Throttling vs. Isentropic Compression:

In the Carnot vapour compression cycle, there is isentropic expansion through state 4 to state 5. It is attained by the utilization of a turbine. Though, in real cycles, the expansion from saturated liquid on state 4 to liquid-vapour mixture on state 5 generated very little work. A turbine functioning under such conditions would have very low efficiency that would not justify the expense involved in utilizing a turbine. Also, the refrigeration system would become very bulky & not appropriate for domestic use.

In real practice, an expansion valve is utilized to attain the desired expansion from state 4 to state 5. The refrigerant throttled in the expansion valve from saturated liquid to liquid-vapor mixture. The expansion not remains isentropic longer. Now the expansion becomes an isenthalpic procedure.

Therefore, we illustrate that the Carnot vapour refrigeration cycle is not appropriate for use in refrigeration systems. A better ideal cycle is the vapour compression refrigeration cycle.