Pfeiffer Vacuum A Passion for Perfection

2.5.1 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 inlet (1) or 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 housing and rotors, as well as between the rotors relative to one another, determine the achievable ultimate pressure of a screw pump. The geometry and the resulting configuration of the gap in connection with the mesh between the rotors also significantly influence ultimate pressure.

(Figure 2.8)
Figure 2.8: Operating principle of a screw pump

Because the dissipated energy that is generated by the pulsing gas flow heats the pump on the outlet side, cooling is required at precisely this location. The gap between housing and rotors is a function of the temperature differential between the warmer rotors and the cooled housing. The amount of heat produced and the temperature are a function of the inlet pressure range. Temperatures are lowest at high inlet pressures (nearly atmospheric), as virtually no compression work is performed here and the displaced air transports sufficient heat out of the pump. In addition, the high gas flow also prevents oscillation of the gas in the last stage. During operation at ultimate pressure (p < 1 mbar), the oscillation of the atmospheric air produces higher temperatures at the outlet area, since no gas is passing through the pump, and no heat is thus being transported out of the pump.

HeptaDry pumps are dry screw pumps with internal compression. The screw rotors have a symmetrical geometry with variable pitch. These pumps do not have an end plate with control openings; instead, the gas is discharged axially against atmospheric pressure. Because of the internal compression, the volume of pulsing gas is low.

This results in lower power consumption, quiet operating, uniform temperature distribution within the pump and low cooling water consumption. This makes these pumps extremely cost-effective, in spite of their robust design.

(Figure 2.9)
Figure 2.9: HeptaDry™ rotors

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