By Ron Marshall for the Compressed Air Challenge
A PVC window components manufacturer upgraded their compressed air system a few years ago by adding a VSD controlled compressor. To provide backup capability, they kept their old fixed speed compressor. Measurement and verification of the new installation showed that the company initially gained energy savings of 55% worth $20,700 annually.
The company was originally told that to ensure savings levels were maintained, the VSD compressor had to be the main compressor that was always running. But, as time went on, the operating hours of this new unit accumulatedβwith the backup compressor having minimal run time. This bothered the maintenance manager, because he liked his compressors to have equal run hours to make calculating the regular maintenance checks easier. He also wanted the fixed-speed backup to be regularly exercised, to keep the moving parts well lubricated. To make this happen, he instructed his technicians to regularly run the backup.
Initially, this run time was only to be a few hours a month. But, after a few years, the personnel at the plant changed and people forgot the initial reason why the new compressor was installed. Since the VSD had most of the run time hours, plant personnel decided to run the fixed-speed compressor full time, so it could catch up to the same number of hours, too. About that time, the power utility performed a random audit to check if the expected savings of the previous project had been maintained.
The audit showed that running the fixed-speed unit rather than the VSD was very costly because the unit was accumulating a significant number of unloaded run hours, where the compressor consumes about 35% of its full load power, but produces no air. The efficiency calculations showed that the system was running at poorer efficiency than before the project due to lighter loading on the fixed speed compressor. Running the VSD compressor had previously eliminated this wasted power consumption, but because it was turned off, it was ineffective. After a long talk with the maintenance crew, the VSD compressor was switched back to full-time duty, and is now saving even more electrical costs than before because power rates are higher.
Learn more about VSD compressor controls in our next Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org
