Good elements fail early when they are installed wrong. These five rules cover most of it.
Installation problems account for a large share of early element failures. None of these five rules is complicated, but all five get skipped under schedule pressure.
An element expands about 1.5% from room temperature to 1,000 °C. A 2 m element grows roughly 30 mm.
Fix one end, let the other end slide. Never weld or clamp both ends.
For horizontal helical elements, support spacing should be 30–50 times the wire diameter.
A φ6 mm wire needs supports every 180–300 mm. Closer than that wastes material; further apart lets the element sag into the charge or the hearth.
Helical elements change pitch when hot. Design the cold pitch slightly larger than the hot pitch so adjacent turns do not touch at temperature.
A useful rule: cold pitch ≥ hot pitch × 1.02.
The terminal end runs hot. Keep it away from the furnace wall and out of the insulation. If the terminal is buried, it will oxidise and the contact resistance will climb.
In a three-phase furnace, every phase should have the same wire diameter, length and number of turns. Any mismatch shifts the current split and one phase will fail first.
On a new furnace or a new set of elements, run a dry burn-in before the first production load. Take the furnace to 100–200 °C below working temperature and hold for one to two hours. This burns off oil and adsorbed moisture that would otherwise contaminate the first batch.
Welding both ends of the element "to make it secure". It looks solid, but at temperature the element has nowhere to expand, so it arches or cracks. We see this more often than any other installation error.
