Power Heating

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CLASSIFICATION OF HEATING METHODS

Classification of heating methods

Power Frequency Heating High Frequency Heating

Resistance Heating Arc Heating Induction Heating Dielectric

Heating

Direct

Direct Indirect Indirect


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RESISTANCE HEATING

Electric furnace is a device which converts electric energy into

heat energy for industrial applications.

Various types of heating methods are employed here, which

we discussed earlier.

One such method is POWER FREQUENCY HEATING.

Power frequency method is, in turn, divided into 3 methods:

1) resistance heating

2) arc heating3) electron bombardment heating


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RESISTANCE HEATING

Working principle

When electric current passes through the resistance, I2R loss

takes places in the resistance. According to Joules law (H=I2Rt Joules) it appears in the form

of heat.

In case of resistance heating, low voltage and low frequency

A.C supply is used to produce heat. In some applications heatis also obtained by D.C supply.


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TYPES OF RESISTANCE HEATING

1. Direct resistance heating2. Indirect resistance heating


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DIRECT RESISTANCE HEATING

H = I2RT joules

R = Resistance of charge

I = Current through charge A t = Time

T = Temperature of furnace = 1000C to 1500C

V = Supply voltage required = 2 to 15 V ( AC orDC )

I = Current up to 3000 A


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CONSTRUCTION

The furnace is constructed by using heat insulating material

such as firebrick and is supported on metal framework.

The electrodes are inserted from top or from both sides of the

furnace according to the requirement.

Between these two electrodes the charge (material to be

heated ) is placed . This charge maybe in form of powder,

pieces or liquid.


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DIRECT RESISTANCE FURNACE


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OPERATION

In direct resistance heating, charge acts as a conductor

between two electrodes hence electric current passes

through it.

Heat is generated in the charge according tojoules law, i.e.

H = I2RT joules

R = Resistance of charge

I = Current through charge A

t = Time This method is most efficient method as the heat is generated

in charge itself thus uniform heating of the charge is obtained.


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Heating in this furnaces eliminates oxidation and gives rapidand uniform heating.

As the temperature of bath increases , its resistance goes

down. Therefore these furnaces are started with highest tap, as

heat proceeds, this is brought to lower taps to limit the current.

When highly conductive materials are to be heated, it acts as

direct circuit. To avoid this, highly resistive powder is sprinkled

between the surfaces of pieces.

The temperature of furnace is maintained between 1000C to1500C.

The supply voltage required is between 2 to 20V and current

up to 3000A.


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APPLICATIONS

This method employed in salt bath furnaces and in electrode

boiler for heating water.

Generally these types of furnaces are used for the purposes ofcarbonizing, tempering, quenching and hardening of steel

tools.


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INDIRECT RESISTANCE HEATING

In this type of heating, electric current is passed through

resistive element.

When electric current is passed through resistance it produces

IR loss, further according to joules law heat is produced

H = IRTJoules

Where R = Resistance of heating element.

I = Current through heating element.A

t = Time Heat generated in the heating element is then transferred to

the charge mainly by radiation and convection.

e.g. room heater, hair dryer, electric kettle, water heater etc.


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INDIRECT RESISTANCE HEATING


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APPLICATIONS

The low temperature furnaces (300C to 1050C) are used for

drying varnishes, vulcanising and hardening of synthetic

material.

High temperature of 1050C to 1350C are used for hardening

purpose.


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ELECTRIC ARC HEATING

When the sufficient high voltage is applied between two

electrodes containing air as a media between them, then air

gets ionised and current flows through the air is known as arc.

The heat generated thus can be utilized to melt the material.

A very high voltage is required to generate the arc but small

voltage is sufficient to maintain the arc.

In arc furnace arc is produced by short circuiting the two

electrodes momentarily and withdrawing them back. Thus

high voltage is not required at start.

Temperature obtained by these furnaces arc 1,000C and

3500C depending upon the electrodes used.


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TYPES OF ARC FURNACE

According to method of producing thearc, furnaces are classified as:

1. Direct arc furnace

2. Indirect arc furnace

3. Submerged arc furnace


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In direct arc furnace arc is struck between the electrodes andcharge

In this type of furnace heat is produced by two means.

1. Due to the arc which is struck between electrode and arc is in

direct contact with charge2. Since the circuit getting completed through charge; therefore

current flows through the charge. this current produces heat

as IRT Joules where,

I= Current flowing through charge in AMPR= Resistance of charge in s

T= Time in sec

Due to these two reasons charge can be heated to highest

temperature.

DIRECT ARC FURNACE


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DIRECT ARC FURNACE


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In case single phase arc furnaces two electrodes are taken

vertically downward as shown in fig. Through the roof of

furnace to the surface of charge.

In case of three phase arc furnace three electrodes areseparated at the corners of an equilateral triangle, thus three

arcs are formed.

Stirring action are automatic as current passing through

charge produces electromagnetic field.

The size of furnace is about 5 to 10 tonnes.

For one tonne furnace power required is 200 kW.

Power factor of these furnaces are about 0.8 lagging.

These furnaces are commonly used for production steel.


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INDIRECT ARC FURNACE

The furnace is made of cylindrical shape and electrodes

projecting through the chamber from each end as shown

in fig.

In these furnace arc is struck between the two electrode

above the charge and heat generated is transferred to the

charge by radiation.

Temperature obtained by these furnaces is lower than

that of direct arc furnace.


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INDIRECT ARC FURNACE


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Since the current does not flow through the charge there is

no stirring action.

These furnaces are used for melting of non powers metals.

These furnace are available in 0.25 tonnes to 3 tonnes.

Power factor of these furnace is about 0.85 lagging.

Power consumption of these furnaces is about 400 kWhper tonne.


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SUBMERGED ARC FURNACE

These are cylindrical furnaces in which arc is formed between

the carbon electrodes and hearth electrodes.

The hearth lining is of magnesite which becomes

comparatively good electrical conductor when hot.

The stirring action is inherent due to electromagnetic force

set up by the current resulting in uniform heating of charge.

These furnaces are used for the manufacture of Ferro-alloys

like Ferro-chrome and Ferro-manganese.

Power factor of these furnace is about 0.8 lagging.


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