Overheat protection

Cause and effect of a kiln overheat.

Kiln overheat ccurs most commonly as a result of the failure of a power switching device to remove power from one or more of the elements, or less frequently due to malfunction of another control component, a kiln can continue to heat up until the element fails through overheating at temperatures above 1400°C. A sensibly powered glass kiln should be able to increase temperature at 1.5 to 2°C per minute at around 800°C. This means that, should an overheat fault occur at top soak, then the kiln could be close to 900°C in 1 hour after the fault occurs, mid 900’s in 2 hours, 1000°C in 3 hours.

Just imagine where this very fluid glass could flow and the damage it could do.

In modern kilns there are three components which control power to the elements:

• A controller which determines when power should go to the elements,

• Solid State Relays or Contactors which switch power in response to instructions from the controller.

• A thermocouple which measures the temperature in the kiln.

A fault in any one of these can create an overheat situation.

Overheat shutdown systems 

Industrial kilns and furnaces have long been fitted with systems to protect them against overheating.

Functions of a modern Overheat Shutdown system should include:

1. detect an overheat situation and remove power from the elements;
2. prevent power from being re-applied when the temperature drops into the safe range  (prevent recycling continuously at the set overheat temperature); and
3. make the operator aware that an abnormal situation had occurred.

An effective system can be fitted to most kilns for a fraction of the cost of repair and downtime should a fault occur.
It should comprise:

• a temperature switch to sense the overheat temperature,
• an independent contactor to isolate the elements and,
• an independent thermocouple which was often mounted high up in the enclosure.

Temperature switches

Until very recent times an analogue temperature switching device was used. This usually had an engraved dial on which the approximate cut-off temperature could be set. This setting could often be plus or minus 10 degrees or so.

With the introduction of digital indicators there was an apparent immediate increase in preciseness of reading; and much operator confusion. With two digital instruments displaying the readings of two independent thermocouples it would be a minor miracle if both read the same. To minimise confusion, analogue temperature switches continued to be used on Riley kilns until the introduction of the Brainchild L91 limit switch. (see more below). 

A Temperature Switch (TS) usually has an internal relay which is energised so long as the Process Value (PV, kiln temperature) is below the Set Value (SV, overheat temperature setting). 

When the kiln is initially turned on the TS will be set at some temperature 50°C to 100°C above the normal top firing temperature. In kilns for slumping and fusing it is common to set the top operating temperature in the controller to 850°C or 900°C and the overheat temperature at 50°C higher.

If an over temperature condition is sensed the TS contacts will change over, dropping out the isolating contactor and removing power from the elements. The kiln will cool down. This meets function 1 above.

Unfortunately, when the temperature drops to below the set value on the TS the contacts will change back and re-energise the contactor, re-applying power to the kiln. It will continue to cycle in this manner until turned off by the operator.

Different methods have been used to overcome this and meet function 2 above, including lock-up circuits using miniature relays or a combination of latching and standard relays. Sometimes an indicator lamp showed when an overheat condition had occurred.

Brainchild L91 Limit Switch

More recently a special device, the BTC L91, was introduced by the Brainchild Instrument people. It incorporates an electronic latching system which prevents the recycling and which must be manually reset by the operator after an overheat situation occurs.

It is a stand alone device and does not need any additional components to perform functions 1 and 2 above. The L91 Limit Switch is distributed in Australia by  ECE Fast. Currently (2010 ) this is the most suitable and cost effective device known to me for this function.

The L91 is a digital device. To prevent operator confusion from two digital devices showing different readings it can be set to display SAFE so long as the kiln is operating within the safe temperature range. Only when an overheat condition is sensed will it display the temperature sensed by its own thermocouple.

Riley Glass Kilns circuits

There can be confusion between contactors, relays and power relays.

To further clarify the above, here is a description of the electrical system used in all Riley Glass kilns to which an overheat shutdown system was fitted.

The control system comprised:

• A programmable controller supplying a 12Volt DC output signal. (Most recently a Shinko PCD-33A S/M)
• D2440 Solid State Relays, one or more per phase with each limited to 32Amps. Each SSR is mounted on an oversize heatsink with fins exposed externally and without forced ventilation.
• Type K thermocouple.

The Overheat Shutdown system comprised:

• A Brainchild L91 limit switch.
• A contactor with an AC1 rating at least equal to the current rating of the kiln elements.
• A separate type K thermocouple mounted beside the first.

Prior to the arrival of the L91 an analogue temperature switch was used with two 2 pole miniature relays using relay logic to lock up the system when tripped and to prevent cycling at overheat temperature. 

Electrically, the active conductors are connected into the isolating contactor located ahead of the SSR's. A  supply is taken from the live side of one phase to a 2Amp circuit breaker to power the controls.

The contactor will energise when the kiln is first turned on so that the points will rarely switch under load. The normal switching of power is done by the SSR's. The cycle time of the controller output is 3 seconds so that short bursts of power of from 1/100ths of a second to 3 seconds duration are supplied.

Using smaller contactors

The normal current rating for a contactor is the maximum current it can carry as an inductive load, called AC3. That is for electric motors and similar loads. Heating elements are a resistive load, called AC1. A given size contactor can carry a higher AC1 load than it can AC3, so take advantage of this if installing an overheat shutdown system; it will save money and space.

When choosing a contactor for a kiln using resistive elements look at the AC1 rating.