High Temperature Heater

The atmosphere decides as much as the temperature

At these temperatures there is no protective sheath. The element itself sits in whatever the furnace is full of, and that changes what will survive. Air is fine for silicon carbide and molybdenum disilicide. Molybdenum in air above about 400 °C simply oxidises and disappears, so it belongs only in a vacuum or an inert-gas furnace.

Carbon-rich, sulphurous or halogen-bearing atmospheres shorten the life of every type, sometimes dramatically. Say what goes in the furnace as well as how hot it gets — glazes, fluxes and binders all give off something at temperature.

These elements age, and the furnace has to be built for it

A silicon carbide element does not fail suddenly one day. It gets steadily more resistive over months, so the same supply pushes less power and the furnace takes longer to come up. Left alone, that is read as a control fault and chased around the panel for weeks.

The fix is designed in from the start: a transformer with several tappings, and a maintenance habit of measuring the resistance of each circuit once or twice a year and stepping up a tap when the numbers say so.

Never mix a new element into an old circuit

Because they age, elements in service are not interchangeable with new ones. Put one new element in series or parallel with worn ones and it takes the wrong share of the current — too much in a parallel bank, too little in series — and it either burns out or leaves a cold zone. Replace a circuit at a time. Keep the old elements that still work, matched by resistance, as spares for their own group.

Handle them as ceramics, because that is what they are

Silicon carbide and molybdenum disilicide rods are brittle, and a used molybdenum disilicide element is far more brittle cold than it was when new. They break from being levered, from being clamped hard, and from the weight of their own length if unsupported. Support long elements at the middle, let them hang or slide freely as they expand, and never force one that has bonded to its bush.

The contact ends do the failing

Most furnace element failures are at the terminals, not in the hot zone. The cold end is where a braided strap, an aluminium-sprayed contact and a clamp all meet, and where oxide grows between them. A poor contact heats, oxidises further, and heats more, until the end of the element burns off. Clean and re-tension the clamps at every service, and keep the terminal area ventilated and free of dust.

Spacing and radiation

At 1500 °C an element radiates fiercely to anything near it. Too close to the wall, it overheats the lining; too close to the work, it marks it. Keep at least twice the element diameter clear all round, and remember that an element radiating onto another element makes both hotter than the design allowed.