The definition of a dried out vacuum pump is a 
pump that does 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 extra attributes that define a dry vacuum pump.
Principle of Operation
Dry 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. … In addition, it results in a lesser temperature load of the compressed gas.
