A large trade school replaced their older reciprocating compressor with a newer lubricated screw compressor. The staff was tired of the noise of the piston compressor, hammering away during daytime activities. The newer and quieter screw compressor was an easy replacement — just drop it in and turn it on! It purred like a kitten. The unit ran no problem during busy days where the compressor would run near full load. The previous reciprocating compressor always ran hot, and never liked to run at full flow.
To save space, the compressor was purchased with an internal refrigerated air dryer to condition the air so no water formed in the piping that fed the complicated CNC machining tools the students were learning to use. No extra storage tank was added.
As part of a facility energy conservation program, a compressed air auditor was invited in to do a leakage audit. The survey found a few minor leaks in the facility, but a quick check of the compressor hours revealed some concerning data. The compressor had been running loaded only 240 of its 4,000 lifetime run hours. The unit was only about 6% loaded; the remainder of the time the unit ran unloaded, with short periods in automatic standby. This type of operation is very inefficient, as a result the compressor was running at a specific power of about 70 kW per 100 cfm. Normal is about 22 kW per 100 for an optimized system. Data logging in the graphic at the beginning of this article shows an average draw of 13.5 amps.
The old reciprocating compressor was still in place, so it was run as a test to compare operation. Under similar conditions, this unit ran with an average input of about 1.7 amps, about one-eighth that of the screw compressor. In this case, a screw compressor was much less efficient than a reciprocating compressor. This screw compressor consumed about $2,300 per year in electrical costs, with the internal non-cycling dryer consuming about $700 per year in power.
Unknown to the purchasers, this compressor should have been installed with larger storage receiver capacity sized at between 5 and 10 gallons per cfm capacity. With larger storage tanks, this unit could have been set to start and stop just like the reciprocating compressor, eliminating the wasteful unloaded run time that consumed 70% of the total compressor energy. Also, the internal dryer had the capability to be set to energy saver mode, where it would turn off, rather than waste energy when the compressor was not running.
Plans are to add more capacity and adjust the compressor settings for a 75% savings in power consumption.