When sizing air compressors, the typical strategy is to err on the side of caution. Nobody wants to be the dummy who sized the compressor too small, causing low pressure production shut downs. So when compressors are installed, the usual act is to buy the next size bigger, without regard to the actual compressed air demand. Sometimes the compressor will be further upsized for potential future growth, or purchased bigger because one or more safety factors were applied to the sizing calculations.
The result of all this is oversizing is that the air compressor may be twice, three times or even five times the required size — with significant extra energy cost penalty due to this mistake. These days, VSD compressors are often installed — and these units can automatically correct for this oversizing problem, and reduce energy consumption — by turning down the speed. But if the compressor is grossly oversized, causing the compressor to run below its minimum speed, problems may result.
When a VSD compressor runs below minimum speed, the control typically changes the operation to start/stop mode. This saves energy, but the resulting heat of compression is not enough to keep the compressor air end at high enough temperature to boil off the water that forms in the compressor lubricant. This can cause free water to collect in the compressor lubricant, leading to damaged bearings due to poor lubrication characteristics. And operation in start/stop mode can cause the compressor to throw off more lubricant into the airstream, contaminating downstream piping and equipment.
If you find your compressor is always running below minimum speed, some right sizing may be required. Often times, a smaller pony compressor can be used to supply low flows, using the larger compressor only when peak output flows are required. Ask your compressor service provider for some help with this, and consider having your compressed air flow profile measured to make sure things are going smoothly … and that your compressor size is okay.