Chapter:

# 40+ Air Compressors Solved MCQs

Chapters

42
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Chapter: Air Compressors
1.

## What is the correct formula for net work done of reciprocating engine?

A. Wnet = mean effective pressure × clearance volume
B. Wnet = mean effective pressure × total volume of cylinder
C. Wnet = mean effective pressure × displacement volume
D. none of the above
Answer» C. Wnet = mean effective pressure × displacement volume
2.

A. 0.75
B. 1
C. 1.27
D. 1.35
3.

## Compressed air coming out from a punctured football

A. becomes hotter
B. becomes cooler
C. remains at the same temperature
D. may become hotter or cooler depending upon the humidity of the surrounding air
4.

## The capacity of a compressor is 5 m /min. 5 m /min refers to

A. standard air
B. free air
C. compressed air
D. compressed air at delivery pressure
5.

## The overall isothermal efficiency of compressor is defined as the ratio of

A. isothermal h.p. to the BHP of motor
B. isothermal h.p. to adiabatic h.p.
C. power to drive compressor to isothermal h.p.
D. work to compress air isothermally to work for actual compression
Answer» A. isothermal h.p. to the BHP of motor
6.

## The- most efficient method of compressing air is to compress it

A. isothermally
C. isentropically
D. isochronically
7.

## Maximum work is done in compressing air when the compression is

A. isothermal
C. polytropic
D. any one of the above
8.

## The pressure and temperature conditions of air at the suction of compressor are

A. pressure sightly less than atmospheric and temperature slightly more than atmospheric.
B. slightly more than atmospheric
C. slightly less than atmospheric
D. pressure slightly more than atmospheric and temperature slightly less than atmospheric
Answer» A. pressure sightly less than atmospheric and temperature slightly more than atmospheric.
9.

## Isothermal compression effeicency can be attained by running the compressor

A. at very high speed
B. at very slow speed
C. at average speed
D. at zero speed
Answer» B. at very slow speed
10.

## The compressor capacity with decrease in suction temperature

A. increases
B. decreases
C. remains unaffected
D. may increase or decrease depending on compressor capacity
11.

## Isothermal compression efficiency, even when running at high speed, can be approached by using

A. multi-stage compression
B. cold water spray
C. both (a) and (b) above
D. fully insulating the cylinder
Answer» C. both (a) and (b) above
12.

## Compression efficiency is compared against

A. ideal compression
C. isothermal compression
D. isentropic compression
13.

## Aeroplanes employ following type of compressor

B. axial flow
C. centrifugal
D. combination of above
14.

## Inter cooling in compressors

A. cools the delivered air
B. results in saving of power in compressing a given volume to given pressure
C. is the standard practice for big compressors
D. enables compression in two stages
Answer» B. results in saving of power in compressing a given volume to given pressure
15.

## An ideal air compressor cycle without clearance on p-v diagram can be represented by following processes

A. one adiabatic, two isobaric, and one constant volume
B. two adiabatic and two isobaric
C. two adiabatic, one isobaric and one constant volume
D. one adiabatic, one isobaric and two constant volume
16.

## An ideal air compressor cycle with clearance on p-v diagram can be represented by following processes

A. one adiabatic, two isobaric, and one constant volume
B. two adiabatic and two isobaric
C. two adiabatic, one isobaric and one constant volume,
D. one adiabatic, one isobaric and two constant volume
17.

A. 1
B. 1.2
C. 1.3
D. 1.4
18.

## Isothermal compression though most efficient, but is not -practicable because

A. it requires very big cylinder
B. it does not increase pressure much
C. it is impossible in practice
D. compressor has to run at very slow speed to achieve it
Answer» D. compressor has to run at very slow speed to achieve it
19.

## Ratio of indicated H.P. and brake H.P. is known as

A. mechanical efficiency
B. volumetric efficiency
C. isothermal efficiency
20.

## The ratio of work doen per cycle to the swept volume in case of compressor is called

A. compression index
B. compression ratio
C. compressor efficiency
D. mean effective pressure
21.

## Cylinder clearance in a compressor should be

A. as large as possible
B. as small as possible
C. about 50% of swept volume
D. about 100% of swept volume
Answer» B. as small as possible
22.

## Ratio of compression is the ratio of

A. gauge discharge pressure to the gauge intake pressure
B. absolute discharge pressure to the absolute intake pressure
C. pressures at discharge and suction corresponding to same temperature
D. stroke volume and clearance volume
Answer» B. absolute discharge pressure to the absolute intake pressure
23.

## Clearance volume in actual reciprocating compressors is essential

A. to accommodate Valves in the cylinder head
B. to provide cushioning effect
C. to attain high volumetric efficiency
D. to provide cushioning effect and also to avoid mechanical bang of piston with cylinder head.
Answer» D. to provide cushioning effect and also to avoid mechanical bang of piston with cylinder head.
24.

## The net work input required for compressor with increase in clearance volume

A. increases
B. decreases
C. remains same
D. increases/decreases depending on compressor capacity
25.

## Ratio of indicated h.p. to shaft h.p. is known as

A. compressor efficiency
B. isothermal efficiency
C. volumetric efficiency
D. mechanical efficiency
26.

## Volumetric efficiency is

A. the ratio of stroke volume to clearance volume
B. the ratio of the air actually delivered to the amount of piston displacement
C. reciprocal of compression ratio
D. index of compressor performance
Answer» B. the ratio of the air actually delivered to the amount of piston displacement
27.

## Volumetric efficiency of a compressor with clearance volume

A. increases with increase in compression ratio
B. decreases with increase in compression ratio
C. is not dependent upon compression ratio
D. may increase/decrease depending on compressor capacity
Answer» B. decreases with increase in compression ratio
28.

## Volumetric efficiency of a compressor without clearance volume

A. increases with increase in compression ratio
B. decreases with increase in compression ratio
C. is not dependent upon compression ratio
D. may increase/decrease depending on compressor capacity
Answer» C. is not dependent upon compression ratio
29.

## The clearance volume of the air compressor is kept minimum because

A. it allows maximum compression to be achieved
B. it greatly affects volumetric efficiency
C. it results in minimum work
D. it permits isothermal compression
Answer» B. it greatly affects volumetric efficiency
30.

## Which is false statement about multistage compression.

A. Power consumption per unit of air delivered is low
B. Volumetric efficiency is high
C. It is best suited for compression ratios around 7:1
D. The moisture in air is condensed in the intercooler
Answer» B. Volumetric efficiency is high
31.

## Reciprocating air compressor is best suited for

A. large quantity of air at high pressure
B. small quantity of air at high pressure
C. small quantity of air at low pressure
D. large quantity of air at low pressure
Answer» B. small quantity of air at high pressure
32.

## Rotary compressor is best suited for

A. large quantity of air at high pressure
B. small quantity of air at high pressure
C. small quantity of air at low pressure
D. large quantity of air at low pressure
Answer» D. large quantity of air at low pressure
33.

## After-cooler is used to

A. cool the air
B. decrease the delivery temperature for ease in handling
C. cause moisture and oil vapour to drop out
D. reduce volume
Answer» C. cause moisture and oil vapour to drop out
34.

## The compressor performance at higher altitude compared to sea level will be

A. same
B. higher
C. lower
D. dependent on other factors
35.

A. better lubrication is possible advantages of multistage
B. more loss of air due to leakage past the cylinder
C. mechanical balance is better
D. air can be cooled perfectly in between
Answer» B. more loss of air due to leakage past the cylinder
36.

## Losses in a centrifugal compressor are due to

A. inlet losses
B. impeller channel losses
C. diffuser losses
D. all of the above
Answer» D. all of the above
37.

A. centrifugal
B. reciprocating
C. axial
D. screw
38.

## For minimum work in multistage compression, assuming same index of compression in all stages

A. work done in first stage should be more
B. work done in subsequent stages should increase
C. work done in subsequent stages should decrease
D. work done in all stages should be equal
Answer» D. work done in all stages should be equal
39.

## Diffuser in a compressor is used to

A. increase velocity
B. make the flow stream-line
C. convert pressure energy into kinetic energy
D. convert kinetic energy into pressure energy
Answer» D. convert kinetic energy into pressure energy
40.

## Phenomenon of choking in compressor means

A. no flow of air
B. fixed mass flow rate regardless of pressure ratio
C. reducing mass flow rate with increase in pressure ratio
D. increased inclination of chord with air steam
Answer» B. fixed mass flow rate regardless of pressure ratio
41.

## Axial flow compressor has the following advantage over centrifugal compressor

A. larger air handling ability per unit frontal area
B. higher pressure ratio per stage
D. higher average velocities
Answer» A. larger air handling ability per unit frontal area
42.

## Rotary compressors are suitable for

A. large discharge at high pressure
B. low discharge at high pressure
C. large discharge at low pressure
D. low discharge at low pressure
Answer» C. large discharge at low pressure
43.

## The vloumetric efficiency of compressor with increase in compression ratio will

A. increase
B. decrease
C. remain same
D. may increase/decrease depending on clearance volume