Electrical Heating 02-02

DIELECTRIC HEATING

Mr. Vijay Balu Raskar - Electrical Engineer

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
 Plywood Industry
 Sand Core Baking
 Plastic Industry
 Tobacco Industry
 Bakeries
 Electronic Sawing
 Dehydration of food
 Electro medical application
 Book Binding

Advantages of Dielectric heating
1) Heating is very quick
2) The efficiency is higher
3) Heating is uniform
4) Being free from smoke, dust,
process is very clean
5) There are no flue gases, no risk of
pollution
6) Heat is produced due to dielectric
loss occurs in the material itself

ARC HEATING
• 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 conducting. 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


Direct ARC Furnance


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.

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

Direct ARC operates at 0.8 p.f. Lagging


INDIRECT ARC:


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. lagging
have to provide rocking motion
through a motor to distribute heat
uniformly.
USE:- melting non-ferrous metals.

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

SUBMERGED ARC FURNACE


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

Principle of transformers
In 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

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.

In this, get heated due to eddy
currents and then heat transferred to
charge by radiation or convection.
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.


Operation

When the primary winding is connected to the supply , the
eddy currents gets induced in the metal container forming th
walls of the oven.
Due to eddy currents, metal container gets heated and then i
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:-
1) Simple and foolproof method of temperature control
2) No external temperature control equipments required

Limitations:-
1) Poor Power factor
2) Complicated Construction

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


Electrical Heating 02-02

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