Proper Insertion Length for MoSi2 (Super) Elements
One of the more common issues we see with MoSi2 heating elements is improper insertion length into the furnace cavity. Often times, there really isn’t an instruction manual that comes with elements explaining how to properly assemble the heating element into the furnace. Given the high cost of heating elements, it is worth this quick read to make sure that you are installing your elements properly, and in turn, getting the best life from them possible.
If you take a look at your heating elements, you will see that there is a taper between the terminal section and the heated section. If you recall from when you had to measure your elements to use our element sizing tool, the beginning of the taper (starting from the skinny side) is also the starting point for measuring the Le and Lu of the element.
This taper also coincides with where the hot part of the element starts to transition into the cooler, terminal portion of the element. The difference in outer diameter of the terminal and the heated length is related directly to Ohm’s law – at the same electrical power, the fatter terminal section has more cross section for the electrical current to flow through, reducing the resistance, and thereby reducing the temperature.
The skinny hot zone section has a smaller cross section, so the resistance will be higher, creating more heat.
The Bottom Line
Luckily, correct installation of the MoSi2 element is simple – just make sure that the taper is fully inside the furnace. Since the skinnier section of the element runs far hotter than the terminal section of the element, if the skinny section of the element is inside of the insulation and not able to radiate heat into the furnace cavity, it will form a hot spot, exceeding the maximum temperature of the element. In short, the heating element will burn out sooner rather than later.
You want your element installation to look as so (where the block represents your furnace insulation):
Note that the entire taper is inside of the hot box of the furnace. Good luck, and if you have any questions, don’t hesitate to drop us a line.