By Ron Marshall
A new maintenance manager was concerned about the compressor control in his plant. To him, it appeared that the performance of his three screw compressors was substandard, especially this variable speed drive compressor. And, he was running out of compressor capacity, the plant pressure was dropping to unacceptable levels, which was affecting production. To more fully investigate he called in a compressed air auditor to check things out.
The auditor dutifully placed measurement instruments throughout the plant to see how things were going. On inspecting the 50-hp VSD compressor, things didn’t seem right. The unit, running at full load, was drawing less amps than expected, and sounded like it was running at fixed speed. The nameplate and model number sticker on the side of the compressor enclosure said it was a variable speed compressor—yet on careful inspection, no electronic drive could be found. It was discovered that at some point in the past the drive had failed, so the company’s service firm removed it to get the compressor going, but never replaced it.
This particular compressor manufacturer runs their VSD compressor motors at 80 Hz—at the top end of the compressor variable range when at full output capacity. To get 80 Hz, you need an electronic drive. Now, since the unit was running only on 60 Hz mains power, the modified compressor has only 75% of the full rated output capacity of the original 50 hp set-up.
This industrial plant has quite a dusty environment, this is a characteristic of the particular industry involved. Keeping the compressor room free of dust, and at normal temperatures will be difficult if a new VSD compressor is installed. In this case, it is questionable whether it is an appropriate environment for a VSD compressor. Another strategy being considered is to use multiple smaller fixed speed compressors, running in start/stop automatic mode to provide more efficient trim capacity.