Dielectric Heating And Arc Furnace
Transcript of Dielectric Heating And Arc Furnace
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