Avoid the compressor control gap

Anybody who has maintained cars for teenage drivers knows that the teen approach to life, full throttle alternating with both feet on the brakes, doesn’t translate well into automotive asset health or fuel economy. Not surprisingly, the same applies to compressed air systems. Most are driven by single speed motors that prefer to be on or off, not constantly moving between those two states.

Unfortunately, plant air use varies dramatically with time of day, almost stopping at night in some factories. The result with single speed compressors is a power bill that may be over 90% of what it would cost to run the system at full speed 24 hours a day. Wear and tear, including introduction of leaks everywhere air surges can be felt, can be a large multiple of what full-power operation would have cost.
In recent years variable speed drives have dropped in price and improved in reliability to the point where they are viable competitors for the old, single speed systems. In fact, given the fact that power cost is something like 95% of total life cycle cost for electric motors, the new drives can often pay for themselves quickly by reducing power requirements and system wear.

On most compressed air systems it usually makes economic sense for least one of the motors to be a variable speed drive (VSD) unit that enables the system to adjust output to the changing plant usage by tapering output rather than offering just two speed settings. Vendors sometimes quote power cost reductions in the 30-40% range for drives and controls that keep compressed air systems out of the control gap.

Control gap arises when the drive array is forced to cycle, sometimes as often as every 20 seconds, between full on and full off in order to maintain plant pressure within specifications. Of course this administers a pounding to the equipment and introduces variability to any processes that use the air. If the largest drive is a VSD, then it can taper its output between full power and a very low partial power to match plant usage. The result is much closer to a steady state, which makes life much easier for the equipment.

In order to be effective in the evening and other low power times, the VSD must be the first-on unit in the system. Therefore, it must be as big as any of the other compressors that will come online after the VSD achieves full power. That way it can back down to near zero output and ramp up as demand increases. Then when a single speed unit comes online, the VSD can back down again and continue the ramping process for each subsequent compressor. If the VSD can’t ramp clear up to the power of a compressor that is coming on, the classic on and off jolting, called “uncontrolled” in the trade, will ensue, sometimes with all units, including the VSD, participating. This is, of course, a worst case scenario; call it “teen compression.” Like most teen things, it is to be avoided.

A white paper in the Plant Services library covers this topic in more detail. Download it at http://www.plantservices.com/whitepapers/2014/variable-speed-compressors/.

Read Stanton McGroarty's monthly column Strategic Maintenance.

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