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