EE037-3.5-2 EPU L07 Electric Heating-P1

8/13/2019 EE037-3.5-2 EPU L07 Electric Heating-P1

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ELECTRICAL POWER

UTILIZATION

ELECTRIC HEATING


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EE037-3.5-2-ELECTRICAL POWER UTILIZATION L7-ELECTRIC HEATING SCHOOL OF ENGINEERING

INTRODUCTION

Electric heating is extensively used both for domesticand industrial applications.

Domestic applications include:

(i) room heaters(ii) immersion heaters for water heating

(iii) hot plates for cooking

(iv) electric kettles

(v) electric irons

(vi) electric ovens for bakeries

(vii) electric toasters etc.


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INTRODUCTION

Industrial applications of electric heating include:

(i) melting of metals

(ii) heat treatment of metals like annealing, tempering,

soldering and brazing etc. (iii) moulding of glass

(iv) baking of insulators

(v) enamelling of copper wires etc.


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ADVANTAGES

As compared to other methods of heating using gas,

coal and fire etc., electric heating is far superior for

the following reasons :

(i) Cleanliness. Since neither dust nor ash is produced inelectric heating, it is a clean system of heating requiring

minimum cost of cleaning. Moreover, the material to be

heated does not get contaminated.

(ii) No Pollution. Since no flue gases are produced inelectric heating, no provision has to be made for their exit.


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ADVANTAGES

(iii) Economical. Electric heating is economical because

electric furnaces are cheaper in their initial cost as well asmaintenance cost since they do not require big space for

installation or for storage of coal and wood. Moreover,

there is no need to construct any chimney or to provideextra heat installation.

(iv) Ease of Control. It is easy to control and regulate the

temperature of an electric furnace with the help of manual

or automatic devices. Temperature can be controlledwithin 5C which is not possible in any other form of

heating.


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ADVANTAGES

(v) Special Heating Requirement. Special heating

requirements such as uniform heating of a material orheating one particular portion of the job without affecting

its other parts or heating with no oxidation can be met

only by electric heating.

(vi) Higher Efficiency. Heat produced electrically does not

go away waste through the chimney and other by

products. Consequently, most of the heat produced is

utilised for heating the material itself. Hence, electricheating has higher efficiency as compared to other types of

heating.


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ADVANTAGES

(vii) Better Working Conditions. Since electric heating

produces no irritating noises and also the radiation lossesare low, it results in low ambient temperature. Hence,

working with electric furnaces is convenient and cool.

(viii) Heating of Bad Conductors. Bad conductors of heatand electricity like wood, plastic and bakery items can be

uniformly and suitably heated with dielectric heating

process.



ADVANTAGES

(ix) Safety. Electric heating is quite safe because it

responds quickly to the controlled signals.

(x) Lower Attention and Maintenance Cost. Electric

heating equipment generally will not require muchattention and supervision and their maintenance cost isalmost negligible. Hence, labour charges are negligibly

small as compared to other forms of heating.



METHODS OF HEAT

TRANSFER

Conduction

The flow of heat from one part of the body to another isdependent upon the temperature their difference.

Heated molecules of the substances transfer their heat tothe adjacent molecules.

Heat flow will take place as long as there is a difference in

temperature and continuity.

Fig. 7.1: Conduction



METHODS OF HEAT

TRANSFER

Conduction

Consider a solid material of cross-section A sq.m. andthickness x metre as shown in Fig. 7.1.

If T1 and T2 are the temperatures of the two sides of theslab in K, then heat conducted between the two oppositefaces in time t seconds is given by:

where K is the thermal conductivity of the material.



METHODS OF HEAT

TRANSFER

Convection

Heat is transferred by the actual motion of molecules ofthe substances.

The quantity of heat absorbed by the body by convectionprocess depends on:

The temperature of the heating element above the

surroundings

The size of the surface of the heater The position of the heater.



METHODS OF HEAT

TRANSFER

Convection

The amount of heat dissipated is given by,

where,

a and b = constants for the fluid

T1= the temperature of the heating surface in K

T2

= the temperature of the fluid in K

b



METHODS OF HEAT

TRANSFER

Radiation

Heat is transferred by means of heat waves.

These waves do not heat the medium in between the two

bodies but heat the bodies which intercept the waves. The amount of heat transmitted is given by Stephens Law:

Where,

K = radiating efficiency, a constant


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METHODS OF HEAT

TRANSFER

Radiation

where,

K = radiating efficiency, a constant; its value depends upon

the number of heating elements and their dispositione = emissivity; is 1 for black body, 0.9 for resistance heating

elements

T1= temperature of the heating surface

T2= temperature of the substance to be heated

EE037-3.5-2 EPU L07 Electric Heating-P1

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