Comparison of compound refrigeration system with simple vapour compression system
Answers
Vapour compression cycle consists of phase change from gas to liquid in condenser and the compressor pumps liquid phased refrigerant. Making it more efficient as compression work per unit of mass flow is low. Also as the enthalpy of phase change is very high as compared to specific enthalpy, the overall cooling capacity per unit system size is also more making it a compact package. It is generally used in refrigeration purposes and air conditioning in buildings. The general refrigerants are R134a (in case of domestic) , ammonia (in case of industrial).
Air cycle uses air as working fluid and doesn't have phase change in any part of cycle. Meaning air always remains in gas state. Hence the fluid circulation per KW of cooling is very high. Also the compression work is more. One of very few applications which satisfy its suitability is air conditioning in aircraft. Since compressed air is readily available from engine, it is cooled with air flow and then directed in cabin for air conditioning purpose.