This is of a dried out vacuum pump is a pump that does not use any fluids to make a vacuum or contact the process gas and will also discharge to atmosphere. … Timing gears with essential oil reservoirs and 
close clearances between the rotors and housing are additional attributes that make up a dry vacuum pump.
Principle of Operation
Dry screw vacuum pumps operate with two screw rotors rotating in opposing directions. This traps the medium 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 heat load of the compressed gas.