There are several choices in the heating element category that are widely chosen for various ranges of heating operations. In this article the information is provided briefly to enable the users to choose the specific heating element like nichrome heating wire and others after evaluating their use in the certain application and the validity of metallic element in the proposed solution. But obtaining the comprehensive specification and design of heating element is the best way to choose the high quality electric furnace, by consulting the system expert or heating element supplier who have achieved expertise in the different classes of heating elements.
However, the detailed specification and design of electric heating elements is best left to either a highly-qualified electric furnace/kiln builder, an experienced and unbiased electric heating system consultant, or a full-range electric heating element supplier who has no bias toward a particular class of element.
The kanthal heating wire elements were introduced by kanthal located in Sweden and these metals are popular with the trade name of kanthal super resistance alloys for performing in the high temperatures in the sensitive atmospheres, kanthal super ER is made for extremely high temperatures in the altering environments, Kanthal super HT is made to perform in the elevated temperatures and quick cycling in the laboratory and elevated temperature sintering furnaces. The Kanthal super NC is introduced for contamination active procedures followed in large temperatures.
The heating elements are introduced in the nichrome heating wire and strip forms that can easily produced by following the traditional manufacturing procedures in the different sizes and shapes. The heating elements can be produced to perform on line potential during the systems using silicon carbide and molybdenum disilicide that generally need transformers. Moreover the nichrome heating elements are less costly than the silicon carbide and molybdenum disilicide materials for providing the same results and performances.
The mechanical properties of nichrome resistance wire are specified as :
|Wire size||Yield strength||Tensile strength||Elongation||Hardness|
|mm (in.)||MPa (ksi)||MPa (ksi)||%||Hv|
|0.5 (0.020)||390 (56,6)||800 (116)||35||190|
The density of nichrome heating element is 8.10 g/cm3 or 0.293 lb/in3 and these offer electrical resistivity of 1.19 Ω mm2/m or 716 Ω/cmf at 20 degree C.
The temperature factor of resistivity for nichrome resistance wire is:
The coefficient of thermal expansion for nichrome elements is :
|Temperature °C (°F)||Thermal Expansion x 10-6/K|
|20 – 250 (68-482)||14|
|20 – 500 (68-932)||15|
|20 – 750 (68-1382)||16|
|20 – 1000 (68-1832)||17|
However nichrome elements have lower MET than silicon carbide and molybdenum disilicide elements. These are not self supportable. Also the resistance of elements increases with the passage of time due to decrease in the cross section area and increase in length that affect the output of the system leads to ultimate failure of equipment.