Experimental Comparison of the Performance of Refrigerants R134a and R32/R134a in Dry Expansion and Liquid Recirculation Refrigerating Systems

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1 Prde University Prde e-pbs International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 1998 Experimental Comparison of the Performance of Refrigerants R134a and R32/R134a in Dry Expansion and Liqid Recirclation Refrigerating Systems G. Giliani F. Marchesi Donati S. Nobili F. Polonara Follow this and additional works at: Giliani, G.; Donati, F. Marchesi; Nobili, S.; and Polonara, F., "Experimental Comparison of the Performance of Refrigerants R134a and R32/R134a in Dry Expansion and Liqid Recirclation Refrigerating Systems" (1998). International Refrigeration and Air Conditioning Conference. Paper This docment has been made available throgh Prde e-pbs, a service of the Prde University Libraries. Please contact epbs@prde.ed for additional information. Complete proceedings may be acqired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 EXPERIMENTAL COMPARISON OF THE PERFORMANCE OF REFRIGERANTS Rl34a AND R32/134a IN DRY EXPANSION AND LIQUID RECIRCULATION REFRIGERATING SYSTEMS G. Giliani, F. Marchesi Donati, S. Nobili, and F. Polonara Dipartimento di Energetica,, via Breece Bianche, Ancona, Italy ABSTRACT The ability of zeotropic mixtres with a temperatre glide to operate in liqid recirclation systems is investigated in this paper, and the preliminary reslts of an experimental comparison between the performance of R134a and of zeotropic mixtre R32/Rl34a, (25/75 by mass) are presented. The perfonnance of Rl34a operating in dry expansion mode are also presented as baseline data. INTRODUCTION Conventional refrigerating systems operate in the so called "dry expansion" configration: the same refrigerant flow rate is encontered throghot the system and a sperheated vapor exits the evaporator, nder the control of the expansion valve. In liqid recirclation systems a liqid/vapor separator spplies satrated liqid to the evaporator and satrated vapor to the compressor; the low pressre side of the system and the high pressre side work with different refrigerant flow rates. The evaporator is overfed with respect to the compressor and the condenser: as a conseqence the refrigerant flowing throgh the evaporator does not evaporate completely and, at the otlet of that heat exchanger, the vapor qality is always less than one. The ratio between the refrigerant flow rate in the low pressre side (evaporator) and the flow rate in the high pressre side (compressor and condenser) is called "recirclation ratio". When operated with pre refrigerants, liqid recirclation systems take advantage of a better tilization of the evaporator srfaces, so that the heat exchange coefficients are higher. Frthermore, the compressor is fed with satrated vapor, allowing for lower discharge temperatres, whilst the presence of the liqid/vapor separator prevents the compressor from receiving liqid in the vapor [1]. If the same systems operate with a zeotropic mixtre as working flid, the vapor and the liqid inside the liqid/vapor separator will have different compositions. T-herefore the mixtre flowing in the high pressre side and the mixtre flowing in the low pressre side have different compositions. In addition, the liqid at the inlet of the evaporator is richer of the less volatile components of the mixtre, while the vapor at the inlet of the compressor is enriched with the more volatile components of the mixtre. This change in composition has been claimed responsible for the redction in any of the advantages of sing a refrigerant with a temperatre glide dring the phase changes [2], [3], [4). Also there is an implication in the design of sch systems in that there are nknown circlating compositions and therefore the properties of any sch flids are nknown. The prpose of this research is to stdy and compare the behavior of two different refrigerants, one characterized by a temperatre glide dring the phase changes and the other showing constant temperatre (at constant pressre), when they operate in dry expansion and liqid recirclation systems. A cstomized test facility has been bilt to evalate the relative performances of R134a and of the mixtre R32/R134a (in the composition 25/75 by mass). The comparison is based on the. evalation of for parameters: the compressor discharge temperatre, the pressre ratio across the compressor, the coefficient of performance and the refrigerating capacity. Baseline testing reslts with Rl34a are presented in this paper, bt only preliminary reslts with R32/134a mixtre are reported. Experimental work is still in progress and involves gaschromatographic analyses of mixtre composition within the liqid/vapor separator. 63

3 TEST FACILITY The experimental facility is a vapor compression refrigerating machine tilizing a two-cylinder reciprocating compressor driven by a variable speed electric motor. A schematic flow diagram of the facility, with the location of measrement sensors, is shown in Fig. 1. nomenclatre Cd condenser Ev evaporator F filter F# flowmeter M electric motor Mt torqe sensor pr# sampling point P# pressre sensor rpm speed sensor Tls R liqid receiver ==Cd====;::;:!:::::;:========.,.:;-l@-- S vapor/liqid separator : -- : ----n;r-1 i -----mr-@l--1 Fig. 1 schematic view of the experimental test bed Sc sbcooler So oil separator Sr sperheater T# thermocople Vg float valve Vm manal expansion valve Vp pilot operated valve Vt thermostatic expansion valve The condenser and the evaporator are conterflow heat exchangers, the refrigerant flowing throgh the srronding annli. The sction line to the compressor is provided with a refrigerant tbe-in-tbe sperheater which prevents liqid refrigerant from flowing throgh the compressor. Additional liqid sbcooling is obtained before the refrigerant enters the expansion device by a brazed plate heat exchanger, in which the working flid transfers heat to a water circit. Heat to the evaporator is provided by a water/ethylene-glycol mixtre circit; heat prodced in the condenser is transferred to a water circit. The temperatres of the cold and the hot tanks are controlled by two motorized threeport valves, which reglate the flows of the water and the water/ethylene-glycol mixtre in separate plate heat exchangers, while the flow rate of the heat sorce and the heat sink are controlled by proportioning valves on the pmps. The test facility can be operated either in a dry expansion mode or a liqid recirclation mode, depending on the expansion device and associated circitry chosen. In the dry expansion mode, the system ses either a manal expansion valve or a thermostatic expansion valve, while in the liqid recirclation mode, the expansion is performed by a low pressre side float valve which maintains a constant level on the liqid/vapor separator vessel. Type T thermocoples measre the temperatre of water and water/ethylene-glycol mixtre entering and leaving the varios heat exchanger sections and the temperatre of the refrigerant flowing throgh the varios parts of the plant The cold jnctions are maintained at 0 C in an ice-water bath and each thermocople is monted on the srface of the copper tbes and are inslated from any ambient effects. Refrigerant pressres are measred sing twelve absolte pressre strain-gage transdcers based on a Wheatstone bridge arrangement. Since the compressor is an open arrangement, the speed of the compressor is measred by a toothed wheel arrangement. The toothed wheel, with 60 teeth, is magnetically copled to an electronic analog voltage transmitter. Compressor torqe is measred by a strain-gage apparats, placed between the variable-speed drive and the compressor. The instrment is connected to the motor and compressor by two flexible coplings which allow an imperfect arbor alignment. A Coriolis-effect mass flowmeter, located downstream of the condenser, is sed to measre the refrigerant mass flow- 64

4 rate. A trbine flowmeter measres the volmetric flow rate of the refrigerant in the low pressre side of the circit, and it is located downstream of the hermetic pmp, while three electromagnetic flowmeters measre the volmetric flow-rate of the water and the water-glycol mixtre which flow throgh the condenser, the evaporator, and the sperheater. The data acqisition system sed is connected to a PC which collects measred data, maniplated by programs rnning in Labview software environment. The data are then processed in real time in order to calclate both the thermophysical properties of the refrigerant and the refrigerating system performance. The evalation of the thermophysical properties of refrigerant has been performed with REFPROP [5] which has been interfaced with Labview. As performance parameters are sally not directly measred bt calclated from measred qantities, a thorogh investigation has been carried ot, as proposed in [6], to evalate how the measrement ncertainties propagate into the performance parameters. The calclated ncertainties are reported as error bars within the graphs showing or experimental reslts. DRY EXPANSION AND LIQUID RECIRCULATION PERFORMANCE COMPARISON FOR R134a The dry expansion configration is the most conventional and therefore the reslts obtained with this type of configration are sed as baseline data. The refrigerant leaves the evaporator as satrated vapor or as slightly sperheated vapor, and the sperheat is controlled to the desired level by means of the sperheater heat exchanger. The experimental rns have been performed maintaining constant the temperatre of the hot sorce and varying the temperatre of the cold tank, i.e. the sitation where the heat sink has a fixed temperatre, and the heat sorce, the ambient to cool, can vary its temperatre depending on the specific plant demands. The tested parameters are the pressre ratio across the compressor, the discharge temperatre, the refrigerating capacity and the Coefficient Of Performance (COP). The refrigerating capacity incldes the heat exchanged in the sperheater. The COP has been calclated dividing the refrigerating capacity by the power absorbed by the compressor, since in this way is it possible to take into accont also the thermal and mechanical losses at the compressor, e.g. the heat which is dispersed into the srrondings and the internal friction. The tests were performed at 3 different hot tank temperatre, namely 35 C, 45 C and 50 C; for each of them the cold tank temperatre was set at three vales sccessively: 4 C, 8 C, l2 C. The compressor speed was maintained at 800 rpm, and the sperheat level at the compressor inlet was set at 9 C. Dring the liqid recirclation configration tests a recirclation ratio of 2 was sed. The reslts for the test in dry expansion configration are presented in Fig. 2, where the variations of the refrigerating capacity and COP at different cold tank and hot tank temperatres are shown. As expected, both the refrigerating capacity and the coefficient of performance are decreasing when the hot tank temperatre is increasing, and are increasing when the cold tank temperatre is increasing. The same reslts for the liqid recirclation configration are displayed in Fig. 3. The refrigerating capacity and the COP have the same trend as in the dry expansion tests. The beneficial characteristic of the liqid recirclation system are not evident from the analysis of Fig. 2 and 3. However, they can be nderlined when the two cycles are compared on a pressre-enthalpy chart. The cycles are ahnost the same, except for the evaporation process: the liqid recirclation system has a higher evaporation temperatre, which means a smaller power is reqired from the compressor. This is a conseqence of the better heat exchange process that is the characteristic of this configration, since the heat tranferred is the same, bt the potential for this process, i.e. the temperatre difference between the refrigerant and the heat transfer flid, is lower. The presence of an additional heat exchange device downstream the condenser (the sbcooler), whose effect is to 65

5 diminish e vapor qality t the ex?ansion device otlet, improves the performance of the dry expansion configration. Therefore, the difference m performance between the two configration is less evident o::l (,l 12 1:>0 0 p t:: 1.8.::::: 10 o::l t 1:>0 ;E Cold tank temperatre [oc] Cold tank temperatre [ 0 C] 0 Hot tank temp.=35 C 0 Hot tank temp.=45 C 0 Hot tank temp.=50 C Fig. 2 Experimental reslts for R134a operating in dry expansion configration (heat exchanged at sperheater i. inclded) o o::l ' (,l 12 1:>0 2.0 t:: t 1:> Cold tank temperatre [ 0 C) Cold tank temperatre [ C] 0 Hot tank temp.=35 C 0 Hot tank temp.=45 C 0 Hot tank temp.==50 C Fig. 3 Experimental reslts for Rl34a operating in liqid recirclation configration (heat exchanged at sperheater i. inclded) Figre 4 shows the comparison between the experimental reslts for the two configrations on the tests at 45 C hot tank temperatre: discharge temperatre, pressre ratio, refrigerating capacity and COP are the compared parameters. Here the refrigerationg capacity does not inclde the heat which is absorbed by the sperheater, so to focs on the difference of the heat transfer processes in the evaporator in the two different configration. It can be noticed that the refrigerating capacity and the COP have, as expected, higher vales for the liqid recirclation system, de to a better heat exchange. LIQUID RECIRCULATION RESULTS FOR R32/134a AND COMPARISON WITH Rl34a Tests for the mixtre R32/134a (25175 by mass) where rn at the same conditions presented in the previos paragraph for Rl34a, bt at lower (27, 32 and 35 C) hot tank temperatres in order not to exceed allowable pressres. The refrigerant charge was the same as in Rl34a tests. Figre 5 shows, in the sal pattern, the reslts for the refrigerating capacity and for the COP at different hot and cold tank temperatres. 66

6 In comparison with R134a, the tested mixtre presents a sensible increase in the vales of the refrigerating capacity bt similar vales of the COP. The compared vales of refrigerating capacity and COP at 32 C hot tank temperatre and for different vales of cold tank temperatre are presented in Fig. 6. The fact that the COP is almost the same, while the refrigerating capacity varies, is de to the fact that the mixtre has better thermodynamic properties, bt needs higher power from the compressor. G t B v OJ) 76...s:: 74 i '-...J---'--...J '---J..---l r---r---r--.-,..---,;-,.-,--, 6.8 e..;:s 64 v :; 6.0 "' ,. '---'-...., ,...--,---r---r---r--'..,...--r---r--r---r_,...--r Q., ,_..._ _..._ Cold tank temperatre [ 0 C] Dry expansion configration Cold tank temperatre [ 0 C] 0 Liqid recirclation configration Fig. 4 Comparison between performance of Rl34a in dry expansion and liqid recirclation configration (heat exchanged at sperheater is not inclded) (reslts obtained at 45 C Hot tank temperatre) Of great interest is also the comparison between the two refrigerants when working in dry expansion mode and the comparative analysis of performance in both configrations: in other words it is necessary to evalate if, dring dry expansion operations, the mixtre related improvements are of the same magnitde or higher than those experienced dring liqid recirclation operations. These reslts are not yet available: work on this sbject is still nder way, copled with a systematic analysis of mixtre composition in different vapor/liqid separator locations n o:l ' p = 1.8 OJ) 10 : OJ) Cold tank temperatre [ 0 C] Cold tank temperatre [ 0 C] D Hot tank temp.=32 C 0 Hot tank temp.=35 C Fig.5 Experimental reslts for R32!134a (25/75 by mass) operating in liqid recirclation configration (heat exchanged at sperheater inclded) 67

7 n ca g. p = 1.8 bl) 10 ;s bl) Cold tank temperatre [ 0 C] Cold tank temperatre [ C] 0 R32/134a 0 Rl34a Fig. 6 Comparison of experimental reslts for Rl34a and R32/134a (25n5 by mass) in liqid recirclation configration (heat exchanged at sperheater!. inclded) (reslts obtained at 32 C Hot tank temperatre) CONCLUSIONS The reslts of an experimental investigation carried ot with a prpose-bilt test bed on the behavior of pre and mixed refrigerants operating in liqid recirclation systems is presented in this paper. Reslts for pre refrigerant Rl34a have been obtained either with the dry expansion configration and with the liqid recirclation configration. They have been presented as baseline data. Preliminary reslts for the performance of mixtre R32/134a (25n5 by mass) in liqid recirclation configration are also presented and compared with Rl34a. More experimental work is nder way in order to better nderstand the behavior of azeotropic mixtres, sch as R32/134a, in dry expansion and liqid recirclation systems. ACKNOWLEDGMENTS This work has been spported by the Eropean Union as part of the Jole Project within the IV Framework for RTD and by the Italian "Ministero dell'universita e della Ricerca Scientifica e Tecnologica". REFERENCES [1] Stoecker W. F., 1988, "Indstrial refrigeration", Bsiness News Pblishing Co., Troy, Mich. [2} Cavallini, A., 1996, "Working flids for mechanical refrigeration", lnternationa Jornal of Refrigeration, Vol. 19, No. 8, pp [3) Domanski P. A. and Didion D. A., 1993, "Thermodynamic evalation of R22 alternative refrigerants and refrigerant mixtres", ASHRAE Transactions, 99 (2) pp [4] Low R. E., Gilbert B. E., Davies M., Morrison J. D., and Mrphy F. T., 1995, "Handling zeotropic refrigerants", Proceedings of the International CFC and Halon Alternatives Conference, Washington D.C. pp [5) Hber, M., Gallagher, J., McLinden, M., and Morrison, G., NIST Thermodynamic Properties of Refrigerants and Refrigerant Mixtres Database (REFPROP) Version 5.0, National Institte of Standards and Technology, Thermophysics Division. [6] Moffat, R., 1988, "Describing the ncertainties in experimental reslts", Experimental thermal and flid sciences, Vol. 1, pp