Content:
Wire temperature
Surface load
Surface and resistance
Coil parameters
Metal-sheathed tubular element
Examples
Domestic applications include functions such as cooking, fluid heating, drying, ironing, space heating, and specialized uses like heating beds, aquariums, saunas, soldering irons, and paint strippers.
Industrial applications include functions such as heat treatment, hardening, and drying of inks, paints, and lacquers. In vehicles, heating elements are used for seats, engines, and rearview mirrors.
The device and the heating element must meet performance, raw material, manufacturing cost, life expectancy, and safety requirements. Often, these requirements conflict. A longer lifespan and increased safety require a lower wire temperature, leading to longer heating times and higher raw material costs.
Designs for domestic and industrial applications must ensure safety and avoid harm to individuals or property damage. Safety specifications vary by market and can influence the design and temperature limitations of the device and its elements.
The lifespan of a well-designed element depends on the make and type of wire used. Our FeCrAl and NiCr(Fe) wires excel at high temperatures and offer the longest lifespan. Remember, wire life increases with a thicker wire and lower operating temperature.
Wire temperature
For embedded or supported element types, wire temperature is influenced by both the wire and the element's surface load. For suspended elements, surface load typically cannot be defined. In addition to surface load, factors such as ambient temperature, heat dissipation, and the presence or position of other elements will affect wire temperature, which in turn influences the selection of the wire and element surface load.
Surface load
When designing an element, the voltage and power rating are typically known. The surface load of the heating element is calculated by dividing the power rating by the surface area of the energized wire. A range of surface loads, rather than a single value, is usually listed in the tables. Selection within this range depends on the element’s requirements, as well as the available voltage, power rating, and dimensions.
For example, high voltage and low power ratings result in a thinner wire, which has a shorter lifespan at the same temperature. Therefore, a lower wire surface load is required. The wire surface area is calculated as the ratio of the power rating to the wire surface load.
Surface and resistance
Once the cold-state resistance is calculated, the ratio of surface area to resistance is determined. This ratio is listed for all wire types and dimensions in this handbook allowing for easy selection of the correct wire size.
Coil parameters
The ratio of coil diameter to wire diameter (D/d) must be calculated to ensure the coil can be manufactured easily. The recommended ratio (D/d) ranges from 5 to 12. For supported elements, this ratio should be compared to the deformation curve under "Elements for electric appliances".
When both coil length and diameter are known, the coil pitch (s) can be estimated using the formula [17] under "Symbols, formulas and definitions". The coil pitch is typically 2 to 4 times the wire diameter (d). For quartz tube heaters, a smaller pitch is usually applied. Pre-oxidized coils made from Kanthal® FeCrAl can be tightly wound in such elements.
For straight wire on a threaded ceramic rod or many suspended elements, the wire length is fixed. The resistance per meter can then be calculated, and the appropriate wire size can be selected from the handbook.
Metal-sheathed tubular element
Calculating a metal-sheathed tubular element is more complex, as the resistance is reduced by 10 to 30% due to the compression of the element. First, determine the tube surface load based on the element’s use. The wire surface load is typically 2 to 4 times higher. After calculating resistance from the power rating and voltage, increase the resistance by 10 to 30% to account for coiling. The wire surface area decreases by 2 to 7% after compression. Since tube length increases due to compression during rolling, the tube surface area usually remains unchanged.
Examples
Design calculations for heating elements: Examples