Electric Heating

Electric Heating

Prof. Tejas H. Panchal

Electric Heating

Heating is required for

1. Domestic Purpose

Hot plates for cooking

Room heaters

Immersion heaters for water heating

Electric irons

Electric toasters

Electric ovens for bakeries

Pop-corn plants etc.

Electric Heating

2. Industrial Purpose

Melting of metals

Heat treatment processes

Moulding of glass

Baking of insulators

Enameling of copper wires

Welding etc.

The use of electrically produced heat is alwayseconomical proposition on account of present low

cost and availability of electrical energy.

Advantages of Electric Heating

Free from dirt. It is a clean system requiring min. costof cleaning.

Does not product any flue gas. No provision isrequired for their exit.

Simple & accurate temp. control can be made.

It is economical as electric furnaces are cheaper ininitial cost and in maintenance cost.

Automatic protection against overcurrents andoverheating can be provided.

Overall efficiency of electric heating is much higher.

Safe and responds quickly.

No upper limit to temp. obtainable except the abilityof the material to withstand heat.

Modes of Heat Transfer

Heat transfer is defined as the transmission of heatenergy from one region to another as a result of temp.

gradient.

It takes place by the following three modes:

(i) Conduction (ii) Convection (iii) Radiation.

Methods of Electric Heating

Power Frequency

Heating

Resistance Heating

Direct Resistance

Heating

Indirect Resistance

Heating

Arc Heating

Direct Arc

Heating

Indirect Arc

Heating

High Frequency

Heating

Induction Heating

Direct Core-type Induction

Heating

Coreless type Induction Heating

Dielectric Heating

Infrared Heating

Methods of Electric Heating

Direct Resistance Heating

Electric current is passed through the body to beheated.

This method has high efficiency since the heat isproduced in the material itself.

This principle of heating is employed in:

Resistance welding;

Electrode boiler for heating water

Indirect Resistance Heating

Electric current is passed through a resistanceelement which is placed in an electric oven.

Heat produced is proportional to I2R losses in theheating element.

The heat produced is delivered to the charge/bodyeither by radiation or convection or by combination oftwo.

This method is used in:

Immersion heaters;

Resistance ovens

Domestic and commercial cooking

Heat treatment of metals etc.

Arc Heating

The arc drawn between two electrodes develop high temp.(about 3000 3500 C) depending upon material ofelectrode.

The arc may be used in following different ways:

1. By striking arc between the charge & electrode orelectrodes. In this method heat is directly conducted andtaken by the charge. The furnace operating on thisprinciple is known as direct arc furnace.

2. By striking arc between two electrodes. In this methodheat is transferred to the charge by radiation.

3. By striking an arc between an electrode and the twometallic pieces to be joined, as in arc welding.

Direct Induction Heating

In this method the current is induced by electro-magnetic action in the body to be heated.

The induced currents when flowing through theresistance of the body to be heated develop the heat &

thus raise the temp.

In induction furnace heat is used to melt the charge.

Eddy current heaters are employed for heat treatmentof metals.

Indirect Induction Heating

In this heating method the eddy currents areinduced in the heating element by electro-

magnetic induction.

Eddy currents set up in heating element producethe heat which is transferred to the body to be

heated up, by radiation and convection.

This principle is employed in certain ovens whichare employed for heat treatment of metals.

Dielectric Heating

It is also called high-frequency capacitive heatingand is used for heating insulators like wood,

plastics and ceramics etc. which cannot be heated

easily and uniformly by other metals.

The supply frequency required for dielectricheating is between 10-50 MHz and the applied

voltage is 20 kV.

The overall efficiency of dielectric heating is about50%.

Infrared or Radiant Heating

In this method of heating, heat energy from anincandescent lamp is focused upon the body to be

heated up in the form of electromagnetic

radiations.

This method is employed to dry the wet paints onan object.

Resistance Heating

This method of heating is based upon I2R effect andhas wide applications such as heat treatment of

metals(e.g. annealing, normalizing, hardening,

tempering etc.), drying and baking of potteries,

domestic cooking etc.

In oven where wire resistance are employed forheating, temp. to the time of 1000 C can be obtained.

Following are the two methods of heating:

1. Direct resistance heating

2. Indirect resistance heating

Resistance Heating


In this method the material or charge to be heated istreated as a resistance and current is passed through

it.

The charge may be in the form of powder, small solidpieces or liquid.

The electrodes are inserted in the charge andconnected to either A.C. or D.C. supply.

In case of D.C. or single phase A.C. supply twoelectrodes will be required, while in case of 3-phase

A.C. supply three electrodes will be used.

This method of heating has high efficiency becausethe heat is produced in the charge itself.

Resistance Heating


This heating method is employed in

Salt bath furnace

Resistance welding

Electrode boiler for heating water

Resistance Heating

2. Indirect resistance heating

In this method the current is passed through a highresistance wire know as heating element.

The heat produced due to I2R loss in element istransmitted to the body to be heated by one or more modesof heat transfer viz. conduction, convection and radiation.

This method of heating is used in

Room heaters

Bimetallic strips used in starters

Immersion water heaters

Various types of resistance ovens used in domestic andcommercial cooking

Salt bath furnaces

For industrial purposes where a large amount of chargeis to be heated

Properties of a good heating element

High specific resistance

High melting temperature

Low temperature coefficient of resistance

High oxidising temperature

Positive temperature coefficient of resistance

High ductility and flexibility

High mechanical strength of its own

Every heating element; with passage of time; breaksopen and becomes unserviceable. Some of the

factors responsible for this failure are:

Format of hot spots which shine brighter during operation

Oxidation

Corrosion

Mechanical failure

Materials of Heating Element

1. The materials commonly employed for low and

medium temperature services are:

Alloy of nickel and chromium: Ni = 80%, Cr = 20%

Alloy of nickel, chromium and iron: Ni = 65%,Cr=15%, Fe = 20%

Ni-Cr is suitable for temp. upto 1150 C.

Ni-Cr-Fe alloy is suitable for temp. upto 850 C.

2. For temp. above 1150 C resistors are made of silicon

carbide, molybdenum, tungsten and graphite.

Silicon Carbide is the basis of a resistor materialfor operating in air for temperatures upto about 1500

C. The material is formed into rods of diameters and

lengths for combination into circuits of the required

electrical rating.

Molybdenum resistors are suitable for temp. upto1650 C. This metal is ductile enough at room temp.

for drawing into wire for resistor windings. Owing to

its high vapour pressure, molybdenum is not suitable

for resistors of vacuum furnaces.

Materials of Heating Element Cont

Tungsten resistors can be employed for temp. upto2000 C. The maximum temp. is limited by the

refractory supports of the resistor. The low vapour

pressure of tungsten makes it useful for resistors of

vacuum furnaces.

Graphite resistors are suitable for any temp. thatcan be used. The resistors require protection against

oxidation above about 600 C. Due to the chemical

activity of carbon, special consideration need to be

given to the surrounding atmosphere.

Materials of Heating Element Cont

Resistance Furnaces or Ovens

They are suitable-insulated closed chambers with aprovision for ventilation.

Used for heat treatment of metals, pottery work,commercial and domestic heating.

Power frequency voltage is utilized as the supplysource.

Temp. upto 1000 C can be obtained by heatingelement made of nickel, chromium and iron.

Ovens using heating elements made of graphite canproduce temp. upto 3000 C. Heating element may

consist of circular wires or rectangular ribbons.

An enclosure for charge which is heated by radiationor convection or both is called a heating chamber.

The chambers are used to:

i. Control the distribution of heat within the chamber

ii. Control the cooling rate of charge, if required

iii. Confine the atmosphere around the charge

iv. Store as much of heat supplied as may be practicableand economical

Heating chambers may be of batch or continuoustype.

In batch type the charge remains stationary duringthe heat application. The cycle may include cooling thecharge in the chamber.

In continuous type the charge is heated as it movesthrough the chamber. It is extended for more or less

cooling of the charge before it leaves the chamber.

It is recommended where flow of material isreasonably uniform and continuous.

Temperature control of resistance furnaces

Following are the three ways by which thetemperature (I2Rt or V2t/R) can be controlled:

(i) Voltage (ii) Time (iii) Resistance

1. Tapped Transformer: Voltage can be varied by

using tapped transformer for supply to oven or by

using a series of resistance so that some voltage is

dropped across this series resistor.

2. On-off switch: An on-off switch can be employed to

control the temp. The time for which oven is

connected to supply and time for which it remains

isolated from supply will determine the temp.

3. Variation in circuit configuration: The temp. canbe controlled by switching in various combinations ofgroups of resistances used in the oven.

In single phase supply, various series and parallelcombinations along with some resistances being in thecircuit, others out of the circuits will give varioustemperatures.

For 3-phase ovens, different connection with star-deltaarrangements will give different temperatures.

If the temp. is to be controlled automatically someform of thermostat should be used in the circuit sothat it operates and switches out or switches in theoven whenever temp. goes above or below a certainpredetermined value respectively.

Protective equipment:

An instantaneous overload relay to trip the circuit at10 or 15% above normal current

Fuses to provide protection in case of failure ofautomatic control system

Maximum operating voltage: It is limited byelectrical insulation at high temperatures and from

safety consideration to 600 V.

Efficiency and Losses: The heat produced in theheating elements, besides raising the charge torequired values, is also to overcome the lossesmentioned below:

i. Heat used in raising temperature of oven orfurnace: This loss can be calculated by knowingmass of the refractory material, its specific heat andrise of temp. (mcT).

ii. Heat used in raising temp. of the containers orcarriers: This loss is calculated exactly the sameway as for oven or furnace.

iii. Heat conducted through walls: This source ofheat loss is most important since the heat iscontinuously conducted through the walls.

Heat loss by conduction through walls =

Where k = Thermal conductivity of walls, W/m K,

A = Area, m2

T1, T2 = Inside and outside temperatures, K

t = Thickness of the walls, m.

iv. Escapement of heat due to opening of door:

Although there is no specific formula fordetermination of loss occurring due to opening of door

for inspection of the charge, however, this loss may be

taken as 0.6 to 1.2 MJ/m2 of the door area if the door

is opened for a period of 20 to 30 seconds.

1 2( )k A T T

t

The heat required to raise the temperature of thecharge to the required value,

Where, m = Mass of charge, kg

c = specific heat of charge, J/kg K,

T = Temperature rise, K

The efficiency lies between 60 and 80%.

.

arg

.

arg

Heat required to raisetemp of

thech e tothe required valueEfficiency of the oven

Heat required to raisetemp of

the ch e tothe required value losses

Q m c T joules

Design of Heating Element

The heating elements are normally made of wires ofcircular cross-section or rectangular conducting

ribbons/strips.

Under steady state conditions, a heating elementdissipates as much heat from its surface as it receives

the power from the electric supply.

If P is the power input and H is the heat dissipated byradiation, then

P = H under steady-state conditions

Heat radiated by a body, as per Stefans law ofradiation, is given by

Where rad = Radiating efficiency

e = Emissivity

T1 = Temperature of hot body, K

T2 = Temperature of cold body(or cold

surroundings), K.

Design of Heating Element Cont

4 4

21 25.67 / __(1)100 100

T TH e W m

Now

If H is the heat dissipated by radiation per second perunit surface area of the wire, then,

Heat radiated per second = (d)lH __(4)


2

22

2 2 2

2

2

2

4

4

__(2)4 4

__(3)4

V l l lP and R

R a dd

V d VP

l l

d

l V

d P

From eq. (2) and (4), we get

From eq. (3) and (5), we can find the values of l and d.


2 2

2 2

( )

( )4

4__(5)

P d l H

d Vd l H

l

d H

l V

Ribbon type element:

If w and t are the width and thickness of the ribbonrespectively, then

Heat lost from ribbon surface = 2wl H __(8)

(neglecting side area 2tl, as thickness is negligible)


2 2 2 2

__(6)/ / ( )

V V V V wtP

R l a l w t l

2

__(7)l V

wt P

Equating eqn. (6) and (8), we have

The value of l and w can be solved by solvingequations (7) and (9).


2

2 2

2

2__(9)

V wtwlH

l

t H

l V

Induction Heating

The process of induction heating makes use ofcurrents induced by electro-magnetic action in the

charge to be heated.

Induction heating is based on the principle oftransformer working.

The primary winding which is supplied from an A.C.source is magnetically coupled to the charge which

acts as a short-circuited secondary of a single turn.

When A.C. voltage is applied to the primary, itinduces voltage in the secondary i.e., charge.

The secondary current heats up the charge in the sameway as any electric does while passing through aresistance.

If V is the voltage in the charge and R is the resistance ofthe charge, then heat produced = V2/R.

So to develop heat sufficient to melt the charge, theresistance of the charge must be low, which is possible onlywith metals, and voltage must be higher, which is obtainedby employing higher flux and higher frequency.

Types of Induction furnaces:

1. Core type or low frequency induction furnace

(i) Direct core type (ii) Vertical core type

(iii) Indirect core type

2. Coreless type or high frequency induction furnace

Direct Core Type Induction Furnace

It consists of a transformer in which charge to beheated forms a single-turn short circuited secondary.

The secondary is magnetically coupled to the primaryby iron core.

The furnace consists of a circular hearth whichcontains the charge to be melted.

When there is no molten metal in the ring, thesecondary becomes open-circuited thereby cutting offthe secondary current. Hence, to start the furnace,molten metal has to be poured in the annular.

Since the magnetic coupling between primary &secondary is very poor, it results in high leakage fluxand poor power factor. For this reason, the furnace isoperated at low frequencies of the order of 10 Hz or so.

Direct Core Type Induction Furnace

The melting is rapid and clean and the charge iscapable of accurate control as far as temp. and

alloying elements are concerned.

The inherent stirring of the melt ensures a greateruniformity of the end product.

However, if the current density exceeds about500 A/cm2, the current flowing around the cross-

section of the melt, interacts with the alternating

magnetic field and exerts constricting forces on the

cross-section of the metal which may squeeze it to the

extent that a complete interruption of the secondary

takes place. This is known as Pinch effect(formation of

bubbles and voids).

This type of furnace has the following drawbacks:

i. Leakage reactance is high and so power factor is low

on account of poor magnetic coupling.

ii. Low frequencies have to be employed as normal freq.

causes turbulence of the charge. This requires motor-

generator set or frequency converter.

iii. The crucible for the charge is of odd shape and not

convenient from the metallurgical point of view.

iv. The furnace cannot function if the secondary circuit

is not closed. This requires a complete ring of the

charge around the core. For starting the furnace,

either molten metal is poured into the crucible or

sufficient molten metal is allowed to remain in the

crucible from a previous operating.

v. It suffers from pinching effect.

Such furnaces are not suitable forintermittent services.

On account of these drawbacks thesefurnaces have become obsolete these

days.

Vertical Core Type Induction Furnace

Or The Ajax-Wyatt Furnace


It is also known as Ajax-Wyatt Furnace and is animprovement over the core type of furnace.

It has vertical channel for the charge so that thecrucible used is also vertical which is convenient from

metallurgical point of view.

The top is closed by an insulated cover which can beremoved for charging.

Since it is a vertical core type furnace the tendency ofthe currents to interrupt the secondary circuit due to

Pinch effect is avoided due to weight of the charge in

the main body of the crucible.

The circulation of molten metal is kept up round theVee portion by convection currents and by

electromagnetic forces between the currents in the two

halves of the Vee.

It is to be noted that the Vee must be kept full ofcharge in order to maintain continuity of the

secondary circuit. For this reason this furnace is

useful for continuous operation.

The p.f. of the furnace is of the order of 0.8 to 0.83 andit can be operated at power frequency.

This is normally used for melting and refining brassand other non-ferrous metals.


With normal supply frequency its efficiency is about75% and its standard size varies from 60-300 kW, allsingle phase.

Advantages:

i. Consistent performance and simple control.

ii. Accurate temperature control, uniform castings,reduced metal losses and reduction of rejects.

iii. Highly efficient heat, low operating costs andimproved production.

iv. High power factor(0.8 to 0.85).

v. Local working conditions in a cool atmosphere withno dirt, noise or fuel.

vi. Absence of crucibles.


Indirect Core Type Induction Furnace

In this type of furnace, a suitable element is heated byinduction which, in turn, transfers the heat to the

charge by radiation.

In fig, the secondary consists of a metal containerwhich forms the walls of the furnace proper. The

primary winding is magnetically coupled to this

secondary by iron core.

When primary winding is connected to A.C. supplysecondary current is induced in the metal container by

transformer action which heats up the container.

The metal container transfers this heat to the charge.


The part LM of the magnetic circuit situated insidethe oven chamber consists of a special alloy which

loses its magnetic properties at a particular temp. but

regains them when cooled back to the same temp.

As soon as the chamber attains the critical temp.,reluctance of the magnetic circuit increases manifold

there by cutting off the supply of heat.

The bar LM is detachable and can be replaced byother bars having different critical temperature.


Coreless Type or High Frequency

Induction Furnace

Construction: It consists of three main parts:

(i) Primary coil

(ii) Ceramic crucible(container) containing chargewhich forms the secondary and

(iii) Frame which includes supports and tiltingmechanism.

It contains no heavy iron core. So there is nocontinuous path for the magnetic flux. The containerand the coil are relatively light in construction andcan be conveniently tilted for pouring.

Coreless Type or High Frequency Induction Furnace

Working: The charge is put into the crucible andprimary winding is connected to high frequency A.C.

supply. The flux created by primary sets up eddy

currents in the charge.

These eddy currents heat up the charge to its meltingpoint and also setup electro-magnetic forces producing

stirring action which is essential for obtaining uniform

quality of metal.

Since flux density is low(due to absence of themagnetic core) high frequency supply has to be used

because eddy current loss, Pe B2 f2.


However this high frequency increases the resistanceof primary winding due to skin effect, therebyincreasing primary copper losses.

Hence the primary winding is not made of copper wirebut consists of hollow copper tubes which are cooled bywater circulating through them.

As the magnetic coupling between the primary andsecondary windings is low, the furnace p.f. liesbetween 0.1 and 0.3.

Static capacitors are, therefore, invariably employedin parallel with such a furnace in order to improve thep.f.


Applications:

1. Steel production (Energy consumption is 600 to 1000

kWh per tonne of steel)

2. Melting non-ferrous metals like brass, bronze, copper

and aluminium etc. along with various alloys of these

metals

3. Vacuum melting

4. Melting in controlled atmosphere

5. Melting in precision casting

6. Electronic industry

7. Industrial activities like soldering, brazing,

hardening and annealing in instruments.


Advantages:

1. Fast in operation.

2. Low erection cost.

3. Low operating cost.

4. Can be operated intermittently.

5. Operation is free from smoke.

6. Charging and pouring is simple.

7. Less melting time.

8. Precise control of power.

9. Possibility of employing vacuum heating necessary for

precious metal melting.

10.Most suitable for production of high grade alloy steels.


High Frequency Eddy Current Heating

In order to heat an article by eddy currents, it isplaced inside a high frequency A.C. current-carrying

coil.

The alternating magnetic field produced by the coilsets up eddy currents in the article, which

consequently, gets heated up.

Such a coil is known as heater coil or work coil and thematerial to be heated is known as charge or load.

Primarily it is the eddy current loss which isresponsible for the production of heat although

hysteresis loss also contributes to some extent in the

case of magnetic materials.

As eddy current loss Pe B2 f2, this loss can be

controlled by controlling flux density B and supply

frequency f.


This loss is greatest on the surface of the material butdecreases as we go deep inside.

The depth of penetration(d) of eddy currents into thecharge is given by

Where = Resistivity of the molten metal

f = Supply frequency

r = Relative permeability


91 10

2 rd cm

f

Since eddy current heating can be restricted

to any desired depth of the material to be heated by

judicious selection of frequency of the heating.

The supply frequency is usually employed between10 kHz to 40 kHz.


1d

f

Advantages of eddy current heating:

Temperature control is very easy.

The heat can be made to penetrate into the metalsurface to any desired depth.

This heating method is quick, clean and convenient.

Very less wastage of heat(as heat is produced in thebody to be heated up directly)

The equipment can be operated even by unskilledoperator.

The surface area over which heat is produced can beaccurately controlled.


Advantages of eddy current heating:

The amount of heat produced can be accuratelycontrolled by suitable timing devices.

It can easily take place in vacuum or other specialatmosphere(whereas other conventional types of

heating are not possible in such places)

The work coils are not required to fit closely aroundthe object being treated.

Demerits:

The generation of heat is costly.

Efficiency of equipment is quite low(less than 50%)

Initial cost of the equipment is high


Applications of eddy current heating:

1. Surface Hardening: The bar whose surface is to be

hardened by heat treatment is placed within the

working coil which is connected to an A.C. supply of

high frequency.

After a few seconds, when surface has reached theproper temperature, A.C. supply is cut off and the bar

is at once dipped in water.

2. Annealing: In conventional method of annealing the

process takes long time resulting in scaling of the

metal which is undesirable. But in eddy current

heating, time taken is much less so that no scale

formation takes place.

By this method temp. of the order of 750 C can beattained in one minute(approx.) upto a depth of 25

mm.

3. Soldering: Eddy current heating can be

economically employed for soldering precisely for

high temperature soldering where silver, copper and

their alloys are used as solders.


Other applications of eddy current heating include thefollowing:

i. Drying of paints.

ii. Welding.

iii. Melting of precious metals.

iv. Sterilization of surgical instruments.

v. Forgings of bolt heads and rivet heads.


Arc Heating Arc Furnaces

If a sufficiently high voltage is applied across the air-gap, the air becomes ionized and starts conducting in

the form of a continuous spark or arc thereby

producing intense heat.

When electrodes are made of carbon/graphite, thetemp. obtained is in the range of 3000 - 3500 C.

The high voltage required for striking the arc can beobtained by using step-up transformer fed from a

variable ac supply(fig. .

An arc can also be obtained by using low voltageacross two electrodes initially in contact with each

other as shown in fig. (b).


The low voltage required for this purpose can beobtained by using a step-down transformer.

Initially, a low voltage can be applied, when the twoelectrodes are in contact with each other. Next, when

the two electrodes are gradually separated from each

other, an arc is established between the two.


Arc furnaces are of two types:

Direct arc furnace

Indirect arc furnace

Direct Arc Furnace

In this type of furnace, arc is formed between the twoelectrodes and the charge in such a way that electric

current passes through the body of the charge as

shown in fig.

Such furnaces produce very high temperatures.

Direct Arc Furnace

Single phase direct arc furnace

Direct Arc Furnace

Three phase direct arc furnace

Direct Arc Furnace

A 3-ph direct arc type furnace consists of a circularsteel casting lined inside with refractory material. The

roof is provided with three holes through which three

electrodes are passed.

The electrodes used may be of graphite or amorphouscarbon.

Graphite has double the conductivity than amorphouscarbon and will carry 2.5 times higher current than it.

Though graphite electrodes are costly they are used

for these advantages.

To maintain desired length of arc the electrodes areraised and lowered individually by electric motors

operated by automatic regulators.

Direct Arc Furnace

The voltage between steel and electrodes may be 40-145volts.

The longer the arc, higher the voltage require and the lessthe input of heat to the furnace.

Electric power is supplied in a bulk in the form of threephase ac current at 6.6 or 10 kV.

A transformer set up close to the furnace reduces thevoltage down to that required for the arcs.

Its primary windings have tapping which allow foradjustments to the arc voltage.

As the power supply is a three phase circuit, threeelectrodes are arranged in an equilateral triangle over themetal.

Direct Arc Furnace

The usual size of the furnace is between 5 to 10tonnes, though 50 to 100 tonnes furnaces have beenproduced. This type of furnace is used for making alloysteels such as stainless steel.

The advantage of this furnace is that purer product isobtained and composition can be exactly controlledduring refining process. Even though initial andoperating cost of this furnace is higher than otherfurnaces, it is preferred over other types of furnaces.

Due to its higher cost its use is restricted to refiningthan melting.

It operates at a power factor about 0.8 lagging.

Indirect Arc Furnace

In an indirect are furnace arc is formed between twoelectrodes above the charge and heat is transmitted to

the charge by radiation.

Fig. shows a 1- indirect arc furnace which iscylindrical in shape. The arc is struck by short

circuiting the electrodes manually or automatically for

a moment and then, withdrawing them apart.

The heat from the arc and the hot refractory lining istransferred to the top layer of the charge by radiation.

The heat from the top layer of the charge is furthertransferred to other parts by conduction.


In this type of furnace, since no current passesthrough the body of the charge, there is no inherent

stirring action due to electromagnetic forces setup by

the current.

Hench such furnaces have to be rocked continuouslyin order to distribute heat uniformly. For this

different layers of the charge is exposed to the heat of

the arc.

An electric motor is used to operate suitable grindersand rollers to impart rocking motion to the furnace.

Rocking motion also increases furnace efficiency andthe life of the refractory lining material.


In this furnace, since the charge is heated by radiationonly, its temperature is lower than that obtainable ina direct arc furnace.

Power input is regulated by adjusting the arc lengthby moving the electrodes.

The power factor is about 0.85 lagging.

The capacity of furnace varies from 0.25 to 3 tonnes.

These furnaces are mainly employed for meltingnon-ferrous metals. However they can be used in ironfoundaries where small quantities of iron are requiredfrequently.


Advantages of indirect arc furnaces:

1. Lower overall production cost per tonne of molten

metal.

2. Sound castings in thin and intricate designs can be

produced.

3. Metal loses due to oxidation and volatilisation are

quite low.

4. Flexible in operation.

Electric Heating

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