Carbolite VCF Series - Manual (Page 9)

VCF
Replace and reconnect the solid state relay ensuring that the heat-conducting thermal pad is
sandwiched between the relay and the plate. Alternatively a thin layer of white, heat-conducting
silicon paste may be applied between the new relay and the plate.
The new solid state relay contains a built-in MOV which protects it from short periods of excess
voltage. If the old relay had a separate disc-shaped "MOV" connected between the high voltage
terminals of the old relay, discard the old MOV.
Replace the removed panel
5.6 Thermocouple Replacement
Disconnect the furnace from the supply, lay the furnace on its back and remove the bottom 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 K) positive (type R)
white green orange
Disconnect the thermocouple from its terminal block.
Unscrew the fixing screw, or bend the metal tag (as appropriate), 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 fixing screw is replaced (or the metal
tag is bent back) to grip the sheath.
5.7 Element Replacement
See section 5.2 - wearing a face mask is recommended.
The chamber is formed by four separate slabs comprising coils of heating wire embedded in
refractory cement. All the connections are in the bottom of the furnace casing.
Disconnect the furnace from the supply. Lift up the door and ensure that it is safely held open.
Remove the top panel. On the left is a removable frame strut: this may need to be removed.
Remove the left panel.
Inside the left hand side, observe that the inner chamber (“brickbox”) has a removable side; loosen
this side - it should not be necessary to remove it completely.
Turn the furnace onto its back and remove the bottom panel.
Make a written plan showing ALL the element and thermocouple connections. It is important to
make this plan for each individual furnace.
Disconnect all the element wire tails and thermocouple cables from their terminal blocks.
Straighten all the element tails. Note that these become brittle with use.
Working from the top, ease the failed slab element(s) out. They may have become fused together,
so it may be difficult to remove just one of them.
Insert the new element(s), carefully threading the tails through the terminal blocks. Cut off excess
tails, and retighten. Remake all connections according to the plan previously made.
Refit all panels and return the furnace to the vertical position.
Run the furnace for 30 minutes at 800°C without interruption to ensure complete burn-off of any
organic binders. Some smoke may be observed during this process, which should be carried out in
a well ventilated area.
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.