A lumber mill manager proudly showed off his plant’s new variable speed drive controlled compressor. The unit had been purchased to increase system efficiency and to give the mill more capacity. A compressed air auditor, who had been sent by the power utility to verify the savings, checked the new compressor control history. He was surprised to see the compressor was running mostly at full load, with very little time in its variable range.
Data loggers were placed on all the compressors, one on the new variable speed unit and three more on each of three the fixed speed units that had served the plant for many years. Sure enough, the loggers quickly showed that the new VSD compressor was not properly coordinated within the pressure band of the three fixed speed load/unload setpoints. During production time, the VSD compressor was running at full load, one of its least efficient points, and two of the fixed speed units were loading and unloading to support the pressure. And, on occasion, the third fixed speed compressor started but ran unloaded, producing no air, but consuming significant power.
A check of the control settings showed the compressors should have been working correctly. However, it was discovered the pressure control points were taken from different sources. The fixed-speed units used local control on the upstream side of the air dryers and filters, but the VSD had remote sensing taken from the downstream side. Corrections had to be made to compensate for the pressure drop across the air dryer. A permanent fix will be to move all sensing points to the downstream location to make it easier to set up the compressors.
The controls were adjusted — now the VSD does the trimming and the fixed speed units run at full load, then quickly turn off when not required, minimizing wasteful unloaded run time. The adjustments, which required only a few pushes of buttons, immediately saved the plant 15% on energy, worth roughly $24,000 per year.