EE3029D Electric Power Utilization
MODULE 02
Reference Books:
1. Taylor E Openshaw, Utilisation of Electric Energy, Orient Longman,1986.
2. J B Gupta, Utilization of electric power and electric traction, S K Kataria & Sons, 2002.
Electric Heating
Electric heating is extensively used both for domestic and industrial applications. Domestic applications include (i)
room heaters (ii) immersion heaters for water heating (iii) hot plates for cooking (iv) electric kettles (v) electric irons
(vi) pop-corn plants (vii) electric ovens for bakeries and (viii) electric toasters etc.
Industrial applications of electric heating include (i) melting of metals (ii) heat treatment of metals like annealing,
tempering, soldering and brazing etc. (iii) moulding of glass (iv) baking of insulators (v) enamelling of copper wires
etc.
Basically, heat is produced due to the circulation of current through a resistance. The current may circulate directly
due to the application of potential difference or it may be due to induced eddy currents. Similarly, in magnetic
materials, hysteresis losses are used to create heat. In dielectric heating, molecular friction is employed for heating
the substance. An arc established between an electrode and the material to be heated can be made a source of heat.
Bombarding the surface of material by high energy particles can be used to heat the body.
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Electric Heating
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Electric Heating
Direct Resistance Heating
In this method the material (or charge) to be heated
is treated as a resistance and current is passed
through it. The charge may be in the form of powder,
small solid pieces or liquid. The two electrodes are
inserted in the charge and connected to either a.c. or
d.c. supply. Obviously, two electrodes will be required
in the case of d.c. or single-phase a.c. supply but
there would be three electrodes in the case of 3-
phase supply. When the charge is in the form of small
pieces, a powder of high resistivity material is
sprinkled over the surface of the charge to avoid
direct short circuit. Heat is produced when current
passes through it. This method of heating has high
efficiency because the heat is produced in the charge
itself.
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Electric Heating
Indirect Resistance Heating
In this method of heating, electric current is
passed through a resistance element which placed
in an electric oven. Heat produced is proportional
to I2R losses in the heating element. The heat so
produced is delivered to the charge either by
radiation or convection or by a combination of the
two.
Sometimes, resistance is placed in a cylinder
which is surrounded by the charge placed in the
jacket as shown in the Fig. This arrangement
provides uniform temperature. Moreover,
automatic temperature control can also be
provided.
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Electric Heating
Requirement of a Good Heating Element
Indirect resistance furnaces use many different types of heating elements for producing heat. A good heating element
should have the following properties :
(1) High Specific Resistance. When specific resistance of the material of the wire is high, only short length of it will be
required for a particular resistance (and hence heat) or for the same length of the wire and the currrent, heat produced will
be more.
(2) High Melting Temperature. If the melting temperature of the heating element is high, it would be possible to obtain
higher operating temperatures.
(3) Low Temperature Coefficient of Resistance. In case the material has low temperature coefficient of resistance, there
would be only small variations in its resistance over its normal range of temperature. Hence, the current drawn by the
heating element when cold (i.e., at start) would be practically the same when it is hot.
(4) High Oxidising Temperature. Oxidisation temperature of the heating element should be high in order to ensure longer
life.
(5) Positive Temperature Coefficient of Resistance. If the temperature coefficient of the resistance of heating element is
negative, its resistance will decrease with rise in temperature and it will draw more current which will produce more
wattage and hence heat. With more heat, the resistance will decrease further resulting in instability of operation.
(6) Ductile. Since the material of the heating elements has to have convenient shapes and sizes, it should have high ductility
and flexibility.
(7) Mechanical Strength. The material of the heating element should posses high mechanical strength of its own. Usually,
different types of alloys are used to get different operating temperatures. For example maximum working temperature of
constant an (45% Ni, 55% Cu) is 400°C, that of nichrome (50%, Ni 20% Cr) is 1150°C, that of Kantha (70% Fe, 25% Cr, 5% Al)
is 1200° C and that of silicon carbide is 1450°C.Module 02 _ EE3029D Electric Power Utilization 6
Electric Heating
Resistance Furnaces or Ovens
These are suitably-insulated closed chambers with a provision for ventilation and are used for a wide variety of
purposes including heat treatment of metals like annealing and hardening etc., stoving of enamelled wares, drying and
baking of potteries, vulcanizing and hardening of synthetic materials and for commercial and domestic heating.
Temperatures upto 1000°C can be obtained by using heating elements made of nickel, chromium and iron. Ovens using
heating elements made of graphite can produce temperatures upto 3000°C. Heating elements may consist of circular
wires or rectangular ribbons. The ovens are usually made of a metal framework having an internal lining of fire bricks.
The heating element may be located on the top, bottom or sides of the oven. The nature of the insulating material is
determined by the maximum temperature required in the oven.
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Electric Heating Temperature Control of Resistance Furnaces
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Electric Heating
Design of Heating Element
Under steady-state conditions, a heating element dissipates as much heat from its surface as it receives the power from
the electric supply. If P is the power input and H is the heat dissipated by radiation, then P = H under steady-state
conditions. As per Stefan’s law of radiation, heat radiated by a hot body is given by
heat dissipated by radiation per second per unit
surface area of the wire
where T1 is the temperature of hot body in °K and T2 that of the cold body (or cold surroundings) in °K
e: Emmissivity K: Radiating efficiency
Assuming heating element as wires of
circular cross-section
4 P
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Electric Heating
Total surface area of the wire of the element = (πd) × l
If H is the heat dissipated by radiation per second per unit surface area of the wire, then heat radiated per second
Ribbon Type Element
If w is the width of the ribbon and t its thickness, then
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Electric Heating
Heat lost from ribbon surface = 2wl H (neglecting the side area 2tl )
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For further tutorials and other topics,
Refer Class Note and Reference Books
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