- The basic principle of the oil-free screw compressor is same as for oil-injected compressors. But as the name suggests, there is no oil injected during compression. No oil means that there is no oil for sealing the rotors and for cooling the compressed air, elements and rotors. Because there is no oil for sealing, the rotors need to be very precise and have very small tolerances. The rotors don’t touch each other, but the air-gap between the two is very small (for optimal performance).
The element is cooled by cooling water that flows through special pockets in the element casing. Of course this is less efficient as injecting relatively cold oil, and only the casing is cooled, not the rotors or the air itself.
For this reason, the pressure ratio of the oil-free screw element is much lower compared to the oil-injected element. Remember, the pressure ratio is the outlet pressure divided by the inlet pressure (around 13 for oil-injected compressor, about 3.5 for oil-free elements).
If the oil-free chamber was to compress air directly to 7 bar, the element will get too hot and grind to a stop (literally). Hence two stages are installed in series or tandem. Further to sustain the high temperature, the rotors are coated with Teflon or ceramic material.
The first element (stage 1) compresses the air to about 3.5 bar. The air is cooled down by the intercooler. The second element (stage 2) compresses the air further to the end pressure of 7 bar.
Each compression chamber has two screw shaped helical rotors. Also, they require a gear box to drive two elements from one compressor. On top of that, the compressor elements used in oil-free types are more expensive than oil-injected types, since they are manufactured with much smaller clearances compared to oil-injected compressor elements.
The two compressor elements, stage 1 and stage 2 work together to produce the required output pressure. The first stage pumps air to the intercooler. The second takes the air from the intercooler and compresses it to the final pressure. The two stages are designed so that they work in a perfect balance.
