The oil-free screw compressor element
No 
oil implies that there is absolutely no oil for sealing the rotors and for cooling the compressed atmosphere, components and rotors. … The element is certainly cooled by cooling water that flows through unique pockets in the component casing.
In a Oil-free Screw Air Compressor china 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 break down of the lubrication properties due to internal conditions.