A mine had a large 10,000-cfm air compressor installed to feed underground operations. Busy in its heyday, the mine operations had been reduced substantially, and the compressed air demand was on average 3,300 cfm. Unfortunately, even with the reduction in flow over the years, the electrical demand of the air compressor did not drop off substantially.
A compressed air auditor was called in to have a look. While viewing the large machine, and verifying that it was fully loaded, he wondered aloud where all the air was going. The local machine operator beaconed him over to the outside door and pointed up. There was the answer: a blow-off pipe expelled all the excess air.
The compressor was controlled with a blowoff valve, always keeping the compressor fully loaded; the valve was automatically opened or closed to maintain a set discharge pressure.
Of the 10,000 cfm the compressor produced, on average about 6,700 cfm was wasted by blowing off. The cost of operation of this continuously running compressor was estimated at $930,000 per year!
A project was initiated to replace this compressor with a system of four smaller 300-hp compressors controlled by a compressor sequencer. The sequencer ensured that only the required number of compressors were running at any one time. With this efficient control, the electrical operating costs fell by 65%, saving about $600,000 per year. A substantial utility incentive helped pay for 50% of the cost of the retrofit.
Sometimes compressor operators know the locations of compressed air waste — all it takes is to ask the right questions!
