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.
Motor power
The motors supplied with the rotary vane and rotary plunger pumps deliver enough power at ambient temperatures of 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).
Fig. 2.9 Cross section of a single-stage rotary plunger pump (monoblock design)
Fig. 2.10 Operating cycle of a rotary plunger pump (for positions 1 to 9 of the plunger)
Fig. 2.11 Motor power of a rotary plunger pump (pumping speed 60 m3 /h) as a function of intake pressure and operating temperature. The curves for gas ballast pumps of other sizes are similar.