A refrigeration machine uses R134a as the refrigerant. The refrigerant enters the adiabatic compressor as saturated vapor at 120 kPa and exits the compressor at 800 kPa and 50°C. The refrigerant exits the condenser as saturated liquid at 800 kPa. Then, the refrigerant passes through an adiabatic expansion valve that reduces the pressure back to the evaporator pressure of 120 kPa. The refrigerant mass flow rate is 0.025 kg/s. The refrigeration system is designed to maintain a room at -15°C and rejects heat to the surroundings at 20°C. Tasks: a) Calculate the isentropic efficiency of the compressor and draw a T-s diagram of the cycle.b) Calculate the second-law efficiency of the compressor.c) Calculate the exergy destruction rate in the evaporator in kW.d) Calculate the coefficient of performance (COP) and the second-law efficiency of the cycle. What is the minimum power requirement (in kW) needed to achieve the same cooling effect between -15°C and 20°C?
A refrigeration machine uses R134a as the refrigerant. The refrigerant enters the adiabatic compressor as saturated vapor at 120 kPa and exits the compressor at 800 kPa and 50°C. The refrigerant exits the condenser as saturated liquid at 800 kPa. Then, the refrigerant passes through an adiabatic expansion valve that reduces the pressure back to the evaporator pressure of 120 kPa. The refrigerant mass flow rate is 0.025 kg/s.
The refrigeration system is designed to maintain a room at -15°C and rejects heat to the surroundings at 20°C.
Tasks:
a) Calculate the isentropic efficiency of the compressor and draw a T-s diagram of the cycle.
b) Calculate the second-law efficiency of the compressor.
c) Calculate the exergy destruction rate in the evaporator in kW.
d) Calculate the coefficient of performance (COP) and the second-law efficiency of the cycle. What is the minimum power requirement (in kW) needed to achieve the same cooling effect between -15°C and 20°C?
Step by step
Solved in 2 steps with 6 images
