PuK - Process Technology & Components 2024
A technical trade magazine with a history of more than 60 years.
A technical trade magazine with a history of more than 60 years.
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Vacuum technology<br />
Screw spindle vacuum pumps<br />
Potential of surface structures for the<br />
reduction of vacuum gap flows<br />
Sven Brock, Heiko Pleskun, Gero Polus, Jannis Saelzer, Prof. Dr.-Ing. Prof h.c. Dirk Biermann,<br />
Prof. Dr.-Ing. Andreas Brümmer<br />
Abstract<br />
This article presents a patented approach<br />
for the fluid-mechanical and<br />
thermodynamic improvement of screw<br />
spindle vacuum pumps [Brü21]. These<br />
machines belong to the group of rotary<br />
positive displacement vacuum pumps<br />
and have two parallel rotors that convey<br />
the gas along the rotor axes. Chambers<br />
are formed between the rotors and<br />
the surrounding housing. Due to rotation<br />
the fluid is carried in axial direction<br />
and expelled on the high-pressure side<br />
(which is typically atmosphere) by reducing<br />
the chamber volume. The main<br />
loss mechanism of such machines is<br />
identified as the operational gaps,<br />
through which the fluid can flow in the<br />
opposite direction. These gap flows are<br />
usually minimized by selecting the lowest<br />
possible gap height.<br />
As the reduction of the gap height<br />
is limited by the operational safety due<br />
to manufacturing tolerances and thermal<br />
expansion, this study concentrates<br />
on the reduction of gap mass flow<br />
rates through surface structures without<br />
simultaneously reducing the gap<br />
height. These structures have a profile<br />
depth that is significantly less than the<br />
minimal gap height between the housing<br />
and the rotor. The main objective<br />
is to specifically manipulate the reflection<br />
properties of molecules in the region<br />
of rarefied gas flows where gassurface<br />
interactions dominate. The<br />
strategic arrangement of these surface<br />
structures should increase the rate of<br />
back scattering of the molecules in opposite<br />
direction of flow. This is intended<br />
to achieve a reduced gap mass flow<br />
without jeopardising the operational<br />
safety of the machine.<br />
they are able to generate a technically<br />
clean vacuum and at the same<br />
time have a good tolerance for dirt<br />
particles and small amounts of liquid.<br />
As only a few machine parts are required<br />
due to their design, the assembly<br />
and maintenance costs of<br />
these machines are comparatively<br />
low. Together with their high suction<br />
speed (up to S eff<br />
= 2500 m³/h), these<br />
machines are particularly interesting<br />
for industrial purposes. They offer<br />
suction pressures from p suction<br />
= 0.1 Pa<br />
up to atmospheric pressure p at<br />
and<br />
are therefore suitable for low and<br />
medium vacuum applications. In<br />
many applications, they are used as<br />
fore vacuum pumps in combination<br />
with roots pumps or other vacuum<br />
pumps for high suction speeds in the<br />
fine or high vacuum regime [Jou18].<br />
The most important parameter for<br />
SSVPs is the effective pumping speed<br />
S eff<br />
, which describes the volume flow<br />
on the low-pressure side. The lowest<br />
achievable pressure that can be<br />
reached in a recipient with a vacuum<br />
pump without external leakage is<br />
referred to as the ultimate pressure<br />
[Jou18]. The characteristic curve describing<br />
the machine is the so-called<br />
suction speed curve, which describes<br />
the suction speed as a function of the<br />
suction pressure, whereby the discharge<br />
pressure corresponds to the<br />
atmospheric pressure. In measurements,<br />
Dreifert and Müller have observed<br />
that the suction speed curve<br />
of an SSVP is significantly influenced<br />
by the clearance between the rotors<br />
and the enclosing housing (the socalled<br />
housing gap). Fig. 2 shows that<br />
even a ten per cent change in the gap<br />
height causes a significant change in<br />
the characteristic curve, with an increasing<br />
effect for lower suction pressures<br />
[Dre14]. Accordingly, minimising<br />
the housing gap mass flow rate is<br />
essential for the efficiency of the machine,<br />
whereby the reduction of the<br />
gap height has limits in terms of operational<br />
safety, as the clearance must<br />
be guaranteed minus the manufacturing<br />
tolerances, possible vibrations<br />
and, in particular, thermal expansion<br />
for friction-free operation.<br />
In general, the suction speed of<br />
the machine initially increases with<br />
decreasing suction pressure until<br />
a maxi mum - the so-called nominal<br />
suction speed - is reached. The<br />
suction speed then drops to zero as<br />
the pressure is lowered further and<br />
the machine’s ultimate pressure is<br />
Introduction<br />
Screw spindle vacuum pumps (SSVP)<br />
(see Fig. 1) have become increasingly<br />
important in recent years, because<br />
Fig. 1: Principle sketch of a screw spindle vacuum pump (SSVP)<br />
46 PROCESS TECHNOLOGY & COMPONENTS <strong>2024</strong>
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