Compressor controls

About compressor controls

Since compressed air is generally required in varying quantities at the relatively constant pressure, compressor regulators or controls are used to vary the  delivery of the compressor.

if the pressure rise too high , The pilot (pressure responsive device) operate to reduce the air delivery.

If pressure falls too Low, The pilot operated to increase the delivery of air.

Several mechanisms are used as unloading devices to very the delivery of compressed air.

(1) the inlet valves may be held open during both the suction and compression stroke -  thus no air  will be compressed in the cylinder.

(2) clearance pockets may be used which are automatically controlled by the pilot ,reducing the volume of compressed air delivery.

(3) on rotary compressor, a valve which completely closed  the intake line, prevent the intake of air.

(4) on some compressors the speed is varied in response to pressure changes on the pilot.  as the  volume of output varies  directly with the compressor Speed, the output follows the demand. This is a common method with reciprocating steam driven and engines or gasoline and diesel engines.

(5) on small compressors, a type of control known as " automatic start and stop", which responds to a pressure actuated switch, opens or closes the motor circuit as the demands varies.  large compressors are usually kept running when unloaded.

The method of control used for a compressors demands on the type of service encountered. systems with nearly constantly demand are usually  supplied by a compressor which is operated continuously at nearly full capacity.

While on system with  variable demand, either the compressor( start and stop) or its pumping(unloader-control) action maybe stopped part of the time to minimise power consumption.

Unloader control is well suitable where a demand of 75 % or less of the compressor capacity.

Start stop operation is best suitable for application where the normal system demand is less than 40% of the compressor capacity.

Clearance pocket or clearance space is to avoid piston hitting the valve while closing and opening in a reciprocating compressor.

Unloader is provided to avoid compression while starting.This reduces the starting torque and avoid prime mover to draw overcurrent when starting.However unloader is also used when stopping the compressor,to avoid sudden twisting of the coupling.