Energy Performance Assessment of R32/R125/R600a Mixtures as Possible Alternatives to R22 in Compression Refrigeration Systems

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 12 Energy Performance Assessment of R32/R125/R600a Mixtures as Possible Alternatives to R22 in Compression Refrigeration Systems N.Suguna Ramu Department of Mechanical Engineering Chettinad College of Engineering and Technology Karur , India P.Senthil kumar Department of Mechanical Engineering KSR College of Engineering Tiruchengode India. M.Mohanraj Department of Mechanical Engineering Hindustan College of Engineering and Technology Coimbatore India Abstract In this work, the energy performance assessment of compression refrigeration systems has been theoretically assessed with R22 and the ternary mixtures composed of R32, R125 and 600a as alternative refrigerants. The energy performance assessment of the air conditioner was made for three different condensing temperatures such as 35, 45 and 55 o C with evaporator temperatures between -10 and 10 o C (which covers the medium temperature refrigeration and air conditioning applications). The assessment was made in terms of standard energy performance parameters such as coefficient of performance (COP), compressor power consumption, compressor discharge temperature and volumetric cooling capacity (VCC). Total equivalent global warming impact (TEGWI) of the air conditioner was assessed for fifteen years life time. The results obtained showed that the VCC of new refrigerant mixture composed of R32/R125/R600a (in the ratio of 0.4:0.4:0.2, by mass) is closer to R22. Hence, new refrigerant mixture can be used as drop in substitute in existing R22 systems. The COP of the mixture was found to be lower than that of R22 by about 16-20% at all condenser and evaporator temperatures. The compressor discharge temperature of the new refrigerant mixture was observed to be 6-11 o C lower than that of R22, which confirms that improved compressor life can be expected with new refrigerant mixture. TEGWI of new refrigerant mixture was found to be higher than that of R22 by about 20% due to its higher compressor power consumption. The results confirmed that the new refrigerant mixture is an ozone-friendly alternative to phase out R22 in the existing refrigeration systems to extend its life without modifications. Additionally, the suggestions to improve performance of the system are discussed. Index Term-- R22, R32/R125/R600a mixture, Compression based refrigeration system, Total Equivalent Global Warming Impact. I. INTRODUCTION R22 has been widely used in compression based refrigeration, air conditioning and heat pump systems due to its good thermodynamic and thermo-physical properties. Due to its poor environmental properties, it was phased out in many developed countries, whereas the developing countries are in transient to phase out R22 [1]. During last decade, many R22 alternatives refrigerant mixtures have been developed, which are summarized and reported in review articles [2-4]. Among the alternatives, the hydrocarbons (HCs) such as R290, R1270 and its mixtures R432A, R433A, hydroflurocarbon mixtures (HFCs) such as R404A, R407C and R410A and HFC/HC mixtures such as R417A and R422A are identified as the leading replacements for R22 in refrigeration, air conditioning and heat pumps units. The properties of such R22 alternatives are compared in Table 1. The hydrocarbons such as R290 and R1270 are reported as the possible alternatives to R22 for residential air conditioners and heat pumps [5-8]. Similarly, the hydrocarbon mixtures such as LPG mixture composed of R290, R170, R600a (in the ratio of 98.95: 1.007: , by mass) [9], R290/R170 mixture (in the ratio of 94:6, by mass) [10], R432A (near azeotrope mixture composed of R1270 and RE170, in the ratio of 80:20, by mass) [11], R433A (near azeotrope mixture composed of R1270 and R290, in the ratio of 70:30, by mass) [12], mixtures composed of R1270, R290, RE170 and R152a [13] are reported as alternatives to R22 in compression based refrigeration and air conditioning units. The reported studies confirmed that hydrocarbon based refrigerant mixtures are the good energy efficient and environment friendly alternative option to replace the R22 in residential air conditioners. However, due to unavailability of hydrocarbon refrigerant mixtures reported in this section, it is not possible to replace in the existing refrigeration, air conditioning and heat pump systems. The flammability is another drawback to use it in existing compression based refrigeration systems. Similarly, many investigators tried with HFC mixtures such as R404A, R407C and R410A as leading substitutes for replacing R22 in compression based refrigeration, air conditioning and heat pump systems [4]. Out of these three substitutes, 404A is a good R22 replacement for low temperature applications [14-16]. Similarly, R407C was reported as a possible R22 alternative for compression based systems used for refrigeration, air conditioning and heat pump systems by changing the lubricant [17-20]. A number of investigators tried with R410A as a possible alternative to R22 in air conditioners and heat pumps [21-23].

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 13 TABLE I. PROPERTIES OF R22 AND ITS ALTERNATIVES Refrigerant Composition Molecular Weight Boiling Point ( o C) Critical Temperature ( o C) ODP GWP R22 Pure fluid R290 Pure fluid R1270 Pure fluid R404A R125/R143a/R134a R407C R32/R125/R134a R410A R32/R R417A R125/R134a/R R422A R125/R134a/R600a R431A R290/R152a R432A R1270/RE R433A R290/R The major problem associated with R410A is its lower critical temperature, which restricts its usage in compression based systems working at higher condensing temperatures. Wu et al. [24] investigated the performance of HFC mixture composed of R152a, R125 and R32, in the ratio of 48:18:34, by mass in a R22 based domestic air conditioner. Similarly, the performance of binary R32/R134a mixture was investigated for air conditioning [25] and heat pump applications [26]. The two major problems faced by HFC refrigerant are its GWP [27] and its immiscible nature with conventional mineral oil [28]. Hence, polyol ester oil (POE) is recommended for the compression systems working with HFC refrigerants. The high hygroscopic nature of POE demands stringent service practices to avoid moisture absorption [29]. Due to the above limitations with POE, mineral oil lubricants are preferred. To overcome the drawbacks with HC and HFC refrigerants, the mixtures composed of HC and HFC was developed [30-43]. Park et al. [30] investigated the performance of residential air conditioner working with R22 and R431A mixture composed of R290/R152a (in the ratio of 71:29, by mass). In similar work, Jabaraj et al. [31, 32] used HC composed of R290 and R600a (in the ratio of 45.2:54.8, by mass) to tackle the miscibility issue of R407C with mineral oil in a residential air conditioner. In another work, Mohanraj et al. [33, 34] used LPG mixture as an additive with R407C to overcome the miscibility issue with mineral oil lubricant. Similarly, the low volatile hydrocarbon component (R600) in the R417A mixture tackles the miscibility issue with mineral oil [35]. The performance of R417A was evaluated for cold storage, heat pump, chiller and residential air conditioners [36-39]. Similarly, 422 series of refrigerant mixtures were used as alternatives to R22 in a compression based refrigeration and air conditioning systems [40-44]. In another work, Nanxi et al. [45] studied the performance, thermodynamic properties, miscibility with mineral oil and flammability of a near azeotropic refrigerant mixture composed of R124/R142b/R600a in the ratio of 0.9:0.08:0.02 (by mass fraction) in a heat pump. The unavailability of R417A and R422 series of refrigerants in Indian market is the major problem facing in R22 replacement. In India, the mixture composed of R32 and R125 (in the ratio of 50:50, by mass) is a readily available under the commercial name of R410A. In this work, an attempt has been made to blend the R410A with R600a to tackle the miscibility issue and the drawbacks associated with R410A. Additionally, the energy performance of the residential air conditioner is theoretically assessed with new refrigerant mixtures composed of R32/R125/R600a with different mass fractions of R600a. II. REFRIGERANT SELECTION Only few pure hydrocarbon refrigerants are having properties closer to R22. However, the refrigerant mixtures provide much flexibility in searching new alternatives to match the required properties with R22. In this section, selection of new refrigerant mixtures based on thermodynamic, thermo-physical, chemical and environmental properties are discussed in detail. A. Hydrocarbon additive with HFC mixtures The HFC/HC refrigerant mixtures tackle the miscibility issue with mineral oil. Formeglia et al. [46] reported that hydrocarbon refrigerants are possible to blend with HFC refrigerants. The commercially available R410A mixture composed of R32 and R125 (0.5:0.5, by mass) has higher

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 14 saturation pressure compared to R22. The hydrocarbons refrigerants are the possible additives with R410A to reduce its saturation pressure. The possible hydrocarbons for blending with R32/R125 mixture are R290, R1270, R600 and R600a [31-34]. Out of these four HC refrigerants, R290 and R1270 are having highly volatile with high saturation pressure, whereas, R600 and R600a are less volatile with less saturation pressure. If R290 or R1270 is blended with R32/R125 mixture, the saturation vapour pressure will gets increased, which results in poor system performance. Hence, low volatile and low pressure components like R600 or R600a are preferred as additives with HFC refrigerants. Earlier studies confirmed that it is possible to blend R32 and R125 with R600a [47]. In this work R600a is taken as a viable additive to tackle the oil miscibility issue in the compressor. It is designated in this paper as NRM10, NRM20 and NRM30 according to the mass fraction of R600a in the mixture (10%, 20% and 30% of R600a in the mixture). The compositions of the mixtures are listed in Table 2. In this work the maximum percentage of R600a was limited to 30% (by mass) of total mixture to avoid the flammability risk. B. Thermodynamic and thermo-physical properties The properties of the mixtures obtained from REFPROP database have been used in this work. The thermodynamic and thermo-physical requirements such as, vapour pressures, liquid density, latent heat, specific heat and specific heat ratio, critical pressure, critical temperature, boiling point, etc are essential for choosing an alternative refrigerant [48]. TABLE II. COMPOSITION OF MIXTURES R32 R125 R600a Ref Mix. (by mass) (by mass) (bymass) NRM NRM NRM R600a mass fraction. At low temperatures (0-20 o C), the latent heat of NRM10, NRM20 and NRM30 are found to be higher than that of R22 by about 3-7%, 7-10% and 12-14%, respectively, which ensures more refrigeration effect in the evaporators. Similarly, at higher temperatures, the latent heat of NRM10, NRM20 and NRM30 are found to be lower than that of R22 by about 5-35%, 8-20% and 4-9%, respectively at higher temperatures between 40 and 60 o C. Hence, low heat rejection is expected with refrigerant mixtures in the condensers. The latent heat of R22 and all the mixtures gets reduced with increase in temperature. 3. Density The variation of liquid and vapour densities of R22 and NRMs against temperature are compared in Fig. 3 and Fig. 4, respectively. From Fig. 4, it is observed that, increase in mass fraction of R600a in the refrigerant mixture will reduce the liquid density. The liquid densities of all the NRM10, NRM20 and NRM30 were found to be 23-43%, 37-56% and 49-65% lower than that of R22 across the considered range of temperatures between 0 and 60 o C, respectively. Due to the lower liquid density of NRMs, the mass charge requirement would be reduced compared to that of R22. The lower liquid density values of the refrigerant mixtures have lower friction in the tubes of heat exchangers, which ensures better heat transfer coefficients compared to R22 in the condensers and evaporators. The vapour density of NRM20 is found to be 2-7% higher than that of R22 in the temperature ranges between 0 and 25 o C, where as NRM10 has higher magnitude for vapour density by about 27% and NRM30 has lower values by about 40%, which demands compressor modifications. 1. Vapour pressure Fig. 1 shows the variation of vapour pressure for R22 and NRM10, NRM20 and NRM30 against the temperature between 0 and 60 o C, which covers the normal operating temperature range in a residential air conditioner. From figure 1, it was observed that the vapour pressure of NRM10 has about 25% higher than that of R22 for wide range of temperatures. Whereas, NRM20 has vapour pressure about 6-14% higher than that of R22 for the considered range of operating temperatures. The vapour pressure of NRM30 has about 6% lower vapour pressure at lower temperatures, whereas at higher temperatures, the vapour pressure is about 12% higher than that of R22. The increase in R600a mass fraction has reduced the vapour pressure of the mixtures. The higher vapour pressure of NRMs will increase the compressor power consumption compared to R Latent heat Fig. 1. Vapour pressure versus temperature The variations of latent heat for R22 and NRMs against temperatures between 0 and 60 o C are compared in Fig. 2. The latent heat of NRMs gets increased with increase in

4 4. Specific heat International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 15 Fig. 2. Latent heat versus temperature Fig. 3. Liquid density versus temperature The specific heat of R22 and NRMs influences both condensing and boiling heat transfer coefficients in the condensers and evaporators, respectively. The specific heats in both liquid and vapour phases of R22 and NRMs are compared in Fig. 5 and Fig. 6, respectively. From Fig. 5, it is observed that all the NRMs are having higher specific heats in the range of 26-55%, 30-52% and 33-48% for NRM10, NRM20 and NRM30, respectively compared to R22 for range of temperatures between 0 and 60 o C. Similarly, from figure 6, it is confirmed that vapour specific heat of NRMs are found to be higher than that of R22 by about 37-56%, 36-52%, 36-38%, respectively. 5. Viscosity The refrigerant viscosity is a major source of irreversibility, which influences condensation and boiling heat transfer coefficients. A low viscosity is desirable to achieve higher heat transfer coefficients. The liquid and vapour viscosities of R22 and NRMs are compared in Fig. 7 and Fig. 8, respectively. From Fig. 7, it is observed that liquid viscosities of NRM10, NRM20 and NRM30 were found to be about lower than that of R22 by about 40-76%, 46-77% and 48-74%, respectively, which yields better heat transfer coefficients in the condensers. Similarly, the vapour viscosities of new refrigerant mixtures are compared in Fig. 8. The vapour viscosity of NRM10 was found to be higher than that of R22 by about 16-24%. Whereas, the viscosity of NRM20 is found to be 3% lower than that of R22 at low temperatures, which confirmed that NRM20 yields better heat transfer coefficients. NRM30 has 2-10% lower vapour viscosity compared to R Critical properties The critical temperature should be high to achieve better COP. Also the critical pressure of the alternatives should be lower to achieve low condensing pressure. The critical properties of R22 and NRMs are compared in Table 3. It is observed that the critical temperature of NRM10, NRM20 and NRM30 were found to be lower than that of R22 by about 18 o C, 12 o C and 5 o C, respectively. Similarly, the critical pressure of NRM10, NRM20 and NRM30 are found be lower than that of R22 by about 0.31, 0.41 and 0.51 MPa, respectively. 7. Boiling and freezing points TABLE III. PROPERTIES OF R22 AND NEW REFRIGERANT MIXTURES Critical Temp. ( o C ) Critical Pr. (M Pa) Mol. Wt. (Kg/ Kmol) The boiling points of the NRM10 and NRM20 are found to be lower than that of R22 by about 6 o C and 2 o C, respectively. Whereas, RM30 has higher boiling point of about 2 o C compared to R22. Molecular weight of the refrigerant affects the compressor size because the specific volume of the vapour is directly related to it. A low molecular weight refrigerant is preferred for the reciprocating refrigerant compressor. The molecular weight of NRM10, NRM20 and NRM30 is found to be lower than that of R22 by about 2, 6 and 8 kg/kmol, respectively, which are listed in Table III. Boiling point ( o C ) ODP CFC=1 GWP R NRM 10 NRM

5 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 16 Fig. 7. Liquid viscosity versus temperature Fig. 4. Vapour density versus temperature Fig. 8. Vapour viscosity versus temperature Fig. 5. Liquid specific heat versus temperature Fig. 6. Vapour specific heat versus temperature C. Behavior of refrigerant mixture The mixture composed of R32/R125/R600a is ternary zeotropic mixture. When zeotrope evaporates inside the tubes, more volatile components in the mixture evaporates first and it becomes rich in less volatile component (R600a). Because of increase in less volatile component (R600a) in the mixture, the saturation temperature gets decreased; as a consequence the pressure drop in the evaporator is compensated [49]. The low volatile component (R600a) in the mixture is miscible with mineral oil and returns the lubricant back to the compressor. Unlike pure refrigerants, the phase change process of zeotropic mixtures is non isothermal and the compositions do not remain constant, which leads to composition shift and temperature glide [50]. HFC/HC mixtures reduce the flammable nature of HC refrigerants and the GWP of HFC refrigerants. D. Chemical properties The HFC and HC mixtures used in the present analysis are found to be chemically stable and non-reactive with compressor materials and tubes of the refrigeration systems for wide range of operating temperatures in the air conditioners. The main advantage of HC refrigerants are

6 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 17 their solubility with mineral oil, which is commonly used as a lubricant for chlorine based refrigeration systems [51]. E. Environmental properties Ozone depletion potential (ODP) and global warming potential (GWP) are the two major environmental properties considered for choosing an alternative refrigerant. Table 3.3 shows the environmental properties of R22 and new refrigerant mixtures. The ODP of the mixtures are found to be zero due to the absence of chlorine. The GWP of HC/HFC mixtures is less than one third of HFC, when it is used alone [52]. The GWP values of NRM10 is found to be higher than that of R22 by about 6%, whereas, NRM20 and NRM30 are having lower GWP by about 6% and 20% than that of R22, respectively. In summary, the properties discussed in this section confirmed that NRMs are found to be the good replacement for R22 in air conditioners. The energy performance of air conditioners is theoretically assessed to investigate their feasibility. Fig. 9. Schematic diagram of compression based refrigeration cycle III. THERMODYNAMIC CYCLE ANALYSIS The detailed configuration of a compression refrigeration systems and its pressure-enthalpy diagrams are shown in Fig. 9 and Fig. 10, respectively. The compression refrigeration system consists of a hermetically sealed reciprocating compressor, a finned tube air cooled condenser, a sealed type refrigerant drier, a capillary tube and a finned tube evaporator. The processes, 1-2, 2-3, 3-4 and 4-1 representing the various processes such as compression, condensation, expansion and evaporation, respectively. The processes 1-1 ', and 3-3' represents the superheating and sub-cooling processes, respectively. The two possible ways to achieve the super heating and sub cooling effects in the air conditioners are (i) by attaching the capillary tube with suction line and (ii) by increasing the length of the condensers and evaporators. The points 1, 2, 3 and 4 represent the thermodynamic state of the refrigerant at compressor inlet (superheated vapor at evaporator pressure), compressor outlet (superheated vapor at condenser pressure), condenser outlet (sub cooled liquid at condenser pressure) and two phase fluid at evaporator pressure. The energy performance and the environmental impacts of an air conditioner are theoretically assessed. The following assumptions are made based on preliminary experimental investigations with one ton capacity compression based refrigeration systems using R22 as refrigerant: (a) Steady state processes within the system, (b) Pressure drop in condenser is 10 kpa and in the evaporator is 20 kpa. (c) Space temperature: Evaporator temperature +15 o C, (d) Isentropic efficiency of the compressor is 75%, (e) Sub cooling in the condenser and super heating in the evaporator are 5 o C, (f) life of the air conditioner: 15 years, leakage rate: 25% every year, Fig. 10. Pressure enthalpy diagram for a compression based air conditioning cycle (g) Charge requirement of R22: 900 g and for the mixtures: 750 g due to its lower liquid density. Based on the above assumptions, the thermodynamic performance of an air conditioner has been assessed with three different condensing temperatures 35, 45 and 55 o C and evaporator temperatures between -10 o C and 10 o C (which covers the medium temperature applications like walk in coolers and air conditioners). IV. TOTAL EQUIVALENT GLOBAL WARMING IMPACT The environmental impact caused by the air conditioner throughout its life can be represented in terms of TEGWI [53]. It is the combination of direct emission caused due to leakage of refrigerants from the systems and the indirect emissions caused in the power plants to power the air conditioners. TEGWI is calculated by the following equations. TEGWI ( GWP L N m) ( GWP m (1 )) +365 N ( n E ) (1) Here, GWP is the global warming potential of refrigerant, L is the leakage rate in the system (in terms of kg/year), m is the mass of refrigerant (in Kg), N is the life of the system (in years), n is the system running time per day, E is the energy consumption per day (in kw.h/year), α is the recycling factor of refrigerants, β is the CO 2 emission factor. In this work, the CO 2 emission factor is assumed as 0.9 (tons of CO 2 ) [54].

7 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 18 V. RESULTS AND DISCUSSIONS The results obtained in the theoretical assessment of a residential air conditioner using R22 and its alternative refrigerant mixtures composed of R32/R125/R600a with three different mass fractions of R600a at three condensing temperatures such as 35, 45 and 55 C with reference to the evaporator temperature of -10 o C are discussed in this section. A. Effect of condensing temperatures 1. Pressure ratio The compressor pressure ratio is the important factor considered for choosing an alternative. It influences the volumetric efficiency of the compressor. Increase in pressure ratio results in decrease in volumetric efficiency of the compressor. The variations of pressure ratio at three condensing temperatures 35, 45 and 55 o C with -10 o C evaporator temperature are shown in Fig. 11. From the figure, it was observed that, NRM20 and NRM30 has higher compressor pressure ratio by about 8-11% and 14-19%, respectively compared to R22. The pressure ratio of NRM10 was found to be lower than that of R22 by about 4.5% for the condensing temperatures between 35 and 55 o C. Increase in mass fraction of R600a will increase the compressor pressure ratio. 2. Compressor discharge temperature The compressor discharge temperature is an important factor considered for choosing an alternative refrigerant. Higher the compressor discharge temperature will reduce the life of the compressor due to thermal degradation of lubricants, causing excessive wear in compressors. The compressor discharge temperature of NRM10, NRM20 and NRM30 against 35 o C, 45 o C and 55 o C condensing temperatures are compared in Fig. 12. From Fig. 12, it was found that compressor discharge temperature of NRM10, NRM20 and NRM30 are found to be lower than that of R22 by about 5-8 o C, 8-12 o C, and o C, respectively, which are similar to R417A and R422D [36-44]. Hence, higher compressor life can be expected with NRMs. The compressor discharge temperature gets increased with increase in condensing temperature due to increase in compressor pressure ratio and ambient temperature. It is also observed that the compressor discharge temperature of the mixture gets decreased with increase in mass fraction of R600a. 3. Volumetric cooling capacity VCC is the major factor considered for choosing the alternative, which influences the compressor size. The variation of VCC of NRMs at three condensing temperatures 35 o C, 45 o C and 55 o C are compared in Fig. 13. From the figure, it was found that VCC of NRM20 was found to be closer to R22 in the range of % for considered condensing temperatures. Hence, NRM20 can be used as drop-in substitute in the existing R22 systems. NRM10 has 16-25% higher VCC, whereas NRM30 has 30-38% lower VCC compared to R22. NRM10 and NRM30 are not possible to use as drop in substitutes due to its mismatch in VCC. The VCC of all the refrigerants gets decreased with increase in condensing temperatures. 4. Coefficient of performance The COPs of the investigated refrigerants against condensing temperatures are compared in Fig. 14. The COPs of NRMs are found to be lower due to its higher vapour pressure and higher vapour density with lower critical temperature compared to R22. The COP of NRM10, NRM20 and NRM30 are found to be lower than that of R22 by about 2-14%, 16-20% and 25-30%, respectively. The COP of the NRM20 is found to be similar to R417A and R422D for refrigeration and air conditioning applications [36-44]. The COP of all the refrigerants is found to be increased with an increase in condensing temperature. 5. Compressor power consumption The compressor power consumption of R22 and NRMs versus condensing temperature is compared in Fig. 15. The compressor power consumption of NRM10, NRM20 and NRM30 are found to be higher than that of R22 by about 46-48%, 19-22% and 2-9%, respectively for the condensing temperatures between 35 and 55 o C due to its higher vapour pressure and vapour density with lower critical temperature. The compressor power consumption of all the investigated refrigerants is found to be increased with increase in condensing temperature. The compressor power consumption can be reduced by using higher speed blower to increase the volume flow rate of air over the condenser coil [55]. 6. Condenser heat rejection Fig. 16 depicts the variation in condenser heat rejection with reference to condensing temperatures between 35 and 55 o C. The new refrigerant mixtures NRM10 and NRM20 have about 33% and 14% higher condenser heat rejection compared to R22 due to its higher mass flow rate through the condenser. Hence, the size of the condenser needs to be modified to accommodate the increase in heat rejection. Whereas, NRM30 has about 26% lower condenser heat rejection compared to R22 due to its lower condenser heat rejection. The rate of heat rejection is controlled by optimizing the condenser face area and volume flow rate of air [55]. B. Effect of R600a mass fraction The presence of R600a in the NRMs overcomes the miscibility issue of R32/R125 mixture with mineral oil and also improves the thermodynamic and thermo-physical properties. The presence of R600a in the NRMs has reduced its GWP. However, the compressor pressure ratio has major influence with increase in R600a mass fraction, which affects the volumetric efficiency of the refrigeration compressor. The higher compressor ratio has also influence the condenser pressure and compressor power consumption, which leads increase in global warming potential. Liquid receiver is suggested in the refrigerant circuit after the condenser to control the condenser pressure.

8 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 19 Fig. 11. Pressure ratio versus condensing temperature at -10 o C evaporator temperature Fig. 14. Coefficient of performance versus condensing temperature at - 10 o C evaporator temperature Fig. 12. Compressor discharge temperature versus condensing temperature at -10 o C evaporator temperature Fig. 15. Compressor power consumption versus condensing temperature at -10 o C evaporator temperature Fig. 13. Volumetric cooling capacity versus condensing temperature at - 10 o C evaporator temperature Fig. 16. Condenser heat rejection versus condensing temperature at -10 o C evaporator temperature Amount of heat rejection in the condenser is gets reduced with increase in mass fraction of R600a. The VCC of NRMs gets reduced with increase in R600a mass fraction. The VCC of the NRM20 is found to closer to R22. The mass flow rate of the refrigerant is also gets reduced with increase in R600a

9 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:02 20 mass fraction due to its reduction in vapour density at compressor suction. Based on the performance comparisons discussed in this section, NRM20 is identified as an interim R22 replacement in existing compression based systems to extend its life. C. Total equivalent global warming impact The environmental impacts of refrigerants caused by the system are estimated for fifteen years life time. The direct emissions CO 2 due to the emission of refrigerants are depicted in figure 24. NRM10, NRM20 and NRM30 are having lower direct emissions by about 13%, 23% and 32%, respectively due to its lower mass charge requirement in the system and its lower GWP. Similarly, the NRM10, NRM20 and NRM30 are having higher indirect emissions by about 47%, 22% and 4%, respectively due its higher power consumption, which are illustrated in figure 25. TEGWI of the two refrigerants are compared in figure 26. TEGWI of NRM10, NRM20 and NRM30 are found to be higher than that of R22 by about 42%, 20% and 5%, respectively due to its higher compressor power consumption, which is similar to the earlier reported work for R422D [42]. However, zero ODP of the refrigerant mixtures makes it feasible to use it in exiting R22 based refrigeration systems to extend its life. The performance of the system can be improved by optimizing the compressor and condenser operating parameters. Fig. 17. Direct CO 2 emissions from compression refrigeration system at different leakage rates VI. CONCLUSION Following conclusions are drawn based on the theoretical computations with R32/R125/R600a mixtures and R22 in a compression based refrigeration systems. NRM20 is identified as an alternative to R22 for compression based refrigeration and air-conditioning applications due to its good thermodynamic and thermophysical properties. The compressor pressure ratio of new refrigerant mixture NRM20 was found to be higher than that of R22 by about 6-11%, which may influence the volumetric efficiency of the compressor. The NRM20 has lower compressor discharge temperature in the rage of 6-11 o C, which ensures better compressor life compared to R22. Volumetric cooling capacity of the NRM20 was found to be lower than that of R22 by about 4-9% for the evaporator temperatures between -10 o C and 10 o C. The COP of the mixture was found to be lower than that of R22 by about 16-20% across the wide range of evaporator temperatures between -10 o C and 10 o C. Refrigerant mass flow rate of NRM20 is found to be higher than that of R22 by about 2-9% across the wider range of evaporator temperature temperatures, which may increase the compressor power consumption. Fig. 18. Indirect CO 2 emissions from compression refrigeration system at different evaporator temperatures with 10% leakage every year Fig. 19. TEGWI from compression refrigeration system at different evaporator temperatures with 10% leakage every year TEGWI of NRM20 was found be higher than of R22 by about 20%. The ODP of the new refrigerant mixtures are found to be zero due to the absence of chlorine. The results discussed in this section confirmed that NRM20 is found to be ozone friendly drop-in substitute for replacing R22

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