# Refrigeration Questions and Answers – Theoretical VCR with Superheated Vapour before and after Compression

This set of Advanced Refrigeration Questions and Answers focuses on “Theoretical VCR with Superheated Vapour before and after Compression”.

1. The net effect of superheating is to have ________ COP.
a) higher
b) lower
c) moderate
d) higher or lower depending on the refrigerant used

Explanation: Superheating increases the refrigeration effect as well the work done in compressor, as the increase in refrigeration effect is less than the increase in work done for some refrigerants and more in other refrigerants, the net effect of superheating is to increase or decrease the COP.

2. The COP of vapor compression when vapor is superheated after compression having refrigeration effect (Re) of 360 and work done (W) of 40 is?
a) 8
b) 6.7
c) 7.8
d) 9

Explanation: The COP is given by Refrigeration effect by work done,
COP=$$\frac{Re}{W}=\frac{360}{40}$$=9.

3. What is the mass flow rate of the refrigerant when the heat required is 100 MJ/hr, enthalpy at discharge H2 is 434.5 KJ/kg and enthalpy of liquid Hf3 in KJ/kg at point 3 in p-h diagram is 271.97?
a) 10.254 kg/hr
b) 15.56 kg/hr
c) 10.254 kg/min
d) 15.56 kg/min

Explanation: mass flow rate = $$\frac{Q}{H_2 – Hf_3} = \frac{100×1000}{434.5 – 271.97}$$
= 10.254 kg/min.

4. The cycle in which the evaporation starts at point 4 and continues up to point 1 where it is dry-saturated is called?

a) Theoretical VCR with dry vapor compression
b) Theoretical VCR with wet vapor compression
c) Theoretical VCR with super – heated vapor after compression
d) Theoretical VCR with super – heated vapor before compression

Explanation: In theoretical VCR with super – heated vapor before compression, the vapor is superheated before entering the compressor up to point 1.

5. What is the COP of performance of the plant when the vapor is superheated 5 degree C before entering the compressor and
H1 – Enthalpy before compression = 180
H2 – Enthalpy after compression = 215
Hf3 – Enthalpy at the end of condensation = 55
H4 – Enthalpy at the end of expansion = 55

a) 5.7
b) 4.8
c) 4.7
d) 3.5

Explanation: The COP of performance of the plant when the vapor is superheated 5 degree C before entering the compressor = $$\frac{H_1 – Hf_3}{H_2 – H_1}=\frac{180 – 55}{215 – 180}$$ = 3.57.

6. In domestic vapor compression refrigeration, the refrigerant commonly used is?
a) Freon – 12
b) CO2
c) H2O
d) Ammonia

Explanation: In domestic vapor compression refrigeration, the refrigerant commonly used is Freon – 12 because of its safe properties, stability, miscibility with oil and suitable to wide range of operating temperatures.

7. Superheating before compression increases the refrigeration effect.
a) True
b) False

Explanation: Superheating before compression increases the refrigeration effect and the work done as well, as the superheating increases the enthalpy of the vapor entering the compressor and thus increases the refrigerating effect.

Sanfoundry Global Education & Learning Series – Refrigeration & Air Conditioning.

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