TAB Technician Manual Module 1 HVAC Fundamentals
Disclaimer:
This document contains the expression of the professional opinion of Robert J Blanchard, P. Eng as to the matters set out herein, using its professional judgment and reasonable care. The opinions and recommendations outlined in this document are to be read and understood in its complete context. Robert J Blanchard, P. Eng disclaims any liability to third parties in respect of the publication, reference, quoting, or distribution of this report or any of its contents to and reliance thereon by any third party.
FORWARD A significant percentage of the activities performed by a Sheet Metal Worker are the manufacture of duct systems, the installation of duct systems, and the installation of terminal devices. The installation of the components is just the preliminary step towards the successful operation of a HVAC system as per the designer’s intent. The verification of airflow rates or adjustment of airflow rates to the designer’s values is the second step towards successful operation. The third, and final step, is the controls contractor starting and stopping of equipment and their manipulation of dampers installed by the Sheet Metal Workers. This program will focus on enhancing the skill set of Sheet Metal Workers and Balancing Trainees to perform the activities related to the verification and/or adjustment of Air flow rates. The trade term is “Air Balancing”. The level of HVAC knowledge required for this program is independent of that required for system design, duct design, or equipment selection. Our focus is strictly on the understanding of the Testing, Adjusting, or Balancing, TAB, of Air flow rates. This Module is the first of 10 Modules designed to guide a Sheet Metal Worker or a Balancing Trainee along his path of understanding the process of balancing of HVAC systems to the level of TAB Technician, but not to the level of a Balancing Contractor. The order of the Modules was arranged to present some fundamentals then an application of those fundamentals. The fundamentals become progressively more separated from the Sheet Metal Workers curriculum and the balancing concepts become progressively more complicated. The purpose of this approach is to start building to student’s confidence from the start; and then slowly increase the level of complexity so any student can enhance his skill set up to his level of comprehension. Some statements presented early may be over simplified or generalized in order to ease the student’s understanding of a concept. Some statements may appear to be contradicted later in the text as a concept is more fully developed. Module 1: HVAC Fundamentals Module 2: Air Flow Measurement Module 3: Fans and Fan Systems Module 4: Balancing Constant Flow Rate Systems Module 5: Electrical Fundamentals and Measurement Module 6: Balancing Variable Air Systems Module 7: Hydronic Fundamentals Module 8: Hydronic Balancing Module 9: Preparation for Balancing Module 10: Expanding your Skill Set
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The primary reference manual for this set of Modules is “TESTING ADJUSTING BALANCING MANUAL FOR TECHNICIANS” by NEBB There are other excellent reference sources for the material and we strongly advise reviewing other texts for slightly different explanations of material, especially if an explanation in the NEBB manual is difficult to follow. One readily available example is the TECHNICIAN Training Manual published by the AABC.
1 READING Read this Module along with Chapter 1, HVAC Fundamentals in the text book. The sequence of information in the Module does not necessarily parallel the sequence of information presented in the text book. There is a reason for the sequencing of the information in this Module
2 AIRFLOW TERMINOLOGY The Sheet Metal Worker is quite familiar with measuring tangible, two dimensional, items such as flat metal or architectural distances with a ruler or a tape measure. He is familiar with the measurement of speed in k/hr. by reading the speedometer in his vehicle. The measurement of fluid flow rate, however, is a bit more complex because we are measuring the movement of a volume of something we cannot see over a time period of a minute or a second. This concept is rate of flow or flow rate. The calculation will first be reviewed using Imperial units and Air as the fluid. The flow rate of air using SI units will be reviewed in Module 2. The flow rate of water will be reviewed in Module 7. The first Equation of fluid flow rate is Q = VA Where: V represents the Velocity of the fluid in fpm, ft. / min, A represents the Area through which the fluid flows in ft² Q represents the Volume flow rate in Cubic Feet per Minute, ft³/ min,
x ft² = “A” can be measured with a standard tape measure and converted to ft² (Width x Height corrected to square feet)
“V” is determined with the use of instruments. Some instruments display velocity. Other instruments display air pressure. The Technician then converts the measured pressure to a velocity (speed) of air.
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Simple Example #1: Air is measured flowing at 1200 fpm through a 42” x 24” duct. A = w x h = (42”÷12) x (24”÷12) = 3.5’ x 2’ = 7 ft² Or: A = w x h = = .= 7 ft. Q = VA = 1200 ft. / min x 7 ft² = 8,400 ft³/ min The measured flow rate is 8,400 cfm The Second Equation of fluid flow rate is V = 4005√
, Or VP = (
)
Where: √ is the square root of the measured velocity pressure in in. w.c., 4005 is the conversion constant used for standard air, V represents the Velocity of the fluid in fpm, or,
Example #2: Air is measured at 0.25” w.c. flowing through a 44” x 15” duct A = w x h = (44”÷12) x (15”÷12) =3.67’ x 1.25’ = 4.59ft² V = 4005√ =4005√ = 4005 x 0.5 = 2003 ft. / min Q = VA = 2003 ft. / min x 4.59 ft² = 9,178 ft³/ min The measured flow rate is 9,178 cfm
There are three terms you should understand before reading further in this Module: 1. Velocity Pressure, VP, is the fan energy which pushes the air through the duct system. VP increases as the square of velocity. Velocity is used when referring to air speed, although true Velocity is a Vector quantity. VP acts in the direction of air flow. 2. Static Pressure, SP, also referred to as Friction Loss is the fan energy consumed to overcome the obstacles to air flowing straight through a duct at a uniform speed (velocity) in a duct. i.e. elbows, tapers, filters, coils, dampers, etc. SP acts in the direction of airflow and perpendicular to air flow. SP establishes the shape of a balloon. A Ductolator or Friction Loss Tables can be used to determine the Friction Loss per every 100 ft. of duct, given the duct size and air flow rate. 3. Total Pressure, TP, is the sum of Velocity Pressure and Static Pressure. Total pressure decreases in value (becomes less positive) from a fan discharge to the furthest outlet in a duct system. Total pressure decreases in value (becomes more negative) from the farthest inlet terminal to the fan inlet. TP = VP + SP Perspective: If a 200 ft. long straight duct had Friction Loss of 0.12”/ 100 ft., a VP of 0.16” and a SP of 1.35” at the start of the system. The Total Pressure at the start of the system would be 1.35” + 0.16” = 1.51”. The TP at the end of the 200 ft. duct would be 1.35” - (2 x 0.12) + 0.16” = 1.27” The Velocity Pressure would still be 0.16”, but the SP would drop to 1.11” w.g. TAB Technician Manual Module 1
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A couple of poorly designed elbows with a pressure drop (resistance to flow) of 0.20” would decrease the Total Pressure at the end of the system to 1.35” - (2 x 0.12) - (2 x 0.20) + 0.16” = 0.87”. The VP would still be 0.16”
Below is a table which shows how the VP increases with Velocity Low Velocity
Medium Velocity
High Velocity
Downstream of VAVs and Return or WR exhaust duct
VAV Supply Ductwork
Exhaust Ducting and Dust Collector
Velocity 250 fpm 400 fpm 566 fpm 1,000 fpm 1,200 fpm
VP 0.0039” 0.01” 0.022” 0.0623” 0.0898”
Velocity VP Velocity VP 1,500 0.14” 2750 fpm 0.471” fpm 1,800 0.202” 3,000 fpm 0.561” fpm 2,000 0.25” 3,500 fpm 0.764” fpm 2,250 0.316” 4,000 fpm 0.998” fpm 2,500 0.39” 5,000 fpm 1.559” fpm Note: Low velocity, Medium velocity, and High velocity are quite arbitrary terms and are quite independent from Duct Construction standards. Duct Construction standards refer to the design Static Pressure of a duct system. A very efficient filter enclosure may be constructed to 10” w.c., but experience a Velocity less than 400 ft. / min. (0.01 “ w.c.) The inlet velocity of a dust collector hood may be greater than 5,000 ft./ min, but there is no measurable Static Pressure
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3 AIRFLOW MEASUREMENT There are two unique measurement procedures: Duct Traverse and terminal device. This Airflow Measurement information will be presented in greater detail in Module 2 “Air Flow Measurement”
3.1 DUCT TRAVERSE A Duct Traverse, when conducted with care, is considered about as accurate an evaluation of air flow rate as can be achieved in the field. (±5% at velocities > 650 ft./min) The principal of calculating the flow rate of air flowing in a duct is to measure the size of the duct and to measure how fast the air is flowing. That is: measure the cross sectional Area, A, and the Velocity, V. Then Q = VA as we documented in Section 3 While this may appear quite straight forward but the procedure relies on determining the average velocity of the air and measuring the net cross sectional area (inside internal duct insulation) of the duct. Duct traverse are not performed in tapers. The recommended location for a duct traverse is where there are about 10 diameters of straight ductwork upstream and a minimum of two duct diameters downstream. A series of Velocity Pressure samples are recorded, converted to Velocity, and then averaged. Note: the velocities are averaged - not the Velocity Pressures. A Probe to sense pressure inside the duct and an Instrument to display a pressure are required to perform a duct traverse. The probe is called a Pitot Tube. The Pitot Tube is inserted through a small hole in the duct wall with the Total pressure port facing into the flowing air stream. One tube may be connected to the Pitot Tube to read either Static pressure or Total pressure. Two tubes are connected between the Pitot Tube connections and an instrument to display the difference between Total Pressure, TP, and Static Pressure, SP: Velocity Pressure, VP.
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TP = SP + VP VP = TP – SP
The diagram above depicts a Pitot Tube connected to a Manometer to display Static Pressure, or Velocity Pressure, or Total Pressure. Manometers are “Old School. Today we use a Magnehelic gauge or a Micromanometer to display the pressure. Micromanometers are the preferred instrument because they may display to a greater accuracy, they may display imperial or SI units, they may convert VP readings to Velocity, and they may display the Average Velocity for a number of VP samples. Unfortunately, they require a battery where a MAGNEHELIC or a Manometer require no external power,
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The air flow rate determined by Duct Traverse is the product of Average Velocity and duct area. Q = V average x A “A” can be measured with a standard tape measure and converted to ft² (Width x Height corrected to square feet)
“V” is the calculated average of all of the Velocities sampled. The number of samples is dependent on the size of the duct. Imagine the rectangular duct divided into small segments of equal area between 6” square and 8” square. Drill holes in the duct wall so the Pitot Tube will sample the Velocity Pressure at the centre of each of those little segments. Usually the holes are drilled along the longer side of the duct. Sometimes it is necessary to drill holes along one short side, both short sides, or a combination of long side and short side depending on the length of your Pitot Tube and obstructions such as pipes, lights, or other ductwork.
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The same principle is applied to Round ductwork except the spacing of the samples is to record the velocity at the centre of Equal Annular areas across perpendicular diameters.
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The next step depends on the instrument used: If the Micromanometer is able to display “Average Velocity”: then Q = V average x A If the Micromanometer is only capable of displaying “Velocity”: then sum the velocities and calculate the Average Velocity and Q = V average x A If the instrument only displays “Velocity Pressure”: then convert each VP to a Velocity, sum the velocities, and calculate the Average Velocity and Q = V average x A Always record the Static Pressure at the Duct Traverse location for future reference. Case Study: I completed a balancing project for a partial floor in an office building and submitted my report. I was called to the site to verify my readings with the Project Engineer. The measurements that day were quite a bit less than documented in my report. I showed the Project Engineer that the measured SP in the duct feeding that area had dropped from 0.36” w.c.to 0.24” w.c. Another TAB Technician had stolen my air. The Project Engineer agreed to have the other TAB Technician return my air.
3.2 DIFFUSERS In Section 4.1 we used the equation Q = V average x A to measure the flow rate of air in a duct. “A” was easy to calculate from the duct dimensions. How would you calculate “A” for a multi-ring diffuser? The manufacturers of diffusers have published a set of values for each style and each neck size of diffuser they market. These values are called Ak factors. The flow rate of air from a diffuser is Q = V average x Ak. The table to the left indicates which ring their Ak factor is applicable. The airflow rate is the product of their Ak factor and the average of four velocity readings recorded from the appropriate ring. Note: The Ak factors are specific for a style, a neck size, a ring, and an instrument. Usually the tables are for an Alnor Velometer.
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This picture shows a student measuring the velocity at the appropriate ring of a round diffuser with an Alnor Velometer. Note: the wand is oriented as per the manufacturer’s diagram and the Velometer body is held with the face vertical with the base horizontal. Laying the face horizontal or skewing the base left or right of horizontal will yield an inaccurate velocity.
The more common method of measuring air flow from a diffuser is the Flow Hood. There are two benefits to using a Hood:
There is no need to consult the book for the appropriate AK factor. There is no need to calculate the Average velocity of four readings.
There are also some drawbacks:
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Not well suited for no ceiling, A Flow hood costs more than a Velometer, You require several different sized hoods for other than standard 24” x 24” diffusers, The case for a hood is far larger than the case for a Velometer, You need to be relatively tall for the scale to be at your eye level. Most Hoods require batteries
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3.3 GRILLES Grilles, Registers, and other non-standard terminals are measure using a similar equation as a diffuser: Q = V average x Ak. The Ak factor for a diffuser was obtained from a manufacturer’s catalogue. However, there are an infinite number of styles, sizes, and damper arrangements for grilles so deriving a table of Ak factors is not practical. Instead the manufacturers publish a “K” factor for each style of grille they manufacture. Their “K” factor is multiplied by the Core Area to derive the equivalent of an Ak factor. The airflow rate from a Grille is Q = V average x K x Core Area. The instrument used to measure Velocity is usually a rotating vane Anemometer. The tables will give “K” values for different blade angles, damper or no damper, different instruments, etc.
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3.4 RECAP: Q = VA is the basic equation of flow rate Q = V average x A for Duct Traverse Q = V average x Ak for Diffusers Q = V average x K x Core Area for Grilles and Registers
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4 HEAT TRANSFER The primary reason for the duct work we install in buildings is to distribute outdoor air at a sufficient flow rate to dissipate the body odours of the workers. This process is called Ventilation. For many hours of operation the outdoor air must be heated to prevent the occupied space from becoming too cold. The process is called Heating and Ventilating. For some hours of operation the outdoor air must be cooled to prevent the occupied space from becoming too warm. The process is called Heating and Ventilating, and Air Conditioning, HVAC. This module is not intended to be a short course in physics, but there are two different considerations of heat transfer the TAB Technician must understand. When you put your warm beer in the refrigerator to reduce the temperature from room temperature to just above freezing, or when you add heat to a pot of water to prepare coffee you are changing the temperature of a fixed volume of fluid. When cool air passes over a coil with warm water flowing through the tubing you are changing the temperature of both of the flowing fluids. This is the more complex heat transfer concept which the TAB Technician must understand.
4.1 STATIC HEAT TRANSFER Heat energy may be transferred from one body to another body of lesser heat energy by one of the following methods: Conduction: The two bodies must be in contact with each other. Heat one end of a metal rod and hold the other end of the rod in your bare hand. You will quickly understand the concept of heat transfer by Conduction. Convection: Warmer air moves to a place of cooler air. The cooler air is displaced toward a warm surface where the air becomes warmer and moves to a place of cooler air. The term is convection current. Think of a radiator under a window. The air in contact with the fins warms and rises away from the fins, cold air on the floor flows to replace the air that was in contact with the fins. It warms and rises. An air current is created without any mechanical assistance Radiation: The two bodies will not be in contact with each other. An example is the sun heating you through the wind shield of your vehicle travelling at 100km/hr in January. There is a considerable difference in your comfort when the sun is blocked by a cloud. Another example is a campfire at night: your body exposed to the fire will be warm, but the rest of your body will be cold- even with a coat.
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4.2 FLUID FLOW HEAT TRANSFER The majority of Air Balancing is performed with air temperatures between 55ºF and 75ºF (13ºC and 22ºC) and the temperature of the air is not relevant to the balancing process. There are, however, other tasks such as trouble-shooting or maintenance which may require the Testing Technician to report on the Heat Transfer performance of components. Air Side Heat Transfer Equation: Btu/hr = 1.085 x cfm x ΔTair The rate of heat energy gain by a stream of air is the product of: a constant: 1.085; the flow rate of air: cfm; and the difference in temperature between the entering air and the leaving air in Fº. The rate is given in British thermal units per hour, Btu/hr Note: a temperature is measured in degrees ºF or ºC, but a difference in temperature, ΔT, is measured in F degrees, Fº or C degrees, Cº. Just one of those little things to remember.
A house furnace increases the temperature of a flow rate of 1200 cfm from 70 ºF to 135 ºF. What was the rate of heat energy transfer? Btu/hr = 1.085 x cfm x ΔTair = 1.085 x 1200 x (135 ºF- 70 ºF) = 1302 x 65Fº = 84,630 Btu/hr Heat energy is added at the rate of 84,630 Btu for each hour of operation. TAB Technician Manual Module 1
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Note the furnace did not add 84,630 Btu to a volume of1200 cubic feet of air, but added heat energy such that: if a flow rate of 1200 cubic feet of air per minute flowed past the heat chamber for 60 minutes (72,000 ft³) ; the amount of heat energy transferred to that air would be 84,630 Btu. The concept to understand is the rate of heat transfer. The air is flowing at a rate per minute, but the heating device is rated at a heat transfer rate per hour. We calculated the rate of heating energy output of the furnace. If the efficiency of the furnace were rated at 92% then the rate of energy input to the furnace would be 84,630 Btu/hr ÷ 0.92 = 91,990 Btu/hr. Be aware of the whether rating of an energy device is Btu/hr input or Btu/hr Output. Water Side Heat Transfer Equation: Btu/hr = 500 x gpm x ΔTwater A boiler increases the temperature of a flow rate of 200 gpm from 160 ºF to 180 ºF. What was the rate of heat energy transfer? Btu/hr = 500 x gpm x ΔTwater = 500 x 200 gpm x (180 ºF-160 ºF) = 100,000 x 20Fº = 2,000,000 Btu/hr Heat energy is added to the water at the rate of 2,000,000 Btu for each hour of operation of the boiler. This is also referred to as 2,000 Mbh (thousands of Btuh) Note the furnace did not add 2,000,000 Btu to a volume of 200 gallons.
4.3 HEAT BALANCE EQUATION Since: Btu/hr = 1.085 x cfm x ΔTair and Btu/hr = 500 x gpm x ΔTwater Our Equation for Heat Transfer is: 1.085 x cfm x ΔTair = 500 x gpm x ΔTwater You will learn later that this equation is only used for heating applications and not for cooling applications where there is condensation occurring. You will also learn far more accurate methods of measuring water flow rates, but you should understand the principles presented in this section. For many years water balancing was performed using the Heat Balance Equation. Knowing the air flow rate, a TAB technician could measure the four temperatures (Entering Air, Leaving Air, Entering Water, and Leaving Water) and throttle the water valve until the design temperature differences were achieved for the air flow
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or for the water flow. They were seldom lucky enough to achieve both because of the inherent inaccuracy of the process. A heating coil with a flow rate of 1200 cfm increases the air temperature from 70ºF to 95ºF while 1.2 gpm of water at 160ºF enters the coil. What will be the leaving water temperature? 1.085 x cfm x ΔTair = 500 x gpm x ΔTwater 1.085 x 1200 x (95ºF-70ºF) = 500 x 1.2 x ΔTwater 1302 x 25Fº= 600 ΔTwater 32550 = 600 Fº ΔTwater
160 °F-54.25F° =105.75 ºF leaving water temperature Verify the energy balance: Btu/hr = 1.085 x cfm x ΔTair = 1.085 x 1200 x (95 ºF-70 ºF) =32,550 Btu/hr Btu/hr = 500 x gpm x ΔTwater = 500 x 1.2 x (160ºF-105.75ºF) = 32,550 Btu/hr The air flow gained heat energy at the same rate the water flow lost energy. Note the positive and negative sign convention. For the air side we use (LAT-EAT) to yield a positive value, (heat gained). For the water side we could use either (EAT-LAT) to yield a positive value, (heat transferred) or (LAT-EAT) to yield a negative value, (heat lost): 500 x 1.2 x (105.75ºF-160ºF) = - 32,550 Btu/hr Use the convention with which you are more comfortable or understand better. The preceding is a branch of science called Thermodynamics. We called it Heat Transfer in order to not scare the student. Limitation: The accuracy of the Heat Balance Equation is quite dependent on the accuracy of the temperature readings. A single thermometer of appropriate range should be used to measure the EAT and LAT. A Different thermometer of appropriate range should be used to measure the EWT and LWT. Remember it is the temperature differences which are important, not necessarily any actual temperature. Consider measuring 70ºF using a thermometer with a range from 70ºF to 240 ºF.
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Consider measuring 70ºF using a thermometer with an inaccuracy of +2Fº and measuring the 95 ºF using a thermometer with an inaccuracy of -2Fº. The thermometers would read 72ºF and 93ºF for a difference of 21 Fº, only 84% of the actual temperature difference. The accuracy of the Heat Balance Equation is quite dependent on allowing sufficient time after any adjustment before gathering a new set of LAT and LWT. The EAT and the EWT will probably remain consistent but the LAT and LWT will change after each adjustment of water low rate. The rate of heat transfer is nonlinear so the Heat Transfer Equation may only be used to verify the design performance of a coil if the design Air flow rate, the design EAT, and the design EWT are held constant over the evaluation period. This nonlinear performance will be reviewed in Module 6: Balancing VAV Systems.
4.4 MIXED AIR EQUATION A TAB Technician should be comfortable with the Mixed Air Equation: (Cfm Outdoor air x Temp Outdoor air) + (Cfm Return air x Temp Return air) = Cfm Mixed air x Temp Mixed air (Cfm x ºF) Outdoor air + (Cfm x ºF) Return air = (Cfm x ºF) Mixed air (% x ºF) Outdoor air + (% xº F) Return air = ºF Mixed air Example 1. A flow rate of 1,200 cfm of Outdoor air at 15ºF is mixed with a flow rate of 2,000 of Return Air at 72F. What is the calculated Mixed air temperature? (Cfm x ºF) Outdoor air + (Cfm x ºF) Return air = (Cfm x ºF) Mixed air (1,200 x 15) + (2,000 x 72) = (1,200 + 2,000) x ºF mixed air 18,000 + 144,000 =3,200 ºF
= 50.6ºF is the Mixed Air Temperature, MAT Example 2. A flow rate of 15% of Outdoor air at 15ºF is mixed with a flow rate of return Air at 72F. What is the calculated Mixed air temperature? (% x ºF) Outdoor air + (% xº F) Return air = ºF Mixed air (0.15 x 15) + (0.85 X 72) = ºF mixed air = 2.25 + 61.2 = 63.45 ºF is the mixed air temperature Example3. A flow rate of 15% of Outdoor air at 0ºF is mixed with a flow rate of return Air at 72F. What is the calculated Mixed air temperature? (% x ºF) Outdoor air + (% xº F) Return air = ºF Mixed air (0.15 x 0) + (0.85 X 72) = ºF mixed air = 0 + 61.2 = 61.2 ºF is the mixed air temperature
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5 PSYCHROMETRICS Psychometrics is the study of the properties of the mixture of Air and Water over a range of temperatures. The topic is covered extensively during Advanced Level training for apprentices. This section will discuss four terms the TAB Technician must understand. The other three terms are more related to design of cooling systems. A Psychometric Chart is a graphical presentation of seven properties. Knowing any two of those properties it is easy to read the other five properties directly from a Psychometric Chart Those seven properties are: Dry Bulb Temperature Relative Humidity Specific Volume Wet Bulb Temperature Enthalpy Dew Point Grains of Moisture
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5.1 DRY BULB TEMPERATURE
The Dry Bulb Temperature, sometimes called Sensible Temperature, is what you read with any standard thermometer. This is the value everyone refers to unless they specify “Wet Bulb Temperature” or “Dew Point Temperature”. Throughout this set of modules “Temperature” will refer to Dry Bulb Temperature. It is good practice to record the Temperature of the air whenever you perform a duct traverse in case you are required to repeat the traverse at a later date. Dry Bulb temperature is used for the Heat transfer equations: Btu/hr = 1.085 x cfm x ΔTair or Btu/hr = 500 x gpm x ΔTwater All of our equations so far have been presented in Dry Bulb Temperatures with reference to the Fahrenheit scale. The more common temperature scale in our area is the Celsius scale. Freezing point of water is 32ºF or 0ºC; Boiling point of water is 212ºF or 100ºC The TAB Technician must be able to convert quickly between the two scales. ºF = (ºC x 1.8) + 32 or ºC = (ºF - 32) ÷ 1.8 (16ºC x 1.8) + 32 = 28.8 + 32 = 60.8 ºF One point of quick reference: 16ºC≈61 ºF (82º F- 32) ÷ 1.8 = 50÷ 1.8 = 27.78 ºC Another point of quick reference: 28ºC≈82 ºF For ΔT: you do not need to convert before subtracting (ºF - ºF) ÷1.8 = ΔCº and (ºC - ºC) x1.8 =ΔFº TAB Technician Manual Module 1
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5.2 WET BULB TEMPERATURE
Wet Bulb Temperatures are read from a standard thermometer with a wet sock on the sensing bulb. See the picture just above the Psychometric Chart above. The TAB Technician is normally only concerned with measuring Wet Bulb Temperatures when there is a cooling process involved during his balancing. Historically the simple instrument used to measure Wet Bulb Temperatures was the Sling Psychrometer. The same device hanging on a wall is called a hydrometer. Today we use electronic equipment to measure Wet Bulb Temperatures rather than twirling a thermometer with a wet sock attached. The Wet Bulb Temperature will always be less than the Dry bulb Temperature because of a cooling effect produced by the evaporation of moisture from the sock which reduces the temperature of the bulb and thus the temperature reading. The lower the moisture content of the air, Relative Humidity, the quicker the rate of evaporization from the wet sock and thus the greater the difference between the dry Bulb Temperature and the Wet Bulb temperature reading. The difference between the Bulb Temperature reading and the Wet Bulb temperature reading is called Wet Bulb Depression. A table which comes with a Sling Psychrometer will give the Relative Humidity for any Dry Bulb temperature and the Wet Bulb Depression TAB Technician Manual Module 1
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5.3 RELATIVE HUMIDITY
The value of Relative Humidity is not used in any of our calculations, but it is a value most people readily relate to their comfort. The weather forecasters relate rising Relative Humidity values as increased discomfort. We know that rain is probable as the RH approaches 100%. It is of interest that they talk about Temperature and RH in our area, but in the south they talk about Temperature and Dew Point during their weather forecast. The TAB Technician uses the value of Relative Humidity along with temperature to find other properties of air. The least expensive upgrade for an electronic Thermometer is Relative Humidity. More expensive electronic equipment may display all seven properties.
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5.4 SPECIFIC VOLUME=
Fans are machines that perform the work of moving air. As silly as it sounds, the weight of the air influences the energy required to move air. The equation V = 4005√ is based on air that is about 70ºF and about 0% RH where the Specific Volume is about 13.33 ft³ per pound at sea level. Each pound of air occupies 13.33 ft³. This is the same Specific Volume as air at 67ºF & 50%RH or 65ºF &100%RH. If the air at 69ºF and 50% RH where in Denver CO at 5,000ft; SV =16.04 A flow rate of 1,200
= 1,200
÷ 13.33
= 90 lb. of air per minute.
The Reciprocal of SV is Specific Density
which is sometimes more useful data.
13.33
x 0.075
= 0.075
: A flow rate of 1200
The real equation for air velocity is V = 1096.7√ If SV = 13.33
; V = 1096.7√
=1096.7√
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=90 lb. of air per minute
or V = 1096.7 √ =1096.7 √
x 3.65 = 4005√
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Psychometric Example
A flow rate of 2,000 cfm of air at 102 ºF and 20% RH goes through a humidification process which results in the air being at 95 ºF and 60% RH. Dry Bulb Temp Relative Humidity Wet Bulb Temp Grains of moisture Enthalpy Specific Volume
102ºF & 60%RH 102 60 82.707 150.8 46.564 btu/ lb 14.464 lb/ ft³
95.0F & 20% RH 95 20 70.362 60.9 34.106 btu/lb 14.340 lb/ ft³
1. Calculate the change in Sensible Heat. (Right to Left is a loss of Sensible Energy) Btu/hr = 1.085 x cfm x ΔTair =1.085 x 2,000 x (95 – 102) = -15,190 Btu/hr 2. Calculate the change in Latent Heat (Up the Chart is a gain in Energy Btu/hr = 0.69 x cfm x Δ grains =0.69 x 2,000 x (150.8 - 60.9) =124,062 Btu/hr 3. Calculate the change in Total Heat Total Heat = Sensible + Latent =-15,190 Btu/hr +124,062 Btu/hr=108,872 Btuh Btu/hr = 4. 45 x cfm x Δ Enthalpy =4.45 x 2,000 x (46.564 – 34.106) =110,876 Btuh The 1.8% difference can be attributed to the change in Density and to rounding off. TAB Technician Manual Module 1
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6 FANS Fans are the heart of a ventilation system. They are used to increase the energy of a flowing mass of air. A TAB Technician must understand the different types of fans and the mathematics of changing a fan’s performance. Fans are discussed in detail in Module 3. Simple fan systems such as a desk fan, may have no ductwork, they just be used to move air at a greater velocity than would naturally occur. Other fan systems, such as a washroom exhaust fan, will have duct work on the inlet and/or outlet to extract air from a place where it may have odours to a place where the odours are irrelevant. More complex fan systems, such as Heating, Ventilation and Air Conditioning Units, HVAC, W ill have Ductwork on the inlet and on the outlet to extract air from an occupied zone, blend Outdoor Air, filter, heat, humidify, and cool the air before delivering it bac k to the occupied space. These units may be smaller than a car to larger than a transport trailer. All fans have one property in common: they move air from an area of lower pressure to an area of higher pressure.
Lower Pressure → Higher Pressure (Velocity)
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Higher pressure at outlet
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6.1 FAN LAWS
Fan Law #1: cfm ~ rpm: Fan Law #2: SP ~ rpm2:
cfm x√
(
) OR
(
) or
(
) OR
(
)
= cfm
Fan Law #3: Hp ~ rpm3:
6.2 FAN TYPES . In HVAC we use many fans that come in many shapes, sizes, and uses. The fans that a TAB Technician may encounter will have been selected for a particular set of operating parameters. His task is to measure the air flow rate of each fan and to document the performance of each fan in a format acceptable to the industry. There are three basic types of fans: Propeller, Axial, and Centrifugal, each with an endless number of sizes, shapes, and applications. The all have one property in common: they move air from an area of lower pressure to an area of higher pressure.
6.2.1 PROPELLER FAN They may have a frame as pictured to the left or just blades like the one over a kitchen table. Propeller fans are used to deliver large air flow rates with very little resistance to flow. There is no practical method of determining the air flow rate of a propeller fan. The TAB Technician should verify the proper rotation: Clockwise or Counterclockwise and measure the electrical current draw.
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6.2.2 AXIAL FAN
Air flows through the blades: no change in direction. Axial Fans have their blades mounted in a tube. The motor may be inside the tube or external. They may be used to deliver any flow rate of air and overcome high resistance to flow. There is usually ductwork at the inlet and/or the outlet of the fan. Airflow rate is measured via an air traverse in the ductwork. There are a lot details for a TAB Technician to document.
6.2.3 Centrifugal Fan Air changes direction as it passes through the Fan. The fan blades are mounted on a cylinder. The motor may be inside the cylinder or external to the housing. The air may enter one side of the cylinder or both sides. They may be used to deliver any flow rate of air and overcome high resistance to flow. There is usually ductwork at the inlet and/or the outlet of the fan. Airflow rate is measured via an air traverse in the ductwork. There are a lot details for a TAB Technician to document.
6.2.4
Fan Systems
Most of the Fan Systems we encounter will have a Supply Air Fan and a Return Air fan, along with Dampers, Filters, Heating Coils, Cooling Coils, Humidifiers, Control Valves, Pumps, Boilers, Chillers, and Cooling Towers. Our focus as TAB Technicians, for the first five modules, is strictly on documenting the performance of the fans, the supply air diffusers, and the return air diffusers. In Module 7 and Module 8 we will learn to measure the performance of pumps and we will learn how to measure and /or balance the water flow rates through boilers, heat exchangers, chillers, and cooling towers. TAB Technician Manual Module 1
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At the completion of Module 9, a student should have the knowledge to measure and/or to adjust the performance of all of the components in the diagram below.
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7 TAB EQUATIONS Each Module will have a section with a list of the equations presented in that module 7.1 FLOW RATE Our first Equation of fluid flow rate is Q = VA
Q = VA
V represents the Velocity of the fluid in fpm, A represents the Area through which the fluid flows in ft²
V=
Q represents the Volume flow rate in CFM, x ft² =
A=
Our Second Equation of fluid flow rate is V = 4005√
√
V = 4005√
VP =
(
)
,
is the square root of the velocity pressure in inch w.c.
4005 is the conversion constant used for standard air V represents the Velocity of the fluid in fpm,
⁄
,
V = 1096.7√
SD Is the Specific Density of the air (The Reciprocal of SV) V = 1096.7 √
SV Is the Specific Volume of the air (From Psychometric Chart)
7.2 HEAT TRANSFER ºF = (ºC x 1.8) + 32 or ºC = (ºF - 32) ÷ 1.8
SI Equations will be High-lighted in Turquoise
Btu/hr = 1.085 x cfm x ΔTair For Sensible Heat energy of air Btu/hr = 0.69 x cfm x Δ grains For latent Heat energy of air Btu/hr = 4. 45 x cfm x Δ Enthalpy For Total heat energy of air Btu/hr = 500 x gpm x ΔTwater For Sensible Heat energy of 1.085 x cfm x ΔTair = 500 x gpm x ΔTwater is the energy balance equation
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7.3 MIXED AIR (Cfm x ºF) Outdoor air + (Cfm x ºF) Return air = (Cfm x ºF) Mixed air (% x ºF) Outdoor air + (% xº F) Return air = ºF Mixed air or (%OA x OAT) + (%RA x RAT) = MAT Remember: a temperature is measured in degrees ºF or ºC, but a difference in temperature is measured in F degrees, Fº or C degrees, Cº.
7.4 FAN LAWS Subscript 1 is always your measured data; subscript 2 is always the target data regardless of whether you are increasing performance or reducing performance. Fan Law #1: cfm ~ rpm
Fan Law #2: SP ~ rpm2
(
)
cfm = cfm
√
(
)
rpm = rpm
√
(
)
cfm = cfm
√
(
)
rpm = rpm
√
Fan Law #3: Hp ~ rpm3
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8 EQUIVALENTS Base Units
Imperial Units
SI Units
Air Density @ 70ºF / 21ºC
0.07488 lb./ft³
1.2041 kg/m3.
Water Density @ 68ºF/ 20ºC
62.316 lb./ft.³
998.21kg/m³ (1kg/ ℓ)
0 psig
14.7 psia
101.33 kPa
1 psi
2.31 ft. water 27.72 in water (12 x 2.31) 2.04 in Hg (Mercury)
6.8948 kPa 0.068046 Bar
1 ft. of water
12 inch water 0.8843 In Hg
2.9861 kPa 0.3048 m
1 inch water
248.84 Pa
1” Hg
0.5 psi
3.3769 kPa
1 gal water (US)
8.3304 lb 0.8330 gal (Imp)
3.7786 kg 3.7854 ℓ
1 gal water (imp)
10.0004 lb 1.2009 gal US
4.5379 kg 4.5461 ℓ
1 gpm (US)
0.8330 gpm (Imp)
0.06309 ℓ/s
1 gpm (Imp)
0.07577 ℓl/s
1 cu ft., 1ft³
1.2009 gpm US 7.4806 gal of water (US) 6.2290 gal of water (Imp)
Specific Heat of Water
1Btu/ lb/ Fº
4.1868 kJ/kg/Cº
1 Btu/Hr 1 MBh
0.29307 W 1,000 Btu/Hr
0.29307 kW
1 hp (Electric)
746 W
1 hp (Boiler)
809.6 W
1 Ton of Refrigeration
12,000 Btuh
1 watt
3.41212 Btu/h
1 kWh
3412.12 Btu/h
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3516.8w, 3.5163 kw
3,600 kJ
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9 GLOSSARY There are some terms the TAB Technician should understand in the context of their use in the HVAC industry. Air: is a fluid which behaves like water; warm air flows up and cool air flows down. Air, Outdoor: Air introduced from outdoors, and therefore not previously circulated through a ventilation system.
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