A three-cycle cascade refrigeration unit is to use methane (cycle 1), ethylene (cycle 2), and ammonia (cycle 3). The evaporators are to operate at: cycle 1, 115.6 K; cycle 2, 180 K; cycle 3, 250 K. The outlets of the compressors are to be: cycle 1, 4 MPa; cycle 2, 2.6 MPa; cycle 3, 1.4 MPa. Use the Peng-Robinson equation to estimate fluid properties. Use stream numbers from Fig. 5.11. The compressors have efficiencies of 80%.

(a) Determine the flow rate for cycle 2 and cycle 3 relative to the flow rate in cycle 1.

(b) Determine the work required in each compressor per kg of fluid in the cycle.

(c) Determine the condenser duty in cycle 3 per kg of flow in cycle 1.

(d) Suggest two ways that the cycle could be improved.

Transcribed Image Text:

8 Hot Side Condenser XThrottle Valve 5 4 Evaporator/ Condenser XThrottle Valve 1 Evaporator Cold Side Compressor 6 3 Compressor 2 12 Throttle Valve 9 8 Hot Side Condenser 4 XThrottle Valve 5 Evaporator/ Condenser 1 Evaporator/ Condenser Throttle Valve Evaporator Cold Side 11 Compressor 10 7 Compressor 6 3 Compressor 2 Figure 5.11 Binary cycle (left) and three-cycle cascade (right) refrigeration cycles. The refrigerants do not mix in the evaporator/condensers.