Utilization of Electrical Energy Unit 2
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.
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
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.
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 amount 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,
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
where ‘a’ and ‘b’ are the constants whose values are depend upon the heating surface and T1
and 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.
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 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:
High-specific resistance
Material should have high-specific resistance so that small length of wire may be required
to provide given amount of heat.
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.
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
Low temperature coefficient of resistance
From 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
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
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
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. 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-φ supply. 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.
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 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.
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
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.
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
sufficien....
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
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
Induction heating
Heat developed in the disc is depending upon the following factors.
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:
i. Direct core type.
ii. Vertical core type.
iii. Indirect core type.
(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
Direct core type furnace
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
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:
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.
Coreless type induction furnace
It is a simple furnace with the absence core is shown In this furnace, heat developed in the
charge due to eddy currents flowing through it.
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.
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
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 t
is the time for which current flows.
Here, the total resistance offered to the flow of current is made up of:
i. The resistance of current path in the work.
ii. The resistance between the contact surfaces of the parts being welded.
iii. 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.
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.
the energy input to the welding process, welding strength, and welding current vary with
welding time.
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
Utilization of Electrical Energy Unit 2
CHOICE OF WELDING TIME
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.
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 work piece and high
energy expenditure. From, 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:
M.Vinay Kumar ,Asst Prof, EEE Dept,GMRIT,Rajam
