In this article we will discuss about the design of heating elements and causes of its failure.
Design of Heating Elements:
The wire employed for heating element may be circular or rectangular like a ribbon. Knowing the electrical input and its voltage the size and length of wire required as the heating element to produce the given temperature can be calculated.
The temperature of the heating element at the initial stage will increase gradually but after sometimes it attain a constant temperature. At this instant the heat dissipated from the surface of the element is equal to the input of electrical energy. The temperature will be sufficiently high to assume that heat is dissipated by radiation.
Heat dissipated according to Stefan’s Law,
where,
ρ = The specific resistance of wire material
I = The length of the wire
d = Diameter of circular wire
Surface area of the element = πdl cm2, so heat input per cm2 of surface area, 
Heat dissipation is equal to heat input.
Equating equations (i) and (ii) we get,
We also know that
Solving equations (iii) and (iv) length and diameter of wire can be calculated.
Causes of Failure of Heating Elements:
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The main causes of failure of heating elements are given below:
1. Formation of Hot Spot:
Hot spots are the points in the heating element which are formed at higher temperature. One of the reasons of formation of hot spot in heating element is high rate of local oxidation that may reduce the element cross-section, thereby increasing the resistance at that spot and produces more heat locally and causing breakdown the element.
The other causes are wrong fuse material, may result in sagging and wrapping of the material.
2. Contamination and Corrosion:
Gases of the controlled atmosphere prevailing in annealing furnace or fumes from flux used in brazing furnaces or oil fumes caused by heat treatment of components contaminated with lubricant contaminate the elements and produce dry corrosion.