Vacuum Technology Book, Volume II

4.4.1 Design / Operating principle

Two parallel bearing-supported, intermeshing screw rotors (3) having opposite threads synchronously and contactlessly counter-rotate in a cylindrical housing (2) that tightly encloses them, and together form a multi-stage pump. Because of the counter-mesh of the two rotors, the volumes sealed in each thread are advanced along the rotors to the outlet (4). The pump has no valves at either the inlet (1) or the outlet. When a displacement volume reaches the outlet opening, the pressure is equalized with the atmosphere. This means that atmospheric air flows into the displacement volume and is then discharged again as the rotor turns. This pulsing gas flow generates a high level of dissipated energy and heats the pump. The dissipated energy can be minimized by means of internal compression. This internal compression is achieved by reducing the thread pitch in the direction of the outlet. The gaps between the housing and the rotors, as well as between the rotors relative to one another, determine the ultimate pressure which a screw pump can attain. The geometry and the gap configuration which results when the rotors engage with each other also significantly influence the ultimate pressure.



