Dielectric Heating

Employed for Heating of Non-Metals

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.

Polarisation Effect

Basic Principle

When a capacitor is connected to an A.C. source

Practically the phase angle between current and

voltage applied is not 90 degree. 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.

Depending Factors

=

Loss factor e"

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( )

Dielectric properties of some materials

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.

Applications

Plastic welding Vulcanization of rubberGluing of woodElectronic sewing Heating of raw plasticFood Processing Pasteurizing milk

Dehydrating fruits Defrosting Frozen Food

Sewing

Machine

Wood

Gluing

Heating of raw plastic

Food Processing

Milk Pasteurizing

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.

Arc Furnace

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.

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.

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).

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).

Disadvantages

Enclosures to reduce high sound levelsDust collector for furnace off-gasSlag productionCooling water demandHeavy truck traffic for scrap, materials

handling, and productsEnvironmental effects of electricity

generation