A health supplements manufacturer had just improved its compressed air system by adding a variable speed drive compressor to its system of fixed speed load/unload units. The VSD was sized so that it would carry the plant loading in all but the highest peak load conditions. If its capacity was exceeded, a fixed speed compressor would start and load to supplement the system capacity through coordinated pressure band control. Due to air quality requirements, a desiccant air dryer was required for the upgrade and a dryer with dew point dependent switching was installed to save purge air.
After the installation was complete, the local power utility performed a verification audit to ensure things were running as expected—that way, a financial incentive could be granted to assist with the project cost. Surprisingly, the system was consuming much more power than expected because the fixed speed compressor was constantly running unloaded. The compressed air loading was also higher than expected for some reason. The compressor that was running unloaded was consuming 35% of its full load power even though it was producing no air!
Analysis of the data tracked the cause of the running unloaded compressor to regular fluctuations in air pressure exactly every 7 minutes. The compressor was a type that had a special “smart” control that tracked the system pressure and turned the compressor on early in anticipation of a load cycle if the pressure falls at a rapid rate. Some local system fluctuations were causing a momentary rapid fall in pressure at the compressor discharge, confusing the smart controller into making the compressor start.
Investigation revealed the fluctuations were being caused by a timer operated condensate drain installed on the compressor water separator. The timer was set to operate every 7 minutes, and when it did the flow of compressed air it expelled to atmosphere caused the pressure to fall momentarily, tricking the compressor into starting. The compressor did not stop between cycles because it needed to run for at least ten minutes due to an internal timer that protects the compressor from having too many starts per hour. Excessive starts may damage the drive motor and prematurely wear out the compressor.
To improve the situation, an airless drain was installed that expelled only condensate and did not release a large blast of compressed air. This eliminated the pressure fluctuations and allowed the compressor to turn itself off and go into standby mode. In this case, a $400 investment into a more efficient drain saved $6,500 in electrical costs.
The unexpected high compressed air loading mentioned earlier turned out to be as a result of poor commissioning of the new dew point controlled dryer. The supplier supplied the dryer but did not turn the energy efficient dew point control on. As a result, the dryer ran on fixed cycle, wasting purge air—even when the plant was under light load conditions. The supplier felt that it was the responsibility of the customer to activate this feature. The customer assumed that the supplier would activate the control. The result was a waste of expensive compressed air.
This story illustrates the importance of verifying the proper operation of an upgraded system after installation. Often, equipment suppliers have little interest in activating the energy features sold with compressed air equipment. Many times, customers are not aware of the steps that need to be taken to ensure efficient operation.
Learn more about compressed air efficiency in our next Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org for more information. By Ron Marshall for the Compressed Air Challenge.
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