The oil-free screw Oil-free Screw Air Compressor china compressor element
No oil means that there is no essential oil for sealing the rotors and for cooling the compressed atmosphere, elements and rotors. 
… The component can be cooled by cooling drinking water that flows through particular 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.