Comparative study of the performance of an ejector ... comparative data are needed in order to develop substitutes, therefore the present study includes three CFC refrigerants. 2

Comparative study of the performance of an ejectorrefrigeration cycle operating with various refrigerants

Da-Wen Sun 1

Department of Agricultural and Food Engineering, University College Dublin, National University of Ireland, Earlsfort

Terrace, Dublin 2, Ireland

Received 11 February 1998; accepted 15 September 1998

Abstract

Ejector refrigeration systems have attracted many research activities in recent years. These systemstraditionally operate with water as refrigerant with low COP values. Other refrigerants commonly usedin mechanical vapour compression cycles may provide better performance for ejector refrigerationcycles. Eleven refrigerants, including water, halocarbon compounds (CFCs, HCFCs and HFCs), a cyclicorganic compound, and an azeotrope, are chosen as working ¯uids in an ejector refrigeration system,and their performances are compared. The results show that steam jet systems have very low COPvalues, the system using R152a as refrigerant has better performance and the variation in COP valuesfor various refrigerants is almost independent of system operating conditions. # 1999 ElsevierScience Ltd. All rights reserved.

Keywords: Azeotrope; Cooling; Ejector; Entrainment ratio; Halocarbon compound; Heat pump; Jet pump; Model-ling; Organic compound; Refrigeration; Refrigerant; Water; CFC; COP; HCFC; HFC; H2O; R123; R134a; R152a

Nomenclature

A cross-section area_m mass-¯ow rateP pressureQ thermal energyT temperatureW work

Energy Conversion & Management 40 (1999) 873±884

0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.PII: S0196-8904(98)00151-4

PERGAMON

1 Tel.: +353-1-706-7493; fax: +353-1-475-2119.

GreekZ e�ciency of nozzle or di�userz area ratio, constant area section to nozzle throato ¯ow entrainment ratio, secondary stream to primary stream

Subscriptsb boiler (primary)c condenser (back)d di�usere evaporator (secondary)k cross-section of constant area section of ejectorme mechanicaln nozzlet cross-section of minimum area of primary nozzle

1. Introduction

In recent years, ejector refrigeration systems have attracted many research activities [1].These systems have several advantages over the conventional vapour compression system.These include no moving parts (except the pump) and hence no lubrication required, very littlewear and potentially a very reliable system. In addition, these systems are heat powered.Therefore, waste heat, solar heat, biomass or geothermal energy can be utilised via thesesystems. Inexpensive thermal energy sources can make an ejector refrigeration system a viableand economic proposition.Traditionally, ejector refrigeration systems operate with water as refrigerant. The ®rst steam

jet system for cooling and refrigeration dates back to the early 1900 s [2]. Research has beenextensively performed to understand and improve the performance of steam jet systems [3±10].However, steam jet systems su�er the disadvantages of very low COP values and being unableto generate refrigeration below 08C. Therefore, halocarbon compound refrigerants have beenwidely used in ejector refrigeration systems for higher COP values [1]. The earliest reportedresearch on ejector refrigeration using a refrigerant other than water was performed in the1950 s [11]. After that, many common halocarbon compound refrigerants were used and tested,including R11 [12±18], R12 [11, 19], R113 [14, 20±22], R114 [23], R123 [18, 24], R142b [18, 25]and R134a [26, 27].Despite all the above e�orts, little research work has been performed to study systematically

the performance of ejector refrigeration systems using various refrigerants, especially using zeroozone depletion HFC refrigerants, such as R134a and R152a. R134a has been identi®ed as themost popular choice as an alternative to the widely used R12, however R-152a also receives aproduction boost [28, 29]. In the present study, eleven refrigerants, including water (R718),halocarbon compounds, i.e., CFCs (R11, R12, R113), HCFCs (R21, R123, R142b) and HFCs(R134a, R152a), a cyclic organic compound (RC318), and an azeotrope (R500), are chosen asworking ¯uids in an ejector refrigeration system, and their performances are compared. Itshould be pointed out that, although the production and import of CFCs ceased at the end of

D. Sun / Energy Conversion & Management 40 (1999) 873±884874

1995, comparative data are needed in order to develop substitutes, therefore the present studyincludes three CFC refrigerants.

2. Ejector cycle

The basic components for an ejector refrigeration system include an ejector, a boiler, anevaporator, a condenser, an expansion valve and a circulation pump. Fig. 1 shows thearrangement of these components. The operating principle is as follows. Low grade heat Qb isdelivered to the boiler, where liquid refrigerant is vaporised at high pressure. The vapour (theprimary or driving ¯uid) ¯ows through the primary nozzle and accelerates within it. At the exitof the nozzle, the accelerated ¯ow becomes supersonic, which induces a locally low pressureregion at the exit of the nozzle. Hence, the vapour (the secondary or driven ¯uid) from theevaporator is sucked into the ejector. The primary and secondary ¯uids then mix in the mixingsection and undergo a pressure recovery process in the di�user section. The combined stream¯ows to the condenser where it condenses. The heat of condensation Qc is rejected to theenvironment. Then, the condensate is divided into two parts, one is pumped back to the boiler,and the other expands through an expansion valve to a low-pressure state and enters theevaporator, where it is evaporated to produce the necessary cooling e�ect Qe. The vapour is®nally entrained by the ejector, thus completing the cycle. In order to improve the systemperformance, a regenerator and a precooler can be added to the cycle [7, 24]. The regenerator

Fig. 1. Schematic of an ejector refrigeration system.

D. Sun / Energy Conversion & Management 40 (1999) 873±884 875

uses the sensible heat from the ejector exhaust to heat the condensate pumped by thecirculation pump before it ¯ows into the boiler. The precooler uses the cold vapour from theevaporator to cool the condensate before it enters the expansion valve. These two componentsare not included in Fig. 1. Details of their contribution to system performance can be foundelsewhere [7, 24].The system performance is measured by the coe�cient of performance (COP), which is the

ratio of the useful cooling generated by the evaporator over the gross energy input into thecycle. It is given by:

COP � Qe=�Qb �Wme� �1�For general calculation, the work consumed by the circulation pump is very small compared

to the thermal energy required by the boiler, hence it may be omitted.The ejector is the key component in the ejector refrigeration cycle. Fig. 2 shows its structure

and the pressure distribution within it. The ejector is generally divided into four parts: a nozzlesection including a primary convergent-divergent nozzle and a suction duct for the secondary¯uid ¯ow, a mixing section for the primary and secondary ¯uids to mix, a constant areasection in which the mixed stream will normally undergo a transverse shock andsimultaneously produce a sudden static pressure rise, and a di�user section for the mixedstream to di�use inside until its pressure reaches the condenser pressure.The performance of an ejector is measured by its entrainment ratio o which is the mass ¯ow

rate ratio of the secondary ¯ow to that of the primary ¯ow. It is given as:

o � _me

_mb�2�

The ejector structure is normally characterised by the area ratio z which is de®ned as thecross-section area of the constant area section divided by that of the primary nozzle throat. Itis given as

z � Ak

At�3�

Eqs. (1)±(3) are used to predict the ejector and cycle performance. This requiresthermodynamic analysis of the cycle. The main part of the system modelling is based on theejector analysis which can be found elsewhere [7±10, 18, 24, 30]. Other components can be easilyanalysed with the knowledge of thermodynamics.

3. Results and discussion

In order to predict the ejector and cycle performance, a computer simulation program hasbeen developed. Except R718, R123 and R134a, the thermodynamic properties of therefrigerants (R11, R12, R113, R21, R142b, R152a, RC318 and R500) for temperature,pressure, enthalpy and speci®c volume in saturated liquid, saturated vapour and superheatedvapour states are calculated based on the equations given by ASHRAE [31]. The


thermodynamic equations for R718 are taken from published literature [32]. For R123 andR134a, the thermodynamic properties were taken from standard tables [33] and ®tted by theauthor to a set of polynomial equations. All these property equations were veri®ed againststandard thermodynamic tables for their accuracy and then incorporated into the simulationprogram. The ejector e�ciencies were taken as constants of Zn=Zd=0.85 in the program,details of their e�ects on the ejector and cycle performance can be found elsewhere [24]. Thesimulation program predicts the optimum ejector and cycle performance under given operatingconditions and the corresponding optimum ejector area ratio and other dimensions. Therefore,the following entrainment ratios and COPs are the maximum values for given refrigerants and

Fig. 2. Ejector structure and pressure pro®le.


operating conditions, and the area ratios are required by the ejectors to achieve thesemaximum performances.

3.1. Ejector performance

Fig. 3 shows the comparison of ejector entrainment ratio for various refrigerants under theconditions of boiler temperature at 908C, condenser temperature at 358C and evaporatortemperature at 58C. Fig. 3 indicates clearly that, using water as refrigerant, the ejectorentrainment ratio is very low. This has been con®rmed by many experimental results. Amongmany common halocarbon compound refrigerants, R12 gives higher entrainment ratio thanothers. However, R12 is a CFC, and its use will soon be prohibited. For HCFCs, R142b giveshigh entrainment ratios [11, 18]. HCFCs are generally considered as short- to medium-termreplacements for CFCs, particularly R123 as a replacement of R11 and R142b for R114. ForHFC refrigerants, R134a and R152a have comparative ejector performance, with R152a havinga slightly higher entrainment ratio. HFCs cause no ozone depletion and should be used toreplace CFCs if possible. For RC318 and R500, the azeotrope R500 also gives a highentrainment ratio. Since R500 is a combination of R12 (73.8% by mass) and R152a (26.2% bymass), the contribution from its components R12 and R152a is clearly shown in Fig. 3.Fig. 4 shows the comparison of ejector area ratio for various refrigerants under the

conditions of boiler temperature at 908C, condenser temperature at 358C and evaporatortemperature at 58C. The area ratios shown in Fig. 4 are required for the ejectors to achievetheir entrainment ratios in Fig. 3. Fig. 4 illustrates that, if water is used as refrigerant, the

Fig. 3. Comparison of ejector entrainment ratio for various refrigerants.


ejector will require a much larger area ratio than those if other refrigerants are used. This isprobably the reason why steam jet ejectors are commonly used for experiments, since for asmall unit, a bigger ejector is required for a steam jet cycle. This will make the manufacture ofthe ejector easier. For CFCs, R12 requires an ejector with a smaller area ratio than others. ForHCFCs, R142b also requires small area ratio ejectors. The area ratios for ejectors using R134aand R152a are similar. Finally, for RC318 and R500, the area ratio for the ejector using R500is similar to that for R12, which is also the smallest among the ejectors using otherrefrigerants. Comparing Figs. 3 and 4, one can notice that, generally speaking, the smaller theentrainment ratio, the larger is the ejector area ratio. For a given refrigerant, the results shownin Figs. 3 and 4 are only a�ected by the cycle operating conditions. Details of the e�ect ofoperating temperatures on ejector entrainment ratio and ejector geometry can be foundelsewhere [7, 8, 24].

3.2. Cycle performance

Fig. 5 shows the comparison of cycle performance for various refrigerants under theconditions of boiler temperature at 908C, condenser temperature at 358C and evaporatortemperature at 58C. Fig. 5 is generated based on the results given in Figs. 3 and 4. The cycleCOP value for given operating conditions is mainly in¯uenced by the ejector entrainment ratio.Therefore, Fig. 5 shows that the cycle with water as refrigerant has the lowest COP value, andthe ones using R152a and R500 have higher COP values, with R152a giving the bestperformance. The variation of the COP values for various refrigerants is more or less the same

Fig. 4. Comparison of ejector area ratio for various refrigerants.


as that shown in Fig. 3 for entrainment ratio. This con®rms the importance of improvingejector design for maximum entrainment ratio in order to maximise the cycle performance.Comparing the results in Figs. 3 and 5, one can notice that a small increase in ejectorentrainment ratio results in a large increase in COP with the exception of water. In Fig. 3, thedi�erence in entrainment ratio between water and halocarbon compound refrigerants isremarkable, however the COP value di�erence in Fig. 5 is not so remarkable. This is becauseof the contribution of the latent heat of water at the evaporator temperature, which is muchlarger than those for halocarbon refrigerants. This also indicates that, besides the entrainmentratio, the latent heat of a refrigerant also a�ects the cycle performance. Generally speaking, arefrigerant with large latent heat can make full use of the ejector characteristics. With anejector using such a refrigerant, a small increase in entrainment ratio will result in a largeincrease in the system performance.

3.3. E�ect of operating conditions

It has been shown that the entrainment ratio of an ejector and the COP value of the ejectorcycle for a given refrigerant depends on the cycle operating conditions [7, 8, 24]. In order toinvestigate whether operating conditions also a�ect the variation of COP values for the variousrefrigerants shown in Fig. 5, the operating temperatures are varied.Fig. 6 shows the comparison of cycle performance for various refrigerants under the

conditions of boiler temperature at 808C, condenser temperature at 358C and evaporatortemperature at 58C. The results in Fig. 6 are very similar to those in Fig. 5 except that the

Fig. 5. Comparison of system COP for various refrigerants at Tg=908C, Tc=358C and Te=58C.


COP values are lower. This is because of the low entrainment ratio caused by the lowgenerator temperature of 808C. The e�ect of the condenser temperature is shown in Fig. 7. Theresults are very similar to those in Fig. 5, except that the COP values in Fig. 7 are muchhigher, caused by the low condenser temperature. This also shows that the condensertemperature has a strong in¯uence on the ejector and cycle performance. The COP di�erencebetween water and halocarbon refrigerants shown in Fig. 7 is very small because of the e�ectof the large latent heat of water. Fig. 8 shows the e�ect of the evaporator temperature. Again,the variation of COP values is very similar to those in Fig. 5. In Fig. 8, the COP for water iszero because water will freeze in the evaporator if the evaporator temperature is lower thanzero. The COP variation in Fig. 8 is distinct, because all the latent heats for the variousrefrigerants under lower evaporator temperature are larger, and the contributions of theejectors are more obvious. Fig. 8 also shows that the COP values for the low evaporatortemperature of ÿ58C is very low. An ejector refrigeration system can perform much betterunder the high evaporator temperatures shown in Fig. 5, con®rmed by experiments [7, 8].The results shown in Figs. 6±8 indicate that the variation in COP values for various

refrigerants is almost independent of the system operating conditions, and therefore, the sameconclusions as those in Fig. 5 can be drawn for Figs. 6±8. It should be pointed out thatejectors using environmental-friendly HFC refrigerants R134a and R152a perform very well, asshown in Figs. 6±8, with R152a giving the higher COP values. R152a also appears to be abetter alternative for R12 in heat pump applications, although R134a is the most popularchoice as an alternative [34]. This is probably the reason why R152a has received a productionboost [28, 29].




Fig. 8. Comparison of system COP for various refrigerants at Tg=908C, Tc=358C and Te=ÿ 58C.


4. Conclusions

A computer simulation program for ejector refrigeration systems has been developed. Thisprogram can give predictions for the performance of an ejector refrigeration system andprovide optimum ejector design data for the system. The program can be used to compare theperformance of various refrigerants used in an ejector refrigeration system.In the current study, eleven refrigerants were tested using the program. These refrigerants

include water (R718), halocarbon compounds, i.e., CFCs (R11, R12, R113), HCFCs (R21,R123, R142b) and HFCs (R134a, R152a), a cyclic organic compound (RC318), and anazeotrope (R500). The results show that a steam jet refrigeration cycle has the lowest COPvalue. For CFCs, R12 gives better performance; for HCFCs, R142b gives high COP value; theHFC refrigerants tested have comparative performance, with R152a giving the bestperformance among all the other refrigerants. Using HFC refrigerants, which cause no ozonedepletion, also produces extra environmental bene®ts. The system using azeotrope R500 asrefrigerant also performs well. This pattern of performance variation for various refrigerants isalmost independent of the system operating conditions. An ejector refrigeration cycle using arefrigerant with large latent heat can make full use of the ejector performance characteristics.The results also show that a steam jet system is suitable for experimental studies, since smallsteam jet refrigeration units require comparatively large ejectors that are easier to manufacture.

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Documents

Comparative study of the performance of an ejector ... comparative data are needed in order to develop substitutes, therefore the present study includes three CFC refrigerants. 2