A displacement vacuum pump is generally a vacuum pump in which the gas which is to be pumped is sucked in with the aid of pistons, rotors, vanes and valves or similar, possibly compressed and then discharged. The pumping process is effected by the rotary motion of the piston inside the pump. Differentiation should be made between oiled and dry compressing displacement pumps. By using sealing oil, it is possible to attain in a single-stage, high compression ratios of up to about 105. Without oil, “inner leakiness” is considerably greater and the attainable compression ratio is correspondingly less, about 10.
As shown in the classification Table 2.1, the oil sealed displacement pumps include rotary vane and rotary plunger pumps of single and two-stage design as well as single-stage trochoid pumps which today are only of historic interest. Such pumps are all equipped with a gas ballast facility which was described in detail for the first time by Gaede in 1935. Within specified engineering limits, the gas ballast facility permits pumping of vapors (water vapor in particular) without condensation of the vapors in the pump.
Rotary vane pumps (see Fig. 2.6) consist of a cylindrical housing (pumping ring) (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 TRIVAC B range (Fig. 2.6) has only two vanes offset by 180°. 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 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 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 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 – despite the very low ultimate pressure – in practice lead to considerable difficulties and will significantly impair operation of the pump.
I High vacuum stage
II Second forevacuum stage
a – Valve stop
b – Leaf spring of the valve
Shown in Fig. 2.9 is a sectional view of a rotary plunger pump of the single block type. Here a piston (2) which is moved along by an eccentric (3) turning in the direction of the arrow moves along the chamber wall. The gas which is to be pumped flows into the pump through the intake port (11), passes through the intake channel of the slide valve (12) into the pumping chamber (14). The slide valve forms a unit with the piston and slides to and fro between the rotatable valve guide in the casing (hinge bar 13). The gas drawn into the pump finally enters the compression chamber (4). While turning, the piston compresses this quantity of gas until it is ejected through the oil sealed valve (5). As in the case of rotary vane pumps, the oil reservoir is used for lubrication, sealing, filling of dead spaces and cooling. Since the pumping chamber is divided by the piston into two spaces, each turn completes an operating cycle (see Fig. 2.10). Rotary plunger pumps are manufactured as single and two-stage pumps. In many vacuum processes combining a Roots pump with a single-stage rotary plunger pump may offer more advantages than a two-stage rotary plunger pump alone. If such a combination or a two-stage pump is inadequate, the use of a Roots pump in connection with a two-stage pump is recommended. This does not apply to combinations involving rotary vane pumps and Roots pumps.
The motors supplied with the rotary vane and rotary plunger pumps deliver enough power at ambient temperatures of 53.6°F (12°C) and when using our special oils to cover the maximum power requirement (at about 400 mbar). Within the actual operating range of the pump, the drive system of the warmed-up pump needs to supply only about one third of the installed motor power (see Fig. 2.11).