This document provides an overview of electric heating methods. It discusses several types of electric heating including direct resistance heating, indirect resistance heating, and resistance ovens/furnaces. Direct resistance heating involves passing current directly through the material to be heated. Indirect resistance heating uses a heating element in contact with the material. Resistance ovens and furnaces are constructed of insulating materials and use heating elements mounted on the top, sides or bottom. The document also covers properties required for heating elements and the design considerations involved.

1. ONLINE CLASS
ON
ELECTRIC HEATING
SUBJECT : UEET
DATE : 25-03-2020

2. INTRODUCTION
• Heating is required for:
• Domestic purpose such as cooking
• Industrial purpose such as melting of metals, hardening & tempering, drying and welding.
Economical
Cleanliness
Absence of Flue Gases
Ease of Control
Automatic Protection
ADVANTAGES OF ELECTRIC
HEATIING
Upper Limit of temperature
Special Heating Requirements
High Efficiency of Utilization
Better working conditions
Safety

3. MODES OF TRANSFER OF HEAT

4. POWER
FREQUENCY
HEATING
RESISTANCE HEATING DIRECT RESISTANCE
INDIRECT RESISTANCE
INFRARED/RADIANT
ARC HEATING DIRECT ARC
INDIRECT ARC
HIGH
FREQUENCY
HEATING
INDUCTION HEATING DIRECT INDUCTION
INDIRECT INDUCTION
DIELECTRIC HEATING
EDDY CURRENT HEATING
CLASSIFICATION OF ELECTRICHEATINGMETHODS

5. DIRECT RESISTANCE HEATING
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 and three electrodes
are immersed in the charge if it is 3-phase supply.
The current flow throughs the charge and heat is produced.
This method has high efficiency since heat is produced in the charge itself.
As the current is not variable, hence automatic temperature control is not possible.
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.

6. INDIRECTRESISTANCE HEATING
The current is passed through a wire or other high resistance
forming a heating element.
The heat proportional to I2R losses produced in the heating
element is delivered to the charge by one of the mode of
transfer of heat (conduction/convection/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
This arrangement provides a uniform temperature.
Automatic temperature control can be provided in this case.
This method of heating is used in Room Heaters, Immersion Water heater & in various
Resistance Ovens for cooking and salt bath.
For industrial purposes, where a large amount of charge is to be heated, the heating element
is kept in a cylinder surrounded by jacket containing the charge.

7. RESISTANCE OVENS
• Resistance furnaces may be classified according to their operating temperature.
• Low temperature heating chamber with provision for ventilation is termed as oven.
• Resistance ovens are used for drying and baking potteries, drying varnish coatings,
vulcanizing and hardening of synthetic materials and commercial and domestic heating.
They are also employed for tempering hardened steel pieces.
• Medium temperature furnaces having operating temperatures between 300°C and
1,050°C are employed for annealing, normalizing of steel and non-ferrous metals,
melting of non-ferrous metals and stove enameling.
• 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.

8. RESISTANCE FURNACE
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.
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.

9. • Should have HIGH SPECIFIC RESISTANCE, so that small length of
wire may be sufficient to produce required amount of heatHIGHRESISTIVITY:
• Melting point of material to be used for heating element should
be high so that change can be heated to a high temperature.High Melting Point:
• 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.
Low Temperature
Coefficient
• The material for heating should be such that it may withstand the
required temperature without getting oxidized, otherwise it would
have to be replaced frequently.
Free from
Oxidation:
PROPERTIES OF HEATING ELEMENT

10. DESIGN OF HEATING ELEMENT
Heat
dissipated
according to
Stefan’s Law
H=5.72𝑘𝑒
𝑇1
100
4
−
𝑇2
100
4
𝑤𝑎𝑡𝑡𝑠/𝑚2
Electrical input
P=
𝑉2
𝑅
……………………………..(1)
Resistance of
heating
element
R=
𝜚𝑙
𝑎
=
𝜚𝑙
𝜋𝑑2
4
=
4𝜚𝑙
𝜋𝑑2…………(2)

11. DESIGN OF HEATING ELEMENT
Electrical input
P=
𝑉2
4𝜚𝑙
𝜋𝑑
2
=
𝜋𝑑2
𝑉2
4𝜚𝑙
𝑙
𝑑2 =
𝜋𝑉2
4𝜚𝑃
…………(3)
Surface area= 𝜋𝑑𝑙
Heat dissipated=
𝜋𝑑𝑙 H
Since at Steady
Temperature
Power Input=Heat
dissipated
P= 𝜋𝑑𝑙 H
∴
𝜋𝑑2
𝑉2
4𝜚𝑙
= 𝜋𝑑𝑙 H
𝑑
𝑙2 =
4𝜚𝐻
𝑉2 …………(4)

12. DESIGN OF HEATING ELEMENT