[bhhco]

Originally Posted by Dennis Brady

...In all the years I've been doing glass work, the ONLY fire I ever heard of caused by a kiln was when the fool owner placed it directly on a wood table without the kiln's metal stand....

Kilns can withstand and contain 2400 deg temperatures...

This is true for kilns. However, today a lot of folks are using Annealers which are not truly 'Kilns'... and are not designed for use above 1200 F.

The heating element in an annealer is just like the one in a kiln, and heats to 4500 F when energized (BTW... so does a common electric light bulb). The annealer 'controller' cycles the power on/off to the element to ensure the internal box temperature remains below 1200 F. This would be like placing you hand very near a light bulb and turning it on and off frequently enough that it does not burn your hand... but just keeps your hand 'warm'. The annealer's external surface temperature nominally remains at/below 220 F with a internal annealer temperature of ~960F. The controller's primary function is to ensure the internal temperature remains within the parameters set for the specific annealing schedule.

The heating element operates at 4500 F, and thus has the potential of raising the internal box temperature much higher than 1200 F if the controller fails to de-energize the heating coil. This would be like placing your hand very near a light bulb, and then turning it on BUT not turning it off... pretty soon the hand would be burned, first, second, then third degree. You would experience the full potential of a 4500 F, 60W light bulb.

The structural design of some annealers (again... not kilns) assumes the internal temperature will not be allowed to rise above 1200 F. Thus the external box surface temperature nominally remains at/below 220 F. However, if the heating element is constantly on, then the internal temperature can rise to over 2400+ F, and the external box surface temperature will likewise rise several hundred additional degrees above the 220 F.

The actual surface temperature depends on the annealer's thermal design. The important thing is this... for some annealers the external temperature can rise to the ignition point of the surface coating (e.g., the paint) used on the annealer's metal box... and result in a fire. Objects located near the annealer, which would normally be unaffected by a annealer box surface temperature of 220 F or less, will now experience a significantly higher thermal radiation... i.e, heat far above 220 F.

How can this happen? If an auto-controller (e.g., infinite switch) is used, it can fail closed - with heating element energized. If a digital controller is used, it can too... due to either software (PID logic) or hardware (relay) failure. Some controllers include a circuit to detect that failure and shutdown the unit -- more about that later.

How likely is this to happen, and/or does it ever even happen? Let me ask this first. How often do gas torches cause home fires? In 2003, the NFPA statistics show there was an estimated 1,300 residential structural fires due to gas fired torches used by 'hobbyists' in their homes, resulting in 70 injuries and $1.8M property loss. The statistics do not go to that level of detail relative to electrical appliances, non-certified, annealers, used by hobbyists. So, I do not know how often 'it happens'... but it happened to me... and that caused me to look deeper into this.

The annealers used by flameworkers are quite varied (even when we restrict the field to just plain 'Annealers', and exclude all real 'Kilns' ). Many annealers are made by companies and entrepreneurs with a long history of experience and expertise. Some are not. Some annealers are DIY projects using various 'off the net' plans... and modified in various ways. Thus, the design and the parts used are quite... varied, and the design safety is unknown.

All of these annealers, regardless of who makes them, are refractory appliances, meaning they operate at very high temperatures and require specialized parts which can survive temperatures. This is one reason annealers (and kilns) are expensive... the parts and materials are more expensive than those used in a standard kitchen oven or toaster.

Additionally, special considerations must be made in the design to include, if possible, a 'thermal runaway policing circuit' using either hardware or software. Some annealer controllers have that circuit, and the annealer can not go into thermal runaway... or at least if they do, the runaway is arrested and stopped, and the system is shut down. Some do not have this circuit.

Given the plethora of annealers, from various manufacturers, entrepreneurs, and those made by DIY'ers... it would be difficult to say 'carte blanche', that it is safe to leave an annealer unattended - without knowing the specific annealer controller's design. Some folks have come home to a kiln that had 'shutdown early'... others have required hardware change-outs because the annealer 'refused to ramp up'. To me those are potentially examples of systems which may have sensed a runaway and shutdown the system.

However, I may actually be the only person who has ever experienced a thermal runaway... and the resulting fire. Possibly I'm the only one it will ever happen to.

Me

p.s. We cannot rely on element failure or CBs to police and shutdown the system. The heating coil will not simply burn out when the internal temperature reaches 2400 F, nor even when it reaches 4500 F and stays there... anymore than will a light bulb. Additionally, the circuit breaker will not trip if an element burns out, anymore than if a light bulb burns out. CBs trip due to increased resistance, and the resistance actually decreases as the heating element temperature rises.