# Thermal Engineering Questions and Answers – Classification of Air Compressors

This set of Thermal Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Classification of Air Compressors”.

1. On which of the following basis “Air compressors” cannot be classified?
a) According to working of compressor
b) According to action of compressor
c) According to number of stages of compressor
d) According to the type of energy at inlet of compressor

Explanation: Air compressors are classified according to working of compressor, action of compressor and number of stages of compressor. Generally, turbines are classified according to the type of energy at inlet.

2. From the given pairs, which of the pairs is the type of positive displacement compressor?
a) Reciprocating and rotary
b) Reciprocating and centrifugal
c) Rotary and centrifugal
d) Centrifugal and axial

Explanation: Reciprocating and rotary are the types of positive displacement compressor because air is trapped in between two relatively moving components and then compression takes place by the squeezing action of the relatively moving components.

3. Which of the following is not a type of reciprocating compressor?
a) Single acting compressor
b) Double acting compressor
c) Diaphragm compressor
d) Scroll compressor

Explanation: Single acting, double acting and diaphragm compressor employs reciprocating motion of piston/diaphragm for the purpose of air compression, whereas Scroll compressor is a type of rotary compressor which uses rotary motion of moving scroll inside a fixed scroll for the purpose.

4. Which of the following are the types of rotary compressor?
a) Lobe compressor, liquid ring compressor, screw compressor, scroll compressor
b) Lobe compressor, diaphragm compressor, screw compressor, scroll compressor
c) Centrifugal compressor, liquid ring compressor, screw compressor, scroll compressor
d) Centrifugal compressor, diaphragm compressor, screw compressor, scroll compressor

Explanation: Out of the given list, lobe compressor, liquid ring compressor, screw compressor, scroll compressor are the type of rotary compressor, whereas diaphragm compressor is a type of reciprocating compressor and centrifugal compressor is a type of dynamic compressor.

5. Axial compressor comes under the category of which type of compressor?
a) Positive displacement compressor
b) Reciprocating compressor
c) Rotary compressor
d) Dynamic compressor

Explanation: Dynamic compressors are of two types centrifugal and axial. In axial compressor the air flows parallel to the axis of compressor and during the flow over a series of fixed and moving vanes it gets compressed, whereas reciprocating and rotary are the types of positive displacement compressor.
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6. Vane compressor is a type of reciprocating compressor.
a) True
b) False

Explanation: Vane compressor is a type of rotary compressor because vane compressor consists of a cylindrical casing, valves and a rotor placed eccentrically inside the casing. It works by the rotary action of the rotor and not by the reciprocating motion; therefore it is a rotary compressor.

7. In which of the following compressors is the delivery of compressed air intermittent?
a) Rotary compressor
b) Reciprocating compressor
c) Centrifugal compressor
d) Axial compressor

Explanation: In reciprocating compressor during the suction stroke air is sucked into the cylinder, at this instant suction valve is open and delivery valve is closed, followed by a compression stroke and at this instant suction valve is closed and delivery valve is open, therefore delivery of compressed air is intermittent i.e. discontinuous.

8. Which of the following compressors consists of piston-cylinder assembly?
a) Rotary compressor
b) Reciprocating compressor
c) Centrifugal compressor
d) Axial compressor

Explanation: Reciprocating compressor consists of cylinder, piston, inlet and discharge valves. And due to the reciprocating motion of the piston inside the cylinder, the compressor is named as reciprocating compressor.

9. Which of the following compressors consist of diffuser casing?
a) Centrifugal compressor
b) Axial compressor
c) Screw compressor
d) Reciprocating compressor

Explanation: At the inlet of centrifugal compressor, the impeller produces suction through which air is sucked in and then it imparts kinetic energy to the air, then the air passes through the diffuser casing in which the kinetic energy of air is converted to pressure energy because of the continuously diverging cross-sectional area.

10. The following diagram is of which type of compressor?

a) Liquid ring compressor
b) Axial compressor
c) Reciprocating compressor
d) Centrifugal compressor

Explanation: The diagram contains piston-cylinder assembly and reciprocating compressor is the only type of compressor which consists of piston-cylinder assembly, whereas centrifugal and axial compressor consists of impeller and vanes encased inside a casing.

11. In which of the following the flow of air is parallel to the axis of the compressor?
a) Rotary compressor
b) Reciprocating compressor
c) Centrifugal compressor
d) Axial compressor

Explanation: In axial flow compressor the flow of air is parallel to the axis of compressor and during the flow over a series of fixed and moving vanes the air gets compressed, whereas in centrifugal compressor the flow of air is perpendicular to the axis of compressor.

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