The comparison of running versus unloaded hours shows this compressor is inefficient, running at only 30% duty.
By Ron Marshall
A paint manufacturer had a small compressed air system consisting of four lubricated screw compressors feeding general plant compressed air flows. As part of a compressed air assessment, the compressor operations were monitored and found that all four of the compressors were running during production but they were at reduced duty cycle. This was easily confirmed by observing the compressor running versus loaded operating hours (see photo above right).
A check of the collected data showed that all the compressors were fighting with each other for control. This caused all the compressors to load and unload, seemingly at random—and although each compressor had automatic shutdown, none of the compressors stopped at any time during production hours. A calculation showed that, rather than four compressors, only two were required to run during average conditions.
Checking the compressor pressure settings showed that all the compressors were set to exactly the same load and unload set points, rather than the typical cascade scheme. These settings were adjusted and two of the compressors immediately unloaded and shut down. One of the remaining compressors went to full load, its most efficient point, with the other running in trim duty, taking the remainder of the load. Data logging showed that a savings of 25% was gained just by a push of a few buttons. The power of measurement and observation of a compressed air system was confirmed again!
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Ron Marshall is a compressed air energy efficiency expert at www.compressedairaudit.com
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