A seed cleaning and distribution center had installed a small 20-hp lubricated screw compressor, a desiccant dryer and a large 1060-gal storage receiver installed in their facility. Even though the storage receiver was more than sufficient to reduce the compressor load/unload cycles down to a low frequency, the compressors continued to load and unload rapidly, making the compressors consume much more power than normal.
In addition, even though the compressed air load was low—and the moisture loading was much less than rated due to cool ambient temperatures—the desiccant dryer continued to purge at 100% of its flow, wasting compressed air energy.
The system was data logged by a compressed air auditor and it was determined that the air compressors could not “see” the storage tank. The air compressor sensed the system pressure right at its discharge. The storage receiver was located on the downstream side of the air dryer. The dryer had check valves installed inside it to prevent the reverse flow of compressed air.
Whenever the air compressor unloaded and stopped producing air, the downstream system pressure that it was supposed to “see” could not make its way past the check valves. Because the air dryer continued to purge, it consumed all the remaining air in the small length of pipe between it and the air compressor in less than a second. When this happened, the compressor sensed low pressure and loaded back up, even though the air it produced was not yet required by the main system. After a few seconds, the discharge pressure would build back up to the unload point and the compressor would unload. This pattern continued with the compressor loading and unloading at a high frequency, making the system extremely inefficient.
Dew point control was installed on the air dryer to reduce the purge flow to a lower level. The air compressor was outfitted with remote sensing that allowed it to “see” past the air dryer. These two measures reduced the compressor cycle frequency significantly and dropped system power by 45%. Future plans call for the installation of a VSD controlled compressor that will reduce compressor unloaded run time by 75% and save a further 30% for a total 75% savings.
Learn more about the effects of compressor cycle frequency 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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