Working Principles of Ceramic Heating Elements
Millenia after it was first discovered, mankind is still obsessed with fire. We rely on combustion fuels for countless industrial and everyday processes, from domestic heating to thermal processing. Yet it can be an inconvenient method of generating heat – particularly in industrial markets. Natural gas furnaces are still broadly used in heat treatment markets, but manufacturers more commonly rely on the comparatively sophisticated method of generating heat from electricity.
Metal and ceramic heating elements operate on the principle of electric resistance heating, which is defined as the heat generated by a material with high electrical resistance as a current are passed through it. When a current flows through metal or ceramic heating elements, the material resists the flow of electricity and generates heat. This is a basic explanation of a complex concept, but the principle generally holds true for common metal and ceramic heating elements in industrial furnaces.
In this blog post, Thermcraft explores the basic working principles of ceramic heating elements in more detail.
Ceramic Heating Elements and Resistance Heating
While there are myriad heating element types available to furnace designers, ceramic heaters generally fall into one of two groups: exposed ceramic rods; or coils, ribbons, and wires of an alloy embedded in a plate of ceramic insulation. At the simplest level, these heating element types operate on the same principle.
The material’s coefficient of electrical resistance determines its ability to generate heat proportional to the amount of current flowing through it. A ceramic heating element’s thermal output, therefore, is determined by its electrical load and its intrinsic resistive properties. Under ideal conditions, the element will resist the flow of current and generate heat which will radiate outwards into the heat treatment chamber. The primary benefit of this compared to combustion is vastly increased efficacy, as 100% of electricity supplied is theoretically converted into heat.
Yet there are numerous interconnected factors that can affect these two underlying properties. Alloy composition, element dimensions, watt loading, voltage, and device architecture are just a selection of these fundamentally important properties.
For example a typical exposed ceramic heating element material is high-purity silicon carbide (SiC), which can be arranged in rods, multi-leg, and spiral-cut heaters. The lengths and diameters of these elements can be customized to specific furnace dimensions, while the outstanding thermomechanical stability of the material means it always retains its rigidity. This simplifies installation of the heater, as it does not need to be mounted on nor embedded into the furnace wall. This reduces the risk that that generated heat will propagate through the furnace to damage sensitive equipment. Silicon carbide ceramic heating elements also boast improved electrical efficiency, converting 100% of all electricity supplied into heat with a shallow drop-off in terms of watt loading.
The drawback of exposed ceramic heating elements comprised of silicon carbide is that the material is not fully densified, which makes it susceptible to cross-reactivity with atmospheric gases at elevated temperatures. These reactions can affect the conductive cross-section of the element, which gradually causes an increase in electrical resistance over time. In fact, the resistance of a silicon carbide ceramic heating element could increase by up to 300% before the end of its service life.
Ceramic Heating Elements from Thermcraft
The performance of industrial heating elements varies significantly, not just based on the architecture of the device, but the operating conditions of the furnace itself. Designing an appropriate heating element subsequently requires an intrinsic knowledge of material performance under given conditions, and how best to optimize that performance for desired specifications.