Compressed air fail: Saving money on compressor purchase


By Ron Marshall

This modulating compressor consumes 70% of its full load capacity at 2% load.

This modulating compressor consumes 70% of its full load capacity at 2% load.

A mine located in northern Canada has a compressed air system used to generate 150 psi compressed air for diesel engine starting duty. Due to the failure of the original compressor, a new compressor was purchased as a replacement. The previous compressor operated in load/unload mode with timed shutdown. This ensured the unit had minimal wasteful unloaded run time, saving power.

When the replacement compressor was selected, the local purchasers looked to keep the cost of the new compressor down. A supplier offered an attractive price for a screw compressor similar to the original—but unknown to them, the compressor operated in a different control mode called modulation. This compressor did not operate in load/unload, nor did it have timed shutdown … these options were not installed to save costs.

A compressed air auditor assessed the new compressor operation and found that it was consuming 27 kW while outputting about 10 cfm of average flow. This works out to a specific power of 270 kW/100 cfm. Normal operation would be 25 kW/100 cfm, less than one-tenth the energy intensity. The mine, located in a remote location, pays 32 cents per kWh. This compressor is costing $81,000 per year in electricity!

A second system using load/unload control with timed shutdown exists in another location in the mine, also producing starting air. This system was also monitored and found to consume about $6,500 per year in electricity because of better control energy efficiency. After comparing the two systems, it would appear the attractively low price of the replacement compressor has not generated the desired savings for the company. They are now considering another replacement, this time with better control.

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Ron Marshall is a compressed air energy efficiency expert at


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