Modified conductance model of the turbomolecular pump

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Modified conductance model of the turbo-molecular
pump
The use of classical flow and conductance
calculations to estimate the performance of
turbo-molecular pumps in novel configurations

• Andy Pearce
• Oxford Vacuum Science Limited
• 26th October 2005

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History
Compact GCMS application
PEI Cell
Panalyser
3
History
GCMS split flow application
Pressure calculations
Optimise
for maximum sensitivity
Not Known
4
History
Compact GCMS application
CEI-Analyser
PEI Cell
Panalyser
He
CRSplit?
Compression Ratio
?
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History
GCMS split flow application
Pressure calculations (ii)
Need some way of measuring / calculating split
flow pumping speed and compression ratio -
Turbo pump model
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How does a turbo pump work?

• Traditional laymans explanation uses cricket bat
  description
• Molecule deflected down pump by impact with blade

• Alternative approach is to consider frame of
  reference of molecule
• Blade geometry appears different according to
  motion of molecule

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Blade angle modification

• Conductance for molecules entering pump is
  increased

• Conductance for molecules in contra-flow is
  decreased

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Derivation of pump characteristics
Pinlet
C
C
Pstage_1
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Derivation of pump characteristics
P1 not known
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Derivation of pump characteristics
S expressed as function of backing pressure
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Compression ratio, R
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Effect of stator stages

• Impact between blade and molecule imparts
  momentum to molecule such that it enters frame of
  reference of blade
• Stator stage in motion relative to new frame of
  reference of molecule
• Stator stages count as rotor stages
• Stator stages necessary to allow continued
  modification for multiple pump stages

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Derivation of (modified) conductances

• Measure geometry of pump!

• Calculate blade angles
• Calculate mach angle and therefore modified blade
  angles

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Derivation of (modified) conductances

l/s _at_ 20C

• Inlet conductance of duct not yet considered
• Some ducts have less than zero length
• More fundamental approach probably better

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Derivation of (modified) conductances

• Angle of blades casts shadow over inlet
• Integral performed over molecular angles of
  incidence to calculate resultant reduction in
  conductance

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Results

• Pumping speed typically within 20 agreement
• R within order of magnitude
• Better accuracy for light gases
• Generally model works very well for this pump

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Results

• Speed, compression ration and pressures
  calculated throughout pumping cell

• Able to balance compression and speed to maximise
  EI Cell throughput

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Results
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Results
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Trans-molecular flow

• Model assumes molecular flow regime
• Speed and compression ratio constant vs pressure
• At higher pressures molecule-molecule collisions
  dominate
• Molecules do not enter frame of reference of each
  stage
• Model no longer works
• Viscous flow conductances applied without
  modification
• Characteristic speed / compression vs pressure
  curves obtained

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Trans-molecular flow
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Trans-molecular flow