized Collection of Convenient Fire Safety Computations. NBSIR 863308. Gaithersburg, MD: National Institute of Standards and Technology.

Friedman, R. (1992). “An International Survey of Computer Models for Fire and Smoke.” Journal of Fire Protection Engineering 4: 81–92.

Olenick, S. M., and D. J. Carpenter. (2003). “An Updated International Survey of Computer Models for Fire and Smoke.” SFPE Journal of Fire Protection Engineering 13 (2): 87–110.

International Survey of Computer Models for Fire and Smoke.

(2008). http://www.firemodelsurvey.com/surveyresults.html.

Beyler, C. L., P. J. DiNenno, D. J. Carpenter, and J. M. Watts, Jr. (2008). Introduction to Fire Modeling. In: Fire Protection Handbook, 20th ed., Cote, A. E., ed. Quincy, MA: National Fire Protection Association.

Kuligowski, E. D., and R. D. Peacock. (2005). A Review of Building Evacuation Models. Technical Note 1471.

Gaithersburg, MD: National Institute of Standards and Technology.

McAllister, T. P., et al. (2008). Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7. NIST NCSTAR 1-9, Volumes 1 and 2. Gaithersburg, MD: National Institute of Standards and Technology. Available at http://wtc.nist.gov.

Babrauskas, V. (2008). Heat Release Rates. In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

Fire Growth and Smoke Transport Modeling with CFAST. (2010). http://cfast.nist.gov.

Peacock, R. D., W. W. Jones, P. A. Reneke, and G. P. Forney. (2013). CFAST: Consolidated Model of Fire Growth and Smoke Transport (Version 6) User’s Guide. Special Publication 1041r1. National Institute of Standards and Technology.

Peacock, R. D., W. W. Jones, P. A. Reneke, and G. P. Forney. (2013). CFAST: Consolidated Model of Fire Growth and Smoke Transport (Version 6) Technical Reference Guide. Special Publication 1026r1. National Institute of Standards and Technology.

Clarke, F. B. III, R. W. Bukowski, S. W. Stiefel, J. R. Hall, Jr., and S. A. Steele. (1990). The National Fire Risk Assessment Research Project Final Report. Quincy, MA: National Fire Protection Research Foundation.

Levin, B. M. (1987). EXITT: A Simulation Model of Occupant Decisions and Actions in Residential Fires. NISTIR 873591. Gaithersburg, MD: National Institute of Standards and Technology.

McGrattan, K., and S. Miles. (2008). Modeling Enclosure Fires Using Computational Fluid Dynamics (CFD). In: SFPE Handbook of Fire Protection Engineering, 4th ed., DiNenno, P. J., et al., eds. Quincy, MA: National Fire Protection Association.

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APPENDIX

A

248

FESHE Correlation Guide Principles of Fire Behavior and Combustion FESHE Course Outcomes

Principles of Fire Behavior and Combustion, Fourth Edition Chapter Correlation

1. Identify physical properties of the three states of matter.

3, 7, 8, 9

2. Categorize the components of fire.

5, 6, 7

3. Explain the physical and chemical properties of fire.

2, 3, 5, 6, 7, 8, 9, 10, 12, 14

4. Describe and apply the process of burning.

5, 6, 7, 8, 9, 11

5. Define and use basic terms and concepts associated with the chemistry and dynamics 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 of fire.

6. Discuss various materials and their relationship to fire as fuel.

2, 6, 7, 8, 9, 10, 11

7. Demonstrate knowledge of the characteristics of water as a fire suppression agent.

6, 13

8. Articulate other suppression agents and strategies.

6, 13

9. Compare other methods and techniques of fire extinguishments.

6, 9, 13

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APPENDIX

B

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Imperial and Metric Conversions Table B-1 Length 1 inch = 0.08333 foot, 1,000 mils, 25.40 millimeters 1 foot = 0.3333 yard, 12 inches, 0.3048 meter, 304.8 millimeters 1 yard = 3 feet, 36 inches, 0.9144 meter 1 rod = 16.5 feet, 5.5 yards, 5.029 meters 1 mile = (U.S. and British) 55,280 feet, 1.609 kilometers, 0.8684 nautical mile 1 millimeter = 0.03937 inch, 39.37 mils, 0.001 meter, 0.1 centimeter, 100 microns 1 meter = .094 yards, 3.281 feet, 39.37 inches, 1,000 millimeters 1 kilometer = 0.6214 mile, 1.094 yards, 3,281 feet, 1,000 meters 1 nautical mile = 1.152 miles (statute), 1.853 kilometers 1 micron = 0.03937 mil, 0.00003937 inch 1 mil = 0.001 inch, 0.0254 millimeters, 25.40 microns 1 degree = 1/360 circumference of a circle, 60 minutes, 3,600 seconds 1 minute = 1/60 degree, 60 seconds 1 second = 1/60 minute, 1/3600 degree

Table B-2 Area 1 square inch = 0.006944 square foot, 1,273,000 circular mils, 645.2 square millimeters 1 square foot = 0.1111 square yard, 144 square inches, 0.09290 square meter, 92,900 square millimeters 1 square yard = 9 square feet, 1,296 square inches, 0.8361 square meter 1 acre = 43,560 square feet, 4,840 square yards, 0.001563 square mile, 4,047 square meters, 160 square rods 1 square mile = 640 acres, 102,400 square rods, 3,097,600 square yards, 2.590 square kilometers 1 square millimeter = 0.001550 square inch, 1.974 circular mils 1 square meter = 1.196 square yards, 10.76 square feet, 1,550 square inches, 1,000,000 square millimeters 1 square kilometer = 0.3861 square mile, 247.1 acres, 1.196,000 square yards, 1,000,000 square meters 1 circular mil = 0.7854 square mil, 0.0005067 square millimeter, 0.0000007854 square inch

Table B-3 Volume (Capacity) 1 fluid ounce = 1.805 cubic inches, 29.57 milliliters, 0.03125 quarts (U.S.) liquid measure 1 cubic inch = 0.5541 fluid ounce, 16.39 milliliters 1 cubic foot = 7.481 gallons (U.S.), 6.229 gallons (British), 1,728 cubic inches, 0.02832 cubic meter, 28.32 liters 1 cubic yard = 27 cubic feet, 46,656 cubic inches, 0.7646 cubic meter, 746.6 liters, 202.2 gallons (U.S.), 168.4 gallons (British) 1 gill = 0.03125 gallon, 0.125 quart, 4 ounces, 7.219 cubic inches, 118.3 milliliters 1 pint = 0.01671 cubic foot, 28.88 cubic inches, 0.125 gallon, 4 gills, 16 fluid ounces, 473.2 milliliters 1 quart = 2 pints, 32 fluid ounces, 0.9464 liter, 946.4 milliliters, 8 gills, 57.75 cubic inches 1 U.S. gallon = 4 quarts, 128 fluid ounces, 231.0 cubic inches, 0.1337 cubic foot, 3.785 liters (cubic decimeters), 3,785 milliliters, 0.8327 Imperial gallon

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1 Imperial (British and Canadian) gallon = 1.201 U.S. gallons, 0.1605 cubic foot, 277.3 cubic inches, 4.546 liters (cubic decimeters), 4,546 milliliters 1 U.S. bushel = 2,150 cubic inches, 0.9694 British bushel, 35.24 liters 1 barrel (U.S. liquid) = 31.5 gallons (various industries have special definitions of a barrel) 1 barrel (petroleum) = 42.0 gallons 1 millimeter = 0.03381 fluid ounce, 0.06102 cubic inch, 0.001 liter 1 liter (cubic decimeter) = 0.2642 gallon, 0.03532 cubic foot, 1.057 quarts, 33.81 fluid ounces, 61.03 cubic inches, 1,000 milliliters 1 cubic meter (kiloliter) = 1.308 cubic yards, 35.32 cubic feet, 264.2 gallons, 1,000 liters 1 cord = 128 cubic feet, 8 feet × 14 feet × 4 feet, 3.625 cubic meters

Table B-4 Weight 1 grain = 0.0001428 pound 1 ounce (avoirdupois) = 0.06250 pound (avoirdupois), 28.35 grams, 437.5 grains 1 pound (avoirdupois) = the mass of 27.69 cubic inches of water weighed in air at 4°C (39.2°F) and 760 millimeters of mercury (atmospheric pressure), 16 ounces (avoirdupois), 0.4536 kilogram, 453.6 grams, 7,000 grains 1 long ton (U.S. and British) = 1.120 short tons, 2,240 pounds, 1.016 metric tons, 1016 kilograms 1 short ton (U.S. and British) = 0.8929 long ton, 2,000 pounds, 0.9072 metric ton, 907.2 kilograms 1 milligram = 0.001 gram, 0.000002205 pound (avoirdupois) 1 gram = 0.002205 pound (avoirdupois), 0.03527 ounce, 0.001 kilogram, 15.43 grains 1 kilogram = the mass of 1 liter of water in air at 4°C and 760 millimeters of mercury (atmospheric pressure), 2.205 pounds (avoirdupois), 35.27 ounces (avoirdupois), 1,000 grams 1 metric ton = 0.9842 long ton, 1.1023 short tons, 2,205 pounds, 1,000 kilograms

Table B-5 Density 1 gram per millimeter = 0.03613 pound per cubic inch, 8,345 pounds per gallon, 62.43 pounds per cubic foot, 998.9 ounces per cubic foot Mercury at 0°C = 0.1360 grams per millimeter basic value used in expressing pressures in terms of columns of mercury 1 pound per cubic foot = 16.02 kilograms per cubic meter 1 pound per gallon = 0.1198 gram per millimeter

Table B-6 Flow 1 cubic foot per minute = 0.1247 gallon per second, 0.4720 liter per second, 472.0 milliliters per second = 0.028 m3/min, lcfm/ft2, 0.305 m3/min/m2 1 gallon per minute = 0.06308 liter per second, 1,440 gallons per day, 0.002228 cubic foot per second 1 gallon per minute per square foot = 40.746 mm/min, 40.746 l/min · m2 1 liter per second = 2.119 cubic feet per minute, 15.85 gallons (U.S.) per minute 1 liter per minute = 0.0005885 cubic foot per second, 0.004403 gallon per second

Table B-7 Pressure 1 atmosphere = pressure exerted by 760 millimeters of mercury of standard density at 0°C, 14.70 pounds per square inch, 29.92 inches of mercury at 32°F, 33.90 feet of water at 39.2°F, 101.3 kilopascal

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1 millimeter of mercury (at 0°C) = 0.001316 atmosphere, 0.01934 pound per square inch, 0.04460 foot of water (4°C or 39.2°F), 0.0193 pound per square inch, 0.1333 kilopascal 1 inch of water (at 39.2°F) = 0.00246 atmosphere, 0.0361 pound per square inch, 0.0736 inch of mercury (at 32°F), 0.2491 kilopascal 1 foot of water (at 39.2°F) = 0.02950 atmosphere, 0.4335 pound per square inch, 0.8827 inch of mercury (at 32°F), 22.42 millimeters of mercury, 2.989 kilopascal 1 inch of mercury (at 32°F) = 0.03342 atmosphere, 0.4912 pound per square inch, 1.133 feet of water, 13.60 inches of water (at 39.2°F), 3.386 kilopascal 1 millibar (1/1000 bar) = 0.02953 inch of mercury. A bar is the pressure exerted by a force of one million dynes on a square centimeter of surface 1 pound per square inch = 0.06805 atmosphere, 2.036 inches of mercury, 2.307 feet of water, 51.72 millimeters of mercury, 27.67 inches of water (at 39.2°F), 144 pounds per square foot, 2,304 ounces per square foot, 6.895 kilopascal 1 pound per square foot = 0.00047 atmosphere, 0.00694 pound per square inch, 0.0160 foot of water, 0.391 millimeter of mercury, 0.04788 kilopascal Absolute pressure = the sum of the gage pressure and the barometric pressure 1 ton (short) per square foot = 0.9451 atmosphere, 13.89 pounds per square inch, 9,765 kilograms per square meter

Table B-8 Temperature Temperature Celsius = 5/9 (temperature Fahrenheit Ð 32°) Temperature Fahrenheit = 9/5 × temperature Celsius + 32° Rankine (Fahrenheit absolute) = temperature Fahrenheit + 459.67° Kelvin (Celsius absolute) = temperature Celsius 273.15° Freezing point of water: Celsius = 0°; Fahrenheit = 32° Boiling point of water: Celsius = 100°; Fahrenheit = 212° Absolute zero: Celsius = 273.15°; Fahrenheit = Ð 459.67°

Table B-9 Sprinkler Discharge 1 gallon per minute per square foot (gpm/ft2) = 40.75 liters per minute per square meter (Lpm/m2) = 40.75 millimeters per minute (mm/min)

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GLOSSARY absolute zero The lowest possible temperature, at which all molecular motion has ceased. This temperature is 0 K, −273.15°C, and −459.67°F. absorptivity The fraction of radiant energy striking a surface that is absorbed, rather than reflected or passed through the material. accuracy 1. The ability of a computation to match the actual value of the quantity being measured. 2. The degree of closeness of measurements of a quantity to that quantity’s actual (true) value. acute toxic effect(s) The effect(s) on a person during exposure to smoke from a single fire. addition polymer A polymer formed without the loss of any atom or molecule. adiabatic flame temperature The maximum possible temperature for a combustible mixture, reached when there is complete combustion of the fuel and all of that heat is applied to raising the temperature of the mixture. aerosol mist Fine solid particles or liquid droplets dispersed in air or another gas. air entrainment The drawing of air into a fire or fire plume due to the buoyant flow of the plume. alcohol An organic compound containing one or more hydroxyl (–O–H) groups attached to carbon atoms. aldehyde An organic compound in which a carbon atom has a double bond to an oxygen atom and a single bond to a hydrogen atom. alkane An organic compound in which all the carbon–carbon bonds are single bonds. alkene (olefin) An organic compound in which there are one or more carbon–carbon double bonds. alloy A mixture of two or more metals. amorphous solid A solid that lacks the geometric order of its atoms, molecules, or ions that is inherent in a crystal. anaerobic pyrolysis Pyrolysis in the absence of oxygen. anion A negatively charged atom or molecule. aqueous film-forming foam (AFFF) A low-viscosity, water-based foam that spreads rapidly across the surface of hydrocarbon fuels, cooling the fuel surface and blocking fuel vaporization. aromatic compound An organic compound containing a ring of conjugated unsaturated bonds. asphyxiant gas (narcotic gas) A gas whose inhalation can cause an adverse physiological effect due to lack of oxygen. atmospheric lifetime The estimated average time a chemical compound remains in the troposphere before being removed by chemical or physical processes. atom The smallest characteristic unit of a chemical element, consisting of a nucleus with a positive electric charge, surrounded by negatively charged electrons. atomic mass The average mass of atoms of an element, calculated using the relative abundance of the naturally occurring isotopes. atomic number An integer equal to the positive electric charge on the nucleus of an atom of each of the 118 elements. autoignition (nonpiloted ignition, thermal ignition) Ignition resulting from heating a fuel without the presence of a flame or spark.

23

Avogadro’s number The number of atoms (6.022 · 10 ) in the gram atomic mass of any element. backdraft Intense flames emanating from a just-opened doorway that introduced fresh air to a fire that had been oxygen starved. balancing The process of adding coefficients to the atoms and molecules in a chemical equation, such that the number of each type of atoms is the same on the left (reactant) side of the equation as on the right (product) side of the equation.

5

2

bar A unit of pressure equal to 10 Pa or 10 kPa. black body An energy emitter (absorber) that radiates (absorbs) the maximum energy per unit area at all wavelengths. boiling liquid/expanding vapor explosion (BLEVE) A violent pressure release that occurs when a closed container of liquefied gas is heated externally, resulting in vaporization of the liquid, creating an internal pressure that exceeds the strength of the container material. boilover The rapid overflow or expulsion of burning liquid fuel from an open container when water, located below the fuel surface, boils and expands.

Bouguer’s law (Beer-Lambert law) A mathematical relationship describing the decrease of intensity of a beam of light as it passes through a semitransparent medium, such as smoke. boundary layer The slow-moving layer of fluid in the immediate vicinity of a surface where the effects of viscosity are significant. buoyancy The upward force exerted by a fluid that opposes the weight of an immersed object. burning velocity The speed with which a flame moves through unburned gas. cation A positively charged atom or molecule. ceiling jet The radially outward flow under a ceiling resulting when a fire plume impinges on a ceiling. cellulosic material A material composed entirely or mostly of a polymer of glucose.

Celsius scale (°C) A temperature scale in which 0°C and 100°C are the freezing point and boiling point of water, respectively. chain branching reaction A chemical reaction in which there is a net increase in the number of free atoms or free radicals.

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chain propagation reaction A chemical reaction in which the number of free atoms or free radicals is unchanged. chain reaction A self-sustaining sequence of chemical reactions in which reactive atoms or free radicals are formed, leading to further reactions. chain termination reaction A chemical reaction in which the number of free atoms or free radicals is decreased. change of state The transformation of a compound from one phase to another (e.g., liquid to gas) without a change in chemical composition. chemical bond The attractive force between two atoms that allows the formation of chemical substances that contain two or more atoms. chemical compound A pure chemical substance consisting of molecules containing atoms of two or more different chemical elements. chemical equilibrium The state in which all chemical species are present at concentrations that have no further tendency to change with time. chemical kinetics The study of the rates at which chemical systems change their compositions in their approach to equilibrium. chemical thermodynamics The flow of enthalpy or energy associated with chemical reactions leading to a state of chemical equilibrium. chronic toxic effect The accumulated damage from exposure to smoke in multiple fires. clean agent A fire suppressant that does no harm to the protected facility and leaves no residue. coagulation The adhesion of particles to form larger particles, generally due to forces weaker than chemical bonding. composite A product consisting of more than one material or a material consisting of multiple solids that remain physically distinct. computational fire model A fire model that requires the use of a computer for its execution. concentration The quantity of a substance in a mixture per unit volume of the mixture. condensation polymer A polymer that splits out small molecules, usually water, during its formation. conduction The transfer of heat from a warmer region to a cooler region of a medium by means of molecular or atomic diffusion and collisions. conjugated bonds A sequence of double (or triple bonds) alternating with single bonds, leading to notably higher bond strengths.

Consolidated Model of Fire and Smoke Transport (CFAST) The most widely used zone fire model. convection The transfer of heat from a warmer region to a cooler region by the movement of one or more fluids. copolymer A polymer made of two or more different monomers. critical temperature The temperature of a pure substance above which distinct liquid and vapor phases cannot coexist, regardless of the pressure. cross-linked polymer A polymer in which the long chains are bonded to one another at intermediate points. crystal A solid consisting of atoms, molecules, or ions fixed in a regular geometric pattern extending in all three spatial dimensions. cup burner A laboratory apparatus used to measure the volume fraction of a gaseous fire suppressant needed to quench a small laminar diffusion flame. deflagration Combustion propagating through a gas or an explosive material at a subsonic velocity, driven by the transfer of heat. density Usually, the mass of a substance per unit volume. However, when an extinguishing agent is applied to a surface, the term density is used to mean the mass rate of application of agent per unit of surface. deterministic model A mathematical model in which the outcome is determined by fixed physical and chemical relationships, with given input always producing the same output. detonation Combustion of a gas or explosive material propagating at a supersonic velocity and driving a shock front directly in front of it. diffusion flame A flame whose propagation is governed by the interdiffusion of the fuel and oxidizer. dry chemical powder Any of several powders used to extinguish fires. electrochemical CO sensor A device that measures the volume fraction of carbon monoxide in the environment by the current the gas induces in a galvanic cell. electron The negatively charged particle common to all atoms. element Any of the 118 kinds of atoms of which all matter is composed. Each element consists of atoms unique to that element and different from the atoms of all other elements. emissivity (surface emissivity) The fraction of radiative energy emitted by an object, relative to that emitted from a black body at the same temperature. The emissivity is between 0 and 1. endothermic reaction A chemical change that absorbs heat. energy The capacity to do work or effect a change within a system at constant volume. enthalpy The capacity to do work or effect a change for a system at constant pressure. The enthalpy released is equal to the energy released plus the product of the pressure and the volume change. exothermic material A material that can undergo chemical reaction that releases heat without an additional oxidizer, such as oxygen from the air. exothermic reaction A chemical change that releases heat. explosive spalling The violent fracturing of a porous material, especially when moisture is liberated within the pore structure and thermally expands at a rate faster than it can migrate to the surface. extensive property A material property whose value is proportional to the quantity of the material.

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extinction coefficient A parameter, obtained using Bouguerâ&#x20AC;&#x2122;s law, that defines how strongly a substance absorbs light at a given wavelength or over a given wavelength range. field model [computational fluid dynamics (CFD) model] A computer fire model that predicts the three-dimensional distributions of velocity, fire products, radiant flux, and temperature. fire barrier material A protective layer that prevents or severely retards the contribution to a fire from any subsurface material(s). fire consequence A specific, quantified fire hazard. fire control The containment of the hazards of a fire.

Fire Dynamics Simulator (FDS) The most commonly used computational fluid dynamics fire model. fire effluent Smoke from a fire. fire extinguishment The point at which flames are no longer visible and the combustible item no longer generates heat or combustion products. fire hazard A condition with the potential to create harmful consequences under a specified fire scenario. fire inerting The creation of an environment in which ignition cannot occur. fire model A representation of one or more fire phenomena. fire point The minimum temperature to which a liquid must be heated, in a standardized apparatus, so that sustained combustion results when a small pilot flame is applied, as long as the liquid is at normal atmospheric pressure. fire retardant A chemical additive that slows the ignition and/or burning rate of a material. fire risk The undesired consequences of a fire multiplied by the likelihood of their happening. fire scenario A specified set of fire conditions, including details of the fire site and its condition, the combustible items, the number and characteristics of the occupants, and anything else that might affect the outcome of the fire. fire suppression Fire extinguishment. flame arrestor A device installed in a pipe or duct to prevent the passage of flame. flash point The minimum temperature to which a liquid must be heated, in a standardized apparatus, so that a transient flame moves over the liquid when a small pilot flame is applied. flashover The often-sudden transition from local burning to almost simultaneous ignition of (nearly) all of the exposed combustibles in a confined area. fluoroprotein foam A firefighting foam that contains a fluorinated surfactant. force The influence on a body that causes it to accelerate if it is free to move. fractional effective concentration (FEC) The sum of the volume fraction of each irritant gas divided by its concentration that causes a harmful effect in the average person. fractional effective dose (FED) The accumulated product of the volume fractions of narcotic gases and the time interval over which they are inhaled divided by the volume fraction-time product that causes incapacitation, death, or any other harmful effect in the average person. free atom An atom that would be stable when combined with another atom of the same type, but which at the moment is unattached, either because it has just been formed by a chemical reaction or because it is at very low pressure or frozen in an inert matrix. free radical A molecular fragment with unsatisfied chemical valences, i.e., the capacity to form one or more covalent bonds. free stream velocity The velocity in the part of a flow that is not disturbed by any object or boundaries. fuel lean A mixture of fuel(s) and oxidant(s) in which there is an excess of oxidant, relative to a stoichiometric mixture. fuel rich A mixture of fuel(s) and oxidant(s) in which there is an excess of fuel, relative to a stoichiometric mixture. glass transition temperature A point within the temperature range over which an amorphous solid liquefies, corresponding to the substance attaining a specified value of a physical property, such as viscosity. global equivalence ratio In a volume or flow, the overall ratio of fuel vapor to air, relative to the stoichiometric ratio for the same fuel. global warming potential (GWP) A measure of how much heat a substance traps in the Earthâ&#x20AC;&#x2122;s atmosphere over a designated time interval, relative to a similar mass of carbon dioxide. glowing A descriptor of smoldering combustion when it is accompanied by visible thermal radiation. gravity The attraction of all physical bodies for each other, most commonly experienced as the force that gives weight to objects with mass and causes them to fall to the ground when dropped. gray body An object with an emissivity (absorptivity) less than 1 at all wavelengths; it emits (absorbs) a proportionately lower fraction of the maximum possible radiant energy. gray body approximation The treatment of an object as having a constant emissivity (absorptivity) at all wavelengths, whether or not this is the case. gross heat of combustion The value of the heat of combustion when the water formed is condensed to liquid form; also called the higher heat of combustion.

Haberâ&#x20AC;&#x2122;s rule The empirical finding that, for a particular gas and toxicological effect, the product of the volume fraction and the exposure time is a constant. halogen Atoms or molecules of the elements fluorine, chlorine, bromine, iodine, or astatine. halon An organic compound containing one or more halogen atoms. heat The enthalpy or energy that travels from a higher temperature source to a lower temperature sink. heat of combustion (at constant pressure) The enthalpy released when 1 mole of a combustible item reacts completely with oxygen at atmospheric pressure 101 kPa and 298 K to form combustion products at 298 K. heat of fusion The energy absorbed when a unit mass of a solid melts without chemical change.

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heat of gasification (of a solid) The enthalpy absorbed by a solid when it vaporizes or decomposes to form gases. heat of solidification The enthalpy released when a unit mass of a liquid solidifies without chemical change. heat of sublimation The enthalpy absorbed when a unit mass of a solid gasifies directly, without forming a liquid and without chemical change. heat of vaporization The enthalpy absorbed when a unit mass of a liquid vaporizes without chemical change. heat transfer The flow of energy from matter at a higher temperature to matter at a lower temperature. homopolymer A polymer that contains only one type of repeat unit. hydroxyl radical (OH) A free radical consisting of a hydrogen atom and an oxygen atom, —OH. ignition The onset of combustion. incapacitation The inability to effect one’s own escape or to progress to a place of refuge, making it unlikely that one would survive without assistance. intensive property A material property whose value is independent of the quantity of the material. ion An atom or molecule in which the number of protons is not equal to the number of electrons. ionization smoke alarm A device that detects fire-generated aerosols by their transport of electric charge. irritant gas A gas that causes a physiological effect by affecting the eyes and/or upper respiratory tract. isomers Two or more molecules, each consisting of the same number and kinds of atoms that are bound together in different ways. isotopes Two or more atoms of the same chemical element, which have the same number of protons but different numbers of neutrons in the nucleus. joule (J) The basic SI unit of energy or enthalpy. It is equal to a force of 1 N acting through a distance of 1 m.

Kelvin scale (K) A temperature scale in which 0 K is absolute zero and 1 K equals 1°C. kilogram (kg) The basic unit of mass in the SI system. Its magnitude is defined as the mass of an object, called the international prototype kilogram, made of an alloy (90% platinum and 10% iridium by mass), machined into a rightcircular cylinder, 39.17 mm in both diameter and height. kinetic energy The energy associated with the motion of an object or a fluid. laminar flame A flame in which the flow streamlines are smooth and fluctuations in the velocity components are negligible. laminar flow (streamline flow) The movement of a fluid in smooth, parallel layers, with no disruption among the layers. lean flammability limit (lean limit) The lowest volume percent of a gas or vapor in air capable of ignition. limiting hazard The fire threat that first reaches a level at which a person’s life is in peril. limiting oxygen index (LOI) The minimum volume percent of oxygen that will support flaming of a material, as measured in a standard apparatus. linear burning rate The rate at which the surface of a liquid pool recedes as it burns. luminous flame height The distance between the base of a flame and the point at which the plume is luminous half the time and transparent half the time. mass The fundamental inertial property of an object. mass fraction The mass of a substance in a mixture per unit mass of the mixture. medium- and high-expansion foam A firefighting foam with an expansion ratio between 20 and 200 (medium-expansion) and over 200 (highexpansion). meter (m) The basic SI unit of length. It is defined as the length of the path traveled by light in 1/299,792,458 of 1 second. methyl radical (CH3) A free radical containing three hydrogen atoms bound to a single carbon atom, —CH3. mole (mol) The amount of a substance that contains Avogadro’s number of atoms or molecules. molecule The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms. monomer A small molecule that can combine with other molecules of the same kind (or different kinds) to form a repeating chain molecule, or polymer.

N-gas model The empirical finding that death or incapacitation from smoke inhalation can be attributed to just a few of the numerous components of the smoke. net heat of combustion The value of the heat of combustion when the water formed remains as a vapor; also called the lower heat of combustion. neutral plane The horizontal (slightly wrinkled) interface between the hot upper layer and the cool lower layer in a fire room or a room into which there has been flow of the hot fire effluent. neutron An electrically neutral particle with nearly the same mass as the proton. Neutrons are part of the nuclei of all atoms except the most common isotope of hydrogen.

2

newton (N) The basic SI unit of force. It is the force needed to accelerate a mass of 1 kg at the rate of 1 m/s .

1

nucleus The positively charged mass at the center of each atom, composed of protons and (with the exception of H) neutrons and containing more than 99.9 percent of the mass of the atom. oligomer A molecule consisting of a few monomer units. optical density The logarithmic ratio of the radiation incident on a material to the radiation transmitted through the material. optically thick A semitransparent or opaque, hot radiating region (such as flame or smoke), for which the intensity of the emerging radiation is independent of the thickness of the region. optically thin A semitransparent, hot radiating region (such as flame or smoke), for which the intensity of the emerging radiation is dependent on the thickness of the region. organic compound A molecule whose structure is based on carbon.

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oxidation The reaction of a compound with oxygen. (More generally, the loss of electrons from an atom or molecule during a reaction, with a resulting increase in its valence.) oxidative pyrolysis Pyrolysis in which atmospheric oxygen plays a role. oxygen consumption calorimetry The determination of heat release rate in combustion using measurement of the depletion of oxygen from the incoming air. ozone depletion potential (ODP) The ratio of the global degradation of the Earth’s ozone layer due to a given substance over the global loss of ozone due to a similar mass of CCl3F (trichlorofluoromethane). pascal (Pa) The basic SI unit of pressure, or force per unit area. It is equal to a force of one N exerted over an area of one square meter. perfluorocarbons An organic compound consisting only of carbon and fluorine atoms. performance-based design (PBD) A methodology for the design of a facility based on meeting a set of functional objectives for the facility. phase change A change of state. phenyl radical An aromatic ring of six carbon atoms, five of which are bonded to hydrogen atoms, with the sixth carbon atom having a free valence, C6H5—. photoelectric smoke alarm A device that detects fire-generated aerosols by their scattering of light. physical fire model An apparatus, including the operating procedure, test specimen configuration, and combustion environment, that is intended to represent a certain stage of a fire. piloted ignition Ignition that results from the presence of a flame or spark. plunge test An apparatus and procedure for measuring the response time of an automatic fire sprinkler following its sudden immersion in a heated air flow. polymer A large molecule consisting of very many repeated units, called monomers. postexposure toxic effect The delayed effect on a person attributable to exposure to smoke from a single fire. potential energy The energy in an object or fluid due to its position in a gravitational field. A compressible fluid may also have potential energy because of its pressure. power The rate at which enthalpy or energy is expended. precision The degree to which the correctness of a quantity is expressed. premixed flame A flame in which the fuel and oxidizer are mixed prior to ignition. pressure Force per unit area. probabilistic model A computer model that treats fire and fire hazard development as a series of states, each of which has a distribution of values. propagating flame A flame that is moving to an adjacent region containing oxygen and fuel that has not yet ignited. protein foam A firefighting foam based on a high-molecular-weight polymer of natural proteins. proton The positively charged particle that is common to the nuclei of all atoms and that comprises the nucleus of the most common isotope of hydrogen. The number of protons in a nucleus defines the element to which the atom belongs. pyrolysis The thermal anaerobic, or oxidative decomposition of a gas, liquid, or solid into other molecules when heated.

The verb form, both transitive and intransitive, is “pyrolyze.” radiant flux The radiant power emitted from or incident on a surface. radiation The transfer of heat in the form of electromagnetic energy. response time index (RTI) A measure of the sensitivity of an automatic fire sprinkler to actuation in a heated air flow. rich flammability limit (rich limit) The highest volume percent of a gas or vapor in air capable of ignition. rollover (flameover) The stage of a structure fire when fire gases in a room or other enclosed area ignite. saturated compound An alkane. semiconductor electrical resistance CO detector A device that detects carbon monoxide by the change in electrical resistance resulting from the gas being adsorbed on a semiconductor. sensitivity The degree of effect on the output of a computation that results from uncertainty in the input data or from alternative simplifications of the physics and chemistry of the fire.

SI units The units used in the metric system of measurement. significant figures In a number, those digits that carry meaning contributing to the number’s precision. smoke The airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, together with the quantity of air that is entrained or otherwise mixed into the mass. smoke-point height For a combustible gas or vapor, the height of the shortest laminar diffusion flame that will just release black smoke from its tip.

Smokeview Software used to visualize the output from CFAST and FDS. smoldering The slow, low-temperature, flameless combustion of a solid that is sustained by the heat evolved when oxygen reacts directly with the fuel surface. smoldering (nonflaming combustion) The slow, low-temperature, flameless combustion of a solid. soot Carbonaceous particles resulting from the incomplete combustion of organic fuels. specific gravity The ratio of the density of a substance to the density of a reference substance at a specified temperature and pressure. For gases, the reference substance is generally taken to be dry air. For liquids, the reference substance is water. spontaneous ignition (self-heating) Ignition of a material by a localized heat-increasing reaction between the material and an oxidant and not involving the addition of heat from an outside source.

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stack effect The vertical flow of air in a building, driven by vertical pressure differences developed by thermal buoyancy due to a difference between the building interior temperature and the outdoor temperature. stationary flame A flame whose location is not changing over time.

Stefan-Boltzmann law An equation that specifies the intensity of radiation emitted by a black or gray body, in terms of its absolute temperature. stoichiometric mixture A mixture of reactants in which the quantities of the reactants are exactly those needed for the formation of the specified products, with no excess of any reactant remaining after the reaction has been completed.

For combustion processes, the reactants are a fuel and oxygen. streaming fire suppressant A firefighting chemical that is applied to a fire in the form of a directed flow. sublimation The evaporation of molecules from a solid to form a gas in the absence of a liquid and without chemical change. thermal conductivity The characterization of a material’s propensity to conduct heat, typically represented using Fourier’s law of heat conduction. thermal runaway Self-heating that rapidly accelerates to high temperatures. thermally thick material A material for which, during a heat transfer process at one surface, all of the other surfaces remain at ambient temperature. thermally thin material A material for which, during a heat transfer process at one surface, the temperature at one or more other surfaces exceeds the ambient temperature. thermoplastic polymer A polymer that softens upon heating and returns to its original state upon cooling. thermoset polymer A polymer that, upon heating, undergoes irreversible change. total flooding fire suppressant A gas or aerosol that quenches a fire by filling the entire volume in which a fire has occurred. triple point The unique temperature and pressure at which all three phases (gas, liquid, and solid) of a pure substance can coexist. turbulent flame A flame in which the fluid movement and the temperature are characterized by irregular fluctuations. turbulent flow Fluid movement characterized by irregular fluctuations. unsaturated compound A carbon compound containing one or more double or triple bonds between carbon atoms. valence The number of bonds an atom can form with other atoms. Some atoms have multiple valences. validation The process of determining that a computer model, within its domain of applicability, possesses a satisfactory range of accuracy consistent with the intended application of the model. vapor pressure The pressure of gaseous molecules over their liquid phase at equilibrium—that is, when the rate of evaporation is equal to the rate of condensation. verification The process of confirming that the physical phenomena in a model are correctly implemented. vinyl polymer A polymer synthesized from monomers that contain a carbon–carbon double bond. viscosity The thickness of a fluid due to friction between neighboring regions of the fluid that are moving at different velocities. vitiation The depletion of oxygen and incorporation of the resulting combustion products in air by a fire. volume fraction The volume of a gas in a mixture of gases per unit volume of the mixture. water mist A dispersion of very small water droplets used as a fire suppressant. watt (W) The fundamental SI unit of power. It is equal to the expenditure of one joule for one second. yield The mass of smoke or a component of the smoke per mass of fuel combusted or pyrolyzed. zone model A computational fire model based on the premise that the smoke concentrations and temperatures generated by a fire can be sufficiently simulated by dividing each compartment as a small number of homogeneous subvolumes.

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INDEX The index that appeared in the print version of this title was intentionally removed from the eBook. Please use the search function on your eReading device to search for terms of interest. For your reference, the terms that appear in the print index are listed below.

A ABS copolymer absolute zero absorptivity acceleration of rigid body accuracy acetylene (C2H2) acidâ&#x20AC;&#x201C;base pairs acrylic window panel active halogenated agents acute effects addition polymer adiabatic flame temperature aerosol mist formation aerosols, smoke mist formation smoke particles, quantity of soot formation types of visibility through smoke yields, measurement of AFFF. See aqueous film-forming foam air entrainment air flow air foam generation, steps in aircraft hangar alcohol-resistant foaming agents alcohols aldehydes alkali metals alkanes alkenes alkynes alloy aluminum ammonia (NH3) amorphous substances/glasses anaerobic pyrolysis anion Apollo 1 fire (1967) aqueous agents aqueous foams enhanced water water aqueous film-forming foam (AFFF) aqueous foams benefits of principal application of area units aromatic compounds asphyxiant effects asphyxiant gases atmospheric lifetime

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atomic mass and dimension atomic number atomizer atoms atomic mass and dimension stability of autoignition automatic sprinkler system average soot curve Avogadro’s number

B backdraft demonstration balanced equations balancing ball-and-stick model balloon model bar barrier formation barrier material bases Beer-Lambert law. See Bouguer’s law bench-scale tests black body radiation, wavelength of blue heat-loss curve boiling liquid/expanding vapor explosion (BLEVE) boilover Bouguer’s law boundary layer aminar to turbulent flow in degree of turbulence and thickness of branched polymers bromine atoms bromotrifluoropropene (BTP) building fire, life safety in Bunsen burner buoyancy, fluid behavior buoyant diffusion flames, zones of burning of solid fuel burning rates of liquid pools test methods for burning velocity premixed flames

C calcium carbonate calorimeters candle-like flame carbon dioxide (CO2) fire extinguisher minimum required volume ratios of phase diagram of triple point of carbon monoxide (CO) carbonaceous solid particles carbonated beverages carbon–fluorine bond carboxyhemoglobin (COHb) cation

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caustics ceiling jet cellulose, structures of cellulosic materials Celsius scale (°C) CFAST zone model. See Consolidated Model of Fire and Smoke Transport zone model CFD models. See computational fluid dynamics models “cgs” system chain branching reaction chain propagation reaction chain reactions chain termination reactions change of state. See phase change char formation chemical bonds, concept of chemical compound chemical elements, abridged list of chemical equilibrium chemical kinetics chemical reaction combustion in in flames chemical thermodynamic procedures chlorinated dioxin, structure of chlorine atoms chlorine gas chlorofluorocarbons chronic effects classes of liquids Clausius-Clapeyron equation clean agents CO. See carbon monoxide coagulation process coefficient of viscosity COHb. See carboxyhemoglobin combustion process community energy enthalpy, fractions of flaming and nonflaming of gases elementary chemistry hydrocarbon fuels hydrogen oxidation premixed methane–oxygen flame chemistry spreads combustion products gaseous CO2 and H2O combustion gases HCN hydrogen halides nitrogen oxides partially oxidized organic molecules smoke aerosols mist formation smoke particles, quantity of soot formation visibility through smoke yields, measurement of smoke alarms composite materials computational fire model CFAST field models

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and limiting hazard concept types of users of values and limitations of zone models computational fluid dynamics (CFD) models computer fire model construction of types of concentration condensation polymer conduction conductive heat conductive heat transfer cone calorimeter conjugated bonds Consolidated Model of Fire and Smoke Transport (CFAST) zone model convection component cooling, rate of convective heat transfer conventional sprinklers conversion factors coolant, fire termination copolymer creeping flame spread critical temperature cross-linked polymers cross-linking process crystals cup burner cyano group

D decomposition process decorative laminates deep tank liquid fuel fire deflagrations density units detectors, types of deterministic model detonations diffusion flames premixed flames vs. distinct modes for ignition downward flame spread droplet diameter distributions dry chemical agents application of types of dry chemical powders dry powder system Dupont Plaza Hotel_fire (1986)

E elastomeric polymers elastomers electrochemical CO sensor electromagnetic spectrum electron micrograph of soot aggregate electronic equipments electrons elements endothermic reactions energetics of chemical change

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energy enhanced water enthalpy_ units equivalence ratio ethene ethylene (C2H4) exothermic chemical reaction exothermic materials exothermic reactions explosions explosive spalling extensive property extinguishing metal fires

F fast-response flame suppression systems FDS. See Fire Dynamics Simulator FEC. See fractional effective concentration FED. See fractional effective dose field models characteristics of example FDS fire-blocking layer fire consequence fire control Fire Dynamics Simulator (FDS) simulation of smoke flow fire effluent fire environment, toxic potency of fire extinguishment by absorption of heat, demonstration deep tank liquid fuel fire of flowing gas flames with foam shallow liquid fuel spill fire ultrafast extinguishment of fires of wax candle flame fire fighting chemicals aqueous agents foams water categories of fire suppressants nonaqueous agents active halogenated agents dry chemical agents inert gases. See inert gases special considerations for fire extinguishment fire gases movement fire plume. See fire plume smoke. See smoke toxicity of carbon dioxide carbon monoxide HCl and HBr HCN nitrogen oxides organic irritants fire hazards assessment of fire hose fire inerting fire initiation

264

fire measurement conversion factors energy and enthalpy units force and pressure units length, area, and volume units mass and density units time units fire plume under ceiling structure, zones in fire point of liquid fire protection specialists fire resistance fire retardants (FR) fire risk fire scenario fire smoke fire spread fire stages and metrics char formation and melting flame spread rate test methods for flaming combustion, ignition to mass burning rate materials and products nonflaming combustion, ignition to pyrolysis solids vs. gases and liquids uncommon fire environments, effects of fire suppressants categories of chemicals, examples of properties of fire suppression effectiveness fire termination fire tetrahedron fire ventilation flame arrestor flame/fire suppression flameover flames categorization of laminar vs. turbulent flames premixed vs. diffusion flames hydrocarbon oxidation reactions interaction mechanism quenching efficiency radiation intensity of pyrolyzes spread rates flaming combustion ignition to flaming fires flammability limits for acetylene of premixed flames flammable liquids flammable organic fluids flash point of liquid flashover fluid behavior, basic elements of buoyancy force and pressure

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viscosity fluid flows fluid behavior, basic elements of kinetic energy laws governing motions, rigid body potential energy fluorinated organic molecules fluorinated polymers fluoropolymer pyrolyzes fluoroprotein foams foam formulations force fluid behavior units Fourier equation for steady-state heat conduction FR. See fire retardants fractional effective concentration (FEC) fractional effective dose (FED) free atoms free radicals free stream velocity freezing-point depressants fuel/air equivalence ratio fuel composition fuel in combustion fuel lean fuel-lean mixtures fuel rich fuel–air mixture

G gas density, calculation of gas flames, extinguishment of flowing gas-phase flame inhibition gas-phase process gas temperatures gas/vapor–air mixture gaseous combustibles chemical mechanisms flames categorization hazardous gases ignition of gases premixed flames. See premixed flames gaseous combustion elementary chemistry hydrocarbon fuels hydrogen oxidation premixed methane–oxygen flame chemistry products CO2 and H2O combustion gases HCN hydrogen halides nitrogen oxides partially oxidized organic molecules gaseous diffusion flames, burners for stabilizing gaseous fuels noncombustibility of gases generator, hybrid propellant-driven ignition of properties of gasification of wood gasoline

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gasâ&#x20AC;&#x201C;soot mixture, emissivity of glass transition temperature global equivalence ratio global warming potential (GWP) glowing combustion glucoses molecule, portrayals of structures of glycerol Grashof number gravitation effect on rigid body gravity gray body gray body approximation green heat-loss curve gross heat of combustion GWP. See global warming potential

H Haberâ&#x20AC;&#x2122;s rule halogenated hydrocarbons halogens acid production halons cold streams of decomposition extinguishing agents, physical properties and chemical formulas of flame-quenching efficiency of stockpiles of Hartford circus fire (1944) hazardous gases HBr. See hydrogen bromide HCFCs. See hydrochlorofluorocarbons HCl. See hydrogen chloride HCN. See hydrogen cyanide heat generation, rate of heat-loss curves heat of combustion heat of condensation heat of fusion heat of gasification heat of solidification heat of sublimation heat of vaporization heat release rates of solid rates vs. time for particleboard of upholstered furniture for various combustible items heat transfer hazards from modes of temperature and heat hemicelluloses higher flame temperature Hindenburg disaster home structure fire with upholstered furniture homopolymer horizontal flame spread horizontal PMMA slab burning in air, heat balance for human exposure data hybrid propellant-driven gas generator, schematic of hydrated alumina

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hydrocarbon fuels gases combustion hydrochlorofluorocarbons (HCFCs) hydrogen (H2) atoms flammability limits of oxidation, gases combustion hydrogen bromide (HBr) hydrogen chloride (HCl) hydrogen cyanide (HCN) hydrogen halides hydrogen–fluorine bon hydroxyl radical (OH) hyperbaric chambers for medical treatment

I ideal gas law ignition of liquid mode and behavior sources in home structure fire test methods for measurement immiscible layers incapacitation incompressible fluids inert gases carbon dioxide effects of to hydrogen–air mixture n-heptane flames in cup burner by nitrogen noble gases infrared radiation insulation factor intensive property International Standards Organization (ISO) intramolecular oxidation–reduction ionization smoke alarm ions iron irritants ISO. See International Standards Organization isocyanurate cross-linked rigid foams isomers chemical properties of examples of second pair of third pair of isotopes

J joule (J)

K Kelvin scale (K) ketones kilogram (kg) kinetic energy

L laminar burning velocities of combustibles in air laminar flames

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vs. turbulent flames laminar flow large fire calorimeter large-scale calorimeters large-scale tests lean flammability limit lean limit length units life safety in building fire light-emitting sign light in combustion lignin limiting hazard concept limiting oxygen index (LOI) linear burning rate pool diameter on linear mass burning rate linear polymers liquefied gases liquefied natural gas (LNG) spills liquefied petroleum gas (LPG), leak of liquid combustibles burning rates of liquid pools flame spread rates over liquid surfaces ignition of liquids liquid fuel fires, hazards of droplets fires, flowing fuels fires, hazards of ignition of pool, burning rates of propane properties of surface, flame spread rates over lithium LNG spills. See liquefied natural gas spills LOI. See limiting oxygen index LPG. See liquefied petroleum gas luminous flame height

M magnesium manometers mass burning rates of square mass fractions mass optical density mass rate of burning mass units materials densities of measurement, defined medium- and high-expansion foams melamines metals meter (m) meterâ&#x20AC;&#x201C;kilogramâ&#x20AC;&#x201C;second system (MKS) units methane molecule ball-and-stick model balloon model chemical formula perspective structure

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planar structure MGM Grand Hotel fire microgravity minimum radiative flux miscible organic compound mock-up design modacrylics mole (mol) molecules and compounds, chemical bonds and valence methane. See methane molecule molten magnesium momentum of rigid body monomer multiply ionized

N N-gas model n-hexane NaK narcotics National Fire Incident Reporting System (NFIRS) National Fire Protection Association (NFPA) natural gas natural materials near-stoichiometric mixture net heat of combustion neutral plane neutron (N) newton (N) Newton, Isaac Newtonian fluids, viscosities of NFIRS. See National Fire Incident Reporting System NFPA. See National Fire Protection Association nitrogen minimum required volume ratios of nitrogen dioxide nitrogen oxides nomenclature non-piloted ignition nonaqueous agents active halogenated agents dry chemical agents inert gases. See inert gases nonflaming combustion ignition to nonthermal smoke damage nucleus Nusselt number (Nu)

O octanes OH. See hydroxyl radical olefins oligomer optical density per meter optically thick organic chemistry nomenclature organic compounds, basic groups of organic irritants organic molecules partially oxidized oxidation of metal

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of nitrogen oxidative pyrolysis oxidizers in combustion oxidizing agent oxygen deficiency percentage, effect of oxygen consumption calorimetry Oxygen Index Test ozone depletion

P particleboard curves pascal (Pa) PBD. See performance-based design PCBs. See polychlorinated biphenyls pendant groups pentachlorobiphenyl, structure of perfluorocarbons performance-based design (PBD) peroxides phase change phases, characterization of phenolics phenyl radical phlogiston photoelectric detectors photoelectric smoke alarm physical fire model pill test piloted ignition planar structure plunge test PMMA. See polymethylmethacrylate polychlorinated biphenyls (PCBs) polycyclic aromatic hydrocarbons (PCAHs) polyesters polyethylenes polymers polymethylene polymethylmethacrylate (PMMA) polystyrenes polyurethanes polyvinyl chloride (PVC) pyrolyzes pool diameter, on linear burning rate postexposure effects potential energy power units Prandtl number precision premixed flames burning velocity vs. diffusion flames explosions, deflagrations and detonations flammability limits stabilizing techniques for premixed methaneâ&#x20AC;&#x201C;oxygen flame chemistry pressure fluid behavior gradients units probabilistic model product

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propagating flames propane, leak of proportions, principle of combining prospective users of fire model protein foams proton pure iron powder PVC pyrolyzes. See polyvinyl chloride pyrolyzes pyrolysis

Q quenching flames

R radiant flux radiant heat flux radiation radiation flux radiative component radiative enhancement, effect of radiative flux intensity, effect of radiative heat transfer rate of convective cooling rate of heat release per unit area from two bedroom fire reaction zone red heat-loss curve reducing agent residential fire losses in U.S. residential smoke alarms residue enhancement response time index (RTI) retrospective users of fire model Reynolds number rigid body, motions of gravitation effect momentum and acceleration rigid solid materials risk, assessment of rollover room fires, examples RTI. See response time index rubbers

S saturated compounds self-contained breathing apparatus self-polymerization semiconductor electrical resistance detector sensitivity sensory effects of irritant gases sensory irritant gases shallow liquid fuel spill fire SI system of units adoption of fire measurement and. See fire measurement length units significant figures single chemical bonds singly ionized smoke

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aerosols mist formation smoke particles, quantity of soot formation types of visibility through smoke yields, measurement of alarms components exposure, hazards of filling of fire compartment by flow from compartment with opening layer measurements, comparison of movement in buildings particles, quantity of smoke-laden gases smoke-point heights smoke toxic potency measurement LC50 and IC50 principles of smokeview frames from smoldering mattress sodium chloride, atomic arrangement in solid-phase heat absorption solids combustibles acidâ&#x20AC;&#x201C;base pairs cellulosic and natural materials composite materials exothermic materials fire stages and metrics. See fire stages and metrics FR metals synthetic polymeric materials. See synthetic polymeric materials fuel, burning of vs. gases and liquids properties of soot formation values from small-scale burning of fuels spark ignition, indicators for specific gravity speed spontaneous ignition, thermal process of sprinkler spray sprinkler system sprinkler water, application of stack effect states of matter gases, properties of liquids, properties of phases, characterization of solids, properties of Station and Kiss nightclub fires (2003 and 2013) stationary flames steel Stefan-Boltzmann law stoichiometric mixture streaming sublimation substantive heat in combustion supertoxicants

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surface emissivity synthetic polymeric materials addition polymer condensation polymers definitions synthetic polymers

T temperature units thermal conductivity of metals values of thermal damage thermal ignition indicators for temperatures thermal instability thermal process of spontaneous ignition thermal radiation flux, function of thermal runaway thermally thick thermally thin thermite reaction thermodynamic temperature scale thermoplastic polymers thermoset polymers time units titanium dust total flooding toxic gases, classes of toxicity of fire gases carbon dioxide carbon monoxide HCl and HBr HCN nitrogen oxides organic irritants triple point of carbon dioxide turbulent ceiling jet turbulent diffusion flames turbulent flames laminar flames vs. turbulent flow turbulent fuel-jet flame, radiative fraction for two-zone model CFAST

U ultrafast extinguishment of fires uncommon fire environments, effects of unsaturated compounds upward flame spread uranium, isotopes of U.S. Clean Air Act 1994 U.S. Consumer Product Safety Commission U.S. residential fire losses

V valence validation vapor vapor pressures

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of liquids velocity verification vinyl monomers vinyl polymers vinyl radical viscosity, fluid behavior visibility through smoke vitiation volume fraction volume units

W wall temperatures water density of enhanced freezing point of water-based fire suppression system water mist, mechanisms watts (W) weight

Y yield

Z zero gravity zone models CFAST zone approximation

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