It is known that turbo-molecular pumps are to be operated at already low levels of pressure. So the turbo is always backed by another pump (usually a scroll, rotary pump) that ensures this. One should not operate a turbo pump at atmospheric pressure.

Why? Is that because of air resistance? The blades would get really hot and cook the bearings?

Something that is usually also specified for turbo pumps is the backing pressure. Is this the pressure below which the turbo can be operated? Does this affect the vacuum that the turbo can achieve, or does it only relate to the blades not getting too hot?

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    $\begingroup$ Turbos work as a momentum transfer device. They are designed for pressures where the mean free path of a gas molecule is larger than the blade spacing in order to 'direct' the molecules in a particular direction. They are not fan blades designed to pump a fluid (even though they kind of look like fan blades). Trying to pump a higher pressure will require more torque than the motor/bearings are designed for, leading to the temperature interlock (for the motor) failing and a little (usually red) light on the front panel of the controller. $\endgroup$ – Jon Custer Mar 30 '16 at 23:11
  • $\begingroup$ Thanks. So is the backing pressure just a 'safety of operation' thing? Is there any relation between the backing pressure and the high vacuum pressure after the turbo? $\endgroup$ – SuperCiocia Mar 31 '16 at 0:18
  • $\begingroup$ The turbo will not pump effectively (or at all) without a low enough backing pressure. $\endgroup$ – Jon Custer Mar 31 '16 at 0:30
  • $\begingroup$ Say I have 0.2 mbar or 0.05 mbar behind the turbo pump. Both values under the 10 mbar nominal pressure below which my turbo can be operated. Will the vacuum reached be different? Is there any back streaming or equilibrium reached between the two sides (behind and in front of the turbpo)? $\endgroup$ – SuperCiocia Mar 31 '16 at 0:36
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    $\begingroup$ No - once below the spec limit you should be just fine. $\endgroup$ – Jon Custer Mar 31 '16 at 0:57

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