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Materials Used for Heating Elements and the Causes of their Failure
Materials Used for Heating Element
An electric heating element is a device which converts electrical energy input into heat energy output. No single element, metallic or non-metallic, is used for designing the heating elements. Different alloys have been tried to obtain different temperature. The various heating alloys are given as follows −
The alloys commonly used for low and medium temperature services are −
Alloy of Nickel and Chromium, where Ni = 80 % and Cr = 20%. The Ni-Cr alloy is suitable for temperature up to 1200 °C.
Alloy of Nickel, Chromium and Iron, where Ni = 65%, Cr = 15% and Fe = 20%. The addition of iron to the alloy decreases the temperature at which oxidation takes place and hence the cost of product also reduces. The Ni-Cr-Fe alloy is suitable for temperature up to 850 °C.
For temperatures above 1200 °C, the heating elements are made of silicon carbide, molybdenum, tungsten and graphite.
The following table gives the names of alloys, composition and their safe operating temperature for different alloys used as heating element −
| Name of Alloy | Composition | Safe Operating Temperature |
|---|---|---|
| Constantan | Ni = 45%, Cu = 55% | 400 °C |
| Nichrome | Ni = 80%, Cr = 20% | 1150 °C |
| Kanthal | Chromium, Aluminium, Cobalt, Iron | 1300 °C |
| Silicon Carbide | Carbon = 30%, Silicon = 70% | 1450 °C |
Causes of Failure of Heating Elements
Following are the factors responsible for the failure of heating elements −
Mechanical Failure
The mechanical failure of the heating element is mainly due to production of more heat. During the process of alloying, a portion may have a higher content of the higher resistivity constituent. This portion of the element will produce more heat and get failed.
Oxidation
The outermost surface of the heating element gets oxidized at high temperatures. During switching operation, the oxide layer gets flack off and exposing the inner surface to the atmosphere. Now, for the further operation, the inner surface gets oxidized and thus, the effective cross-section at such spots becomes comparatively smaller resulting into formation of hot-spots and finally breakdown.
Formation of Hot-Spots
In case of formation of hot spots, the point of failure of the heating element is the place where the element shines brightest during the operation. This shows that there is a comparatively higher operating temperature at the particular spot as compared to the rest of the element.
Corrosion
If the heating element directly comes in contact with the chemical fumes during industrial operations, this will corrode the region of the element where fumes touch the element. This portion of the element will finally get failed.
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