Tech Stuff

Air compressor technology: Match the compressor to your specific needs

Cost of rotary screw vs. piston type: Myth buster

If you’re like many shop owners, your initial reaction may be, “Gee, I just can’t afford a rotary screw compressor.” Chances are, you’re fooling yourself. Yes, a rotary screw compressor carries a higher initial purchase price tag as compared to a piston type. However, one of the common mistakes that many shops make is to not consider the long-term cost. As but one example, many shops quickly tire of the noise that a piston type compressor generates. In order to cut down on noise, a shop may have a special room built to “hide” the compressor.

When you consider the cost of having a local contractor build a small room addition inside the shop to the tune of, let’s say, $3,000 to $5,000 or more, the total money spent (the piston compressor, filters, separators and the enclosed room) could have easily purchased a rotary screw type compressor that’s plug ‘n’ play ready. Yes, you could perform the construction yourself, but in reality, who has the time? From the outset, you could have a compressor that’s quiet and more efficient for the same or even less money.

Also, consider the wear and tear aspect. A piston compressor eventually requires rebuilding due to long-term wear of piston rings, cylinder walls, pistons and valves. As parts wear, more oil loss (pass-through) occurs, making the compressor less efficient and more prone to contaminate output air. This translates into down-time and more repair expense.

A rotary screw compressor’s screws don’t contact each other (no blade wear) and are able to deliver compressed air on a continuous basis when required (capable of 100% duty cycle with no drop in pressure, as compared to the approximately 60% to 70% duty cycle of a typical piston type compressor).

According to industry experts, a rotary screw compressor will last about 10 times longer as compared to a piston type, before requiring major service.

Summary

There’s nothing wrong with a piston type compressor. Either style (piston or screw) will get the job done. If you can talk yourself into the initial higher purchase price of a screw type, you’ll enjoy the lower noise level and the long-term operating benefits that this type offers.

Rotary compressors offer distinct advantages in noise, heat and air quality, but if these factors don’t directly impact productivity or workplace comfort/safety, they shouldn’t drive your buying decision.

If you don’t need much air, or don’t need it often, a piston compressor will be more cost effective. The more air you need, the stronger the argument for a rotary screw or rotary vane compressor. ●

Air compressor sources (some leading examples)

Atlas Copco, (866) 546-3508, www.atlascopco.us

Kaeser Compressors Inc., (540) 834-4520, www.kaeser.com

Snap-on, (877) 762-7664, www.snapon.com

Sullair Corp., (219) 861-5115, www.sullair.com

Comparison of applicable compressor types

Piston type Rotary screw Rotary vane

HP range up to 25 hp 3 - 650 hp 2 - 200 hp

Pressure (psi) up to 175 up to 215 up to 145

Volume (cfm) up to 100 9 - 3,000 8 - 1,100

How a typical rotary screw air compressor works

Atmospheric air is drawn through a pre-filter into the dry type air intake filter (1) where it is cleaned prior to compression. The atmospheric air is then compressed with the efficient screw airend (2). The airend is driven by an electric motor (3).

In this example, oil is injected (4) into the rotor housing to lubricate, seal and remove the heat generated by compression. As a result of this internal cooling mechanism, the temperature during compression reaches only 180 degrees Fahrenheit under normal operating conditions. Oil is separated from the compressed air stream by a three-stage oil/air separator (5). The separated oil is cooled in an oil cooler (6), flows through a micro oil filter (7) and returns to the injection point. This oil circulates solely by internal pressure, eliminating the need for an oil pump. The oil temperature is controlled over the entire range from cold start to full load or idle operation by an oil thermostatic valve (8).

The compressed air is separated from the injected oil in an oil separator cartridge (9), and is passed to an aftercooler (10) via the minimum pressure check valve (11). This valve maintains minimum system pressure to ensure continuous oil injection into the airend. The temperature of the compressed air is cooled to within 10 to 15 degrees F of ambient temperature by the air-cooled aftercooler, which removes a large percentage of moisture from the compressed air.

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