HEATING

Syllabus
Electric heating and welding: Introduction & Advantages, Electric
heating methods, Resistance heating: Methods, requirements of
heating elements, methods of temperature control, Heating element
design; Induction heating: Principle, types of induction furnaces, direct
core type, vertical core type, indirect core type, core less type; Dielectric
heating: Principle, advantages, applications, Arc-furnace: Principle,
types, direct and indirect arc furnaces,
Modes of Transfer of Heat
• Conduction
• Convection
• Radiation
 Heating
◦Resistance Heating
◦ Direct
◦ Indirect

◦Induction Heating
◦ Direct
◦ Indirect

◦Dielectric heating
◦Arc Heating
 Resistance Heating
 Resistance heating is based upon the I2R effect.
 This method of heating has wide applications
such as heat treatment of metals (annealing,
hardening etc.), drying and baking of potteries,
stoving of enamelled ware and commercial and
domestic cooking.
 Temperature up to about 1,000°C can be ob-
tained in ovens employing wire resistances for
heating elements.
 Methods: Direct and Indirect
1. Direct Resistance Heating
 In this resistance heating method, the material or charge to
be heated is taken as resistance and current is passed through
it.

The charge may be in the form of powder, pieces or a liquid.

Two electrodes are immersed in the charge and connected to
the supply in case of availability of direct current or single
phase ac supply
 Three electrodes are immersed in the charge for 3-Φ ac.
When some pieces of metals are to be heated some highly
resistive powder is sprinkled over the surface of pieces to avoid
direct short circuit.
The current flows through the charge and heat is produced.
 Used in salt bath furnaces and in the electrode boiler for
heating water.
2. Indirect Resistance Heating
 In this resistance heating method, the current is passed
through a wire or other high resistance material forming a
heating element.
 The heat proportional to I2R loss produced in the heating
element is delivered to the charge by one or more of the
modes of transfer of heat viz. conduction, convection and
radiation.
 If the heat transfer is by conduction the resistor must be
in contact with the charge. An enclosure, known as heating
chamber, is required for heat transfer by radiation and
convection for the charge.
 For industrial purposes, where a large amount of charge
is to be heated, the heating clement is kept in a cylinder
surrounded
 Used in room heaters, immersion water heaters,
resistance ovens and salt bath furnaces
Resistance Ovens and Furnaces
 Low temperature furnace is termed as oven.

 Medium temperature furnaces having operating

temperatures between 300°C and 1,050°C are
employed for annealing, normalising of steel and
non-ferrous metals, melting of non-ferrous metals
and stove enamelling.
 High temperature furnaces having operating
temperatures between 1,050°C and 1,350°C are
employed for hardening applications.
 The resistance oven is constructed of firebricks or other
heat insulating material supported on a metal framework.
The heating elements are mounted on the top, sides or
bottom of the oven, according to circumstances.
 The resistance furnace is an enclosure with a refractory
lining : a surrounding layer of heat insulation and outer
casing of steel plate, bricks or tiles.
 The inside proportions of a heating chamber are made to
suit the character of the charge and type of furnace or oven.
 The nature of material required for the insulation is
determined by the maximum temperature of the inner face
of the layer of insulation of a heating chamber.
 The heating elements are mounted on top, sides or
bottom of the oven as the circumstances permit.
Heating Chamber
An enclosure of a charge to be heated by radiation or convection or by
the joint effect of these modes of heat transfer is known as heating
chamber.
The functions of the heating chambers are :
to control the distribution of heat within the chamber:
to control the cooling rate of charge, if required:
to confine the atmosphere around the charge: and
to store as much of the heat supplied as may be practicable and
economical.
Heating Element Materials
High resistivity: The material to be used for heating element should be
of high specific resistance so that a small length of wire may be
sufficient to produce the required amount of heat.
High melting point: Melting point of material to be used for heating
element should be high so that charge can be heated to a high
temperature.
Low temperature coefficient: The material for heating element should
have low temperature coefficient so that resistance may not vary with
the change in temperature otherwise starting current would be high.
Free from oxidation: The material for heating element should be such
that it may withstand the required temperature without getting
oxidised, otherwise it would have to be replaced frequently.
Maximum Operating Voltage
Maximum operating voltage is limited, by electrical
insulation at high temperatures and from safety
consideration, to 600 volts. This value may exceed
in special cases. Resistor winding may be in one,
two or more circuits and may be connected either
to single or to polyphase supply system.
Design of Heating Element
Efficiency and Losses
The heat produced in the heating elements is also to overcome the
losses occurring due to
heat used in raising the temperature of oven or furnace;
heat used in raising the temperature of the containers or carriers;
heat conducted through the walls;
escapement of heat due to opening of door; in addition to heat
required to raise the temperature of the charge to the required value.
Resistance Heating-Flow Tubes
Temperature Control of
Resistance Furnaces
In resistance ovens/furnaces heat developed depends upon
I2R t or V2/R t. So there are three ways in which the
temperature can be controlled.
1. by varying the applied voltage to the elements or current
flowing through the element;
2. by varying the resistance of elements and
3. by varying the ratio of on and off times of supply.
Temperature can also be controlled by switching the various
combinations of groups of resistances used in the ovens or furnaces in
the following ways.
Use of Variable Number of Elements: In this method, the number of
heating elements in working are changed; so total power input or heat
developed is changed. This method does not provide uniform heating
unless the number of heating elements in the circuit at any particular
instant are distributed over the surface area, which requires
complicated wiring.
Change of Connections: In this method the elements are arranged to be
connected either all in series or all in parallel or combination of both or
in star or in delta by means of switches at different instants according to
the requirements. This is the simplest and most commonly used
method of control.
Causes of Failure of Heating Elements
1. Formation of Hot Spots
2. Oxidation and Intermittency of Operation
3. Embrittlement Due to Grain Growth
4. Contamination and Corrosion
 Induction heating
Induction heating is a non-contact heating process. It uses high
frequency electricity to heat materials that are electrically conductive.
Placing a metal body in an alternating magnetic field creates eddy
currents, causing losses through which the metal is heated.
 Why induction heating ?
All electrically - conductive materials can be heated
quickly and cleanly with pollution free induction
heating.
Features include :
◦ Selected parts of the work piece can be heated.
◦ Heating time and temperature can be precisely controlled.
◦ There is no smoke pollution.
◦ Heating operation can be integrated in semi - automatic
production sequences.
◦ Induction equipment can usually be operated by unskilled
personal.
 Induction Furnaces
For Curing, Brazing, Soldering, Melting and Drying
Melting ferrous and non - ferrous metals with
temperatures up to 18000c.
Heating for forging up to 12500c.
Surface hardening of steel or cast iron workpieces
at 8500c to 9500c.
Soft soldering and brazing up to 11000c.
Recrystallisation.
Tube and Pipe Welding.
ADVANTAGES
Fast Startup & Shutdown
Low Energy Cost
Compact Size
Precise Heating Control
Easy to Adjust
On-Off Operation
Environmentally Sound
 Principle of operation
The induction (generation) of the electrical current in a
conductive metal (charge) placed within a coil of
conductor carrying electrical current is known as
electromagnetic induction of secondary current.
The alternating current applied to the coil produces a
varying magnetic field which is concentrated within the
helical coil. This magnetic field passing through the
charge induces secondary current in the charge piece.
The current circulating in the charge produces electrical
(I2R) losses which heat the charge and eventually melt it.
Basic elements of induction heating
 How does Induction Heating work ?
Electromagnetic Induction: Induction heating uses electromagnetic fields
to heat materials.
Electric Current: An alternating current (AC) passes through a coil,
creating a changing magnetic field around it.
Induced Currents: The changing magnetic field induces electric currents
(called eddy currents) in the material placed inside or near the coil.
Heat Generation: The resistance of the material to these eddy currents
generates heat.
Heating Effect: The material heats up due to this resistive heating,
allowing for various applications like metal hardening or melting.
Control: The amount of heat can be controlled by adjusting the
frequency of the AC or the strength of the magnetic field.
Core Type Furnaces
1. Direct Core Type Induction Furnace
2. Vertical Core Type Induction Furnace
3. Tama Furnace
4. Indirect Core Type Induction Furnace
1. Direct Core Type Induction Furnace
It consists of an iron core, crucible of some insulating material and
primary winding connected to an ac supply. The charge is kept in the
crucible, which forms a single turn short-circuited secondary circuit. The
current in the charge is very high, of the order of several thousand
amperes.
2. Vertical Core Type Induction Furnace
 Known as Ajax Wyatt vertical core type furnace employs a
vertical channel instead of horizontal one for the charge.
 The convection currents keep the circulation of molten
metal round the V portion. As V-channel is narrow, so even
a small quantity of charge is sufficient to keep the
secondary circuit closed. Hence the chances of discontinuity
of the circuit are less.
Due to pinch effect the adjoining molecules carrying
current in same direction will try to repel each other, but
because of the weight of the charge they will remain in
contact and chances of interruption will be reduced.
AJAX Furnace
16 TON STUDLESS Furnace Coil
 3. Tama Furnace
The main drawback of Ajax Wyatt furnace is that it cannot be used for
melting aluminium and its alloys. This is mainly because of tendency of
anderence, of any aluminium oxide formed, to the walls of V channel. This
makes the operation of furnace impossible any longer. This difficulty,
however, can be overcome by using a ring type furnace, known as
the Tama furnace
4. Indirect Core Type Induction Furnace

 In such a furnace an inductively heated clement is made to transfer its

heat to the charge by radiation.
 It consists of an iron core linking with the primary winding and
secondary also. In this case secondary consists of a metal container
forming the walls of the oven proper. Primary winding is connected to
the ac supply, inducing currents and heating the metal container.
 Heat is transmitted to the charge by radiation.
 Coreless Induction furnace
It essentially consists of three main parts (i) the primary coil

(ii) the refractory container and

(iii) the frame which includes supports and a tilting mechanism.

Coreless Induction Furnace
Working Principle
 Induction Coil: The furnace uses a coil made of conductive material,
through which an alternating current (AC) is passed. This generates a
rapidly changing magnetic field.
 Induction Heating: The magnetic field induces eddy currents in the
metal charge placed inside the coil. The resistance of the metal to these
eddy currents generates heat, melting the metal.
 Molten Metal: The metal heats up and melts due to this resistive
heating, and can be maintained at the desired temperature for various
processes, such as alloying or casting.
 Arc Furnace
 When a high voltage is applied across an air gap, the air in the gap gets
ionised under the influence of electrostatic forces and becomes conducting
medium. Current flows in the form of a continuous spark, called the arc.
 It is to be noted that a very high voltage is required to establish an arc across
an air gap but to maintain an arc small voltage may be sufficient.
 Alternatively an arc can also be produced by short circuiting the two
electrodes momentarily and then withdrawing them back.
In this method of striking an arc, high voltage is not required.
Arc drawn between two electrodes produces heat and has a temperature
between 1,000 °C and 3,500 °C depending on the material of the electrodes
used. The use of this principle is made in electric arc furnace.
Types of Arc furnaces
1. Direct Arc Furnace
2. Indirect Arc Furnace
Direct Arc Furnace
It is commonly used for production of steel. The usually size of such a
furnace is between 5 and 10 tonnes.
 Electrodes and Charge: In a direct arc furnace, the charge
(material to be melted) acts as one of the electrodes, and
the arc is formed between the electrodes and the charge.
 Arc Heating: The arc, which is in direct contact with the
charge, heats the charge to very high temperatures.
Additional heat is produced by the current flowing through
the charge itself.
 Electrode Arrangement:
Single-Phase Furnace: Uses two electrodes placed vertically
downward through the furnace roof, touching the charge surface.
 Three-Phase Furnace: Uses three electrodes arranged in an
equilateral triangle configuration, forming three arcs that heat the
charge uniformly.

Electromagnetic Stirring: The current passing through the

charge generates an electromagnetic field that creates
stirring action, promoting uniform heating of the charge.
Indirect Arc Furnace
In an indirect arc furnace, the arc is formed between two
electrodes positioned above the charge, with heat
transmitted to the charge solely through radiation.
Since the current does not flow through the charge, it heats
up to lower temperatures compared to a direct arc furnace.
The furnace requires mechanical rocking for stirring, as no
electromagnetic stirring is present. This rocking action
promotes thorough mixing of the charge.
Low melting point. So these furnaces are used for melting
non-ferrous metals.
The capacity of furnace varies from 0.25 tonne to 3 tonnes.
Direct-current electric arc furnace
A.C. electric arc furnace with eccentric bottom tap hole
Conventional continuous casting of slabs (discharge roller table)
Dielectric Heating
 Dielectric Heating
High Frequency Capacitive Heating
Used for heating of insulating materials: Wood, Plastic,
Ceramic, Glass
Insulating materials ← High Voltage
Dielectric Loss↑
Heat ↑
Temperature ↑
Dielectric Loss α Frequency, Voltage
20 KV & 10-30 MHz
 Applications
Food Processing
Wood Processing
Chocolate Industry
Plastic Industry
Plywood Industry
Book Binding
Bakeries
Coffee Roasting
 Advantages
High Uniformity
Best Quality
Limited Space
Economical
Easy Maintenance
Easy Operation
Increase in Production