Materials Used for Heating Elements

Most heating equipment or appliances such as electric furnace, electric oven, electric heaters etc. utilize electrical energy to produce heat. In these equipment or appliances, the heating element is used to convert electrical energy into the form of heat. The working of heating elements is based on heating effect of electric current. When a current is passed through a resistance, it produces the heat.

To produce the heat, the electric energy consumed by resistance is given by,

E = I²Rt Joules

Where,

‘I’ is the current through the resistance (in A)
‘R’ is resistance of element (in ?)
‘t’ is the time (in seconds)

The performance and the operating life of a heating element depends on the properties of the material used for manufacturing the heating element. The required properties in the material that is used for a heating elements are:

1. High melting point.
2. Free from oxidation in open atmosphere.
3. High tensile strength.
4. Sufficient ductility to draw the metal or alloy in the form of wire.
5. High resistivity.
6. Low temperature coefficient of resistance.

Following material are used for manufacturing heating element

1. Nickel-Chromium (NiCr) Alloy
2. Iron-Chrome-Aluminium (FeCrAl) Alloy
3. Copper Nickel (CuNi) Alloy
4. Platinum Metal

Nickel-Chromium (NiCr) Alloys

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Composition of Nickel-Chromium (NiCr) Alloys

Ni = 80% + Cr = 20%

Properties of Nickel-Chromium (NiCr) Alloys

• Resistivity at 20°C: 40 μΩ-cm
• Temperature coefficient of resistance at 20°C: 0.0004 /°C
• Melting point: 1400°C
• Specific gravity: 8.40 gm/cm³
• High resistance to oxidation

Use of Nickel-Chromium (NiCr) Alloys

Nickel-Chromium (NiCr) Alloy is used in making heating elements for electric heaters and furnaces.

Note
Nickel-Chromium (NiCr) Alloy is best suited and the ideal material for making heating elements. It has a comparatively high resistance makeing it suitable to convert electrical energy into heat. When the heating element is heated the first time, the chromium in the alloy reacts with oxygen in the atmosphere and forms a layer of chromium oxide on outer surface of the heating element. This layer of chromium oxide works as a protective layer for heating element and protects the material beneath this layer against oxidation, preventing the heating element wire from breaking and burning out. Heating elements made of Nickel-Chromium (NiCr) Alloy can be used for continuous operation at a temperature upto 1200°C.

Iron-Chrome-Aluminium (FeCrAl) Alloys

FeCrAl Alloy is the name for alloys composing of Iron-Chromium-Aluminum (Fe-Cr-Al). These Iron-Chrome-Aluminium (FeCrAl) alloys are used in a wide range of resistance and heating applications.

Composition of Iron-Chrome-Aluminium (FeCrAl) Alloys

Fe = (62.5% - 76%) + Cr = (20% - 30%) + Al = (4% - 7.5%)

Properties of Iron-Chrome-Aluminium (FeCrAl) Alloys

• Resistivity at 20°C: 145 μΩ-cm
• Temperature coefficient of resistance at 20°C: 0.000001 /°C
• Melting point: 1500°C
• Specific gravity: 7.10 gm/cm³
• High resistance to oxidation

Use of Iron-Chrome-Aluminium (FeCrAl) Alloys

Iron-Chrome-Aluminium (FeCrAl) Alloy is used in making heating elements for electric heaters and furnaces.

Note
When a heating element made of Iron-Chrome-Aluminium (FeCrAl) Alloy is heated for the first time, the aluminum in the alloy reacts with the oxygen in the atmosphere and forms a layer of aluminum oxides over the heating element. This layer of aluminum oxide is an electrical insulator but has very good thermal conductivity. This electrical insulating layer of aluminum makes the heating element shock proof. Heating elements made of Iron-Chrome-Aluminium (FeCrAl) Alloy can be used for continuous operation at a temperature up to 1400°C. Therefore, it is suitable for making heating elements for electric furnaces used for heat treatment in ceramics, steels, glass and electronic industries.

Copper Nickel (CuNi) Alloys

Copper Nickel (CuNi) Alloy is also called as copper-nickel. It an alloy made by alloying copper, nickel and strengthening elements such as iron and manganese.

Composition of Copper Nickel (CuNi) Alloys

Cu = 66% + Ni = 30% + Fe = 2% + Mn = 2%

Properties of Copper Nickel (CuNi) Alloys

• Resistivity at 20°C: 50 μΩ-cm
• Temperature Coefficient of Resistance at 20-500°C: 0.00006 /°C
• Melting Point: 1280°C
• Specific Gravity: 8.86 gm/cm³
• High resistance to oxidation

Use of Copper Nickel (CuNi) Alloys

Copper Nickel (CuNi) Alloy is used in making heating elements for electric heaters and furnaces, for making coins.

Note
Copper Nickel (CuNi) Alloy has high electrical resistance, high ductility and good corrosion resistance. Heating elements made of Copper Nickel (CuNi) Alloy can be used for continuous operation at a temperature up to 600°C.

Platinum Metal

Platinum is a chemical element and not an alloy like the rest of the materials. It has the chemical symbol Pt and atomic no. 78. Platinum is least reactive metal. Platinum has remarkable resistance to corrosion, even at very high temperature, therefore, it is considered as noble metal.

Composition of Platinum Metal

Pt = 100%

Properties of Platinum Metal

• Resistivity at 20°C: 10.50 μΩ-cm
• Temperature Coefficient of Resistance at 20°C: 0.00393 /°C
• Melting Point: 1768.30°C
• Specific Gravity: 21.45 gm/cm³
• High resistance to oxidation
• High ductility
• Highly malleable
• Good mechanical strength
• Good stability with temperature and mechanical stress

Use of Platinum Metal

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 1300°C, 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.