Vacuum Technology Know How - Triumf

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Analytical applications
In many cases, mass spectrometers are used in analysis devices today. Fluids are often
injected and evaporated in the inlet chamber of the vacuum system. Pressure is reduced in
several stages, and the individual chambers are isolated from one another by orifices. Since
each chamber must be pumped, the objective is to combine the gas flows via taps on the
turbopump through skillful combination of backing pumps and turbopumps. Specially modified
turbopumps with taps are used for series applications.
Helium leak detectors, too, are equipped with turbopumps. In this case, the counter-flow
principle is often used; i.e. a mass spectrometer is arranged on the high vacuum side of the
pump. Due to the lower compression ratios of turbopumps for helium than for nitrogen or
oxygen, the pump acts as a selective amplifier for the helium partial pressure.
Pumps with high gas loads in vacuum processes
The turbopump offers two advantages when pumping high gas loads for vacuum processes:
It generates clean vacuum at the beginning of each process step, and can then pump down
process gas without any harmful backflow. In the second step, the primary objective is to
maintain the given pressure at which the desired vacuum process should run. In this process,
gas throughputs and working pressure will be determined by the application in question; i.e.
a given volume flow rate will be pumped at a given gas throughput. Moreover, it should be
possible to quickly achieve clean intermediate vacuum when changing workpieces. Since
these are conflicting requirements, a turbopump of sufficient size for the required gas throughput
and the required intermediate vacuum will be selected. The process pressure will be
regulated via an inlet (butterfly) valve. An example of how to dimension this kind of pumping
station is shown in Chapter 7. The maximum permissible gas loads specified in the technical
data should be taken to mean permissible continuous loads. This applies subject to the
assurance of sufficient cooling in accordance with the specification and a backing pressure
that is less than 50 % of the critical backing pressure.
Pumping corrosive and abrasive substances
When pumping corrosive gases, measures must be taken to protect the motor / bearing areas
and the rotor, in particular, against corrosion. To do this, all surfaces that come into contact
with corrosive gas are either provided with a coating or made from materials that can withstand
attacks by these gases. A defined inert gas flow is admitted into the motor / bearing
area in the backing-vacuum via a special sealing gas valve. From there, the gas flows through
labyrinth seals to the backing-vacuum area, mixes with the corrosive gas and is pumped
down together with the corrosive gas.
The blades can wear mechanically should dust accumulate; this could necessitate repairs and
replacement of the rotor. It should also be noted that deposits can be expected to form in the
pump, which will necessitate shorter service intervals. In particular, it is necessary to ensure
that deposits in the pump do not react with the moisture in the air to become aggressive
substances. Consequently, the pumps should be vented with dry inert gases only, and should
be fitted with sealed backing-vacuum and high vacuum flanges. Turbopumps for these applications
are always classical turbopumps without a Holweck stage, as the narrow gaps and
pump channels in the Holweck stage would quickly clog with dust deposits and the
rotor would seize.
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