How to buy an air compressor

Every workshop should have an air compressor for powering nailers, sprayers, and other pneumatic woodworking tools, as well as other do-it-yourself jobs, such as inflating tires and breaking lug nuts loose with an impact wrench. But because compressors come in countless sizes, shapes, and capabilities, how do you know which one to get for your shop?

As with many tools and machines, the answer to that question depends on how you plan to use it. But each task and tool have different demands, and not every compressor can meet those demands. Here's what you need to know to get the compressor that will do all you want it to.

An air compressor, as its name suggests, uses a pump to pack a large volume of air into a small amount of space—the tank—increasing the air pressure (expressed in pounds per square inch, or psi) so it can be put to work in a tool.

Take a look at the chart, below, to see which pneumatic tools you'll want to operate with your compressor. Once you've figured out which tool on your list demands the most airflow, search for a compressor with a pump rated for the needed airflow, or one close to it with a tank large enough to hold plenty of air in reserve. For example, if you want to use a paint sprayer, which requires about 7–12 cfm of continuous air flow, a compressor with at least a 30-gallon tank will likely have a sufficient pump as well.

There are several compressor configurations, shown below, but tank capacity (rated in gallons) matters more than style.

To be considered portable, a compressor needs to be light enough to carry, or have one or more wheels so you can roll it around. Hand-carry units typically have 1–6-gallon tanks and can deliver 4–5 cfm at 90 psi. Wheeled compressors go up to 30-gallon tanks with airflow up to 10–12 cfm at 90 psi.

Consumer-level stationary compressors generally have 60-gallon or larger tanks, with pumps capable of sustaining 15–30 cfm. (Industrial models can have bigger tanks and generate far greater airflow.) Many stationary models use a two-stage pump (see "Just going through a stage or two" at end of article) to maximize efficiency. If you go with a compressor of this size, dedicate a space in the shop and anchor it to the floor or wall to prevent movement.

All that pumping generates a lot of friction, so the pumps on most compressors use an oil bath for lubricating the piston(s). This helps to both increase longevity (less piston and cylinder wear) and decrease noise. But that oil needs to be changed regularly depending on the hours of use. And if you use the compressor in cold-weather conditions, you should change to thinner oil in the winter months.

Oil-free compressor pumps are mostly maintenance-free, but tend to be louder than oil-lubricated pumps because the metal-piston-in-metal-cylinder action resonates more. Many oil-free models produce noise well above the level where you should wear hearing protection (85 decibels).

However, some manufacturers now make small, portable, oil-free compressors that create remarkably low noise levels (less than 70 dB), even quieter than comparable oil-lube models. They achieve this by using a slower motor speed and two pistons, enabling these machines to create the same airflow as a single-piston, high-speed compressor. This lower noise can make a huge difference in a workshop.

A gas-powered model works great on a job site, but inside a workshop, you'll obviously want an electric-powered compressor. Most electric compressors with 30-gallon or smaller tanks run on 110-volt service; larger units require 220 volts.

And these days, you can buy air compressors powered by rechargeable lithium-ion battery packs. These models tend to have smaller motors, tanks, and airflow, and the battery runtime proves respectable. They work great for powering a finish nailer, but not for any tool requiring more sustained airflow than that.

Most compressors don't come with a hose, so plan on buying one when budgeting for the compressor. For shop use, you can use either a ¹⁄₄ "- or ³⁄₈ "-diameter hose. Most come in 50' and 100' lengths; if that's too long, either search the market for a 25' hose, or cut a longer one to fit and reattach the fitting. Similarly, quick-connect fixtures do not come with hoses, so you'll have to get those, too. Supple polyurethane and rubber hoses tend to lie flatter on the floor after uncoiling than do stiff plastic or PVC-reinforced hoses.

Make sure to get the right quick-connect fittings for your hose.

A single-stage compressor draws air into its one-cylinder pump—a few models use two cylinders—and then compresses the air directly into the tank. These usually operate on 110-volt electrical service.

A two-stage unit has two pistons housed in a cast-iron pump to divide the workload, and requires 220 volts. The first piston compresses air to about half the desired pressure level, then sends it through an intercooler (removing built-up heat from friction) to the second cylinder, where the air gets further compressed and then sealed inside the tank at a pressure higher than a single-stage compressor.

Two-stage compressors usually have 80-gallon or larger tanks and can produce about twice the airflow of single-stage models. This allows you to use high-demand tools, such as large sprayers, impact wrenches, and dual-action sanders, without running low on air pressure. For most home woodworking shops, one of these would be overkill