There are five different capacity control modes available for use with North American-built large industrial air compressors. It pays to know how each mode works, especially if you operate or service air compressors or happen to have your compressors operating in one of the least efficient modes.
The accompanying photograph shows just one design that was available in the past for variable displacement compressors. The dial shows that this compressor is running with the variable displacement valve (this manufacturer calls it a spiral valve) that is partly open at about 80% capacity. In the background we can see its inlet valve is wide open, the preferred condition.
For this compressor, the operation of this control is problematic because the site operators are running out of compressed air capacity. The control system on this compressor has a fault, reducing the output capacity of the unit. A sharp eye in this case can detect the problem and fix it.
The problem with newer controls is that valve position is not so easy to detect, however, because the spiral valve indicator is simply a bolt that sticks out of the screw element, and the inlet valve for this manufacturer is now a poppet style with no mechanical indication of its position.
In these cases, we must measure the compressor input power to determine if it is producing full flow — and if possible, measure the output flow with a properly installed flow meter. Lower than rated amps (when the discharge pressure is lower than the compressor setting) usually means low output flow, possibly pointing to a control problem.
The five different capacity control modes for compressors are:
- Start/stop — simply starting and stopping the compressor using pressure switch control. Very efficient but can only be used on small compressors.
- Modulation — choking of the inlet flow of air using an inlet valve. Very inefficient at part loads.
- Load/unload — Loading and unloading a compressor using inlet valve control and sump blowdown. Moderately efficient if the compressor has access to large storage.
- Variable displacement — Opening up ports in the screw to bypass the compression element. Good efficiency if set up correctly.
- Variable speed drive — Speeding up or slowing down the compressor to vary its output flow. Most efficient operation at part load if applied properly.
In the next few blogs, we will discuss the various operating modes.
