How much air, at what pressure?

Before buying an air compressor, you should know the pressure and capacity that you require.

If you don't know the pressure or capacity you need, go back to my air compressor buying guide start page where I explain in detail what pressure and what capacity you need. Maybe you find out that a rotary screw compressor is better suited for you!

With reciprocating compressors, it's always better to buy a (slightly-) bigger air compressor than you need. As your business and air demand grows, you have some extra capacity that you can use. Reciprocating air compressor don't mind standing still, so this is not bad for your compressor.

Pressure

Two stage air compressors can get up to to 175 psi (12 bar). You need to figure out for yourself how much pressure you need. If you only need 125 psi, it's better to look for compressor with a lower maximum pressure, but at an higher capacity (cfm): more air for your bucks!

Of course, check the maximum pressure. Keep in mind that if you buy a compressor with a higher maximum pressure than you need, you can always lower the pressure setpoint. Or, you can keep the higher pressure and use the pressure reducer to reduce the pressure to the pressure you need.This way you have an extra buffer. Great if you have big air tools that require a lot of compressed air and you only need them for a minute or two.

Keep in mind, that pumping up to an higher pressure will costs you a lot more electricity. If you don't use it frequently this is not a problem, but keep it in mind if you use compressed air frequently. If you don't know what pressure you need, start at my air compressor buying guide main page.

Capacity (CFM, not tank size)

Like with buying any type of compressor, you need to know (approximately) the amount of air that you will be using.

If you don't know what capacity you need, be sure to start at the air compressor buying guide main page. Two stage reciprocating air compressors go up to about 50 CFM (cubic feet per minute). If you need more, a rotary screw compressor might be a good option.

The capacity of the compressor is the amount of air that is produced by the compressor. Most people will know what pressure they need, but are unsure of the capacity. If you don't know what capacity you need, start at my air compressor buying guide main page. A lot of online air compressor stores will advertise by the size of the air tank, for example 'this is a 60 gallon air compressor'. I also see this a lot in online forums. Yes, it's an air compressor with a 60 gallon tank, but what is the capacity? Remember, capacity is in cubic foot per minute (cfm). Tank size is in cubic foot, or gallons.

Tank size

Most industrial/commercial grade single-stage air compressors come with a 60 gallons air tank. I can fit a very small compressor on a very big tank. Or I can fit a very big compressor on a very small tank. In fact, I see this a lot: a very small capacity air compressor, installed on a very big air tank. The unit is than sold at the price of an high-capacity unit! The size of the tanks says nothing about the capacity of the compressor! It is simply the compressed air buffer you have.

Bigger tank vs smaller tank

With a bigger tank, the compressor will start less frequently, but it takes longer to fill up the tank to the required pressure. On the other hand, you can use your air tools longer, before the compressor starts up again. With a small tank, the compressor will start frequently and the tank is filled up very quickly. A bigger tank size is better. But don't just look at the tank size only, the capacity (cfm) of the compressor is more important.

Duty cycle

For industrial buyers — duty cycle is not really a buying consideration.

Real industrial-grade compressors — Atlas Copco, Ingersoll Rand, Quincy, the proper brands — are 100% duty cycle, full stop. They don't even talk about it because there's nothing to talk about. The machine runs as long as you want it to run. The whole duty cycle conversation (50%, 75%, 100%) is a relic of the workshop and hobby compressor market.

If a vendor is quoting you anything less than 100% duty cycle, you're not looking at an industrial machine. Either walk away or know exactly what you're buying.

That said, here's the standard explanation you'll see in the wider market — useful context if you're comparing a workshop-grade piston unit against an industrial one, or shopping at the boundary between the two:

The duty cycle of almost all two stage air compressors is 100%. That's one of the reasons why they are viewed as 'more powerfull' and durable. 100% duty cycle means that the compressor can run non-stops, without needing time to cool down.

In contrast, most single-stage air compressors have a duty cycle of 50% or 75% (see single stage vs two stage vs duplex)