As you know,Air compressor is used to increase the air pressure by reducing its volume.Every compressor has to discharge some specific pressure so we can use this utility to operate pneumatic tools etc.Basically electro-pneumatic control is a part of instrument engineering.
To control discharge air pressure of Air Compressor,some controlling devices as like M-type unloader,W-type unloader etc.are used in old days.
The accuracy of such devices are not perfect,to overcome this problem" Electro-Pneumatic control" is introduced.
This is an automatic type capacity control used to replace mechanical unloader.The discharge pressure may be control in 2-step,3-step or 5-step depending upon the model & capacity of the compressor.
It consist of 2 way or 3 way solenoid valves operated by the pressure switches,actuating at receiver pressure.Copper or Nylon tube is used for flow of air from receiver to suction or inlet valve through pressure switch & solenoid valve.
This system also provides the compressor to operate at no-load,till the electric motor attains normal speed & lubricating oil builds up to the required pressure,at the time of initial starting.
At starting of Air Compressor,the air pressure inside the receiver is low so pressure switches energises the solenoid valves.This will stop air supply to inlet or suction valve & compressor comes to full speed,which is known as 100% load.As the receiver pressure increases than the set pressure of pressure switch-1,the contact of switch opens & cut the electric supply of respective solenoid valve & de-energises the same.
Due to this air supply flows to inlet or suction valve and one set of inlet valve open thus compressor unloades partially.i.e. 50% load.If the consumption of compressed air is low than production then air pressure in receiver further increase to the set pressure of another pressure (switch-2) again the contact of switch opens & de-energies the respective solenoid valve.
This unloads the other set of inlet or suction valve & compressor gets fully unloaded which is known as 0% load.
Similarly,fall in receiver pressure causes the pressure switches to energises the solenoid valve & stops air supply to inlet or suction valve & compressor comes on load in steps i.e. 50% load & 100% load.
The pressure switches may be acting direct at receiver pressure or at reduced pressure where process controller is fitted.
As this is modern type of capacity controller,it is essential to take following precautions for smooth operation of capacity control.
01)Keep control filters clean,by draining the moisture collected in the filter bowl.otherwise,this will mix up with dust,oil & other materials and forms paste and restrict the air flow of air or sometimes enter into the inlet valves.
02)Stop air leakage in the impulse line.
03)Do not disturb the setting of pressure switches.
04)Set minimum 0.5 kg/cm square as differential pressure to operate the switch.
05)Use only genuine quality pressure switch & solenoid valves.