Luoyang Judian Metal Hot Processing Equipment Co., LTD is mainly engaged in the manufacture of complete sets of equipment in the metal thermal processing industry and the integration of the entire production line.
Heating Elements and Non-Metallic Heating Materials
Efficiency of Direct Resistance Heating
When direct resistance heating causes the core temperature of the bar to become too high, power consumption also increases, and efficiency is reduced accordingly.
In this case, efficiency refers to the ratio between “the increase in heat content of the bar at the end of heating” and “the heat equivalent of the input electrical energy.”
The following comparison data for resistance heating of mild steel shows the influence of size:
A bar with a cross section of 60 × 60 mm and a length of 3 m is heated in 117 seconds, with an efficiency of 84%.
A bar with a cross section of 100 × 100 mm and a length of 3 m is heated in 582 seconds, with an efficiency of 60%.
A typical resistance heating arrangement for bars is usually used to illustrate this kind of equipment.
If the cross section of the bar varies along its length, direct resistance heating cannot produce a uniform temperature.
2. Heating Elements
A heating element maintains the required temperature through the balance between energy absorption and heat loss.
The electrical contact point is located outside the furnace. The heating element must pass through the furnace wall, where heat loss is very small. To prevent overheating, the section of the heating element located inside the furnace wall should have a larger cross-sectional area, or it should be made of a material with high electrical conductivity.
3. Non-Metallic Heating Elements
At the high temperature of white heat, refractory materials become electrical conductors. Their electrical conductivity varies with composition, density and temperature.
However, only a very small number of refractory materials are suitable as resistance heating elements, because before use they must first be heated to a high incandescent temperature by another heat source. Even so, some resistance materials are used in practice and must be preheated before operation. These materials include glass and salts.
Granular carbon, such as coke, was one of the earliest non-metallic materials used as a heating element. It is mainly used in pit-type furnaces, also known as soaking pits. In the furnace, it burns slowly and maintains a reducing atmosphere that does not produce oxide scale. To maintain a sufficient current-carrying cross section, new coke must be added from time to time. Damage to the container and burning of the electrodes that supply current to the carbon particles can both cause failures.
The method of using carbon particles as heating elements has been reapplied in the heating of special steels. In this method, steel is placed in a silicon carbide trough equipped with suitable electrodes, mainly to maintain a reducing or controllable oxidizing atmosphere.
Graphite Heating Elements
In a protective atmosphere, carbon in the form of graphite is an excellent material for making heating elements.
At atmospheric pressure, incandescent graphite does not melt. It sublimates when the temperature exceeds 3600°C. Graphite cannot be rolled, forged or drawn. Heating elements made of graphite are formed by mechanical cutting.
For the reasons described above, graphite heating elements are mainly used where temperatures of about 1400°C or higher are required.
