Refrigeration Questions and Answers – Effect of Various Parameters on VCR System – 2

This set of Refrigeration MCQs focuses on “Effect of Various Parameters on VCR System – 2”.

1. What is the use of a flash chamber in VCR system?
a) To increase pressure
b) To decrease pressure
c) To separate vapour and liquid refrigerant
d) To evaporate refrigerant partially

Explanation: Flash chamber is used to separate vapour and liquid refrigerant. It is an insulated container and separates both forms by using the centrifugal effect.

2. What is the amount of mass of vapour refrigerant flowing directly from flash chamber to suction of the compressor?

a) m1
b) m2
c) m1 + m2
d) m2 – m1

Explanation: As, m1 = Mass of the liquid refrigerant supplied to the evaporator
m2 = Mass of refrigerant (liquid and vapour) circulating through the condenser
Hence, the amount of mass flowing through the flash chamber to the compressor is given by,
= m2 – m1.

3. What is the ratio of the mass of refrigerant supplied to evaporator and mass of refrigerant circulating through condenser i.e., m1 / m2?

a) h1 – h2 / h1 – h4
b) h1 – h4 / h1 – hf4’
c) h2 – h1 / h4 – h1
d) h1 – hf4’ / h1 – h4

Explanation: As flash chamber is an insulated cylinder. Heat transfer between flash chamber and surroundings is zero. So, by considering thermal equilibrium of flash chamber,
Heat taken by the flash chamber = Heat given out by the flash chamber
m2 h4 = m1 hf4’ + (m2 – m1) h1
m2 (h1 – h4) = m1 (h1 – hf4’)
m1 / m2 = [h1 – h4 / h1 – hf4’].

4. What is the use of accumulator or pre-cooler in VCR system?
a) To supply dry saturated vapour and pure liquid refrigerant to the respective equipment
b) To decrease pressure
c) To increase pressure
d) To condense refrigerant partially

Explanation: Accumulator or pre-cooler does almost the same work as a flash chamber. It receives the discharge of a mixture of liquid and vapour refrigerant from expansion valve and supplies liquid refrigerant only to the evaporator. This discharge again sent to the accumulator, and only dry saturated vapour refrigerant is further sent to the compressor.

5. What is the use of a liquid pump in the VCR system containing accumulator?

a) To maintain the circulation of the refrigerant
b) To provide liquid refrigerant
c) To expand the liquid refrigerant
d) To pre-evaporate the liquid refrigerant

Explanation: Accumulator separates the mixture of refrigerants, and provides dry saturated vapour to the compressor and pure liquid refrigerant to the evaporator. So, to maintain the circulation of the liquid refrigerant in the evaporator, a liquid pump is used.

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6. What is the ratio of the mass of refrigerant supplied to evaporator and mass of refrigerant circulating through condenser i.e., m1 / m2?

a) h1 − h2 / h1 − h4
b) h1 − h4 / h1’ − hf4’
c) h2 − h1 / h4 − h1
d) h1’ − hf4’ / h1 − h4

Explanation: As accumulator is an insulated vessel. Heat transfer between flash chamber and surroundings is zero. So, by considering thermal equilibrium of flash chamber,
Heat taken by the accumulator = Heat given out by the accumulator
m2 h4 + m1 h1’ = m2 h1 + m1 hf4’
m1 (h1’ − hf4’) = m2 (h1 − h4)
m1 / m2 = [h1 − h4 / h1’ − hf4’].

7. What is the use of heat exchanger in the following schematic?

a) Compression
b) Expansion
c) Subcooling of liquid by vapour refrigerant
d) Subcooling of vapour by liquid refrigerant

Explanation: Liquid leaves condenser at a higher temperature than vapour leaving the evaporator. So, liquid refrigerant leaving the condenser is able to be subcooled by passing it through a heat exchanger which is supplied with the vapour refrigerant from the evaporator. Hence, this heat exchanger is used to do the subcooling of liquid refrigerant by the vapour refrigerant.

8. What is the ratio of the mass of the vapour refrigerant and mass of the liquid refrigerant i.e., m1 / m2?

a) 1
b) h1 − h4 / h1’ − hf4’
c) 0
d) h1’ − hf4’ / h1 − h4

Explanation: In the case of this kind of heat exchanger, heat transfer takes place between vapour and liquid refrigerant due to the change of temperature and specific heat. As the mass flowing through condenser and evaporator is same. Due to heat transfer, mass does not get affected. Hence,
m1 = m2 or m1 / m2 = 1.

9. What is the use of heat exchanger in the following schematic?

a) Compression
b) Subcooling of liquid by liquid refrigerant
c) Subcooling of liquid by vapour refrigerant
d) Subcooling of vapour by liquid refrigerant

Explanation: Liquid leaves the condenser at a higher temperature than liquid leaving the expansion valve. So, the liquid refrigerant leaving the condenser is able to be subcooled by passing through the heat exchanger which is supplied with the liquid refrigerant from the expansion valve. Hence, a heat exchanger is used to do the subcooling of liquid refrigerant at a higher temperature by the liquid refrigerant at a lower temperature.

10. What is the amount of mass of refrigerator leaving the evaporator?

a) m2 − m1
b) m1 − m3
c) m2 + m3
d) m2 − m3

Explanation: As, m1 = Mass of refrigerant leaving the evaporator
m2 = Mass of liquid refrigerant passing through the condenser
m3 = Mass of liquid refrigerant supplied to the heat exchanger from the expansion valve
By taking the help of schematic,
Mass of refrigerator leaving the evaporator, m1 = m2 − m3.

11. What is the ratio of the mass of liquid refrigerant passing through the condenser and mass of liquid refrigerant supplied to the heat exchanger i.e., m3 / m2?

a) h1 − h2 / h1 − h4
b) h1 − h4 / h1’ − hf4’
c) hf3 − hf3’ / h1 − hf4’
d) h1 − hf4’ / hf3 − hf3’

Explanation: Considering the thermal equilibrium of the heat exchanger,
Heat lost by liquid refrigerant from condenser = Heat gained by liquid refrigerant from expansion valve
m2 hf3 + m3 hf4’ = m3 h1 + m2 hf3’
m2 (hf3 − hf3’) = m3 (h1 − hf4’)
m3 / m2 = [hf3 − hf3’ / h1 − hf4’].

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

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