This set of Refrigeration Multiple Choice Questions & Answers (MCQs) focuses on “Refrigerant Compressors – 1”.
1. What is the purpose of using a refrigerant compressor?
a) Raise the temperature of refrigerant
b) Reduce the temperature of refrigerant
c) Reduce the pressure of refrigerant
d) Expand the refrigerant
Explanation: Refrigerant compressor is multifunctional equipment used for work done. The compressor raises the temperature and pressure of the refrigerant. It also helps to circulate the refrigerant through the refrigerant system.
2. What must be the state of refrigerant entering the compressor?
a) Dry saturated vapour refrigerant
b) Dry saturated liquid refrigerant
c) Vapour and liquid mixture
d) Superheated vapour refrigerant
Explanation: For better performance in the refrigeration system, the refrigerant must enter the compressor in a superheated vapour state. For any other state of refrigerant, extra work is to be done to reach the condenser pressure. This increase in work can reduce the coefficient of performance. So, the superheated state will give the best results from the refrigeration system.
3. What is the pressure at the outlet of a refrigerant compressor?
a) Critical pressure
b) Suction pressure
c) Discharge pressure
Explanation: The absolute pressure at which the refrigerant comes out of the compressor is called Discharge pressure. This pressure may not be equal to condenser pressure in practical systems.
4. What is the pressure at the inlet of a refrigerant compressor?
a) Critical pressure
b) Suction pressure
c) Discharge pressure
Explanation: The absolute pressure at which the refrigerant enters the compressor is called Suction pressure. This pressure may not be equal to evaporator pressure in practical systems.
5. 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 vapour 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.
6. Which represents compressor ratio or pressure ratio among the following?
a) Relative discharge pressure / Relative suction pressure
b) Relative discharge pressure / Absolute suction pressure
c) Absolute discharge pressure / Relative suction pressure
d) Absolute discharge pressure / Absolute suction pressure
Explanation: Compressor or pressure ratio is the ratio of pressure after compression to the pressure before compression i.e., absolute pressure at discharge to the absolute pressure at suction. This can be represented as Absolute discharge pressure / Absolute suction pressure.
7. How is the compressor ratio defined in terms of volume?
a) Clearance volume / Total volume of the cylinder
b) Clearance volume / Swept volume of the cylinder
c) Total volume of cylinder / Clearance volume of the cylinder
d) Total volume of cylinder / Swept volume of the cylinder
Explanation: Swept volume is the volume covered by a piston in the cylinder and clearance volume is the extra space in the cylinder which is kept for easy circulation of the refrigerant. The total volume of the cylinder is the summation of swept and clearance volume. Compressor ratio is the ratio of the total volume of the cylinder to the clearance volume of the cylinder.
8. What is the formula for swept volume?
If, D = Diameter of cylinder
L = Length of the piston stroke
a) π D L
b) π D L / 2
c) π D L / 4
d) π D2 L / 4
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.
9. What is the value of the clearance factor?
If, VC = Clearance volume and VP = Piston displacement volume
a) VC x VP
b) VC / VP
c) VP / VC
d) VC + VP
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.
10. What is the volume of the amount of refrigerant passing through compressor called?
a) Compressor rating
b) Compressor ratio
c) Compressor capacity
d) Compressor efficiency
Explanation: The volume of the actual amount of refrigerant, which passes through the compressor in unit time is called compressor capacity. It is equal to the suction volume and has unit m3/s.
11. What the meaning of VS?
a) Surface volume
b) Suction volume
c) Swept volume
d) Secondary piston volume
Explanation: The volume of the refrigerant sucked by the compressor in the suction stroke is called suction volume. Suction volume is denoted by VS. And swept volume is the piston displacement volume, which is denoted by VP.
12. What is the volumetric efficiency?
a) VP / VS
b) VC / VS
c) VS / VC
d) VS / VP
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 %.
13. Clearance volume can be zero.
Explanation: Though it is possible in ideal conditions, a cylinder with no clearance volume will have very less efficiency due to contamination of newly entering and outgoing refrigerant in the cylinder. Optimum compression will not be possible without clearance volume. Volumetric efficiency will be affected if VC = 0. It is a very crucial parameter to have and to get optimum results in the refrigeration system.
14. Mixture of liquid and vapour state of refrigerant reduces overall work done.
Explanation: Dry saturated vapour or superheated vapor state of refrigerant is desired when entering the compressor. If the mixture enters the compressor, then the compressor has to covert the liquid to vapour and later compress the vapour refrigerant till the desirable pressure near to the condenser pressure. So, overall work is increased, and this will affect the C.O.P. of the system.
15. Pressure or compression ratio is always __________
a) equal to one
b) less than one
c) greater than one
Explanation: Compression or pressure ratio is the ratio of pressure after compression to the pressure before compression i.e., absolute pressure at discharge to the absolute pressure at suction. This can be represented as Absolute discharge pressure / Absolute suction pressure. As the discharge pressure is always more than suction pressure, the value of the compression ratio will always be greater than unity.
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