ELECTRIC HEATING
Heating by means of converting electricity into heat is called electric heating. Heating is required for domestic purposes such as cooking and heating of buildings, as well as for industrial purposes such as melting of metals, hardening and tempering, drying and welding. Practically all the heating requirements can be met by some form of electric heating equipment. The main advantages of electric heating over other systems of heating i.e., gas, coal, oil heating are given below.
ADVANTAGES OF ELECTRIC HEATING OVER OTHER SYSTEM OF HEATING
The main advantages of electric heating over other system of heating such as coal, gas or oil heating are given below.
1. Economical: Electric energy is cheaper to use as it is produced on a large scale. The electric furnaces run by electric energy are cheaper in initial cost. The maintenance cost is also low as compared to other system.
2. Cleanliness: It is free from ash, dust and soot, so it is clean system of heating.
3. Ease of temperature control: Desired temperature can be controlled either by hand or by fully automatic switches such as thermostat in electric system. 4. Absence of flue gases: When we use electrical energy to generate heat, no flue gas is produced. So the surrounding atmosphere is not polluted.
5. Saving in space: With electrical heating, no storage of fuel is required as may be the case with coal, oil or gas heating.
6. Uniform heating: In electric heating, the heat can be generated inside the core of the material which achieves uniform heating.
7. Maintenance cost: The maintenance costs are lower for electric equipments used for heating.
8. Less attention required: No attention is required in general as the process can be made fully automatic, whereas for use of coal, or gas etc., an attendant is required.
9. Location of heat: If necessary, the heat can be localised in a particular portion of the article to the heated.
10. Safety: Electric heating is quite safe and respond quickly.
11. Absence of Noise: Noise is entirely eliminated in case of electric heating.
12. Quick operation: Heat by electricity can be produced more quickly than by any other process.
13. Efficiency: The overall efficiency of electrically produced heat is comparator higher, because the source can be brought directly to the point where heat is required. Hence losses can be reduced.
14. Automatic Protection: Automatic protection against over current or over heating can be provided through suitable switch gears in the electric heating system.
Methods of Transfer of Heat
Heat is transferred from a hot body to cold body by the following three methods normally.
1. Conduction: Heat is transferred from one surface area to the other by way of conduction.
Solid get heated according to this mode heating.
2. Convection: In this mode of heating, heat is transferred by actual motion of the molecules. Liquids are usually heated due to convection. Immersion type water heater is the application of this process.
3. Radiation: In this mode of heat, the heat reaches the substances to be heated from the source without heating the medium in between.
METHODS OF ELECTRIC HEATING
The various methods of electric heating are enlisted below: Considering that it is both economical and desirable to produce heat with the help of electricity, again there are various methods of heating. The merits of each method depend upon the degree of temperature required as well as the purpose of it. For example, resistance type heating element is used in a room heater, in an oven or in a press. For melting charge of a metal, an induction or arc furnace is used. The various methods of producing heat are classified as below:
(i) Resistance Heating:
(a) Direct resistance heating
(b) Indirect resistance heating
(ii) Induction Heating or Eddy Current Heating:
(a) Direct induction heating
(b) Indirect induction heating
(iii) Electric Arc:
(a) Direct arc furnaces.
(b) Indirect arc furnaces
(iv) Dielectric or Capacitive Heating:
The comparative study of each type is given as below:
RESISTANCE HEATING
Direct Resistance Heating
In case of direct resistance heating, the material or charge to be heated is taken as resistance and current is passed through it. This current produces FR losses in the form of heat within the body itself. The charge may be in the form of powder, pieces or liquid.
This principle is made use of in resistance welding and in heating water by means of an electrode Boiler. In case of electrode boiler, the electrodes are lowered into the tank filled with water. The current flows through electrodes into the water and the water gets heated up by FR losses. The current must be adjusted from time to time since the resistance of water lowers down as its temperature rises. A water load may also be prepared in the lab, that works on the same principle. It is important to note that only AC is suitable for this purpose as DC will cause electrolysis of water.
AC supply is used having voltage varies from 2 to 20 volts and currents upto 2500 amps. Automatic stirring action is produced in the charge to be heated and no external method of stirring is required to get uniform heating.
Indirect Resistance Heating
So many domestic appliances work on the principle of Indirect Resistance Heating. In this case the current does not flow through the body to be heated but it flows through resistance elements which get heated up. The heat is then carried to the body by convection or radiation. The appliances that work on this principle include room heater, cooking ovens, immersion rod, electric kettles, electric iron and various types of resistance ovens used for bright annealing and Salt Bath heating etc.
INDUCTION HEATING
In induction heating effect of currents induced by electromagnetic action in the charge is employed.The heat developed depends upon the voltage and resistance of the charge, because power drawn is equal to V2/R so to develop heat sufficient of melt the charge, the resistance of the charge must be low which is possible only with metals and voltage must be higher, which is obtained by employing higher flux and higher frequency.
Types of Induction Heating
(i) Direct induction heating
(ii) Indirect induction heating
Direct Induction Heating
The induction heating works on the transformer principle. It is also known as eddy current heating The currents are induced by the principle of Electromagnetic Induction. The induction heating may be low frequency as in case of core type induction furnace of high frequency as is the case with coreless induction furnace or various processes utilising high frequency eddy current heating.
Again the induction heating may be Direct Induction heating when the eddy currents are produced within the material itself that is to be heated or it may be indirect induction heating in which case the currents are produced in a coil which gets heated up and the heat is carried to the charge through convection and chiefly by Radiation.
The examples of Direct Induction heating are the high frequency eddy current heating used for case hardening or tempering of various machine parts, annealing of stell strip and soldering. The core type induction furnace used for melting non-ferrous metals such as Copper, Zine, Brass and the coreless induction furnace used for preparing various high grade steels also work on the same principle.
Indirect Induction Heating
The example of indirect induction heating is the indirect induction oven which is indirect competition with resistance oven and is preferred over it due to its fine temperature control. It is used for the same purpose as the resistance ovens.
Moreover in the indirect induction heating method eddy current are induced in the heating elements by electromagnetic induction which produces heat in the heating elements. The heat thus produced is transferred to the body to be heated by radiation.
EDDY CURRENT HEATING
This is also known as the induction heating, The material to be heated is placed inside the coil. The heat in the material to be heated is produced by eddy currents. Power loss due to eddy currents is eddy current loss and appears in the form of heat. The metal to be heated is placed within a high frequency current carrying coil. By doing so an alternating magnetic field is set up, eddy currents are induced in the metal piece and heating is affected. The power loss due to eddy current in the metal piece depends upon the power drawn by the metal piece.
The frequency may vary from 50 Hz to 8 MHz depending upon the type of work done. This method is frequently used for forging, annealing. The process is economical for continuous heating. It may also be used for welding, brazing and soldering.
Advantages of Eddy Current Heating
1. It is quick, and clear.
2. There is little wastage of heat as heat is produced in the body to be heated up directly.
3. Temperature control is easy i.e., by controlling the supply frequency and flux density.
4. The heat can be made to penetrate into the metal surface to any desired depth.
5. The equipment can be operated even by unskilled worker.
Disadvantage of Eddy Current Heating
1. It is a costly method for the production of heat.
2. Low efficiency.
3. Initial cost of the apparatus is high.
Uses: Eddy current heating is used for the heat treatment of metals i.e., annealing, tempering. surface hardening etc.
ELECTRIC ARC HEATING
The arc furnaces depend upon the principle of heat generated by electric arc. The electric arc heating may be used in the following different ways.
(i) By striking the arc between the charge and electrode. In this method the heat is directly conducted and taken by the charge. The furnaces operating on this principle are known as direct arc furnaces. These furnaces are used for production and refining of various grade of steel.
(ii) By striking the arc between two electrodes. In this method the heat is transferred to the charge by radiation. The furnaces operating on this principle are known as indirect arc furnaces. These types of furnaces are used for melting of non-ferrous metals such as brass, copper and zinc.
Arc Furnaces
It has already been mentioned that the arc furnaces are of two types viz., the direct are furnace and indirect are furnace. These are usually operated from 3 phase supply.
Direct Arc Furnace
The Arc is struck between the electrode and the charge. Three electrodes are used for three phase supply. In each case the arc is between tip to electrode and the charge which forms the star point.
Since the arc is struck on the charge itself, it is possible to produce the highest temperatures by this method.
The Refractory lining used in the inside of arc chamber is for high quality fire clay bricks which do not soften or melt at the melting temperatures of steel for which purpose this furnace is mostly used.
The electrodes may be of carbon or graphite. The graphite electrodes are of about half the size of carbon electrodes for the same power, Graphite electrodes permit production of higher temperatures.
The arc has a-ve resistance characteristic i.e., the resistance falls with increase in temperature. Thus some sort of current limiting device is essential in the circuit in order to prevent short circuits. This may be in the form of a Reactor. Alternatively the supply transformer may be so wound that the voltage is adjusted from it. This limits the current also.
A more uniform product is obtained by this method since the automatic string action is produced when the arc is focused on the charge itself.
This type of furnace is extensively used for melting of various types of ferrous alloys, in production of high quality steels and for refining purposes when the steel is produced in cupola and refined in arc furnace.
There is a charging door from where the charge is supplied and also there is an outlet for molten metal.
The sizes of furnaces in common use are between 5 to 10 tons.
Indirect Arc Furnace
In this case the arc is struck between two electrodes. The chief mode of transfer of heat is through radiation. The temperature attained is lower than the direct arc furnace. So these furnaces are suitable for melting metals having lower melting points e.g., Non ferrous metals such as brass, copper, zinc, bronze etc. Furnace supported by steel frame work and lined with refractory material.
The arc is struck between the electrodes so only two electrodes are required. The supply is therefore single phase.
Since during the process of heating the electrodes are consumed, so the feeding of electrodes to the furnace is automatic. The furnace is cylindrical in shape.
Since the arc does not come in contact with the charge so the automatic stirring action which is present in direct arc furnace is absent. The furnace may be equipped with automatic rocking equipment. The power factor varies from 0.7 to 0.8.
The electrode material and the electrical equipment is similar to that used for direct arc furnaces.
DIELECTRIC HEATING
When non-metallic parts such as wood, plastics, bones ceramics are subjected to an alternating electrostatic field, dielectric loss occurs. These losses are used in dielectric heating which appears in the form of heat. The material to be heated is placed as a slab between two metallic electrodes across which high frequency voltage is applied.
To ensure sufficient loss and to give an adequate amount of heating, frequencies between 10 to 30 mega cycle per second must be used and the voltage needed may be as high as 20 kV. The necessary high-frequency supply voltage is obtained from a valve oscillator.
The current drawn by the capacitor, when an ac supply voltage is applied across its two plates, does not lead the supply voltage by 90° exactly. Due to this component of current, heat is always produced in dielectric material placed in between the two plates of the capacitor.
The electric energy dissipated in the form of heat energy in the dielectric material is known as dielectric loss. The dielectric loss is directly proportional to the frequency of ac supply.
This method of heating is also employed for drying of textiles, manufacture of plywood, paper etc. The overall efficiency in case of dielectric heating is about 50%.
Advantages of Dielectric Heating
1. Since the heat is produced throughout the whole mass of material, we get uniform heating. By conventional method of heating, it is not possible to achieve this.
2. Short time is required to complete the process as compared to other methods.
3. Materials heated by this method are non-conducting, so by other methods heat cannot be conducted inside so easily.
Disadvantages of Dielectric Heating
1. Only those materials can be heated which have high dielectric loss.
2. The cost of equipment required for dielectric heating is so high that it is employed only where other methods are impracticable.
Applications of Dielectric Heating
1. It is used in drying tobacco, paper, wood, gluing and bonding of wood.
2. Welding of PVC.
3. Sterilization of medical supplies.
4. For producing artificial fibers, heating of bones and tissues etc.
5. Food processing.
6. It is employed for dyeing textiles.
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