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