3 or 4 stage compressor - if you would compress air in a single cylinder down to 1/200 of the original volume, the air would be extremely hot, and the forces on the material would make it hard to construct something that one can handle.
So, the air is compressed in stages. After each stage, the air can cool and the water vapor that condensates due to the higher pressure, can drip into water collectors. A 4-stage compressor has an extra cooling stage (and a longer lifespan).
When you run your own compressor, you have to think about these things:
- Every 15 minutes the condensate needs to be drained, from all condensate collectors. Not more time, not less time. That is done by opening the drain valves, which will cause a pressure drop. The materials are designed for a certain number of pressure cycles (eg. 20000), so sticking to these 15 minutes prolongs the life of the parts.
- Once the condensated water has been removed, the air is still not dry enough. It could still cause corrosion. To get it about 7 times dryer than the air in the Sahara, the compressed air is pushed through a Molecular Sieve filter. That filter material can absorb moisture to the amount 20% of it's own weight. In order to do this job, the air pressure needs to be above 140bar in the filter housings, also when you start filling a tank that's only pressurized to 50bar.
- And finally, since the compressor uses oil for the moving parts, the smell has to be removed from the compressed air. That's done by pushing it through active carbon.
So, these are three things to check when you start looking for a compressor.
First of all, an Automatic Condensate Drain (ACD) will do that 15 minute job for you without ever forgetting the time. Once you have seen a couple of compressors, you'll recognize an ACD: the condensate drain valve is replaced with a connection to a block that has at least one solenoid valve and a silencer that collects the moisture from all stages. There's a timer that counts down 15 minutes and activates the solenoid for 6 seconds.
Second, a Pressure Maintaining Valve (PMV) or back pressure valve. It's located after the last filter housing and before the fill whip.
Third, the filter housing(s) and filter cartridge. Look at the type of filter, the size of it. Can you refill these yourself, or do you have to buy filter cartridges? This is basically where the compressor manufacturers make money, by selling filter cartridges. Many older compressors have filter cartridges that are too small to meet todays air requirements. Or you need to change the cartridge every couple of hours (especially when there's no PMV). You have to keep track of the time a filter has been in use, so a clock on the compressor is handy as well.
This story goes for every single compressor out there. You've already mentioned Bauer and L&W, another common brand is Coltri.
It's all from Europe. Everything is metric.
