Isoprime User’s Guide
Version 1.02 Background Information Page 142 of 219
THE TURBOMOLECULAR PUMP
A turbomolecular pump is essentially a high speed axial compressor. Gas is taken from the low
vacuum side, compressed and exhausted to the vacuum foreline, and then pumped away by the
rotary pump.
If the turbomolecular pump is turned off while open to the vacuum system, hydrocarbons from
the foreline and motor bearing will diffuse through the rotor/stator interspaces to the high
vacuum part of the pump. To prevent this back diffusion, a vent valve that admits vent gas to
the turbo pump is opened when the pumping system is turned off and the pump speed has
dropped below 50% of the full rotational speed.
The turbo will only operate if the rotary pump is working. The pump is self-regulating and
accelerates as the pressure drops.
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