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UNIT 2
Electric Heating

2.1 INTRODUCTION

Heat plays a major role in everyday life. All heating requirements in domestic purposes such as
cooking, room heater, immersion water heaters, and electric toasters and also in industrial
purposes such as welding, melting of metals, tempering, hardening, and drying can be met easily
by electric heating, over the other forms of conventional heating. Heat and electricity are
interchangeable. Heat also can be produced by passing the current through material to be heated.
This is called electric heating; there are various methods of heating a material but electric heating
is considered far superior compared to the heat produced by coal, oil, and natural gas.

ADVANTAGES OF ELECTRIC HEATING

The various advantages of electric heating over other the types of heating are:

(i) Economical

Electric heating equipment is cheaper; they do not require much skilled persons; therefore,
maintenance cost is less.

(ii) Cleanliness

Since dust and ash are completely eliminated in the electric heating, it keeps surroundings
cleanly.

(iii) Pollution free

As there are no flue gases in the electric heating, atmosphere around is pollution free; no need of
providing space for their exit.

(iv) Ease of control

In this heating, temperature can be controlled and regulated accurately either manually or
automatically.

(v) Uniform heating

With electric heating, the substance can be heated uniformly, throughout whether it may be
conducting or non-conducting material.

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(vi) High efficiency

In non-electric heating, only 40–60% of heat is utilized but in electric heating 75–100% of heat
can be successfully utilized. So, overall efficiency of electric heating is very high.

(vii) Automatic protection

Protection against over current and over heating can be provided by using fast control devices.

(viii) Heating of non-conducting materials

The heat developed in the non-conducting materials such as wood and porcelain is possible only
through the electric heating.

(ix) Better working conditions

No irritating noise is produced with electric heating and also radiating losses are low.

(x) Less floor area

Due to the compactness of electric furnace, floor area required is less.

(xi) High temperature

High temperature can be obtained by the electric heating except the ability of the material to
withstand the heat.

(xii) Safety

The electric heating is quite safe.

MODES OF TRANSFER OF HEAT

The transmission of the heat energy from one body to another because of the temperature
gradient takes place by any of the following methods:

1. conduction,
2. convection, or
3. radiation.

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Conduction

In this mode, the heat transfers from one part of substance to another part without the movement
in the molecules of substance. The rate of the conduction of heat along the substance depends
upon the temperature gradient.

The amount of heat passed through a cubic body with two parallel faces with thickness ‘t’
meters, having the cross-sectional area of ‘A’ square meters and the temperature of its two
faces T1°C and T2°C, during ‘T’ hours is given by:

where k is the coefficient of the thermal conductivity for the material and it is measured in
MJ/m3/°C/hr.

Ex: Refractory heating, the heating of insulating materials, etc.

Convection

In this mode, the heat transfer takes place from one part to another part of substance or fluid due
to the actual motion of the molecules. The rate of conduction of heat depends mainly on the
difference in the fluid density at different temperatures.

Ex: Immersion water heater.

The mount of heat absorbed by the water from heater through convection depends mainly
upon the temperature of heating element and also depends partly on the position of the heater.

Heat dissipation is given by the following expression.

H = a (T1 – T2)b W/m2,

where ‘a’ and ‘b’ are the constants whose values are depend upon the heating surface
and T1and T2 are the temperatures of heating element and fluid in °C, respectively.

Radiation

In this mode, the heat transfers from source to the substance to be heated without heating the
medium in between. It is dependent on surface.

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Ex: Solar heaters.

The rate of heat dissipation through radiation is given by Stefan's Law.

where T1 is the temperature of the source in kelvin, T2 is the temperature of the substance to be
heated in kelvin, and k is the radiant efficiency:

= 1, for single element

= 0.5–0.8, for several elements

e = emissivity = 1, for black body

= 0.9, for resistance heating element.

From Equation (4.1), the radiant heat is proportional to the difference of fourth power of the
temperature, so it is very efficient heating at high temperature.

ESSENTIAL REQUIREMENTS OF GOOD HEATING ELEMENT

The materials used for heating element should have the following properties:

o High-specific resistance
Material should have high-specific resistance so that small length of wire may be
required to provide given amount of heat.
o High-melting point
It should have high-melting point so that it can withstand for high temperature, a small
increase in temperature will not destroy the element.
o Low temperature coefficient of resistance
From Equation (4.1), the radiant heat is proportional to fourth powers of the
temperatures, it is very efficient heating at high temperature.
For accurate temperature control, the variation of resistance with the operating
temperature should be very low. This can be obtained only if the material has low
temperature coefficient of resistance
o Free from oxidation

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The element material should not be oxidized when it is subjected to high temperatures;
otherwise the formation of oxidized layers will shorten its life.
o High-mechanical strength
The material should have high-mechanical strength and should withstand for
mechanical vibrations.
o Non-corrosive
The element should not corrode when exposed to atmosphere or any other chemical
fumes.
o Economical

The cost of material should not be so high.

MATERIAL FOR HEATING ELEMENTS

The selection of a material for heating element is depending upon the service conditions such as
maximum operating temperature and the amount of charge to be heated, but no single element
will not satisfy all the requirements of the heating elements. The materials normally used as
heating elements are either alloys of nickel–chromium, nickel–chromium–iron, nickel–
chromium–aluminum, or nickel–copper.

Nickel–chromium–iron alloy is cheaper when compared to simple nickel–chromium alloy.

The use of iron in the alloy reduces the cost of final product but, reduces the life of the alloy, as
it gets oxidized soon. We have different types of alloys for heating elements. Table 4.1 gives the
relevant properties of some of the commercial heating elements.

Table : Properties of some heating elements

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The properties of some commercial heating element materials commonly employed for low
and medium temperatures up to 1,200°C are Ni–Cr and an alloy of Ni–Cr–Fe composition of
these alloys are given in Table 4.1. For operating temperatures above 1,200°C, the heating
elements are made up of silicon carbide, molebdenum, tungsten, and graphite. (Ni–Cu alloy is
frequently used for heating elements operating at low temperatures. Its most important property
is that it has virtually zero resistance and temperature coefficient.)

CAUSES OF FAILURE OF HEATING ELEMENTS

Heating element may fail due to any one of the following reasons.

1. Formation of hot spots.

2. Oxidation of the element and intermittency of operation.
3. Embrittlement caused by gain growth.
4. Contamination and corrosion.

Formation of hotspots

Hotspots are the points on the heating element generally at a higher temperature than the main
body. The main reasons of the formation of hotspot in the heating element are the high rate of the
local oxidation causing reduction in the area of cross-section of the element leading to the
increase in the resistance at that spot. It gives rise to the damage of heating element due to the
generation of more heat at spot. Another reason is the shielding of element by supports, etc.,
which reduces the local heat loss by radiation and hence the temperature of the shielded portion
of the element will increase. So that the minimum number of supports should be used without
producing the distortion of the element. The sagging and wrapping of the material arise due to
the insufficient support for the element (or) selection of wrong fuse material may lead to the
uneven spacing of sections thereby developing the hotspots on the element.

Oxidation and intermittency of operation

A continuous oxide layer is formed on the surface of the element at very high temperatures such
layer is so strong that it prevents further oxidation of the inner metal of the element. If the
element is used quite often, the oxide layer is subjected to thermal stresses; thus, the layer cracks
and flakes off, thereby exposing fresh metal to oxidation. Thus, the local oxidation of the metal
increases producing the hotspots.

Embrittlement causing grain growth

In general, most of the alloys containing iron tend to form large brittle grains at high
temperatures. When cold, the elements are very brittle and liable to rupture easily on the slightest
handling and jerks.

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contamination and corrosion

The heating elements may be subjected to dry corrosion produced by their contamination with
the gases of the controlled atmosphere prevailing in annealing furnaces.

DESIGN OF HEATING ELEMENTS

By knowing the voltage and electrical energy input, the design of the heating element for an
electric furnace is required to determine the size and length of the heating element. The wire
employed may be circular or rectangular like a ribbon. The ribbon-type heating element permits
the use of higher wattage per unit area compared to the circular-type element.

Circular-type heating element

Initially when the heating element is connected to the supply, the temperature goes on increasing
and finally reaches high temperature.

Let V be the supply voltage of the system and R be the resistance of the element, then electric

power input, .

If ρ is the resistivity of the element, l is the length, ‘a’ is the area, and d is the diameter of the
element, then:

Therefore, power input,

By rearranging the above equation, we get:

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where P is the electrical power input per phase (watt), V is the operating voltage per phase
(volts), R is the resistance of the element (Ω), l is the length of the element (m), a is the area of
cross-section (m2), d is the diameter of the element (m), and ρ is the specific resistance (Ω-m)

According to Stefan's law, heat dissipated per unit area is

where T1 is the absolute temperature of the element (K), T2 is the absolute temperature of the
charge (K), e is the emissivity, and k is the radiant efficiency.

The surface area of the circular heating element:

S = πdl.

∴ Total heat dissipated = surface area × H

= Hπdl.

Under thermal equilibrium,

Power input = heat dissipated

P = H × πdl.

Substituting P from Equation (4.2) in above equation:

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By solving Equations (4.3) and (4.4), the length and diameter of the wire can be determined.

Ribbon-type element

Let ‘w’ be the width and ‘t’ be the thickness of the ribbon-type heating element.

We know that, (for ribbon or rectangular element, a = w × t)

The surface area of the rectangular element (S) = 2 l × w.

∴ Total heat dissipated = H × S

= H × 2 lw.

∴ Under the thermal equilibrium,

Electrical power input = heat dissipated

P = H × 2 lw

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By solving Equations (4.7) and (4.8), the length and width of the heating element can be
determined.

Example 4.1: A 4.5-kW, 200-V, and 1-φ resistance oven is to have nichrome wire heating
elements. If the wire temperature is to be 1,000°C and that of the charge 500°C. Estimate the
diameter and length of the wire. The resistivy of the nichrome alloy is 42.5 μΩ-m. Assume the
radiating efficiency and the emissivity of the element as 1.0 and 0.9, respectively.

Solution:

Given data

Power input (P) = 4.5 kW

Supply voltage (V) = 200 V

Temperature of the source (T1) = 1,000 + 273

= 1,273 K.

Temperature of the charge T2 = 500 + 273

= 773 K.

According to the Stefan's law,

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The heat dissipation is given by:

By solving Equations (1) and (2):

d3 = 0.7466

d = 0.907 mm.

Substitute the value of ‘d’ in Equation (2):

l = 135.14 m.

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Example 4.2: A20-kW, 230-V, and single-phase resistance oven employs nickel—chrome strip
25-mm thick is used, for its heating elements. If the wire temperature is not to exceed 1,200°C
and the temperature of the charge is to be 700°C. Calculate the width and length of the wire.
Assume the radiating efficiency as 0.6 and emissivity as 0.9. Determine also the temperature of
the wire when the charge is cold.

Solution:

Power supplied, P = 20 × 103 W.

Let ‘w’ be the width in meters, t be the thickness in meters, and ‘l’ be the length also in
meters. Then:

According to the Stefan's law of heat radiation:

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The total amount of the heat dissipation × the surface area of strip = power supplied

P=H×S

= H × 2 lw (S = surface area of strip = 2lw)

From Equations (1) and (2):

Substitute the value of ‘w’ in Equation (2) then:

l = 7.435 m.

When the charge is cold, it would be at normal temperature, say 25°C.

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Example 4.3 Determine the diameter and length of the wire, if a 17-kW, 220-V, and 1-
φresistance oven employs nickel-chrome wire for its heating elements. The temperature is not
exceeding to 1,100°C and the temperature of the charge is to be 500°C. Assume the radiating
efficiency as 0.5 and the emissivity as 0.9, respectively.

Solution:

For a circular element:

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According to Stefan's law of heat dissipation:

At steady temperature, crucial power input = heat output:

Solving Equations (1) and (2), we get:

Substitute the value of ‘d' in Equation (2) gives:

l = 21.198 m.

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METHODS OF ELECTRIC HEATING

Heat can be generated by passing the current through a resistance or induced currents. The
initiation of an arc between two electrodes also develops heat. The bombardment by some heat
energy particles such as α, γ, β, and x-rays or accelerating ion can produce heat on a surface.

Electric heating can be broadly classified as follows.

(i) Direct resistance heating

In this method, the electric current is made to pass through the charge (or) substance to be
heated. This principle of heating is employed in electrode boiler.

(ii) Indirect resistance heating

In this method, the electric current is made to pass through a wire or high-resistance heating
element, the heat so developed is transferred to charge from the heating element by convection or
radiation. This method of heating is employed in immersion water heaters.

Fig. Classification of electrical heating

Infrared (or) radiant heating

In this method of heating, the heat energy is transferred from source (incandescent lamp) and
focused upon the body to be heated up in the form of electromagnetic radiations. Normally, this
method is used for drying clothes in the textile industry and to dry the wet paints on an object.

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Direct arc heating

In this method, by striking the arc between the charge and the electrode or electrodes, the heat so
developed is directly conducted and taken by the charge. The furnace operating on this principle
is known as direct arc furnaces. The main application of this type of heating is production of
steel.

Indirect arc heating

In this method, arc is established between the two electrodes, the heat so developed is transferred
to the charge (or) substance by radiation. The furnaces operating on this principle are known as
indirect arc furnaces. This method is generally used in the melting of non-ferrous metals.

Direct induction heating

In this method of heating, the currents are induced by electromagnetic action in the charge to be
heated. These induced currents are used to melt the charge in induction furnace.

Indirect induction heating

In this method, eddy currents are induced in the heating element by electromagnetic action.
Thus, the developed heat in the heating element is transferred to the body (or) charge to be
heated by radiation (or) convection. This principle of heating is employed in induction furnaces
used for the heat treatment of metals.

Dielectric heating

In this method of electric heating, the heat developed in a non-metallic material due to inter-
atomic friction, known as dielectric loss. This principle of heating usually employed for
preheating of plastic performs, baking foundry cores, etc.

RESISTANCE HEATING

When the electric current is made to pass through a high-resistive body (or) substance, a power
loss takes place in it, which results in the form of heat energy, i.e., resistance heating is passed
upon the I2R effect. This method of heating has wide applications such as drying, baking of
potteries, commercial and domestic cooking, and the heat treatment of metals such as annealing
and hardening. In oven where wire resistances are employed for heating, temperature up to about
1,000°C can be obtained.

The resistance heating is further classified as:

1. direct resistance heating,

2. indirect resistance heating, and

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3. infrared (or) radiant heating.

Direct resistance heating

In this method, electrodes are immersed in a material or charge to be heated. The charge may be
in the form of powder, pieces, or liquid. The electrodes are connected to AC or DC supply as
shown in Fig. 4.1(a). In case of DC or 1-φ AC, two electrodes are immersed and three electrodes
are immersed in the charge and connected to supply in case of availability of 3-φsupply. When
metal pieces are to be heated, the powder of lightly resistive is sprinkled over the surface of the
charge (or) pieces to avoid direct short circuit. The current flows through the charge and heat is
produced in the charge itself. So, this method has high efficiency. As the current in this case is
not variable, so that automatic temperature control is not possible. This method of heating is
employed in salt bath furnace and electrode boiler for heating water.

Fig. (a) Direct resistance heating

(i) Salt bath furnace

This type of furnace consists of a bath and containing some salt such as molten sodium chloride
and two electrodes immersed in it.

Such salt have a fusing point of about 1,000–1,500°C depending upon the type of salt used.
When the current is passed between the electrodes immersed in the salt, heat is developed and
the temperature of the salt bath may be increased. Such an arrangement is known as a salt bath
furnace.

In this bath, the material or job to be heated is dipped. The electrodes should be carefully
immersed in the bath in such a way that the current flows through the salt and not through the job
being heated. As DC will cause electrolysis so, low-voltage AC up to 20 V and current up to
3,000 A is adopted depending upon the type of furnaces.

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The resistance of the salt decreases with increase in the temperature of the salt, therefore, in
order to maintain the constant power input, the voltage can be controlled by providing a tap
changing transformer. The control of power input is also affected by varying the depth of
immersion and the distance between the electrodes.

(ii) Electrode boiler

It is used to heat the water by immersing three electrodes in a tank as shown in Fig. 4.2. This is
based on the principle that when the electric current passed through the water produces heat due
to the resistance offered by it. For DC supply, it results in a lot of evolution of H2 at negative
electrode and O2 at positive electrode. Whereas AC supply hardly results in any evolution of gas,
but heats the water. Electrode boiler tank is earthed solidly and connected to the ground. A
circuit breaker is usually incorporated to make and break all poles simultaneously and an over
current protective device is provided in each conductor feeding an electrode.

Fig. 4.2 Electrode boiler

Indirect resistance heating

In the indirect resistance heating method, high current is passed through the heating element. In
case of industrial heating, some times the heating element is placed in a cylinder which is
surrounded by the charge placed in a jacket is known as heating chamber is shown inFig. 4.3.
The heat is proportional to power loss produced in the heating element is delivered to the charge
by one or more of the modes of the transfer of heat viz. conduction, convection, and radiation.
This arrangement provides uniform temperature and automatic temperature control. Generally,
this method of heating is used in immersion water heaters, room heaters, and the resistance ovens
used in domestic and commercial cooling and salt bath furnace.

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Fig. 4.3 Indirect resistance heating

Resistance ovens

According to the operating temperatures, the resistance furnaces may be classified into various
types. Low-temperature heating chamber with the provision for ventilation is called as oven. For
drying varnish coating, the hardening of synthetic materials, and commercialand domestic
heating, etc., the resistance ovens are employed. The operating temperature of medium
temperature furnaces is between 300°C and 1,050°C. These are employed for the melting of non-
ferrous metals, stove (annealing), etc. Furnaces operating at temperature between 1,050°C and
1,350°C are known as high-temperature furnaces. These furnaces are employed for hardening
applications. A simple resistance oven is shown in Fig. 4.4.

Fig. 4.4 Resistance oven

Resistance oven consists of a heating chamber in which heating elements are placed as shown
in the Fig. 4.4. The inner surface of the heating chamber is made to suit the character of the

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charge and the type of furnace or oven. The type of insulation used for heating chamber is
determined by the maximum temperature of the heating chamber.

Efficiency and losses of resistance ovens

The heat produced in the heating elements, not only raises the temperature of the charge to
desired value, but also used to overcome the losses occurring due to:

1. Heat used in raising the temperature of oven (or) furnace.

2. Heat used in raising the temperature of containers (or) carriers,
3. Heat conducted through the walls.
4. Heat loss due to the opening of oven door.

1. The heat required to raise the temperature of oven to desired value can be calculated by knowing the
mass of refractory material (M), its specific heat (S), and raise of temperature (∆T) and is given by:
Hoven = MS∆TJ.
In case the oven is continuously used, this loss becomes negligible.
2. Heat used in rising the temperature of containers (or) carriers can be calculated exactly the same way as
for oven (or) furnaces.

3. Heat loss conducted through the walls of the container can be calculated by knowing the area of the
container (A) in square meters, the thickness of the walls (t) in meters, the inside and out side
temperatures of the container T1 and T2 in °C, respectively, and the thermal conductivity of the container

walls ‘k’ in m3/°C/hr and is given by: Heat loss by conduction

Actually, there is no specific formula for the determination of loss occurring due to the opening
of door for the periodic inspection of the charge so that this loss may be approximately taken as
0.58–1.15 MJ/m2 of the door area, if the door is opened for a period of 20–30 sec.

The efficiency of the oven is defined as the ratio of the heat required to raise the temperature of
the charge to the desired value to the heat required to raise the charge and losses.

The efficiency of the oven:

The efficiency of the resistance oven lies in between 60% and 80%.

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Infrared or radiant heating

In this method of heating, the heat transfer takes place from the source to the body to be heated
through radiation, for low and medium temperature applications. Whereas in resistance ovens,
the heat transfers to the charge partly by convection and partly by radiation. In the radiant
heating, the heating element consists of tungsten filament lamps together with reflector and to
direct all the heat on the charge. Tungsten filament lamps are operating at 2,300°C instead of
3,000°C to give greater portion of infrared radiation and a longer life. The radiant heating is
mainly used for drying enamel or painted surfaces. The high concentration of the radiant energy
enables the heat to penetrate the coating of paint or enamel to a depth sufficient to dry it out
without wasting energy in the body of the workpiece.

The main advantage of the radiant heating is that the heat absorption remains approximately
constant whatever the charge temperature, whereas with the ordinary oven the heat absorption
falls off very considerably as the temperature of the charge raises. The lamp ratings used are
usually between 250 and 1,000 W and are operating at voltage of 115 V in order to ensure a
robust filament.

TEMPERATURE CONTROL OF RESISTANCE HEATING

To control the temperature of a resistance heating at certain selected points in a furnace or oven,
as per certain limits, such control may be required in order to hold the temperature constant or to
vary it in accordance with a pre-determined cycle and it can be carried out by hand or
automatically.

In resistance furnaces, the heat developed depends upon I2 R t (or) t. Therefore, the
temperature of the furnaces can be controlled either by:

1. Changing the resistance of elements.

2. Changing the applied voltage to the elements (or) current passing through the elements.
3. Changing the ratio of the on-and-off times of the supply.

Voltage across the furnace can be controlled by changing the transformer tapings. Auto
transformer or induction regulator can also be used for variable voltage supply. In addition to the
above, voltage can be controlled by using a series resistance so that some voltage dropped across
this series resistor. But this method is not economical as the power is continuously wasted in
controlling the resistance. Hence, this method is limited to small furnaces. An on-off switch can
be employed to control the temperature. The time for which the oven is connected to the supply
and the time for which it is disconnected from supply will determine the temperature.

Temperature can be controlled by providing various combinations of groups of resistances

used in the furnace and is given as follows:

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(i) Variable number of elements

If ‘R’ be the resistance of one element and ‘n’ be the number of elements are connected in
parallel, so that the equivalent resistance is R/n.

Heat developed in the furnace is:

i.e., if the number of elements connected in parallel increases, the heat developed in the furnace
also increased. This method does not provide uniform heating unless elements not in use are well
distributed.

(ii) Series parallel (or) star delta arrangement of elements

If the available supply is single phase, the heating elements can be connected in series for the
low temperatures and connected in parallel for the high temperature by means of a series—
parallel switch.

In case, if the available supply is three phase, the heating elements can be connected in star for
the low temperature and in delta for the high temperatures by using star—delta switch.

Example 4.5: Six resistances, each of 60 ohms, are used in a resistance; how much power is
drawn for the following connections.

1. Supply is 400 V, AC, and single phase and the connections are:

1. Three groups in parallel, each of two resistance units in series.

2. Six groups are in parallel, each of one resistance unit.

2. With the same three-phase supply, they are connected in delta fashion.

0. Two resistance units in parallel in each branch.

1. Two resistance units in series in each branch.

3. Supply is 400 V and three-phase while the connection is a star combination of:

0. Two resistance elements in series in each phase.

1. Two resistance elements in parallel in each phase.

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4. If the supply is a 25% tapping with an auto transformer, calculate the output of the oven.

Solution:

1.

1. The power consumption of the two resistances in series is:

The power consumed by the three units in parallel is P = 3 × 1,333.33 =

4,000 W.
2. The power consumed by each resistor is:

The power consumed by the six resistors in parallel is:

P = 6 × 2,666.67
= 16,000 W.
2. Since in delta fashion, line voltage = phase voltage = 400 V:

0. The power consumed by the each branch:

The power consumed by the three units is:

P = 3 × 5,333.34
= 16,000 W.
1. The power consumed by the each unit, when they are commuted in series is:

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The power consumed by the three units is:

P = 4,000 W.
3. For the star connection,

0. The power consumed by the two resistors in series is

p = 444.44 W.
The power consumed by the three units is:
P = 1,333.33 W.
1. The power consumed by the two resistors in parallel is:

The power consumed by the three units in series is:

P = 3 × 1,777.77
= 5,333.32 W.
4. The power is proportional to the square of the voltage. Hence, the voltage is 25%. So that, the power

loss will be th of the values obtained as above.

ARC HEATING

If the high voltage is applied across an air gap, the air in the gap gets ionized under the influence
of electrostatic forces and becomes conducting medium, current flows in the form of a
continuous spark, known as arc. A very high voltage is required to establish an arc but very
small voltage is sufficient to maintain it, across the air gap. The high voltage required for striking
an arc can be obtained by using a step-up transformer fed from a variable AC supply.

Another method of striking the arc by using low voltage is by short circuiting the two
electrodes momentarily and with drawing them back. Electrodes made up of carbon or graphite
and are used in the arc furnaces when the temperature obtained is in the range of 3,000–3,500°C.

Electrodes used in the arc furnaces

Normally used electrodes in the arc furnaces are carbon electrodes, graphite electrodes, and self-
baking electrodes. Usually the carbon and graphite electrodes are used and they can be selected
based on their electrical conductivity insolubility, chemical inertness, mechanical strength,
resistance to thermal shock, etc. The size of these electrodes may be 18–27 cm in diameter. The

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carbon electrodes are used with small furnaces for manufacturing of ferro-alloys, aluminum
phosphorous, etc. The self-baking electrodes are employed in the electrochemical furnaces and in
the electrolytic production of aluminum.

The salient features of carbon and graphite electrodes are:

1. Resistivity: The graphite electrodes have low-specific resistance than the carbon electrodes, so the
graphite required half in size for the same current resulting in easy replacement.
2. Oxidation: Graphite begins to oxides at 600°C where as carbon at 400°C.
3. Electrode consumption: For steel-melting furnaces, the consumption of the carbon electrodes is about
4.5 kg of electrodes per tonne of steel and 2.3–to 6.8 kg electrodes per tonne of steel for the graphite
electrodes.
4. Cost: The graphite electrodes cost about twice as much per kg as the carbon electrodes. The choice of
electrodes depends chiefly on the question of the total cost. In general, if the processes requiring large
quantities of electrode, carbon is used but for other processes, the choice depends on local conditions.

Types of arc furnaces

There are two types of arc furnaces and they are:

1. direct arc furnace and

2. indirect arc furnace.

(i) Direct arc furnace

When supply is given to the electrodes, two arcs are established and current passes through the
charge, as shown in Fig. 4.5. As the arc is in direct contact with the charge and heat is also
produced by current flowing through the charge itself, it is known as direct arc furnace.

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Fig. Direct arc furnace

If the available supply is DC or 1-φ, AC, two electrodes are sufficient, if the supply is 3-φ,
AC, three electrodes are placed at three vertices of an equilateral triangle. The most important
feature of the direct arc furnace is that the current flows through the charge, the stirring action is
inherent due to the electromagnetic force setup by the current, such furnace is used for
manufacturing alloy steel and gives purer product.

It is very simple and easy to control the composition of the final product during refining
process operating the power factor of arc furnace is 0.8 lagging. For 1-ton furnace, the power
required is about 200 kW and the energy consumed is 1.0 MWh/ton.

(ii) Indirect arc furnace

In indirect arc furnace, the arc strikes between two electrodes by bringing momentarily in contact
and then with drawing them heat so developed, due to the striking of arc across air gap is
transferred to charge is purely by radiation. A simple indirect arc furnace is shown inFig. 4.6.

Fig. 4.6 Indirect arc furnace

These furnaces are usually l-φ and hence their size is limited by the amount of one-phase load
which can be taken from one point. There is no inherent stirring action provided in this furnace,
as current does not flow through the charge and the furnace must be rocked mechanically. The
electrodes are projected through this chamber at each end along the horizontal axis. This furnace

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is also sometimes called as rocking arc furnace. The charge in this furnace is heated not only by
radiation from the arc between electrode tips but also by conduction from the heated refractory
during rocking action; so, the efficiency of such furnace is high. The arc is produced by bringing
electrodes into solid contact and then withdrawing them; power input to the furnace is regulated
by adjusting the arc length by moving the electrodes.

Even though it can be used in iron foundries where small quantities of iron are required
frequently, the main application of this furnace is the melting of non-ferrous metals.

Example 4.6: Calculate the time taken to melt 5 ton of steel in three-phase arc furnace having
the following data.

Current = 8,000 A Resistance = 0.003 Ω

Arc voltage = 50 V Reactance = 0.005 Ω

Latent heat = 8.89 kcal/kg Specific heat = 0.12

Initial temperature = 18°C Melting point = 1,370°C

The overall efficiency is 50%. Find also the power factor and the electrical efficiency of the
furnace.

Solution:

The equivalent circuit of the furnace is shown in Fig. P.4.1.

Arc resistance per phase

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Fig. P.4.1 Equivalent circuit of arc furnace

RA = 0.00625 Ω.

Drop due to the resistance of transformer, I Rt = 8,000 × 0.003 = 24 V and drop due to the
reactance, I Xt = 8,000 × 0.005 = 40 V.

From the phasor diagram (Fig. P.4.2):

Fig. P.4.2 Phasor diagram

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The amount of heat required per kg of steel:

= Specific heat × (t2 - t1) + latent heat

= 0.12 × (1,370-18) + 8.89

= 171.13 kcal.

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Example 4.7: A 100-kW Ajax Wyatt furnace works at a secondary voltage of 12 V at power
factor 0.6 when fully charged. If the reactance presented by the charge remains constant but the
resistance varies invert as the charge depth in the furnace; calculate the charge depth that
produces maximum heating effect when the furnace is fully charged.

Solution:

Secondary power, P = V2I2 cos φ

When the crucible is fully charged, then the secondary impedance is:

From the impedance triangle:

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Let ‘H’ be the height of the crucible when the crucible is full of charge and ‘Hm’ be the height of
the charge at which maximum heating effect is possible.

Given that the height of the charge is inversely proportional to the resistance. Let ‘Rm’ be the
maximum resistance at which maximum heating effect will be possible.

At Rm = X2, the heat produced will be maximum.

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HIGH-FREQUENCY HEATING

The main difference between the power-frequency and the high-frequency heating is that in the
conventional methods, the heat is transferred either by conduction convection or by radiation, but
in the high-frequency heating methods, the electromagnetic energy converted into the heat
energy in side the material.

The high-frequency heating can be applied to two types of materials. The heating of the
conducting materials, such as ferro-magnetic and non-ferro-magnetic, is known as induction
heating. The process of heating of the insulating materials is known as dielectric heating. The
heat transfer by the conventional method is very low of the order of 0.5–20 W/sq. cm. And, the
heat transfer rate by the high-frequency heating either by induction or by dielectric heating is as
much as 10,000 W/sq. cm. Thus, the high-frequency heating is most importance for tremendous
speed of production.

INDUCTION HEATING

The induction heating process makes use of the currents induced by the electromagnetic action in
the material to be heated. To develop sufficient amount of heat, the resistance of the material

must be low , which is possible only with the metals, and the voltage must
be higher, which can be obtained by employing higher flux and higher frequency. Therefore, the
magnetic materials can be heated than non-magnetic materials due to their high permeability.

In order to analyze the factors affecting induction heating, let us consider a circular disc to be
heated carrying a current of ‘I’ amps at a frequency ‘f’ Hz. As shown in Fig. 4.9.

Fig. 4.9 Induction heating

Heat developed in the disc is depending upon the following factors.

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o Primary coil current.

o The number of the turns of the coil.
o Supply frequency.
o The magnetic coupling between the coil and the disc.
o The high electrical resistivity of the disc.

If the charge to be heated is non-magnetic, then the heat developed is due to eddy current loss,
whereas if it is magnetic material, there will be hysteresis loss in addition to eddy current loss.
Both hysteresis and eddy current loss are depended upon frequency, but at high-frequency
hysteresis, loss is very small as compared to eddy currents.

The depth of penetration of induced currents into the disc is given by:

where ρ is the specific resistance in Ω-cm, f is the frequency in Hz, and μ is the permeability of
the charge.

There are basically two types of induction furnaces and they are:

1. Core type or low-frequency induction furnace.

2. Coreless type or high-frequency induction furnace.

Core type furnace

The operating principle of the core type furnace is the electromagnetic induction. This furnace is
operating just like a transformer. It is further classified as:

1. Direct core type.

2. Vertical core type.
3. Indirect core type.

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(i) Direct core type induction furnace

The core type furnace is essentially a transformer in which the charge to be heated forms single-
turn secondary circuit and is magnetically coupled to the primary by an iron core as shown
in Fig. 4.10.

Fig. 4.10 Direct core type furnace

The furnace consists of a circular hearth in the form of a trough, which contains the charge to
be melted in the form of an annular ring. This type of furnace has the following characteristics:

o This metal ring is quite large in diameter and is magnetically interlinked with primary winding, which is
energized from an AC source. The magnetic coupling between primary and secondary is very weak; it
results in high leakage reactance and low pf. To overcome the increase in leakage reactance, the furnace
should be operated at low frequency of the order of 10 Hz.
o When there is no molten metal in the hearth, the secondary becomes open circuited thereby cutting of
secondary current. Hence, to start the furnace, the molten metal has to be taken in the hearth to keep the
secondary as short circuit.
o Furnace is operating at normal frequency, which causes turbulence and severe stirring action in the
molten metal to avoid this difficulty, it is also necessary to operate the furnace at low frequency.
o In order to obtain low-frequency supply, separate motor-generator set (or) frequency changer is to be
provided, which involves the extra cost.
o The crucible used for the charge is of odd shape and inconvenient from the metallurgical viewpoint.
o If current density exceeds about 500 A/cm2, it will produce high-electromagnetic forces in the molten
metal and hence adjacent molecules repel each other, as they are in the same direction. The repulsion
may cause the interruption of secondary circuit (formation of bubbles and voids); this effect is known
as pinch effect.

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The pinch effect is also dependent on frequency; at low frequency, this effect is negligible, and
so it is necessary to operate the furnace at low frequency.

(ii) Vertical core type induction furnace

It is an improvement over the direct core type furnace, to overcome some of the disadvantages
mentioned above. This type of furnace consists of a vertical core instead of horizontal core as
shown in Fig. 4.11. It is also known as Ajax–Wyatt induction furnace.

Fig. 4.11 Vertical core type furnace (Ajax–Wyatt induction furnace)

Vertical core avoids the pinch effect due to the weight of the charge in the main body of the
crucible. The leakage reactance is comparatively low and the power factor is high as the
magnetic coupling is high compared to direct core type.

There is a tendency of molten metal to accumulate at the bottom that keeps the secondary
completed for a vertical core type furnace as it consists of narrow V-shaped channel.

The inside layer of furnace is lined depending upon the type charge used. Clay lining is used
for yellow brass and an alloy of magnesia and alumina is used for red brass.

The top surface of the furnace is covered with insulating material, which can be removed for
admitting the charge. Necessary hydraulic arrangements are usually made for tilting the furnace
to take out the molten metal. Even though it is having complicated construction, it is operating at
power factor of the order of 0.8–0.83. This furnace is normally used for the melting and refining
of brass and non-ferrous metals.

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Advantages

o Accurate temperature control and reduced metal losses.

o Absence of crucibles.
o Consistent performance and simple control.
o It is operating at high power factor.
o Pinch effect can be avoided.

(iii) Indirect core type furnace

This type of furnace is used for providing heat treatment to metal. A simple induction furnace
with the absence of core is shown in Fig. 4.12.

Fig. 4.12 Indirect core type furnace

The secondary winding itself forms the walls of the container or furnace and an iron core links
both primary and secondary windings.

The heat produced in the secondary winding is transmitted to the charge by radiation. An oven
of this type is in direct competition with ordinary resistance oven.

It consists of a magnetic circuit AB is made up of a special alloy and is kept inside the
chamber of the furnace. This magnetic circuit loses its magnetic properties at certain temperature
and regains them again when it is cooled to the same temperature.

When the oven reaches to critical temperature, the reluctance of the magnetic circuit increases
many times and the inductive effect decreases thereby cutting off the supply heat. Thus, the

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temperature of the furnace can be effectively controlled. The magnetic circuit ‘AB’ is detachable
type that can be replaced by the other magnetic circuits having critical temperatures ranging
between 400°C and 1,000°C. The furnace operates at a pf of around 0.8.

The main advantage of such furnace is wide variation of temperature control is possible.

Coreless type induction furnace

It is a simple furnace with the absence core is shown in Fig. 4.13. In this furnace, heat developed
in the charge due to eddy currents flowing through it.

Fig. 4.13 Coreless induction furnace

The furnace consists of a refractory or ceramic crucible cylindrical in shape enclosed within a
coil that forms primary of the transformer. The furnace also contains a conducting or non-
conducting container that acts as secondary.

If the container is made up of conducting material, charge can be conducting or non-

conducting; whereas, if the container is made up of non-conducting material, charge taken
should have conducting properties.

When primary coils are excited by an alternating source, the flux set up by these coils induce
the eddy currents in the charge. The direction of the resultant eddy current is in a direction
opposite to the current in the primary coil. These currents heat the charge to melting point and
they also set up electromagnetic forces that produce a stirring action to the charge.

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∴ The eddy currents developed in any magnetic circuit are given as:

We ∝ Bm2f2,

where Bm is the maximum flux density (tesla), f is the frequency in (Hz), and We is the eddy
current loss (watts).

In coreless furnace, the flux density will be low as there is no core. Hence, the primary supply
should have high frequency for compensating the low flux density.

If it is operating at high frequency, due to the skin effect, it results copper loss, thereby
increasing the temperature of the primary winding. This necessitates in artificial cooling. The
coil, therefore, is made of hollow copper tube through which cold water is circulated.

Minimum stray magnetic field is maintained when designing coreless furnace, otherwise there
will be considerable eddy current loss.

The selection of a suitable frequency of the primary current can be given by penetration
formula. According to this:

where ‘t’ is the thickness up to which current in the metal has penetrated, ‘ρ’ is the resistivity in
Ω-cm,'μ’ is the permeability of the material, and ‘f’ is the frequency in Hz.

For the efficient operation, the ratio of the diameter of the charge (d) to the depth of the
penetration of currents (t) should be more than ‘6’, therefore let us take:

Substitute above in Equation (4.11).

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Following are the advantages of coreless furnace over the other furnaces:

o Ease of control.
o Oxidation is reduced, as the time taken to reach the melting temperature is less.
o The eddy currents in the charge itself results in automatic stirring.
o The cost is less for the erection and operation.
o It can be used for heating and melting.
o Any shape of crucible can be used.
o It is suitable for intermittent operation.

Example 4.8: Determine the amount of energy required to melt 2 ton of zinc in 1 hr, if it
operates at an efficiency of 70% specific heat of zinc is equals to 0.1. The latent heat of zinc =
26.67 kcal/kg, the melting point is 480°C, and the initial temperature is 25°C.

Solution:

Weight of zinc = 2 × 1,000 = 2,000 kg.

The heat required raising the temperature from 25°C to 480°C:

H = w × S × (t2 - t1)

= 2,000 × 0.1 × (480-25)

= 91,000 kcal.

The heat required for melting:

=w×l

= 2,000 × 26.67

= 53,340 kcal.

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Example 4.9: A high-frequency induction furnace that takes 20 min to melt 1.9 kg of
aluminum, the input to the furnace being 3 kW, and the initial temperature is 25°C. Then,
determine the efficiency of the furnace.

The specific heat of aluminum = 0.212.

Melting point = 660°C.

The latent heat of the fusion of aluminum = 76.8 kcal/kg.

Solution:

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DIELECTRIC HEATING

When non-metallic materials i.e., insulators such as wood, plastics, and china glass are subjected
to high-voltage alternating electric field, the atoms get stresses, and due to interatomic friction
caused by the repeated deformation and the rotation of atomic structure (polarization), heat is
produced. This is known as dielectric loss. This dielectric loss in insulators corresponds to
hysteresis loss in ferro-magnetic materials. This loss is due to the reversal of magnetism or
magneto molecular friction. These losses developed in a material that has to be heated.

An atom of any material is neutral, since the central positive charge is equals to the negative
charge. So that, the centers of positive and negative charges coincide as long as there is no
external field is applied, as shown in Fig. (a). When this atom is subjected to the influence of the
electric field, the positive charge of the nucleus is acted upon by some force in the direction of
negative charges in the opposite direction. Therefore, the effective centers of both positive and
negative charges no longer coincident as shown in Fig. (b). The electric charge of an atom
equivalent to Fig.(b) is shown in Fig. (c).

Fig. Polarization

This gives raise to an electric dipole moment equal to P = q d, where d is the distance between
the two centers and q is the charge on the nucleus.

Now, the atom is said to be polarized atom. If we apply alternating voltage across the
capacitor plate, we will get alternating electric field.

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Electric dipoles will also try to change their orientation according to the direction of the
impressed electric field. In doing so, some energy will be wasted as inter-atomic friction, which
is called dielectric loss.

As there is no perfect conductor, so there is no perfect insulator. All the dielectric materials
can be represented by a parallel combination of a leakage resistor ‘R’ and a capacitor ‘C’ as
shown in Fig. 4.15 (a) and (b).

Fig. Dielectric heating

If an AC voltage is applied across a piece of insulator, an electric current flows; total current
‘I’ supposed to be made up of two components IC and IR, where IC is the capacitive current
leading the applied voltage by 90° and IR is in phase with applied voltage as shown in Fig.
4.15(c).

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where ‘V' is the applied voltage in volts, ‘f’ is the supply frequency in Hz, ɛ0 is the absolute
permittivity of the medium = 8.854 × 10-12 F/m, ɛr is the relative permittivity of the medium = 1
for free space, A is the area of the plate or electrode (m2), d is the thickness of the dielectric
medium, and δ is the loss angle in radian.

From Equation (4.14):

Normally frequency used for dielectric heating is in the range of 1–40 MHz. The use of high
voltage is also limited due to the breakdown voltage of thin dielectric that is to be heated, under
normal conditions; the voltage gradient used is limited to 18 kV/cm.

The advantages of the dielectric heating

o The heating of the non-conducting materials is very rapid.

o The uniform heating of material is possible.
o Heat is produced in the whole mass of the material.

The applications of the dielectric heating

o The drying of paper, wood, etc.

o The gluing of wood.
o The heat-sealing of plastic sheets.
o The heating for the general processing such as coffee roasting and chocolate industry.
o The heating for the dehydration such as milk, cream, and vegetables.
o The preparation of thermoplastic resins.
o The heating of bones and tissues.
o Diathermy, i.e., the heat treatment for certain body pains and diseases, etc.
o The sterilization of absorbent cotton, bandages, etc.
o The processing of rubber, synthetic materials, chemicals, etc.

Example 4.12: A piece of insulating material is to be heated by dielectric heating. The size of
the piece is 10 × 10 × 3 cm3. A frequency of 30 mega cycles is used and the power absorbed is

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400 W. Determine the voltage necessary for heating and the current that flows in the material.
The material has a permittivity of 5 and a power factor of 0.05.

Solution:

The capacitance offered by the material is given by:

In the phasor diagram, δ is called the dielectric loss angle and φ is called the power factor angle.

From the phasor diagram (Fig. P.4.3):

Fig. Phasor diagram

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SHORT QUESTIONS AND ANSWERS

1. Give any two advantages of electric heating.

1. Electric heating equipment is cheaper; it does not require much skilled

persons so maintenance cost is less.
2. In this heating, the temperature can be controlled and regulated accurately
either manually or automatically.

What are the modes of the transfer of heat?

The modes of the transfer of heat are:

0. Conduction.
1. Convection.
2. Radiation.

What is an oven?
Oven is mean that a low-temperature heating chamber with provision for ventilation.
Define conduction.
The process of heat transfers from one part of a substance to another part without
movement in the molecules of substance. The rate of conduction of heat along the
substance depends upon temperature gradient.
Define convection.
The process of heat transfer takes place from one part to another part of a substance or
a fluid due to the actual motion of the molecules. The rate of conduction of the heat
depends mainly on the difference in the fluid density at different temperatures.
Define radiation.
The process of heat transfers from the source to the substance to be heated without
heating the medium in between the source and the substance.
What are the essentials requirements of heating elements?
The materials used for heating element should have:

0. High-specific resistance.
1. High-melting point.
2. High-mechanical strength.
3. Free from oxidation.

What is the Stefan's formula for heat dissipation?

Stefan's law for heat dissipation is:

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What are the causes of the failure of the heating elements?

The failure of the heating element may cause due to:

0. The formation of hotspots.

1. The oxidation of the element and the intermittency of operation.
2. The embitterment caused by gain growth
3. Contamination and corrosion.

What is meant by resistance heating?

The process of heating the charge or substance by the heat produced due to the
resistance offered by the charge or heating element.
What is meant by induction heating?
The process of heating the material due to the heat developed by the currents induced
in the material by electromagnetic induction process.
What is meant by dielectric heating?
The process of heating non-metallic materials, i.e., the insulators such as wood,
plastics, and china clay due to the heat developed in the material when they are
subjected to high voltage alternating electric field, the atoms get stresses and due to
inter-atomic friction caused by the repeated deformation and rotation of atomic
structure.
What are the various losses occurring in resistance oven?
The heat produced in the heating elements, not only raises the temperature of charge to
desired value, but also used to overcome the losses occurring due to:

0. The heat used in raising the temperature of oven (or) furnace.

1. The heat used in raising the temperature of containers (or) carriers.
2. The heat conducted through the walls.
3. The heat loss due to the opening of oven door.

List out various methods of controlling the temperature of resistance heating.

The temperature of the furnaces can be controlled either by:

0. Varying the resistance of elements.

1. Varying the applied voltage to the elements or the current flowing through
the elements
2. Varying the ratio of the on-and-off times of supply.

What are the types of arc furnaces?

There are two types of arc furnaces and they are:

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0. Direct arc furnace.

1. Indirect arc furnace.

What is the condition for the maximum power output of electric arc furnace?
The condition for the maximum power output of electric arc furnace is:

1.What is pinch effect?

The formation of bubbles and voids in the charge to be heated by the electromagnetic
induction due to high-electromagnetic forces, which causes the interruption of
secondary circuit. This effect is known as pinch effect.

2.What is high-frequency eddy current heating?

The process of heating any material by the heat developed due to the conversion of
electromagnetic energy into heat energy.

3.How amount of heat is controlled in high-frequency eddy current heating?

The amount of heat is controlled by controlling the supply frequency and the flux
density in high-frequency eddy current heating.

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UNIT 3
Electric Welding

INTRODUCTION

Welding is the process of joining two pieces of metal or non-metal together by heating them to
their melting point. Filler metal may or may not be used to join two pieces. The physical and
mechanical properties of a material to be welded such as melting temperature, density, thermal
conductivity, and tensile strength take an important role in welding. Depending upon how the
heat applied is created; we get different types of welding such as thermal welding, gas welding,
and electric welding. Here in this chapter, we will discuss only about the electric welding and
some introduction to other modern welding techniques. Welding is nowadays extensively used in
automobile industry, pipe-line fabrication in thermal power plants, machine repair work,
machine frames, etc.

ADVANTAGES AND DISADVANTAGES OF WELDING

Some of the advantages of welding are:

o Welding is the most economical method to permanently join two metal parts.
o It provides design flexibility.
o Welding equipment is not so costly.
o It joins all the commercial metals.
o Both similar and dissimilar metals can be joined by welding.
o Portable welding equipment are available.

Some of the disadvantages of welding are:

o Welding gives out harmful radiations and fumes.

o Welding needs internal inspection.
o If welding is not done carefully, it may result in the distortion of workpiece.
o Skilled welding is necessary to produce good welding.

ELECTRIC WELDING

It is defined as the process of joining two metal pieces, in which the electrical energy is used to
generate heat at the point of welding in order to melt the joint.

The classification of electric welding process is shown in fig.

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Fig. Classification of electric welding

The selection of proper welding process depends on the following factors.

o The type of metal to be joined.

o The techniques of welding adopted.
o The cost of equipment used.
o The nature of products to be fabricated.

RESISTANCE WELDING

Resistance welding is the process of joining two metals together by the heat produced due to the
resistance offered to the flow of electric current at the junctions of two metals. The heat
produced by the resistance to the flow of current is given by:

H = I2Rt,

where I is the current through the electrodes, R is the contact resistance of the interface, and tis
the time for which current flows.

Here, the total resistance offered to the flow of current is made up of:

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1. The resistance of current path in the work.

2. The resistance between the contact surfaces of the parts being welded.
3. The resistance between electrodes and the surface of parts being welded.

In this process of welding, the heat developed at the contact area between the pieces to be welded
reduces the metal to plastic state or liquid state, then the pieces are pressed under high
mechanical pressure to complete the weld. The electrical voltage input to the welding varies in
between 4 and 12 V depending upon area, thickness, composition, etc. and usually power ranges
from about 60 to 180 W for each sq. mm of area.

Any desired combination of voltage and current can be obtained by means of a suitable
transformer in AC; hence, AC is found to be most suitable for the resistance welding. The
magnitude of current is controlled by changing the primary voltage of the welding transformer,
which can be done by using an auto-transformer or a tap-changing transformer. Automatic
arrangements are provided to switch off the supply after a pre-determined time from applying the
pressure, why because the duration of the current flow through the work is very important in the
resistance welding.

The electrical circuit diagram for the resistance welding is shown in Fig. 5.2. This method of
welding consists of a tap-changing transformer, a clamping device for holding the metal pieces,
and some sort of mechanical arrangement for forcing the pieces to form a complete weld.

Fig. Electric circuit for resistance welding

Advantages

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o Welding process is rapid and simple.

o Localized heating is possible, if required.
o No need of using filler metal.
o Both similar and dissimilar metals can be welded.
o Comparatively lesser skill is required.
o Maintenance cost is less.
o It can be employed for mass production.

However, the resistance welding has got some drawbacks and they are:

o Initial cost is very high.

o High maintenance cost.
o The workpiece with heavier thickness cannot be welded, since it requires high input current.

Applications

o It is used by many industries manufacturing products made up of thinner gauge metals.

o It is used for the manufacturing of tubes and smaller structural sections.

Types of resistance welding

Depending upon the method of weld obtained and the type of electrodes used, the resistance
welding is classified as:

1. Spot welding.
2. Seam welding.
3. Projection welding.
4. Butt welding.

(i) Spot welding

Spot welding means the joining of two metal sheets and fusing them together between copper
electrode tips at suitably spaced intervals by means of heavy electric current passed through the
electrodes as shown in Fig. 5.3.

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Fig. 5.3 Spot welding

This type of joint formed by the spot welding provides mechanical strength and not air or
water tight, for such welding it is necessary to localize the welding current and to apply
sufficient pressure on the sheet to be welded. The electrodes are made up of copper or copper
alloy and are water cooled. The welding current varies widely depending upon the thickness and
composition of the plates. It varies from 1,000 to 10,000 A, and voltage between the electrodes is
usually less than 2 V. The period of the flow of current varies widely depending upon the
thickness of sheets to be joined. A step-down transformer is used to reduce a high-voltage and
low-current supply to low-voltage and high-current supply required. Since the heat developed
being proportional to the product of welding time and square of the current. Good weld can be
obtained by low currents for longer duration and high currents for shorter duration; longer
welding time usually produces stronger weld but it involves high energy expenditure, electrode
maintenance, and lot of distortion of workpiece.

When voltage applied across the electrode, the flow of current will generate heat at the three
junctions, i.e., heat developed, between the two electrode tips and workpiece, between the two
workpieces to be joined as shown in Fig. 3.3. The generation of heat at junctions 1 and 3 will
effect electrode sticking and melt through holes, the prevention of electrode striking is achieved
by:

1. Using water-cooled electrodes shown in Fig. 5.4. By avoiding the heating of junctions 1 and 3 electrodes
in which cold water circulated continuously as shown in Fig. 5.3.
2. The material used for electrode should have high electrical and thermal conductivity. Spot welding is
widely used for automatic welding process, for joining automobile parts, joining and fabricating sheet
metal structure, etc.

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Fig. Water cooled electrode

(ii) Seam welding

Seam welding is nothing but the series of continuous spot welding. If number spots obtained by
spot welding are placed very closely that they can overlap, it gives rise to seam welding.

In this welding, continuous spot welds can be formed by using wheel type or roller electrodes
instead of tipped electrodes as shown in Fig. 5.5.

Fig. 5.5 Seam welding

Seam welding is obtained by keeping the job under electrodes. When these wheel type
electrodes travel over the metal pieces which are under pressure, the current passing between
them heats the two metal pieces to the plastic state and results into continuous spot welds.

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In this welding, the contact area of electrodes should be small, which will localize the current
pressure to the welding point. After forming weld at one point, the weld so obtained can be
cooled by splashing water over the job by using cooling jets.

In general, it is not satisfactory to make a continuous weld, for which the flow of continuous
current build up high heat that causes burning and wrapping of the metal piece. To avoid this
difficulty, an interrupter is provided on the circuit which turns on supply for a period sufficient to
heat the welding point. The series of weld spots depends upon the number of welding current
pulses.

The two forms of welding currents are shown in Fig. 5.6(a) and (b).

Fig. 5.6 Welding current

Welding cannot be made satisfactorily by using uninterrupted or un-modulated current, which

builds up high heat as the welding progress; this will over heat the workpiece and cause
distortion.

Seam welding is very important, as it provides leak proof joints. It is usually employed in
welding of pressure tanks, transformers, condensers, evaporators, air craft tanks, refrigerators,
varnish containers, etc.

(iii) Projection welding

It is a modified form of the spot welding. In the projection welding, both current and pressure are
localized to the welding points as in the spot welding. But the only difference in the projection
welding is the high mechanical pressure applied on the metal pieces to be welded, after the
formation of weld. The electrodes used for such welding are flat metal plates known as platens.

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The two pieces of base metal to be weld are held together in between the two platens, one is
movable and the other is fixed, as shown in Fig. 5.7.

Fig. 5.7 Projection welding

One of the two pieces of metal is run through a machine that makes the bumps or projections
of required shape and size in the metal. As current flows through the two metal parts to be
welded, which heat up and melt. These weld points soon reach the plastic state, and the
projection touches the metal then force applied by the two flat electrodes forms the complete
weld.

The projection welding needs no protective atmosphere as in the spot welding to produce
successful results. This welding process reduces the amount of current and pressure in order to
join two metal surfaces, so that there is less chance of distortion of the surrounding areas of the
weld zone. Due to this reason, it has been incorporated into many manufacturing process.

The projection welding has the following advantages over the spot welding.

o Simplicity in welding process.

o It is easy to weld some of the parts where the spot welding is not possible.
o It is possible to join several welding points.
o Welds are located automatically by the position of projection.
o As the electrodes used in the projection welding are flat type, the contact area over the projection is
sufficient.

This type of welding is usually employed on punched, formed, or stamped parts where the
projection automatically exists. The projection welding is particularly employed for mass
production work, i.e., welding of refrigerators, condensers, crossed wire welding, refrigerator
racks, grills, etc.

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(iv) Butt welding

Butt welding is similar to the spot welding; however, the only difference is, in butt welding,
instead of electrodes the metal parts that are to be joined or butted together are connected to the
supply.

The three basic types of the butt welding process are:

1. Upset butt welding.

2. Flash butt welding.
3. Percussion butt welding.

(a) Upset butt welding

In upset welding, the two metal parts to be welded are joined end to end and are connected
across the secondary of a welding transformer as shown in Fig. 5.8.

Fig. 5.8 Upset butt welding

Due to the contact resistance of the metals to be welded, heating effect is generated in this
welding. When current is made to flow through the two electrodes, heat will develop due to the
contact resistance of the two pieces and then melts. By applying high mechanical pressure either
manually or by toggle mechanism, the two metal pieces are pressed. When jaw-type electrodes
are used that introduce the high currents without treating any hot spot on the job.

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This type of welding is usually employed for welding of rods, pipes, and wires and for joining
metal parts end to end.

(b) Flash butt welding

Flash butt welding is a combination of resistance, arc, and pressure welding. This method of
welding is mainly used in the production welding. A simple flash butt welding arrangement is
shown in Fig. 5.9.

Fig. 5.9 Flash butt welding

In this method of welding, the two pieces to be welded are brought very nearer to each other
under light mechanical pressure. These two pieces are placed in a conducting movable clamps.
When high current is passed through the two metal pieces and they are separated by some
distance, then arc established between them. This arc or flashing is allowed till the ends of the
workpieces reach melting temperature, the supply will be switched off and the pieces are rapidly
brought together under light pressure. As the pieces are moved together, the fused metal and slag
come out of the joint making a good solid joint.

Following are the advantages of the flash butt welding over the upset welding.

o Less requirement of power.

o When the surfaces being joined, it requires only less attention.
o Weld obtained is so clean and pure; due to the foreign metals appearing on the surfaces will burn due to
flash or arc.

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(c) Percussion welding

It is a form of the flash butt welding, where high current of short duration is employed using
stored energy principle. This is a self-timing spot welding method.

Percussion welding arrangement consists of one fixed holder and the other one is movable.
The pieces to be welded are held apart, with the help of two holders, when the movable clamp is
released, it moves rapidly carrying the piece to be welded. There is a sudden discharge of
electrical energy, which establishes an arc between the two surfaces and heating them to their
melting temperature, when the two pieces are separated by a distance of 1.5 mm apart. As the
pieces come in contact with each other under heavy pressure, the arc is extinguished due to the
percussion blow of the two parts and the force between them affects the weld. The percussion
welding can be obtained in two methods; one is capacitor energy storage system and the other is
magnetic energy storage system. The capacitor discharge circuit for percussion welding is shown
in Fig. 5.10.

Fig. 5.10 Capacitor discharge circuit for percussion welding

The capacitor ‘C’ is charged to about 3,000 V from a controlled rectifier. The capacitor is
connected to the primary of welding transformer through the switch and will discharge. This
discharge will produce high transient current in the secondary to join the two metal pieces.

Percussion welding is difficult to obtain uniform flashing of the metal part areas of the cross-
section grater than 3 sq. cm. Advantage of this welding is so fast, extremely shallow of heating is

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obtained with a span of about 0.1 sec. It can be used for welding a large number of dissimilar
metals.

Applications

o It is useful for welding satellite tips to tools, sliver contact tips to copper, cast iron to steel, etc.
o Commonly used for electrical contacts.
o The metals such as copper alloys, aluminum alloys, and nickel alloys are percussion welded.

CHOICE OF WELDING TIME

The successful welding operation mainly depends upon three factors and they are:

1. Welding time.
2. Welding current.
3. Welding pressure.

Figure 5.11 shows how the energy input to the welding process, welding strength, and welding
current vary with welding time.

Fig. 5.11 Performance characteristics of electric welding

The heat developed during welding process is given by H = I2Rt. Here both welding current
and welding time are critical variables.

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Greater the welding current, the shorter the welding time required is; usually longer welding
time produces stronger weld but there is lot of distortion of workpiece and high energy
expenditure. From Fig. 5.11, it is to be noted that, from 0 to t1 sec, there is appreciable increase
in welding strength, but after t2 sec, the increase in the welding time does not appreciably result
in the increase in strength; therefore, ‘t2’ is the optimum welding time. This optimum time varies
with the thickness of the material. The optimum times of material (sheet steel) with different
thickness are given as:

Dimensions of material Optimum time

2 × 24 SWG 8 cycles

2 × 14 SWG 20 cycles

2¼″ 2 sec

Therefore, from the above discussion, it is observed that shorter welding times with strength and
economy are always preferable.

Electromagnetic storage welding circuit is shown in Fig. 5.12. In this type of welding, the
energy stored in the magnetic circuit is used in the welding operation.

Fig. 5.12 Magnetic energy storage welding circuit

In this system, rectifier is fed from AC supply, which is converted to DC, the DC voltage of
rectifier is controlled in such a way that, voltage induced in the primary without causing large
current in the secondary of transformer on opening the contactor switch, DC on longer flows,
there is rapid collapse of magnetic field, which induces very high current in the secondary of a
transformer. Induced currents in the secondary of the transformer flow through the electrodes
that develop heat at the surface of the metal and so forming the complete weld.

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ELECTRIC ARC WELDING

Electric arc welding is the process of joining two metallic pieces or melting of metal is obtained
due to the heat developed by an arc struck between an electrode and the metal to be welded or
between the two electrodes as shown in Fig. 5.13 (a).

Fig. Arrangement of electric welding equipment

In this process, an electric arc is produced by bringing two conductors (electrode and metal
piece) connected to a suitable source of electric current, momentarily in contact and then
separated by a small gap, arc blows due to the ionization and give intense heat.

The heat so developed is utilized to melt the part of workpiece and filler metal and thus forms
the weld.

In this method of welding, no mechanical pressure is employed; therefore, this type of welding
is also known as 'non-pressure welding’.

The length of the arc required for welding depends upon the following factors:

o The surface coating and the type of electrodes used.

o The position of welding.
o The amount of current used.

When the supply is given across the conductors separated by some distance apart, the air gap
present between the two conductors gets ionized, as the arc welding is in progress, the ionization
of the arc path and its surrounding area increases. This increase in ionization decreases the
resistance of the path. Thus, current increases with the decrease in voltage of arc. This V-
I characteristic of an arc is shown in Fig. (b), it also known as negative resistance characteristics
of an arc. Thus, it will be seen that this decrease in resistance with increase in current does not

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remain the arc steadily. This difficulty cab be avoided, with the supply, it should fall rapidly with
the increase in the current so that any further increase in the current is restricted.

For the arc welding, the temperature of the arc should be 3,500°C. At this temperature,
mechanical pressure for melting is not required. Both AC and DC can be used in the arc welding.
Usually 70–100 V on AC supply and 50–60 V on DC supply system is sufficient to struck the arc
in the air gap between the electrodes. Once the arc is struck, 20–30 V is only required to
maintain it.

However, in certain cases, there is any danger of electric shock to the operator, low voltage
should be used for the welding purpose. Thus, DC arc welding of low voltage is generally
preferred.

Electric arc welding is extensively used for the joining of metal parts, the repair of fractured
casting, and the fillings by the deposition of new metal on base metal, etc.

Various types of electric arc welding are:

1. Carbon arc welding.

2. Metal arc welding.
3. Atomic hydrogen arc welding.
4. Inert gas metal arc welding.
5. Submerged arc welding.

Carbon arc welding

It is one of the processes of arc welding in which arc is struck between two carbon electrodes or
the carbon electrode and the base metal. The simple arrangement of the carbon arc welding is
shown in Fig. 5.14.

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Fig. Carbon arc welding

In this process of welding, the electrodes are placed in an electrode holder used as negative
electrode and the base metal being welded as positive. Unless, the electrode is negative relative
to the work, due to high temperature, there is a tendency of the particles of carbon will fuse and
mix up with the base metal, which causes brittleness; DC is preferred for carbon arc welding
since there is no fixed polarity maintained in case of AC.

In the carbon arc welding, carbon or graphite rods are used as electrode. Due to longer life and
low resistance, graphite electrodes are used, and thus capable of conducting more current. The
arc produced between electrode and base metal; heat the metal to the melting temperature, on the
negative electrode is 3,200°C and on the positive electrode is 3,900°C.

This process of welding is normally employed where addition of filler metal is not required.
The carbon arc is easy to maintain, and also the length of the arc can be easily varied. One major
problem with carbon arc is its instability which can be overcome by using an inductor in the
electrode of 2.5-cm diameter and with the current of about of 500–800 A employed to deposit
large amount of filler metal on the base metal.

Filler metal and flux may not be used depending upon the type of joint and material to be
welded.

Advantages

o The heat developed during the welding can be easily controlled by adjusting the length of the arc.
o It is quite clean, simple, and less expensive when compared to other welding process.
o Easily adoptable for automation.
o Both the ferrous and the non-ferrous metals can be welded.

Disadvantages

o Input current required in this welding, for the workpiece to rise its temperature to melting/welding
temperature, is approximately double the metal arc welding.
o In case of the ferrous metal, there is a chance of disintegrating the carbon at high temperature and
transfer to the weld, which causes harder weld deposit and brittlement.
o A separate filler rod has to be used if any filler metal is required.

Applications

o It can be employed for the welding of stainless steel with thinner gauges.
o Useful for the welding of thin high-grade nickel alloys and for galvanized sheets using copper silicon
manganese alloy filler metal.

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Metal arc welding

In metal arc welding, the electrodes used must be of the same metal as that of the work-piece to
be welded. The electrode itself forms the filler metal. An electric arc is stuck by bringing the
electrode connected to a suitable source of electric current, momentarily in contract with the
workpieces to be welded and withdrawn apart. The circuit diagram for the metal arc welding is
shown in Fig. 5.15.

Fig. 5.15 Metal arc welding

The arc produced between the workpiece and the electrode results high temperature of the
order of about 2,400°C at negative metal electrode and 2,600°C at positive base metal or
workpiece.

This high temperature of the arc melts the metal as well as the tip of the electrode, then the
electrode melts and deposited over the surface of the workpiece, forms complete weld.

Both AC and DC can be used for the metal arc welding. The voltage required for the DC metal
arc welding is about 50–60 V and for the AC metal arc welding is about 80–90 V

In order to maintain the voltage drop across the arc less than 13 V, the arc length should be
kept as small as possible, otherwise the weld will be brittle. The current required for the welding
varies from 10 to 500 A depending upon the type of work to be welded.

The main disadvantage in the DC metal arc welding is the presence of arc blow, i.e., distortion
of arc stream from the intended path due to the magnetic forces of the non-uniform magnetic
field with AC arc blow is considerably reduced. For obtaining good weld, the flux-coated

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electrodes must be used, so the metal which is melted is covered with slag produces a non-
oxidizing gas or a molten slag to cover the weld, and also stabilizes the arc.

Atomic hydrogen arc welding

In atomic hydrogen arc welding, shown in Fig. 5.16, the heat for the welding process is produced
from an electric arc struck between two tungsten electrodes in an atmosphere of hydrogen. Here,
hydrogen serves mainly two functions; one acts as a protective screen for the arc and the other
acts as a cooling agent for the glowing tungsten electrode tips. As the hydrogen gas passes
through the arc, the hydrogen molecules are broken up into atoms, absorbs heat from the glowing
tungsten electrodes so that these are cooled.

Fig. 5.16 Atomic hydrogen arc welding

But, when the atoms of hydrogen recombine into molecules outside the arc, a large amount of
heat is liberated. This extraheat is added to the intense heat of arc, which produces a temperature
of about 4,000°C that is sufficient to melt the surfaces to be welded, together with the filler rod if
used. Moreover hydrogen includes oxygen and some other gases that might combine with the
molten metal and forms oxides and other impurities. Hydrogen also removes oxides from the
surface of workpiece. Thus, this process is capable of producing strong, uniform, smooth, and
ductile welds.

In the atomic hydrogen arc welding, the arc is maintained between the two non-consumable
tungsten electrodes under a pressure of about 0.5 kg/cm2. In order to obtain equal consumption of
electrodes, AC supply is used. Arc currents up to 150 A can be used. High voltage about 300 V
is applied for this welding through a transformer. For striking the arc between the electrodes the
open circuit voltage required varies from 80 to 100 V.

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As the atomic hydrogen welding is too expensive, it is usually employed for welding alloy
steel, carbon steel, stainless steel, aluminum, etc.

Inert gas metal arc welding

It is a gas-shielded metal arc welding, in which an electric arc is stuck between tungsten
electrode and workpiece to be welded. Filler metal may be introduced separately into the arc if
required. A welding gun, which carries a nozzle, through this nozzle, inert gas such as beryllium
or argon is blown around the arc and onto the weld, as shown in Fig. 5.17. As both beryllium and
argon are chemically inert, so the molten metal is protected from the action of the atmosphere by
an envelope of chemically reducing or inert gas.

Fig. 5.17 Inert gas metal are welding

As molten metal has an affinity for oxygen and nitrogen, if exposed to the atmosphere, thereby
forming their oxides and nitrides, which makes weld leaky and brittle.

Thus, several methods of shielding have been employed. With the use of flux coating
electrodes or by pumping, the inert gases around the arc produces a slag that floats on the top of
molten metal and produces an envelope of inert gas around the arc and the weld.

Advantages

o Flux is not required since inert gas envelope protects the molten metal without forming oxides and
nitrates so the weld is smooth, uniform, and ductile.
o Distortion of the work is minimum because the concentration of heat is possible.

Applications

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o The welding is employed for light alloys, stainless steel, etc.

o The welding of non-ferrous metal such as copper, aluminum, etc.

SUBMERGED ARC WELDING

It is an arc welding process, in which the arc column is established between above metal
electrode and the workpiece. Electric arc and molten pool are shielded by blanket of granular
flux on the workpiece. Initially to start an arc, short circuit path is provided by introducing steel
wool between the welding electrode and the workpiece. This is due to the coated flux material,
when cold it is non-conductor of the electricity but in molten state, it is highly conductive.
Welding zone is shielded by a blanket of flux, so that the arc is not visible. Hence, it is known
as 'submerged arc welding’. The arc so produced, melts the electrode, parent the metal and the
coated flux, which forms a protective envelope around both the arc and the molten metal.

As the arc in progress, the melted electrode metal forms globules and mix up with the molten
base metal, so that the weld is completed. In this welding, the electrode is completely covered by
flux. The flux may be made of silica, metal oxides, and other compounds fused together and then
crushed to proper size. Therefore, the welding takes place without spark, smoke, ash, etc. Thus,
there is no need of providing protective shields, smoke collectors, and ventilating
systems. Figure 5.18 shows the filling of parent metal by the submerged arc welding.

Fig. 5.18 Submerged arc welding

Voltage required for the submerged arc welding varies from 25 to 40 V. Current employed for
welding depends upon the dimensions of the workpiece. Normally, if DC supply is used
employing current ranging from 600 to 1,000 A, the current for AC is usually 2,000 A.

Advantages

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o Deep penetration with high-quality weld is possible.

o Job with heavy thickness can be welded.
o The weld so obtained has good ductility, impact strength, high corrosion resistance, etc.
o The submerged arc welding can be done manually or automatically.

Applications

o The submerged arc welding is widely used in the heavy steel plant fabrication work.
o It can be employed for welding high strength steel, corrosion resistance steel, and low carbon steel.
o It is also used in the ship-building industry for splicing and fabricating subassemblies, manufacture of
vessels, tanks, etc.

ELECTRON BEAM WELDING

It is one of the processes of the electric welding, in which the heat required for carrying out the
welding operation is obtained by the electron bombardment heating.

In the electron bombardment heating, continuous stream of electron is produced between the
electron emitting material cathode and the material to be heated. The electrons released from
cathode possess KE traveling with high velocity in vacuum of 10-3-10-5 mmHg. When the fast
moving electrons hit, the material or workpiece releases their KE as heat in the material to be
heated. This heat is utilized to melt the metal.

If this process is carried out in high vacuum, without providing any electrodes, gasses, or filler
metal, pure weld can be obtained. Moreover, high vacuum is maintained around the (filament)
cathode. So that, it will not burn up and also produces continuous stable beam. If a vacuum was
not used, the electron would strike the small partials in the atmosphere, reducing their velocity
and also the heating ability. Thus, the operation should be performed in vacuum to present the
reduction of the velocity of electron. That's why this is also called as'vacuum electron beam
welding’. The power released by the electron beam is given by:

P = nqv watts,

where n is the number of charged particles, q is the charge in coulombs per meter, and v is the
voltage required to accelerate the electrum from rest.

The electron beam welding (Fig. 5.19) process requires electron-emitting heating filament as
cathode, focusing lens, etc.

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Fig. 5.19 Electron beam welding

Advantages

o Heat input to the electron beam welding can be easily controlled by varying beam current, voltage, the
position of filament, etc.
o The electron beam welding can be used to join high temperature metals such as columbium.
o It can be employed for the welding of thick sections, due to high penetration to width ratio.
o It eliminates contamination of both weld zone and weld metal.
o Narrow electron beam reduces the distortion of workpiece.

Disadvantages

o The pressure build up in the vacuum chamber due to the vapor of parent metal causes electrical break
down.
o Most of the super alloys, refractory metals, and combinations of dissimilar metals can also be welded.

LASER BEAM WELDING

The word laser means 'light amplification stimulated emission of radiation’. It is the process of
joining the metal pieces by focusing a monochromatic light into the extremely concentrated
beams, onto the weld zone.

This process is used without shielding gas and without the application of pressure. The laser
beam is very intense and unidirectional but can be focused and refracted in the same way as an
ordinary light beam. The focus of the laser beam can be controlled by controlling the lenses,
mirrors, and the distance to the workpiece. Ablock diagram of the laser beam welding system is
shown in Fig. 5.20.

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Fig. 5.20 Laser beam welding

In laser beam welding system, flash tube is designed to give thousands of flashes per second.
When capacitor bank is triggered, the electrical energy is injected into the flash tube through
trigger wire. Flash tube consists of thick xenon material, which produces high power levels for
very short period. If the bulb is operated in this manner, it becomes an efficient device, which
converts electrical energy to light energy. The laser is then activated.

The laser beam emitting from the flash tube, passing through the focusing lens, where it is
pinpointed on the workpiece. The heat so developed by the laser beam melts the work-piece and
the weld is completed. The welding characteristics of the laser are similar to the electron beam.

The laser beam has been used to weld carbon steel, low-alloy steel, aluminum, etc. The metals
with relatively high-electrical resistance and the parts of different sizes and mass can be welded.

TYPES OF WELDING ELECTRODES

An electrode is a piece of metal in the form of wire or rod that is either bare or coated uniformly
with flux. Electrode carries current for the welding operation. One contact end of the electrode
must be clean and is inserted into the electrode holder, an arc is set up at the other end.

The electrodes used for the arc welding are classified as follows (Fig. 5.21).

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Fig Classification ofelectrods

Non-consumable electrodes

Electrodes, which do not consume or fuse during the welding process, are called non-
consumable electrodes.

Ex: Electrodes made up of carbon, graphite, or tungsten do not consume during welding.

Consumable electrodes

Electrodes, which are consumed during the welding operation, are consumable electrodes. These
are made up of various materials depending upon their purpose and the chemical composition of
metal to be welded.

The consumable electrodes are made in the form of rod having diameter of about 2–8 mm and
length of about 200–500 mm. They act as filler rod and are consumed during welding operation.

Bare electrodes

These are the consumable electrodes, which are not coated with any fluxing material. Bare
electrodes are in the form of wire. During welding operation, an arc is struck between the
workpiece and the electrode wire, then the electrode is melted down into the weld.

When the molten metal electrode and the workpiece are exposed to the atmosphere of oxygen
and nitrogen, they form their oxides and nitrides and cause the formation of some non-metallic
constituent, which reduces the strength and ductility of the deposited weld. The bare electrodes
are usually employed in automatic and semiautomatic welding. With bare electrode, the welding

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can be done satisfactorily with DC supply only if the electrode should be connected to the
negative terminal of the supply.

Coated electrodes

Depending upon the thickness of flux coating, the coated electrode may classified into:

1. lightly coated electrodes and

2. heavily coated electrodes.

For obtaining good weld, the coated electrodes are always preferred.

(i) Lightly coated electrodes

These electrodes are coated with thin layer of coating material up to less than 1 mm. This coating
is usually consists of lime mixed with soluble glass which serves as a binder. These electrodes
are considered as improvement over bare electrodes.

The main purpose of using the light coating layer on the electrode is to increase the arc
stability, so they are also called as stabilizing electrodes. The mechanical strength of the weld
increased because slag layer will not formed on the molten weld. For this reason, lightly coated
electrodes may only be used for welding non-essential workpieces.

(ii)Heavily coated electrodes

These electrodes have coating layer with heavy thickness. The heavily coated electrodes
sometimes referred to as the shielded arc electrodes. The materials commonly used for coating
the electrodes are titanium oxide, ferromanganese, silica, flour, asbestos clay, calcium carbonate,
etc. This electrode coating helps in improving the quality of weld, as if the coating layer of the
electrodes burns in the heat of the arc provides gaseous shield around the arc, which prevents the
formation oxides and nitrites.

Advantages

o Arc is stabilized due to the flux compounds of sodium and potassium.

o The weld metal can be protected from the oxidizing action of oxygen and the nitrifying action of
nitrogen due to the gas shielded envelope.
o The impurities present on the surface being welded are fluxed away.
o The electrode coating increases deposition efficiency and weld metal deposition rate through iron
powder and ferro alloy addition.
o In case of AC supply arc cools at zero current and there is a tendency of deionizing the arc path.
Covering gases keep the arc space ionized.
o The welding operation becomes faster due to the increased melting rate.
o The coated electrodes help to deoxidize and refine the weld metal.

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The type of electrode used for the welding process depends upon the following factors.

o The nature of the electric supply, either AC or DC.

o The type of the metal to be welded.
o The welding position.
o The polarity of the welding machine.

COMPARISON BETWEEN RESISTANCE AND ARC WELDING

Resistance welding Arc welding

1 The source of supply is AC only. The source of supply is either AC (1-φ or 3-φ) or DC.

2 The head developed is mainly due to the The heat developed is mainly due to the striking of arc between
flow of contact resistance. electrodes or an electrode and the workpiece.

3 The temperature attained by the workpiece The temperature of the arc is so high, so proper care should be taken
is not so high. during the welding.

4 External pressure is required. No external pressure is required hence the welding equipment is more
simple and easy to control.

5 Filler metal is not required to join two Suitable filler electrodes are necessary to get proper welding strength.
metal pieces.

6 It cannot be used for repair work; it is It is not suitable for mass production. It is most suitable for repair
suitable for mass production. works and where more metal is to be deposited.

7 The power consumption is low. The power consumption is high.

8 The operating power factor is low. The operating power factor is high.

9 Bar, roller, or flat type electrodes are used Bare or coated electrodes are used (consumable or non-consumable).
(not consumable).

ELECTRIC WELDING EEQUIPMENT

Electric welding accessories required to carry out proper welding operation are:

1. Electric welding power sets.

2. Electrode holder to hold the electrodes.
3. Welding cable for connecting electrode and workpiece to the supply.
4. Face screen with colored glass.
5. Chipping hammers to remove slag from molten weld.

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6. Wire brush to clean the weld.

7. Earth clamp and protective clothing.

COMPARISON BETWEEN AC AND DC WELDING

AC welding DC welding

1 Motor generator set or rectifier is required in case of Only transformer is required.

the availability of AC supply.

2 The cost of the equipment is high. The cost of the equipment is cheap.

3 Arc stability is more. Arc stability is less.

4 The heat produced is uniform. The heat produced is not uniform.

5 Both bare and coated electrodes can be used. Only coated electrodes should be used.

6 The operating power factor is high. The power factor is low. So, the capacitors are necessary
to improve the power factor.

7 It is safer since no load voltage is low. It is dangerous since no load voltage is high.

8 The electric energy consumption is 5–10 kWh/kg of The electrical energy consumption is 3–4 kWh/kg of
deposited metal. deposited metal

9 Arc blow occurs due to the presence of non-uniform Arc blow will not occur due to the uniform magnetic
magnetic field. field.

10 The efficiency is low due to the rotating parts. The efficiency is high due to the absence of rotating parts.

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