Analytical computation of thermodynamic performance parameters of actual vapour compression refrigeration system with R 22 , R 32 , R 134 a , R 152 a , R 290 and R 1270

The present work focuses on analytical computation of thermodynamic performance of actual vapour compression refrigeration system by using six pure refrigerants. The refrigerants are namely R22, R32, R134a, R152a, R290 and R1270 respectively. A MATLAB code is developed to compute the thermodynamic performance parameters of actual vapour compression system such as refrigeration effect, compressor work, COP, power per ton of refrigeration, compressor discharge temperature and volumetric refrigeration capacity at condensing and evaporating temperatures of 54.4oC and 7.2oC respectively. Analytical results exhibited that COP of both R32 and R134a are 15.95% and 11.71% higher among the six investigated refrigerants. However R32 and R134a cannot be replaced directly into R22 system. This is due to their higher compressor discharge temperature and poor volumetric capacity respectively. The discharge temperature of both R1270 and R290 are lower than R22 by 20-26oC. Volumetric refrigeration capacity of R1270 (3197 kJ/m3) is very close to that of volumetric capacity of R22 (3251 kJ/m3). Both R1270 and R290 shows good miscibility with R22 mineral oil. Overall R1270 would be a suitable ecofriendly refrigerant to replace R22 from the stand point of ODP, GWP, volumetric capacity, discharge temperature and miscibility with mineral oil although its COP is lower.


Introduction
Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs) was extensivenly used in vapour compression based refrigeration industries from past several years due to their better thermophysical and thermodynamic properties [1].However CFCs and HCFCs contains ODP (ozone depletion potential) and high GWP (global warming potential) and hence CFCs and HCFCs were eventually phase out in all the developing nations by the year 2010 and in all the developed nations by the year 2030 respectively in accordance to Monetreal protocal [2][3].Therefore it is essential to develop new ecofriendly refrigerants.
Formerly several theoretical and expermintal studies were carried to find an appropriate fluids to replace R22 and R134a respectively.Experimental peformance tests were carried out in a vapour compression plant with R134a, R152a, R290, R600a, R1234yf and R1234ze(E) respectively [4].Tests results revealed that refrigerants R152a and R1234yf were suitable drop in replacements to R134a.Expermimental studies were done in a vapour compression test setup with R513A and R134a [5].Results reported that superior peformance of R513A would be obtained only with the adjustment of thermostatic expansion valve.Peformance tests were conducted in a heat pump tester under various test conditions with hydrocarbons like R290 and R1270 [6].Experimental results showed that performance of both R1270 and R290 were better than the R22.Particulary R290 exhibited 6-12% higher COP under various test conditions compared to R22.Therotical thermodynamic analyis was carried out in a standard vapour compression system with RE170, R290, R1270, R600a, R32, R134 and R152a respectively [7].Reults exhibited that peformance of RE170 was higher among other studied refriferants and hence it was a viable alternative to R134a.Theoretical and expermintal investiagtions were done in an air conditioner under different operating conditions with R161 and R22 [8].Both theoretical and exprimental results exhibited that energy efficiency ratio of R161 was superior than the R22.A thorough review study reported that hydrcarbons and its mixtures were viable refrigerants to refrigeration related industries [9].Therortical performance studies were carried out in a simple saturation vapour compression system with R1234yf, R161, R152a and R134a respectively [10].Results revealed that R152a would be a better choice to replace R134a from the stand point of COP, power savings, volumetric capacity and GWP.The present work emphasis on performance computation of six pure refrigerants based on actual vapour compression cycle in order to find an appropriate candidate to replace R22.

Properties of investigated refrigerants
In this study six pure refrigerants like R22, R32, R134a, R152a, R290 and R1270 are considered and thermodynamic properties of all the six pure refrigerants are must require to do the thermodynamic analysis of vapour compression refrigeration system.Hence a computer MATLAB program is written to develop the thermodynamic properties of six pure refrigerants by using Martin-Hou equation of state [11]. (1) The developed properties of all the six studied refrigerants shows good agreement with the experimental reported properties available in ASHRAE hand book [12].The deviation of computed properties of all the six pure refrigerants from that of ASHRAE are within 1.5 to 3% for the given operating conditions.Hence the developed MATLAB code is reliable and therefore the same developed thermodynamic properties are used in this study.The basic physical and critical characteristics of six pure refrigerants are taken from the ASHRAE hand book and it is given below in table 1. 3 Thermodynamic performance analysis In the present methodology analytical computation of thermodynamic performance of six pure refrigerants (R22, R32, R134a, R152a, R290 and R1270) is done based on actual vapour compression refrigeration cycle.In this cycle superheating, subcooling, various pressure and heat losses or heat gain occurred at different components of the system are considered and they are given in table 2 and 3

Mathematical computations
The mathematical computations involved in the thermodynamic analysis of investigated refrigerants are given below.
Refrigerant mass flow rate is computed by Isentropic compressor work is computed as Refrigerating effect (Cooling effect) is calculated as (4) .Coefficient of performance (COP) is calculated as (5) Energy (Power) consumption by the compressor per ton of refrigeration is computed by (6) Volumetric cooling capacity is calculated as (7) The compressor discharge temperature (Td) is computed using superheated property tables and interpolating for the degree of superheat corresponding to the entropy difference.

Analytical validation
In the present study a MATLAB code was developed and it is used to compute the thermodynamic performance of R22 and its alternatives.To validate the MATLAB code, results are compared with the previously published data [7,17].Dalkilic AS and Wongwises S has computed the thermodynamic analysis of simple saturation vapour compression system with R22 at Tk = 50 o C and Te=-10 o C whereas Baskaran A and Koshy Mathews P has computed the thermodynamic performance of standard vapour compression system with R134a at Tk = 50 o C and Te=-10 o C by considering degree of superheating and subcooling as 10 o C and 5 o C respectively.For the validation of present MATLAB code, the same refrigerants R22, R134a and operating conditions are used as that of the previous authors.The results of thermodynamic performance parameters of refrigerants R22 and R134a obtained from the MATLAB code are compared with published data results and they are given in table 5 and 6 respectively.The deviation of MATLAB code results with Dalkilic AS, Wongwises S, Baskaran A, and Koshy Mathews P for both the R22 and R134a is less than 1%.Therefore the developed MATLAB code is reliable and hence it can be used for the computation of thermodynamic performance of any refrigerants.

Results and discussions
The results and discussions of various thermodynamic performance parameters of six pure studied refrigerants are given below.

Refrigeration effect
Figure 2 shows the cooling effect (refrigeration effect) of various refrigerants.From the figure 2 it is noticed that refrigeration effect of refrigerants like R32, R152a, R290 and R1270 are higher than the R22.This is due to their high latent heat of vaporization when compared to R22.

Compressor work
Figure 3 shows the compressor work of six studied refrigerants.Referring to figure 3 it is observed that the compressor work of both R290and R1270 refrigerants are higher than the R22.This is due to high vapour enthalpies of refrigerants (R290 and R1270) compared to R22.

Coefficient of performance
Figure 4 shows shows the coefficient of performance of various considered refrigerants.COP is measured as an energy efficiency index of the device charged with particular refrigerant.From the table 4 it is seen that both cooling (refrigeration effect) effect and compressor work increase for the refrigerants R32, R152a, R290 and R1270 respectively.Therefore net effect of refrigeration effect and compressor work of these refrigerants on coefficient of performance may be either increases or decreases or remains same which depends on the type of the refrigerant and also on the operating conditions of the device.
From the figure 4 it is clear that the COP of R32 and R134a is 15.95% and 11.71% higher than R22 respectively.

Compressor discharge temperature
Before introducing the new refrigerant into the device, it is essential to study the steadiness and lifetime of compressor.This can be done by computing the compressor discharge temperature of all the studied refrigerants.Referring to figure 5 it is noticed that compressor discharge temperature of both R1270 and R290 are lower than R22 by 20-26 o C.This is due to lower adiabatic index of refrigerants.The lower compressor discharge temperature is advantageous from the stand point of durability of the compressor life.Hence both R290 and R1270 fluids exhibits better reliability and lifespan of compressor motor.However R32 is not preferable from the stand point of compressor life because its discharge temperature is very high.

Power per ton of refrigeration
Figure 6 shows shows the power (energy) per ton of refrigeration for various studied refrigerants.It indicates the electrical work input required to the compressor to produce per ton of refrigeration.Referring to figure 6 it is observed that energy spent by the compressor per ton of refrigeration for the refrigerant R32 is 13.75 % lower among six investigated refrigerants.This is due to refrigerant type and also due to operating conditions of the given device.

Volumetric cooling capacity
Figure 7 shows shows the volumetric refrigeration capacity of various refrigerants.Volumetric refrigeration capacity depends on density of refrigerant vapour and refrigeration effect of the given fluid.It denotes the cooling capacity per unit volume of vapour refrigerant at the outlet of evaporator .And also it denotes the volume of refrigerant handled by the compressor for the given fluid.From the figure 7 it is evident that volumetric refrigeration capacity of R134a and R152a are lower than R22 and hence these refrigerants requires larger size of compressor whereas volumetric capacity of R32 is very high among six studied refrigerants.Therefore R32 requires redesign or smaller size of compressor.
Volumetric cooling capacity of refrigerant R1270 is very close to R22 and hence same size of compressor can be used for R1270 as that of R22.Volumetric capacity of R290 is in between R22 and R1270.

Conclusions
From the thermodynamic performance computation of six pure refrigerants the following conclusions can be drawn.
 Eventhough COP of R32 and R134a were 15.95% and 11.71% higher among the six investigated refrigerants, they cannot be replaced directly into R22 system.This is due to their higher compressor discharge temperature and poor volumetric capacity respectively. Among the six investigated refrigerants the discharge temperature of both R1270 and R290 were lower than R22 by 20-26 o C. Hence both R290 and R1270

Fig 1 .
Fig 1. P-h diagram of actual vapour compression refrigeration cycle.
respectively[13][14].The p-h diagram of actual vapour compression refrigeration cycle is shown in figure1[15][16].A MATLAB code is developed to compute the thermodynamic performance parameters of six pure refrigerants at ARI conditions (Tk=54.4oC and Te=7.2oC) respectively.The various performance parameters are refrigeration effect, compressor work, coefficient of performance, compressor discharge temperature, power per ton of refrigeration and volumetric refrigeration capacity.The capacity and operating conditions of the system considered for all the six refrigerants are Q=1.5 TR, Tk=54.4oC and Te=7.2oC respectively.

4 bar 3
Pressure drop occurred in the suction line 0.1 bar 4 Pressure loss occurred in the discharge line 0.1 bar 5 Pressure drop in the evaporator 0.1 bar 6 Rise in temperature due to heat gain at the inlet of compressor 5 o C 7 Temperature drop due to heat loss at the outlet of compressor 5

Table 2 .
Explanation of different state points of the cycle

Table 3 .
Description of various losses considered in the cycle

Table 4 .
Summary of results for the pure refrigerants

Table 6 .
Comparison of thermodynamic performance parameters of MATLAB code results withBaskaran A and Koshy Mathews P results for the refrigerant R134a