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Experimental study of a series-connected two-evaporator refrigerating system with propane (R-290) as the refrigerant

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Experimental study of a series-connected

two-evaporator refrigerating system with propane (R-290)

as the refrigerant

Chao-Jen Li, Chin-Chia Su

*

Department of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan, ROC

Received 18 November 2002; accepted 16 March 2003

Abstract

The performance of a refrigerating system with an environment-friendly refrigerant, propane (R-290) as the refrigerant, was experimentally studied. There were two evaporators connected in series within the system under study.

The results show that with both lengths of the two capillary tubes fixed, both the mass flow rate of the refrigerant and the suction pressure of the system increase with the condensing pressure. In addition, the cooling capacity of the high-temperature evaporator decreases, but that of the low-temperature evaporator increases. As the condensing pressure is fixed and the length of the capillary tube for the high-temperature evaporator is increased while that for the low-temperature evaporator is fixed, the cooling capacity of the high-temperature evaporator increases while that of the low-temperature evaporator decreases. On the other hand, as the capillary tube for the low-temperature evaporator is lengthened while that for the high-temperature evaporator is fixed, the variations in the cooling capacity of these two evaporators reverse. The enthalpy changes of the refrigerant within the evaporators are strongly affected by the length of the high-temperature capillary tube, while the evaporating pressures are influenced mainly by the length of the low-temperature capillary tube.

Ó 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Propane; R-290; Two-evaporator; Refrigerating system

*Corresponding author. Tel./fax: +886-2-2368-7352. E-mail address:chinchiasu@ccms.ntu.edu.tw(C.-C. Su).

1359-4311/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-4311(03)00082-6

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1. Introduction

It is well known that the impacts of the refrigerant on the ozone depleting potential (ODP) and the global warming potential (GWP) are determined by the chlorine and fluorine, respectively, contained in the refrigerants. Refrigerants containing chlorine or fluorine, such as R-22, HFC-134a, etc., should therefore be replaced by environment-friendly substances, containing neither chlorine nor fluorine, hydrocarbons (HCs) represent a zero ODP and an extremely low GWP solution. From technical and thermodynamic points of view, some HCs are good alternatives. The refrigerating properties of propane (R-290) are close to those of R-22. R-290 is, therefore, a

Nomenclature

Ai inside heat transfer area (m2)

Ao outside heat transfer area (m2)

c specific heat (kJ/(kg K))

d inside diameter of capillary tube (mm) Di;i inside diameter of the inner tube (mm)

Do;i outside diameter of the inner tube (mm)

Di;o inside diameter of the outer tube (mm)

Do;o outside diameter of the outer tube (mm)

H enthalpy (kJ/kg)

h heat transfer coefficient (W/(m2K))

L length of capillary tube (m) Le length of the evaporator (m)

LMTD logarithmic-mean temperature difference (°C)

M mass flow rate (kg/h)

Pc condensing pressure (kPa)

Psuc suction pressure (kPa)

Q cooling capacity (W)

R conduction resistance (K/W)

T temperature (K)

U overall heat transfer coefficient (W/(m2K))

W power input (W) Subscripts H high-temperature evaporator Llow-temperature evaporator r Refrigerant t Total hm heating medium in inlet out outlet

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potential alternative to R-22. In addition, the compatibility of R-290 with the conventional mineral oil used in R-22 systems is very good.

The performance of the refrigerating system with HCs as the refrigerant has been investigated (e.g. [1–5]). However, the investigation was concentrated on the conventional system with only one evaporator. A refrigerating system with two or more evaporators connected in series may give better performance than that with only one evaporator. From the thermodynamic viewpoint, decreasing the temperature difference between evaporation and condensation increases the energy efficiency. The advantage may be availed with two evaporators, in which the temperature dif-ference for the high-temperature evaporator is smaller. For example, a two-evaporator refriger-ating system charged with nonazeotropic refrigerant R22/R11 (50%/50% by weight) shows a power saving of 20% compared with that of R-12 with only one evaporator [6]. Similar results were obtained by Rose et al. [7]. The computer simulation of a two-evaporator refrigerating system charged with pure and mixed refrigerants, conducted by Jung and Radermacher [8], shows a significant increase in COP. The cycle proposed by Lorenz and Meutzner [6] was modified by Simmons et al. [9,10] with some bypasses to the compressor. Again the experimental results showed that the energy saving potential is high with two evaporators in series.

It should be pointed out that although there are two evaporators in both the original cycle proposed by Lorenz and Meutzner [6] and the modified one [9,10], there is only one capillary tube in each system. Different evaporating temperatures are achieved through the nonazeotropic characteristics of the mixed refrigerants. With a single refrigerant, different evaporating temper-atures can only be obtained by the use of multiple capillary tubes and evaporators. To this end, an experimental system was established to explore the performance of the two-evaporator refriger-ating system with pure R-290 as the refrigerant.

2. Experimentation

Fig. 1 shows the schematic of the experimental system setup for specific purpose. The experi-mental apparatus can be divided into three subsystems: one refrigerant loop and two heat-exchange fluid loops. The states of the working fluids at appropriate locations were monitored with T-type thermocouples and pressure gauges as shown.

As shown in Fig. 1, the refrigerant loop is composed of a reciprocating compressor (L51B562DBL B, Bristol), a condenser, a filter–dryer, a refrigerant flow meter, a sight glass, an electromagnetic valve, two capillary tubes, two evaporators, and some valves. A capillary tube is arranged before each evaporator.

The power input of the three-phase, 220 V, reciprocating compressor is controlled by the fre-quency converter. The output of the converter can be adjusted between 0 and 120 Hz, but the range for stable operation is from 40 to 80 Hz. The lubricant of the compressor is a 150 SSU mineral oil. The condenser is a finned-tube heat exchanger. The condensation of the refrigerant in the tube is effected by the cooling air, which is induced by the action of a fan inside the condenser. The airflow rate is controlled by the rotating speed of the fan through a voltage transformer.

Both evaporators are of double-tube type in which the refrigerant flows in one direction through the inner tube while the heating medium flows in the opposite direction through the annular space between the inner and outer tubes. The heating medium of the high-temperature

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evaporator is water while that of low-temperature evaporator is water/glycol solution to avoid icing in the annular space between the tubes.

The selection of the capillary tubes involves some preliminary calculations and a trial-and-error test procedure for appropriate flow rates and operating temperatures. The cooling capacity of the low-temperature evaporator, QL, may cover that of freezer of small to medium size. Note that the

capillary tube for the high-temperature evaporator is not only smaller but also longer than that for the low-temperature counterpart. This situation is induced by the effect that part of the re-frigerant flowing within the high-temperature capillary tube is in liquid state, while that within the low-temperature capillary tube is all in two-phase. The evaporators and the capillary tubes are all made of copper and are all insulated outside. The dimensions of the capillary tubes and the evaporators are listed in Table 1.

evaporator (high T) capillary tube (high T) electromagnetic valve

sight glass flow meter

filter-dryer 1 5 8 T P T frequency converter power meter P T 4 P T compressor P T condenser P 2 T 3 P T capillary tube (low T) 7 T 6 water tank pump T flow meter T water/glycol tank flow meter T pump T evaporator (low T) P P

Fig. 1. Schematic diagram of the experimental system.

Table 1

Dimensions of capillary tubes and evaporators

dH (mm) dL(mm) Le(m) Di;i(mm) Do;i(mm) Di;o(mm) Do;o(mm)

1.1 1.4 2 8.7 9.525 14.85 15.875

LH (m) LL(m)

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Both the heat-exchange loops are composed of a pump, a flow meter, and an electrically heated unit within the tank. Depending on the operation conditions, the refrigerant within the high-temperature evaporator is controlled between 15 and 20°C, while that within the low-temperature evaporator is below )10 °C.

T-type thermocouples with monitor (APL7000&S.B767, Jenco) with accuracy 0.1 °C and analogical pressure meters (316 SS TUBE&SOCKET, Imperial) with accuracy 1% of the reading are used. Both the temperatures and pressures of the refrigerant before and after the compressor, both capillary tubes, and both evaporators are monitored. In addition, the tem-peratures of the water and the water/glycol solution of the heat-exchange loops are also recorded. The flow rates of the refrigerant, water, and water/glycol solution are measured with flow meters. The accuracy of the refrigerant flow meter (102-5T, Macmillan) is 3% in the range of the reading.

The controlled variables are the condensing pressure, the frequency of compressor, and the lengths of both capillary tubes, while the flow rates of both heating media are kept constant.

3. Results and discussion 3.1. Pressure-enthalpy diagram

Based on some test data and the thermodynamic properties of the refrigerant [11] and the heating media, the state of the refrigerant at different locations along the system is shown in Fig. 2,

200 300 400 500 600 700 800 0 200 400 600 800 1000 1200 1400 1600 L /d L /d 1363.64 142.86 2045.45 142.86 1590.91 285.71 1590.91 428.57 6 7 5 3,4 2 1,8 50? 30? -30? P(kPa) H(kJ/kg)

Fig. 2. P–h diagram of the refrigerant within the system and the effect of lengths of capillary tubes on states of refrigerant (Pc¼ 1372 kPa).

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in which the encircled numbers indicate the corresponding locations as shown in Fig. 1. The details of Fig. 2 will be discussed later. To determine the states 6 and 7, the following equations for heat transfer are adopted:

Qr ¼ Mr ðHr;out Hr;inÞ ð1Þ

Qhm¼ Mhm chm ðThm;in Thm;outÞ ð2Þ

Eq. (1) gives the actual heat absorbed by the refrigerant. As both evaporators are insulated, Qr

is taken as equal Qhm. Hr;in within the high-temperature evaporator, i.e. H5, equals H4 which is

achievable through the measured P4 and T4. Hr;out within the high-temperature evaporator, i.e. H6,

can thus be obtained through Eq. (1). The location of point 6 can then be fixed by H6 and the

measured P6. Knowing state 6, state 7 can be allocated by the measured P7 and the fact that

H7 ¼ H6. Similar situations occur for all other running conditions in the study.

3.2. Effect of condensing pressure

Fig. 3 shows the variation of the mass flow rate of the refrigerant (Mr) and the suction pressure

(Psuc) with the condensing pressure (Pc). It can be seen that both Mr and Psucincrease with Pc for

different length of high-temperature capillary tube, LH. For a given LH, the total pressure drop

available for the refrigerant flowing through the system increases with Pc. However, the pressure

ratio across the compressor, Pc=Psuc, varies with Mr, which is a characteristic of the reciprocating

compressor. Therefore Psucincreases with Pc. On the other hand, for a given Pc, Mrdecreases with

LH since more pressure drop is required for a longer capillary tube.

1100 1200 1300 1400 1500 1600 1700 7 8 9 10 11 12 13 Psuc LH/dH 1363.64 1590.91 1818.18 2045.45 M L /d 1363.64 1590.91 1818.18 2045.45 P c(kPa) M r (k g /h ) 200 220 240 260 280 300 320 340 360 P su c (kPa)

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The effect of Pcon the cooling capacity of the two evaporators and COP is shown in Fig. 4. The

heat-exchange rate between the working media within an evaporator may be obtained with Eqs. (1) and (2). However, it may also be expressed as

Q¼ ðU  AÞ  ðLMTDÞ ð3Þ

where the products of the overall heat transfer coefficients and the heat transfer areas and LMTD are defined as ðU  AÞ ¼ 1 hr Ai  þ R þ 1 hhm Ao 1 ð4Þ

ðLMTDÞ ¼ðThm;out Tr;inÞ  ðThm;in Tr;outÞ ln ðThm;out Tr;inÞ

ðThm;in Tr;outÞ

  ð5Þ

Q and LMTD in the high-temperature evaporator are taken as equal QH and LMTDH,

re-spectively, while those in the low-temperature evaporator are taken as equal QL and LMTDL. Tr;in

in the high- and low-temperature evaporators are the saturation temperatures of the refrigerant corresponding to the measured P5 and P7, respectively, while Tr;out in the low-temperature

evap-orator is measured directly.

In the study, the conduction resistance, R, is fixed. In addition, the heat transfer coefficients of the heating media, hhm, is more or less constant since the flow rates of the heating media are kept

constant and the variation in the temperatures of the heating media entering the evaporator is negligible. It is clear that both QH and QL are affected by LMTD and the heat coefficient of the

refrigerant, hr, which is dominated by Mr.

1100 1200 1300 1400 1500 1600 1700 0 100 200 300 400 500 600 700 800 COP L /d 1363.64 1590.91 1818.18 2045.45 CO P Q +Q L /d 1363.64 1590.91 1818.18 2045.45 Q L /d 1363.64 1590.91 1818.18 2045.45 Q L /d 1363.64 1590.91 1818.18 2045.45 P c(kPa) Q( W ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

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Note that the average quality of the refrigerant in the high-temperature evaporator is lower than that in the low-temperature counterpart as shown in Fig. 2. With the criteria suggested by Shin et al. [12], the phase change in the high-temperature evaporator is thus mainly in the nucleate boiling region, while that in the low-temperature evaporator is mainly in the film boiling region. It has been pointed out that for nucleate boiling hrdoes not change with Mr, while for film boiling, hr

increases with Mr [12]. QHis, therefore, dominated completely by LMTDH, while QLis affected by

hr;L along with LMTDL.

With LH and LL fixed, the refrigerant enters each evaporator with higher pressure when Pc is

increased. Therefore, both LMTDHand LMTDL decrease with increasing Pc. The decrease of QH

with Pcshown in Fig. 4 is then induced entirely by the decreasing LMTDH. On the other hand, the

increase of QL with Pc indicates that the effect of hr;L dominates that of LMTDL. It is noticeable

that the total cooling capacity of the two evaporators, Qt, increases with Pc as shown in Fig. 4.

The variation of COP with Pc is shown in Fig. 4. COP is defined as

COP¼Qt Wr

ð6Þ where

Wr ¼ Mr DH12¼ Mr ðH2 H1Þ ð7Þ

The specific enthalpies, H1 and H2, are determined through the states of the refrigerant just before

and after the compressor, i.e. states (P1; T1) and (P2; T2). Note that since the study is concentrated

on the effect of two evaporators with individual capillary tubes, isentropic compression is assumed in obtaining H2. The increases of Mr, Qt and Wr with Pcare shown in Figs. 3–5, respectively. The

net effect of Qtand Wr makes COP of the system also decrease slightly with Pc.

1200 1400 1600 1800 2000 2200 8 9 10 11 12 13 14 15 Wr Pc 1568 1470 1372 1274 Mr Pc(kPa) 1568 1470 1372 1274 L H/dH M r (kg/h) 200 250 300 350 400 W r (W )

Fig. 5. Effects of length of capillary tube for high-temperature evaporator on mass flow rate of refrigerant and com-pression work (LL=dL¼ 142:86Þ.

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3.3. Effect of the length of capillary tubes

The physical properties of the refrigerant at the inlet of the high-temperature capillary tube are more or less the same for different lengths of capillary tubes as shown in Fig. 2. Mr is therefore

affected mainly by the length of capillary tube. Fig. 5 shows that for a given Pc, both Mr and Wr

decrease with increasing LH. Similar variation in Mrversus LLcan be found in Fig. 6. However, Wr

seems to increase with LL at high Pc. The increase of Mr versus Pc is discussed in the previous

subsection. For the case of high-temperature capillary tube, higher frictional loss is expected for longer tube, so Mr decreases with LH. For the case of low-temperature capillary tube, a check has

been made to see whether the flow is chocked with the criterion suggested by Pate and Tree [13]. The results indicate that indeed choke occurs within the low-temperature capillary tubes for all of the four different Pc. With choke occurring at the exit of the tube, the flow speed of the refrigerant

there is more or less fixed. Lower density of the refrigerant at the exit of the low-temperature capillary tube can thus be expected at lower Psucinduced by increasing LL. Therefore, Mrdecreases

with LL. On the other hand, DH12 increases with LH. Since Wr, is a product of DH12 and Mr, its

variation versus LHis thus more gradual than that of Mr. It can be seen from Fig. 6 that a similar

phenomenon occurs for the variation of Wr versus LL.

For a given Pc, QHincreases, while QLdecreases with LH as shown in Fig. 7. QH decreases and

QLincreases with LLas shown in Fig. 8. As explained earlier, LMTDHincreases with LHand hr;H

is not affected by Mr. On the other hand, hr;L decreases with decreasing Mr. However, when LHis

increased, the increasing LMTDLmakes heat transfer higher. It seems that the effect of hr;Lon QL

is stronger than that of LMTDL.

0 100 200 300 400 500 600 700 800 7 8 9 10 11 12 13 14 Wr Pc(kPa) 1568 1470 1372 1274 Mr Pc(kPa) 1568 1470 1372 1274 L L/dL M r (kg/h) 200 250 300 350 400 W r(W )

Fig. 6. Effects of length of capillary tube for low-temperature evaporator on mass flow rate of refrigerant and com-pression work (LH=dH¼ 1590:91).

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For a given LL, the increasing LH does not lower the pressure in the high-temperature

evapo-rator, P5 shown in Fig. 2. For an increase of 50% in LH (from 1.5 to 2.25 m), the pressure drop

1000 1200 1400 1600 1800 2000 2200 0 100 200 300 400 500 600 700 800 QH+QL Pc(kPa) 1568 1470 1372 1274 QL PC(kPa) 1568 1470 1372 1274 Q H Pc(kPa) 1568 1470 1372 1274 LH/dH Q(W )

Fig. 7. Effects of length of capillary tube for high-temperature evaporator on cooling capacities (LL=dL¼ 142:86).

0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 QH+QL Pc(kPa) 1568 1470 1372 1274 Q L Pc(kPa) 1568 1470 1372 1274 QH Pc(kPa) 1568 1470 1372 1274 LL/dL Q(W)

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induced by the high-temperature capillary tube increases by 39.2 kPa, while that by the low-temperature capillary tube decreases by 4.9 kPa. The relative changes of the pressure drops in comparing with their own original values within the high- and low-temperature capillary tubes are thus +7.1% and )0.9%, respectively, while the total pressure difference between states 4 and 7 increases by only 3.1%. The decreasing Mrwith the increasing LHmay be the main reason for such

a small increase in pressure drop. However, the effect of increasing LL on the pressures of states 5

and 6 is more dramatic. With LH fixed, an increase of 50% in LL (from 0.4 to 0.6 m) enlarges the

pressure drop induced by the high- and low-temperature capillary tubes by )14.3% and 12.1%, respectively. However, the total pressure difference increases by only 0.9%. It seems that the changes of the pressure difference in the individual capillary tubes induced by varying LLare more

than those by varying the high-temperature counterpart.

Although lengthening LH does not induce much pressure drop between states 5 and 6, it does

bring about an apparent increase in the enthalpy of state 6 as shown in Fig. 2. Compared with the original enthalpy change in the high- and low-temperature evaporators, the increases induced by lengthening LHby 50% are 39.5% and)15.5%, respectively, while those induced by lengthening LL

are)6.9% and 4.7%, respectively. The details of the variations may be worthy of further investi-gation.

4. Conclusions

With Pc, LH and LL as the varying factors, the following conclusions may be drawn from the

experimental study of the series-connected, two-evaporator refrigerating system with R-290 as the refrigerant.

1. Both Mrand Psucincrease with Pcfor fixed LHand LL. In addition, QHdecreases but QLincreases

with Pc.

2. With other factors fixed, QH increases with LH, while QL and Mr decrease. On the other hand,

QL increases with LL, while QH and Mr decrease.

3. QH and QL are mainly affected by the two effects: hr and LMTD. For the case of the

high-temperature evaporator, LMTD is the dominant factor. However, hr also plays an important

role in the case of the low-temperature evaporator.

4. The enthalpy changes of the refrigerant within the evaporators are strongly affected by LH.

However, the evaporating pressures within the evaporators are influenced mainly by LL.

The variations of Mr, Wr, QH, QL and COP versus LH, LL and Pc studied in this work may be

applied to the design of a two-evaporator refrigerating system. However, the operating ranges of Pc and the dimensions of the capillary tubes may be enlarged for clearer understanding of the

effect of these factors. In addition, works on other environment-friendly refrigerants or the blends of them are also worthy of further investigation.

References

[1] M.S. Kim, W.J. Mulroy, D.A. Didion, Performance evaluation of two azeotropic refrigerant mixtures of HFC-134a with R-290 (Propane) and R-600a (Isobutane), J. Energy Resour. Technol., Trans. ASME 116 (1994) 148–154.

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[2] D. Jung, C.B. Kim, B.H. Lim, H.W. Lee, Testing of a hydrocarbon mixture in domestic refrigerators, ASHRAE Trans.: Symposia 102 (1) (1996) 1077–1084.

[3] M.A. Alsaad, M.A. Hammad, The application of propane/butane mixtures for domestic refrigerators, Appl. Thermal Eng. 18 (1998) 911–918.

[4] Y. Zhao, M. Yitai, L. Yie, C. Zhonghai, M. Lishan, The performance of some substitutes for HCFC22 under varying operating conditions, Appl. Thermal Eng. 19 (1999) 801–806.

[5] B. Tashtoush, M. Tahat, M.A. Shudeifat, Experimental study of new refrigerant mixtures to replace R12 in domestic refrigerators, Appl. Thermal Eng. 22 (2002) 495–506.

[6] A. Lorenz, K. Meutzner, On application of nonazeotropic two-component refrigerants in domestic refrigerators and home freezers, in: XIV International Congress of Refrigeration, Moscow, 1975.

[7] R.J. Rose, D.S. Jung, R. Radermacher, Testing of domestic two-evaporator refrigerators with zeotropic refrigerant mixtures, ASHRAE Trans. 98 (1992) 216–226.

[8] D.S. Jung, R. Radermacher, Performance simulation of a two-evaporator refrigerator–freezer charged with pure and mixed refrigerants, Int. J. Refrig. 14 (1991) 254–263.

[9] K.E. Simmons, K. Kim, R. Radermacher, Experimental study of independent temperature control of refrigerator compartments in a modified Lorenz–Meutzner cycle, ASME AES 34 (1995) 67–73.

[10] K.E. Simmons, I. Haider, R. Radermacher, Independent compartment temperature control of Lorenz–Meutzner and modified Lorenz–Meutzner cycle refrigerators, ASHRAE Trans. 102 (1) (1996) 1085–1092.

[11] M.O. McLinden, E.W. Lemmon, S.A. Klein, A.P. Peskin, NIST thermodynamic properties of refrigerants and refrigerant mixtures database (REFPROP). US Department of Commerce, National Institute of Standards and Technology, Gaithersburg, MD, 6.0, 1998.

[12] J.Y. Shin, M.S. Kim, S.T. Ro, Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube, Int. J. Refrig. 20 (4) (1997) 267–275.

[13] M.B. Pate, D.R. Tree, An analysis of choked flow conditions in a capillary tube-suction line heat exchanger, ASHRAE Trans. 93 (1) (1987) 368–380.

數據

Fig. 1. Schematic diagram of the experimental system.
Fig. 2. P–h diagram of the refrigerant within the system and the effect of lengths of capillary tubes on states of refrigerant (P c ¼ 1372 kPa).
Fig. 3 shows the variation of the mass flow rate of the refrigerant (M r ) and the suction pressure (P suc ) with the condensing pressure (P c )
Fig. 4. Effects of condensing pressure on cooling capacities and COP (L L =d L ¼ 142:86).
+4

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