43958566 electric heating

7/29/2019 43958566 Electric Heating

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LECTUREBY

EPSITA CHAKRABARTI


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WHAT IS ELECTRIC HEATING ?WHAT IS THE PRINCIPLE BEHINDIT ?Electric heating is any process in whichELECTRICAL ENERGY is converted to HEATENERGY.

Electric heating works on the principle ofJOULE HEATING: an electric current througha resistor converts electrical energy intoheat energy.

Lesson Plan 32


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EXAMPLES OF ELECTRICHEATING

Lesson Plan 32

DomesticImmersion Heaters forWater HeatingElectric ToastersElectric IronsElectric OvensRoom Heaters

IndustrialMelting of MetalsMoulding of GlassEnamelling ofCopper WiresWeldingHeat TreatmentProcess


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Lesson Plan 32

ADVANTAGES OF ELECTRICHEATINGFree from dirt

No flue gas is producedResponds quicklyAccurate temperature control can bemade easily

Comparatively safeOverall efficiency is much higher


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TRANSFER OF HEATConductionThis phenomenon takes place in solid, liquid and gasHeat transfer is proportional to the difference oftemperatures between two faces.No actual motion of molecules.

ConvectionThis phenomenon takes place in liquid and gasHeat is transferred due to actual motion of molecules

RadiationThis phenomenon is confined to surfacesRadiant energy emitted or absorbed is dependent on thenature of the surface.

Lesson Plan 32


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Low Temperature Heating up to 400 C

Medium Temperature Heatingfrom 400 Cto 1150 C

High Temperature Heatingabove 1150 C

Lesson Plan 32


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CALCULATION OF QUANTITIES OFHEAT

Heat dissipated (Q) in a body depends upon

Mass of the body [m in Kg]Nature of Material [specific heat, s in KJ/KgC]

Difference in Initial & Final Temperature[ (2- 1)inC ]

Formula Q = m s (2 - 1)


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Q. A large auditorium has an air circulationof 50,000 m3 for 20 hr. What is the rating ofthe heater that raises the temperature by24C.Specific Heat of air = 1.34 KJ/ m3C

Ans: 22.33 kW.


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Lesson Plan 33

CLASSIFICATION OF METHODS OFELECTRIC HEATING

PowerFrequency/OhmicHeating method(50-60 Hz)

High Frequencymethod/Radio Frequency(kHz) (5-500KHz)Microwave Heating(0.3 GHz 300 GHz)

InductionHeatingDirectIndirect

ResistanceHeatingDirectIndirect

Arc HeatingDirectIndirect

DielectricHeating/ HighFrequencyCapacitiveHeating

InfraredHeating/RadiantHeating


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RESISTANCE HEATINGThe generation of heat is done by electricresistor carrying current.

Characteristic Features of HeatingElements

high resistivityable to withstand high temperatures without

deteriorationlow temperature coefficient of resistancepositive temperature coefficient of resistancefree from oxidation at high temperatures


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ELECTRIC WATER HEATER- ANEXAMPLE OF RESISTANCEHEATING


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Lesson Plan 33

DIRECT & INDIRECT RESISTANCEHEATINGDirect

Electric current is passedthrough the body (charge)to be heated

High efficiency Example- Electrode boiler for heating

water Resistance Welding

Indirect

Electric current is passedthrough highly resistivematerial (heating element)

placed inside an oven Heat produced due to I2R loss

in the element is transmittedto the body

Mode of heat transfer isConduction &/or Convection

&/or Radiation Example- Room Heaters Domestic & commercial

cooking Heat treatment of metals


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Lesson Plan 33

Q.WHICH TYPE OF HEATING ISUSED IN BIMETALLIC STRIPS OFFLUOROSCENT LAMP STARTERS-DIRECT/INDIRECT?


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Lesson Plan 33

MATERIALS OF HEATING ELEMENTS

Low & Medium

Temperature (up to1150 C)

Alloy of nickel &

chromiumNi= 80%, Cr= 20%Alloy of nickel,chromium & ironNi= 65%, Cr=15%, Fe=

20%

High Temperature

(>1150 C)

Silicon carbide Molybdenum

Tungsten Graphite


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Lesson Plan 33

ELECTRIC RESISTANCE FURNACEThese are insulated closed chambers witha provision for ventilation

Heating elements may consists of circularswire or rectangular ribbons

Heating elements placed either on top or

sides of the oven

Charge is placed inside a heating chamber


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Lesson Plan 33

TEMPERATURE CONTROL OF HEATINGFURNACETemperature of Furnace depends upon----

Voltage (V) can be controlled by tapped transformer

Resistance (R)- can be controlled by switching in variouscombinations of groups of resistance of the furnace

Time (t)- can be controlled by an on-off switch, whichdetermines the time for which the furnace is connectedto supply or remain isolated from the supply.


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Lesson Plan 32

LOSSESBesides raising the temperature of the Charge, heatenergy is lost in following operations:

In raising the temperature of furnace

In raising the temperature of the container/carrier

Heat conducted through wall

Escapement of heat due to opening of door


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Lesson Plan 33

EFFICIENCYHeat required to raise the temperature ofthe charge to the required value

Efficiency =

Heat required to raise the temperature of thecharge to the required value + losses


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Lesson Plan 33

DESIGN OF HEATING ELEMENTIf P = Power input

H= Heat dissipated by radiation thenAt steady state P= H

22

4

4d

l

d

ll

aR

R

VP

2

(I)

.(II)

])100()

100[(67.5 4241

TTewhereH in W/m2

= radiating efficiency, e=emissivity,T1=temp of hotbody in K, T2= temp of cold body in K.

For circular wire a= d2/4


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Lesson Plan 33

Therefore,

l

VdP

4

22

Or,

P

V

d

l

4

2

2

Heat Dissipated/sec = (d) l H

dlH

l

Vd

4

22

22

4,

V

H

l

dor

.(III)

(IV)

From equation (III) & (IV) we can find values ofl& d.

For rectangular ribbons a=t where =width,t=thickness & surface area=2l, neglecting side areaof small thickness t.


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Lesson Plan 33

Q. A resistance oven employing nichrome wire isto be operated from 220 V single phase supplyand is to be rated at 16 KW. If the temperature ofthe element is to be limited to 1170C andaverage temperature of the charge is 500C findthe diameter and length of the element wire. = 0.57, e=0.9, specific resistance of nichrome=10910-8m

Ans:2.723mm.


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Lesson Plan 34

ARC HEATINGDue to application of sufficiently high voltage across an air gap causes

the air in the gap to get ionized and forms ARC

ARC drawn between two electrodes develop high temperature (3000-3500C) depending upon material

ARC may be used in following ways:

by striking between charge and electrodes-principle behind DirectARC Furnace

by striking between two electrodes-principle behind Indirect ARC

Furnace

by striking between an electrode and the two metallic pieces to bejoined - principle behind ARC welding


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Lesson Plan 34

ARC FURNACE & ELECTRODESTypes of ARC Furnaces :1. Direct2. Indirect3. Submerged

Electrodes:1. Carbon Electrode- used with small furnaces less costly than graphite electrode its large area allows uniform heating

2. Graphite Electrode size is 1/4th that of carbon electrodes owing to lower

resistivity oxides at higher temperature than carbon

consumes less than carbon electrode

3. Self Baking Electrode made up of special paste contained in a steel cylinder heat due to flow of current bake the paste to form electrode


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Lesson Plan 34

DIRECT ARC FURNACEelectric current passes through the body of the charge and ARC isformed between charge and electrodepurer product is obtainedoperates at 0.8 p.f laggingcostly

Use a)composition can be controlled during refining processb) used for making alloy steels like stainless & high speed

steel


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Lesson Plan 34

ARC is formed between two electrodes and heat is transmitted to thecharge by radiationlower temperature than direct ARC furnaceoperates at 0.85 p.f lagginghave to provide rocking motion through a motor to distribute heat

uniformly

Use usually employed for melting non-ferrous metals

INDIRECT ARC FURNACE


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Lesson Plan 34

SUBMERGED ARC FURNACE ARC is formed between carbon electrodes placed at top and

hearth electrode or the conducting hearth itself the number of electrodes placed at top depends on the type ofsupply power is controlled either by varying the applied voltage to theelectrodes or by varying the distance between electrodes better distribution of heating better mixing of charge current under short circuit is less than indirect ARCfurnace p.f is 0.8 lagging

Use-Manufacture of ferro alloy like ferro-chrome and ferro-manganese


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Lesson Plan 34

POWER SUPPLY REQUIREMENTSPower consumption is very highARC voltage is quite low (50-150V) resulting in a highcurrent (several hundred amperes)

Advantages of High Current Low voltage Supply1. High current is essential as Heating is proportional to

the current2. From view point of Insulation & Safety low voltage is

needed3. As voltage is small electrode must be placed very nearer

to charge making the ARC length small increasing life of

the roof4. This arrangement causes Higher Voltage Gradientbetween charge and electrode causing nitrogen offurnace atmosphere ionized and absorbed by thecharge, which produces embrittlement.


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Lesson Plan 35

INDUCTION HEATINGPrinciple behind Induction HeatingPrinciple of TRANSFORMERA.C source is provided to primary windingPrimary winding is magnetically coupled to the charge Charge acts as secondary winding

Voltage is induced from primary side to the chargeCharge is heated up by the secondary current Heat is proportional to V2/R (V= voltage induced in charge,R= resistance of the charge)

For sufficient development of Heat R should be low possible with metals onlyEmploying Higher frequency & Flux to raise voltage level


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Lesson Plan 35

TYPES OF INDUCTION FURNACECore type orLow Frequency

Coreless type orHigh Frequency

DirectCoretype

VerticalCoretype

IndirectCoretype


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Lesson Plan 35

DIRECT CORE TYPE INDUCTIONFURNACEConsists of a vertical furnace with a V shaped portion at the bottom Consists of a Transformer charge is magnetically coupled to theprimary winding and forms single-turn short circuited secondarySecondary channel should not be emptied Magnetic coupling between primary & secondary is weak resulting inlarge leakage reactance poor p.fLeakage reactance is nullified using low frequency supplyDrawbacks

Leakage reactance is high & p.f is lowLow frequency requirement Suffers from Pinching EffectPinching Effect current, density above 500 A/cm2, flowing around the

melt interacts with the alternating field and produces force to squeezethe melt of a section and results in complete interruption of current.Use-preferable for non- intermittent service, are obsolete these days


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Lesson Plan 35

VERTICAL CORE TYPE INDUCTIONFURNACE Also known as AJAX-WYATT FURNACE Vertical crucible is used for the charge Bottom is usually V-shaped or U-shaped Molten metal is kept circulated round the V

Pinch effect is counteracted by the weight of the charge Capable of continuous operation Normal frequency can be used Use-melting & refining of non-ferrous metals Advantage- Highly efficient

Low operating cost Temperature control is simple Comparatively higher p.f


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Lesson Plan 35

INDIRECT CORE TYPE INDUCTIONFURNACECharge is heated indirectly A metal container forms the secondary of the transformer Container is heated up by transformer action Temperature control is done using different materials for thedetachable bar P.f is comparatively poor (w.r.t direct resistance furnace, approx-0.8)


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Lesson Plan 35

CORELESS TYPE OR HIGH FREQUENCY Consists of 3 parts-Primary coilCeramic crucibleFrameNo iron corePrimary coil is fed with ac supplyEddy current is induced within the chargeEddy current produces heating of the chargeHigh frequency supply is to be used (flux density is low)High frequency increases resistance due to skin effect resulting in highprimary copper lossNo magnetic core results in very low p.f (between 0.1-0.3)Static r is employed to improve p.f


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Use-Steel productionMelting non-ferrous metalsElectronic industry

Advantages

Fast operationCan be used intermittentlyLow costPrecise control of powerLow melting time

CORELESS TYPE OR HIGH FREQUENCY

43958566 electric heating

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