A large automobile auction house on the outskirts of a city was having problems with its small reciprocating air compressor. The 15-hp compressor was used to feed the compressed air tools used in the automotive repair shops within the facility, which previously required about 7 cfm of average flow. At one time, the compressor was more than adequate for all the activities within the shops, but now the unit was running fully loadedβthis caused it to overheat, damaging internal compressor components and causing hot oily compressed air to overwhelm the air dryer and contaminate downstream tools and equipment. Operating costs for the system were measured at $7,230 per year.
Since the facility was far outside the range of the city sewage system, the site was required to set up an effluent treatment pond for their grey water runoff. Local standards required that this pond be aerated. The easiest way to aerate the pond was to use 125 psi compressed air, regulated down to the few psi required to bubble air through the grey water. This aeration required a flow of 30 cfmβthe equivalent to a 7.5-hp air compressor at a supply pressure of 125 psi.
An investigation of the aeration nozzle depth revealed that a pressure of just under 3 psi was required for 6 feet of water. This compressed air application was determined to be an βinappropriate useβ of compressed air. A 3-hp aeration blower consuming 1.3 kW at 6 psi was installed instead to provide about 45% more low pressure air for better aeration.
The failing reciprocating compressor was replaced by a variable speed drive rotary screw compressor with large storage, a cycling dryer, low differential filter, and pressure/flow controller. A small amount of leaks were repaired. The new average compressed air loading fell to 4 cfm, and due to the lower flow and more efficient compressed air equipment, the total operating cost for both the compressed air system and blower was reduced to $1,300 per year. By completing the project, this company gained a substantial utility incentive to help them with the equipment replacement costs.
A savings of 82% was achieved by replacing βeasyβ with βsmart.β
Learn more about inappropriate uses of compressed air in the next Compressed Air Challenge webinar. Visit for more information.
By Ron Marshall, for the .
Warning! If the new average flow is down to 4 cfm, there could be a future bearing and air element failure in the future. Hopefully your new VSD sees periods where it runs in the upper end of the capacity range so the moisture can be cleared out of the lubricant. Should this compressor be doing an excessive number “short cycles”, which unload the air element, your energy savings go out the window and your condensate build up in the lubricant will cause corrosion in the bearings, clog the cooler, and foul up the separator filter. Your energy savings just cost you a major repair bill, plus with most utilities, you can’t get rebates on a new replacements for a period of 10 to 12 years…. but the new Atlas Copco VSD+ small compressors don’t un-load, so they might not suck in as much moisture under long periods of running at the bottom end of the load range. But even then, the VSD+ will need to run warm for part of the week..
Paul,why did the company use compressed air through a dryer to aerate the water?Would a blower giving high flow at low pressure had been more suitable for this application ,or was the water for some special process?
Regards Garry Evans
This compressor uses a lubricant that prevents water from pooling and ruining bearings, and is installed in a dry climate. The installation has large storage installed so the compressor will not cycle more than 15 times per hour in worst case conditions. Compared to the previously installed load/unload compressor the unit will pay for itself in energy savings every 3 or 4 years. There are hundreds of similar installations running in the territory with similar savings.
The Atlas Copco you mention is still new and yet to be proven out and are not available in some locations. Lets hope those units live up to their claims.
Why did they use commpressed air for aeration….because it was cheap and easy to hook up!