4.5.2
Emergency shutdown
To prevent the dissipation of all of the energy stored in the rotating rotor into the safety bearings
during touchdown, you must vent the EXT250M.
·
With a manual system, vent the EXT250M and switch off the backing-pump to shorten the
run-down time and reduce the energy dissipated in the safety bearings.
·
In the case of vacuum failure, leave the EXC300M Controller switched to On so that the
electromagnetic bearing will continue to operate and lift the rotor from the safety bearings as
quickly as possible.
If the rotor decelerates from full rotational speed to standstill on the safety bearings, leave the
EXT250M switched off for approximately one hour to allow it to cool before you restart it.
4.5.3
Excessive vibration
If external vibration or shocks cause the rotor assembly to continuously contact the safety
bearings, the ‘Emergency’ LED on the EXC300M Controller goes on, the pump-motor stops
immediately. The entry of debris into the pump can unbalance the rotor and cause the same
effect.
4.6
Bakeout
CAUTION
When you bake the EXT250M to above 70
o
C at the inlet-flange, you must cool the pump
by forced air or water cooling, to prevent the pump from overheating.
If you heat your EXT250M pump (and your vacuum system), you will speed up the degassing
process so that the pump will reach ultimate vacuum in the shortest possible time. If you heat the
pump, this will also prevent condensation of vapours inside the pump. You can use the BOC
Edwards BX bakeout band to heat the pump (refer to Section 7). Fit the band around the pump,
just below the inlet-flange. When you bake the pump, or the system, make sure that the
temperature of the inlet-flange does not exceed 100
o
C.
If you bake the vacuum system and the temperature of the system exceeds 200
o
C, you must put a
radiation shield between the system and the EXT250M. The radiation shield will reduce the heat
radiated onto the pump rotor.
Typically, a bakeout of four hours is long enough to remove water condensation from the pump.
However, the bakeout time will depend on the amount of condensation in the pump and the
vacuum system, and the ultimate pressure you want to achieve.
EXT250M Turbomolecular Pump
25
