1. The process of under cooling the refrigerant below the saturation temperature happens after?
a) Compression
b) Before condensation
c) Evaporation
d) Before throttling
Explanation: We can see from the below diagrams, of theoretical VCR with under cooling that the under cooling process takes place before throttling.
T – s diagram
P – h diagram
2. The process of under cooling can only be brought about by circulating more quantity of cooling water through the condenser or by using water colder than the main circulating water.
a) True
b) False
Explanation: There are various processes by which under cooling can be brought about some of them are by circulating more quantity of cooling water through the condenser, by using water colder than the main circulating water and employing a heat exchanger.
3. What effect does the under cooling process have on piston displacement of the compressor?
a) Increases
b) Reduces
c) No effect
d) Can’t say from the given data
Explanation: Though the compressor work remains same per kg of refrigerant, but, it reduces the compressor power per ton of refrigeration due to reduced mass flow rate of refrigerant. Also it reduces the piston displacement of the compressor.
4. The effect of liquid sub cooling is less pronounced in case of?
a) Water
b) CO2
c) NH3
d) Effect is same for all the liquids
Explanation: The effect of liquid sub cooling is less pronounced in case of NH3 as refrigerant, since it has large latent heat of vaporization as compared to its liquid specific heat.
5. The difference between the saturation temperature T3 of liquid refrigerant at pressure p3 = p2 and the actual temperature of liquid refrigerant T3′ is called?
a) Degree of freedom
b) Degree of subcooling
c) Degree of superheating
d) Degree of condensation
Explanation: The difference between the saturation temperature T3 of liquid refrigerant at pressure p3 = p2 and the actual temperature of liquid refrigerant T3′ is called degree of sub cooling or degree of undercooling i.e. (T3 – T3′).
6. Which of these is not an effect of superheat in suction vapour?
a) increase in refrigeration effect
b) increase in compression work
c) decrease the capacity of compressor per ton of refrigeration
d) condenser size decreases
Explanation: The effects of superheat in suction vapour are increase in refrigeration effect, increase in compression work, decreasing the capacity of compressor per ton of refrigeration and increasing the condenser size as the heat rejected in the condenser increases.
7. The COP of the cycle may increase or decrease or remain unchanged depending upon the range of pressure and the degree of superheating.
a) True
b) False
Explanation: The COP of the cycle may increase or decrease or remain unchanged depending upon the range of pressure and the degree of superheating as it can affect the refrigeration effect, compression work, heat rejected in condenser etc. and thus the COP.
8. In which case will the refrigeration effect will not increase and the COP of cycle with superheated vapour in suction phase will be less than the COP of ideal cycle?
a) Superheating the vapour from state 1 to 1′
b) Superheating the vapour from state 1 to 2
c) Superheating the vapour from state 1′ to 2′
d) COP always increases when superheating vapour in suction vapour
Explanation: As we can see from the below picture of a cycle with superheated vapour in suction vapour, the refrigeration effect is not increased and thus the COP will be less than that of ideal COP.
9. Which one of the following is not effect of liquid sub cooling?
a) increase the refrigeration effect
b) reduce the piston displacement
c) increase the COP
d) increase the compressor power per ton of refrigeration
Explanation: Sub cooling the liquid reduces the compressor power per ton of refrigeration due to reduced mass flow rate of refrigerant. It also increases the refrigeration effect, reduce the piston displacement and increase the COP.
10. Which one of the following is not effect of change in suction pressure?
a) reduces the net refrigeration effect
b) reduces the mass flow rate of refrigerant per ton of refrigeration
c) reduces the COP
d) increases the compressor power per ton of refrigeration
Explanation: Change in suction pressure reduces the net refrigeration effect thus increasing the mass flow rate of refrigerant per ton of refrigeration.