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Heater Replacement Procedure
Figure 29 shows the components of the heater element
assembly. The heater replacement procedure is the same
for all large diffusion pumps*.
NOTE
During heater replacement, the crush
plate* must be replaced if the heaters
are removed for inspection or
maintenance.
* The NHS-35 does not use a crush
plate. Additional studs are used to
assure adequate contact between the
heater and the boiler plate.
Figure 29 Heater Element Assembly
WARNING
High voltage (up to 480 V) can kill.
Always break the primary circuit to the
power supply before starting to work on
the heater or its wiring.
CAUTION
Poor clamping resulting in inadequate
thermal contact may result in reduced
heater life and poor pump performance.
Tighten nuts finger-tight on clamping
plate, then gradually and evenly tighten
them to 250 inches-pounds of torque.
To replace a heater:
1. Use an ohmmeter at the electrical box to determine
which heater element has failed.
2. Disconnect its leads from the terminal strip.
3. Pull the leads out of the terminal box and then out of
the elbow cover plate.
4. Remove the heater cover from the bottom of the
diffusion pump. Move the insulation to expose the
faulty heaters.
High Throughput Diffusion Pumps
50
5. Spray heater clamp nuts with penetrating oil.
6. Remove the nuts holding down the clamping plate.
7. Lower the entire heating unit (heater element,
clamping plate and crush plate (except for the
NHS-35 which does not have a crush plate) until
it is off the mounting studs.
Installing the new heater:
1. Apply an anti-seize compound such as Loctite C5-A
to the top 3/4" of the studs.
CAUTION
You must apply anti-seize compounds
to the studs during heater replacement.
Failure to do so will cause the nut to
seize to the stud and increase the
possibility of stud breakage during the
next heater replacement.
2. Assemble the new unit consisting of the heater
element, clamping plate, and crush plate (except for
the NHS-35).
3. Support the heating unit by the clamping plate, line
up the holes with the boiler studs, and push the unit
up against the boiler plate. Use two nuts finger-tight
to hold it in place.
4. Screw on the remaining nut(s) finger-tight, then
tighten all nuts evenly to 250 inches-pounds of
torque.
5. Check the heater alignment with the crush plate. The
heater must be located inside the crush plate tabs on
all three sides.
6. Label the heater leads according to the schematic
diagram.
7. Run the wires around the periphery of the pump,
under the wire retaining clips, into the conduit, and
up into the junction box.
8. Connect the leads to the terminal strip then replace
the elbow cover plate.
9. Replace the heater insulation (not shown if Figure 29)
between the high temperature nickel wires and the
clamping plate.
10. Replace the heater cover.
CAUTION
Heater wiring and terminal lugs must
not use copper or copper-plated wire
which could oxidize rapidly and fail
due to the high temperatures near the
heaters. Use #10 strand nickel wire
with high temperature insulation.
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