Dielectric heating employs the polarization effect to heat non-metallic materials. When an alternating electric field is applied, the molecules in the material align with the changing field, causing internal heating through molecular friction. The power loss from the lag between the current and voltage applied to a capacitor is used to heat the dielectric medium. The ease of heating depends on the material's loss factor, which represents power dissipation and is dependent on temperature and electric field strength. Dielectric heating has applications in plastic welding, rubber vulcanization, food processing, and more. An electric arc furnace uses an electric arc to melt scrap metal by forming an arc between graphite electrodes and the charged material.

1. Dielectric Heating Employed for Heating of Non-Metals

2. Polarisation Effect When a changing electrical field is applied , the molecules of the material to be heated will try to align with the field and follow the field changings. At well-defined frequencies ‘rubbing’ between the individual molecules, leads to internal heat development and an increase of the temperature in the material.

4. Basic Principle When a capacitor is connected to an A.C. source Practically the phase angle between current and voltage applied is not 90degree. There is some leakage current flows between the two plates through the dielectric medium and the power is lost . This power is utilized for heating the charge.

5. Depending Factors =

6. Loss factore" The ease with which a dielectric material can be heated is represented by what is known as the loss factor. In the above relation is Loss Angle. For a given material, the loss factor is not a constant value . These are dependent on temperature. With increasing temperature, the loss factor will often increase. e "=e '×tan( )

7. Dielectric properties of some materials

8. Effect of the field strength The power dissipation is proportional to the square of the field strength. The upper limit is that of the breakdown voltage of air. Dry air (1atm) breaks down at approximately 3kV/mm but for safety reasons, radio frequency installations operate mostly with a field strength between 80 and 160 (300) V/mm.

9. Applications Plastic welding Vulcanization of rubber Gluing of wood Electronic sewing Heating of raw plastic Food Processing Pasteurizing milk Dehydrating fruits Defrosting Frozen Food

10. Sewing Machine

11. Wood Gluing

12. Heating of raw plastic

13. Food Processing

14. Milk Pasteurizing

16. Electric arc furnace An electric arc is an electrical breakdown of a gas resulting from a current flowing through normally nonconductive media such as air. An electric arc furnace (EAF) heats charged material by means of an electric arc.

17. Arc Furnace

18. Structure of an Electric Arc Furnace The furnace consists of a spherical hearth (bottom), cylindrical shell and a swinging water-cooled dome-shaped roof. The roof has three holes for consumable graphite electrodes held by such mechanism that provides independent lifting and lowering of each electrode. The electrode and the scrap form the star connection of three-phase current, in which the scrap is common junction. Oxygen lancing is used on some furnaces to increase the rate of carbon removal from the melt.

19. The furnace is mounted on a tilting mechanism for tapping the molten steel through a tap hole with a pour spout located on the back side of the shell. A Hydraulic rocker is used for stirring purpose. The charge door, through which the slag components and alloying additives are charged, is located on the front side of the furnace shell. The charge door is also used for removing the slag. Refractory linings of Electric Arc Furnaces are made generally of resin-bonded magnesia-carbon bricks.

20. Physical processes in an Electric Arc Furnace Melting process starts at low voltage (short arc) between the electrodes and the scrap. The arc during this period is unstable. When the electrodes reach the liquid bath the arc becomes stable and the voltage may be increased (long arc). Temperature of the arc reaches 6300ºF (3500ºC).

21. Some Facts & Advantages Energy consumption of a typical EAF varies from 350-700 kWh/ton of steel produced. The main advantage of the Electric Arc Furnaces over the Basic Oxygen Furnaces (BOF) is their capability to treat charges containing up to100% of scrap. About 33% of the crude steel in the world is made in the Electric Arc Furnaces (EAF).

22. Disadvantages Enclosures to reduce high sound levels Dust collector for furnace off-gas Slag production Cooling water demand Heavy truck traffic for scrap, materials handling, and products Environmental effects of electricity generation