Choosing a compressor for plasma cutting

To add to what Tommy has said, lets look at what the specs for my Thermal Dynamics A120 are:
Operating Pressure: 60-95 psi (4.1-6.5 bar)
Gas Flow: 300-500 SCFH (142-235 Litres per minute).

So what that immediately tells me my air compressor will consume up to 235 lpm
Fortunately here in Australia, compressors must be sold in their Free air delivery (FAD) which is more or less similar to what the quoted gas flow is.

So ignoring the pressure for a moment, the minimum sized compressor I need is 235 lpm FAD and that ignores any losses. But that would be running at 100% of the time while I was cutting. That would never do the job. According to one air compressor manufacturer I spoke to, you really should double that number. So I should have bought a 470 litre FAD machine. But this would have required 3 phase power.I already had the biggest single phase machine I could find which was 320 litres per minute FAD. This only gives me 36% headroom if I am cutting 16-20 mm plate at 120 amps. But most of the time, I'm cutting much thinner stuff so its proved adequate.

If you have insufficient headroom above your requirements, the cycle time will be greater and your air will be hotter. Hotter air will carry more water vapor. Plasma cutters don't like hate wet air!.

In a commercial setting (eg sand blasting, powder coating, paint shop) they would normally fit a precooler heat exchanger after the compressor and an auto drain before the air goes into the tank to minimise the amount of water in the stored air and bring the temperature down to ambient. Then as the air exits the storage tank, it will go through a refrigerated drier before it is used. These driers need to reach a minimum dew point temperature to remove all the water. But there is also a maximum range that they can cool the air so if the air is too hot it will not be able to do its job with 100% efficiency. With a precooler, the stored air will be at ambient temperature which should ensure the dew point is hit and the maximum amount of water will be removed from the near frozen air.

With a horizontal or vertical compressor with built in storage tank, this is not normally done. The stored air is therefore hotter so it will be wetter. It is possible to use an automotive transmission cooler between the compressor and the tank. Ideally this cooler should be located so the fan on the compressor pulley draws air through it.

In my climate, before I added a refrigerated drier, the simple dessicant drier I had could not cope and I found water pooled on the floor below a filter regulator. No wonder the torch was not cutting!

So this is yet again why plasma cutting to my mind is not an entry level sport! Its expensive! You need a CNC machine, a decent air compressor and most likely an air drier of some kind before you can cut reliably. In my case the money I saved buying a discounted plasma cutter was quickly consumed when I sorted out the air supply.

Just to add to Toms wisdom, When in doubt buy one of these


And dont forget the dryer!

I have learnt the hard way about how much air plasma uses. Our larger compressor has a very hard time keeping up with running workshop tools and the plasma at the same time. When the plasma is working all day it really cops a flogging!

Rotary screw compressors are amazing if you truly need lots of air.

Very true. But the up front cost turns people away. When you look at the fact that they produce dryer air and you can get away with a much smaller dryer they start to look better.

And they don't like running backwards if you hook them up the wrong way.