give three characteristic of material for heating of elements
Answers
ANSWER-
i) It should have a high resistance so that a major amount of electric energy is converted into heat energy.
(ii) It should not get oxidised easily up to a temperature of 1000°C.
(iii) It should have a high melting point and a low thermal Expansion so that it does not get loosened on getting heated
Answer:
High melting point.
Free from oxidation in open atmosphere.
High tensile strength.
Sufficient ductility to draw the metal or alloy in the form of wire.
High resistivity.
Low temperature coefficient of resistance.
Following material are used for manufacturing heating element-
Nichrome
Kanthal
Cupronickel
Platinum
Nichrome
Composition of Nichrome
Properties of Nichrome
Resistivity : 40 µΩ -cm
Temperature coefficient of resistance: 0.0004 /oC
Melting point: 1400oC
Specific gravity: 8.4gm /cm3
High resistance to oxidation
Use of Nichrome
Used in making heating elements for electric heaters and furnaces.
Note
Nichrome is best suitable and ideal material for making heating element. It is having comparatively high resistance. When the heating element is heated first time, chromium of alloy react with oxygen of atmosphere and form a layer of chromium oxide on outer surface of heating element. This layer of chromium oxide works as a protective layer for element and protect the material beneath this layers against oxidation, preventing the element wire from breaking and burning out. Heating elements made of Nichrome can be used for continuous operation at a temperature up to 1200oC.
Kanthal
“Kantahl” is trademark name for alloys made by composing Iron-Chromium-Aluminum (Fe-Cr-Al). These alloys are used in wide range resistance and heating applications.
Composition of Kanthal
Properties of Kanthal
Resistivity at 20oC: 145 µΩ -cm
Temperature coefficient of resistance at 20oC: 0.000001/oC
Melting point: 1500oC
Specific gravity: 7.10 gm /cm3
High resistance to oxidation
Use of Kanthal
Used in making heating elements for electric heaters and furnaces.
Note
When the element made of “Kanthal” is heated first time, the aluminum of alloy react with oxygen of atmosphere and form a layer of aluminum oxides over heating element. This layer of aluminum oxides, is an electrical insulator but have good thermal conductivity. This electrical insulating layer of aluminum make the heating element shock proof. Heating elements made of Kanthal can be used for continuous operation at a temperature up to 1400oC. Therefore, it is very much suitable for making heating elements for Electric Furnaces used for heat treatment in ceramics, steels, glass and electronic industries.
Cupronickel
Cupronickel is also called as copper-nickel. It an alloy made by composing copper, nickel and strengthening elements such as iron and manganese.
Composition of Cupronickel
Properties of Cupronickel
Resistivity at 20oC: 50 µΩ -cm
Temperature coefficient of resistance at 20-500oC : 0.00006 /oC
Melting point: 1280oC
Specific gravity: 8.86gm /cm3
High resistance to oxidation
Use of Cupronickel
Used in making heating elements for electric heaters and furnaces, for making coins.
Note
Cupronickel is having high electrical resistance, high ductility and good corrosion resistance. Heating elements made of “Cupronickel” can be used for continuous operation at a temperature up to 600oC.
Platinum
Platinum is a chemical element. It is having the chemical symbol Pt and atomic no. 78. Platinum is least reactive metal. It has remarkable resistance to corrosion, even at high temperature. Therefore it is considered as noble metal.
Properties of Platinum
Resistivity at 20oC: 10.50 µΩ -cm
Temperature coefficient of resistance at 20oC: 0.00393 /oC
Melting point: 1768.30oC
Specific gravity: 21.45gm/cm3
High resistance to oxidation
High ductility
Highly malleable
Good mechanical strength
Good stability with temperature and mechanical stress
Use of Platinum
Platinum is an incredible material with high resistivity and melting point. It is very much suitable for electrical heating elements, rheostats. But due to very high cost, its use in electrical engineering is limited to laboratory furnaces with a working temperature of 1300oC, rheostats, and resistance thermometers.
Platinum is a precious metal, it is very popular for making jewelry.
In medical platinum is used in chemotherapy for treatment of certain types of cancers.
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