DIELECTRIC HEATING

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DIELECTRIC HEATING

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Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy.Dielectric heating is a special way of transforming electric current into heat.By the method of dielectric heating, generally, foils, plates and profiles with a thickness of 0,1-2,0 mm is are welded.

DIELECTRIC HEATING

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We understand dielectric heating as the generation of thermal energy (heat) in a non-conducting material by the application of an electromagnetic force or field t it. This is the way a microwave oven heats things placed in it.Wasted energy appears as heat called dielectric loss.The non metallic material with poor thermal conductivity can be very effectively heated by dielectric heating.Dielectric loss is proportional to frequency and square of the supply voltage.Frequency can be selected between 10 to 30kHz and voltage about 20kV

.When a solid dielectric material (Insulating) is subjected to an alternating electric field , it is not supposed to carry any current. However, in practice some leakage current passes through it and power loss is takes place. This loss is called as dielectric loss and result into heating of dielectric material.

APPLICATION OF DIELECTRIC HEATING

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Plywood IndustrySand Core BakingPlastic IndustryTobacco IndustryBakeries Electronic SawingDehydration of food Electro medical applicationBook Binding

Advantages of Dielectric heating

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1) Heating is very quick2) The efficiency is higher3) Heating is uniform4) Being free from smoke, dust, process is very clean5) There are no flue gases, no risk of pollution 6) Heat is produced due to dielectric loss occurs in the material itself

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ARC HEATING

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• One of the popular method of heating.• Principle:- When voltage is applied between the two

electrodes separated by small distance in air is increased, a stage is reached when the air gets ionized and air act like conductingmedium. Hence, current flows between the electrodes in the form of continuous spark called ARC. This self sustained discharge of electricity between 2 electrodes through air is known as “Electric ARC”.

An ARC drawn between 2 electrodes has a temperature between 3000°C to 3500°C depending upon the electrode material.

ARC Heating Types:-1) Direct ARC heating 2) Indirect ARC heating

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Direct ARC Furnance

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Direct ARC :- ARC is established between the charge and the electrode.The heat is directly conducted to the charge which charge absorb. Hence, this method is called Direct ARC heating.This consist of Carbon or Graphite electrode.The power is controlled by adjusting the ARC length by moving the electrodes manually or automatically.

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USE (Direct ARC)1) Making of Alloy steels like stainless and high speed steel2) Composition can be controlled during refining process.

Direct ARC operates at 0.8 p.f. Lagging

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INDIRECT ARC:

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INDIRECT ARC:ARC is formed between two electrodes and heat is transmitted to the charge by radiation.Lower temperature than direct ARC.It Operates at 0.85 p.f. lagginghave to provide rocking motion through a motor to distribute heat uniformly.USE:- melting non-ferrous metals.

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Due to application of sufficiently high voltage across an air gap causes the air in the gap to get ionized and forms ARCARC drawn between two electrodes develop high temperature (3000°C-3500°C)depending upon materialARC may be used in following ways:by striking between charge and electrodes-principle behind Direct ARC Furnaceby striking between two electrodes principle behind Indirect ARC Furnaceby striking between an electrode and the two metallic pieces to be joined -principle behind ARC welding

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SUBMERGED ARC FURNACE

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ARC is formed between carbon electrodes placed at top and earth electrode or the conducting earth itself.Number of electrodes depends on type of supply.Power is controlled by varying supply or varying distance between electrodes.Better mixing of Charge. P.f. is 0.8 Lagging

USE:-Manufacturing of ferro-chrome and ferro-manganese

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Principle of transformersIn the transformer, supply is utilized by secondary.

•Induction method is based on Principle of “Electromagnetic Induction”•When alternating Current flows in a conductor it produces alternating flux.•If any other conducting material is placed in this magnetic flux emf gets induced in it •This induced emf drives eddy current in that piece and power loss due to eddy current appears as heat.

Factors for Induction heating:-

1) it is proportional to relative permeability. Heating produced in magnetic material is more than non magnetic material. 2) Heating is proportional to MMF. Force can be vary by changing current or number of turns.3) Heating effect can be increased by employing high frequency supply.

DIRECT INDUCTION HEATING

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In this, currents are induced in the charge itself. This is usually used in furnaces for smelting (extraction of metal from ore), melting of metals etc.This requires very high frequency supply.They are classified as core and coreless type induction furnaces.

INDIRECT INDUCTION HEATING (Example :- Oven)

In this, eddy currents are induced in the heating element.Thus heat produced by heating element is then transferred to the charge by radiation or convection.

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Secondary winding is metal container.Below part is situated in the oven chamber which is made up of special alloy which losses its magnetic property and regain when they cooled.

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Operation

When the primary winding is connected to the supply , the eddy currents gets induced in the metal container forming the walls of the oven.Due to eddy currents, metal container gets heated and then is transferred to the charge by radiation.

ACTION OF SPECIAL ALLOY:-Oven reaches its critical temperature . Magnetic circuit looses its magnetic property. Due to this reluctance of the magnetic circuit becomes very high and inductive effect corrosponding decreases.

Advantages:-

Limitations:-

Applications:-

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1) Simple and foolproof method of temperature control2) No external temperature control equipments required

1) Poor Power factor2) Complicated Construction

It is used for general heat treatment of metallic and other charges.

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