All compressors operate in the same basic way. A motor and compressor pump work together to draw in air, compress the air, and then force it into a tank. Once the pressure in the tank reaches a specific level (the cut-out pressure level), the compressor stops running. The compressed air is then available for use by the air tool. An air hose attaches to a quick connect out­let on the compressor and onto an air plug on the air tool, enabling air to flow between the two. A dial on the compres­sor (the regulator) enables you to adjust the amount of air delivered to the tool. Once the stored air in the tank drops to a specified level (the cut-in pressure level), the compressor starts up again to refill the tank. A tank pressure gauge shows the air pressure inside the tank, while a regulated tank pressure gauge shows the pressure of the air coming out of the air hose. In the event of a malfunction, a safety valve automatically opens to release air from the tank. A drain valve on the bottom of the tank enables mois­ture to be released. See “Know Your Tools” for a more detailed view of the parts that make up an air compressor.

The function of a compressor is to provide a source of air power (compressed air) for use by air tools. It’s somewhat analogous to the service panel in your shop that provides elec­trical power that is used by your machinery. Ideally, you want to know what kind of machinery you’ll be using in your shop, and the power requirements for that machinery, before you install (or upgrade) the service panel.

The same principle applies before you purchase a com­pressor. You’ll want to think about the kind of work that you plan to do with that compressed air, and the types of air tools you’ll need, before you buy the compressor. This is important because each air tool has a specific requirement for the vol­ume of air, measured in CFM (cubic feet per minute), that it requires to function properly. However, because CFM varies with atmospheric pressure, temperature, and humidity, manu­facturers often use the SCFM (standard cubic feet per minute) rate, which takes these variables into account.

The SCFM rating on tools can vary quite a bit, from as little as 2 SCFM for a brad nailer, and as high as 25 or 30 SCFM for a sander. Compressors also have a specific maximum SCFM they can deliver. It’s a good idea to list the specific air tools you’re likely to want, along with the SCFM requirement for each tool. The highest SCFM airflow requirement in your tool list is the magic num­ber – the compressor you choose should, minimally, match this number. Of course, if you plan to run two air tools simultane­ously, you’ll need to add the SCFM requirements of both tools to arrive at your magic number.

Along with air flow, compressors are also rated by their pres­sure level, as measured in PSI – the pressure level at which air is delivered to the tool. Most of the air tools that woodwork­ers use will operate at a pressure of between 70 and 120 PSI. Quite often you’ll see the PSI level listed at 90 PSI, which is a common working pressure. For example, my 18-gauge brad nailer consumes 2.4 SCFM at 90 PSI. In order to use the nailer, my compressor will have to deliver at least 2.4 SCFM of air.

Various Shapes and Sizes – Small compressors generally come in three varieties – single, twin and pancake. The differences are mainly aesthetic, though the single and pancake types are often lighter, in terms of weight, and will be easier to carry around your shop and home.

While a large stationary compressor can be run continuously for hours on end, smaller compressors need to cool down. The duty cycle specifies the amount of time that the compressor motor has to rest. A common duty cycle for compact compres­sors is 50 percent. This means that for every 15 minutes of use, the compressor has to rest for 15 minutes.

If you use air tools intermittently, then you don’t have to worry about exceeding the duty cycle. However, extended over­use is likely to cause excessive wear on the compressor pump or overheat the motor. Unfortunately, not all manufacturers list the duty cycle, though you’re probably safe in assuming a 50 percent level for a compact unit. In general, choose the com­pressor with the higher duty cycle. In most small hobby shops this is not an issue.

Basically, there are four ways to categorize compressors: electric and gas; single- and two-stage; oil-less and oil-lubed; and portable and stationary.

Most people will opt for an electric compressor, unless you’ll be working outdoors on a job site where there is no, or limited access to, electricity.

Likewise, you’ll want to choose a single-stage compressor, which compresses air once before it’s forced into a tank. As you might have guessed, air is compressed twice in a two-stage compressor – this provides greater storage in the tank, giving a higher working pressure. Almost all smaller, compact compres­sors are single-stage. However, if several people were going to use the compressor at the same time, then you would want to consider a two-stage unit.

Choose an oil-lubricated compressor if you want a quieter machine, and one that will give you the longest service life. Steer towards an oil-less unit if you don’t want to deal with the added maintenance that an oil-lube machine requires, and you want to spend less money.

If you’ll be using air tools intermittently, in short bursts, then a smaller tank will do quite nicely. Compressors with tanks up to about 6 gallons are generally small enough to be hand-carried, though units under 3 gallons are much easier to carry. They’re easy to move around the shop and transport to a job site, and they can be easily stored under a workbench. These compact (aka “hand-carry”) compressors are the most common size you’ll find in smaller, one-person woodworking shops. Renovators, cabinet and flooring installers, finish carpenters, hobbyist woodworkers, and DIYers prefer this size as well. They come in a variety of tank configurations – single (or “hot dogs”), twin (“double”, or “pon­toon”), and pancakes. Singles and pancakes tend to be the lightest in weight, and also the easiest to carry around.

Mid-sized compressors (aka “roll-aways”) have tanks up to about 40 gallons. These heavier, larger units are also consid­ered portable as they are usually mounted on wheels. But don’t try lifting one up. Anything over 40 gallons is likely meant to remain stationary. These are best suited for more continuous operation, and can accommodate multiple users simultaneously.

The vast majority of readers of this magazine will likely best be served by an electric, single-stage, oil-less, portable com­pressor. An exception will be those wanting to run sanders, or other air tools with huge air appetites, or shops where two or more people might need to use air tools at the same time. In these cases, an electric, two-stage, oil-lube, stationary unit would be the better choice.

Staying Put – Though some larger compressors have wheels, they are generally meant to be kept stationary. Wheels will make it easier to move a mid- to large-size compressor when needed, but don’t count on taking one of these up a flight of stairs, as they weigh far too much. Larger compressors also have the ability to power air tools with high SCFM ratings, and have the ability to work continuously.

Once you’ve settled on the SCFM rating you’ll need, and picked out a tank size range, it’s time to narrow your selection by looking at specific model features. To help you out, we’ve given some of the most popular compact compressors a once over (see “Compressors 3 Gallons and Under” on page 41). Whether you are looking at a compressor 3 gallons or less, or a unit much larger, here are a few things you’ll want to consider:

Horsepower (HP) isn’t an overly useful factor. All other things being equal, choose a compressor with the higher HP rating – only if the rating is specified as “running HP” and not “peak HP”. However, amperage draw is worth considering if the compressor will be on a shared circuit. Amp draw can vary widely, from as low as 2 or 3 amps up to 15 amps. The greater the draw, the more likely the circuit breaker will trip when the compressor kicks on to replenish air in the tank.

When air in the tank drops to a preset level, around 90 or 100 PSI, the motor starts up to refill the tank. When it reaches a preset cut-out level, typically about 150 PSI, the motor stops. The time it takes to refill the tank is called the recovery time, and it can vary from 15 or 20 seconds to a minute or more. The longer the recovery time, the more noise you have to put up with. Some of the newer compact com­pressors on the market have a cut-out pressure of 200 PSI. These compressors can store about one-third more air than a compressor with an equivalent tank size that cuts out at 150 PSI. Air tools will run longer before the motor comes on to refill the tank.

Large, preferably upward facing gauges are easier to read, and you won’t have to haunch down every time you want to read the pressure level.

The drain valve needs to be opened and closed every day; otherwise, moisture can rust the inside of tank, and foul your air hose. Some compressors have the valve underneath the tank, requiring you to lift, or tilt the unit to access the valve. Others place the valve at the back of the tank – if you store the compressor under a workbench or up against a wall, you’ll have to pull the compressor forwards to access the valve. A valve positioned at the bottom of the front of the tank makes for easiest access.
The air filter is somewhat less of an issue, as you only have to clean or replace it a few times a year. The frequency depends on how often you use the unit, and how dusty your shop is. The easier the filter is to access, the more likely you are to actually change it.
Air compressors are noisy, but intermittently so. Most seem to be in the 80-90 decibel range. If you work in a small shop tucked in the basement of your house, or in an attached garage, it can prove quite irritating. To deal with the noise, you could place the compressor outside (in mild weather), in a sec­ond room nearby, or place it in a shop-built sound-deadening box – although you will need to ensure that the compressor has access to sufficient cool air so as not to overheat.

A roll cage design – heavy-duty tubing that surrounds the motor and compressor pump – will be more useful if you’ll be transporting and using the compressor on a job site, and less so if the compressor stays parked in your shop.

Dual quick-connect outlets are useful if you plan on run­ning two hoses to the compressor; otherwise, the second outlet becomes redundant.

Price is always important, but shouldn’t be a deciding fac­tor. The cost of ownership for a compact compressor, prorated over a 10-year period, is likely to be less than a large pizza and a six pack.

Even if you only use a couple of air tools with your com­pressor, you will quickly find it to be an indispensible shop machine.

There are pros and cons when comparing air-powered tools to conventional electric or cordless tools. Still, air tools offer some serious advantages, which is why they’re a staple in so many workshops.

Pros

• Because air tools lack motors and only have a few moving parts, they run cooler, and have a much longer service life.
• They are noticeably lighter in weight and produce less vibration, making them easier to use over longer periods of time.
• They are quieter in use than their watt-powered equivalents.
• In general, air tools cost less, and because they last longer, their cost of ownership is considerably lower.

Cons

• Air tools are, quite literally, tethered to a compressor – where one goes, the other must follow.
• It can be somewhat annoying hearing the compressor motor and pump cycling on and off throughout the day.
• Having to deal with an air hose snaking around the shop can be frustrating.