Calculate the mass flow rate of CO2 and the C.O.P of the system.

Question

A cascade refrigeration system of 100 tons (350 kW) capacity uses ammonia and carbon dioxide the evaporating and condensing temperatures of CO₂ are         –40°C and 5°C, respectively. The evaporating temperature of NH₃ is –7°C. The power supplied to the ammonia compressor is 96.5 kW. In the CO₂ circuit, the liquid leaving the condenser is saturated, the vapor leaving the evaporator is dry and saturated, and compression is isentropic. Calculate the mass flow rate of CO₂ and the C.O.P of the system. The following data for CO₂ may be used:

Temperature °C	Pressure, bars	Enthalpy, kJ/kg		Entropy, kJ/kg K
		Liquid	Vapor	Liquid	Vapor
–40	10.05	332.7	652.8	3.8531	5.2262
5	39.70	43.10	649.8	4.2231	5.0097

Specific heat of superheated vapor            cp = 0.85 kJ/kg K.