Carbolite HRF (MF05) - Manual (Page 9)

HRF
MF05 – 3.25
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 K)
White green
Disconnect the thermocouple from its terminal block.
Re-assemble with a new thermocouple observing the colour coding and ensuring that any
porcelain spacers are repositioned as before.
5.6 Element Replacement – HRF 7/22
See section 5.2 - wearing a face mask is recommended.
The mineral insulated type of element consists of a resistance wire which is centred in a metal tube
by a compressed powder. Such elements may be bent into "U" or other shapes without losing the
electrical insulation.
Disconnect the furnace from the supply. Remove the furnace back panel. Carefully note how the
elements are connected, and remove the electrical leads from them.
Unscrew the gland caps on the faulty element, and carefully prise open the split compression
collars, which should be retained for fitting to the new element.
Remove the chimney, open the furnace door and slide out the chamber liner. Withdraw the
element through the front of the furnace.
Insert the new element through the glands and refit the compression collars. Their centres should
be approximately 40 mm from the end of the element. Firmly tighten the gland cap, ensuring the
element lies centrally between the chamber liner and the insulation.
Reconnect the electrical leads, refit the chamber liner and back panel, and reconnect the supply.
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.
Note on 208V elements: these elements are marked “240V 2000W”. These elements correctly give
1500W when used at 208V.
5.7 Element Replacement – Industrial Models
The resistance wire elements are able to withstand temperatures considerably higher than the
limit for this furnace, and are expected to fail infrequently or not at all.
In the event of the need for replacement, it is recommended that the furnace roof be removed and
the stainless steel liner lifted out. The furnace upper back panel should be removed to allow
access to the terminal blocks.
Element replacement is then straightforward, if the following hints are observed:
- when disconnecting elements from the porcelain terminal blocks, use two spanners and do not
overtighten, to avoid cracking the blocks.
- to feed element tails through the back insulation, first push through the insulation, from the
back, lengths of thin polythene tubing; slip the element tails into the tube, and ease the tube
with the tails backwards through the insulation.
- avoid damaging the grooved fibre carriers in which the elements sit; these carriers are fragile.