The basic workings are the same as the oil-injected screw compressor, only this time, there’s no oil, only air!
Because there’s no oil injected during compression, the compression is usually done in two stages.
If we would compress the air in one go from 1 to 7 bars, it would get really, really hot.
Stage one compresses the air to a few bars (say 2,5 bars). The air will be very hot at this time, so it flows through an inter-cooler first before entering the second stage. Stage two will compress the air further from 2,5 bar to the end-level, mostly 7 bar.
Normally the two stages will be built on 1 gearbox, with one electro motor driving them at the same time.
Here’s more info on how an oil-free rotary screw compressor works.
If you need 100% oil-free air and lots of it, the oil-free rotary screw compressor is the way to go. Of course, it comes with a bigger price-tag, but if you really need 100% oil free air, there’s not really a choice.
100% oil-free air
More expensive than oil-injected type
Servicing/repairing more difficult, and more
expensive than oil-injected type
More noise than oil-injected compressors