The limitations of achieving UHV with turbomolecular vacuum pumps

Posted by Vacuum Science World News on Sep 18, 2019 11:00:00 AM

There are several types of pumps that can deliver high and ultra-high vacuum pressures; diffusion pumps, cryo pumps; ion getter pumps (IGP); titanium sublimation pumps (TSP); non-evaporable getter (NEG) pumps; and turbomolecular pumps (TMP).

The methods whereby these pumps are capable of producing high and ultra-high vacuum pressures (between 10-3 and 10-11 mbar) are either by momentum transfer of gas molecules or by capturing them (either physically or chemically).


Of these pump types, TMP pumps fall into the genre of kinetic-type vacuum pumps whereby, an exceptionally rapidly moving turbine-type rotor rotates at between 24,000 and 90,000 rpm within a chamber. As the blades of the “turbine” strike the gas molecules, they are projected towards the exit portal of the chamber. As gas molecules are expelled, the chamber pressure drops. TMPs are capable of producing vacuum pressures of between 10-3 and 10-11 mbar, employing pumping speeds of between 10 and 4,000 l/s.

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Advantages & Disadvantages of Turbomolecular Pumps

TMPs have numerous benefits:

  • they are easy to operate
  • they are low maintenance
  • they provide a hydrocarbon-free operation
  • they require no re-generation
  • and, they operate at high pumping speeds in the high and ultra-high vacuum range

However, TMPs are not without their disadvantages and limitations including: moving parts (which mean that they produce vibration and electrical noise); reduced pumping speeds for light gasses; they are sensitive to mechanical shocks and are intolerant of particulate contamination.

 

Achieving high vacuum pressures with turbomolecular pumps

In addition, due to the high vacuum pressures involved, other factors (and thus limitations) become apparent. A TMP’s main gas load is from outgassing (i.e. the “liberation” of gas molecules found near the external surface of metals, including from the chamber itself, as well as gas molecules that permeate from within the other metallic, rubber and/or plastic components). Whilst the majority of these newly liberated molecules originate from the chamber, they are also “harvested” from any and all materials and objects including pipes, flanges and valves.

 

Achieving ulta-high vacuum (UHV) pressures with turbomolecular pumps


Whilst acknowledging that each system is different, generally speaking vacuum equilibrium is achieved when the load from outgassing is matched to the pumping speed of the TMP. Furthermore, water vapour, which is preferably bound to the materials from which vacuum systems are fabricated, constitute an almost infinite source of molecules thus making them the dominant load in terms of high vacuums.

To be able to achieve higher vacuum pressures, baking of the system at 150 to 250 ͦC for between 24 and 48 hours is required. Up to this point water has been the major factor, but with baking the percentage of hydrogen increases and usually becomes the dominant residual gas.

But that is not the end of the story in terms of TMPs’ limitation and complications. Even after baking, water vapour can still become a significant part of the load in the backing pump and, gas ballasting will be required to prevent an additional build-up of condensation in the pump itself. In addition--and as already mentioned--as hydrogen becomes the dominant gas in the chamber, the partial pressure will rise in the backing line. This pressure increase can be further raised in oil-seal pumps from worked hydrocarbon stock oil, with the result that there is a partial upwards drift in pressure above the TMP as the partial pressure of the hydrogen builds up in the backing lines.

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