What is the relation between Pressure and Temperature of Refrigerant?

Before jumping to answer this question we should discuss some basics.The pressure & condensing temperature of any vapor (say refrigerant here) are directly proportional to each other when the volume is fixed. What does it mean? It means that if the pressure of the enclosed system( refrigeration system) increases, the condensing temperature of the refrigerant vapor also increases. The below chart for R-134a refrigerant (refrigerant gas used in air conditioner) lists out the condensing temperatures for various pressures. Did you notice that at 22psig the condensing temperature is -62.38 Degree F and at 37psig it is 42 Degree F? The temperature is increasing in relation to pressure.

Now the question is, why we have to increase the pressure of refrigerant?

The refrigerant has to perform two major functions.

• It should absorb the heat from the room air which is at 23 Degree C.
• It should discharge the heat absorbed from the room to the ambient which will be around 35 to 45 Degree C.

To perform the first function, the refrigerant should be less than room air by at least 9 to 10 Degree C i.e 23–10 = 3 Deg C or 37.4 Degree F. The corresponding pressure should be 33 psig.

To perform the second function, the refrigerant should be more than ambient air by at least 13 to 15 Degree C i.e 35+15 = 50 Deg C or 122 Degree F. The corresponding pressure should be 180 psig.

Summary

To discharge the heat which is absorbed from the room, the refrigerant temperature should be above the ambient air temperature. To increase the temperature of refrigerant we should increase the pressure of the refrigerant by an equipment called compressor.