as the low temperature heat reservoir

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1 Analyses on the circulation o heat pump using the lue gas as the low temperature heat reservoir Tian Guansan Fu Lin Jiang Yi Dept. o building science, Tsinghua University, Beijing. China Abstract:According the combustion theory, ist law o thermodynamics and gas law, analyses are made on the dew-point temperature, saturated humidity and enthalpy o lue gas o combusted natural gas exhaust. The calculating equations or these parameters are derived and the corresponding calculating results are given in patterns o curves, which can be used as the base or actual engineering design. A new technical low chart is put up what have a heat exchanger and three parallel heat pumps. Basing on it the actual COP o each heat pump can exceed 3. I the lue gas temperature is cooled down rom 1 ~ to 4 ~3, the use o the sensible heat and latent heat o partial condensation o lue gas can be at a high level, and the thermal eiciency o natural gas can be raised 1%~%. Keywords:lue gas, dew point, condensation, heat pump, low chart. LIST OF SYMBOLS V volume o air or lue gas air actor d speciic humidity o lue gas T temperature o air or lue gas (K) t temperature o air or lue gas ( ) M molecule weight P pressure, partial pressure (Pa) condensation ratio speciic density I enthalpy relative humidity o air H heat value o natural gas r heat o vaporization o water R gas constant C isobar speciic heat Subscripts a air water vapor o standard state lue gas act actual speciic air requirement 1. INTRODUTION It is known that heat pump have enormous potential or saving energy, particularly in building 54

2 heating processes. They are the only heat recovery systems, which enable the low temperature heat to be raised to more useul levels or heating. It is very popular or air heat pump in the regions o the east and south o china in winter. But in north china the using o air heat pumps has been hampered by the act their eiciency and capacity will become very low when the outdoor temperature is below -1. So that using waste heat is the main method to overcome the disadvantages o heat pumps. Along with the adjustment o energy structure o china. Natural gas is used more and more, particularly in district heating, CHP and many other commercial uses. A lot o natural gas exhaust has a emission temperature o 15 ~, some time even reaches ~3. A high return water temperature level in heating system has restricted the using o sensible heat and latent heat o lue gas. For the conventional water heating system has a standard supply water temperature 95 and a standard return water temperature 7, The temperature dierence between medium and lue gas is generally equal to or more than 8 in natural gas boilers, so that heat eiciency and economic are very low by using o sensible heat and latent heat o lue gas directly. As the lue gas o natural gas is very clean, so that the corrosion o equipment produced by lue gas is very small. A new technology low chart is put up in this paper, which can largely raise the heat using eiciency o natural gas.. Humidity O Flue Gas The condensation o water vapor begins when the state o saturation or the steam in the lue gas is reached while cooling the lue gas. This point called the dew point, and its temperature is the dew-point temperature. At the dew point the steam pressure equals the saturation steam pressure o water, which is, as an approximation, or a total pressure about 1 bar. Determining relationship between the saturation humidity and temperature o lue gas is very important or using the sensible heat and latent heat o lue gas. According the thermodynamics theory the saturation temperature steam pressure o water, lue gas temperature and total pressure inluence the saturation humidity. Because the main composition o natural gas is methane, so that in this paper we assumed natural gas only has one composition o methane. The combustion reaction equation can be described as ollow: CH 4 + (O N ) = CO + H O + 7.5N (1) According the equation (1), the stoichiometric air requirement is V 9. 5,and stoichiometric amount o lue gas is V In order to insure the natural gas burning completely, the actual air requirement is greater than the stoichiometric air requirement. The air actor is deined as the ratio o actual air requirement and the stoichiometric air requirement, hence: V act V () Fig.1 and ig are the results rom equation (1) and (). From the igures the cures o volume and mass humidity o water vapor in lue gas as a unction o the air actor are given. It is shown that the amount o humidity is decreasing with the air actor increasing. The conventional air actor or natural gas combustion changes orm 1. 1to 1. 5, the volume percent o humidity in lue gas changes orm 18% to 1%, and the mass percent o humidity in lue gas changes orm 1% to 8%. About 1.6kg water vapor will produce when 1Nm 3 natural gas burnt 55

3 the mole raction o vapor (%) the mass raction o vapour(%) completely air actor air actor Fig.1 volume composition o humidity in lue gas as a unction o the air actor Fig. mass composition o humidity in lue gas as a unction o the air actor.1 SPECIFIC HUMIDITY OF FLUE GAS We describe the mass o a humid lue gas mixture as an alternative to speaking in terms o m and m. The way is to speciy the mass ratio called speciic humidity or humidity ratio m d kg water vapor/kg dry lue gas (3) m Which represent the number o kilograms o water vapor that correspond to 1 kilogram o dry lue gas in the given mixture. As the pressure o lue gas is near the pressure o atmosphere,the pressure o water vapor in lue gas is very small, the speciic volume is very great, so that the water vapor and lue gas can be assumed as theoretical gas. The pressure-volume-temperature equations can be described as ollow: p p V m R T (4) V m R T (5) combine eq.(3) with eqs. () and (5): m p R d (6) m p R MR R (7) we combine eqs. (6) and (7) to conclude that m M p d (8) m M p according equations (1) and () the molecular weight o lue gas can be calculated by using ormula M ( ( 1)) /( ( 1)),and molecular weight o water vapor is 56

4 the ratio o water vapor condensation M 18. Equation (8) combine with M and M to transorm as: d 18 p ( ( 1)) /( ( 1)) p (9). CONDENSATION RATIO OF WATER VAPOR IN FLUE GAS When the state o lue gas saturated with water vapor is called dew point. The pressure is called dew-point pressure and the temperature is called dew-point temperature. I the temperature o lue gas cooled down below the dew point the condensation occurs. The condensation ratio o water vapor in lue gas is deined as the mass ratio between condensation water and total vapor produced rom the combustion o speciic volume o natural gas. The ambient air state has been assumed to be in state with temperature o, pressure o 1135Pa, and relative humidity o. The total amount o vapor in lue gas is sum o vapor produced in combustion and vapor rom air. According about analyses the condensation ratio o water vapor in lue gas can be calculated by equation: V H H O dv a d( VCO CO VN N V( 1 V H H O ) ) d( ( 1)) (1) 167 Fig. 3 is the result calculation o the condensation ratio against the dew-point temperature o lue gas with dierent air actor and relative humidity o ambient air by equation (1). It shows that the air actor and relative humidity o air inluence the dew-point temperature and the condensation ratio o lue gas, the inluence o air actor is greater than that o relative humidity o air. The dew point temperature changes in the range rom 59 to 38 when the air actor varies in the range rom 1 to 3. The tendency o condensation ratio increases with the lue gas temperature decreasing. I the relative humidity o air is below 1%, the condensation ratio will be less than 1 when the temperature achieved. Because partial water vapor produced in combustion process o natural gas saturates the excess unsaturated air. At the same air actor situation the dew-point temperature and condensation ratio change agree with the relative humidity o air. a = 1., = 1 % a = 1., =. % a = 3., = 1 % a = 3., =. % lue gas dew point temperature( c) Fig.3 the curves o condensation ratio as unction o lue gas temperature a 3 THERMAL EFFICIENCY OF NATURAL GAS According the irst low o thermodynamics the enthalpy o the total lue gas rom 1Nm 3 natural gas burnt completely can be calculated rom: 57

5 eiciency ( h ) I VCO CCO t ( 1 d a / H O VH OCH Ot ) V (11) N C ( 1) N t VCat dv The thermal eiciency o natural gas can be deined as the ratio between the sum o low heat value o natural gas and the latent heat minus the enthalpy o lue gas emission to the surroundings, which can be available: H l VH O H Or I (1) H l The calculation result reveals the thermal eiciency o natural gas changed with the lue gas temperature (Fig. 4) The diagrams show that no condensation occurs when the lue gas temperature is higher than the dew point, and the heat eiciency goes up with the lue gas temperature decreasing in linear pattern. But when temperature is cooled down below the dew point condensation occurs, the heat eiciency increases sharply with the lue gas temperature decreasing in parabola pattern, and the increasing rate is greater when the temperatures are between the dew point that the condensation occurs to than it is when between to. Secondly the air actor inluences the heat eiciency, it declines with the air actor increasing, the temperature is higher, and the eiciency is lower. Now the temperature is about or most emission lue gas. I the outlet temperature o lue gas drops rom to 3,the heat eiciency will increases rom 77%~9% to 13~18%(related the lower caloriic value),the eiciency lit is in the range rom 6% to 16%, especially or great air actor and high outlet lue gas temperature the energy saving is obvious condensation occur a=1. a=1.1 a=1.5 a=. a=.5 a= discharging lue gas temperature (t ) Fig. 4 natural gas heat eiciency as unction o outlet lue gas temperature and air actor 4 FLUE GAS HEAT PUMPS Along with the developing o an coil heating and loor radiant heating technology, the 4 ~ 6 water is widely used to heating. The eiciency to produce 4 ~6 water is very low by using the natural gas exhaust to heat directly. We put up a technical low (ig. 5) to use the waste heat o outlet lue gas completely. First the higher temperature lue gas through a waster boiler 58

6 saturated pressure o R134a (kpa theoretical cop heats some mount o 4 water to 6,and the lue gas temperature drop to about 8 ; Then the lower temperature lue gas through three parallel heat pumps heats 4 water to 6 or heating circulation. Its principle is illustrated in ig VI 15 ~ V I II III IV 3 ~4 Fig.5 lue gas heat pump technical low chat I waster heat boiler;ii III IV lue gas heat pump;van;vi water pump The dierence between the condensing and evaporating temperature ( t t ) is the gross or maximum possible temperature lit. The net temperature lit between heating water and lue gas t t ) is less than the gross temperature lit by the sum o the temperature dierence driving ( w orces in the evaporator and condenser. So that only when the evaporating temperature and the gross temperature lit are both very low, the using level o the lue gas waste and the actual COP o heat pump are both high. For the single stage heat pump the gross temperature lit should be 4 ~5,or else the COP would be very low. For the three parallel heat pumps, the gross temperature lit or each heat pump is designed to be 4 ~5, so the COP is high. The actors should be considered or selecting the working luid or the heat pump: the condensing pressure should not be more than bar, the evaporating pressure should be more than atmospheric pressure to prevent produce vacuum, and the speciic volume delivering teat capacity should be high; the working luid should be security and riendly to environment. So we selected R134a as working co ev saturated temperature ( c) Fig. 6 variation o saturated pressure with saturated temperature or R134a Dt=4 C Dt=45 C Dt=5 C evaporating temperature ( C) Fig.7 theoretical COP against the evaporating temperature or R134a luid. The CSD equation is be used to analyze the thermal thermodynamic properties or the heat 59

7 compression ratio Pco/Pev actual cop pump operating. The ig.6 is a plot o saturated pressure against saturated temperature or R134a. Fig.7 is a plot o theoretical Rankine coeicient perormance (COP) against the evaporating temperature or R134a according dierent gross temperature lits being 4, 45 and 5. Fig.7 shows that the gross temperature lit inluences the COP obviously, i it increases 5 the COP will be cut down in the range 1.6 to 1.8, and the condensing temperature has a little inluence on the COP. In practice the operation o a mechanical vapor compression heat pump consists o two heat exchangers, a compressor, an expansion valve and a working luid. There are low resistance and unorganized heat transer in every components o the heat pump. So that the actual amount o teat delivered, high-grade energy input and COP are dierent rom Rankine cycle heat pump. The gross temperature lit and condensing temperature impact the COP ratio between the practice cycle heat pump and Rankine cycle heat pump. When the gross temperature lit is in the range 4 to 5, and the condensing temperature is in the range 45 to 7, the actual COP is only.6 to.8 times as same as the Rankine cycle heat pump [3]. The practice high grade input energy is used as the base to determine the actual COP,and the work consumption o lue gas to overcome the luid resistance is considered. When the lue gas pressure lit is controlled between the range 1Pa to Pa,the most consumption o work is about 1w per 1m 3 lue gas. Fig. 8 is the plot o compression ratio against evaporation temperature. The gross temperature lit is higher and the compression ratio is higher, the ratio drops along with the evaporating temperature increasing. Fig.9 shows the relationship o actual COP against the gross temperature lit and evaporating temperature, along with the temperature lit increasing the actual COP drops obviously, i the lit increases 5 the actual COP will decrease about.5. The evaporating temperature has a little inluence on the actual COP Dt=4 C Dt=45 C Dt=5 C Dt=4 C Dt=45 C Dt=5 C evaporating temperature ( C) evaporating temperature ( C) Fig.8 compression ratio against evaporation Fig.9 actual COP against evaporation temperature temperature I the technical low chart (Fig.5) is used in practice engineering the actual average COP is greater than or equals 3. It can save 1%~% energy o natural gas when the emission lue gas temperature drops rom the range 1 ~ to 3 ~4 or dierent air actors. It is very important to increase the thermal eiciency and decline the consumption o natural gas. 5 CONCLUSIONS According the combustion theory, ist law o thermodynamics and the gas law, analyses are made on the dew temperature, saturated humidity and enthalpy o lue gas o combusted natural gas. 51

8 The calculating equations or these parameters are derived and the corresponding calculation results are given in patent o curves, which can be used as the base or actual engineering design. A new technical low is put up that have a waste heat boiler and three parallel heat pumps. Basing on the new technical low the COP o each heat pump can exceed 3. I the decrease o temperature drops rom high temperature 1 ~ to low temperature 4 ~3,the use o the sensible heat and latent heat o partial condensation can be at a high level, and the thermal eiciency o natural gas can be raised 1%~%. REFERENCES 1. F. Haase, H. Kehne, Design o Scrubbers or Condensing Boilers, Progress in Energy and Combustion Science 5, 1999, J. Kuck, EFFICIENCY OF VAPOUE-PUMP-EQUIPPED CONDENSING BOILERS, Applied Thermal Engineering, Vol.16. No.3 P33-44, F.A.lland, F.A. Watson and S. Devotta, Thermodynamic Design Data For Heat Pump Systems, oxord: Pergamon Press, 198,p Tongji University et al. Gas Combustion and Application. Beijing:China Architecture and Building Press, Akio Miyara, Shigeru Koyama: Consideration o the Perormance o a Vapor-Compression Heat-pump Cycle Using Non-azeotropic rerigerant Mixtures, Int. J. Rerig.,Vol.15, No.1,