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Basic types of air compressors

Learning the basics of what type of air compressor you need is an important first step in the buying process. Here are some of the decisions you should make before you start talking to air compressor dealers.

Rotary air compressors and reciprocating air compressors

The two most common types of air compressors are rotary compressors and reciprocating compressors. There are other types – super-powered centrifugal compressors, for example – but they're only used in the largest-scale industrial operations.

Reciprocating compressors are also called piston-driven compressors. They use a piston to raise the pressure of air in a cylinder, then store that high pressure air in a holding tank.

Rotary compressors use a slightly more complex mechanical process. Instead of a piston in a cylinder, rotary compressors use two screws rotating inside a sealed compartment to create pressure.

The low end of the compressor market is dominated by reciprocating models: the small compressors you'd buy at a hardware store are almost all piston-driven. One reason for this is that reciprocating compressors are less expensive than other types of compressors, so for low-end applications, they're the most affordable.

However, there are a couple of reasons to choose rotary compressors instead. A basic one is size. We cover choosing the right size air compressor separately, but suffice it to say that rotary compressors are more efficient at larger sizes.

Another important consideration is how you'll be using the compressed air: intermittently or continuously. For intermittent use, like powering hand tools, either type of compressor is fine. However, reciprocating compressors are not suited to continuous use, such as spray painting.

The way to verify this is to check the duty cycle of the compressor you want: reciprocating compressors have a duty cycle that can be as low as 50%, meaning they have to rest just as long as they run. Rotary compressors are more commonly rated at 100% duty cycles, meaning they can run continuously for hours on end.

Electric vs. diesel fuel

In most cases, the environment you'll be using the compressor in will dictate which type of power is best.

If you'll be using the compressor indoors, you'll want an electric model, although there are exceptions. Large, permanently installed compressors can be set up outside your shop or plant with the air piped in, allowing you to choose a diesel compressor if you prefer.

Compressors that will be moved from one job site to another are usually diesel powered. Their diesel engines are reliable and long-lasting, providing a steady source of air for years. In addition, diesel fuel is commonly available at construction sites.

You can choose gasoline powered compressors, but they're generally the smallest models, designed for powering a few small hand tools or inflating tires and the like.

Portable vs. permanent vs. towable compressors

Portable compressors are relatively easy to transport from one location to another: you can wheel them into the back of a pickup truck and carry them to your next job. Many small compressors are portable not by design, but simply because their small motors and compression sets allow them to be moved around.

The opposite end of the spectrum are permanently installed compressors. These are often set up much like a backup generator: installed on a concrete pad just outside the shop or factory that needs the air. Other installations dedicate an interior room to house the compressor.

Keep in mind that compressors are hot and loud. They cause significant problems if installed indoors without sound barriers and airflow considerations. In addition to creating quite a bit of heat through their operation, they also need to draw in considerable amounts of air to compress. Without proper ventilation, a large compressor could cause havoc with your HVAC system.

In between these two options are towable compressors. A common site on job sites across the country, towable compressors are mounted on wheels and include a tow hitch. It's a simple matter to hook them up and tow them to the job site. Once on location, they're freestanding. This approach provides more power and airflow than is available in portable compressors, but still allows you to get the compressed air where you need it.