Electric Heating
Introduction . When the current is pass through a conductor , it gets heated up due to i^2R losses and this heating characteristic of the electric current is being utilized in industrial and domestic appliances
Heating ia required for domestic purposes such as cooking and heating of building where as for industrial purposes and heating is required for melting of metals , Harding and tempering and in welding .
Advantage of Electric heating and over other system of heating :
The main advantage of electric heating over other system of heating (i.e. coal, gas or oil) heating are :
1. Economical. Electric heating is economical as Electrical furnaces are cheaper in initial cost as well as maintenance cost .it does not require any attention so there is a considerable savings in labour cost over other systems of heating more more over the electric energy is also cheap as it is produced on large scale
2.cleanliness. since dust and ash are completely eliminated in electric heating system so it is clean system and cleaning costs are rendered to a minimum.
3. Absence of fuel gases. Since no fuel gases are produced in this system. The atmosphere around is clean and pollution free.
4.Ease of cantrol : Simple accurate and reliable temperature of furnace can be had with the help of manual or automatic devices .desired temperature can be had in electric heating system which is not convenient in other heating systems
5. Efficiency. It has been practically found that 75 to 100% of heat produced by electric heating can be successfully utilized as the source can be bought directly to the point where heat is required there by reducing the losses .
6. Automatic protection. Automatic protection against over current or
Overheating can be provided through suitable Switchgear in electric heating system
7. Better working conditions: Electric heating system produces no irritation noise and also the radiation losses are low . Thus working with electric furnace is convenient and cool
8.safety . Electric heating is quite safe and responds quickly.
9. Upper limit temperature. There is is no upper limit to the temperature obtainable of the except the ability of the material to with stand heat
10.special heating requirement: certain requirements of heating such as uniform heating of one particular portion of the job without effecting others, heating of non conducting materials, heating with no oxidation can be met only in electric heating system
Heating element:
The heating effect of the electric current can be produced by passing electric current through. Heating element and the material used for heating element must have following properties
1.if should have high specific resistance so that a small length of wire (R=pl/a,o=Ra/l) is sufficient to produced the required amount heat
2. It should have high melting point so that high temperature can be obtained
3. It should have low temperature Coefficient since for accurate temperature control the resistance ,should have nearly constant at all temperature and this possible only if the resistance does not change with temperature
4. It should not oxidize at Higher temperatures otherwise it's life is shortened and needs frequent replacement
The most commonly used the heating elements are either lalloy of Nickel and chromium or Nickel chromium iron, Nickel chromium aluminium ,Nickel copper. the use of iron reduces the cost but lower
the life of element
Design of Heating Element.
The heating element is used for electrical heating may be a circular or rectangular like Ribbon but Ribbon type of element requires more wattage per unit area. hence circular heating element is preferred. by knowing the electrical input and its voltage size and length of heating element required to produce the temperature can be calculated
Causes of failure of heating element
There are so many causes are there for the failure of heating element some of them are explained below
1.formation of Hot spots: hot spots are the points in heating element which are at Higher temperature then the main body of the element .hot spots may be due to any of the following causes;
(a). High rate of locker oxidation may reduce the cross section of the element wire thereby increasing the resistance at that spot the thus more heat will be produced locally giving rise to the the break dawn of the element
(b) shielding of element by support etc.will reduce the local heat losses by radiation and causes a rise of temperature of shielded portion of the element therefore minimum number of supports without producing distortion of the element should be used
(a).Due to too high element temperature insufficient support for the element or selection of wrong material . Sagging and wrapping of element may result which may cause uneven spacing of section there by producing hot spots.
2.Oxidation of intermittency of operation . At high temperature , oxide scale is the formed on the heating element which is continuous and tenacious and is so starting that it prevents further oxidation of inner metal of element , However ,if the element used quit often layer is subjected to thermal stresses due to frequent cooling and heating thereby the oxide layer cracks and flakes off exposing further fresh metal to oxidation thereby producing hot spots
3 Embrittlement due to grain ,growth: All heating alloys containing iron tend to form large brittle grains at high temperatures . When cold the elements are very brittle and liable to rupture easily on slightest handling and jerks .
Modes of Transfer of heat :
The heat from one body to another body can be transferred by any one of following method
1. Conduction 2.convection 3.Radiation
1.conduction : in this method ,heat is traveles without the actual movement of practice (molecules) .The flow of heat from one part to other part is dependent upon the temperature difference between these parts . It is also applicable when two bodies at different temperature are joined together . The hearted molecules of the substance tramsfer their heat to the adjacent molecules and this heat flow will invariably take place so long as tnere is different in temperature .
For example when one end of solid is heated the molecules at that end absorb the heat energy and begin to vibrate rapidly when these is molecule collide with during molecular energy is per set them with internal begin to vibrate faster to pass some on energy to their molecule the heat is transfer one molecule to another molecule without their actual movement
If the heat is to be conducted from one object to another object, the following conditions must be met
1. The objects should be bodily in conctact with each other.
2. The temperature ofthe two bodies should be different i.e. temperature gradient should exixt.
Definition of conduction: the process in which heat is transferred from one practical to another in direction of fall of temperature without the actual movement of particles of medium is called conduction.
The rate of conduction of heat along a substance depends upon the temperature gradient and expressed in Mj/hr/m^2/m/c^0 or in watts /cm^2 in case of electric heating.
In a place of thickness t meters having X-sectional area of its two parallel faces A sq.meters and temperature of two faces is T1and T2 absolute, the quantity of heat transferred through it during T hours given by
Q=kA
Where K is coefficient of thermal conductivity for material in Mj/hr/m^2/m/c^0
2. Convection:
Def. The process of heat transference in which heat is transferred from one place another (from hotter to colder one) by actual movement of particles of medium is calles convection.
For example: in cases of heater used for heating buildings, the air in contact with a heat radiator element in a room receives heat from contact with the element. The heated air expands and rises, cold air flowing into takes place. Thus in this way the room gets heated up.
A similar action takes place in an electric water heater, a continuous floe of water passing upwards across the immersed heating element, with the result that the whole of the water in the tank becomes hot.
The quantity of heat absorbed from the heater bay convection depends mainaly upon the temperature of the heating element above the surrounding and upon the size of surface of the heater. It also depends partion of the heater.
Heat dissipation is given the following expression
H=a(T2-T2)b W/m^2
Where a, b - constants whose values depends on the heating surface facilitates for heating etc.
T1, T2- temperature of the heating surface and fluid in 0c.
3. Radiation:
Def. The process of transmission of heat in which heat energy is transferred from hotter body to colder body without heating the medium in between is called radiation.
For example: we receive energy from the sun by radiation through there in distance of about 150
million Kms between sun
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Arc heating and Resistance heating
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