Strip Heater

One strip is a line of heat, not a warm surface

The heater is 30 or 40 mm wide and the plate it is bolted to may be 300 mm wide. Everything outside that strip is heated by conduction along the metal, so the temperature falls away from the heater — steeply in thin plate, more gently in thick. If the job needs an even surface, the answer is more strips further apart, or a thicker plate, not more watts in one strip.

A useful rule when laying out a plate: put strips no further apart than about ten times the plate thickness, and keep the outermost strip about half that distance from the edge, because the edge has metal on one side only.

Finned strips only work where the air moves

Fins are surface area, and surface area only helps if air is passing over it. In a duct with a fan behind it, a finned strip runs comfortably. Bolted into a cabinet with no airflow, the same heater cooks — the air right against the fins warms up, stops carrying heat away, and the element sits in its own hot pocket.

Mount finned strips so the fins run along the airflow, and leave clear space on both sides. A finned element pushed up against a duct wall has lost the fins on that side.

Fit it along the hottest path, not across it

On a sealing bar, the useful heat is at the sealing face. Bolt the strip so its length runs along the bar, directly behind that face, and keep the metal between them as thin as the bar’s strength allows. A strip mounted across a bar, or on the far side of a thick section, heats mostly the bar’s own mass and leaves the face lagging behind the setpoint.

Take the expansion into account, in the wiring too

The case grows by a few tenths of a millimetre in length as it heats, and it does that thousands of times. The slotted holes deal with the case. The leads need the same thought: leave a loop of slack near the terminal so the wire is not tugged on every cycle, and support it a hand’s width from the heater. A lead worked back and forth at a fixed terminal breaks inside its insulation, where nothing can be seen.

Sensor on the work, not on the heater

Clamping the sensor to the heater case gives a reading that is always higher than the surface being heated, and it rises and falls faster. The controller then settles at a case temperature while the work is still cold. Put the sensor in the bar or the plate, in a drilled pocket if there is room, at the point whose temperature actually matters.

Look for the marks a failure leaves

A strip that failed in the middle points at a bowed surface. One that failed near a bolt points at a bolt pulled down too hard. One that failed at the lead end points at heat or a pulled wire, not at the element. The replacement will go the same way unless the mark on the old case is read first.