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Some compressors are not able to stop and start frequently enough due to mechanical limitations, as might demanded by a high volume compressed air use plant. If the cycle time between high and low air pressure set points is too frequent, this is negative to that type of compressor.
If your plant’s demand for air means that your compressor cycles on and off more than 15-20 times per hour, you’ve reached the “frequency benchmark” for considering moving from a 'Stop / Start' type of compressor to continuous run.
Of course, when the compressor runs continuously, it would continue to compress air into it’s receiver well past it’s pressure-safety zone, with potential catastrophic results.
This is where the Pilot Unloader Valve comes into play.
When the high pressure set point is reached in a continuous run compressor, the pressure switch signals the pilot unloader valve to open, relieving pressure from the compressor head. This allows the compressor to continue to run, but without load, which reduces wear, heat, and energy costs, and also prevents the compressor itself from continuing to compress air into the receiver or plant air mains.
When the lower air pressure set point is reached, the pilot unloader valve closes, and air is once again compressed, supplying the receiver and the air mains until, once again, the high pressure set point is reached.
For your interest, stop and start compressors have unloader valves of a different sort, and if you wish, you can read about them here.
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