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 may also discharge to atmosphere. … Timing gears with essential oil reservoirs and close clearances between your rotors and casing are additional attributes that make up a dry vacuum pump.
Principle of Operation
Dry screw vacuum pumps operate with two screw rotors rotating in reverse directions. This traps the moderate to be pumped between the cylinder and the screw chambers and transports it to the gas discharge. … In addition, it results in a lower heat load of the compressed gas.