33-most frequently asked questions & answers on air compressors

1-What are the functions of air compressors in power plants?
The functions of the compressor is to take a definite quantity of fluid (air or gas) & deliver it at a required pressure
2-What are the applications of compressed/pressurized air in power plants?
Operating the control valves
Operating the solenoid valves
Operating the dome valves in ash handling system
Air dampers operation in fans
Operating pneumatic tools
3-How does air compressor work? & what are the effects of compression on air?
An air compressor takes in atmospheric air. Compresses it and delivers this air to a storage vessel from which it will be conveyed by the pipe line to the required equipments.
Effects of compression:
Air pressure & temperature increases & moisture content in air increases
4-What are the various types of air compressors used in power plants?
Generally reciprocating, screw & centrifugal air compressors are used.
5-How does the positive displacement compressors work?
These compressors work by increasing the pressure of the air by reducing its volume
5-How does the centrifugal compressors work?
Centrifugal compressors initially increase the air velocity & then pressure at compressor outlet.
6-What are the various types of reciprocating compressors?

  • Single stage & multi stage
  • Single cylinder & multi cylinder
  • Air cooled & water cooled
  • Lubricated & Non lubricated
7-What are single acting compressors?
It is a compressor that has one discharge per revolution of crankshaft.
8-What are double acting compressors?
Compressor that completes two discharge strokes per revolutions of crankshaft
9-What do you mean by compression ratio in compressors?
It is the ratio of volume of air before compression to the volume of air after compression.
10-What do you mean by air compressor capacity?
It is the quantity of air actually delivered by a compressor in m3/minute or m3/sec.
11-What do you mean by Free Air delivery (FAD)?
It is the volume of air delivered by compressor under the compressor intake conditions like pressure &temperatures. FAD is not calculated on standard atmospheric conditions, it is measured in terms of CFM.It means that, amount of compressed air converted back to actual inlet air conditions.
12-What are the various parts of centrifugal compressors?
Impellers, Vanes, Volutes, Suction Eyes, Discharge lines, Diffuser Plates, Seals, Shaft, Casing
13-What do you mean by swept volume?
The volume displaced or swept by piston when it moves between two ends.
14-What do you mean by clearance volume?
 It is the difference between the total volume and the swept volume.
It is the space between the cylinder end and the piston to allow for wear & for easy movement. And also clearance is for valves reception.
Therefore clearance volume should be small as possible, but cannot be reduced to zero since, for mechanical reasons to avoid contact of moving parts in compressors.
15-Compressors efficiency
1. Volumetric efficiency:-
It is the ratio of actual volume of the FAD at standard atmospheric condition in one delivery stroke (Actual air intake) to the swept volume (theoretical air intake) by the piston during the stroke.
2. Isothermal efficiency:-
It is the ratio of isothermal work done to the actual work done
3. Volumetric efficiency:-
It is the ratio of free air delivered to the displacement of the compressor.
It is also defined as the effective swept volume to the swept volume
4. Mechanical efficiency:
It is the ratio of mechanical output to the mechanical input
Mechanical efficiency = Indicated horse power / Shaft horse power
16-Why do the compressors located at higher elevation consume more power as compared to those of lower elevation compressors?
As the altitude increases, air pressure goes on decrease; hence inlet pressure to the compressor decreases. For this compressor has to consume more power to provide required discharge pressure.
17-What is the relation between Relative humidity & moisture carrying capacity of air?
As the relative humidity (RH) increases, Moisture carrying capacity of the air increases
18-What are the reasons for reduction of volumetric efficiency of a reciprocating compressor?

  • Very high speed of compressor
  • Very large clearance volume
  • Obstruction in inlet valves
  • Malfunctioning of valves
  • Overheating of compressed air
  • Inertia effect of air in inlet pipe
19-Why the FAD is actually less than compressor displacement?

  • The fluid resistance through air intake valve& valves prevent the cylinder being fully charged with air at atmospheric conditions.
  • Losses due to leakages
  • The high pressure air trapped in the clearance space, must expand to a pressure below the atmospheric pressure before the automatic suction valves open
20-What is the significance of inter coolers & after coolers in Heat exchangers?
Intercooler & after coolers are the shell & tube type heat exchangers in which hat is removed from air which has been compressed & its temperature has risen as a result of compression.
The cooler which is placed in between the stages are called Inter coolers & coolers are fitted after the last stage is called after coolers. Generally after coolers are used for removing the moisture from air by reducing its temperature.
21-How do you increase the isothermal efficiency of reciprocating compressors?
Isothermal efficiency can be increased by;
Water jacketing: where water is circulated around the LP & HP cylinders, which helps to cool the air
Inter-cooling: Intercoolers are fitted between the two stages to cool the air coming out from 1st stage. In inter coolers air temperature is maintained around atmospheric temperature.
Adding external fins for smaller compressors
22-Why do you prefer multistage compression rather than single stage?

  • Air can be cooled between interstages
  • For the same pressure & FAD, multi stage compressors consume less power
  • These are having good mechanical balance
  • The pressure & temperature range can be kept as per requirement. This results into reduced wear & tear & increased volumetric efficiency
  • However multistage compressors with coolers arrangement is more expensive than single stage compressor
23-A single stage reciprocating air compressor takes 1.5 M3/min of air at pressure 1.013 bar & 25 Deg C temperatures & delivers at 6 bar. Calculate the final air temperature
Assume law of air compression pVn = Constant
That is pV 1.35 =Constant
P1 = 1.013 bar & T1 = 25 + 273 = 298 K
P2 = 6 bar
We have
T2 / T1 = (P2 / P1) (n-1) / n
T2 = 298 X (6/1.013) (1.35-1)/1.35
T2 =470.36 K
T2 = 470.36 – 273 = 197.36 Deg C
24-Suction pressure of a double stage air compressor is 350 mmwc & discharge pressure 6 kg/cm2 .Then calculate the 1st stage outlet pressure?
P1 = 350 mmwc = (1.03-350/1000) = 0.995 kg/cm2
P2 = √(P1 XP3) = √(0.995 X 6)
P2 = 2.44 kg/cm2
25-What do you mean by perfect cooling in compressors heat exchangers?
The condition, where the inlet temperature of air at each stage of multi-stage machine should be the same as it was at the first stage inlet is called perfect cooling.
26-Is volumetric efficiency of a compressor is less at higher delivery pressure?
Yes, at higher delivery pressure volumetric efficiency of any compressor is less & vice versa
27-What do you mean by loading & unloading in air compressors?
It is the two step control of delivery pressure, where compressor is loaded when there is air demand and unloaded when there is no demand.
During unloading air compressors consume lesser power, say 30 to 40% of full load. Positive displacement compressors consume less power & screw compressors more during unloading.
28-Does incorporating VFDs to reciprocating compressors save energy?
No...Compressors are required to run at full RPM, if run at lower RPM time required for loading will increase. So loading & unloading by using VFDs is not recommended for reciprocating compressors However sometimes it can be applied for centrifugal compressors.
29-Explain the process of loading & unloading in reciprocating air compressors?
In reciprocating air compressors, suction valves of compressors are fitted with un loader. Suction valves get forced open by actuating Solenoid valve (SOV) when discharge goes higher than requirement.
During loading: The SOV de-energies & the suction valves open & close as per the operation of flat springs fitted with SOVs.
During unloading: As the pressure increases more than set pressure, SOV on actuating air line opens to allow air to force open the suction valves. During compression stroke air discharges through already opened suction valves & pressure does not increase during suction stroke.
For 50% unloading 50% of suction valves get forced open & for 100% unloading all the suction valves get opened.
30- Explain the process of loading & unloading in screw air compressors?
Screw compressors are silent in operation & also consume very less power as compared to reciprocating compressors.
Unloading: When the demand of the air reduced, a device known as suction slide valve or suction throttle valve is activated. Then very less quantity of air is allowed to pass through it.
Loading: During loading, suction slide valve remains open 100%.
31- Explain the process of loading & unloading in centrifugal air compressors?
For controlling the discharge pressure of a centrifugal compressor, VFDs are installed. As the air consumption decreases, discharge pressure increases & motor speed decreases accordingly to maintain discharge header pressure.
Inlet guide vanes & Anti surge valves are also used for controlling speed.
Thumb rules on air compressors
Every 4 degree rise in inlet air temperature results in a higher energy consumption by 1%  to achieve equivalent power output
For every 250 mmWC pressure drop in increase across at the suction path will increase the power consumption of air compressor by 2%
An increase 5.5 deg C in the inlet air temperature to the second stage results in a 2% increase in the specific energy consumption.
A reduction in the delivery pressure of a compressor by 1 kg/cm2 would reduce the power consumption by 6-10%
32-What are the functions of air receiver tank?

  • Serves as air reservoir
  • Prevents frequent loading & unloading of air compressor
  • Separates moisture from air
33-What are the different types of air dryers used?
1. Refrigerent type
Adsorption type
a.Desiccant heat less drier
b.Desciccant type hot air drier


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