# Refrigeration Questions and Answers – Refrigerant Compressors – 2

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This set of Refrigeration Objective Questions & Answers focuses on “Refrigerant Compressors – 2”.

1. What is the work done during an isothermal compression?
a) 2.3 mRT1 ln (p2 / p1)
b) 2.3 mRP1 ln (p2 / p1)
c) 2.3 mRT1 ln (p1 / p2)
d) mRT1 ln (p2 / p1)

Explanation: Isothermal process is the process in which temperature remains constant and represented by PV = C. Work done is the area under the curve on PV chart. So, for isothermal process the work done is given by = p1v1 ln (v1 / v2) as by ideal gas equation we can replace PV = mRT and v1 / v2 by p2 / p1 which is called as compression ratio.
Work done = mRT1 ln (p2 / p1) or 2.3 mRT1 log (p2 / p1).

2. What is the work done during a polytropic compression?
a) [n / (n – 1)] mR (T2 – T1)
b) [n / (n + 1)] mR (T2 – T1)
c) [n / (n – 1)] mR (T2 + T1)
d) [n / (n + 1)] mR (T2 + T1)

Explanation: Polytropic process is represented by PVn = C and where n = polytropic index. Work done is the area under the curve on PV chart. So, for polytropic process the work done by using ideal gas equation PV = mRT and v1 / v2 by p2 / p1 which is called as compression ratio.
Work done = [n / (n – 1)] mR (T2 – T1).

3. What is the work done during an isentropic compression?
a) [n / (n – 1)] mR (T2 – T1)
b) [n / (n + 1)] mR (T2 – T1)
c) [n / (n – 1)] mR (T2 + T1)
d) [n / (n + 1)] mR (T2 + T1)

Explanation: Polytropic process is represented by PVn = C and where n = polytropic index. Work done is the area under the curve on PV chart. So, for polytropic process the work done by using ideal gas equation PV = mRT and v1 / v2 by p2 / p1 which is called as compression ratio.
Work done = [n / (n – 1)] mR (T2 – T1).
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4. 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.

5. Where is the centrifugal compressor mostly suitable?
a) Small displacements and low condensing pressures
b) Small displacements and high condensing pressures
c) Large displacements and low condensing pressures
d) Large displacements and high condensing pressures

Explanation: Centrifugal compressor is the type of compressor in which in the simplest form consists of an impeller to which a number of curved vanes are fitted symmetrically. As per the construction, the impeller rotates in an airtight volute casing with inlet and outlet points, and for better performance as per the flow of refrigerant, it is very suitable for larger displacements and low condensing pressures to give the optimum refrigeration.

6. Where is the reciprocating compressor mostly suitable?
a) Small displacements and low condensing pressures
b) Small displacements and high condensing pressures
c) Large displacements and low condensing pressures
d) Large displacements and high condensing pressures

Explanation: Reciprocating compressor is the type of compressor in which vapor refrigerant is compressed by the back and forth motion of the piston. As per the construction of piston and cylinder and for better performance as per the flow of refrigerant it is very suitable for small displacements and high condensing pressures to give the optimum refrigeration.

7. What is the value of swept volume?
If, D = 10 mm
L = 6 mm
a) 188.49
b) 94.2477
c) 47.1238
d) 471.23

Explanation: Swept volume is the volume covered by the piston in the cylinder by back and forth motion. As we know, Volume = Area x Length
Here, D is diameter so for a circle with diameter D area = π D2 / 4
So, swept volume = π D2 / 4 x L = π D2 L / 4 = π x 10 x 10 x 6 / 4 = 471.23.

8. What is the value of the clearance factor?
If, VC = 0.137 and VP = 0.034
a) 0.00465
b) 0.0465
c) 4.029
d) 0.2481

Explanation: Piston displacement volume is the volume covered by the piston by back and forth motion in the cylinder and clearance volume is the extra space in the cylinder which is kept for easy circulation of the refrigerant. Clearance factor is the ratio of clearance volume to the piston displacement volume or swept volume.
Clearance factor = VC / VP = 0.137 / 0.034 = 4.029.

9. What is the value of volumetric efficiency?
VC = 0.445, VS = 0.843 and VP = 1.087
a) 1.289
b) 0.5278
c) 1.894
d) 0.775

Explanation: Volumetric efficiency is the ratio of the compressor capacity or the suction volume to the piston displacement volume. So, volumetric efficiency is represented as VS / VP. A good compressor has a volumetric efficiency of around 70 – 80 %.
VS / VP = 0.843 / 1.087 = 0.775 = 77.5 %.

10. The main disadvantage of centrifugal compressors is surging.
a) False
b) True

Explanation: When refrigeration load decreases due to the reversal flow of refrigerant from compressor to the evaporator and causes stress conditions in the compressor, this phenomenon is called surging which occurs in these types of compressors and is one of the disadvantages.

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