TECHNICAL INFORMATION
PUMP USED WITH CORROSIVE GASES
Purge layout
To prevent damage to the bearings, an inert gas
must flow into the pump body around the upper
bearing towards the forevacuum line. To supply
the inert purge gas (e. g. nitrogen) to the pump
through the purge port, connect a gas purge valve
between the pressure regulator and the pump.
Adjust the pressure regulator in order to read a
gas flow rate of 0.1 to 0.8 mbar l/s.
CAUTION
To prevent bearing damage, Varian suggests a
minimum purge gas flow rate of 10 sccm (0.17
mbar l/s). This value can be exceeded, according
to the process requirements. Please contact Var-
ian for specific applications.
1. Purge gas line
2. Pressure
regulator
3. Gas purge valve
4. Gas purge port
5. Forevacuum
pump
6. Turbopump
7. Vent
valve
The purge gas throughput with the recommended
forepump of 15 m
3
/h (11 CFM) allows to achieve a
high vacuum pressure in the 10
-8
mbar range.
The recommended gas flow maintains a pressure
into the pump body higher than the forevacuum
pressure.
The recommended procedure to vent the system
and the pump avoiding the contact between the
pump bearings and the corrosive gas is described
in the following points:
PUMP USED IN PRESENCE OF MAGNETIC
FIELDS
Magnetic fields induce eddy currents in the rotor of
a turbomolecular pump that tend to oppose to its
rotation.
1. Close the corrosive gas flow into the system.
The result is increased electrical power consump-
tion by the motor, most of which is dissipated in
the rotor.
2. Leaving the Turbo-V pump and the backing
pump running and the purge gas flowing, wait
for enough time to evacuate the corrosive gas
from the system.
Since the rotor is not in contact with the stator the
above power can leave the rotor mainly by radia-
tion and hence the rotor may be overheated while
static parts of the pump remain cool.
3. Open the Turbo-V vent port slowly until to
reach atmospheric pressure in the system.
4. Turn off the Turbopump.
5. When the Turbo-V pump and the backing
pump are stopped and the system is at at-
mospheric pressure, for a better bearing pro-
tection it is advisable to leave the purge gas
flowing into the Turbo-V pump, with the
chamber or the Turbo-V vent valve opened, to
avoid system overpressures. If the vent valve
can't be kept opened, the backing pump
should be left operating.
This effect is strongly dependant from the inten-
sity, time function and distribution of the magnetic
field.
In general, therefore, an increase in pump current
can be expected.
If this increase is lower than 50% of the current
value drawn by the motor in high vacuum opera-
tion, no particular problem should be expected.
However if the effect is grater, than the case
should be carefully reviewed by Varian's specialist.
As a matter of fact, in case of high magnetic fields,
also important forces might be generated and ap-
plied to the rotor.
36
87-900-874-01(B)
