Journal of Innovative research in engineering sciences

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1 Innovative Research in Engineering Sciences Vol 2(2), (2016) Journal of Innovative research in engineering sciences Journal homepage : Optimization of energy consumption in warm air furnace Rahimi Alireza Assistant Professor, Research Institute for Energy, University of Kashan,Kashan,Iran. Abstract: Warm air furnace is used to warm interconnected and integrated spaces. This thermal means works as a gas to gas heat exchanger, and one of its major problems is that it has a low overall heat transmission coefficient. To improve the thermal performance of this kind of heating equipment can increase the level and duration related to the heat exchanging and or with installing of flow barriers makes turbulence in thermal boundary layer of it. On this basis in this research, heat output of new model of warm air furnaces is investigated with installing the flow barrier and increasing in the level of heat exchange and so on. Based on the obtained results Thermal efficiency of new warm air furnace is 79.44% output and in similar warm air furnaces existing in the market this yield is 72.8%. Relevant economic analysis also shows that the time for returning added investment for the new model, with the assumption of the average working hours of warm air furnace (in tee months of winter: every day up to 12 hours and with assuming 25 working days in each month), based on subsidized rates (the price of each cubic meters of gas equal to 100 tomans) will be53 months and based on export rates (every cubic meters of gas is 1200 Tomans) becomes equal to 5.3 months. As it is clear optimizing the gas consumption in hot air furnaces in both cases (assessment with a subsidized rates and or export rates ) is an economic issue. Keywords: hot air furnace-optimization-energy-heat exchange-heat losses 1. Introduction Warming the buildings is done in two methods of direct and indirect [1]. In the direct method, temperature of the building increases without mediator and directly with heat generator. In the indirect method, heat by a mediator factor (such as water), from a heat generator will be transferred to the air of the building. Indirect thermal systems include: Radiator Heating system and hot water boiler, hot water boiler and fan coils, air conditioning system and hot water boiler system and package system and Radiator Heating system, warming system from the floor. Direct thermal systems also include: gas and oil heaters, hot air furnace, radiation heating, two- seasonal equipment and Electric warmers. The warm air furnace regarding to air circulation is divided in two natural (or without fan) and compulsory (or with fan) groups, and based on consuming fuel is classified into either the fuel gas or gasoline combustion and according kind of burner, furnace is divided into with fan or atmospheric. There is also coal and electrical warm air furnaces, however in Iran are not applicable. Thermal efficiency of warm air furnaces is higher than all of other thermal systems and has less energy consumption compared to the other systems (2). Prelaunch time period of warm air furnace, compared to radiator system and or air condition is much less and when the building is not specially used, Corresponding Author. Mail. alireza.rahimi93@yahoo.com 23 Even in the limited times can switch off the warm air furnace, while this regard for warming systems from the floor, radiator and or air condition is not possible. The ability of warm air furnace to create centralized control conditions for its functions one of other benefits of it. Aeration of warm air furnaces is done in two ways horizontal and vertical. Also the air circulation created by the fan is in two forms upward and frontward (2). Done researches in the field of improving thermal efficiency of warm air furnaces show that this has not taken into consideration and the thermal performance of this heating equipment has not been so changed. Based on this issue, increase in thermal efficiency of this heating system in this research has been considered Determining the capacity of warm air furnaces [3] Tee parameters, heat capacity, hot air flow and static pressure of canal network, are the main parameters to determine capacity and selection of hot air furnaces. Heat capacity, based on the calculations of heat losses of a building which is going to be warmed, and air flow carrying heat load will be calculated from the formula 1. q v = (1) 1.08(T c2 T c1 ) v:the value of volumetric air flow carrying thermal load (according to cubic feet per minute) q: Thermal load of the considered space that is obtained from calculations related to heat losses related of building and according to ( Btu ), 1.08: is as the result

multiplication of air density ( lb ft30.0749) in specific heat of air ( Btu 0.24) in unit of time (60min).

of internal outline of space according to degrees Faenheit (equivalent to 75 degree Faenheit or 24 degrees Celsius. Approximately, for each Btu/.

2 multiplication of air density ( lb ft ) in specific heat of air ( Btu 0.24) in unit of time (60min). lb T c2 : The highest temperature of the input air to the room which is considered according to Faenheit degrees (equal to 104 degree Faenheit or 40 degrees centigrade) T c1 : The winter temperature of internal outline of space according to degrees Faenheit (equivalent to 75 degree Faenheit or 24 degrees Celsius. Approximately, for each Btu/ heat capacity, amount of air carrying heat load will be considered between 300 to 400 cubic feet per minute (5.8 to 11 cubic meters per minute). The static pressure of fan device is a function of the amount of air carrying thermal load and dimensions and length of ducts network Description of manufacture warm air furnace Built hot air furnace is an oven of hot air with the vertical air flow that needed air flow is provided by a centrifugal fan (in the form of up-flow). The main pieces of this furnace include: fan, torches (burner), heat exchanger plates, combustion chamber, smoke outlet, and baffles [4]. The baffles act as obstacles against escaping of combustion products and their task is the increase in time of heating exchange between the flowed air with fan and heating exchange plates (and as a result more transferring of heat and increase efficiency of warm air furnaces). The manufactured warm air furnace works as a gas to gas thermal exchanger of countercurrent flow. Therefore, for designing of it, heat exchange equations related to this type of exchange rate used. Output of heat exchanger of the countercurrent flow is higher than that in current flow and for specific amount of heat transfer needs to the small surface area. Basic problem in gas to gas heat exchanger is its low overall heat transfer coefficient. Increase level of the heat exchange and increasing in survival time of combustion products inside of the heat exchanger partly solve the problem. The turbulent flow of the combustion products (inside of the heat exchanger) and moreover the indoor air (in which is flowing inside of warm air furnace) causes an increase in heat Transfer Rate. This case in the manufactured model has been considered and with installing special baffles, both inside and on the exchanger there will be created a turbulent flow of the combustion products and the indoor air flow and in addition to this, baffles act as vane and causes more heat exchange (1 pictures and 2). Picture 1. heat exchangers of hot air furnace 2. Formulas of energy exchange [2] With regard to the pictures 1 and 2, a heat exchanger is considered as sum off our existing exchangers inside of a warm air furnace like picture number 1. Regarding Picture 2. Heat exchangers and baffles to the picture 1, the transfer of heat from the combustion products in the open air of the building is given based on formula (2). q = UA T m (2) q: Heat transferred to indoor air (the air in the building) 24

3 A: Surface area of the heat exchange in the thermal exchanger T m : Difference of effective average temperature according to formula (3) the U: Coefficient of overall heat transfer of exchanger based on formula number (6). T 1 T 2 T m = (3) ln( T 1 T 2 ) T 1 = T h1 T c1 (4) T 2 = T h2 T c2 (5) T c2 : Temperature of the building that for being warm enters to the warm air furnace and after warming its temperature reaches to T c1 T h1 : Temperature of combustion products entering to the thermal exchanger, which is shown in the figure 1 T h2 : Temperature of combustion products output of the chimney of hot air furnace that its measured amount for a model of hot air furnace existing in the market is presented as following. 1 U = 1 h i + t k + 1 h o + R o + R i (6) In formula (6): U: Coefficient of overall heat transfer h i : thermal Transfer coefficient of combustion products of hot air furnace according to Watt per square meters kelvin degree( W 50h m 2 k i =) h o :Heat transfer coefficient of building air based on Watt per square meter degrees Entering of the combustion products from the combustion chamber with temperature T h1 Input air to the building with temperature of Tc1 Input air to the building with temperature of Tc1 Return Air from building with T c2 temperature Return air from building with Tc2 temperature Exit of Combustion products toward the chimney with temperature of T h2 Figure 1. One of the sections related to heat exchange. Kelvindegree ( W 40h m 2 k o =) k Thermal conductivity coefficient of exchanger wall according to the Watt meters kelvin degree ( W 50= k) m k t: Thickness of metal sheet used for manufacturing heat exchanger (equal with 2 millimeter). R o : Thermal resistance of deposits on the outer wall of exchanger (for air equal to m2 k ) W R i : Heat resistance of deposits on the inner wall of exchanger (for gases result of the burning of natural gas equal to m2 k ) W Variables of thermal resistance and coefficients of heat transfer and coefficients of thermal conductivity, which have been pointing out above, have been obtained from reference number (Robert, Boiler 2002).With replacing numerical values of variables in formula(4), coefficient of overall heat conductivity of exchanger would be 18.6 Kilocalorie per square meter hour degree Celsius ( kcal 18.6= W m U). m 2 k On the other hand, amount of heat that a warm air furnace transfer to the indoor air is equal to heat losses of the building. The building heat losses (i.e., heat losses from floor, ceiling, air conditioning, etc.....), with the use of the existing methods has been estimated, therefore the amount ofq in the formula 1 and 2 and also T c1 and also T c2 values are known. Correlation between temperature of combustion products and transferred heat from these products is expressed in the formula 7. m c p (T h1 T h2 ) q (7) m : Mass flow rate of combustion products with unit kg of c p : Heat capacity of combustion products with unit of kcal kg. The values of ḿ and c p are obtained based on the equations related to natural gas combustion with percentages of related surplus air (7). T h1 : Temperature of combustion products in the input of heat exchanger T h2 :Also should be such that in one hand doesn t cause distillation of combustion products and on the other hand it should be enough high from temperature of air surrounding exchanger that leads to transferring of heat to indoor air. The value of T h2 (or temperature of combustion products at the outlet of the warm air furnace chimney) in the existing furnaces is about 185 degree Centigrade.The proposal for minimum value oft h2 can be equal to 100 degrees Celsius which can lead thermal transferring from the combustion products to [5] the building air and also to prevent the condensation of combustion products. Details of calculations along with numerical quantities of the above parameters will be presented in the continuation Calculation of heat losses of combustion products in warm air furnaces (8) 25

4 The heat losses are containing two parts: tangible heat losses associated with products of combustion and heat losses caused by the hydrogen and moisture. British standard 845BS presents these values in percentage according to the formula number (8) and (9). L s = α(t h2 T a ) (8) (20.9 β) L s : Sensible heat losses of gas emissions of the chimney divided by the gross fuel heat value)), according to percentage α: Coefficient for special kind of fuel (for liquid fuels is equal 0.711and for fuel gas is 0.615) Th 2: temperature of combustion products in the outlet of hot air furnace according to degrees Celsius T a : Ambient air temperature in degrees Celsius β: Percent of oxygen in combustion products L w = γ [ (T h2 T a )] (9) L w : The heat Losses due to the hydrogen and moisture divided by the gross heating value of fuel) in percent γ:coefficient of special kind of fuel (for liquid fuels for and for gas fuel equal ) With the combination of formula number (8) and (9), the total heat Losses due to the combustion products of a hot air furnace in percent (L t ) will be given in a formula (10) by. In this project, quantities oft h2 and T a and β has been measured for a number of warm air furnace in the market and the results are as following: L t = L s + L w = α(t h2 T a ) (20.9 β) + γ[ (T h2 T a )] (10) With the use of the standard number , which is based on experimental relations, the heat losses can be calculated (9). In this standard heat loss is calculated from formula number 11. L t = (T h2 T a ) [ ] (11) : Percentage of mass fraction of Dioxide carbon measured with the combustion product analysis tool. 3. Thermal Design of made warm air furnace Required information for designing of warm air furnace is including: heat capacity of hot air furnace, aeration capacity of furnace fan and heat capacity of hot air furnace torch. Other necessary dimensions and measurements are acquired based on the size of fan, thermal exchangers, space of burner and torches [7] Estimation of thermal capacity of built hot air furnace: Heat capacity of the warm air furnace, with considering the relevant coefficients will be equal to heat losses of building. Heat losses calculations related to the building has been separately done and would be announced. This amount of thermal load should be provided with four heat exchangers existing in the warm air furnace (pictures 1 and 2). Therefore, the heat capacity of each of these exchangers is equal with one fourth of building heat losses and the numerical value of q (based on the thermal load of a particular typical building) will be equivalent with ( Btu ) or 6850 ( Kcal ). The quantities of T h1 (for the existing hot air furnace) and T c1 (ambienttemperature) and T c2 (based on the standard) are 400 and 25 and 40 degree Celsius, respectively. The amount of T h2 = 125 degree Celsius is considered and we continue the calculations [6]. With placement of above quantities in the formula 3, we will have T m =204. Now, with the use of formula 2, the level of heat exchange for one thermal exchanger will be 1.8m 2. Of this type of heat exchanger, four pieces have been made and were installed inside of one warm air furnace (pictures 1 and 2) Calculation of heat capacity of torch The torch of hot air furnace should be able to provide the required heat of the building and also heat losses of hot air furnace. According to the calculations done for the needed heat of a typical building is equal to ( Btu ) or (Kcal). In the formula 10, air temperature or outdoor temperature (T a ) is equal to zero and percent of excess air (β) related to the existing hot air furnace is equal to 15%. In this case, amount of heat losses will be equal to20.5 % of burner capacity. If heat capacity of torch is equal withq, then the value of Q will be obtained from the formula number 12. (12) 27400Q = Q + From the formula 12 can get the amount ofq = Kcal. With regard to heating value of natural gas (10000 Kcal 3 ), gas consumption of hot air furnace torch is m acquired equal to 5.3 cubic meters per hour Calculation of the aeration capacity of warm air furnace fan: The aeration capacity of hot air furnace fan (from the formula 1) is equal with 3500 cubic feet per minute. Since, it is possible that this device works continuously therefore to prevent the early amortization; centrifugal fans with ability of presenting bi-situational function are utilized (like fans existing in swamp coolers). Thesefans are able to work with a low speed constantly and without a considerable damage. Therefore, chosen fan for this hot air furnace will be from two- situational type which is able to move the air 3500 cubic feet per minute in a low speed. Selection of such a fan causes that in the beginning of installing of a warm air furnace (in the beginning of administrative hours), its high speed enters to the circuit and heats the building faster and after that, when the temperature of the building becomes a little bit balanced, it is switched to the low speed situation [8],[9]. 4. Economic analysis [10] The designing and construction warm air furnace the subject of this research has been done based on the heat capacity of installed torches on the hot air 26

5 furnaces existing in the market. This issue makes possible investigation and comparison of thermal efficiency of these two hot air furnaces. For analysis of thermal efficiency of these two hot air furnaces, a heat torch has been used. The trend of the experiment for comparison of heat performance of heat these two furnaces is as following: first torch is installed on the existing furnace existing in the market and after providing stable thermal conditions, heat losses will be calculated from the formula 10. Then this torch with the same conditions is installed on built hot air furnaces and again with formula 10, heat losses of built hot air furnace is acquired. With comparison of heat losses in both furnaces, can obtain return period of excess expenses related to the built furnace. Additionally, with using the heat losses in both furnaces, the difference in heat output of both furnaces can be also calculated. Temperature of combustion products in the furnace existing in the market is equal to 185 degree Celsius while in built hot air furnace is equivalent with 125 degree Celsius. With the analyzer tool of the combustion products, amount of excess air in both cases has been measured up to 15%. The reason of the equality of amount of excess air in both states is that for performing this experiment in every of these two hot air furnaces, a same heat torch has been used. The air temperature in both situations has been 25 degree. With replacement of formula 10 variables, heat losses in the hot air furnace existing in the market was obtained 27.3% and heat losses of built hot air furnace was acquired up to 20.56%. The percentage of built warm air furnace was an about 6.74% less than the existing model in the market. Amount of gas consumption in torches was set equal to 5.3 cubic meters per hour. Therefore the amount of saved gas in the built model is equivalent with cubic meters per hour. On this basis, the cost of saved gas based on subsidized rates (per each cubic meter 120 Tomans) is 23.8 Tomans in hour and on the basis of rates of exported gas (per each cubic meter 1200 Tomans) 283 Tomans. The difference between costs of manufacturing of one built hot air furnace model compared to the existing model in the market has been up to 300 thousand Tomans. According to return period of excess expenses related to the built furnace, based unsubsidized rates of gas is equal to hours, and based on the rates of exported gas is up to 1060 hours. If the average function of hot air furnace (in tee month of Winter)per a day is equal to 12 hours, with the assumption of 25 working days in each month, return time period for added investment based on subsidized rates and based on the rates of gas export will be 53 months and 5.3 months, respectively. As it is clear to do optimizing of gas consumption in warm air furnaces in both conditions (evaluation with subsidized rates and or rates of export) is economical. With regard to the heat losses calculated for both models of hot air furnaces, heat efficiency of the hot air furnaces existing in the market, is up to72.8and for optimized furnace is equal to 79.44%. Usually the output of hot air furnace will be announced more than 72.8% and the reason is that the manufacturer companies of these equipment don t consider an intangible losses of heat in thermal output and low thermal value of fuel is considered as calculations criterion and this will cause to give the heat output of these means about 10% higher than the real amount of it. References [1]. Tabatabaei, M. calculations of building facilities, ninth edition, publication of contemporary culture home. [2] [3]. Soltandost. Mechanical installations, first edition, 2012, publication of Yazda. [4]. Hosseinpour, H., Kashani Asl, S. Numerical and laboratory investigation of effects of the flow obstacles in increasing heat output of gas fireplace. Journal of scientific research of fuel and combustion, third year, first number, summer, [5]. Liu, H. Thermal exchangers, selsction, detection of quantity of function translation of industries, s, first edition. University of Science and Industry publications, [6]. Horton, R., Boiler, A. translation of Rafiee Pour Alavi, H., first edition, publisher, designer publication, [7]. Mobini, K. Fuel and combustion, second edition, Sharh publication, [8]. [9]. The institute of standard and Industrial Research of Iran, gas fireplace with chimney, characteristics. Test and instructions for the adhesive energy, national standards of Iran to number , [10]. Seyed Hosseini, S.M. Engineering economy and decision makinganalysis. Second edition, University of Science and Industry publications,

Boiler Efficiency Testing. To understand the operation of a fire tube boiler To determine the operating efficiency of the boiler

Boiler Efficiency Testing. To understand the operation of a fire tube boiler To determine the operating efficiency of the boiler Boiler Efficiency Testing Objectives: To understand the operation of a fire tube boiler To determine the operating efficiency of the boiler Theory Most industrial processes require heating. This is usually