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 can also discharge to atmosphere. … Timing gears with essential oil reservoirs and close clearances between the rotors and casing are extra attributes that make up a dried out vacuum pump.
Principle of Operation
Dried out screw vacuum pumps operate with two screw rotors rotating in opposing directions. This traps the Dry Vacuum Pump medium to end up being pumped between the cylinder and the screw chambers and transports it to the 
gas discharge. … It also results in a lesser high temperature load of the compressed gas.