Research on Intelligent Boiling Water Condensing Gas Furnace

Advances in Energy Engineering (AEE) Volume 2, 2014 www.seipub.org/aee

Research on Intelligent Boiling Water Condensing Gas Furnace Ma Liang‐tao*1, Fu Hao‐ka2 Henan University of Urban Construction, Pingdingshan, Henan, China *1mlt@hncj.edu.cn; 2forhowcar2004@163.com

Abstract This paper explained briefly the objective, significance of the research of the intelligent condensing gas furnace of boiling water, the principles of use of condensing heat recovery and heat exchanger of condensing gas furnace with boiling water, combustion research for the combustion flue gas. Excluding condensate and intelligent programmable control methods, the main technical indicators are given based on experiment and achieve satisfactory results.

is Japan, and the Netherlands, France, the United States and Canada, etc. Compared with the traditional gas device, the manufacture of condensing gas device is little, less than 10 percent of the total. The condensing gas device is mainly minitype water heaters, and also very few, our country still does not have large capability and heavy load condensing gas furnace of boiling water. Therefore, the study of this project can narrow the gap with meaning.

Keywords Boiling Water Gas Furnace; Condensing; Heat Exchanger; Burner; Intelligent Control

Introduction With the development of our gas, the boiling water gas furnace has been becoming widely used in factories, agencies, hotels, restaurants, schools, hospitals and other institutions. For a long time, our country attaches great importance to energy conservation and emphasizing the strategy of sustainable development; 2. The fuel prices is steady rising; 3. The environmental requirements are becoming increasingly strict; 4. At present, the thermal efficiency of the boiling water gas furnace has already reached 83% in domestic market, traditional way to improve further thermal efficiency has been very difficult. We must reduce the exhaust flue gas temperature and fully recovery the waste heat even latent heat to improve the thermal efficiency.

Accordingly, this paper point out the research of the intelligent condensing gas furnace of boiling water, the purpose is: developing efficiency gas equipment which can recovery mostly condensing latent heat from flue gas and all the energy can be recycled; researching new burner and reasonable combustion process to reduce the concentration of harmful gas in combustion products; realizing equipment intelligent control include combustion, temperature, inlet and outlet water controlling, make the equipment working in best state.

The earliest condensing gas device production country

The Content, Objective of the Research (1) The development of the device can recovery heat from flue gas, the thermal efficiency can increase about 15%.

(2) The technology adopts a lot of new minitype burner to avoid the burner frequently starts‐stops and increase the working time.

(3) Lower pollutants of combustion products CO content of burning nature gas is less than 300ppm (our standard is less than 2000ppm), NOX content is less than 60ppm (At present there is no criteria in China, Europe is 60~90ppm).

(4) Intelligent programmable control automatic; solve the combustion problem it has the protective function of hypoxia, overheating, flameout protection); solve the problem of water temperature, water level, automatic water‐feeding, the number of continuously boiling(only allowed three times).

(5) Using new corrsion‐resistan material to prevent the condensation water corrosion.

(6) Using horniness plastic foam for insulating to improve its insulating performance.

Research Reviews Working Principle ——latent heat of condensing The condensing gas furnace of boiling water is fully

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recovery the sensible heat and latent heat of water vapor from the flue gas by specific design. The calorific value of gas is 1m3 gas fully combustion discharged heat. It was divided into high calorific value and low calorific value. High calorific value is the heat discharged from 1m3 gas fully combustion which the flue gas is cooled to the temperature of the water vapor in a state of frozen water; low calorific value is the heat discharged from 1m3 gas fully combustion which the flue gas is cooled to the temperature of the water vapor in a state of steam.

Thus, the difference is the latent heat of vaporization.

Take nature gas for example, the main component is CH4 which the burning chemical reaction as follows: CH4+2O2=CO2+2H2O+△H It is clear from the formula that burning 1m3 CH4 can obtain 2m3 steam and discharged some of heat on the assumption that all kinds of gas in the same molar volume.

2m3 steam condensing can release 3.96×103 kJ/m3 latent heat.

The high calorific value of CH4 is 39842kJ/m3 and low calorific value is 35906kJ/m3.

The percentage of the vaporization latent heat and low calorific value is 11%.

In other words, when the gas combustion releases 100kw sensible hot, it also provides 11kw latent heat.

The flue gas from the condensing gas furnace of boiling water is cooled to the dew point temperature 45~65℃, recovery the latent heat and get the most sensible heat. The thermal efficiency can improve more than 10%~15%, even over 100% if calculated according to the low calorific value.

——The dew point temperature depends on the amount of excess‐air, the fewer excess‐air, the higher dew point temperature and the higher efficiency of the heating apparatus.

——The maximum moisture of each volume exhaust gas depends on the gas temperature. The more condensing steam, the more recovery energy and lower gas exit temperature of burner.

——The saturated atmosphere of water vapor condenses on the surface, there are two kinds of mechanism which is a continuous ʺfilmʺ and ʺdropletʺ.

The diameter of droplet increases rapidly in the falling of 3 centimeters. The significantly higher heat transfer coefficient of ʺdropletʺ than continuous ʺfilmʺ,

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decrease the heat transfer area and reduce the total volume of heat exchanger.

——The heat exchange conditions in condensing furnace of water boiling is: when condensing the water vapor of exhaust gas, part of the latent heat transfer to heat transfer medium and the other section is discharged with exhaust gas in saturated state. The build up condensation in the furnace includes condensation water and various kinds of impurity, so is the acid.

Application Technology 1)

Heat Exchange

(1) heat exchange Studying design on the condensing heat exchanger when the main issues taken into consideration are as follows: (a) Make sure condense the water vapor in flue gas, full recovery the waste heat and improve the thermal efficiency. (b) Reasonable construction and friendly use. To ensure improve the efficiency at the same time can obtain good results of compact structure, beauty and convenience. (c) Low cost, and easily manufacture. The production costs should not be considerably increased, if the price leads to lose the market, it violates the principle of technical innovation.

From these principles, we can use two ways: ①Increase the heat exchange areas In fact, it could condense only depend on expand the heat exchange areas between waste gas and heat transfer medium. We designed in a way to effectively solve the problem of heat exchange.

——A ʺsuper heat exchangerʺ refers to the condensation being carried out in heat exchanger as shown in FIG. 1. FIG. 1(a) is a traditional water heater, FIG. 1(b) is a burner of boiling water.

——series heat exchanger(Generally series two groups heat exchanger as shown in FIG. 2(a)), the first group use of the steel is identical to the old type, its ʺoperationʺ obvious above the dew point temperature and no condensation happened. The waste gas temperature of the second group is cooled to condensing ingand recovery the energy, so we should adopt the well corrosion‐resistant

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steel material. This project uses the form as shown in FIG. 2(b).

than the dew point temperature. In addition, make sure the normal flow of condensate in wall. blower

blower fan combustor

air

flue gas

security control

condensated heat exchanger

Intercourse heater

gas hot water single heat exchanger

primary heat exchanger

cold water waste gas siphon

condensated water discharge (a)

combustor

combustion exhaust gas

gas security control

siphon

intercourse heater

condensated water discharge

gas

(a)

combustion flue gas air

the first group heat exchanger gas

common steel tube stainless steel tube condensated water (b)

air

blower fan combustor

FIG. 1 A ʺSUPER HEAT EXCHANERʺ FORM

Increase the heat exchange areas will increase the pressure drop of circulation in the flue gas, If not add the shape size, it is to use the fans (Placed before or behind). ②Increase the heat transfer coefficient When there is only a heat exchanger, if increase the heat transfer coefficient between waste gas and metal will increase the pressure drop of exhaust circulation, the condensation happened in heat exchanger so that the volume of the device can reduce considerably (the device of 144W thermal energy can reduce about 2 liters) It is possible to reduce the temperature difference between waste heat and heat transfer medium in order to reduce the exhaust gas temperature. The heat exchanger use smooth tubes are better than winged‐tubes because of obvious temperature difference between theʺ tip and end ʺof the wings, the temperature of the end of the wings is lower

intercourse heater condensated water

the second group heat exchanger use pure aluminium or copper material

(b)

FIG. 2 THE PRINCIPLE OF THE CONDENSING GAS FURNACE OF BOILING WATER

(2) the shape of the heat exchanger (3) material The condenser should have the capacity of continuous corrosion‐resistant although condensate is weak acid. There are two ways for anti‐corrosion: ——adopt corrosion‐resistant material: the condensate with corrosive because of the flue gas contains RO2 which the main ingredients are CO2

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and SO2. Therefore, all the parts including waste heat collector, neutralized, condensation water collector, fan and flue pipe which contact with the condensation water must be appropriate for use corrosion‐resistant material. The waste heat collector in this project select the 99.5% purity pure aluminum or stainless steel, can also use ALMgSi alloy rolled by aluminum and cast iron alloy. In contrast, pure aluminum and aluminum alloy were widely used by better conduction and corrosion‐ resistant.

improve the effectiveness of combustion and so on.

This project used the new type burner which pre‐ mixed the air, lower content of NOX in combustion products and fewer amount of excess‐air. The difference between this kind of combustor and normal type is showed in FIG. 3.

——Use the paint, particularly applicable to copper as copper with good thermal conductivity and easy cold working. We can also use protective metal or synthetic resin coating. When using the paint, we must be careful to no cracks in it, or else it would corrode.

2)

air inlet

gas

excess‐air ratio λ=1.2~1.8

atmospheric combustor

primary air gas

Combustor ejector

Firstly pay attention to the dew point temperature which the exhaust steam begins to condense, it depends largely on the amount of excess‐air. Make sure hold it on minimum to achieve health standards and increase the dew point temperature.

combustor plate

Mixed air and gas in proportion before burning then put into burner for combustion. Pre‐mixed play an important role on complete combustion.

The pre‐mixed system design the location of fan as needed, the fan located in exhaust outlet is called toinduced draft which usually used in heat exchange between air, the fan located in air intake is called to force pattern which usually used in heat exchange between air and liquid. This design use forced pre‐mixed system based on the lower working temperature of the device, the longer service life, fully mixed between gas and air,

gas fan

mixing tube

——Atmospheric combustor: This combustor generally used in old‐fashioned stove, and also used in condensing type, it has device to extract waste gas and the surplus air is about 40% (minimum 35%), so it is difficult to run adjust.

——Mechanical pre‐mixed combustor: the surplus air of this type is about 30% (minimum 10%), and is relatively easy to adjust. The design used the pre‐ mixed combustor with air/gas controlled in proportion device.

secondary air excess‐air ratio λ=1.4~2.0

mechanical blowing‐type combustor

Make sure the burner not incomplete combustion in minimum excess‐air conditions working. There are two kinds of combustor to choose:

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baffle

fan

air inlet excess‐air ratio λ=1.1~1.5 the new combustor adopted in this project FIG.3 THE COMPARISON BETWEEN THECOMBUSTOR OF THIS PROJECT AND TRADITIONAL COMBUSTOR

According to the component of natural gas in our country, the net wobbe number ranges from 120 to 15.7 KW/m3, the excess‐air coefficient is 1.1‐1.5 and the content of NOX in combustion products is extremely low.

3)

The Discharging of Waste Gas

There are two questions need to consider: ——For the exhaust temperature is lower, so the extract force is smaller.

——Condensating on the vertical wall is caused by the moisture of waste gas saturated.

(1) natural extraction(chimney)

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The lower the exhaust temperature, the smaller the extract force. So smoke discharge pipes need have adequately diameter. It should not be set windshield, but the security devices to prevent exhaust back to the heating apparatus. Aluminum, stainless steel and others are using for flue pipe or chimney by the saturation of moisture in exhaust.

(aluminum, stainless steel, plastic). flue pipe connection pipe (pitch>3%) boiling water furnace

(2) Balancing flue outlet: it should be located at a tilt to return the condensate into furnace.

polyvinyl chloride pipe siphon

(3) Mechanical extraction manual control exhaust with two connecting flue pipe outlets

siphon

exhaust

connect sewer

pitch>3%

FIG.5 CONDENSATED WATER COLLECTER APPARATUS

It shows that in some parts of a siphon in FIG. 5.

The pipe of condensation use polyvinyl chloride material to prevent corrosion.

mechanical ventilation th1

th2 220V

The condensate which is weak acid can mixed with water diluted into the sewer (connected with home life wastes shaker, use polyvinyl chloride pipes)

add‐air inlet thermosensitive detector

5)

PWB

condensing gas furnace of boiling water

FIG.4 MECHANICAL EXTRACTION APPARATUS OF CONDENSING GAS FURNACE OF BOILING WATER

The security devices should be set to prevent exhaust back to the heating apparatus when lack of the extract force. At present, the wind meters (FIG. 4) are widely located in additional air intake when the exhaust speed is lower and control the amount of extract twice about the burner load. The mechanical extraction device should use corrsion‐ resistant material (aluminum, stainless steel, others), the pipe also use the material to prevent condensate leakage.

4)

discharging pipe

Condensed Water Removal

There are two questions need to solve: ——how to collect the condensed water ?

——how to blow off ?

FIG. 5 was designed to collect the condensed water, they usually use corrosion‐resistant material

Water Temperature Control

Water temperature and output quantity in water container are controlled by changing input and output quantity. There is a controller for input water which is dominated by voltage deviation, voltage deviation is determined by actual water temperature which is measured by sensors plunged into water container and given water temperature. The water temperature controlling system is shown in figure6.

ONS——turn‐on signal sensor OFFS——shut‐off signal sensor HES——heat exchange sensor P1——given potentiometer P2——adjusting potentiometer for turn‐on and shut‐off temperature ——The function of Phase control triggers is adjusting the input water by the difference between given water temperature and the actual temperature in water container.

——The furnace control is lighting or shutoff combustor by the turn‐on signal sensor, shut‐off signal sensor and the signal acted on functionality controller.

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characteristic, progress and other parameters, then translate the calculation results into pwm signal to drivers the blower motor turning in new speed, the new pressure drop control the valve put appropriate amount of air and gas into burner. The control panel detectes the turning of fans by hall element and hold the proper proportion of air and gas. Water temperature changes finally back to control panel, close the circuit.

——Light‐emitting diode digital indicator shows the temperature measured by turn‐on signal sensors and form a control unit with the signal lamp which figure water hotter is running. water temperature digital indicator light‐emitting diode digital indicator the controller of the signal lamp of boiling water furnace

Valves control the quantities of air and gas in different burning power based on pressure conditions. The required pressure of valves is determined by wind pressure switch which are usually filter or venturi types. This project adopted the venturi wind pressure switch because it has minimization loss of gas compared with other types. In practical application, the tubes added in fan intake such as absorbing duct reduced the pressure drop caused by venturi pipes and increased the intake quantity. The pressure drop of venturi pipes is the difference between pipe orifice and the narrowest place where the gas in take. Fans adjust the turning speed by the pressure drop of venturi pipes, so should use the sensitive pipes.

the signal lamp of boiling water furnace ONS

the boiling water furnace controller

combustor control terminal electric relay

OFFS

Through electronic components such as electrode or CO2 probe to monitor the gas quality, adjust the gas supply by electric control valve, the fans which is lower power, smaller size and more exactly control of pressure and blowing rate become possible.

220V

T water inlet control terminal P2

Performance—Actual Efficiency

P1 two‐way control silicon switch

HES

The performance characteristic of condensing gas equipment is distinguished two cases: (1) The testing in laboratory condition:in these conditions to test the capability parameter is very exact, it is for sale in the legal standards. In the laboratory tests, there are several equipments for testing the efficiency in different water temperature at the same time.

phase control trigger

FIG.6 THE BLOCK DIAGRAM OF WATER TEMPERATURE CONTROLLING SYSTEM

This control system has three main functions: 6)

Combustion Control

The control of pre‐mixed condensing burner of boiling water is typical open circuit control, the control panel detected the water supply requirement, to calculate the time, date, starting

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(2) We processed much individual testing in traditional burner of boiling water and condensing burner of boiling water, these results are: ——effective efficiency of condensing type at least 12% improved than traditional type.

——Traditional burner will reduce the efficiency under overload running but the condensing burner can keep the efficiency.

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Conclusion, Applications and Economy The project about “Research on intelligent boiling water condensing gas furnace” is started by Henan province scientific department and be run by Henan university of urban construction. The development of the efficiency equipment can recovery mostly condensing latent heat from flue gas and all the energy can be recycled. The intelligent condensing gas furnace of boiling water has the advantages of equipment efficient, energy‐saving, environmental protection and intelligent control. Efficient means the heat efficiency is over 95%; energy‐saving mainly means fuel saving, at least could save 10% of the fuel charging and reduce 10% of CO2 and NOX discharging; environment protection means the harmful gas content of the combustion products is far below the national standards.

The price of the condensating bolling, water furnace is more expensive, in the same power and the same operating conditions, the average installation costs are about 1.5 times the traditional burner. The costs fluctuate between 1.2‐1.8 times traditional burner according to the chimney form. But on the compared, the total costs should include: investment, maintenance and energy costs, the high cost could be recovery in three to five years by saving of fuel cost.

The results show that the condensing furnace of boiling water has significant quantity of energy saving and economic.

The boiling water condensing furnace is the high stage of the improvement of the traditional boiling water furnace. Development of the boiling water condensing furnace has an important significance in saving the gas energy, reducing the environment pollution, improving the technology content and market

competition of the boiling water furnace, so it has great appliance prospect in the future.

REFERNCES

J.C.Kalunta. Civilian condensing heating equipments. In: 20th International gas paper, 1998,59‐62.

Luo Xiancheng. Exploration on the Application of Half‐ Injection Swirl‐Flow Natural Gas Burning Installation[J].

Journal of Chongqing Jianzhu University.2000, 73‐77.

P.Waler. Design and operation on condensing gas equipments. 20th International gas paper.1998, 16‐21.

R.Waikemen,

J.A.Waiyanuta.

The

condensing

gas

equipments and disposal order standard of its combustion products. In: 20th International gas paper, 1998, 9‐13.

T.B.Yanaman. CS Volume type heater. 20th International gas paper, 1998, 63‐65.

Tan Shunmin, Luo Xiancheng, Zheng Liping. Research on condensing type gas water‐heater, Natural Gas Industry.

2001, 98‐101.

Tan Shunmin, Zheng Liping, Luo Xiancheng. Energy Saving Analysis of Gas Water Heater with Condensation[J]. Gas & Heat, 2003, 23(5):287‐289.

Zhang Yumei,Liu Yongzhi. Research for Thermal Technical Properties of Condensation Gas‐fired Water Heater[J].

Gas & Heat, 1998, 18(3):38‐40.

Ma Liangtao, male, 1964. 09, professor, principal teaching and research areas of Energy &Power Engineering.

Fu Haoka, male, 1986. 10, assistant, principal teaching and research areas of Energy &Power Engineering.

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