Module 4: Heat transfer calculations, miscellaneous topics and carbon credit
  Lecture 38: Electric Resistance Heating
 

Non- metallic

Molybdenum disilicide also known as super-kanthal can be used upto . The safe temperature depends on temperature. In air, molybdenum disilicide heating elements can be used upto , in reducing atmosphere upto  .

Silicon carbide heating elements are used in hydrogen and reducing atmospheres upto . In air, the temperature is 1500oC.

Silicon and graphite are used either in reducing or in an inert atmosphere. Maximum temperature is .

Element type Advantages Disadvantages
Metallic i) Available in strip and wire form so that fabrication is easy

ii) Less expensive than SiC and MoSiO2 elements for the same power input

iii) Line voltage can be used		 i) Relatively low maximum element temperature


ii) Need supporting mechanism 



iii) Resistance increases with time due to reduction in cross section caused by oxidation and creep, these results in decreased power output and failure.

Silicon Carbide

i) Elements are self supportive. They can be used in furnaces that are too wide or too long

ii) Available in wide range of shapes and sizes.

iii) Capable to operate at higher watt loading than metallic elements and hence higher temperature can be obtained

iv)Elements can be changed while the furnace is hot

i) Costly power control equipment is required than metallic elements (multi-tap transformers are needed)

ii) Brittle in nature and hence breaks easily


iii) Electrical resistance increases with time, maintenance is required
Molybdneum disilicide

i) Available in a variety of shapes and sizes

ii) Without material degradation fast thermal cycling is possible

iii) Relatively easy to change while the furnace is hot

iv) Stable resistance, that means new and old elements can be connected.

v) Can operate at higher watt loading and temperature than metallic elements.

i) More costly power control equipment than metallic elements

ii) Elements are expensive

iii) Risk of fracture.