Rotary vane pumps (TRIVAC A, TRIVAC B, TRIVAC E, SOGEVAC)
Rotary vane pumps (see also Figs. 2.5 and 2.6) consist of a cylindrical housing (vacuum generator) (1) in which an eccentrically suspended and slotted rotor (2) turns in the direction of the arrow. The rotor has vanes (16) which are forced outwards usually by centrifugal force but also by springs so that the vanes slide inside the housing. Gas entering through the intake (4) is pushed along by the vanes and is finally ejected from the pump by the oil sealed exhaust valve (12).
The older range of TRIVAC A pumps (Fig. 2.5) from LEYBOLD has three radial vanes offset by 120°. The TRIVAC B range (Fig. 2.6) has only two vanes offset by 180°. In both cases the vanes are forced outwards by the centrifugal forces without the use of springs. At low ambient temperatures this possibly requires the use of a thinner oil. The A-Series is lubricated through the arising pressure difference whereas the B-Series pumps have a geared oil pump for pressure lubrication. The TRIVAC B-Series is equipped with a particularly reliable anti-suckback valve; a horizontal or vertical arrangement for the intake and exhaust ports. The oil level sight glass and the gas ballast actuator are all on the same side of the oil box (user friendly design). In combination with the TRIVAC BCS system it may be equipped with a very comprehensive range of accessories, designed chiefly for semiconductor applications. The oil reservoir of the rotary vane pump and also that of the other oil sealed displacement pumps serves the purpose of lubrication and sealing, and also to fill dead spaces and slots. It removes the heat of gas compression, i.e. for cooling purposes. The oil provides a seal between rotor and pump ring. These parts are “almost” in contact along a straight line (cylinder jacket line). In order to increase the oil sealed surface area a so-called sealing passage is integrated into the pumping ring (see Fig. 2.4). This provides a better seal and allows a higher compression ratio or a lower ultimate pressure. LEYBOLD manufactures three different ranges of rotary vane pumps which are specially adapted to different applications such as high intake pressure, low ultimate pressure or applications in the semiconductor industry. A summary of the more important characteristics of these ranges is given in Table 2.2. The TRIVAC rotary vane pumps are produced as single-stage (TRIVAC S) and two-stage (TRIVAC D) pumps (see Fig. 2.7). With the two-stage oil sealed pumps it is possible to attain lower operating and ultimate pressures compared to the corresponding single-stage pumps. The reason for this is that in the case of single-stage pumps, oil is unavoidably in contact with the atmosphere outside, from where gas is taken up which partially escapes to the vacuum side thereby restricting the attainable ultimate pressure. In the oil sealed two-stage displacement pumps manufactured by LEYBOLD, oil which has already been degassed is supplied to the stage on the side of the vacuum (stage 1 in Fig. 2.7): the ultimate pressure lies almost in the high vacuum range, the lowest operating pressures lie in the range between
medium vacuum / high vacuum. Note: operating the so called high vacuum stage (stage 1) with only very little oil or no oil at all will – in spite of the very low ultimate pressure – in practice lead to considerable difficulties and will significantly impair operation of the pump.
Fig. 2.5 Cross section of a single-stage rotary vane pump (TRIVAC A)
Fig. 2.6 Cross section of a rotary vane pump (TRIVAC B)
Table 2.2 Rotary vacuum pump ranges
Fig. 2.7 Cross section of a two-stage rotary vane pump, schematic