In part one we reviewed the fundamentals of resistance heating and discussed the most important factors affecting the design and selection of heating elements. This time we will take a closer look at the various heating element materials, and compare their temperature ratings, cost, and suitability for different process applications.
Heating element materials can be broadly divided into categories of metal, graphite, and silicon carbide (a semi-metallic material sometimes characterized as a ceramic). These materials have varying degrees of reactivity to oxygen in that some can tolerate elevated temperatures in the presence of oxygen while others must be protected from oxygen. The elements of tungsten, molybdenum, tantalum, and graphite all fall into the category of oxygen-sensitive materials. This is particularly important for the furnace owner to understand because even temporary exposure to oxygen can cause oxidation that will permanently impact the heating elements performance.
Maximum Heating Element Temperature and Watt Density
Before beginning our comparison of heating element materials, let’s take a moment to point out that when rating the materials for maximum temperature, we are referring to the maximum element temperature, not the maximum furnace temperature. If the temperature of the heating element exceeds the maximum allowable temperature of the material from which it is made it can embrittle, decompose, or change phase – all of which can cause failures or reduce life expectancy. Because the heating elements transfer heat by radiation to the furnace interior and to the load, the element temperature is by definition higher than the temperature of the furnace or the load. Therefore, a heating element must be selected whose maximum temperature is safely higher than the required furnace or load temperature. This safety range is typically 50 – 200°C (122 – 392ºF) but can vary significantly depending on the heating rate, cycle time and other factors. Only heating element materials with a maximum temperature well above the furnace maximum temperature should be considered.