Dry screw pumps are increasingly popular as an alternative to oil-sealed rotary vane pumps for many medium and high vacuum applications. Vacuum furnaces end-users and designers must have a good understanding of how claw and screw pumps operate. This includes the principles of operation and operating characteristics, pump design and features, sealing, purging, and ancillary devices. A dry screw pump is used alone when a high vacuum is not required. When used in series with other pumps, this primary pump is also referred to as a “backing” pump.
“Screw pumps operate using two counter-rotating screw rotors which are engineered so that they rotate “towards each other”. This traps the gas in the space between the “screws” of their rotors. As the screws rotate, this trapped volume decreases which not only compresses the gas but moves it towards the exhaust.”
Source: Vacuum Science World