The definition of a dry vacuum pump is a pump that will not use any fluids to create a vacuum or contact the process gas and may also discharge to atmosphere. … Timing gears with oil reservoirs and close clearances between your rotors and housing are additional attributes that define 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. … In addition, it results in a lower heat load of the compressed gas.
