PuK - Process Technology & Components 2024

Vacuum technology
Screw spindle vacuum pumps
Fig. 2: Suctions speed curve of an SSVP with different housing gap heights [Dre14].
reached. As less and less mass enters
the machine at lower pressures,
an equilibrium is established at this
pressure between the intake mass
flow rate and the mass flow rate that
returns through the gaps, so that
the machine effectively no longer
conveys anything, thus preventing
a further reduction in pressure. The
charac teristic of the machine with initially
increasing suction speed results
from the fluid mechanical properties
within the gap. At high suction pressure,
the molecular density is high,
so that a molecule collides very frequently
with other molecules until
it encounters a boundary (rotor and
housing). In this way, the momentum
and energy transport are dominated
by intermolecular collisions and the
fluid moves as a continuum. When
the pressure is reduced, the molecular
density also decreases so that a
particle can travel a greater distance
until it collides with another particle.
This increases the proportion of gassurface
collisions, which leads to increased
friction. If the pressure is so
low that the number of intermolecular
collisions is negligible compared to
the particle-wall collisions, this is referred
to as a molecular flow. A qualitative
course of the normalised mass
flow rate of a gap through which air
flows is shown in Fig. 3. The normalisation
is chosen so that a normalised
mass flow rate of one corresponds to
an isentropic choked nozzle flow. The
gap height is set to h = 0.3 mm and
T = 293 K is assumed for the ambient
temperature. It is shown that the normalised
mass flow rate with an initial
continuum flow decreases significantly
with decreasing inlet pressure
until a minimum is reached and then
increases again asymptotically with
further pressure reduction towards
molecular flow. The greater proportion
of gas-surface interactions also
increases the influence of relative
wall movement on the mass flow
rate. Such a wall movement is caused
by the rotational speed of the rotors.
Accordingly, the red line results from
a wall movement in the direction of
flow, the green line for a wall movement
against the direction of flow
and the black line describes a purely
pressure-driven flow with static
boundaries.
In order to realise the highpressure
ratios over the machine
(e. g. p at
/p suction
= 10 5 ), the rotors have
a significantly larger wrap than conventional
screw machines in high
pressure applications. This results in
a kind of multi-stage design with several
encapsulated working chambers
in axial direction, which leads to a reduction
in the pressure ratio between
individual working chambers. In order
to avoid continuous gap connections
from the high-pressure to the lowpressure
side, the number of teeth is
limited to a maximum of two, or even
one for some profiles. Internal compression
is usually achieved by successively
reducing the chamber volume
by changing the rotor pitch, but
more recently also by using conical
rotors [Moe23]. The advantage of reducing
the chamber volume continuously
instead of using an end plate is a
more uniform compression along the
rotor and therefore better heat distribution
in the machine. Furthermore,
throttling losses can be reduced by
avoiding control edges [Jou18]. One
problem with the internal compression
of the machines is that the machine
has to cover very large pressure
ranges. In nominal oper ation, a very
large internal compression would
be desirable, which would reduce
the energy consumption on the one
hand and possibly also the size on
the other. For example, a large suction
chamber takes up a lot of mass,
which can then be compressed to a
small volume and then pushed out
on the high-pressure side at a low
Fig. 3: Schematic course of the normalised mass flow rate through the housing gap of an
SSVP as a function of the inlet pressure and the influence of relatively moved walls.
PROCESS TECHNOLOGY & COMPONENTS 2024
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