Theoretical analysis of large-temperature-difference of chilling water system

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1 heoretical analysis of large-temperature-difference of chilling water system Sui Jun *, Fu Lin, Li Zhen, Jiang Yi * Postdoctor, Department of Building Science School of architecture, singhua university, Beijing 00084, hina bstract his paper performs three absorption refrigeration cycle modes in order to achieve large temperature difference of chilling water, including single absorption cycle, two evaporators cycle and two evaporators-two absorbers cycle. he combined arnot cycle models is established for each cycles. he OP (coefficient of performance) of ideal cycle was introduced. uantitative analysis and comparison of each mode is investigated. Introduction urrently, the absorption systems are widely used in refrigeration, heating and heat recovery of industry, residential and business buildings. he urgent need is to improve the energy-saving and environment-protecting performance of absorption-cycling products. he adoption of large temperature-difference system is a new important trend. he large temperature-difference system can compensate the energy loss caused by the transportation of chilling water in district or central cooling systems. On the other hand, although the OP decreases with the increase of temperature difference, the large temperature-difference system can save transport energy by reducing the flowrate of chilling water. hat is more, the initial investment of equipment is reduced because of the smaller pipe diameters of transport system. he energy saving analysis, operation characters and engineering designs of large temperature-difference systems have been discussed(yin.p. 000,00)(Zhou Y.S. and hen P.L..999)( Huo X.P..996). onsidering the OP (coefficient of performance) of thermodynamics cycle, this paper attempt to find possible approaches to save the energy costs of large temperature-difference systems. hen some reference opinions on practical projects are given. he OP of bsorption ycle bsorption refrigeration is based on the combined cycle of thermal engine and heat pump. xergy, which is outputted when high-grade heat is discharged in the engine cycle, was filled into the heat pump cycle to extract part of heat from the target system at a temperature grade which is lower than the environment temperature, to a even higher temperature grade. Meanwhile the 97

2 target system maintains lower temperature than the environment. bsorption refrigeration systems storage the usable work produced in the process into the cycle fluid as chemical energy. orking medium couples, as carriers, transfer and transform energy during the process of absorption/lixiviation of solution and generating/condensation of refrigerant. herefore a reversible combined arnot cycle drived by a heat engine is adopted to analyse the thermodynamic process of absorption refrigeration. hree modes of system achieving large temperature difference of chilling water are performed to single-action absorption refrigeration.. Mode (Simple absorption cycle) In a simple absorption cycle, large temperature difference can be achieved by reducing the evaporating temperature, which can reduce supply temperature or increase the return temperature of chilling water of refrigerator. he diagrammatic sketch of practical cycles and the combined arnot cycle are showed in Fig. here,, and are the temperatures of the medium in the generator, condenser, absorber and evaporator respectively, and is the internal flow work of the medium. By modeling the arnot engine cycle and reverse arnot cycle, considering a we can obtain: c g condenser generator 4 HX c c a evaporator 6 absorber e a Fig Simple absorption cycle and the conbined arnot cycle 98

3 c condenser g generator splitter e evaporator(h) 0 HX evaporator(l) 9 mixer 4 absorber e a c c b a H L Fig Double-evaporator absorption cycle and the conbined arnot cycle hen we obtain the following equation. OP (). Mode (Double-evaporator absorption cycle) ompared to conventional absorption cycles, large temperature-difference system requires lower supply temperature or higher return temperature of chilling water. By enlarging the temperature difference, it is possible to realize stepped refrigeration at different evaporation temperatures which can reduce the irreversible loss produced by the heat transfer of chilling water and 99

4 refrigerant. e can also deduce from equation () that OP can be reduced by reducing the evaporation temperature while can be improved by increasing the evaporation temperature. herefore we introduce a high-temperature evaporator in mode. he diagrammatic sketch of double-evaporator absorption cycle and combined arnot cycle are showed in Fig. here K a b Using the same processing method as mode, we can obtain the following equations : c condenser g generator e evaporator(l) 9 5 a absorber HX evaporator(h) 0 absorber e a c c 4 b a H Fig Double-evaporator double-absorber cycle and the conbined 940

5 OP K H H H () K. Mode (Double-evaporator and double-absorber cycle) In mode, the vapor of refrigerant created in the high-temperature evaporator need to be throttled in order to be utilized at a lower pressure, which cause the loss of exergy. herefore a high-pressure absorber matching with the high-temperature evaporator is introduced based on the model of mode, forming a double-evaporator and double-absorber cycle. he diagrammatic sketch of double-evaporator double-absorption cycle and the combined arnot cycle are showed in Fig. here K By modeling the arnot engine cycle and reverse arnot cycle, considering a b we can obtain: OP K K H H H K () Results and discussion. omparing mode to mode he temperature difference of supply and return chilling water is 8~0 while the difference is often 5 in conventional air conditioning projects. onsidering a central air conditioning system whose return chilling-water temperature is 7. o achieve a large temperature difference, the difference of supply and return chilling water temperature is set to be 8, therefore the supply temperature is 9. ccordingly the temperature of the evaporator is set to be 7, the temperature of absorber is and the temperature of condenser is 4. ssuming that the latent phase-change heat of the refrigerant varies little with the changing of temperature, / can approximate to be.0. heoretical OP of mode and mode can be obtained from quation () and quation (). he theoretical OP of mode and mode at different K and H, the temperature of the high-temperature evaporator, are showed in Fig 4. It is seen that: 94

6 OP..9.8 mode mode(k=.0) mode(k=0.5) mode(k=.0).7 Fig 4 omparing mode to mode he OP increase with increasing of H ; H(K) H, then OP OP ; H, then OP OP ; H, then OP OP ; onsidering quation (), if K =0,which means that the high-temperature evaporator of mode is not used, the cycle of mode is just the same as mode. hen, OP fall down with the increasing of K while H increase when. K is an infinitely large quantity we can obtain H the following from quation (): OP H It is equal to the cycle of mode cycle removed the low-temperature evaporator. here K H herefore increasing K means increasing the ratio of the refrigerating 94

7 output of high-temperature and low-temperature evaporator. he ratio is limited by the requirement of the quality of the output. onsequently we can conclude that adopting a high-temperature evaporator in the cycle can improve the OP of the system. hat is more, the mentioned above discussion is based on the theoretical reversible cycle. In practical processes, where irreversible loss exists during the heat-transfer between the chilling water and the refrigerant, adopting a high-temperature evaporator can reduce irreversible loss by reducing the mean heat-transfer temperature difference in the evaporator and thus improve the OP of the practical systems. he quantitative analysis of this problem in not included in this paper and will be discussed in the future.. omparing mode to mode Based on the mode, a high-pressure absorber is introduced into mode. here the fixed generating temperature is 07, the temperature of low-temperature evaporator is 7, the temperature of high-temperature evaporator is, the temperature of low-absorber is, the temperature of condenser is 4, K is equal to and / is equal to.0. he OP of mode and mode at different temperature of absorber and different K are showed in Fig 5 under the conditions mentioned above. It can be seen that: he OP decrease with the increase of, then OP OP ;, then OP OP ;, then OP OP ; onsidering quation (), if K =0,which means that the high-pressure absorber of mode is not used, the cycle of mode is the same as mode. hen, OP increase with the increasing of K while fall down when. It can be concluded that the adoption of a high-pressure absorber is favorable to improve the OP of the system when the temperature of high-pressure absorber is lower than the low-temperature absorber. hen considering the practical cycle, usable energy loss caused by irreversible mass transfer and heat transfer exist in the absorber, which complicate the problem. It can be seen from the sketch of practical absorption cycle in Fig and Fig that in mode irreversible mass-transfer loss is created at the mass-transfer process between the high-pressure vapor and concentrated solution when the vapor of refrigerant at different pressure from the two different evaporators contacted with the homogeneous concentrated solution. hile in mode, the mass transfer occurs at high-pressure vapor and concentrated solution of lower concentration or at low-pressure vapor and 94

8 OP 4.5 mode mode(k=.0) mode(k=0.5) mode(k=.0) higher concentration solution. he mass-transfer process approximate to the countercurrent absorption in countercurrent flow, which can reduce the mass-transfer irreversible loss. Properly arranging the flow of the cooling water in the two absorbers, which can decrease the heat-transfer mean temperature difference in the absorbers, can also reduce the irreversible loss. ll mentioned above are favorable to improve the OP of the system. he related quantitative analysis is not listed in this paper.. onclusion hen considering the energy-saving measures of the cycle of large temperature difference, the methods such as stepped refrigeration are necessary to be involved. () In mode, a high-temperature evaporator is added to the simple cycle and the theoretical OP is improved. In practical project, this mode can also reduce the irreversible heat-transfer loss during the evaporation process. () Based on the mode, a high-pressure absorber is introduced in mode, forming a cycle with two evaporators and two absorbers. he OP of the system is improved only when the high-pressure absorption temperature is lower than the low-pressure absorption temperature. In practical system, the system with two absorbers as mode is more favorable to reduce the irreversible loss than the system of mode. () For typical double-action absorption cycle, the low-pressure generator actually equals to an inside heat regenerator to recover the heat of the vapor produced in the high-pressure generator. he ideal combined 944 (K) Fig 5 he comparison of mode to mode

9 arnot cycle is consistent with the single-action cycle so the results and discussions mentioned above are also applicable for double-action cycles. Reference. Yin P Research of large temperature difference in air conditioning(): conomic analysis methods. Nuanong Kongiao, Vol. 0, (4),pp. 6~66. Yin P. 00. Research of large temperature difference in air conditioning(4): an economical analysis of the system with a large temperature difference between supply and return water. Nuanong Kongiao, Vol., (), pp.68~7. Zhou Y.S. and hen P.L nalysis on energy consumption of large temperature difference in air conditioning chilled water system. JianZhu ReNeng ongfeng Kongiao, (),pp. 8~9 4. Huo X.P Design of water systems with a large temperature difference in high-rise office buildings.(4), pp. 58~60 945

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