This set of Refrigeration Multiple Choice Questions & Answers (MCQs) focuses on “Actual VCR Cycle”.
1. Which of the following is not the difference between theoretical and actual VCR cycle?
a) Pressure drops in the evaporator
b) Pressure drops in the condenser
c) Compression of refrigerant is never polytropic
d) Compression of refrigerant is always isentropic
Explanation: Entropy is the randomness of molecules. Change in entropy cannot be kept constant. As any transfer of energy will provide energy to the particles and increasing the entropy. The entropy of the Universe tends to increase day by day. Hence, the compression process cannot be executed in an isentropic manner. Ideally, it can be assumed, but in the actual cycle, it is impossible to attain such a process.
Explanation: Process 2-3 represents evaporation with superheating. The shift of exit of evaporation from point 2 to point 3 is due to
1) Automatic control of expansion valve
2) Absorbs a larger amount of heat
3) Refrigeration effect and work increases.
Explanation: Due to the frictional resistance offered to the vapor refrigerant entering the compression system pressure drops from point 3 to point 4. Thus, actual suction pressure (PS) lower than the evaporator pressure (PE).
Explanation: After pressure reduction and before compression begins, the temperature of cold refrigerant comes in contact with compression cylinder walls, which are at a higher temperature. Hence, due to heat transfer, the temperature of the refrigerant rises to point 5, which is the heating effect.
Explanation: Actual compression is denoted by process 5-6, which is neither isentropic nor polytropic, due to heat transfer between cylinder walls and vapor refrigerant. The temperature of cylinder walls is between cold suction vapor refrigerant and hot discharge vapor refrigerant. Pressure and temperature are increased in this process.
Explanation: After compression and before Condensation begins, the temperature of hot refrigerant comes in contact with compression cylinder walls which are at a lower temperature. Hence, due to heat transfer, the temperature of the refrigerant falls to point 7, which is a cooling effect.
Explanation: Due to the frictional resistance offered to the vapor refrigerant discharging the compression system pressure drops from point 7 to point 8. Thus, actual discharge pressure (PD) higher than the condenser pressure (PC).
Explanation: Before condensing the refrigerant, to reject maximum heat to get stabilized state of refrigerant, the refrigerant must be at saturated state before condensing. Hence, de-superheating is carried out to get the refrigerant at a saturated level and then condensed.
Explanation: Process 9-10 shows the removal of latent heat where the dry saturated refrigerant is converted into a liquid refrigerant. This process is called Condensation.
Explanation: The process 10-11 represents sub-cooling of the liquid refrigerant in the condenser before it is expanded. Sub-cooling is carried out as it increases the refrigerating effect per kg of the refrigerant flow. It does reduce the volume of refrigerant partially evaporated from liquid refrigerant while going through the expansion valve. The refrigerating effect can be increased by circulating a large amount of water, which is at a much lower temperature than condensing temperature.
Explanation: Process 11-1 represents expansion. The expansion of sub-cooled liquid refrigerant is carried out by throttling (keeping enthalpy constant) from the condenser pressure to the evaporative pressure.
Explanation: Process 1-2 represents evaporation. In this process, absorption of heat is carried out and converting liquid refrigerant to saturated vapor condition. The evaporation process is known as Refrigeration effect.
13. What is the effect of a decrease in suction pressure on C.O.P.?
a) C.O.P. increases
b) C.O.P. decreases
c) C.O.P. remains the same
d) C.O.P. becomes zero
Explanation: As C.O.P. is the ratio of refrigeration effect to work done. Due to the decrease in the suction pressure, the refrigeration effect is decreases and work required for compression increases for the same amount of refrigerant flow. Hence, resulting in a reduction of the C.O.P. of the system and refrigeration cost also increases.
14. What is the effect of an increase in discharge pressure on C.O.P.?
a) C.O.P. decreases
b) C.O.P. increases
c) C.O.P. remains the same
d) C.O.P. becomes zero
Explanation: As C.O.P. is the ratio of refrigeration effect to work done. Due to the increase in the discharge pressure, the refrigeration effect decreases for the same amount of refrigerant flow. Hence, resulting in a small decrease in the C.O.P. of the system.
15. Which of the following factor of the actual VCR affects severely the coefficient of performance?
a) Increase in suction pressure
b) Decrease in discharge pressure
c) Decrease in suction pressure
d) Increase in discharge pressure
Explanation: As C.O.P. is the ratio of refrigeration effect to work done. Due to the increase in the discharge pressure, refrigeration effect decreases, but it is not that severe as is in the case of a decrease in suction pressure. Due to the decrease in the suction pressure, the refrigeration effect severely decreases, and work required for compression increases for the same amount of refrigerant flow. Hence, resulting in a severe decrease in the C.O.P. of the system and refrigeration cost also increases.
Sanfoundry Global Education & Learning Series – Refrigeration & Air Conditioning.
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