Unloading: How to Control Discharge Air Pressure of an Air Compressor

(Last Updated On: 22/11/2017)

When more air is being compressed than is being consumed, the air compressor required to be controlled by any means of unloading.

In engineering plant consumption of electric power is a most sensitive issue.

Unloading the compressor will minimize the power required to drive the compressor. Hence maximum economy can be maintained during the unloading period.

air compressor unloading

Manual operated type unloaders such as “M” type did this control or “W” type unloaders also.

Actually, this is an old technology of unloading and replaced by “Electro-Pneumatic Controls” which is also replaced by “Program Logic Control” (PLC).

  • How an Unloading Achieved by “M” Type Unloader


The operation of the unloader is simple and positive. In this system, the pressure in the air discharge line from the compressor to the receiver reaches the set pressure.

At this preset pressure, unloader is set to operate. This is determined by the position of the adjusting screw.

Here plate valve is forced away from its seat on the unloader body.

When this happens, air from the discharge line from the compressor to the receiver passes through the unloader.

This air further goes to the unloading suction valve covers. This will hold suction valves in open condition.

Hence the compressor operates against the atmosphere pressure only.

unloading piping

When the air pressure in the discharge line is reduced to the predetermined pressure as determined by the number of unloader shims.

Then the compression of the spring overcomes the pressure of the air in the discharge line from the compressor acting against the plate valve.

This will force the valve back against its seat on the unloader body. The air trapped in the unloader piping is then vented out to the atmosphere.

Venting is done by vent hole of the unloader.

The pressure at which the unit will unload is determined by the position of the adjusting screw.

Turning this adjusting screw in, increase the compression pressure on the spring.

Hence increasing the unloading pressure of compressor and vice versa. To decrease the loading-unloading range, add the shims and to increase the range, remove the shims.

  • How to Take Care of Unloader

The most important care that the unloader should receive is to be kept clean. In sluggish operation, the operator should first make sure that all unloader piping is free from leakages.

Since leakage from these places will affect the operation of the unloader.

Also, the vent hole in the unloader valve seat is open. The gauze of the air cleaner in the unloader line should be removed and cleaned.

If a problem is not solved, remove the unloader valve seat and clean.

If the cleaning and adjusting fail to make the unloader operate satisfactorily, the valve may require grinding.

Use fine grinding compound for valve seat grinding. The valve is provided with two small holes on the top surface.

These holes can be used for insertion of the tool to turn it into the grind.

Avoid excess grinding since this may change the point of contact sufficiently to affect operation adversely.

Clean out the entire compound thoroughly after grinding.

  • How to Achieve Two Stage Unloading

A single “M” Type unloader will work either at 0% or 100% load-unload condition. To achieve 0%, 50%, and 100% load-unload condition of air compressor, two numbers of unloaders are required to be used.

This requires Double Stage Reciprocating Air Compressor.

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