A refrigeration unit operating on R-134a is used to maintain a freezer at -15°C. The refrigeration unit operates at a suction pressure of 140kPa and a discharge pressure of 900kPa. The condenser of the refrigeration unit is cooled by a water cooled heat exchanger. Cooling water flows through the condenser heat exchanger at a rate of 50kg/min, entering at a temperature of 20°C and leaving at a temperature of 29 C. It is known that the refrigeration unit can be modelled as an ideal vapour compression cycle for the heat rejection, throttling and heat absorption stages; however the compressor is known to be non-isentropic. The discharge temperature from the compressor is 60°C.
i) Determine the mass flow rate of the refrigerant, assuming a constant specific heat for the cooling water of Cwater=4.184 kJ/(kg.K).
ii) What is the required power input to the compressor and the isentropic efficiency of the compressor?