Air compressor unloader valve

This can be two things: the small unloader valve on reciprocating piston compressor, or the big inloader valve at the inlet of rotary screw compressors.

The first type of air compressor unloader (on reciprocating compressors) is installed on the discharge line of the compressor. It is often combined as a check-valve.

Unloader valve on (small) reciprocating piston compressors

Small unloader valve. Photo: Atlas Copco

How does it work and why is it there?

The output of the piston compressor is connected through a pipe to the air receiver. When the compressor stops, the pressure in this pipe is blown off to 0 bar (atmosphere). This is the ssssssshhh sound you will here when the compressor stops. At the same time, the check-valve makes sure that not the whole air receiver (tank) flows empty, but only this small pipe.

Why? When the compressor starts up again, and this pipe would still be under pressure, the electro motor will not be strong enough to turn the compressor.

On smaller units, this valve is mounted on, or inside the pressure switch. On bigger units, it sits somewhere besides the unit.

Air compressor unloader valve on (big) rotary screw compressors

Rotary screw compressors also have an unloader valve, but this one is completely different. It is located on the air-intake of the compressor. When the set pressure is reached, the compressor will go in 'unload' mode: the unloader valve will almost completely close the air intake of the compressor. This way, no more are can be sucked in and compressed, so the compressor is basically just turning around, doing nothing. At the same time, the pressure that is still inside compressor element is released through a small hole (you will hear a sssssshhh sound).

Big unloader valve mounted on
top of compressor element

Note that there is always a small hole in the air compressor unloader valve, so it will never close 100%. This is to make sure there will be enough oil flowing for lubrication and cooling.

Most unloaders are of the LOAD/UNLOAD type. Which means they are either completely open (compressor loaded) or completely closed (compressor unloaded). In unloaded state, when the unloader is closed, the unit will consume about 30% of it's nominal power.

There is another type: continuous. This way the compressor can work at any point between 0 and 100%. It is not often used in stationary compressors (it's not the most economical way of regulation). But it is often seen in portable diesel engine driven compressors.