2. What is Electric Heating?
• Electric heating is a process in which ELECTRICAL
ENERGY is converted to “HEAT ENERGY”.
• It covers those engineering processes by which this
conversion is used effectively to fulfill our need.
3. Why Electric Heating?
• Clean and atmosphere / Free from dirt.
• No pollution / No flue gas is produced
• Ease of control
• Cost effective
• Comparatively better working condition
• Localized application
• Overall efficiency is much higher
• Uniform heating
• Heating of non-conductive material
4. What are the types?
• Resistance Heating
• Arc Heating
• Induction Heating
• Dielectric Heating
• Electron Bombardment Heating
5. Resistance Heating
• When current passes through a resistance ,Power loss takes
• place there in ,which appears in the form of heat
• Electrical energy converted into heat energy
H = I2Rt
Power loss = I2R watts
= VI Watts
= V2/R watt
Direct Resistance Heating & Indirect Resistance Heating
6. Direct Resistance Heating
• Current is passed
through the material or
charge to be heated
• Charge is considered in
a furnace and two
electrodes or three
electrodes for 3phase
are immersed in the
charge
A.C.or D.C
High resistance powder
charge
furnace
8. Salt Bath Furnace (contd…)
• The furnace has a ceramic or metal container filled with
molten salt.
• Bath’s composition consists of one or more salts, nitrates,
chlorides, carbonates, cyanides, caustics, or additives (
sulphates, fluorides, etc.) in small amounts.
• Heat is transferred between work and molten salt by
conduction.
• The salt lends itself to either heating or cooling within a 300o
to 2400o range, as required.
9. Salt Bath Furnace (contd…)
• Used for
i. Carbonizing
ii. Tempering
iii. Quenching
iv. Hardening
• Eliminates Oxidation
• Rapid and Uniform Heating
10. Salt Bath Furnace (contd…)
• Voltage required : 2 – 20V, Current : upto 3kA
• Supply: Step-down transformer having tap changing Gear
• Temperature increases, resistance goes down
• Starts with highest tap, gradually brought down to lowest tap
• Current is controlled by
Voltage
Depth of immersion
distance between electrodes
11. Salt Bath Furnace (contd…)
• To start with
• Current conducting bath is required between electrode
because the dry lumpy salt does have conductivity
• Bridge by a piece of carbon
• Transferring molten salt from an already running furnace
• For high conductive material, highly resistive powder is
sprinkled
12. Indirect Resistance Heating
• In this method of heating,
current is passed through
a high resistance wire
known as heating element
• The heat produced in the
element is transferred to
the charge by radiation or
convection methods
A.C or D.C
Chamber
Heating element
Charge
Furnace
13. Applications
• Room heaters
• Bimetallic strip used in starters
• Water heater i.e. immersion heater
• Ovens like domestic cooking
• Soldering iron
• Flat iron
• Electric kettle
14. Properties of good heating
elements
• High resistivity
• High melting point
• Should not oxidize in operating temperature
• Low temperature coefficient of resistance
15. Heating Elements
• Metal
• CuproNickel : 75% Cu + 25% Ni , upto 400
o
C
Constantan ( 55% Cu + 45% Ni)
• NiCrFe Alloy : 60% Ni, 24% Fe, 16% Cr, upto 950
o
C
• Nichrome : 80% Ni + 20% Cr, upto 1150
o
C
• Kanthal (FeCrAl) : 65-75% Fe + 20-30% Cr + 5% Al, upto 1425
o
C
16. Heating Elements
• Non Metalic
Molybdenum Disilicide (MoSi2), upto 1800
o
C
• PTC Heating elements
PTC rubber, upto 80
o
C - Due to the exponentially
increasing resistivity the heater can never be warmer than this
temperature.
It is a point wise self-regulating heater and self-limiting
heater.
17. Control of Heat
• By Varying Voltage
By autotransformer or induction regulator
By series impedance
By dedicated variable voltage supply
By using power electronic converter
By periodically switching off the supply (Integral Cycle
Control)
• By varying resistance of elements
By varying number of heating elements
By series-parallel of star-delta arrangement of the
elements
