The oil-free screw Oil-free Screw Air Compressor compressor element
No oil means that there is absolutely no oil for sealing the rotors and for cooling the compressed air flow, components and rotors. 
… The component is cooled by cooling drinking water that flows through particular pockets in the element casing.
In a lubricated screw compressor, the lubricant will come in direct in touch with the compressor inlet air and compressed air. … The oil contains anti-oxidants to attempt to reduce the breakdown of the lubrication properties because of internal conditions.
