This set of Refrigeration Problems focuses on “Theoretical VCR with Under Cooling or Sub-Cooling”.
1. What type of VCR is shown in the T – s diagram?
a) Theoretical VCR with sub-cooling or under cooling
b) Theoretical VCR with wet before compression
c) Theoretical VCR with wet after compression
d) Theoretical VCR with dry compression
View Answer
Explanation: In the shown T – s diagram the refrigerant after condensation process is cooled below the saturation temperature before expansion by throttling. This type of process is called undercooling or subcooling of the refrigerant.
2. The ultimate effect of undercooling is to increase the value of COP under same set of conditions.
a) True
b) False
View Answer
Explanation: It is true that the ultimate effect of undercooling is to increase the value of COP under same set of conditions as the refrigeration effect is increased using superheating and undercooling processes.
3. What is the COP of a theoretical VCR having an undercooling of 5° C. Use the following data.
Enthalpy H1 = 180.5,
Enthalpy H2 = 194.5,
Enthalpy Hf3′ = 45.5,
Specific heat at constant pressure for superheated liquid = 0.95KJ/kg K.
a) 12.332
b) 9.982
c) 8.973
d) 9.879
View Answer
Explanation: We know that enthalpy of liquid refrigerant at point 3,
Hf3 = Hf3′ – cpl × degree of undercooling
Hf3 = 45.5 – 0.95 × 5
= 40.75
COP = \(\frac{H_1 – H_{f3}}{H_2 – H_1} = \frac{180.5 – 40.75}{194.5- 180.5}\) = 9.982.
4. The process of under cooling the refrigerant below the saturation temperature happens after?
a) Compression
b) Before condensation
c) Evaporation
d) Before throttling
View Answer
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
5. 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
View Answer
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.
6. 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
View Answer
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.
7. 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
View Answer
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.
8. 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
View Answer
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′).
Sanfoundry Global Education & Learning Series – Refrigeration & Air Conditioning.
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