By Ron Marshall for the Compressed Air Challenge
A metal foundry had a compressed air audit done that showed their three load/unload compressors were running inefficiently. Even though the units were controlled by a sequencer—which was supposed to coordinate the compressors according to the plant flow—the compressors were running inefficiently.
Data logging showed significant pressure differential across their dryers and filters. This caused the compressors to short cycle because the compressor controller was sensing the control pressure at the compressor discharge. Control problems were also evident; one compressor had a sticking inlet valve which prevented it from going to full load, which caused pressure issues during high plant peaks.
Based on recommendations from the auditor, plant personnel improved the pressure differential across the drying and filtering equipment and upsized all the piping in the compressor room. To eliminate wasteful unloaded run time, a VSD compressor was purchased that was larger than each of the base units to prevent control gap issues. The plant personnel were proud of their improvements and were anticipating significant energy savings.
The air auditor was invited back to the plant and did some additional verification data logging. The measurements showed that the four compressors were running inefficiently. A histogram on the VSD control showed that since start-up, the compressor had never run at partial load, where VSD compressors run best. The VSD compressor had been set so that it was running fully loaded all the time, with the other three compressors rapidly loading and unloading. Analysis revealed that incorrect settings were applied to the VSD and the compressor sequencer after all the changes.
Under the auditors guidance, the settings were corrected so that the base compressors operated in a wider pressure band—with the VSD compressor target pressure set inside the sequencer load/unload pressure band. These settings allowed the VSD to take any partial loads (trim) and the base units to come on and turn off as required, minimizing the wasteful unloaded run time and eliminating inefficient short cycling. Energy savings were estimated at 35%, even though the plant flow had increased.
This was a prime example of inadequate training of the user by the supplier of the VSD equipment. The customer had no idea how to correctly set up his compressors for efficient operation—he had trusted his supplier for correct advice. This is a very common problem when installing VSD controlled compressors because non-traditional pressure setting coordination needs to be used.
Learn more about compressor control in our next Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org for more information.
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