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The rotating-vane pump, known also as
"rotary pump", is constituted of a stator and an eccentric
rotor which has two vanes (blades) in. a diametral slot. The stator
is a steel cylinder the ends of which are closed by suitable plates,
which hold the shaft of the rotor. The stator is pierced by the
inlet and exhaust ports which are positioned respectively a few
degrees on either side of the vertical. The inlet port is connected
to the vacuum system by suitable tubulation usually provided with
some kind of dust filter. The exhaust port is provided with a
valve, which may be a metal plate moving vertically between arrester
plates, or a sheet of Neoprene, which is constrained to hinge
between the stator and a metal backing plate.
The rotor consists of a steel cylinder mounted on a driving shaft (fig. 1).
Its axis is
parallel to the axis of the stator, but is displaced from this
axis (eccentric), such
that it makes contact with the top surface of the stator, the
line of contact lying
between the two ports. This line of contact known as the top seal between
rotor and stator must have a clearance of 2-3 microns. A dia
metrical slot is cut through the length of the rotor and carries
the vanes.
These are rectangular steel plates which make a sliding fit in
the rotor slot and
are held apart by springs which ensure that the rounded ends of
the vanes always
make good contact with the stator wall. The whole of the stator-rotor
assembly
is submerged in a suitable oil.
The action of the pump is shown in Fig. 2. As vane A passes
the inlet port
(fig. 2a), the vacuum system is connected to the space limited
by the stator,
the top seal, the rotor and vane A. The volume of this space increases
as the vane
sweeps round, thus producing a pressure decrease in the system.
This continues
until vane B passes the inlet port (fig. 2b), when the volume
of the gas evacuated is isolated between the two vanes. Further
rotation sweeps the isolated gas around the stator until vane
A passes the top seal (fig. 2c). The gas is now
held between vane B and the top seal, and by further rotation
it is compressed
until the pressure is sufficient (about 850 Torr) to open the
exhaust vafve, and
the gas is evacuated from the pump.
Since both vanes operate, in one rotation of the
rotor a volume of gas equal to
twice that indicated in fig. 2b is displaced by the pump. Thus,
the volume rate
at which gas is swept round the pump, referred to as pump displacement
S, is
S = 2Vu
where V is the volume between vanes A and B (fig. 2b), and
n is the number
of rotations per unit time (usually 350-700 r.p.m.).
The contacts of the vanes and rotor with the stator form three
separate chambers
each containing gas at different pressure. These contacts must
therefore make
vacuum-tight seals, especially for the top seals which must support
more than one
atmosphere pressure difference. For this reason the inner surfaces-
of the stator,
that of the rotor and vane, are very carefully machined. Hence,
great care must
be taken to ensure that no abrasive material or gas which is likely
to corrode the
metal surfaces enters the pump chamber.
In theory, the lowest pressure achieved by the pump
is determined only by the
fact that the gas is compressed into a small but finite "dead
volume". When the
system pressure becomes so low that, at maximum compression, the
gas pressure
is still less than that of the atmosphere it cannot be discharged
from the pump.
Subsequent pumping action re-expands and recompresses the same
gas without
further decreasing the pressure in the system. The ratio of the
exhaust pressure
to the inlet pressure is termed the pump compression ratio. Thus,
to produce pressures of the order of 10-2 Torr, pumps having compression
ratios
of the order of 105 are required. In addition to lubrication and
sealing, the oil
also performs the function of filling the dead volume, thus increasing
the compression ratio.
The lowest {ultimate) pressure achieved by a single stage rotary
pump is about
5 x 10-3 Torr, as measured by a McLeod gauge (permanent gas pressure).
If
the pressure is measured by a Pirani gauge (total pressure), pressures
of about
10-* Torr will be recorded for the same single stage pump.
Parallel connection of two identical rotor-stator
systems will provide twice the
displacement but the same ultimate pressure. Series connection
provides the same
displacement but greater pumping speeds at low pressures (lower
ultimate pres
sure). A two-stage pump may reach 10-4 Torr (McLeod) or 2 x 10-3
(Pirani)
ultimate pressure.
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