This is of a dried out vacuum pump is a pump that will not use any fluids to create a vacuum or contact the process gas and will also discharge to atmosphere. … Timing gears with essential oil reservoirs and close clearances between the Dry Vacuum Pump rotors and casing are extra attributes that make up a dried out vacuum pump.
Principle of Operation
Dried out screw vacuum pumps operate with two screw rotors rotating in reverse directions. This 
traps the medium to become pumped between the cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lower warmth load of the compressed gas.