Air Compressor | Interview Question and Answers

Air Compressor | Interview , viva , Oral Question and Answers

1. What is meant by double acting compressor?

In double acting reciprocating compressor, the suction compression and delivery of air take place on both side of the piston.

2. What is meant by single stage compressor?

In single stage compressor, the compression of air from the initial pressure to the final pressure is carried out in one cylinder only.

3. Define clearance ratio

Clearance ratio is defined as the ratio of clearance volume to swept volume (or) stroke volume.
C = Vc / Vs Vs – swept volume, Vc – clearance volume

4. What is compression ratio?

Compression ratio is defined as the ratio between total volume and clearance volume. Compression ratio = Total volume / Clearance Volume

5. What are the factors that effect the volumetric efficiency of a reciprocating compressor?

a. Clearance volume
b. Compression ratio.

6.Classify the types of air compressors.

According to the number of stages: Single stage and Multi stage
According to the number of cylinder: Single cylinder and Multi cylinder.
According to the method of cooling: Air cooled compressor and Water cooled compressor
According to working: Reciprocating compressor and Rotary compressor
According to the action of air: Single acting compressor and Double acting compressor
According to the pressure limit: Low pressure, Medium pressure and High pressure compressor.
According to the capacity:Low capacity, Medium capacity and High capacity compressor.

7. Compressor capacity is highest, when the intake air temperature is _______

Lowest

8. Compressor capacity is expressed in ___________

m3/min

9. As the compression ratio increases, the volumetric efficiency of air compressor _______

Decreases

10. In gas turbine, type of rotary compressor used is __________

Axial flow compressor

11. In Aero plane, type of rotary compressor used is _

Axial flow compressor.

12. What is the difference between complete (or) perfect inter cooling and incomplete (or) imperfect inter cooling.

Perfect Inter cooling:
When the temperature of air leaving the intercooler (T3) is equal to the original atmospheric air temperature (T1), then this inter cooling is known as perfect inter cooling.

Imperfect Inter cooling:
When the temperature of air leaving the inter cooler (T3) is more than original atmospheric air temperature (T1), then this inter cooling is known as Imperfect inter cooling.

13. What is meant by single stage double acting reciprocating compressor?

In double acting air compressor, air is admitted alternatively to both sides of the piston. When one side is performing the suction stroke, the other side will be performing compression & delivery of compressed air. Theoretically a double acting compressor will deliver twice the amount of compressed air compared to a single acting compressor of the same size and speed.

14. Define volumetric efficiency and clearance ratio

Volumetric efficiency:
Volumetric efficiency is defined as the ratio of volume of free air sucked into the compressor per cycle to the stroke volume of the cylinder.

Clearance ratio:
Clearance ratio is defined as the ration of clearance volume of swept volume (or) stroke volume.

15. What are the drawbacks in single stage compression?

The size of the cylinder will be too large. Due to compression, there will be a rise in temperature of air and it will be difficult to reject the heat in the little time available during compression. Some time the high temperature at the end of compression may heat the cylinder head or burn the lubricating oil.

16. Explain the flow of air controlled in reciprocating compressors

The flow of air is controlled by three methods such as
a. Centrifugal governor mechanisms
b. Maintaining the speed of motor constant
c. Providing air pocket advancement to the cylinder.

17. Define mean effective pressure. How is it related to indicated power?

The mean effective pressure is defined as hypothetical pressure, which is considered to be acting on the piston throughout the compression stroke.
The indicated power IP = Mean effective pressure x L x A x N

18. What is inter-cooler? What does complete inter-cooling mean?

An inter-cooler is a simple heat exchanger. It exchanges the heat of compressed air from the low-pressure compressor to the circulating water before the air enters to the high-pressure compressor. The purpose of inter cooling is to minimize the work of compression. If the air is cooled to temperature equal to the temperature of air at the inlet of the Ist stage cylinder, the inter-cooling is complete.

19. Define Stroke volume and cylinder volume

When a piston traverses one stroke and reaches BDC of TDC, the volume swept by the piston is called stroke volume of swept volume.The addition of stroke volume and clearance volume is called cylinder volume.

20. Define positive displacement rotary compressor and dynamic displacement compressor.

In positive displacement rotary type compressor the air is entrapped in between twosets of engaging surfaces. The pressure rise is either by back flow of air or both by variation inthe volume and back flow.

In the dynamic type rotary compressors, the air is not trapped in specific boundaries but it flows continuously and steadily through the machine. The energy from the impeller is transferred to the air as the air flows through the machine and the pressure rise is primarily due to dynamic effects.

21. Name any six use of compressed air

Drive pneumatic tools such as rock drills
Produce air for cleaning purpose in large industries
Operate brakes in heavy vehicles
To start large diesel engines
Spray painting
Refrigeration and Air-conditioning

22. What are the factors that affect the volumetric efficiency?

Clearance volume
Pressure ratio
Temperature
Stroke volume
Wire drawing effect
Valve fluttering

23. Define Thermodynamic cycle and Air standard cycle

Thermodynamic cycle is defined as the series of processes performed on the system, so that the system attains its original state. If we use air as working substance in the thermodynamic cycles, it is called air standard cycles.

24. Name the various gas power cycles

Carnot cycle, Otto cycle, Diesel cycle, Brayton cycle, Duel combustion cycle, Atkinson cycle, Stirling cycle.

25. What are the assumptions made for air standard cycle analysis?

The working medium is prefect gas throughout i.e. it follows PV=mRT
The working medium has constant specific heats
The working medium does not undergo any chemical change throughout the cycle
The compression and expansion processes are reversible adiabatic i.e. there is no loss or gain in entropy.
Kinetic and potential energies of the working fluid are neglected
The operation of the engine is frictionless.
Heat is supplied and rejected in a reversible manner.

26. What are the factors that influence the power input to the compressor?

a. The mass flow of air
b. The pressure ration of the compressor
c. The inlet temperature
d. Temperature difference between the inlet and outlet
e. The properties of the working medium

27. What are the internal and external loses in centrifugal compressor?

The internal losses are due to
a. friction between air and wall of flow passage
b. Disc friction
c. leakage between impeller and casing
d. turbulence
e. shock and the external losses are mainly due to the bearing friction

28. Define Isothermal efficiency and Roots efficiency

Isothermal efficiency is defined as the ratio of Isothermal work to indicated work. Roots efficiency is the ratio of adiabatic work to roots work.

29. What are the factors that affect the power requirement of a compressor?

a. The total pressure ratio of the compressor
b. The total inlet temperature
c. The total temperature difference between the inlet and outlet
d. The clearance volume
e. The mass flow rate of air

30. What are the difference between rotary air compressor and reciprocating air compressor?

Reciprocating Air compressor Rotary Air compressor

The maximum delivery pressure may be as high as 1000 bar.
The maximum delivery pressure is 10 baronly. They are suitable for low discharge of air at very high pressure.
They are suitable for large discharge of air at low pressure.
The speed of air compressor is low .The speed of air compressor is high.The air supply is intermittent .The air supply is continuous. The size of the compressor is large for the given discharge The size of air compressor is small for the same discharge. The balancing is a major problem here is no balancing problem

31. Why Clearance volume is necessary and explain its importance?

In actual compressor, the clearance volume is provided to give cushioning effect otherwise the piston will strike the other end of the cylinder. It is generally expressed as percentage of piston displacement.

Importance of clearance volume:

To give cushioning effect to the piston
To provide space for valve movement.
The maximum pressure may also be controlled by clearance volume.
The volumetric efficiency and pressure ratio are depends upo9n clearance volume. If clearance volume is more, it reduces the volumetric efficiency.

32. What are the advantages of multi stage compressor over single stage compressor?

Less work is done by the compressor to deliver the same quantity of air.
It improves the volumetric efficiency for the given pressure ratio.
The size of the two cylinder may be adjusted to suit the volume and pressure of the air.
It reduces the leakage losses considerably and provides effective lubrication.
It provides more uniform torque and thus smaller size of the flywheel is required.
It reduces the cost by selecting a cheap material for construction.

33. Define volumetric efficiency of the compressor.

It is the ratio of actual volume of air drawn in the compressor to the stroke volume of the compressor.

34. Define mechanical efficiency.

It is the ratio of indicated power to shaft power or brake power of motor.

35. Define Isentropic efficiency.

It is the ratio of the isentropic power to the brake power required to drive the compressor.

36. What is the purpose of inter cooling and explain its process?

The purpose of inter cooling in multistage compression is to reduce the temperature without reduction in pressure. It is placed between LP cylinder and HP cylinder. When air flows through it, the temperature is reduced by maintaining the water circulation.