An
essential component in any gas plant or gas system, a gas compressor is a
mechanical tool designed to raise the pressure of gas or vapor. The increase
in the pressure of the gas is done by lowering its volume. It is very
natural that compression of a gas increases its temperature. Gas compressors
are widely used in various industrial applications. It is one of the most
essential as well as an expensive piece of equipment and has a predominant
influence on gas processing in gas plants.
Working Principle
The working of gas compressor is similar to pumps. Both pumps and
compressors increase the pressure on a fluid/gas and both are used to
transport the fluid/gas through a pipe. However, as gases are compressible,
the gas compressor can also reduce the volume of a gas which is not the case
with pumps. Since liquids are relatively incompressible, the main purpose of
a pump is simply to transport liquids and that brings out the difference
between compressors and pumps. A gas compressor reduces the volume of a gas
and this is done by pressing the gas molecules close together and storing
them under pressure. The energy caused by this action allows the temperature
of the gas to rise.
Types of Gas Compressors
There are four main types of Gas or Air Compressors:
Reciprocating
compressors:
A positive displacement compressor, reciprocating compressor takes
successive volumes of air, which is confined within a closed space, and
raising this air to a higher pressure. This is done by using a piston. Air
is drawn into the cylinder through the piston's compressing and displacing
element. This compressor is considered single acting when the air
compression is done by using one side of the piston and when both the sides
are used, the reciprocating compressor is considered double acting.
Rotary
vane compressors
Also known as sliding vane compressor, rotary vane compressor make use of a
rotor with sliding vanes and hence the name. These compressors are used in
gas plants for supplying relatively large quantities of air or other gas at
moderate pressures. This is done by the rotation of the rotor. As the rotor
rotates, the sliding vanes form spaces and in one complete turn, there is
expansion and contraction. In doing so, the air is drawn in, compressed and
let out when an exhaust port is exposed. Such type of compressors are
efficient at low loads. There is a wide range of availability in
configuration and in pressure and capacity of such compressors.
Rotary screw compressors:
Rotary
screw compressors are widely used in in the process industry for gas
compression and the working of the compressor depends on two meshing helical
rotors rotating in opposite directions. One rotor is considered male and the
other is considered female and when the free space between the rotors
decreases axially, the air that is trapped between the rotors is compressed.
Centrifugal
compressors:
A dynamic compressor, here air is accelerated by high speed rotating
impellors. The energy is transferred from a rotating impeller to the air.
This is done by changing the momentum and pressure of the air. A continuous
duty compressor, with few moving parts, centrifugal compressors are used in
a wide range of applications, but are most suitable to operations that
require very large quantities of compressed air.
Comparison of Gas Compressors
| Item |
Reciprocating |
Rotary Vane |
Rotary Screw |
Centrifugal |
| Efficiency at full load |
High |
Medium-Low |
High |
High |
| Efficiency at part load |
High due to staging |
Poor-Below 60% at full load |
Poor-Below 60% at full load |
Poor-Below 60% at full load |
| Efficiency at no load (power as % of full
load) |
High (10%-25%) |
Medium (30%-40%) |
High-Poor (25%-60%) |
High-Medium (20%-30%) |
| Size |
Largetab |
Compact |
Compact |
Compact |
| Noise level |
Noisy |
Quiet |
Quiet (when enclosed) |
Quiet |
| Vibration |
High |
None |
None |
None |
| Oil Carry Over |
Moderate |
Low-Medium |
Low |
Low |
| Pressure |
Medium-Very high |
Low-Medium |
Medium-High |
Medium-High |
| Capacity |
Low-High |
Low-Medium |
Low-High |
Medium-High |
Applications of Gas/Air compressors
- Fuel Gas / Power Generation
- Gas Boosting
- Gas Generation / PSA
- Gas Liquefication / Cryogenics
- Gas Transmission / Delivery
|
- Oil Refining / Chemical Processing
- Refrigeration / Gas Drying
- Semiconductor Processing
- Vessel / Cylinder Filling
|
Buying Tips
Depending on your applications, gas compressors are to be chosen.
- The most commonly used compressors are the reciprocating compressors.
They are to be chosen when operations are short term and where the load
fluctuates. These compressors can serve as an alternative to screw
compressors.
- Rotary screw compressors are to be chosen for those applications
where operation is continual and there is the need of constant base load
supply. They are very efficient under these conditions.
- You can choose centrifugal compressors when you require large
quantity of compressed air. · Rotary vane compressors are
preferable in applications which use full load.
Besides this, other factors to take into consideration are as follows:
- Reducing air pressure at the compressor: Always make sure
that air is delivered at the right pressure at the right quality and
quantity.
- Minimize air drop: Consider the efficiency of the components.
- Check for leaks.
- Flow Capacity
- Inlet / Suction Pressure
- Discharge / Operating Pressure
- Driver / Motor Power
Gas
Compressor
