A technical high school had to replace its 12-year-old lubricated screw air compressor after 100,000 hours of operation. The unit had simply worn out after many hours of operation.
The school maintenance staff was unaware that some screw compressors, if installed with very little storage, will run unloaded continuously, consuming significant power but producing no air. This 15-hp unit ran constantly for more than 10 years, never shutting off except briefly for maintenance intervals.
The school only operates its shops for six hours per day, five days per week over 10 months of the year. This calculates out to only about 1,200 hours per year. However, the actual annual operation of this unit was more than 8,000 per year. The compressor was loaded about 2% of the time and consumed 5 kW when unloaded, so the wasted energy calculated to a cost of roughly $4,000 every year. Through the life of the compressor, about $48,000 worth of energy was wasted. The maintenance supervisor was surprised and said he had learned something he did not know — this proved to be a very expensive lesson.
A better choice would have been a compressor that was rated for start/stop operation or VSD control that would consume no unloaded power. A similar-sized compressor running for 1,200 hours (with the unit being shut down at night) would consume about $100 worth of electricity per year in the expected 24 annual hours of loaded run time. And this solution is exactly what the school has done now. They have learned their lesson well.