Carbolite TZF Series - Manual (Page 7)

STF/TZF
MF08 – 3.24
7
C. Independent Control
In this case the three controllers and thermocouples are completely independent, as shown in 7.8.
Note that it is not possible to maintain very different zone temperatures because of heat transfer
within the work tube.
3.4 General Operating Advice
Heating element life is shortened by use at temperatures close to maximum. Do not leave the
furnace at high temperature when not required. The maximum temperature is shown on the
furnace rating label and on the back page of this manual.
Lightweight 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.
3.5 Operator Safety
The ceramic materials used in furnace manufacture become electrically conductive to some extent
at high temperatures. DO NOT use any conductive tools within the work tube without isolating it.
If a metal work tube is used, it must be earthed (grounded). Note that the furnace temperature
should be limited if a metal tube is used.
Ensure that there are no inflammable objects placed in such a way that the work tube could break
and fall on them.
Switch off the Heater switch whenever loading or unloading the furnace. The elements are isolated
when the Heater switch is OFF. This switch cuts both sides of the circuit directly or via a contactor
(a contactor is used in models where the rated current exceeds 16 Amps).
(Note that the heater switch is not fitted in models ordered before May 1999).
3.6 Tube Life
A ceramic work tube may crack if workpieces are inserted
too quickly or at temperatures below 900°C (when the
tube is more brittle). Large pieces should also be heated
slowly to ensure that large temperature differences do not
arise.
Poor thermal contact should be encouraged between the
workpiece and the tube; crucibles or boats should be of
low thermal mass and should have feet to reduce the
contact with the tube (fig. 4).
Do not set too high a heating rate. Large diameter tubes are more susceptible to thermal shock than
smaller. A general rule for maximum heating rate is 400/internal diameter (°C/min); for 75mm i/d
tubes this comes to 5°C per minute. The controller can be set to limit the heating rate.
3.7 Pressure
Work tubes are not able to accept high internal pressure. When gas seals or similar fittings are in
use, the gas pressure should be restricted to a maximum of 0.2 bar (3 psi). A pressure of about half
of that should normally be sufficient to achieve the desired flow rate. The customer must ensure
that the exhaust path from the tube is not blocked, so that excess pressure does not occur.