electric heating

A.Srinivasan,SL/EEE,KLNCE,Madurai

ELECTRIC HEATING

Electric Heating means, Whenelectric current passed throughconductor or medium heat is produced.H= I2Rt Joules

APPLICATION

Heating is required for domestic purposes such as:

Cooking

Water heater

Room heater and

Heating of buildings.

Industrial purposes such as:

Extraction of metal from ores

Melting of metals

Hardening and tempering,

Drying

Making of plywood and welding etc.,


Advantage of Electric Heating

1. Transfer of heat is accurately controlled inheating by electrical methods.

2. Production of heat is not accompanied by anycombustion.

3. Any temperature can be reached, provided thematerial can withstand that temperature.

4. It is quite cleans

5. No flue gases are produced and ambienttemperature is not affected.

6. The desired temperature is obtained fast andtemperature control is easy.

7. It is easy to maintain and is safe.

8. The efficiency of electric heating is high.A.Srinivasan,SL/EEE,KLNCE,Madurai

Methods of Heat transfer

1.Conduction.

2.Convection.

3. Radiation.


Conduction

In this type of heat transfer ,one molecules of the substance getsheated and transfer of heat to theadjacent one and so on.

Thus heat is transferred througha substance from one part to anotheror between two substances in contact


ConvectionThis is a process where the

transfer of heat from one point toanother is duo to the actual motion ofthe molecules of a fluid. This is due tothe different in the fluid density atdifferent temperature. Thus, for natureconvection in air we have

Heat density H=3.875(T1-T2)1.25 Watt/m2


RADIATION

In this mode of heat transfer, the heatreaches the substances to be heatedfrom the source of heat without heatingthe medium in between

Rate of heat radiation is given bystepan’law, according to which


Requirements of heating material

High specific resistance. It should have high

specific resistance so that a short length of wire will be requiredfor a particular resistance or the same length and current theheat produced will be same .

High melting point. So that higher temperature

can be obtained.

Free from oxidation. It should not oxides at

higher temperature, otherwise its life will be shortened.

Low temperature coefficient ofresistance. So that its resistance does not change during

its temp. range of operation. Also with this the current drawn bythe element at cold will not be much different from that when itis hot.


Design of Heating element

Normally, the wires of circular cross section or rectangular crosssection ribbons are used as heating element.

Under steady state condition

A heating element dissipates as much heat from its surface as itreceives the power from the electric supply. Power input P=Heatdissipated

Heat dissipated according to Stefan's law

H=5.72X104Ke((T1/1000)4-(T2/1000)4)Watt/m2 1

Electrical input=V2/R

R=ρl/a= ρl/(πd2/4)=4ρl/ πd2 for a circular wire

Electrical input P=V2/(4ρl/ πd2 )= πd2V2 / 4ρl

l/d2= πV2 / 4ρP ---------------------------- 2

Surface area S=πdl

Heat dissipated=π d l H


Since at steady temperature

Power input P=Heat dissipated

P = π d l H

π d2 V2 / 4 ρ l = π d l H

d / l2 = 4 ρ H / V2 ----------------------- 3

Solving expression 2 and 3 length and diameter of wire can bedetermined.

For ribbon type of conductor let ω be the width and t be thethickness.

Electrical input P=V2/R=V2/(ρl/ a )= V2 /( ρl/ ωt)

= V2ωt / ρl

l / ωt = V2 / ρP

Since at steady temperature

Power input P=Heat dissipated (2ω l H )

V2 /( ρl/ ωt)= 2ω l H

t / l2 = 2 ρ H / V2 ------------------------------------- 4

So by solving the two equations 3 and 4 ,length l and width ωfor a ribbon of thickness t will be evaluated.


CLASSIFICATION OF

ELECTRIC HEATING


Classification of

Electric heating

Power frequency

heating

High frequency

heating

Resistance

heatingArc heating

Direct

resistance

Indirect

resistance

Induction

heating

Dielectric

heating

Direct induction Indirect inductionDirect arc Indirect arc

Infra red

or

radiant


Resistance heating

1. Direct resistance heating

D.C or A.C Supply

High Resistive

Powder

Charge

Electrodes


1. Direct resistance heating

The material or charge to be heated is taken asresistance and current is passed through it.

The charge may be in the form of powder, pieces or aliquid.

Two electrodes are immersed in the charge andconnected to supply in case of D.C or single phase acsupply.

When some pieces of metals are to be heated somehighly resistive powder is sprinkled over the surface ofpieces to avoid direct short circuit. The current flowsthrough the charge and heat is produced.

This method has high efficiency since heat is produced inthe charge itself.

It is used in salt bath furnaces and in the electrode boilerfor heating water.


Indirect heating

D.C or A.C Supply

Resistive

Element

Charge


Indirect heating …

The current is passed through a wire or other higherresistance material forming a heating element.

The heat proportional to I2R loss produced in the heatingelement is delivered to the charge by one or more of themodes of transfer of heat.

If the heat transfer is by conduction the resistor must be incontact with the charge.An enclosure known as heating chamber, is required for heattransfer by radiation and convection for the charge.

This arrangement provides a uniform temperature.Automatic temperature can be provided .

Applications: Room heaters, immersed water heaters,domestic and commercial cooking, and salt bath furnaces.


Infra red or Radiant heating

Heat energy from an incandescent lamp is focused

upon the body to be heated up in the form ofelectromagnetic radiations.

In this method heat is transferred from heatingelements to the charge purely radiation.

Heating element consists of tungsten filamentlamps together with reflectors to direct the whole ofthe heat emitted on to the charge.

The lamps are operated at 2300ºC instead of3000ºC giving greater proportion of infra red radiationand a longer life.

Applications: Dry the wet paints on an object, softeningof thermo plastic sheets, drying of paper, textiles etc..


ARC HEATING

The arc drawn between twoelectrodes develops hightemperature (3000ºC-3500ºC)depending upon the electrodematerial.


Direct arc furnaceIn this furnace , charge acts as one of the

electrodes and the charge is heated by producing arcbetween the electrodes and the charge.

Since, the arc is in direct contact with the chargeand heat is also produced by flow of current through thecharge itself, the charge can be, therefore, heated tohighest temperature.

current flowing thro the charge developselectromagnetic field and necessary stirring action is

automatically obtained by it.

Thus uniform heating.


Charge

Electrodes

ARC

Direct arc furnace


Application: Production of steel (Cupola method)

100% steel scrap which is cheaper thanpig iron where as the cupola requires a proportion ofpig iron in cupola charge.

Size of this furnace is 5 and 10 tonnes.

Power factor is 0.8

Direct arc furnace


Indirect Arc Furnace

The arc is formed between two electrodes above thecharge. Heat is transmitted to the charge solely byradiation.

Temperature of the charge is therefore lower thandirect arc furnace.

Charge

Arc Electrodes

Heating

chamber


Indirect Arc Furnace

Since, In this furnace current does not flowthrough the charge, so there is no stirring action andthe furnace is required to be rocked mechanically.

i.e The furnace is made of cylindricalshape, with the electrodes projecting through thechamber from each end and along the horizontalaxis.

Application: Melting of non-ferrous metals like Cu,

bronze,etc.

Size varies from 0.25 to 3 tonnes.

Power factor is 0.85


Induction heating

This heating process makes use of currentinduced by the electromagnetic action in the chargeto be heated.

Induction heating is based on the principle oftransformer action.

The primary winding which is supplied from an a.csource is magnetically coupled to the charge whichact as the short circuit secondary of single turn.

When an a.c voltage is applied to the primary, itinduces voltage in the secondary (i.e) charge.

Heat produced =V2/R


Types of induction heating

Core type furnaces

1. Direct core type induction furnace

2. Vertical core type induction furnace

3. Indirect core type induction furnace

Core less type furnaces


1.Direct core-type induction furnace

Su

pp

ly v

olt

ag

e

Magnetic core

Secondary

(charge)

Primary


The primary winding which is supplied from an a.csource is magnetically coupled (iron core) to the chargewhich act as the short circuit of single turn. The chargeis kept in the crucible.

The current in the charge is very high, of the order ofseveral thousand ampere.

Drawbacks 1.Magnetic coupling between py and sy circuit is poor,therefore, leakage reactance is high and power factor is low. itis overcome by employing supply of frequencies as low as 10hzfor operation of such furnaces. For obtaining low frequencysupply motor-generator set or frequency changer is required,which involves extra cost.

2. If the current density exceeds about 5A/mm2 the pinch effect(formation of bubbles and voids etc.) due to electromagneticforces may cause complete interruption of the secondarycircuit and so of the supply.

3. The crucible for charge is of odd shape and convenient fromthe metallurgical point of view.


Vertical core type induction furnace

This is an improved form furnace of direct core typeinduction furnace.

It employs a vertical channel instead of horizontal onefor the charge.

The convection currents keep the circulation of molten metalround the V portion.( In certain case U or rectangularshape

This V channel is narrow, so even a small quantity ofcharge is sufficient to keep the secondary circuit closed.

Hence the chances of discontinuity of the circuit are less.

Due to pinch effect the adjoining molecules carryingcurrent in same direction will try to repel to each other, butbecause of the weight of the charge they will remain incontact and chances of interruption will be reduced.


High frequency coreless Ajax-Wyatt furnace

Central

iron core

Outer iron

core

Refractory

lining

Charge

Primary winding

Secondary channel A.Srinivasan,SL/EEE,KLNCE,Madurai

Vertical core type induction furnace….Advantages:

1. Highly efficient heat, low operating cost and improvedproduction.

2. Accurate temperature control, uniform castings, reducedmetal losses and reduction of rejects.

3. Absence of crucibles.

4. Ideal working conditions in a cool atmosphere with no dirt,noise or fuel.

5. Absence of combustion gases resulting in elimination of themost common source of metal contamination

Application: Used for melting and refining of brass and otherheavy non-ferrous metals.

Its efficiency is 75%,Size:60 to 300kw,all1Ф,50hz,upto 600V


Coreless induction furnace

Crucible

Charge

Primary winding


Coreless induction furnace….

The furnace consists of a ceramic crucible cylindrical inshape enclosed within a coil which forms primary oftransformer & the charge in the crucible, the secondary of thetransformer.

The flux produced by the primary winding sets up eddycurrents in the charge which flow concentrically with those inthe primary winding.

These currents heat up the charge to the melting point &provide stirring action to the charge.

Since the frequency of the supply is high, the skin effect inthe primary coil increases the effective resistance of the coil& hence the cu losses tend to be high and artificial cooling isnecessary. The coil is ,therefore, made of hollow cuconductors through which cooling water can be circulated.


Coreless induction furnace….The stray magnetic field due to current in the primary coilmay induce eddy currents in the metal supporting structuresand cause over heating of these structures.

Advantages:

They are fast in operation.

It is used for all industrial applications.

The speed of heating and the precise control of power into thecharge result in uniform quality of product unattainable be anyother method of heating.

The working condition around the furnace are far better thanany other type of furnace.

Application:

Used for steel production.

Used for melting of non-ferrous metals like brass,bronze, Cu, aluminium etc..


DIELECTRIC HEATING

It is also called high frequency capacitive heating.

It is used for heating of insulating material (non-metallic) such as wood, plastics, ceramics, glass etc.

The material to be heated is placed as a slabbetween metallic plates or electrodes connected to highfrequency a.c supply. (from valve oscillator)

A.C Supply


DIELECTRIC HEATING

for producing sufficient heating frequency between 10and 30 MHz is used.

The current drawn by the capacitor, when an a.csupply voltage is applied across its two plates, does notlead the supply voltage by exactly 90º and there isalways an in phase component of the current.

Due to this in phase component of current, heat isalways produced in the dielectric material placed inbetween the two plates of the capacitor.

The electric energy dissipated in the form of heatenergy in the dielectric material is known as dielectricloss


DIELECTRIC HEATING

Equivalent circuit Phasor diagram

I

CR

IcIR

A.C Supply

Ic

IR

V

I

Ф


electric heating

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