A pork processing plant installed a modern compressed air system with excellent components. The compressors were cutting-edge VSD style, connected to a sophisticated controller. Air dryers were desiccant style with dew point control and low differential filtration. Significant storage was installed to assist in keeping the compressors running efficiently without pressure issues.
This system should have run very efficiently—right?
Well, some issues with the plant’s demand, as well as what was done about it have caused the system to become unreliable and inefficient.
Sizing systems is always tricky business—and this site was no exception. The plant engineers did a fairly accurate job in estimating the compressed air demand when in the design stages. However, the growing parent company was lucky enough to win new contracts during construction. The result was more production lines being installed than planned and higher compressed air loading.
A compressed air assessment was done when the plant was commissioned. This showed that the plant was running out of air capacity on occasion, which affected the production lines. A new compressor was needed to augment the two new 75-hp variable speed drive compressors. Plant management was not comfortable purchasing a compressor the same size as the existing compressors. It was felt that a larger fixed speed compressor should be selected, in case there were more production increases.
Little did management know that by selecting a larger fixed speed compressor, they introduced a control gap problem into the system. Whenever VSD compressors are to be used, it is essential that the fixed speed base units be equal to or smaller than the VSDs. If they are not then the fixed speed and VSD compressors will fight for control, with the VSD’s alternating between full load—their least efficient point—and unload at the same time as the fixed speed operates in load/unload … an inefficient way to operate. Far be it for the manufacturer’s sales representative to point this out, because it would ruin a very nice sale.
The result is a system made up of very efficient equipment running with poor efficiency and very unstable pressure. The system pressure control is basically in a state of chaos.
One or more smaller fixed speed units should have been purchased to prevent the control gap from happening. Improvements are pending.
Learn more about compressed air control in our next Compressed Air Challenge webinar. Visit www.compressedairchallenge.org for more information.
By Ron Marshall for the Compressed Air Challenge.
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