Question

A refrigerator using R-12 as the working fluid keeps the refrigerated space at ?16? in an environment at 27?. You are asked to upgrade this refrigerator by replacing R-12 with the ozone-friendly R-134a. What changes in the pressure levels would you suggest in the new system? How do you think the COP of the new system will compare to the COP of the old system?

Answer 1

properties of r-12

At -16C p=175.6 kPa , hf=185.2 KJ/kg hg=345.5 KJ/kg

At 27C p=686.7 kPa hg=364.2 KJ/kg

properties of r-134a

At -16C p=157.3 kPa ,hf=30.69 KJ/kg ,hg=240.87 KJ/kg

At 27C p=707.308 kPa ,hg=413.3 KJ/kg

By changing refrigerent evaporator pressure will reduce while condensor pressure will increase.

Consider latent heat of vaporization of r-12 =160.3KJ/kg

prssure ratio with r-12=3.90

latent heat of vaporization of r134a =210.18

prssure ratio with r134a=4.5

COP is ratio of refrigerating effect to work required.

Now for r134a system refrigerating effect is more mean while work input is more so COP may increase or decrease.