Carbolite GPC Series - Manual (Page 9)

GPC
MF04 3.29
9
5.5 Thermocouple Replacement
Disconnect the furnace from the supply, and remove the furnace back panel.
Make a note of the thermocouple connections. The negative leg of the thermocouple is marked
blue. Compensating cable colour codings are:
negative positive (type R)
white orange
Disconnect the thermocouple from its terminal block.
Remove the screw to release the thermocouple sheath, withdraw the sheath, and shake out any
fragments of thermocouple.
Re-assemble with a new thermocouple observing the colour coding, ensuring that the
thermocouple is not twisted as it is being inserted and that the screw refitted to grip the sheath.
5.6 Element Replacement (Laboratory Models)
See section 5.2- wearing a face mask is recommended.
The chambers have two side-mounted refractory panels in which coiled heating elements are
inserted; each side contains 3 “hairpin” elements giving 6 coils of heating wire per side.
Disconnect the furnace from the electrical supply and remove the back panel.
Make a written plan showing ALL the element and thermocouple connections. It is important to
make this plan for each individual furnace.
Loosen the clamping screws onto the terminal blocks holding the faulty element. If necessary
straighten the bent ends of the wire.
From within the chamber, carefully lift the elements out of the channels in the element carrier.
Each element is retained at the front by the unwound portion being trapped between the front
brickwork and the element carrier.
If necessary, use a sharp knife to relieve the element carrier at the front with a 45° cut on the
“bridge” portion to aid withdrawal of the element.
Take care to minimise damage to the delicate fibre element carrier.
Finally pull the tails out through the back insulation.
Fit the new element. It may be help to feed a flexible plastic or nylon tube, with a bore just larger
than the element wire, through the terminal block into the chamber. Fit the element wire into this
tube; as the tube is withdrawn, the element can be through the terminal block.
When both parts of the element are located in the channels gently pull the ends of the element until
the first spiral is level with the back chamber wall insulation. If the element spirals are pulled too
far into the cavity between the back insulation and the element carrier, overheating may occur.
If necessary, bend the element tails, supporting the wire so as not to damage the insulation. Cut off
any excess wire and firmly tighten the terminal block screws.
Refit the back panel.
Check that the furnace is controlling properly to rule out the possibility that previous element
failed because of a fault elsewhere in the control circuit.
5.7 Element Replacement (Industrial Models)
See section 5.2 - wearing a face mask is recommended; also gloves. This operation requires two
people and a cold furnace. Check that the door counterweights hold the door up safely.
The chambers contain 12 off IAP tubes on which the 6 “hairpin” elements are mounted. The
elements run from back-to-front-to back. The connections are accessed by removal of the upper
back panel. The front end of the hairpin is restrained from sliding along the tubes by a hook.