Electric Heating

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Introduction Modes of Transfer of Heat Classification of Electric Heating Methods Resistance Heating Arc Furnaces Induction Heating Dielectric Heating

Heating is required for domestic purposes as well as industrial purposes.

Industrial applications include 1. Melting of Metals2. Hardening3. Tempering4. Case Hardening5. Drying6. Melting

Economy Cleanliness Absence of Flue Gases Ease of Control Automatic Protection Upper Limit of Protection Special Heating Requirement High Efficiency of Utilization Better Working Conditions Safety

1. Conduction

2. Convection

3. Radiation

In this mode of transfer of heat, one molecule of the substance gets heated and transfer the heat to the adjacent and so on.

Rate of conduction of heat along a substance depends upon temperature gradient.

Q=kA/t(T1-T2)T

Convection is the movement of molecules within fluids (i.e. liquids, gases).

It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids.

Radiation is a process in which energetic particles or energy or waves travel through a medium or space.

It can be classified as: Direct Resistance Heating Indirect Resistance Heating Infra-red or Radiant Heating Arc Heating Direct Induction Heating Indirect Induction Heating Dielectric Heating

It is based upon the I2R effect. Has wide applications in heat treatment

of metals, drying, baking of potteries, stoving of enamelled ware and domestic cooking.

Temperature upto 1000 celsuis can be obtained in ovens.

Two common methods of resistance heating are

1. Direct Resistance Heating2. Indirect Resistance Heating

It is accomplished by the dissipation of an electric current in a material.

Material to be melted is placed inside a furnace with electrodes in the walls.

Alternating or direct electric current is then passed between the electrodes through the material.

Resistance losses are dissipated as heat, causing the material to melt.

Advantages: High efficiency. It gives uniform heat and high

temperature.

Application: It is mainly used in salt bath furnace and

water heaters.

In this method the current is passed through a highly resistance element which is either placed above or below the oven depending upon the nature of the job to be performed.

The heat proportional to I2R losses produced in heating element delivered to the charge either by radiation or by convection.

Both A.C and D.C supplies can be used for this purpose at full mains voltage depending upon the design of heating element.

Application: This method is used in room heater, in

bimetallic strip used in starters, immersion water heaters and in various types of resistance ovens used in domestic and commercial cooking.

It must posses the following properties: High Resistivity High Melting Point Low Temperature Coefficient Free Form Oxidation

An electric arc furnace is a furnace that heats charged material by means of an electric arc.

There are two common types of arc furnaces.

1. Three phase furnace. 2. Single phase furnace.

Three phase furnaces are used in the production of alloy steels.

Single phase furnaces are used for the manufacture of gray iron casting also.

Three phase furnaces are used for power ratings from 250KVA, 10,000KVA and capacities up to 25 tons.

Generally graphite electrodes are used. As they are subjected to volatilization, they are to be replaced.

The arc temperature is between 3000 and 3500˚C, so that the process is carried out between 1500˚C and 2500˚C.

1)Variable ratio power transformer2)Reactors3)Automatic current regulator4)Control panel5)Electric motor and tilting motor6)Circuit breaker and connecting switches.

The arc is struck directly with the charge, when a current flows through it and produces intense heat, which results, in high temperature.

Although some furnaces up to 100 tons are made, generally furnaces up to 25 tons are in general use.

Stirring action is automatic and gives a uniform product. It is used for alloy steel manufacture and gives a purer product.

Advantages: It produces purer products It is very simple and easy to control the

composition of the final product during refining process.

Disadvantages: It is very expensive.Application: The most common application of this

type of furnace is to produce steel.

Electrodes are inserted from the sides and the heat produced is transmitted by radiation to the charge.

As there is no inherent stirring action, the furnace should be rocked.

This furnace is used for only single phase supplies. Also the capacity of the furnace is limited up to 100 tons.

Melting of non-ferrous metals is mostly carried out in this type of furnace.

In both the type of furnaces, large quantities of electrodes are used.

Application:

The main application of this type furnace is melting of non-ferrous metals.

It consists of an iron core, crucible and primary winding connected to an AC supply.

The charge is kept in the crucible, which forms a single turn short circuited secondary circuit.

The current in the charge is very high in the order of several thousand amperes. The charge is magnetically coupled to the primary winding.

The charge is melted because of high current induced in it. When there is no molten metal, no current will flow in the secondary.

To start the furnace molten metal is poured in the oven from the previous charge.

The magnetic coupling between the primary and secondary is very weak, therefore the leakage reactance is very high. This causes low power factor.

Low frequency supply is necessary because normal frequency causes turbulence of the charge.

It is modified type of core type induction furnace. It has a vertical channel for the charge, thus the crucible

used is also vertical. The principle of operation is that of a transformer in which

the secondary turns are replaced by a closed loop of molten metal. The primary winding is placed on the central limb of the core.

Hence leakage reactance is comparatively low and power factor is high.

The top of the furnace is covered with an insulated cover which can be removed for charging. Necessary arrangements are usually made for titling the furnace to take out the molten metal.

When primary is connected to the AC supply, high current will be accumulated at the bottom and even a small amount of charge will keep the secondary completed.

Hence chances of discontinuity of the circuit is less.

Advantages: High efficiency and low operating cost. Since both primary and secondary are on

the same central core, its power factor is better.

The furnace is operated from the normal supply frequency.

Applications: This furnaces is used for melting non

ferrous metals like brass, zinc, tin, bronze, copper etc.

In such furnace an inductively heated element is made to transfer its heat to the change by radiation.

It consists of an iron core linking with the primary winding and secondary.

When the primary winding is connected to the supply, current is induced in the secondary of the metal container.

So heat is produced due to induced current. This heat is transmitted to the charge by radiation.

Coreless induction furnace also operates on the principle of transformer. In this furnace there is no core and thus the flux density will be low.

Hence for compensating the low flux density, the current supplied to the primary should have sufficiently high frequency.

Stirring of the metals takes place by the action of the electromagnetic forces. Coreless furnace may be having conducting or non conducting containers.

To prevent the primary winding from high temperature, refractory linings are provided between primary and secondary windings.

Advantages: Time taken to reach the melting

temperature is less. Accurate power control is possible. Any shape of crucible can be used. The eddy currents in the charge results in

automatic stirring. Absence of dirt, smoke, noise, etc. Erection cost is less.

Dielectric heating is also sometimes called as high frequency capacitance heating.

If non metallic materials i.e, insulators such as wood, plastics, china clay, glass, ceramics etc are subjected to high voltage A.C current, their temperature will increase in temperature is due to the conversion of dielectric loss into heat.

The dielectric loss is dependent upon the frequency and high voltage. Therefore for obtaining high heating effect high voltage at high frequency is usually employed.

Advantages: Uniform heating is obtained. Running cost is low. Non conducting materials are heated

within a short period. Easy heat control.Applications: For food processing. For wood processing. For drying purpose in textile industry. For electronic sewing.

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