Air Compressors and Energy

By: James Hunt

You may discover that, even though you are using a smaller amount of compressed air, the electric energy consumed does not diminish proportionately and occasionally does not lessen at all. In one case, after the leaks were fixed, the system pressure rose and the electric power consumed increased. A frequent reason for this result is that the delivery controls are just not responding to system demand.

The two most effective ways to run an air compressor are at full load and off. Unloading controls that operate continuously restrict the air delivered to the system. This is always a compromise that is never as efficient as operating at full load. A great deal has been written about how capacity controls operate. Now it is time to look at some of the common problems that affect their ability to perform.

When working effectively, unloading controls do several things. They match supply to demand and minimize over-pressure while maintaining the minimum acceptable pressure. They make input power proportional to air flow demand. They turn off and reactivate air compressors as required.

The working pressure band is the pressure range the control can span from fully loaded flow to fully unloaded. Well-planned systems hold as narrow a band as possible. The first control type is automatic start/stop that automatic starts and stops the electric motor or driver. A pressure switch shuts off the motor at the upper pressure limit and restarts the motor at a lower pressure. Heat buildup in AC electric motors limits the number of starts allowed over a given time, particularly for motors larger than 10 to 25 hp.

Unloading with continuous-run controls lets the motor run while the compressor is unloaded. This matches supply to demand, usually on the basis of system pressure. Step controls are also called load/no-load or two-step that keeps the inlet either fully open or fully shut. The unit is at full load from the preset minimum pressure point to the preset maximum pressure.

The basic performance is fully loaded or full flow up to the preset maximum pressure when the air flow shuts off completely. The unit then idles at no flow until pressure falls to the preset minimum when the unit goes to full flow. In this control, the compressor runs at its two most efficient modes-full load and full idle.

When controls, piping or storage is misapplied, not only is there little or no power savings, but short cycling can result. Too much backpressure in the interconnecting system can lead to short cycling or ineffective unloading. A duty cycle of 20 seconds "on" and 20 seconds "off" damages equipment and shortens the life of bearings, coolers, motors and separators.

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