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2.8.1.3 Turbopump performance data<br />
Gas loads<br />
dV<br />
The gas loads q = S (Formula 1-13), that can be displaced with a turbo-<br />
pV<br />
molecular pump increase proportionally to pressure in the range of constant volume flow<br />
rate, and in the declining range reach a maximum that also is a function of the size of the<br />
backing pump. The maximum permissible gas loads depend upon the type of cooling and<br />
gas in question.<br />
. p = . p<br />
dt<br />
Displacing heavy noble gases is problematic, because they generate a great deal of dissipated<br />
energy when they strike the rotor; and due to their low specific heat, only little of it can be<br />
dissipated to the housing. Measuring the rotor temperature and reducing the RPM enables<br />
the pump to be operated in the safe range. The technical data for the turbopumps specify the<br />
maximum permissible gas loads at nominal RPMs for hydrogen, helium, nitrogen and argon.<br />
Critical backing pressure<br />
Critical backing pressure is taken to mean the maximum pressure on the backing-vacuum<br />
side of the turbomolecular pump at which the pump’s compression decreases. This value is<br />
determined as part of the measurements for determining the compression ratio in accordance<br />
with ISO 21360-1 by increasing the backing-vacuum pressure without gas inlet on the intake<br />
side. In the technical data for turbomolecular pumps, the maximum critical backing pressure<br />
is always specified for nitrogen.<br />
Base pressure, ultimate pressure, residual gas<br />
In the case of vacuum pumps, a distinction is made between ultimate pressure and base<br />
pressure (see also 2.1.3). While the pump must reach base pressure pb within the prescribed<br />
time under the conditions specified in the measurement guidelines, ultimate pressure pe can<br />
be significantly lower. In the HV range, base pressure is reached after 48 hours of bake-out<br />
under clean conditions and with a metallic seal. What is specified as the base pressure for<br />
pumps with aluminum housings is the pressure that is achieved without bake-out and<br />
with clean FPM seals.Corrosive gas-version pumps have a higher desorption rate, which<br />
can temporarily result in higher base pressures due to the coating on the rotor surface.<br />
Partial pressure<br />
- 9 10<br />
- 10 10<br />
- 11 10<br />
- 12 10<br />
- 13 10<br />
H 2<br />
C<br />
H O 2<br />
O<br />
OH<br />
N 2 +CO<br />
0 5 10 15 20 25 30 35 40 45 50<br />
CO 2<br />
Relative molecular mass M<br />
Figure 2.23: Typical residual gas spectrum of a turbomolecular pump<br />
Page 63<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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