# Refrigeration Questions and Answers – Refrigerant Compressors – Multi – Stage Compression

This set of Refrigeration Multiple Choice Questions & Answers (MCQs) focuses on “Refrigerant Compressors – Multi – Stage Compression”.

1. Why is multi-stage compression used?
a) To improve C.O.P.
b) To decrease the refrigeration effect
c) To increase work
d) To increase leakage loss

Explanation: By expanding the refrigerant very close to the saturated liquid state refrigeration effect can be improved. And if the refrigerant is compressed very near to the saturated vapour line by using compression in more stages, overall work can be reduced. The relative decrease in the overall work done increases the C.O.P. of the system.

2. What is the expression for optimum intercooler or intermediate pressure P2 if the cooling ratio is fixed in a compound compression refrigeration system with intercooling?
a) P2 = P1 / P3
b) P2 = P3 / P1
c) P2 = √P1 x P3
d) P2 = ∛P1 x P3

Explanation: Putting (n / n – 1) = k and dW / dP2 = 0 we get,
T1 P2k – 1 / P1k = T3 P3k / P2k + 1
For the given conditions T1 = T3,
P2 x P2 = P1 x P3
P2 = √P1 x P3.

3. Multistage compression provides effective lubrication.
a) False
b) True

Explanation: Due to the use of high-pressure compression by a single-cylinder, there are chances of obtaining very high discharge temperatures which might heat the cylinder head or burn the lubricating oil. So, with multistage burning of lubricating oil can be avoided as the discharge temperature will be lower. Hence, Multistage compression provides effective lubrication.

4. What is the value of optimum intermediate pressure is the cooling ratio is fixed in a compound VCR with intercooling having suction and discharge pressure as 2 and 8 bar respectively?
a) 2 bar
b) 4 bar
c) 6 bar
d) 8 bar

Explanation: Putting (n / n – 1) = k and dW / dP2 = 0 we get,
T1 P2k – 1 / P1k = T3 P3k / P2k + 1
For the given conditions T1 = T3,
P2 x P2 = P1 x P3
P2 = √P1 x P3
= √2 x 8
= 4 bar.

5. Which of the following is not true about single-stage compression?
a) Size of the cylinder is very large
b) Difficult to reject a good amount of heat in a lesser time of compression
c) Volumetric efficiency will be low
d) Safe at a higher temperature

Explanation: As a single cylinder is used so for effective results large size cylinder will be used. Less time of compression and relatively volumetric efficiency will be lower. Due to the use of high-pressure compression by a single-cylinder, there are chances of obtaining very high discharge temperatures which might heat the cylinder head or burn the lubricating oil. Hence, it might not be safe in every situation.

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

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