Give them air

Pneumatic tools need clean air to run properly. A well-maintained compressor that's matched to the job will do the job.

Like many types of energy, air is free. What costs you is harnessing it and packaging it into a usable form.

That’s what air compressors do. They take air and “package” it so you can power your tools.

That packaging process is rather straightforward: A piston and cylinder compress air into a tank that gets released through a regulator that maintains pressure at around 90 psi. A compressor that can’t maintain an even flow of pressure works too hard and makes you wait. That’s not good for either of you.

You can minimize the “packaging” cost by selecting the right air compressor for your job, then maintaining it to assure a dependable, long life, say experts.

What contractors really want
Extensive contractor surveys by air compressor manufacturers have found contractors rank durability and performance as the most important criteria in choosing an air compressor.

“The technology has been here for a long time to produce air pressure,” says Brian Strickland, compressor product manager at Stanley-Bostitch. “The new compressor designs make them easier to use and less susceptible to jobsite hazards.”

Today’s portable units feature roll-cage designs that wrap critical compressor components in a protective shell. They also put hose ports, gauges and drain cocks in spots with better protection and more convenient access.

Start with selection
Getting the right compressor for your job starts with looking at the tools you use. “Identify the psi needs of the tool and then balance capacity with portability,” says Strickland.

Manufacturers have improved compressor designs so even larger contractor-grade models are easy to cart around the jobsite. Many models also have a lower center of gravity, making them less likely to fall over in a pickup bed.

Then, it’s a matter of selecting the power source. “Some areas get temporary poles in fast. Electricity isn’t an issue on these jobsites, so electric units are popular. However, some framing contractors like gas-powered units because you usually get more air pressure from the unit’s size,” says Rick Brashears, DeWalt’s compressor product manager.

Users looking to purchase a new compressor often get distracted by horsepower, says Dave Smith, product line manager for Porter-Cable’s compressor division. “Don’t buy on horsepower. Although it’s a legitimate number, it doesn’t tell you the strength of the air compressor. Instead, look at the maximum pressure and the size of the tank,” he says. “It is like the gas tank of your car. It dictates how far you can go before you need to fill up. With a compressor, it is how long the unit can operate a tool before the motor kicks in to refill the tank.”

Typically, tank pressure ranges between 100 and 135 psi. However, some air compressors have ratings as high as 175 psi. “It translates to useful air,” explains Smith. “If the tool operates at 90 psi, more pressure in the tank translates to more stored energy and therefore greater capacity. You can operate longer between compressor cycles,” he says.

Electric hand-carried compressors are built to run about 60 percent of the time for best efficiency, says Brashears. “These units supply between 2.8 and 5 cfm. Through testing, we have determined that 4.5 cfm is about the maximum obtainable cfm and still be able to consistently start the unit using 15-amp service,” he says.

Strickland concurs. “Motor manufacturers say the maximum cycle time for motors used on compressors is about 10 to 15 times per hour. The tank must be large enough for this to happen. Otherwise, the compressor runs too much.”

Oil-lubricated or oil-free?
Contractor-grade compressors are available in oil-lubricated and oil-free models. “The professional contractor market tends to select oil-lubricated units because they have the reputation for greater durability and longer life,” says Brashears.

“Oil-free models are lower-maintenance units. They can run at any angle, which is a plus on many jobsites. The downside is they have a limited life when compared with oil-lubricated units. They use a Teflon O-ring with an open crankshaft that can suck in dirt,” Brashears says.

“Most units can be repaired, which is something contractors don’t always realize. Replacing a seized piston and O-ring is not a hard job,” adds Strickland.

Oil-lubricated units offer higher cfm ratings and longer life when they are operated and maintained properly. Most units must be operated on level surfaces for proper lubrication. Oil-free compressors tend to be lighter than comparable oil-lubricated compressors, making them more attractive for high-mobility work patterns. Oil-free compressors are also better suited for working in enclosed spaces such as finished homes where oil contamination may cause damage.

Hoses and fittings matter
Hose selection affects tool performance. Keep the compressor as close as possible to the power source, then use hose to get the compressed air to the job. “You get more power loss by running an extension cord than by running hose from the compressor to the job,” says Smith.

Match the hose to application. As shown in table 1, the inside diameter of the hose should increase as the distance from the compressor increases and the tool capacity increases.

“If hose weight is a concern, add a section of smaller hose as the last section before it goes into the tool,” says Brashears.

Fittings can also affect tool performance. Low-cost quick-connect fittings can restrict air flow. They can also leak, robbing tool air pressure.

If one compressor must supply pressure to more than one medium- or larger-sized air tool, Smith suggests using high-flow regulators and quick-connect fittings.

Published in the November/December 2001 issue of Contractor Tools and Supplies magazine.

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