Air compressor basics – types & accessories

What is an air compressor?

A compressor is a machine that compresses air or another type of gas from a low inlet pressure (usually atmospheric pressure) to a higher desired pressure level. The compressor increases the air pressure by reducing its volume.

The pneumatic control systems operate with a supply of compressed air, which must be available in sufficient quantity and at a pressure that suits the capacity of the system. The useful life of a pneumatic system depends largely on the preparation of the compressed air. Thus, the compressor is an important device in industries.

There are different types of compressors, classified based on the principle of operation, the number of stages included to compress air and highest pressure developed.

Types of the compressor based on working principle:


Reciprocating Compressor:

Reciprocating compressors have been the most widely used for industrial plant air systems. The two major types are single acting and double acting, both of which are available as one or two stage compressors. The reciprocating compressor is simple to maintain and operate.

A reciprocating compressor compresses the air in a cylinder, against the cylinder head by a reciprocating piston.  A reciprocating compressor can single acting and double acting and single stage and multi-stage compressors. Single acting compressor utilize piston connected to the crankshaft by connecting rod and compressed air will be at piston side only. Double acting piston compress on both side.

Here shows a multistage compressor, there are three cylinders acting to compress the air.

Rotary Screw air compressor:

A rotary screw air compressor utilizes two meshing helical shaped rotors to compress the air. As the rotor turns, air is compressed by advancing the helix. The rotor may be either oil flooded or dry.

The dry rotor compressor requires the use of synchronization gears to maintain proper separation between the rotor. The oil in the oil-filled compressor lubricates and seals the rotor and acts as a refrigerant to remove heat from the compressor.

The rotary screw compressor has smaller moving parts than the reciprocating compressor and provides a smooth, almost pulse-free supply of air. Little maintenance can be achieved in the field by plant personnel.


Main accessories of compressors:

Inlet filter: 

Inlet filter prevents dust and other particles from entering the compressor.


Aftercoolers are implemented in the discharge line to lower the compressed air discharge temperature.


Separators are used to remove entrained liquid from compressed air. The most common type of separators is impingement, centrifugal and cyclone types. The separator should be equipped with trap and drain.


Traps collect liquid that has to removed from the air by separation and condensation and release it.


Dryers are used when dryer air is required than can be provided by an aftercooler system.

Pressure regulating valve:

Pressure regulating valves are used to supply small volumes of air to various pneumatic equipment.

Pressure relief valves:

In every compressor, there will a pressure relief valve for safety. The relief valve should be set to open at no higher than 10% above the maximum working pressure and periodically checked for proper operation.



Instrumentation Engineer

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