Evaluation of retrofitting a conventional natural gas fired boiler into a condensing boiler

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1 Energy Conversion and Management 45 (2004) Evaluation of retrofitting a conventional natural gas fired boiler into a condensing boiler Defu Che a,b, *, Yanhua Liu a, Chunyang Gao a a Department of Thermal Engineering, School of Energy and Power Engineering, Xi an Jiaotong University, Xi an , China b State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, Xi an , China Received 21 June 2003; received in revised form 27 October 2003; accepted 9 January 2004 Available online 11 March 2004 Abstract The exit flue gas temperature of a conventional gas fired boiler is usually high and a great amount of heat energy is lost to the environment. If both sensible heat and latent heat can be recovered by adding a condensing heat exchanger, the efficiency of the boiler can be increased by as much as 10%. In this paper, based on combustion and heat transfer calculations, the recoverable heat and the efficiency improvement potential of different heat recovery schemes at various exit flue gas temperatures are presented by performing design calculations. The payback period method has been used to analyze the feasibility of retrofitting a conventional gas fired boiler into a condensing boiler in a heating system in detail. The results show that the most economical exit flue gas temperature is C when a conventional natural gas fired boiler is retrofitted into a condensing boiler simply by adding a condensing heat exchanger. It is feasible to use the return water of a heating system as the cooling medium of the condensing heat exchanger because the return temperature varies with the ambient temperature and is lower than the dew point of the water vapor in the flue gas in most periods of a heating season in some regions, which has been verified by retrofitted case. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Condensing boiler; Sensible heat; Latent heat; Evaluation; Payback period 1. Introduction A great amount of primary energy sources are consumed by heating boilers, which has led to severe environmental pollution issues. Conventionally, in China, the majority of heating boilers * Corresponding author. Address: Department of Thermal Engineering, School of Energy and Power Engineering, XiÕan Jiaotong University, XiÕan , China. Tel.: ; fax: address: dfche@mail.xjtu.edu.cn (D. Che) /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.enconman

2 3252 D. Che et al. / Energy Conversion and Management 45 (2004) take coal as the fuel. However, due to the rising living standard, the development and maturity of the market economy and the emphasis on environmental protection, gas fired boilers are taking a larger market share. Over the past few years, quite a few large gas fields have been proven, and a huge amount of natural gas has been recovered. China is building a high capacity pipeline for transporting natural gas from the west regions to the east industrialized coastal areas, so natural gas will find wider applications than ever. In quite a few municipalities, such as Beijing and XiÕan, new installations of coal fired boilers have been prohibited and natural gas fired boilers are strongly recommended. However, what worries users is the high operational cost of a natural gas fired boiler because of the relatively high price of the fuel gas, which has given rise to the shillyshally of selecting gas fired boilers. The exit flue gas temperature of a conventional boiler is usually higher than 150 C, sometimes as high as 200 C, to avoid low temperature corrosion. At such temperatures, the water vapor entrained in the flue gases does not condense, and the latent heat cannot be reclaimed, which leads to a considerable heat loss. Since the 1970s, condensing boilers have been developed and have found wide applications in Europe and North America [1 7]. In such a boiler, the exit flue gas is reduced to such a low temperature that the water vapor can be condensed, and the latent heat released can be recovered. As such, the thermal efficiency of the boiler can be significantly increased. If the low heating value is still taken as the calculation basis, the efficiency can be as high as, or higher than 100%. Previous research has shown that SO x,no x, dust and soot, etc., which are the constituents of the flue gas, can be partially, even totally, dissolved in the condensed water, and the pollutants emitted to the environment can be noticeably reduced. Therefore, it is of great significance both to energy saving and environmental protection to utilize condensing boilers. Dann [8] concluded in his paper that the potentially high operating efficiencies offered by condensing boilers can be achieved in practice, and this will ensure that for both new and replacement central heating installations, the condensing boiler will provide substantial savings in running costs when compared to the more conventional boiler. Most of the energy saving benefit of using condensing boilers can be achieved without recourse to excessive additional heat emitter surface or sophisticated controls. However, further developments in systems and controls for these appliances should be performed, and optimization of such schemes is necessary. The investigation of Searle et al. [9] and Pickup [10] showed that many parameters of design and installation influence the performance of condensing boilers in the field. These parameters include pipework design, controls, hot water cylinder design and boiler and system sizing, but the system designs for high efficiency condensing boilers do not need to be very different from current good practice for existing non-condensing boilers. The performance of condensing boilers was examined with water flow and return temperatures of 60 and 40 C, respectively, in addition to the 80 and 60 C used for a non-condensing unit. The results from the field study confirmed that substantially greater annual efficiencies were obtained with condensing boilers than with traditional boilers. A simple relationship was proposed that shows how boilers and radiator sizing affect annual efficiency. A payback of between 3 and 4.5 years for the installation of a condensing boiler can be expected. So far, various schemes for reclaiming the latent heat in flue gas have been put forward [11 18] the general methods for designing condensing heat exchangers have been proposed [19 21] and

3 D. Che et al. / Energy Conversion and Management 45 (2004) Table 1 Constituents of Shanbei natural gas Component CH 4 C 2 H 6 C 3 H 8 C 4 H 10 CO 2 H 2 N 2 Content (vol.%) the effects of various factors on the seasonal efficiency or annual efficiency of the heating system have been examined [22 30]. While natural gas is used as the fuel of the boiler, as high as a 20% volumetric fraction of water vapor in the combustion products will be generated, which is much higher than that while anthracite or bituminous coal is used as the fuel. More water vapor in the flue gas means that more latent heat can be recovered, and the thermal efficiency of the boiler can be more greatly improved by decreasing the exit flue gas temperature. On the other hand, natural gas is much more expensive than coal in China, and the operational cost of a gas fired boiler is generally much higher than that of a coal fired boiler. Thus, it is more profitable to recover the latent heat by condensing the water vapor in the flue gases of a gas fired boiler. As is well known, hydrocarbons are the dominant components of natural gas. For example, the volumetric percentage of methane in Shanbei natural gas of China is as high as 96.32% (see Table 1). According to the partial pressure of the water vapor in the flue gas, the dew point temperature is generally C depending on the excess air ratio. The exit flue gas temperature can be reduced below the dew point temperature by a cooling medium to recover both the sensible heat and latent heat. The higher heating value and the lower heating value of the natural gas in Table 1 are Q gr;t;ar ¼ 38:876 MJ/N m 3 and Q net;t;ar ¼ 35:055 MJ/N m 3, respectively, with a difference of (MJ/N m 3 ). The boiler efficiency can reach a theoretically maximum value of / % ¼ 110.9% based on lower heating value. In China, both condensing boilers and water heaters have been fabricated and put into operation. For the conventional gas fired boilers already in use, many users intend to retrofit them into condensing boilers by adding condensing heat exchangers to reclaim both the sensible heat and the latent heat in the flue gas. In this paper, a natural gas fired boiler is retrofitted into a condensing boiler by adding a surface type condensing heat exchanger. The payback period method is used to perform an economical evaluation of the scheme to obtain the optimum exhaust gas temperature. The feasibility of using the return water of a heating system as the cooling medium of the condensing heat exchanger has been examined. 2. Combustion calculations The constituents of the natural gas for calculations is shown in Table 1, Table 2 presents the results of the combustion calculations. It can be seen that when the excess air ratio is 1.05, the volumetric fraction of water vapor is as high as 19.3%, and the corresponding dew point temperature at thermodynamic equilibrium is 59.3 C.

4 3254 D. Che et al. / Energy Conversion and Management 45 (2004) Table 2 Combustion characteristics of Shanbei natural gas Item Unit Source Result Theoretical air quantity N m 3 /N m þ 0:5CO þ P ðm þ nþc 4 mh n þ 1:5H 2 S O 2 Þ 9.29 Theoretical water vapor volume N m 3 /N m 3 0:01ðH 2 þ H 2 S þ P n 2 mh n þ 120ðd g þ V 0 d a ÞÞ 2.1 Theoretical nitrogen gas volume N m 3 /N m 3 0:79V 0 þ 0:01N RO 2 volume N m 3 /N m 3 0:01ðCO 2 þ CO þ P mc m H n þ H 2 SÞ 1.00 Average excess air ratio Selected by experience 1.05 Actual water vapor volume N m 3 /N m 3 VH 0 2 O þ 0:0161ða 1ÞV Total flue gas volume N m 3 /N m 3 V RO2 þ VN 0 2 þ VH 0 2 O þð1þ0:161þða 1ÞV Volumetric fraction of RO 2 V RO2 =V y 0.09 Volumetric fraction of H 2 O V H2 O=V y Volumetric fraction of tri-atomic r H2 O þ r RO gas Density (standard state) kg/n m Dew point C Water steam property table by volumetric fraction 59.3 of H 2 O Latent heat kj/kg Water steam property table Higher heating value kj/n m Lower heating value kj/n m Heat recovery calculations In order to recover both the latent heat of the water vapor and the sensible heat, the flue gas temperature must be reduced sufficiently Sensible heat Sensible heat can be obtained according to the relationship between temperature and enthalpy for various constituents of the flue gas. The sensible heat recovered relative to the enthalpy at 180 C can be calculated at various exit flue gas temperatures Latent heat The quantity of condensate can be calculated in terms of the partial saturation pressures corresponding to various temperatures, and the recovered latent heat is obtained by the condensate quantity multiplied by the average latent heat of vaporization. The calculated results are listed in Table 3. The boiler efficiency based on lower heating value at various exit flue gas temperatures is given in Fig. 1. It is easily seen from the figure that the boiler efficiency curve can be divided into two considerably different regions. The boiler efficiency varies relatively gradually in the temperature range of C, and it is changed very rapidly in the temperature range of C, which is mainly because the latent heat loss takes a greater role than the sensible heat. If the exit flue gas

5 D. Che et al. / Energy Conversion and Management 45 (2004) Table 3 Results of heat recovery calculations Exit flue gas temperature ( C) Total heat recovery (kj/n m 3 ) Condensate quantity (kg/n m 3 ) Efficiency improvement (%) Note: Efficiency improvement is calculated on the basis of the value at 180 C of the exit flue gas temperature (boiler efficiency is 89.9%) Boiler efficiency, % Excess air ratio α = Exit flue gas temperature, C Fig. 1. Boiler efficiency at various exit flue gas temperatures. temperature is decreased to 20 C, the boiler efficiency based on lower heating value may reach 107.4% theoretically. 4. Evaluation The addition of the condensing heat exchanger leads to an increase of investment cost. The payback period method can be used to perform a static evaluation on the utilization of the condensing boiler. Thus, the feasibility of retrofitting a conventional natural gas fired boiler into a condensing boiler can be studied. In this paper, a WNS /95/70-QT boiler is taken for the evaluation. This is a gas fired shell type boiler with output of 2.8 MW, rated pressure of 1.0 MPa, supply water temperature of 95 C

6 3256 D. Che et al. / Energy Conversion and Management 45 (2004) and return water temperature of 70 C. In the calculations, the cold air temperature is taken as 20 C, the excess air ratio is taken as 1.05 and Shanbei natural gas is used Cost accounting The addition of a condensing heat exchanger can lead to improvement of the boiler efficiency and the conservation of fuel gas but also can cause an increase of the investment cost of the equipment, which is due to the exchanger material, valves, piping, installation, extra maintenance and resistance increase. Fig. 2 shows the schematic arrangement when the condensing heat exchanger is used to heat domestic hot water. While the condensing heat exchanger is in normal operation, valve 2 and valve 3 for maintenance are closed, and valve 1 and valve 4 are open Cost increase calculations (1) Cost of heat exchanger The heat reclaimed from the exhaust gases is utilized to heat domestic water. The heat exchangers are assumed to be made of PTFE (polytetrafluoroethylene) tubes, stainless steel tubes and carbon steel tubes respectively. The tubes are staggered in arrangement with transverse spacing s 1 ¼ 54 mm, longitudinal spacing s 2 ¼ 30 mm, tube outside diameter d ¼ 20 mm and wall thickness d ¼ 3 mm. The flue gas across the tube bank is in counter flow with the cooling medium flow. The entering flue gas temperature is 180 C, the entering cold water temperature is 10 C and the hot water temperature is set to be 50 C. The heat transfer equation for a convective heating surface [31]: Q ¼ KHDt=B j where Q is the heat released to heating surface by 1 m 3 of calculating fuel gas, kj/n m 3 ; K the heat transfer coefficient, kw/(m 2 C); H the calculating heat transfer area, m 2 ; Dt the mean temperature difference, C; B j the fuel consumption rate, m 3 /s. K is calculated by ð1þ Fig. 2. Schematic arrangement of condensing heat exchanger.

7 D. Che et al. / Energy Conversion and Management 45 (2004) K ¼ wk 0 ¼ w 1 ¼ wa 1 1 a 1 þ 1 a 2 1 þ a 1 a 2 ð2þ where w, the effectiveness factor, is equal to the ratio of the heat transfer coefficient K of the fouled surface to the heat transfer coefficient K 0 of the surface without fouling. w ¼ K=K 0, which usually can be taken as 0.9; a 1 is the convective heat transfer coefficient of flue gas, kw/(m 2 C); a 2 the convective heat transfer coefficient of cooling water, kw/(m 2 C). For the convective tube bank, because a 2 is so large in value that a 1 a 2 can be neglected, Eq. (2) reduces to K ¼ wa 1 kw=ðm 2 CÞ ð3þ The convective heat transfer coefficient of the flue gas across the staggered tube bank externally (Re ¼ 1: )is 0:6 k wd a 1 ¼ 0:358C s C n Pr 0:33 kw=ðm 2 CÞ d t where C s, C n are called spacing correction factor and correction factor taking account of the number of rows of tubes along the direction of gas flow, respectively. By the geometric arrangement of the studied heat exchanger, the following results a 1 ¼ 0:436 k 0:6 wd Pr 0:33 kw=ðm 2 CÞ ð4þ d t The velocity of the flue gas through the exchanger is taken as 2.5 m/s by experience. The pertinent properties of the flue gas are calculated as follows: Pr ¼ 0:721, thermal conductivity k ¼ 1:01ð2:4742 þ 0:00703t 1:50233E 6t 2 Þ, kinematic viscosity t ¼ 0:975ð13:48881 þ 0:09388t þ 7:26224E 5t 2 Þ, where t is average bulk temperature, C. According to earlier research of the authors [32], the convective heat transfer coefficient with condensation is times the corresponding convective heat transfer coefficient without condensation. The log mean temperature difference can be obtained from the inlet and outlet temperatures of the flue gas and the inlet and outlet temperatures of the cooling water. Finally, from Eq. (1), the required heating surface area H is determined as shown in Fig. 3 for different exit flue gas temperatures. The weight of the heat exchanger is G ¼ qhðd 2 o d2 i Þ=4d o 10 3 where G is the weight of material, ton; q the material density, kg/m 3 for steel, kg/m 3 for PTFE; d o the outside diameter, mm; d i the inside diameter, mm. The cost of the exchanger is C ¼ GV ð6þ where V is the material price, yuan RMB/t (4000 yuan/t for carbon steel tube; 16,000 yuan/t, 130,000 yuan/t for PTFE); C the material cost, yuan. ð5þ

8 3258 D. Che et al. / Energy Conversion and Management 45 (2004) Convective heating surafce area, m α = Exit flue gas temperature, C Fig. 3. Heating surface area at different exit flue gas temperatures. Table 4 Cost of increased material Exit flue gas temperature ( C) Heat recovered (kj/n m 3 ) Heating surface increase (m 2 ) Material cost (yuan) Carbon steel Stainless steel PTFE The calculated results at different exit flue gas temperatures are given in Table 4. (2) Heat exchanger setting cost The casing of the heat exchanger is made of stainless steel, its size is dependent on the design conditions. In the calculations, an exchanger of 1(width) 1.2(height) 1.5(length) m with thickness d ¼ 2 mm is assumed, its cost is about 2500 yuan. (3) Power consumption increase due to extra resistance The increase of resistance to the flue gas stream due to installation of the condensing heat exchanger is calculated by Dh ¼ 1 qw2 2 ð7þ

9 D. Che et al. / Energy Conversion and Management 45 (2004) Resistance increase, Pa α = Exit flue gas temperature, C Fig. 4. Resistance increase at different exit flue gas temperatures. where 1 is the resistance coefficient; q the density of flue gas, kg/m 3 ; Dh the resistance, Pa, and the draft fan power is!, N ¼ b 1 ðb 2 V y Þ b 3 Dh 1:293 q 0 y ð3: gþ ð8þ where V y is the flow rate of the flue gas through the draft fan, m 3 /h; q 0 y the density of the flue gas at standard conditions, kg/m 3 ; g the efficiency of the draft fan, taken as 0.6; N the power of the draft fan, kw; b 1 the spare factor for the draft fan power, usually can be taken as 1.05; b 2 the spare factor for flow rate, usually can be taken as 1.1; b 3 the spare factor for the pressure head of the draft fan, usually can be taken as 1.2. The resistance is converted into electric power. The electricity price is taken into account by 0.6 yuan/kw h. The resistance increase at different exit flue gas temperatures is given in Fig. 4. It can be easily seen that the resistance increase due to the heat exchanger is small. The maximum value of 195 Pa corresponds to an increase of 0.6 kw in the draft fan power consumption, which will exert an insignificant effect on the heating system. (4) Piping and valve cost: 4 gate valves account for ¼ 2000 yuan. The additional piping made of carbon steel accounts for some 1500 yuan. (5) Manufacture and installation cost: the cost for manufacture of the heat exchanger and the setting accounts for 1/3 of the material cost, and the installation cost is calculated as 1/2 of the material cost.

10 3260 D. Che et al. / Energy Conversion and Management 45 (2004) Table 5 Saved fuel cost Exit flue gas temperature ( C) Heat reclaimed (kj/n m 3 ) Saved gas (N m 3 /h) Saved cost (yuan/h) Note: The price of the natural gas is taken as 1.5 yuan RMB/N m 3. (6) Maintenance and repair cost: in the lifetime (taken as 10 years), this cost can be taken as three times the original value of the equipment Saved cost calculations The cost saved is calculated by the energy saved, i.e. saved cost ¼ (saved heat energy/low heating value of the fuel) fuel price. Table 5 presents the calculated results Payback period The payback period method [33] is used to evaluate the economics PBT ¼ IC ð9þ ANCF where PBT is the payback period, h, IC the investment cost, yuan; ANCF the annual net capital flow, yuan/h; ANCF ¼ RFC ) PC: RFC the cost of recovered heat, yuan; PC the operational cost, yuan/h. The calculation results are shown in Fig. 5 (the boiler is assumed to operate at full load) Payback period, hours PTFE 2-Stainless steel 3-Carbon steel Exit flue gas temperature, C Fig. 5. Payback period versus exit flue gas temperature.

11 D. Che et al. / Energy Conversion and Management 45 (2004) It can be seen that the carbon steel heat exchanger has the shortest payback period, and the PTFE heat exchanger has the longest payback period, which implies that material price has a paramount influence on the payback period. The payback period is longer when the exit flue gas temperature is slightly lower than 180 C. The reason for this is that the investment cost of the heat exchanger (including valves and piping) is, by far, higher than the cost of the saved energy. As the saved energy increases due to the lower exit flue gas temperature, the payback period is greatly shortened. When the exit flue gas temperature is reduced to some particular value, the payback period will rise with further reduction because of the more rapid increase of material cost over the cost of the saved energy. When the exit flue gas temperature approaches the dew point of the water vapor in the flue gas, the payback period is sharply reduced, which is due to the recovery of the latent heat in great quantities. For the carbon steel heat exchanger, the shortest payback period is only 320 h at the exit flue gas temperature of 55 C. For the stainless steel heat exchanger, the shortest payback period is 850 h at the exit flue gas temperature of some 50 C. For the PTFE heat exchanger, the shortest payback period is 1800 h. The calculations above are based on the assumption that the three kinds of condensing heat exchangers have an identical lifetime. As is well known, the condensate is weakly acidic with a ph value of 4 5 unless a particular treatment is conducted. As such, it is often the case that the carbon steel exchanger will have a shorter lifetime because of its poor corrosion resistance. The PTFE material is very corrosion resistant, but it is too expensive in China. Calculations show that the most optimum exhaust gas temperatures at different lifetime lengths for the three kinds of exchangers are unchanged, and the payback periods vary very slightly. Thus, stainless steel exchangers are recommended. In the evaluations above, a sufficient domestic hot water requirement is assumed. If there is not sufficient domestic hot water to be heated, but a radiant floor heating system is used, similar conclusions can be drawn. As pointed out by Haase et al. [34], the temperature of the heating medium must be as low as possible to make a condensing boiler run well with a considerable difference between the flow pipe and return temperature. In new buildings, condensing boilers are often connected to floor radiators, or heating systems with low temperature radiators. Even in old buildings, there must be no limitations for appropriate integration of the condensing boiler technology. The heat demands of the building are less than during construction as a result of subsequent thermal insulation. Because the existing heating system, designed for supply temperatures of 90 C and return temperature of 70 C (German standard), is over sized due to the conservative safety factors, lowering the heating medium temperatures is possible. In China, the most commonly used method for regulating a heating system is the so-called quality regulation, by which the heating medium flow rate is kept constant and the supply temperature is varied according to the ambient temperature. Generally, as the outdoor temperature rises, both the supply temperature and the return temperature decrease, but the supply temperature decreases more rapidly than the return temperature. The following correlations can be used to calculate both temperatures [35]. t g ¼ t n þ Dt s 0Q b þ Dt j 0Q t h ¼ t n þ Dt s 0Q b Dt j 0Q ¼ t g 2Dt j 0Q ð10þ ð11þ

12 3262 D. Che et al. / Energy Conversion and Management 45 (2004) where t g, t h are the actual supply and return temperatures, C; t g 0, t h 0 the standard (design) supply and return temperatures, 95/70 C; t n the indoor temperature, 18 C; Dt s 0 the design mean temperature difference of radiators, Dt s 0 ¼ t g 0 þt h 0 2t n, C; Dt 2 j 0 the design temperature difference between supply and return temperatures, Dt j 0 ¼ t g 0 t h 0, C; Q the relative heat, Q ¼ t n t w ; t 2 t n t w 0 w the daily mean temperature in heating season, C; t w 0 the calculating outdoor temperature, it is different from region to region (it is )5 C inxiõan); b ¼ 1, B an index reflecting the heat transfer coefficient 1þB of the radiator, for the commonly used radiators, B ¼ 0:35. The correlations above have been obtained based on a great quantity of experimental data, and validated by in situ measurements for many years in China. As such, if XiÕan is taken as an example, the actual supply and return temperatures by quality regulation can be expressed as follows: t g ¼ 18 þ 64:5 18 t 0:74 w þ 12:5 18 t w ð12þ t h ¼ 18 þ 64:5 18 t 0:74 w 12:5 18 t w ð13þ According to the meteorological data, substituting into Eq. (13) the monthly mean temperatures of five heating seasons in XiÕan from 1990 to 1995 give the monthly mean return temperatures, as shown in Table 6. The results are diagrammatically presented in Fig. 6. The water vapor in the flue gas cannot be condensed by the return water in a heating system operating at design conditions (95/70 C) because the return temperature of 70 C is higher than the saturation temperature of the water vapor. However, the actual return temperatures are lower than the dew point of water vapor in most periods of a heating season, which demonstrates the possibility of recovering the latent heat in the flue gas. The seasonal mean return temperatures are given in Table 7 and Fig. 7. Table 6 Return temperatures of 95/70 C heating systems from 1990 to 1995 in XiÕan ( C) November December January February March Heating season Heating season Heating season Heating season Heating season Monthly mean temperature Mean return temperature t h Monthly mean temperature ) Mean return temperature t h Monthly mean temperature ) Mean return temperature t h Monthly mean temperature Mean return temperature t h Monthly mean temperature Mean return temperature t h

13 D. Che et al. / Energy Conversion and Management 45 (2004) Dew point of water vapor in flue gas heating season heating season heating season heating season heating season Return temperature, C Nov. Dec. Jan. Feb. Mar. Month Fig. 6. Return temperatures of 95/70 C heating systems from 1990 to 1995 in XiÕan. Table 7 Seasonal mean return temperatures from 1990 to 1995 in XiÕan Season Mean return temperature ( C) From the calculations above, for the 95/70 C heating system, the higher outdoor temperature will lead to a lower return temperature, which has been confirmed in practice. The seasonal mean return temperature in XiÕan varies in the range of C, which is lower than the dew point of 59.3 C of the water vapor in the flue gas produced by Shanbei natural gas. As such, it is possible to recover the latent heat in the flue gas by the return water. It should be pointed out that the outdoor temperature in a heating season varies from region to region, so the mean return temperature will vary. Thus, the degree of recovery of the latent heat in the flue gas will depend on geographical location. In fact, since the water side heat transfer resistance is negligible due to its high heat transfer coefficient, the tube wall temperature is always close to the temperature of the water flowing inside. Therefore, whenever the return water temperature is to some extent (several degrees for instance) lower than the dew point, the steam entrained in the flue gas can be condensed due to the thermodynamic non-equilibrium, and the thermal efficiency of the boiler can be improved, which has been verified by retrofitted practices. The field measurements of a natural gas fired boiler with an output of 2.1 MW are given in Table 8 [36].

14 3264 D. Che et al. / Energy Conversion and Management 45 (2004) Water vapor dew point 2-Mean return temperature Temperature, C Heating season from 1990 to 1995 Fig. 7. Seasonal mean return temperatures from 1990 to 1995 in XiÕan. Table 8 Field measurements Return temperature ( C) Before condensing heat exchanger After condensing heat exchanger 40.8 Efficiency (%) Flue temperature ( C) Condensate (kg/h) Efficiency (%) Flue temperature ( C) Condensate (kg/h) Efficiency (%) Flue temperature ( C) Condensate (kg/h) 30 Efficiency improvement Obviously, for regions that are mild in climate, in the heating season, there will be more days in which the return water temperature is lower than the dew point of the water vapor in the flue gas, and it will be more worthwhile to recover both the latent heat and the sensible heat simply by installing a condensing heat exchanger. It should be pointed out that the most optimum exhaust gas temperature mentioned above cannot be achieved since the return water temperature cannot be very low. Thus, it is still recommended that an additional cooling medium source should be introduced into the system to make the payback period of the condensing heat exchanger as short as possible.

15 D. Che et al. / Energy Conversion and Management 45 (2004) Conclusion The efficiency of a natural gas fired boiler can be greatly improved by adding a condensing heat exchanger to recover the latent heat of the water vapor entrained in the flue gas. The resistance increment to the gas stream due to the addition of the exchanger is quite small, and the impact on the heating system can be neglected. The stainless steel condensing heat exchanger is recommended because of its corrosion resistance and relative long lifetime over carbon steel. Cheap and corrosion resistant material for a condensing heat exchanger is still urgently required. There is a most economical exit flue gas temperature, which is C, when a conventional natural gas fired boiler is retrofitted into a condensing boiler simply by adding a condensing heat exchanger. If stainless steel is used, the optimum temperature is about 50 C. Too low an exit flue gas temperature will give rise to an extreme increase of the exchanger surface area, and consequently, the investment cost of the exchanger. As low a temperature of the cooling medium of the condensing heat exchanger as possible to reclaim the latent heat in the flue gas to the utmost extent is always expected, however, it is possible to use the return water of a heating system as the cooling medium of the condensing heat exchanger because the return temperature varies with the ambient temperature and is lower than the dew point of the water vapor in the flue gas in most periods of a heating season in some regions, in particular in the regions with mild climate. However, the most economical exit flue gas temperature cannot be achieved. It is strongly recommended that an additional cooling medium source with a temperature that is as low as possible should be used to achieve the greatest benefit. Acknowledgements The financial support from The Research Fund of The Doctoral Program of Higher Education of China (RFDP no ) and The Special Funds for Major State Basic Research Projects (G ) are gratefully acknowledged. References [1] Gordon JS. Heat recovery with condensing heat exchangers. Am Dyest Rep 1983;72(10):23 4. [2] Field AA. Reclaiming latent heat in flue gases. Heating, Piping Air Condition 1974;46(11):85 7. [3] Noir D, Houlmann N. European technology in condensing flue-gas systems. In: Symposium on Condensing Heat Exchangers Proceedings, vol. II, March 3 4, 1982, Atlanta, GA. p [4] Shook JR. Recover heat from flue gas. Chem Eng Progr 1991;87(6): [5] Thompson RE. Condensing flue gas water vapor: another way to cut your fuel bill. Power 1983;27(5): [6] Thorn WF. Waste heat recovery from stacks using direct-contact condensing heat exchanger. In: Paper Presented at the 9th World Energy Engineering Congress, p [7] Streatfield L. Are condensing boilers the correct choice for domestic heating systems. Heat Ventilat Engr 1984;58(673):5 6. [8] Dann R. Domestic heating systems and controls for condensing boilers. Heat Ventilat Engr 1984;58(668):1 14. [9] Searle M, Shiret AR. The opportunities for a new generation of high efficiency gas boiler. Gas Eng Mgmt 1986;26(7 8): [10] Pickup G. Innovation in home heating. Gas Eng Mgmt 1983;23(5):171 8.

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