Introduction
Electric heating is regarded as a highly efficient thermal process, capable of converting nearly
100% of electrical energy directly into heat with minimal energy loss. Its widespread use spans
residential, commercial, and industrial sectors covering applications such as space and water
heating, cooking, and high-temperature processing. At the heart of these systems is an electric
heater composed of a resistive element that functions based on the principle of Joule heating,
where electrical energy is converted into heat as current flows through the resistor. Modern
devices often utilize nichrome wire as the primary heating element, supported by ceramic
insulators to enhance thermal performance and electrical safety.
Electric heating methods such as electric radiators, underfloor heating systems, and portable
space heaters are especially favored in regions where gas heating is impractical, due to their
convenience, ease of control, and environmental cleanliness. Beyond domestic use, electric
heating plays a critical role in industrial processes. These include the melting and heat treatment
of metals such as annealing, tempering, soldering, and brazing along with glass moulding, the
baking of insulating materials, and the enamelling of copper wires. The adaptability, cleanliness,
and precision of electric heating make it a preferred solution in modern manufacturing
environment.
Research Objectives
To examine the principles and mechanisms of electric heating, including the modes of heat
transfer such as conduction, convection, and radiation.
To classify and analyze various electric heating methods (resistance, arc, induction, dielectric,
and infrared heating) and their respective operational principles.
To investigate both domestic and industrial applications of electric heating, highlighting specific
use cases like cooking, water heating, metal treatment, and glass moulding.
To evaluate the advantages of electric heating over traditional methods, and analyze the
performance, efficiency, and design requirements of heating elements and resistance furnaces.
To propose practical strategies for enhancing the efficiency, reliability, and sustainability of
electric heating systems in modern applications.
What is electric heating
Electric heating is the process of converting electrical energy into heat energy for domestic,
commercial, or industrial use.
Principles and mechanism
Electric heating operates on the fundamental principle of conversion of electrical energy into
heat energy, primarily through resistance. When an electric current passes through a resistive
material, heat is generated due to Joule’s effect, also known as the I²R loss, where the amount of
heat (H) is proportional to the square of the current (I), the resistance (R), and the time (t) the
current flows.
Electric heating can be achieved through various mechanisms, depending on how the heat is
transferred and the type of material involved.
The major mechanisms include:
1. Conduction
In this mode of heat transfer, one molecule of the body gets heated and transfers some of the heat
to the adjacent molecule and so on, Theraja B L and Theraja A K (2005).
In this mode, heat transfers from one part of the substance to another part without the movement
in the molecules of substance. The rate of conduction of heat along the substance depends upon
the temperature gradient.
Consider a solid material of cross-section A sq.m. and thickness x metre as shown in Fig.1. If T1
and T2 are the temperatures of the two sides of the slab in °K, then heat conducted between the
two opposite faces in time t seconds is given by:
KA ( T 1−T 2) × t
H=
x
where K is thermal conductivity of the material.
Convection
In this mode, heat transfers from one part of the substance to another part of a substance or fluid
due to actual motion of the molecules. The rate of heat depends mainly on the difference in the
fluid density at different temperature. This process is applied in the heating of water by
immersion heater or heating of buildings. The quantity of heat absorbed by the body by
convection process depends mainly on the temperature of the heating element above the
surroundings and upon the size of the surface of the heater. It also depends, to some extent, on
the position of the heater. The amount of heat dissipated is given by
H=a ( T 1 −T 2 )
where a is constants and T1 and T2 are the temperatures of the heating surface and the fluid in
°K respectively. In electric furnaces, heat transferred by convection is negligible.
Radiation
In this mode, the heat transfers from source to the substance to be heated without heating the
medium in between. The rate of heat emission is given by Stefan’s law according to which Heat
dissipated,
[( ) ( ) ]
4
T1 T2
H=5.72 eK − W/m2
100 100
where K is radiating efficiency and e is known as emissivity of the heating element.
Advantages of Electric Heating
The following are the advantages of Electric Heating
Cleanliness: It completely eliminates dust and ash and keeps the surroundings clean.
No pollution: Due to the absence of fur gases, there is no risk and contamination of the
atmosphere.
Ease of control: It is possible to control and regulate the temperature accurately either manually
or fully automatically. This is not possible with non-electrical heating.
Uniform heating: The charge can be heated uniformly throughout whether the charge is
conducting or non-conducting material.
High efficiency: The overall efficiency of electric heating is high since the heat can be produced
directly with the charge itself.
Low attention: Electric heating generally does not require continuous attention.
Localized heating: A particular job can be heated up to a particular depth for heat treatment.
Better working conditions: Electric heating produces no irritating noise.
High Temperature: A particular job can be heated up to a particular depth for heat treatment.
High temperature: High temperature can be obtained by electric heating (the only thing is the
heating element should withstand the heat).
Less floor area is required: Due to the compactness of the electric furnace, the floor area required
is less.
No carrying expense: The heat can be developed at or close to the point of use.
Method of electric heating
Electric heating generates heat by passing current through resistive materials, causing I²R (Joule)
losses. Heat may also be produced via eddy currents, hysteresis in magnetic materials, molecular
friction (dielectric heating), or electric arcs. High-energy particle bombardment is another
heating method. Common types include resistance, arc, induction, dielectric, and infrared
heating.
What is Heating Effect of Electric Current?
When a current-carrying conductor is placed in the circuit, when the circuit is closed, the
conductor dissipates heat inside it, due to the resistance. If there is no insulation then the heat
generated from the conductor is used for heating purposes. This phenomenon is called What is
Heating Effect of Electric Current or Joules law of Heating.
It depends on the:
o The resistance of the conductor
o The time duration of the current applied for
o Amount of current flowing
From the above considerations, the Heat generated is given as the heating effect of the
electric current formula H=I2Rt.
Electric Heating Devices
Electric Heating devices are used for different purposes, some of the devices are:
Electric heating pad
Furnace
Oil paths
Drying oven
Hot plates
Types of Electric Heating
Heat can be produced by the circulation of current through a resistance or induced eddy currents.
Production of an arc between two electrodes also develops heat. Bombardment by some high
energy particles like αα, ββ, γγ, and x-rays or accelerating ions can produce heat on a surface.
There are several types of electric heating systems that can be used to provide warmth in
buildings and homes. Some of the most common types of electric heating include:
o Electric baseboard heaters
o Electric radiant heaters
o Electric furnaces
o Electric heat pumps
o Electric space heaters
Electric baseboard heaters: These are typically installed along the bottom of walls and provide
heat by convection. They work by heating the air near the heater, which then rises and circulates
throughout the room.
Electric radiant heaters: These heaters work by emitting infrared radiation, which heats objects
and people directly. They are often used in outdoor settings, such as patios or decks.
Electric furnaces: These are central heating systems that use electricity to heat air, which is then
distributed throughout a building through ducts.
Electric heat pumps: These systems work by extracting heat from the air outside and transferring
it inside. Electric heat pumps are highly efficient and can be used for both heating and cooling.
Electric space heaters: These portable heaters are ideal for heating small areas or rooms. They
come in a variety of styles, including fan-forced heaters and oil-filled radiators.
Overall, electric heating systems are a convenient and efficient way to provide warmth in homes
and buildings. The type of system you choose will depend on your specific heating needs and
preferences.
Resistance Heating
Resistance Heating
When the current passes through a resistance, power loss takes place therein, which appears in
the form of heat.
2
V
Power loss = I2Rwatt = VI watt = watt
R
where
R = resistance of the element (Ω)
V = voltage (volt)
I = current (ampere)
All the electrical energy given to a resistance heating element will be converted to heat energy.
The loss of energy takes place only in transferring heat from the element to charge or load.
Resistance heating is further classified as
o Direct resistance heating
o Indirect resistance heating
o Infrared or Radiant heating.
Direct Resistance heating
In this method of heating, the current is passed through the material or charges to be heated. The
charge is considered in a furnace and two electrodes (or more electrodes for 3-phase) are
immersed in the charge The supply AC or DC is given to the electrodes as shown in the below
figure.
The resistance offered by the charge to the flow of current causes power loss I2R and it results in
the heating of the charge. The charge may be in the form of solid/melted pieces, powder, or
liquid. When solid/metal pieces are to be heated a powder of high-resistivity material is sprinkled
over the surface of the charge to avoid a direct short circuit. Then the current passes through the
charge and heat is produced. This method has quite a high efficiency since heat is produced in
the charge itself.
This method of heating is employed in resistance welding, in the electrode boiler for heating
water, and in salt bath furnaces, Salt bath furnace is used for hardening steel tools and prevents
oxidation during hardening. This arrangement provides high and uniform temperatures.
Indirect Resistance Heating
In this method, the current is passed through a high-resistance wire known as the heating element
as shown below. The heating element can be placed above or below the furnace/charge. Usually,
charge will enclose the heating element for efficient heat transfer. The heat produced in the
element is transferred to the charge by radiation of convection methods. This method of heating
is used in room heaters, in bimetallic strips used in starters, immersion water heaters, and in
various types of resistance ovens used in domestic coating and industrial wall bath furnaces, etc.
This arrangement provides uniform specific, Automatic temperature control can be provided in
this method.
Infrared or Radiant Heating
In radiant heating, the heating element consists of tungsten filament lamps together with
reflectors to direct all the heat onto the charge. These lamps operate at 2300°C giving a greater
portion of infrared heat rays. Radiant heating is mainly used for drying enamel or painted
surfaces. A high concentration of radiant energy enables 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.
Requirements for a Good Heating Material
The material used for the heating element should possess the following properties:
High specific resistance
Low-temperature coefficient of resistance
High melting point
Free from oxidation
Non-corrosive
Ductile
Positive temperature of the resistance
High mechanical strength
Economical
Materials of Heating Elements
No single metal will satisfy all the requirements of heating material. The materials normally used
as heating elements are either alloys of Nickel-chromium, Nickel- chromium iron, Nickel-
chromium aluminum, or Nickel-copper. The use of iron in the alloy will cheapen the final
product but, reduces the life of the alloy as it gets oxidised soon.
S.no Types of composition Trade or Specific Specific Maximum
alloys commercial Resistance gravity Operating
name at @ 20 C Temperature
in (µƱ-m) C
1 Nickel- 80% Ni Nichrome 1.03 8.35 1150
Chromium 20%Cr
2 Nickel- 60% Ni 1.06 8.27 950
Chromium 16% Cr
Iron 24% Fe
3 Nickel- 45% Ni Constantan 0.49 8.88 400
Copper 55% Cu
4 Iron- 65-75% Fe Kanthal 11.4 7.2 1150-1350
Chromium 20-30% Cr
Aluminum 5% Al
Heating element may fail due to any one or more of the following reasons:
Formation of hotspots.
Oxidation of the element and intermittency of operation.
Embrittlement caused by grain growth.
Corrosion.
Design of Heating Element
The design of the heating element for an electric furnace is to determine the size and length of
the element. The element can be designed only if the operating voltage, ambient temperature,
and heating element temperature, etc. are known. Circular and ribbon resistance elements are the
shapes in which heating element is designed and are in common use,
Let
P= electrical power input per phase (watt)
V = operating voltage per phase (volt)
R = resistance of the element
l = length of the element m
a = cross-sectional area of the element
d = diameter of the element
ρ= specific resistance of the element material
T1 = absolute temperature of the element
T2 = absolute temperature of the charge
e = emissivity= 1.0 for black body and 0.9 for resistance heating elements.
k = radiating efficiency= 1 for a single element and 0.5 to 0.8 for many elements.
H = heat dissipated (W/m^2)
Electrical power input,
According to Stefan’s Law, the heat dissipated by radiation is given as
Total heat dissipation = H x surface area of the element
Applications of Resistance Heating
Resistance heating has various applications as follows:
Direct Resistance heating Applications
Scrap heating: Heating some metal scrap and flue gases
Resistance welding: To join the metals at high temperatures
Salt water bath furnace: For tool hardening, and removing rust
Used as Electrode boiler for water heating etc.
Indirect Resistance Heating
Water heaters: to heat water in required quantity and required temperature
Room heaters: To maintain the temperature of room and warmness in the room
Starters (bimetallic strip): Used in electric irons and starters
Resistance ovens: heating the food items
Hot plate: these are similar to stove for making dishes and cooking
Used in Domestic cooking applications.
Temperature Control of Resistance Heating
The supply voltage and resistance of heating elements are independent parameters whereas the
current is a dependent parameter. Therefore, the temperature of the resistance furnace can be
controlled firstly by varying the supply voltage and later by varying the resistance of heating
elements and thereafter by switching ON and OFF the supply.
Electric Arc Furnace
When the electric supply given to two electrodes is increased, and are separated in the air from
each other, a stage arises that increases the voltage gradient in the air, and it becomes a good
conductor of electricity. This is called an Arc. It exists when current passes through an air gap. It
should be noted that a very high voltage is required to establish an arc across the air gap but a
small voltage may be sufficient to maintain the arc. With high voltage also arc can be established
by short-circuiting the two electrodes momentarily and withdrawing them back slowly. The
temperature of the arc developed will be around 3500°C, the process can be carried out between
1500°C and 2500°C. As mentioned earlier there are two types of Electric Arc furnaces namely:
Direct arc furnaces
Indirect arc furnace
Direct Arc Furnace
The below figure illustrates the direct arc furnace. In Figure (a), there are two electrodes. When
supply is given to the electrodes, two arcs are established and current passes through the charge.
Heat is developed due to radiation from the arcs and a small amount of heat is also developed
due to the electrical resistance (I 2R) of the charge. In Figure (b), only one electrode is used hence
one arc is formed. Single-phase direct arc furnaces are commonly known as baby arc furnaces,
used in R and pilot production plants. For large capacity furnaces, a 3-phase supply is generally
employed and is obtained from the secondary winding of a three-phase transformer. The three
electrodes are placed at the three vertices of an equilateral triangle (the charge forms a star
point).
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 set up by the current. This results
in uniform heating of the charge.
Indirect Arc furnace
In this type of furnace, the arc exists between two electrodes and the heat developed in the
charge is purely by the radiation from the arc as shown in the figure below. Since the charge is
heated due to radiation only, the temperature of the charge is lower than that in the direct arc
furnace. As current does not flow through the charge, there is no inherent stirring action provided
and the furnace must be rocked mechanically. Hence, these furnaces are also referred to as
rocking furnaces. That is why indirect arc furnaces are made of cylindrical or spherical shapes.
An electric motor is used to operate suitable grinders and rollers to provide rocking action to the
furnace.
Fig. 47.11
