Oil-free screw compressors including dry screw compressor screw compressor and spray two categories.
In the dry type screw compressor , since the lubricating oil , and take no small particles , the gas will rotor , the housing wall , airway corrosion , so the screw rotor in the conventional fuel injection based on the material , usually spray corrosion layer , such as spraying Teflon , molybdenum disulfide , Teflon , etc., to ensure that the rotor is not deformed, not corrosion. Screw compressor in water , due to the compression chamber injection , although taking into cooling , sealing and maintaining the purity of the gas , and improve the efficiency of the compressor , but also good control of the exhaust gas temperature , but the water bound to the rotor and other components have a corrosive effect , therefore need to carefully select the rotor material . Rotor material selection generally there are several : (1) NSB steel rotor ( 2 ) high polymer ceramic rotor ( 3 ) plastic rotors for screw compressors , line design is essential, necessary to consider Rotor engagement requires continuous contact line , processing performance and other factors .

Technical Difficulties
Dry screw compressor of the technical difficulties ( 1 ) Dry screw compressor rotor deformation performance largely depends on the rotor engages the sealing gap . The rotor , the rotor and the casing there are multiple leak path between the fluid leakage through the gap directly affects the volumetric efficiency and total efficiency. In actual operation, the rotor is in heat , the force effect of the boundary conditions of thermal , elastic deformation force , which makes the design of the actual running clearance gap varied. Gap caused by leaks over the General Assembly increased volumetric efficiency decreased ; gap is too small and prone to accidents killed burns . Although the temperature difference across the rotor , but the rotor radial deformation along the length differences still exist . This is mainly due to the complex shape rotor , teeth , grooves are helical , rather than a strict axisymmetric structure . On the yin and yang, the rotor along the contact line of the radial deformation analysis found that, although the amount of deformation along the axial length direction there are differences , but the two rotors total deformation is essentially the same , and the total amount of deformation varies linearly with temperature trends. Dry compressor discharge temperature tends to be high , in order to prevent the rotor or the rotor teeth bite severely damaged, should be considered to increase the gap or design appropriate cooling measure .

( 2 ) coating failure grungy rotor coating will fall off after a period of operation , the rotor is exposed to air , exposed to impurities in the air and temperature changes. Finally, the resulting reduction in operating performance , or even damage to the machine . Spraying anti-corrosion coating process is one of the difficulties . Is now used more of molybdenum disulfide , polytetrafluoroethylene, and molybdenum - stone coating.