Carbolite HTF 18 - Manual (Page 7)

MF47-3.05 7
3.9 Explosive Materials
The furnace must not be used to heat materials which could explode, or which could emit gases
that could form explosive mixtures.
3.10 Note on Temperature Control
The furnace is designed for rapid heating and cooling applications 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 control unit, which
incorporates a current limit potentiometer preset by Carbolite, but which will require adjustment in
the event of change of supply voltage (see section 5.4).
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.
3.11 Thermocouples - aging
The output from 1700-1800°C thermocouples when used regularly at temperatures greater than
1650°C will deteriorate and decrease with age faster than if used at temperatures below 1650°C;
this will cause the furnace to run at a higher temperature than indicated.
Customers are advised periodically to check the thermocouple output, either by a calibration test,
or by comparing the output with a new reference thermocouple. Any such comparison must be
done at a high temperature, say 1400°C or more.
Failure to check the thermocouple regularly may result in overheating of the work and the furnace,
with consequent damage to both.
3.12 Thermocouples – low temperatures
The thermocouples fitted to these models give very low outputs below about 600°C, and do not
give accurate readings at low temperatures. They may show a negative temperature when the
furnace is started from cold.
The furnaces are designed for operation over 800°C; they may be run below this temperature, but
accurate temperature measurement and stability is not assured.
3.13 Thermal Cutouts
There is a thermal cutout on the interior case surface at the centre top near the chimney. In the
event of fan failure or any other reason for case overheating, power to the heating elements is cut.
To reset the thermal cutout, first isolate the furnace from the electrical supply. Remove the top
panel and press upwards the button on the cutout device. TAKE CARE – the chimney may be hot.
There is a further thermal cutout in the element circuit transformer which cuts power if the
transformer core overheats. This is self-resetting.
If either thermal cutout is activated, then a fault light on the control panel is illuminated.
3.14 General Operating Advice
Heating element life is shortened by use at temperatures close to maximum, so avoid leaving the
furnace at high temperature when not required. The furnace can be cycled between room
temperature and maximum without a detrimental effect on element life. The maximum
temperature is shown on the rating label and on the back page of this manua1.
Light weight ceramic fibre insulation can easily be marked by accidental contact. Some fine cracks
may be visible on the surface of the insulation, or may develop in the surface of the chamber due to
the progressive shrinkage of the insulation materials. Cracks are not usually detrimental to the
functioning or the safety of the furnace.