If an air compressor ran uncontrolled at full load all the time during periods of light load, the system pressure would rise until something popped. Manufacturers have built-in ways to prevent overpressure, the simplest of which is start/stop and load/unload control, using pressure switches. But for large fixed-speed screw type air compressors, there are two additional methods of turning down the output flow to prevent high pressure: inlet modulation and variable displacement.
Inlet modulation will choke off the intake of an air compressor to reduce the flow output. This is a very inefficient way to control the flow, with the compressor consuming 70% power, even at zero flow. The power turn-down for modulation control is about 3% power for every 10% flow reduction, not very good.
Long ago, some manufacturers invented variable displacement control, where valves open within the screw element to effectively shorten the compression length. This has better power turn-down, but is only effective in turning the flow down by 50%, inlet modulation takes over between zero and 50%. And for a properly adjusted compressor, the compressors might unload at a certain pressure. Proper operation of this type of compressor requires careful adjustment of the inlet, capacity, and pressure switches to ensure coordination for each pressure setpoint of the compressor. If the setpoint changes, all three controls must be adjusted together. In some compressors, this is a manual adjustment.
Having so many methods of flow control in compressors is confusing, and most often a compressed air auditor will find that the compressors are not adjusted properly — or the capacity control have failed — but the compressor operators are not aware of the problem. Sometimes due to these issues, extra compressors are running or rental compressors have been brought in to add system capacity to avoid low system pressure.
A way to detect a problem with the controls is to visually check the mechanical position of the controls, if applicable. But some compressors don’t have any visual way of detecting valve position, so the best way is to detect the compressor amps. At 600 volts (in Canada) one horsepower of air compressor capacity will consume roughly one amp at full load, full pressure (always check if the compressor is loaded). The following table shows the multiplier for other voltages:
Volts |
Multiply by |
400 |
1.5 |
460 |
1.3 |
600 |
1.0 |
This number may vary depending on the type of compressor, liquid cooled units will consume a bit less. Compressors running at lower than rated pressure will consume less amps, by a factor of about 1% for every 2 psi in lower discharge pressure. An example calculation:
A 100 hp compressor, 460 volts, is consuming 100 amps when fully loaded. It should be consuming roughly 130 amps (100 x 1.3). There is likely an issue with compressor capacity, most often the controls (but not always).
Recently, a compressed air assessment was done at a large automotive manufacturer, and 80% of the compressor were found to be operating at low capacity. Some well-meaning technician had mistakenly adjusted all the capacity controls to a low level. Some controls had simply failed. The control in Fig. 1 shows the inlet butterfly valve is closed, reducing the compressor capacity by 60%
If you have compressors with capacity controls, be sure you get yourself trained to make proper adjustments. Watch the compressor amps to ensure full capacity when your system requires it.
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