VOL. 11, NO. 2, JANUARY 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com PERFORMANCE IMPROVEMENT USING SUBCOOLING ON FREEZER WITH R22 AND R290 AS REFRIGERANT FOR VARIOUS AMBIENT TEMPERATURES Cecep Sunardi1, Luga Martin1, Kasni Sumeru1 and Henry Nasution2 1Department of Refrigeration and Air Conditioning, Politeknik Negeri Bandung, Bandung, Indonesia 2Automotive Development Centre, Faculty of Mechanical Engineering Universiti Teknologi Malaysia, Skudai, Johor Malaysia E-Mail:
[email protected]ABSTRACT Vapor compression refrigeration cycle (VCRC) is widely used in freezers. In a simple cycle of the VCRC, refrigerant in the condenser exit is at liquid saturated line. If the refrigerant temperature in the condenser exit is further cooled to the subcooled region results in an increase in the cooling capacity due to low vapor quality of refrigerant entering the evaporator. The lower the refrigerant quality entering the evaporator, the higher the cooling capacity on the evaporator. The increase in the cooling capacity enhances the freezer performance. Subcooling uses liquid suction heat exchanger (LSHX) is applied to transfer heat from the exit of the condenser (liquid line) to the suction of the compressor. Because the temperature of liquid line is higher than that of the suction line, heat flows from liquid line to the suction line, results in the decrease in temperature of the exit of the condenser and the increase in the suction temperature (superheating). Consequently, the use of the LSHX is always followed by superheating on the suction of the compressor. The superheating causes an increase in the input power. As a result, subcooling using the LSHX is not always followed by the increase in the coefficient of performance (COP) of the freezer, depending on the type of refrigerant used. Refrigerant of R22 is widely used as refrigerant in the freezers. Because R22 has a high global warming potential (GWP), the near future this refrigerant must be phased out. Refrigerant of R290 (propane) is recommended as a substitute refrigerant for R22. The R290 is a natural refrigerant, abundant and relatively cheaper than that of R22. This paper investigates the use of LSHX subcooling in freezer with the evaporating temperature of -20 oC for various ambient temperatures, viz., 30, 35 and 40 oC using R22 and R290 as working fluids. It is well known that the COP of the freezer decreases with an increase in the ambient temperature. As a result, the use of the LSHX subcooler is expected to enhance the COP of the freezer for the higher ambient temperatures. There are three parameters, viz., the cooling capacity, the discharge temperature and COP improvement will be investigated in the present study. Numerical results showed that the increase in subcooling and ambient temperature increase the cooling capacity and COP improvements. In addition, the cooling capacity and COP improvements of R290 using the subcooling of LSHX were higher than that of R22 for all ambient temperatures. Also, the increase in degree of subcooling enhanced the cooling capacity and COP improvements. For R22, at the ambient temperature above 35 oC is not recommended using LSHX subcooling in freezers, because it will increase the discharge temperature above 90 oC. However, the use of LSHX subcooling is recommended for all ambient temperatures in freezer using R290, because the discharge temperature on the freezer is still below 80 oC. Keywords: subcooling, heat exchanger, LSHX, R22, R290. INTRODUCTION negative effects to the system. The positive effect if the Because refrigeration system consumes a large degree of superheating is to eliminate liquid refrigerant amount of electrical energy, as a result many methods entering to the compressor, because this can cause damage have been developed by researchers to improve the system to the compressor. The negative effect if the degree of performance. In the supermarket, the refrigeration systems superheating is too high, as a result an increase in the (freezer and air conditioner) consume approximately 50- compressor discharge temperature. The increase in the 60% of energy total [1]. Subcooling uses an internal heat compressor discharge temperature results in an increase in exchanger, which transfers heat from the condenser outlet the power consumption. Consequently, the subcooling to the compressor suction, may lead to an increase in the using LSHX does not always increase the COP [3]. cooling capacity produced by the evaporator. This method Nowadays, refrigerant of R22 is widely used as is called subcooling using liquid suction heat exchanger refrigerant in the freezers. However, because R22 has a (LSHX). The most important effect of the use of LSHX is high global warming potential (GWP), the near future this the increase in cooling capacity because of the lower refrigerant must be phased out and refrigerant of R290 quality of refrigerant entering the evaporator, consequently (propane) is recommended as a substitute refrigerant for the evaporator absorb more heat from the ambient [2]. In R22 [4]. The R290 is a natural refrigerant, abundant and the other hand, the use of LSHC results in superheating in relatively cheaper than that of R22. Many studies reported the compressor suction. The superheating has positive and that replacement of R22 with R290 in freezer resulted in 931 VOL. 11, NO. 2, JANUARY 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com an improvement of the COP [5, 6]. The aim of this study is superheating. Also, from the figure, it can be seen that the to investigate the effect of the LSHX subcooler on the subcooler decreases the exit condenser temperature, from performance of freezer using R22 and R290 with various point 3 to 3’, and this is called subcooling. The ambient temperatures. superheating helps in preventing the liquid refrigerant Potter and Hrnjak [7] carried out an experimental from entering the suction compressor. The liquid phase investigation to study the effect of sub-cooling on various refrigerant entering the suction causes problem in the parameters in the air conditioning system using R134a and compressor bearings and rings, or valve failure in some R1234yf. The experimental results showed that the extreme cases. presence of LSHX in the air conditioner increases the COP The degree of subcooling (Tcond - T3’) and the up to 18% and 9% for R1234yf and R134a, respectively. degree of superheating (T1’-Tevap) is not the same due to The results also showed that there are changes in some different in specific heats of the vapor and liquid phases. parameters due to the presence of subcooler using LSHX, The equation energy balance in the heat exchanger is used as shown in Table-1. The table shows that the use of to calculate the degree of subcooling and superheating and LSHX subcooler is always followed by increasing cooling given in equation. (1). capacity, compressor work and superheating. Because the COP is the ratio of the cooling capacity to compressor power, as a result the increase in the COP depends on their Qsub Qsup h1' h1 h3 h3' (1) increments. Most working fluid generates COP improvement on the system when the LSHX subcooling is The COP of freezer after using subcooler is applied, and only few refrigerants. Qevap ( h1 h3 ) ( h1' h1 ) Table-1. Effect of sub-cooler in the VCRC. COP (2) Wcomp ( h2 h1 ) [( h2 ' h2 ' ) ( h 2 h1 )] Quantity LSHX Cooling capacity Increases It can be seen from equation. (2), both numerators Condensing pressure Constant* and denominators of equation. (2) increase. The net effect, i.e., whether positive, negative or zero depends on the Mass flow rate Vary* working fluid used and the operating temperature Compressor work Increases (evaporating and condensing temperatures). COP Increases** In order to determine the performances of the Superheating Increases freezer, the parameter values in each point in Figure-1 in P-h diagram must be determined. Some assumption are *Depending on the degree of sub-cooling. made to carry out simulation: i.e., (a) steady state **Depending on the working fluid used. condition, (b) no heat transfer except on the condenser and the evaporator, (c) no pressure loss in the pipeline, (d) the THERMODYNAMIC ANALYSIS OF SUBCOOLING condensing temperature is 10 oC above the ambient Figure-1 shows the schematic and P-h (pressure- temperature (e) the evaporating temperature is -20 oC and enthalpy) diagrams of LSHX subcooler in the freezer. (f) the enthalpy value on the discharge temperature (h2) is According to the P-h diagram, the figure shows that the determined using equation. (3) and entropic efficiency of use of heat exchanger increases the suction temperature the compressor (comp) follows equation. (4) from point 1 to 1’, and this phenomenon is called 932 VOL. 11, NO. 2, JANUARY 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com Figure-1. Schematic and P-h diagrams of the LSHX subcooler in freezer. ( h2,is h1 ) The higher the degree of subcooling may lead to h2 h1 (3) the higher the discharge temperature, as shown in Figure- comp 3. The figure shows that the discharge temperatures P2 of R290 were always lower than that of R22 for all the comp 0.874 0.0135 (4) P1 ambient temperatures and the degree of subcoolings. The figure also shows that for R22, at the ambient temperature above 35 oC is not recommended using LSHX subcooling The cooling capacity (CC) improvement due to in freezers because the discharge temperature will be using LHSX subcooler is defined with equation. (5). In above 90 oC. Because of for safety reason, the compressor addition, COP improvements are determined using with the discharge temperature above 90 oC is not equations similar to equation. (5). recommended. However, the use of LSHX subcooling is recommended for all ambient temperatures in freezer CC with CC wo using R290, because the discharge temperature on the CCimp (5) freezer is still below 80 oC. For example, at the ambient CC wo temperature of 40 oC with the degree of subcooling of 10 o C, the discharge temperatures are approximately 80 oC and 114 oC for R290 and R22, respectively. RESULTS AND DISCUSSIONS The main purpose of the subcooling is to increase the cooling capacity. Figure-2 shows that the cooling capacity improvements using R290 were always higher than that of R22. The cooling capacity improvement increases with the increase in ambient temperature and degree of subcooling. The higher the degree of subcooling, the higher the cooling capacity. For the degree of subcooling of 2 oC and 10 oC, the cooling capacity improvement with R22 as refrigerant are 1.83% and 2.09%, respectively, for the ambient temperature of 30 oC. Furthermore, for R290 as working fluid with the degree of subcooling of 2 oC and 10 oC, the cooling capacity improvement are 2.31% and 2.71%, respectively, for the ambient temperature of 30 oC. The cooling capacity improvement also increases with the increase in ambient temperature. For instance, for the degree of subcooling of Figure-2. Effect of ambient temperature on the cooling 10 oC with R22 as working fluid at ambient temperature of capacity. 30 oC and 40 oC are 8.80% and 10.13%, respectively. Meanwhile, for R290 as refrigerant with the degree of subcooling of 10 oC at ambient temperature of 30 oC and 40 oC are 11.29% and 13.29%, respectively. 933 VOL. 11, NO. 2, JANUARY 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com increase in ambient temperature. The cooling capacity and COP improvements of R290 were higher than that of R22. The use of LSHX at the ambient temperature above 35 oC is not recommended in the freezers using R22 as refrigerant, because the discharge temperature will be above 90 oC. Meanwhile, to generate a significant COP improvement on freezer using LSHX subcooler with R290 as refrigerant, the degree of subcooling is above 10 oC. ACKNOWLEDGEMENT This study was supported by Politeknik Negeri Bandung. The authors are also grateful to Automotive Development Center (ADC), Universiti Teknologi Figure-3. Effect of ambient temperature on the Malaysia (UTM) for the facilities and management discharge temperature. support. Figure-4 shows the effect of LSHX subcooler on REFERENCES COP improvement. The figure shows that the COP improvement increases with the increase in ambient [1] B.A. Qureshi, S.M. Zubair. 2012. 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