Answer Happy

The figure below shows a two-stage vapor-compression refrigeration system with ammonia as the working fluid. The system uses a direct-contact heat exchanger to achieve intercooling. The evaporator has a refrigerating capacity of 25 tons and produces -20°F saturated vapor at its exit. In the first compressor stage, the refrigerant is compressed adiabatically to 80 lbf/in.², which is the pressure in the direct contact heat exchanger. Saturated vapor at 80 lbf/in.² enters the second compressor stage and is compressed adiabatically to 250 lbf/in.² Each compressor stage has an isentropic efficiency of 85%. There are no significant pressure drops as the refrigerant passes through the heat exchangers. Saturated liquid enters each expansion valve. a) Determine the ratio of mass flow rates, i.e. the ratio of the mass flow rate through the second stage compressor to the mass flow rate through the first stage compressor. b) Determine the power input to each compressor stage, in horsepower. c) Determine the coefficient of performance. d) Plot the T-s, p-h, and T-h diagrams for this cycle. (You may use EES to get all the data involved, then use Excel to plot the diagrams. The saturated liquid and saturated vapor lines should be presented on the diagrams.) e) Plot each of the quantities calculated in parts a) - c) versus the direct-contact heat exchanger pressure ranging from 20 to 200 lbf/in.². (25 points) Cout 5+ Expansion valve Condenser Direct contact heat exchanger 7+ Expansion valve Comp Qin 3 Comp Evaporator