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Kilns
and Kiln Firing
Technical
Data
Safety Devices for Kilns
There are a range of safety devices available that will help alleviate
some of the concerns with gas firing:
1. High temperature cut-out
This device can be fitted to either gas or electric kilns and will
ensure the kiln will shut down at a preset maximum temperature in
case the operator is not in attendance. It consists of a temperature
controller, probe and a connection to a gas solenoid on a gas kiln
or a wiring bridge to the contactor an electric kiln. The temperature
controller can be either a cheaper "blind" model that does not indicate
temperature or the indicating type. Accurate digital units can be
quite inexpensive and will provide all the advantages of digital
temperature indication.
Cones and a special device called a "kiln sitter" can also be
employed. The kiln sitter monitors the temperature and is activated
when the desired cone bends, operating a switch.
2. Flame Safety
Thermoelectric Safety Devices
The most common approved device is the thermoelectric type. This
incorporates a push button unit and the copper safety probe heated
by the flame. These are quite reliable and simple to operate although
these can be disabled. As long as the probe is kept away from the
hot burner port and the device is sensibly maintained, a long life
should be expected. The main disadvantage is that the unit takes
approx. 10-15 seconds to close in the event of flame out. Although
this is quite acceptable according to the gas regulations, many
work places opt for the quick lockout units for increased safety
and ease of use.
Make sure the kiln is adequately vented prior to a restart
in the event of flame out. The kiln door should always be open when
starting gas burners.
Electronic Flame Safety and Ignition
The electronic safety units utilise a gas solenoid, burner electrodes
and a small control unit. These kits, that are required as standard
equipment in most schools, shut down the gas in less than 1 second.
They also have the added advantage of allowing the burner to be
lit by simply operating a start switch. A relay is closed to power
an ignition transformer and open the gas solenoid. The small spark
will ignite the burner, the safety electrode senses the flame and
allows the gas to safely stay on. The sensing electrical circuit
works on the principal of flame rectification. Briefly, these systems
rely on the ability of a flame to conduct a current when a potential
is applied across it. The flame relay detects the DC voltage that
is produced when the ion flow is larger in a single direction.
These systems have several important advantages:
- The flame failure lockout time is less than 1 second, thereby
ensuring no build up of unignited gas
- Long life for the components as the wire used can withstand
very high temperatures
- Automatic operation ensures the burner can be started remotely,
by a time switch start or a temperature programmer
As long as certain principles are applied, such as ensuring the earthing
area for the current flow is four times the area of the sense rod,
these should provide trouble free service. There are other flame sensing
systems used that are generally more industrial. The most common is
UV (ultra violet) sensing that uses a special globe to sense the UV
radiation emitted from a flame. The components are more expensive
but are preferred for larger burner units where there may be higher
flame temperatures. Many kilns have door switches fitted and electric
kilns can have devices that can restrict the power in unsafe situations.
Next Page > Maintaining
Burner Safety Controls > 5
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