SAFETY DEVICES FOR KILNS
There are a range of safety devices available that will help alleviate some of the concerns with gas firing:
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.
FLAME SAFETY THERMOELECTRIC 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.
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