Question

A cooling plant adopted a two-stage cascade refrigeration system to meet the required cooling load. In both upper and lower cycles, the refrigerant leaves condenser as saturated liquid and enters compressor as saturated vapor. The isentropic efficiency of the upper cycle compressor is 80 %, while the lower cycle compressor is 70% efficient. The pressure limits of the upper and lower cycles are 1 MPa and 0.2 MPa with refrigerant-134a, respectively. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where the lower cycle enters about at 0.5 MPa and the upper cycle enter about at 0.32MPa. If the mass flow rate of the refrigerant in the upper cycle is 0.28 kg/s, determine

i. the mass flow rate of the refrigerant in the lower cycle

ii. the rate of heat removal from the refrigerated space and the power input into the compressors

iii. the COP of the refrigerator

Answer 1