Furnace Heater

The cheapest element in the family, and the one most often ruined by fitting

An open coil is just wire, and that is its strength: any power, any chamber, wound to order, replaced for the cost of the wire. Almost every furnace in a small workshop runs on them. What decides whether they last is entirely in the winding and the fitting — the wire diameter, the coil diameter, how far the coil is stretched, and how it is held.

Get those right and an element runs for years. Get the stretch wrong and it fails in weeks, and it looks like a wire quality problem.

Stretch sets the temperature of the wire

A coil is pulled out to a set length before it goes in, usually two and a half to four times its closed length. If it is too tight, adjacent turns face each other across a very small gap, radiate into one another, and run far hotter than the design intends. If it is too stretched, it has less support per unit length and sags out of its groove onto whatever is below.

Fit a coil to the length it was wound for, and check the pitch is even along the whole run. A short section left bunched is a hot spot that will decide where the element fails.

Iron-chrome-aluminium goes brittle, once, and permanently

This is the single fact that catches people out. FeCrAl wire is soft and workable when new. After its first firing it recrystallises and becomes brittle, and from then on it cannot be bent cold — it snaps. So a used element cannot be straightened, re-pitched or eased back into a groove without heating that section first.

Plan for that at fitting: get the geometry right the first time, and when an old element has to be moved to get at something, warm it or accept that it will break. Nickel-chrome keeps its ductility, which is why it is still chosen for elements that hang free or get handled.

The wall crossing is where elements die

The coil in the middle of the chamber is doing what it was designed to do. The tails that leave through the wall are in the worst place — hot, poorly cooled, and joined to a terminal that can loosen. Bring the tails out thicker than the hot coil so they run cool, pass them through ceramic bushes, and keep the terminals clean and tight. Almost every premature failure in an open-coil furnace is within a hand’s width of the wall.

Grooves, pins and gravity

A coil laid in a groove is held by the groove. One hung on ceramic pins is held by the pins, and it will find any that are too far apart. Support spacing has to suit the coil diameter and the temperature: hot wire has very little strength, and a span that is fine at 900 °C will droop at 1300. If a lining groove has broken away, repair the groove before fitting a new element into it.

Match by resistance when replacing part of a set

Elements in a furnace are wired as circuits, and the circuit only behaves if the elements in it are alike. Measure the resistance of a working element in the same circuit and have the replacement wound to match. Two coils can look identical and differ by half in resistance, and the difference shows up as one glowing zone and one dull one.