The basic workings are the same as that of the oil-injected screw compressor, only this time, there’s no oil, only air! Because of this, the rotors used are of superior quality with very little space between them. They do not touch each other though; otherwise, they would wear down too quickly.
Because there’s no oil injected during compression, the compression is usually done in two stages, because 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 3.5 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 3.5 bar to the end level, mostly 7 bar.
Normally, the two stages will be built on one gearbox, with one electromotor driving them at the same time.
• 100 percent oil-free air
• More expensive than the oil-injected type
• Servicing/repairing more difficult and more expensive than the servicing/repairing of the oil-injected type
• More noisy than oil-injected compressors