Sizing Compressors An air gun consumes small gulps of air with each nail it fires. These small gulps add up fast when banging down sheathing, or while running several nailers at once. If the compressor can’t keep up with the nailer, you get "stair stepping" — the nails will stick up progressively higher like the steps on a stair. To avoid this, you need the right size compressor.

Air delivery.

The capacity of a compressor is rated in a couple of ways — as air displacement and as air delivery. Air displacement (also called cubic feet displaced) measures the size of the piston. However, this spec is only useful to design engineers. For sizing up your compressor needs, look to air delivery. Air delivery (also called performance cfm of free air) describes the amount of usable air that a compressor can produce. Air delivery numbers are always given at a specific psi, since it’s harder to force air into a pressurized tank than into an empty one. To measure your air delivery requirements, you need to know how much air your nailer uses. These specs are available from the gun manufacturer. Framing nailers typically use around .15 cfm per nail. Running straight off the pump, you can drive about 17 nails per minute with a 2.5 cfm compressor. (2.5 divided by .15 = 17). Obviously, this isn’t enough for nailing off sheathing. However, in practice, a 2.5 cfm pump with a 4-gallon storage tank will allow you to shoot about 50 nails per minute without stair stepping. The extra air comes from the tank reserve. In general, a compressor rated at 2.5 to 3 cfm at 100 psi can run one framing nailer. If you’re running two framers, opt for a pump that delivers 4 to 6 cfm at 100 psi. For three framers, step up to an air delivery of 6 to 8 cfm at 100 psi. Or, you might try a daisy-chain technique (see ).

Wheelbarrow Features

Larger wheelbarrow models can be powered by heavy-duty capacitor-start/capacitor-run electric motors, or gas engines. Typically, those with gas engines provide the greatest output.

Gas engines.

Compressor manufacturers have been using small block four-cycle gas engines to drive portable compressor pumps for years. The side-valve Briggs used to be one of the most common engines used on wheelbarrow models, but in recent years it has been nudged out by the Honda overhead valve engine (OHV). An OHV engine has its intake and exhaust valves directly over the piston, allowing for faster intake and exhaust, and overall, more efficient combustion. By contrast, the design of a side-valve engine has inherent problems. Intake and exhaust are pulled out of ports on opposite sides of the combustion chamber, so you end up with a cold spot near the intake and a hot spot near the exhaust. This temperature difference can distort the cylinder bore, which might eventually cause the piston to seize. To avoid this, engine makers must shrink the piston relative to the cylinder, reducing ring tolerances. This creates a less efficient engine that must have a bigger displacement for a given power requirement. On any gas-powered compressor, look for these engine features:

   •Automatic decompression, which automatically opens the exhaust valve to relieve the pressure created in the cylinder by the compression stroke. This makes the engine turn over easier, thus it starts easier.

   •Low-oil shutdown, which automatically shuts off the engine when the oil drops below a safe level.

Belt drives.

Most wheelbarrow portables, whether gas or electric, are belt-driven. The engine or motor turns a large flywheel that drives a smaller flywheel on the compressor pump. The flywheels and belt are typically enclosed by a cage to keep fingers and material from getting snagged when the compressor kicks on. Belts typically loosen as the intense engine vibration rattles loose the bolts that secure the engine and pump to the deck plate. Over time, thin-gauge deck plates and corrosion compromise the connection, causing frequent slippage and a finicky belt drive. Some models have an automatic belt tensioner — a spring mechanism that takes up the slack in the belt as it loosens. While these tensioners may be helpful in the short-term, the best way to avoid problems is to evaluate the construction of the deck plate. Models such as Rol-Airs and the Emglo Master Series are notably rugged. Better still are the Thomas models that feature direct-drive, oil-less compressor pumps. This design eliminates the flywheels and belt drives altogether, resulting in a streamlined rig with no need for future adjustment.

Streamlined design.

All Thomas compressors feature oil-less compressor pumps. On wheelbarrow models, such as the Thomas T2820, the pumps run directly off the motor, eliminating the bulk of flywheel and belt drives.

Better by Design

A bright paint job can make any compressor look compelling. But if you have a chance to inspect a machine before you buy, you can tell a lot about its quality. There are several tangible differences in the construction of different compressors. Look for these top-quality features to distinguish the better models.

Gauges.

Integrated "control panels" are a growing and welcome trend on newer models, such as the Emglo Master Series and latest Bostitch (see photo).

Integrated controls.

The latest Bostitch portable features an integrated control panel that places the gauges and regulator control on top where they can be easily accessed. Here, the hose connects, regulator controls, and gauges have all been brought together in a tight panel. In general, all fittings and gauges should be "inside" the compressor – not hanging out past the contours of the machine where they can catch on your pant leg when you brush by it, or worse, catch on a door jamb when the unit is wheeled or carried into the building. Look for gauges that point upwards, so you can read them from above. You shouldn’t have to get down on your knees just to read a pressure gauge.

Drain valves.

Again Emglo’s Master Series sets a new standard with super-nice "soft-seat" drain valves that can be tightened and loosened by hand. Pay attention to where the drain valves are located. Valves should not be hanging out where they can do damage or be damaged. The valves on twin-tank designs should be oriented in the same direction, so you only have to tip the unit one way when draining the condensate at the end of the day. Also, look for "spud" fittings where any pipe or fitting connects to the tanks. A spud is a short threaded pipe that’s welded to the tank wall. It’s a much stronger connection. Cheaper compressors have fittings that simply tap into the tank wall, providing at best just a few threads for the connection. This connection will hold pressure when new, but can be easily damaged. A piece of lumber that falls onto the compressor, or a pipe that snags on the side of the truck when lifting it into the bed could easily jar the tap connection loose.

Regulators.

Cheap all-plastic regulators have shown up in recent years on lower-end compressors. While nearly all now have plastic knobs, better models have a painted all-metal housing. All regulator knobs should have a locking ring to prevent vibration from slowly changing your air settings.

Hardware.

Look for brass and copper "hardware" (fittings and pipes between pump and tank). Copper discharge pipes and intercooler lines discharge heat better than steel or aluminum tubing. The best models use a flexible braided tubing that can’t be crushed, kinked, or bent if the compressor takes a spill while unloading it, or when materials are thrown on top of it in the truck (unavoidable situations in the life of a job-site compressor). Brass fittings, though softer than steel, won’t corrode as easily.