Enthalpy of Vaporization:

This must be as large as possible to reduce the area under superheat & the area reduction because of throttling. Also, the higher value of enthalpy of vaporization lowers the needed flow rate per ton of refrigeration.

Considering these three factors into account, the T-s & p-h diagrams of an ideal refrigerant would be as indicated in given (a) and 4.3 (b).

 (a) T-S Plot of an Ideal Refrigerant     (b) p-h Plot of an Ideal Refrigerant

Practically these properties are not found in any refrigerant. Therefore, a trade-off has to be done to attain as high a COP as possible.