The oil-free screw compressor element
No oil means that there is no essential oil for Oil-free Screw Air Compressor sealing the rotors and for cooling the compressed air, components and rotors. … The element can be cooled by cooling drinking water that flows through special pockets in the component 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 try to reduce the break down of the lubrication properties due to internal conditions.