This is of a dry vacuum pump is a pump that will not use any fluids to create a vacuum or contact the procedure gas and can also discharge to atmosphere. … Timing gears with essential oil reservoirs and close clearances between your rotors 
and housing are additional 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 moderate to end up being pumped between the cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lower high temperature load of the compressed gas.