Read this first: single stage vs two stage vs duplex is NOT your first buying decision.

Your first decision is: what pressure do I need, what capacity, and from a manufacturer that can service the machine for the next 15 years. Once you know that, you go and look at the machines that fit. The spec sheet tells you whether the machine is single-stage, two-stage or duplex. Whatever it happens to be — that's your machine.

Don't get caught up in the "is two stage always better than single" forum talk. A single stage from Atlas Copco can supply the same pressure and capacity as a two stage from a cheap brand — and it'll outlast it. This page explains what each variant means so you can read a spec sheet, not so you can pick a fight on a forum.

The three variants

There are basically three types of reciprocating air compressors:

  • Single-stage units
  • Two (or multi-) stage units
  • Duplex units

Besides the type of air compressor, we can make a distinction in oil-free and oil-flooded air compressors, and between electric and gas-driven reciprocating compressors.

Single-stage units

Single stage units have only piston/cylinder. Air is compressed in 1 stage, from ambient pressure to working pressure.

Single-stage compressor are cheaper and simpler in design. They go up to about 20 CFM (cubic feet per minute) capacity. And go up to 140 psi (10 bar) pressure.

Single stage compressors are mostly used for home use or light commercial use. However, for commercial use, I'd recommend a two-stage air compressor.

Two stage units

A two stage reciprocating air compressor is ideal for demanding, industrial applications. They are much tougher than single stage compressors and can handle more pressure and more capacity (cfm).

Two stage (or double stage) units have two pistons/cylinders. There is a low-pressure cylinder and a high-pressure cylinder. The low pressure pumps up to about half of the end pressure. The air is cooled and then compressed further by the second high-pressure cylinder.

Two stage units have more capacity and an higher pressure. Capacity goes up to 50 CFM (cubic feet per minute) and pressure up to 175 psi (12 bar).

Very suitable for commercial / industrial use. Perfect for workshops, automotive shops, and other industrial uses.

How two-stage compression works

With two stage air compressors, the air is compressed in two stage. In between the stages, the air is cooled.

Stage 1

Air is compressed to medium level. This is the big cylinder/piston. It moves a high volume of air, but at a low pressure.

Cooler

Air is cooled back to a much lower level. This makes the compressor more efficient and reduced stress on the high pressure stage.

Stage 2

The air is compressed further to the end pressure. This is the small cylinder/piston. It moves a lower volume of air, but at an high pressure.

On some models, you can clearly see the two stages (a v-shaped pump with a big and a small cylinder). On other models, they low pressure and the high pressure cylinder are cast in 1 block. It looks like a single stage compressor, but it is a two stage compressor.

Duplex units

Duplex units are basically just two separate compressors installed on one tank. Usually these are single-stage units.

The capacity goes up to about 70 cfm (cubic feet per minute) and the pressure up to 175 psi (12 bar).

The difference with two stage units? Duplex units have a bigger capacity, two stage compressors have an higher pressure.

One advantage of duplex units is that when one compressor breaks down, you always have the other compressor to work with (same pressure, but at half the capacity).