An electric motor drives an air compressor that delivers air

An electric motor drives an air compressor that delivers air with inlet and outlet conditions as shown on the figure. The motor-compressor set operates at SteadyState conditions. The electric power supplied to the motor is 25 kilowatts at 220 volts ac. and the measured heat transfer rate from the combined motor and compressor is 4.4 kilowatts. Experience shows that changes in kinetic and potential energy are negligible for this system. Determine the mass flow rate through the compressor in kilograms per second. Assuming a power factor of 1 (purely resistive circuit), calculate the electric current drawn by the motor, in amps.

Solution

Power supplied to the motor = 25KW = 25KJ/sec

Heat transfer rate from the combined motor and compressor = 4.4 KW

Enthalpy at the input of the compressor H1=302KJ/Kg

Enthalpy at the output of the compressor = 503KJ/Kg

a) Mass flow rate through the compressor = Work done/(H2-H1)

                                                            = (25-4)/(503-302) = 0.1044Kg/sec