This is 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 can also discharge to atmosphere. … Timing gears with oil reservoirs and close clearances between the rotors and casing are additional attributes that define a dry vacuum pump.
Principle of Operation
Dried out screw vacuum pumps operate with two screw rotors rotating in opposite directions. This traps the medium to be pumped between the cylinder and the screw chambers and transports it to the gas discharge. … In addition, it results in a lower high temperature load of the compressed gas.