Application Tips: Air compressors

Using an air compressor is a versatile, economical way to power tools. In the construction world, tow-behind, rotary-screw units are by far the preferred compressor type because of their efficiency, portability and relative quietness.

When buying an air compressor, think about where your business is going in the future and match the compressor’s specifications to those jobs as well as current ones. Buy too small of a compressor and your tools may not operate correctly or at all, and you could waste time waiting for your air tanks to re-charge. Purchasing too large of a machine means added cost and perhaps extra weight and space, with no performance gains.

Portable, rotary-screw compressors range in air volume production from 65 cfm to 1,600 cfm, and have pressure ratings of 100 to 350 psi. Most applications – powering tools such as concrete breakers, for example – require only 100 psi of air pressure, says Chris Moloney, vice president of portable air products for Atlas Copco. Sandblasting and painting applications may call for 125 to 150 psi of pressure, and heavy-duty drilling jobs can demand 200 psi.

As far as air volume goes, compressor manufacturers agree contractors prefer 185-cfm models. These units are capable of powering two tools and are good for applications ranging from pavement breaking to boring holes underground and testing and cleaning pipelines, says Marc James, product manager for Ingersoll-Rand.

Because fewer contractors do the high-profile, large-capacity jobs – such as extremely deep drilling or sandblasting and painting bridges, tanks and water towers – that require 800 cfm or more, high-cfm air compressors are usually rental items.
If you are in the market for an air compressor, the following tips can help you purchase and effectively use a machine that’s right for your operation.

At the Jobsite
· Keep the machine in a well-ventilated area. Air compressors have to breathe, so to speak. They are designed for air to travel through them – through the coolers, the heat exchangers and then out.
· Inspect hoses. Look for holes or possible rupture points before every use. Make sure hose connections are properly secured as well. On high-pressure units, ensure the safety chain is attached. In the event of a failure in the connection, the safety chain will keep the hose attached to the machine.
The hose should be free of twists and turns, and its length as short as possible. While you don’t lose cfm because of hose length, you can lose pressure due to friction. “Air acts a lot like water,” James says. “The flow rate doesn’t change as you add more length when you’re watering your lawn, but if you get a lot of kinks in it you’re going to start losing pressure.”
· Keep your tools lubricated. You can oil them through ports on the tools or through in-line oilers – vessels that fit on the hose between the tool and the hose or between the compressor and the hose. Manufacturers sell in-line oilers as an option or after-market item.
· Set psi on your system to meet tool requirements. This is essential for tool longevity. One rule of thumb states that for every 20 psi over the manufacturer’s recommendation, tool life decreases by half.
· Don’t shut off the machine while the service valves are open. In a sandblasting application, for example, you’re running air through the hose to the abrasive pot, which is under pressure. If you shut down the compressor, that pressure has to go somewhere. It can go back toward the compressor and bring the sand along with it, causing a major malfunction if it happens enough times.
· Don’t use the emergency stop unnecessarily. A lot of operators do this because it shuts down the unit immediately, but over time the rubber coupling that connects the compressor element to the engine will wear out prematurely. The correct procedure is to unload the machine, allowing it to cool down, then turn it off.

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Another Option:
Truck-mounted air compressors

A truck-mounted air compressor can be positioned under the hood or on the truck’s deck.
Two underhood sizes are available – 70 cfm and 150 cfm – and both are rated up to 175 psi. The truck’s engine powers underhood compressors. Deck-mounted compressors, which can be independently powered or hydraulically driven through the truck’s engine, range from 25 cfm to 185 cfm.

A vehicle-mounted compressor will run the same tools as a tow-behind unit with equal cfm. Dan Hutchinson, VMAC marketing, says contractors use truck-mounted air compressors and tow-behinds in the same applications.

Use the following information to decide whether a truck-mounted compressor will work for you.
Underhood air compressors: On the plus side, these units don’t take up any space in the work area of the truck or body, they don’t require maintenance on a separate engine and they don’t need a fuel type different from that of the truck, which is a possibility with tow-behind units.

The downside is you can’t separate the truck from the compressor. And because the compressor operates off the truck’s engine, the truck has to be running in order for the compressor to work.

Deck-mounted air compressors: When equipped with its own engine, this kind of compressor will give you the alternative of leaving the machine at the site. A deck-mounted unit does, however, take up space on the truck’s body. And like tow-behinds, deck-mounted, stand-alone compressors require fuel and engine maintenance separate from the truck.

Hydraulic deck-mounted compressors have the same mobility minuses and engine-related pluses as underhood compressors.