SINGLE STAGE TRIPLE PRESSURE LEVEL ABSORPTION CYCLE BASED ON REFRIGERANTSR22, R32, R125, R134a AND R152a WITH DMEU

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1 SINGLE STAGE TRIPLE PRESSURE LEVEL ABSORPTION CYCLE BASED ON REFRIGERANTSR22, R32, R125, R134a AND R152a WITH DMEU A. Levy 1, M. Jelinek 1, I. Borde 1 & F. Ziegler 2 1 Mechanical Engineering Department, Ben-Gurion University o the Negev,P.O. Box 653, Beer- Sheva, 845, Israel. Phone: , Fax: , avi@bgumail.bgu.ac.il 2 Technische Universität Berlin, Institut Für Energietechnik, Ernst-Reuter-Platz 1, D-587, Berlin, Germany ABSTRACT Current developments in absorption technology include the research or new working pairs and new advanced cycles that would acilitate increased eiciency o absorption units and extend applicability to dierent temperature ranges. Four HFC rerigerants dilouromethane (R32), pentaluoroethane (R125), 1,1,2-tetraluoroethane (R134a) and 1,1-dilouroethane (R152a) which are alternatives to HCFC, such as colorodilouromethane (R22), in combination with absorbent dimethylenurea (DMEU) were evaluated or possible utilization in absorption chillers powered by low temperature heat sources. A computerized simulation program was used to compare the dierent rerigerant-absorbent pairs. The program was based on an advanced single-stage triple pressure level (TPL) cycle containing a jet ejector. The absorption cycle was represented in terms o heat and mass balances or each component and the calculations were based on the thermophysical properties o the rerigerantabsorbent pairs measured and evaluated in our laboratory. The aim o the cycle analysis was to evaluate the highest coeicient o perormance () and the lowest circulation ratio (), which can be obtained or dierent generator temperatures or a given evaporating and cooling water temperatures. At the maximum o each working pair, the eects o changes in the evaporator and the cooling water temperatures on the and the circulation ratio,, were also examined. The perormances o the working luids based on HFC rerigerants-dmeu pairs were compared with the perormance o R22-DMEU. It was obtained that the generator temperature o R125- DMEU at maximum is lower than those obtained with the other alternative working luids. KEYWORDS Absorption rerigeration, Organic working luids, Triple pressure cycle. INTRODUCTION Utilization o available heat sources or cooling and rerigeration can be implemented by various types o absorption heat pumps, both single and multi-stage. However, the utilization o low potential heat sources or cooling and rerigeration (<0 C) is limited by the properties o the working luids and the cycle coniguration o the heat pump. or utilization o low potential heat sources ( C) or cooling and rerigeration to bellow 0 C, a single-stage absorption heat pump based on organic working luids is preerable, since conventional working luids such as ammonia-water or water-lithium bromide are limited to the above described operation conditions. The temperature o the heat source and the cooling or rerigeration demands are usually the actors that determine the type o working luid to be used and the type o the absorption heat pump system required single or multistage. The commonly used working luids are ammonia-water or water-lithium bromide. The ammonia-water combination requires a heat source temperature above 120 C or cooling and rerigeration to <0 C. Such a system is a high-pressure system that requires a rectiication column (Engler et al., 1997). Ammonia has acceptable thermophysical properties, but it is a lammable luid, toxic, irritant and is corrosive to copper. The water-lithium bromide solution can be used with a heat source temperature above 70 C or air-conditioning but not or cooling and rerigeration due to the limitation o the evaporator temperature (>0 C). This system operates under vacuum. The water-lithium bromide solution is highly corrosive and extremely viscous and viscosityreducing agents are requently required. The limitations o using common working luids (Thioye, 1997) or utilizing o low potential heat sources ( C) or cooling and rerigeration (<0 C) are thus sel-evident. To overcome these limitations, working luids based on luorocarbon (HFC) rerigerants and organic absorbents have been chosen (Borde et al., 1995, Borde et al., 1995, Borde et al., 1997, Jelinek & Borde, 1999 and Sawada et al., 1994). The rerigerants are not toxic or corrosive and the organic working luids are environmentally acceptable. In these high-pressure systems, the condenser and the absorber are water cooled, and a rectiication column is not needed, since the dierence between the normal boiling points o the absorbent and the rerigerant is above 200 C. In order to evaluate the perormance o the candidate rerigerant-absorbent pairs in a rerigeration or heat pump cycle in terms o the coeicient o perormance,, and

2 2 the circulation ratio,, the thermophysical properties o the pure components and the mixtures at equilibrium have to be determined rom experimentally, rom published data or by prediction methods. An advanced single-stage absorption heat pump, i.e., a triple-pressure-level (TPL) cycle was presented (Borde & Jelinek, 1986, Borde & Jelinek, 1988, Chen, 1988 and Jelinek et al., 2002) to utilize a low-potential-heat source or cooling by integrating a specially designed jet ejector at the absorber inlet. The jet ejector served two major unctions: it acilitated pressure recovery and improved the mixing between the weak solution and the rerigerant vapor coming rom the evaporator. These eects enhanced the absorption o the rerigerant vapor into the solution drops. The inluence o the jet ejector on the perormance o the TPL absorption cycle was evaluated (Levy et al., 2002), and the perormance o the TPL absorption cycle was compared with that o the well known double pressure level (DPL) cycle (Jelinek et al., 2002). PRESENT STUDY Based on the thermophysical properties o the rerigerant-absorbent pairs measured and evaluated in our laboratory, the and can be calculated with the aid o a computerized simulation program o a triple-pressurelevel single-stage absorption cycle (TPL) containing a jet ejector mixer (Jelinek et al. 2002, Levy et al. 2002). The results o the present investigation are presented with working luids based on dimethylenurea (DMEU) as absorbent with ive rerigerants; chlorodiluoro-methane (R22) as a reerence, diluoromethane (R32), pentaluoroethane (R125), 1,1,1,2 tetraluoroethane (R134a), and 1,1 diluoroethane (R152a). A comparison between the working luids based on the HCFC (R22) and the HFC rerigerants (R32, R125, R134a and R152a) was carried out. The basic data o the pure components, i.e., molecular weight, normal boiling point, critical temperature and critical pressure, are given in Table 1. The perormance characteristics o the investigated working luids in terms o the coeicient o perormance,, and circulation ratio,, were based on the thermodynamic calculation o a triple-pressure-level singlestage absorption cycle (TPL) containing a jet ejector mixer. The cycle is presented schematically in igure 1. Table 1: The basic data or the pure components Formula Mw Tb [ C] Tc [ C] Pc [bar] R22 CHClF R32 CH2F R125 CHF2CF R134a CH2FCF R152a CH3CHF DMEU C5H N2O Figure 1: Schematic representation o a single-stage absorption cycle. G - generator, A - absorber, C - condenser, E - evaporator, HR- rerigerant heat exchanger, HS - solution heat exchanger, P - solution pump, M - jet ejector. THEORETICAL PERFORMANCE The absorption cycle (igure 3) was evaluated in terms o the heat and mass balance or each component with the aid o a computerized simulation program and the thermophysical properties o the rerigerant-absorbent pairs measured and evaluated at our laboratory. From the cycle analysis, and were calculated at dierent temperatures o the components. The (based on the irst law o thermodynamics) and circulation ratio,, where calculated by; =Qe/(Qg+Wp) and = m! s / m! r where Qe, Qg and Wp are the heat supplied to the evaporator, the heat supplied to the generator, and the energy supplied to the pump, respectively, m! s is the mass low rate o the strong solution (rom the absorber to the generator); and m! r is the mass low rate o the rerigerant. For purposes o illustration, the absorption cycle was calculated under the ollowing operating conditions: generator temperature o C, evaporator temperature o -5 C and cooling water at 25 C (condenser temperature o 32 C and absorber temperature o 28 C). The calculated and as unctions o the generator temperature or the investigated working luids are given in igures 2a and 2b. Table 2: Circulation ratio and generator temperature at maximum o each solution. Solution Max. Gen. temp. o C R32-DMEU R22-DMEU R152a-DMEU R125-DMEU R134a-DMEU

3 3 5 te=-5 o C ; tw=25 o C R22 R32 R152a ound to be the worst among the investigated working luids. At the above range o generator temperatures where the maximum was achieved, the variation o and with changes o evaporator temperature, te and cooling water temperature, tw are shown in igures 3a and 3b and igures 4a and 4b. R R134a tw=25 o C tg o C 5 15 te=-5 o C ; tw=25 o C R152a 5 R134a R te o C 5 R125 R tw=25 o C tg o C 12 Figure 2: Variation o and with generator temperature while te=-5 o C and tw=25 o C. 8 6 As can be seen in igure 2a and Table 2, the maximum was achieved with the solution R32-DMEU ollowed by R22-DMEU, R152a-DMEU, R125-DMEU and R134a- DMEU. The circulation ratio and the generator temperature at this maximum calculated or each working luid are summarized in Table 2. The preerable working luid can be considered as a solution with the highest, lower needed generator temperature and circulation ratio as low as possible. As can be seen in igures 2a and 2b, the solution R22- DMEU matches this deinition but it includes HCFC rerigerant (R22). Among the HFC rerigerants, the solution R125-DMEU is preerable (the generator temperature was above C and the circulation ratio in the range o 4 5). Although the solution R32-DMEU shows the highest (about 95), it is not recommended to be used because the high generator temperature (above 120 C) and the high circulation ratio (in the range o 9 ) in comparison with R22-DMEU and R125-DMEU. Similar conclusion can be obtained or the solution R152a-DMEU in spite o its close value o to those o R125-DMEU. The solution R134a-DMEU is te o C Figure 3: Variation o and with evaporator temperature while tw=25 o C. Variation o and the circulation ratio with evaporator temperature can be seen in igure 3a and 3b when the cooling water temperature (25 C) and the generator temperature or each o the working luids remains constant, (generator temperature o 95 C or R22- DMEU, 125 C or R32-DMEU, 90 C or R125-DMEU, 115 C or R134a-DMEU and 130 C or R152a-DMEU). Changing the evaporator temperature by ± 2 C (around - 5 C) causes to changes in values in the range o ± 2% 5% and to changes in values in the range o % 25%.

4 te=-5 o C tw o C te=-5 o C tw o C Figure 4: Variation o and with cooling water temperature while te=-5 o C. Variation o the and the circulation ratio,, with cooling water temperature tw can be seen in igure 4a and 4b when the evaporator temperature (-5 C) and the generator temperature or each o the working luids remains constant, (generator temperature o 95 C or R22- DMEU, 125 C or R32-DMEU, 90 C or R125-DMEU, 115 C or R134a-DMEU and 130 C or R152a-DMEU). Changing the cooling water temperature by ± 2 C (around 25 C) causes to changes in values in the range o 2% 5% and to changes in values in the range o ± 16% 27%. DISCUSSION AND CONCLUSION With the aid o a computerized simulation program or a triple-pressure-level single-stage absorption cycle (TPL), the perormance characteristics o the working luids R22- DMEU, R32-DMEU, R125-DMEU, R134a-DMEU, and R152a-DMEU in terms o and were calculated at the same conditions and then compared. The investigation showed that maximum value o is obtained at dierent generator temperatures depending on the working luid (see Table 2). As mentioned, the preerable working luid can be considered as a solution with the highest, lower generator temperature and circulation ratio as low as possible. Among the HFC rerigerant-absorbent pairs, the solution o R125-DMEU can be taken as the preerable replacement solution or the solution R22-DMEU (as can be seen in igures 2a and 2b), because its lower generator temperature in the range o C and the circulation ratio in the range o 4 5. The eects o changes o the evaporator temperature and the cooling water temperature on the and the circulation ratio,, were also examined at the generator temperatures where the is at maximum or each o the investigated working luids. It was ound that increasing the evaporator temperature results in increasing the and in decreasing the circulation ratio. An inverse eect was obtained while changing the cooling water temperature, increasing the cooling water temperature results in decreasing the and increasing the circulation ratio. REFERENCES Borde, I. and Jelinek, M., 1986, Utilization and recovery o waste heat by absorption heat pumps, Final Scientiic Report Joint German Israel Project. BGUN-ARI Borde, I. and Jelinek, M., 1988, Absorption heat pumps with organic rerigerant-absorbent luid pairs 2nd. International Workshop on Research Activities on Advanced Heat Pumps, Graz, Austria. Borde, I., Jelinek, M., Daltrophe, N. C., 1995, Absorption system based on the rerigerant R134a, Int. J. Rerig. Vol. 18, No. 6, Borde, I., Jelinek, M., Daltrophe, N. C., 1995, Working substances or absorption heat pumps based on R32, Proc. Int. Ins Re. Vol. 1Va, Borde, I., Jelinek, M., Daltrophe, N. C., 1997, Working luids or an absorption system based on R124 (2- chloro-1,1,1,2,-tetraluoroethane) and organic absorbents, Int. J. Rerig. Vol. 20, No. 4, Chen L.T., 1988, A new ejector-absorber cycle to improve the o an absorption rerigeration system, Applied Energy, 30: Daltrophe, N. C., Jelinek, M., Borde, I., 1994, Heat and mass transer in a jet ejector or absorption systems. AES-V0l. 31, Proceedings o the International Absorption Heat Pump Conerence ASME 1993, New Orleans, Louisiana, USA, pp Engler, M., Grossman, G., Hellman H.-M., 1997, Comparative simulation and investigation ammoniawater absorption cycles or heat pump applications, Int. J. Rerig. Vol. 20, No. 7, pp Jelinek, M., Borde I, 1999, Working luids or absorption heat pumps based on R125 (pentaluoroeothane) and organic absorbents, International Sorption Heat Pumps Conerence, Munich, pp

5 5 Jelinek, M., Levy, A. and Borde I., 2002, Perormance o a triple-pressure level absorption cycle with r125- n,n'- dimethylethylurea Applied Energy, 71: , Levy, A., Jelinek, M. and Borde, I., Numerical Study On The Design Parameters O a Jet Ejector For Absorption Systems, Applied Energy,72: , 2002 Thioye M., 1997, Etude comparative de la perormance des machines rigoriiques a absorption utilisant de l'energie thermique a tres aible valeur exergetique, Int. J. Rerig. Vol. 20, No. 4, pp Sawada N., Tanaka T., Mashimo K., 1994, Development o organic working luids and application to absorption systems, International Absorption Heat Pumps Conerence, New Orleans, ASME, AES-Vol. 31, pp NOMENCLATURE - Coeicient o perormance - - Circulation ratio ( m! s m! r ) m! - mass low rate (kg/s) P - Pressure (bars) Q - Heat Source/Sink (Watt) t - - Temperature ( o C) T - - Temperature (K) W - Work (Watt) Subscripts a - - Absorbent c - - Condenser e - - Evporator g - - Generator p - - Pump r - - Rerigerant s - - Solution w - Water

as the low temperature heat reservoir

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