Air Compressors: Types, Uses and Features

An electric motor drives a pump — a piston moving back and forth in a cylinder, in the small reciprocating compressors common in home shops — that draws air in through an intake filter and pushes it through a check valve into the tank. A pressure switch stops the motor when the tank reaches its cut-out pressure and starts it again when the pressure falls to its cut-in pressure; a gauge shows the tank pressure. Between the tank and the hose, a regulator lowers the pressure to what the tool needs, often with a second gauge showing the regulated outlet pressure. A safety valve protects the tank if the pressure rises too far, and a drain valve at the bottom lets out the water that condenses inside. California Air Tools' manual labels each of these parts on its own compressor; layouts vary by model.

• Oil-free and oil-lubricated pumps. DEWALT's DWFP55126 has an oil-free pump; Makita's MAC2400 has an oil-lubricated cast-iron pump with an oil sight glass. Each needs the care its own manual sets out. Neither is better everywhere, and form factor — pancake, twin-stack or hot dog — says nothing by itself about performance.
• Portable and stationary. Small portable units like these plug into an ordinary outlet; large stationary compressors are a different class of machine, with installation set by their makers and local rules, and are beyond this page.

Pressure, in PSI, is how hard the air pushes. Airflow, in cubic feet per minute, is how much air the compressor delivers. They are different quantities and cannot be compared with each other: a tool's airflow need is matched against a compressor's airflow, never against its pressure. Airflow also depends on the pressure it is measured at — Makita rates its MAC2400 at 4.8 CFM at 40 PSI but 4.2 CFM at 90 PSI — so a rating means nothing without its pressure. Makers also use different terms: DEWALT quotes SCFM (standard cubic feet per minute, airflow corrected to defined reference conditions, here tested per ISO 1217), while Makita quotes CFM. The two are not the same measure, and one cannot be converted into the other without knowing the conditions behind each.

• Maximum pressure is the tank's cut-out pressure, not the pressure a tool runs at. A higher maximum stores more air in the same tank, but it is not more airflow.
• Horsepower is stated in different ways — Makita gives a running and a maximum figure — and is not a measure of the air delivered. Compare delivered airflow at a stated pressure instead.
• Volts and amps describe the motor's electrical supply, not the air it makes.
• Duty cycle, where a maker states one, is that unit's limit on how long it may run in a period. Neither maker here states one, and it cannot be worked out from the tank or the motor.

The tank stores compressed air between the pump and the tool. A larger tank holds a larger reserve, so the motor starts and stops less often when a tool uses air in bursts. It does not make more air. A tool that uses air continuously will, sooner or later, use it faster or slower than the pump delivers it; once the reserve is used, only the pump's delivered airflow counts. How long a given tank lasts, or how fast it refills, depends on the pump's delivery, the switch pressures and the tool's demand together, and is not given here.

Spray guns and finishing need more than air: the finish's own instructions govern equipment, ventilation and ignition sources, as set out in wood finish safety. How to run a nailer or a sander belongs to that tool's manual and its own BenchRoot page; the random orbital sander guide covers electric sanders.

Tank pressure rises and falls between the switch's cut-in and cut-out pressures. The regulator holds the outlet at a lower, steadier pressure, which you set for the tool, reading the outlet gauge where there is one. Set it to the tool maker's operating pressure — not to the tank's maximum. Gauges are only as accurate as their makers state. The pressure switch's cut-in and cut-out are set at the factory: California Air Tools' manual says never to adjust the factory-set pressures, and to stop and contact the dealer if the switch seems to need adjusting.

• Rated parts. California Air Tools' manual calls for hoses, fittings and couplings designed for use with air compressors, inspected for leaks and wear and replaced rather than repaired. Check that every part's pressure rating covers the compressor's maximum pressure, as its maker states it.
• Hose length and size. A longer or narrower hose loses more pressure on the way to the tool. How much depends on the hose, the fittings and the airflow, so no limit is given here; follow the tool and hose makers' recommendations.
• Couplers. Quick couplers come in more than one plug profile. A fitting described as 1/4 in NPT tells you its pipe thread, not which plugs it accepts, and two profiles that seem to fit are not necessarily compatible. Match plugs to couplers by the profile their maker states.
• Fixed piping. Running air lines through a shop is a separate engineering job, beyond this page. Do not use ordinary plastic water pipe: OSHA's 1988 bulletin describes a PVC compressed-air line rupturing and injuring a worker, and quotes the Plastics Pipe Institute's recommendation against thermoplastic pipe for compressed air; California Air Tools' manual also rules out plastic pipe for a discharge line because of heat.

Compressing air squeezes out the water vapour in it, and that water collects in the tank. Left there, it rusts the tank from inside and can be carried into the air line. Drain it through the drain valve as your compressor's manual directs. Makers' schedules differ, and even one manual can give two: California Air Tools' safety section says to drain the tank daily or after each use, while its maintenance section gives an average of every 2 to 3 days, depending on conditions and use — so following the stricter instruction is the cautious reading. If the tank is rusted or leaking, that manual says to replace it immediately and never to repair, weld, drill or modify it. A damaged tank is not something to patch or keep using.

• Electrical supply. Follow your compressor's manual and local electrical rules. California Air Tools, for its 4620AC, recommends a dedicated circuit for best results and limits any extension cord to 25 ft and at least 14 gauge, warning that a long or thin cord can damage the motor; other compressors set their own requirements.
• Placement. That manual warns that the motor and pressure switch normally spark and can ignite gasoline or solvent vapours, so keep a compressor away from flammable vapours and finishing areas, use it in a well-ventilated place, keep its intake clear, and do not cover it while it runs: its surfaces get hot. Clearance and temperature limits come from your own manual.
• Noise. DEWALT states 75.5 dBA and Makita 79 dB for their units, but neither page gives the distance or conditions of the measurement, so the figures are not directly comparable. Hearing protection is covered in woodworking safety equipment.

Blowing sawdust off a bench or a machine with compressed air does not remove it: it lifts fine dust into the air you breathe and spreads it around the shop. A dust collector or a shop vacuum captures it instead. Never blow dust off your skin or clothes, and never point an air stream at a person or animal, as California Air Tools' manual warns; the air from a compressor is not fit to breathe. In workplaces, OSHA's rule for employers is that compressed air may be used for cleaning only where it is reduced to less than 30 p.s.i., and then only with effective chip guarding and personal protective equipment. That rule governs employers, not a home shop, and it is not a general limit on compressor pressure — but it shows how seriously cleaning with air is treated.