Venting Turbomolecular Pumps Dieter Mueller Institute of Physics London, January 18, 2006

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Venting Turbomolecular PumpsDieter
MuellerInstitute of PhysicsLondon, January
18, 2006
Dieter Müller International Business Development
Marketsegment RD
Nachdruck, auch auszugsweise, verboten
Proprietary Confidential /? by Leybold Vakuum
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- function principle of turbomolecular pumps -
what happens in a TMP during venting ? - what
would happen in a TMP if we stop it but do not
vent it? - venting TMPs with mechanical
bearings - oil lubricated - grease
lubricated - delayed venting - venting
magnetically levitated TMPs - classic mistakes

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Turbomolecular Pumps - classic designwith
mechanical bearings
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Turbomolecular Pumps MAG Ultra Linewith magnetic
bearings
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What happens in a TMP during venting ?
1) rotor will try to take off like a
helicopter ? forces on the bearings ? broken
cages of the bearings 2) rotor blades will
flutter like a bird (above supersonic speed!) ?
forces on the wings ? deformation of the outer
end of the blades 3) process gas/deposition in
pump will be mixed with venting gas ? chemical
reaction possible 4) pump will be polluted
with moisture and/or debris from chamber or
ambient air ? risk of bearing pollution
state-of-the-art TMPs are mechanically safe
against shock-venting today
Dieter Müller International Business Development
Marketsegment RD
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what would happen in a TMP if we stop it but do
not vent it?
- deceleration in high vacuum will need very long
(gt 30 min) ? pump could operate near dangerous
resonance frequencies too long ? bearing damage
possible - oil vapour from bearings could
contaminate rotor if remaining in molecular
flow regime (remember the bearings may be hot
after operation of the TMP! Grease lubrication
has 1000 times lower partial pressure than
oil) ? Hydrocarbons may reach vacuum chamber -
oil back streaming from the fore vacuum line (by
pressure difference and in molecular flow
regime) ? oil contamination of complete TMP
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venting TMPs with mechanical bearings
- avoid oil (grease) contamination ? venting is
recommended through venting port into the lower
stages of the rotor to have laminar flow
in direction of roughing pump - avoid forces on
bearing/rotor ? use venting valve of pump
supplier to follow recommended pressure
rise curve (see next page) or vent slowly through
chamber - if vacuum process is dirty protect
bearing from particles and/or chemical reactive
process gases (during venting, the bearings
have the lowest pressure) ? vent through
bearing ! (purge- and venting valve) - avoid
condensation in fore vacuum side of TMP ?switch
off water cooling before venting
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venting TMPs with mechanical bearings
recommended venting time for different sizes of
pumps (not valid if vacuum vessel or long
roughing line is vented via the TMP!)
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- use automatic venting devices when automising
processes- use optional delayed venting devices
if you have frequent short power failures- put
TMP never below vacuum chamber (debris does not
fall upwards in a vacuum)- vent with dry Ar to
avoid chemical reaction- vent with dry gas to
improve ultimate pressure of pump- vent with
dry gas and seal pump if stored for longer
times, but operate pump every 3 months for 2-3
hrs (aging of bearing lubricant!)
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venting MAG pumps
- MAG pumps dont have lubricants no risk of
oil contamination - if operated with dry roughing
pump no risk of oil back streaming from
roughing line - no corrosion possible in bearings
(except in dry touch-down bearings which are
coated and protected) ? venting becomes less
important
shock venting may force rotor to touch emergency
bearing ? rotor is decelerated electronically (3
9 minutes) even in case of power failure
(rotor acts as generator)
if pump was operated with corrosive process gas
- use purge gas to decelerate and vent - operate
pump 1 hr with purge gas before stopping - seal
pump and stop cooling
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some classic mistakes
venting of complete vacuum chamber through
TMP permanent cooling water flow when pump is
shut off venting through roughing line
(especially after corrosive processes)
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Marketsegment RD
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Proprietary Confidential /? by Leybold Vakuum