A pump manufacturer implemented a new compressor control strategy that was intended to save energy. The c0mpany had three compressors running — one fixed speed unit and two VSD style units — and found after studying the system they only needed two compressors to feed the load. Savings could be gained by automatically turning a compressor off using an automatic control system.
The compressor control system was installed, and data collection was done, to verify proper operation. The verification study found that the control system was doing its job. One compressor was turned off most of the time except during high peaks, saving energy; however there was a strange 4.5 kW power demand when the compressors were off.
Investigation revealed the compressors had air dryers installed internal to the enclosures. When the compressors turned off, the dryers continued to run, consuming extra power. It can be seen on the load duration plot in the graphic that the dryer ran unnecessarily 96% of the time while zero air flowed. This cost $7,300 in extra electricity charges!
Fortunately, a fix for this was easy. The control in each compressor can be set by a technician to turn off the air dryer when the compressor is off. A few click of some buttons solved the extra power consumption problem.
It is important to pay attention to air dryers when shutting down compressors, they can sometimes eat up part of the savings you expect from better compressor control. Often, installing cycling style dryers that turn off or turn down when no air flows can save significant electrical costs.