Carbolite BLF 18/21 - Manual (Page 6)

BLF
6
MF23 – 3.11
3.3 Atmospheres & Corrosive Materials
In oxidising atmospheres, metal oxides react with the silica layer on the surface of the elements
and may lead to premature failure. Protect the elements from splashes of molten metal and dust
when charging the furnace, and from fumes developed when melting, especially from fluxes. Also
avoid compounds with a high alkali content.
The furnace is designed for use in oxidising atmospheres, but can be operated successfully in
neutral or carburising atmospheres. It may be used with nitrogen, argon or helium atmospheres to
1600°C. Reducing atmospheres are not recommended. Sulphur dioxide in normal concentrations is
not harmful, but chlorine, and more so fluorine, strongly attack even oxidised elements and should
be avoided.
The furnace is not recommended for burning off carbonaceous materials. Other Carbolite furnaces
are available for this application.
When an optional gas inlet is fitted there is a label near the inlet saying "INERT GAS ONLY". In
practice inert or oxidising gases may be used, but not combustible or toxic gases. The furnace is
not gas tight, so gas usage may be high, and that the chamber is likely always to contain some air.
Residual oxygen levels of 1% are to be expected. If an inverted crucible is fitted as an atmosphere
container, then gas-tightness is improved, but there is still some leakage around the seal.
The hearth can be protected from abrasion, if required, by the provision of a secondary plate. This
consumable item can either be in light weight ceramic fibre or in dense, hard-wearing alumina
refractory.
3.4 Operator Safety
The furnace incorporates a safety switch which interrupts the heating element circuit when the
hearth is opened. This prevents the user touching a live heating element, but also prevents the
furnace from heating up if the hearth is left open.
Avoid burns. Carbolite can supply tongs, face masks, and heat resistant gloves. Before you remove
a hot object from the furnace make sure you have a safe place to put it down.
When the hearth is lowered with the furnace hot there is considerable radiative heat. Do not keep
near the furnace any inflammable objects or anything which could be damaged by heat.
3.5 Note on Temperature Control
The furnace is designed for rapid heating and cooling applications up to 1700°C or 1800°C
(depending on model). The programmer enables the furnace to heat or cool at slower rates as
desired, and variable "hold" (dwell) periods can be programmed as required.
The programmer is used in conjunction with a phase angle thyristor Eurocube, which incorporates
a current limit potentiometer preset by Carbolite.
The elements are connected in series across the low voltage output of a transformer housed in the
furnace case. Molybdenum disilicide elements do not age, so if an element fails it is not necessary
to replace the complete set.
3.6 Thermocouples - Warning
(1) The output from thermocouples when used regularly at temperatures greater than 1650°C can
deteriorate and decrease with age.
Customers are advised periodically to check the thermocouple output, either by a calibration test,
or by comparing the output with a new reference thermocouple which has been subjected to high
temperatures for a minimum length of time. Failure to check the thermocouple may result in
overheating of the work and the furnace.
(2) The thermocouples fitted to these models give very low outputs below about 600°C, and do not
give accurate readings at low temperatures. The furnaces are not intended to be operated below
600°C.