The value of COP in vapour compression cycle is usually:
A) Always less than unity
B) Always more than unity
C) Equal to unity
D) None of the above

Hint:COP is short for the coefficient of performance. It is the ratio of cooling or heating to the work done. COP is usually associated with heat pumps, air conditioners and refrigerators. It is similar to the efficiency of a heat engine. The COP gives the efficiency of a system.

Complete step by step solution:
The refrigerant will undergo a phase change from liquid to vapour phase and from vapour to liquid phase in a vapour compression system. This phase change is done by absorbing heat in the evaporator and by ejecting heat at the condenser. If the system has higher COP it will have lower operating costs.

COP will be more than one in heat pumps because all the work will be converted into heat that itself gives $100\% $ efficiency. For vapour compression cycles the COP will be always greater than unity. Because the value of COP being less than one means that the efficiency of the system will be greater than one. Since having an efficiency greater than $100\% $ is impractical, the COP of a vapour compression cycle is always taken to be more than unity.

The answer is Option B. Always more than unity.

Note:Coefficient of performance will depend on the operating conditions, especially the relative temperature and the absolute temperature of the system and the sink. COP can be improved by reducing the gap between the hot and the cold temperatures of the system. For refrigeration, cycle COP will be the ratio between energy usage and the amount of useful cooling.