Heating Element Material:
Some of the ideal characteristics of heating element materials are: High resistivity, High Melting Point, Low temperature coefficient and material should able to withstand high temperatures without getting oxidized
Resistance heating is based on I2R effect and has wide applications such as heat treatment of metals annealing , hardning, drying and baking of potteries, and commercial and domestic cooking etc. Temperatures of about 1000oC can be attained in ovens using wire resistance for heating. There are two methods of resistance heating
Direct Resistance Heating: In this method of heating, current is made to pass through the body of heating. These methods have very high applications
Indirect Resistance Heating:In this method of heating heat is transferred to material through heating element through heat transfer. Immersion heaters, resistance ovens, domestic and commercial cooking and heat treatment of metals are some of the applications
Infra-red or Radiant Heating:
In this method of heating heat is transferred to material through by electromagnetic radiation by focusing incandescent lamps. Some of the advantages of this method are: Rapid heating, Compactness in heating unit, Safety, flexibility. Applications includes preheating of plastics prior to moulding, drying of pottery, paint stoving, drying of radio cabinets and wood furniture etc.
Electrical arc is established between two electrodes which can develop very high temperature depends on the electrode material. This electric arc produced is made to heat the required material. One method of heating is by strucking the arc to the charge which is placed between the electrode. Direct Arc Furnace operates based on the above principle and is commonly employed in steel industry.
Other method of heating is by transferring the heat generated due to arc through radiation. This type of heating is used in indirect arc furnace. Indirect arc heating is used for heating non-ferrous metals.
This type of heating is based on the principle of electromagnetic induction principle. The current is induced into the charge which produces heat (I2R loss). Power drawn is proportional to the square of the applied voltage and inversely proportional to the resistance of the conductor (P= V2/R). Hence in order to develop sufficient heat, the resistance of the material or charge should be low. Therefore using this method, metals (magnetic materials) can be heated more efficiently compared to nonmagnetic materials because of high permeability of the magnetic flux.
High Frequency Eddy Current Heating:
In this method of heating, the material to be heated is placed within a high frequency current carrying coil. When supply is given, alternating magnetic field is set up and eddy currents are induced in the material to be heated and this eddy currents will generate heat (eddy current loss) is responsible for heating the material. Some of the applications are: annealing and soldering, surface hardening, drying of paints, melting of precious materials, sterilization of surgical instruments
In dielectric heating, the material to be heated is placed between metallic plates or electrodes supplied with high frequency ac supply. The heat is produced within the material. The cost of heating with this method is costly and is performed where other methods are not possible. Some of the applications includes: preheating of plastic performs, baking of foundry cores, textile industry, electronic sewing, food processing etc.