LATENT HEAT was discovered by Scottish physician and chemist JOSEPH BLACK
He was born on 16th April 1728.
He was also known for his discoveries of magnesium, specific heat and carbon dioxide. He was a professor of Anatomy and Chemistry at the University of Glasgow for 10 years from 1756, and then Professor of Medicine and Chemistry at the University of Edinburgh from 1766, teaching and lecturing there for more than 30 years. In 1761 he deduced that the application of heat to ice at its melting point does not cause a rise in temperature of the ice/water mixture, but rather an increase in the amount of water in the mixture. JOSEPH Black's theory of latent heat was one of his more-important scientific contributions. The theory of latent heat marks the beginning of thermodynamics. He also showed that different substances have different specific heats. The theory ultimately proved important not only in the development of abstract science but in the development of the steam engine. The latent heat of water is large compared with many other liquids, so giving impetus to James Watt's attempts to improve the efficiency of the steam engine invented by Thomas Newcomen. Joseph Black and Watt became friends after meeting around 1757 while both were at Glasgow. Black provided significant financing and other support for Watt's early research in steam power. He died peacefully at his home in Edinburgh in 6th December 1799. The chemistry buildings at both the University of Edinburgh and the University of Glasgow are named after Black. Joseph Black lived for 71 years in this planet. Even today he lives in the form of latent heat and carbon dioxide.
PROBLEM – 01
The following data relate to a three phase arc furnace:
Quantity of steel to be melted in one hour = 5 tons
Specific heat of steel = 0.5kJ/kg-degree centigrade
Latent heat of steel 37.3 kJ/kg
Melting point of steel = 1370 degree centigrade
Initial temperature of steel = 20 degree centigrade
Overall efficiency = 60 percentage
Input current = 5700 A
Resistance of transformer referred to secondary = 0.008 ohms
Reactance of transformer referred to secondary = 0.014 ohms.
Determine the following:
1. Average kW input to the furnace
2. Arc voltage
3. Arc resistance
4. Power factor of the current drawn from the supply and
5. Average kVA input to the furnace.
PROBLEM – 02
The following data relate to a three phase arc furnace:
1. Current drawn = 4000 A
2. Arc voltage = 60 V
3. Resistance of the transformer referred to secondary = 0.0025 ohms
4. Reactance of the transformer referred to secondary = 0.005 ohms
Calculate the following
1. Power factor and KW drawn from the supply.
2. If the overall efficiency of the furnace is 70 percent, find the time required to melt 3 ton of steel.
The latent heat of steel = 37.2 kJ/kg
Specific heat of steel = 0.5 kJ/kg.K
Melting point of steel - 1370 degree centigrade and
Initial temperature of steel = 20 degree centigrade
CALORIMETRY
Calorimetry is the science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state due for example to chemical reactions, physical changes, or phase transitions under specified constraints. Calorimetry is performed with a calorimeter.
CALORIMETER
A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity.
A calorimeter is an experimental device in which a chemical reaction or physical process takes place. The calorimeter is well-insulated so that, ideally, no heat enters or leaves the calorimeter from the surroundings. For this reason, any heat liberated by the reaction or process being studied must be picked up by the calorimeter and other substances in the calorimeter.
FIRST LAW OF THERMODYNAMICS
The change in a system's internal energy is equal to the difference between heat added to the system from its surroundings and work done by the system on its surroundings.
LATENT HEAT
The quantity of heat absorbed or released by a substance undergoing a change of state, such as ice changing to water or water to steam, at constant temperature and pressure. Also called heat of transformation. [Heat absorbed or radiated during a change of phase at constant temperature and pressure]
SENSIBLE HEAT
Sensible heat is heat exchanged by a body or thermodynamic system that has as its sole effect a change of temperature. MELTING POINT
The temperature at which a solid becomes a liquid. For a given substance, the melting point of its solid form is the same as the freezing point of its liquid form. The melting point of ice is 32°F (0°C); that of iron is 2,797°F (1,535°C).
FREEZING POINT
The temperature at which a liquid becomes a solid. For a given substance, the freezing point of its liquid form is the same as the melting point of its solid form. The freezing point of water is 32°F (0°C); that of liquid nitrogen is -345.75°F (-209.89°C).
STANDARD PRESSURE
A pressure at which standardized measurements may be made, equal to 105 pascals or 1 atmosphere. [the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade]
STANDARD TEMPERATURE
A condition for standardized measurements, usually equal to either 273 K (0°C) for properties of gases or 298 K (25°C) for thermodynamic measurements. [Exactly zero degrees centigrade]
SMELT (V) - Extract (metals) by heating; (Metallurgy) (tr) to extract (a metal) from (an ore) by heating
PROBLEM – 01
In a furnace 150 kg of tin is to be melted in one hour. What should be the rating of the furnace if smelting temperature (melting temperature) of tin = 23.5 degree centigrade,
Specific heat capacity of tin = 0.05, latent heat of tin = 13.3 kcal/kg and initial temperature of tin is 35 degree centigrade.
PROBLEM – 02
A low frequency induction furnace has a secondary voltage of 15 V and takes 600 kW at 0.6 power factor when the hearth is full. If the secondary voltage is maintained at 15 V, determine the power absorbed and the power factor when the hearth is half full. Assume the resistance of the secondary circuit to be thereby doubled and the reactance to remain the same. PROBLEM – 03
Determine the efficiency of a high frequency induction furnace which takes 10 minutes to melt 2 kg of aluminium, the input current to the furnace being 5 kW and initial temperature 15 degree centigrade.
Specific heat of aluminum = 0.212
Melting point = 660 degree centigrade
PROBLEM
– 01
– The slab material 2 cm thick and 150 square centimetre in area having
relative permittivity 4 and power factor of 0.04 is to be heated using
dielectric heating. The power frequency is 250 watts are a frequency of 30 MHz.
Determine the voltage and the current that will flow through the material. If
the voltage were to be limited to 600 V. What would be the frequency for the
same power requirement?
PROBLEM
– 02
– A piece of an insulating material is to be heated by dielectric heating the
size of the piece is 10 x 10 x 3 cm. A frequency is 500 W. Calculate the
necessary for heating and the current that follows in the material. The
material has a relative permittivity of 5 and power factor of 0.05.
PROBLEM
– 03
– The wooden board 30 x 15 x 2 cms is to be heated from 20 degree centigrade to
180 degree centigrade in 10 minute by dielectric heating using 50 MHz supply.
Specific heat of wood is o.35 and density 0.55 gm/c.c relative permittivity is
5 and power factor is 0.05. Estimate the voltage across the specimen and
current during heating. Assume loss of energy by conduction, convection and
radiation as 10%.
Ajax
Wyatt vertical core furnace [This is an improved version of direct and indirect
core type furnace]
1.
This furnace makes use of vertical crucible instead of a horizontal one for the
charge.
2.
The shell of this furnace is made up of heavy steel.
3.
The top of the furnace is covered with an insulated cover which can be removed
for charging.
4.
The inside of the furnace is clay lined for yellow brass and alumina for red
brass and bronze.
5.
Apart from V-Shaped channel, U-shaped and rectangular channels are also
employed.
6.
The circulation of molten metal is kept up round the V-shaped portion by
convection currents in the two halves of the V-shaped.
7.
Since it is a vertical core type furnace the tendency of the currents to
interrupt the secondary circuit is due to pinch effect is avoided due to weight
of the charge in the main body of the crucible.
8.
For continuous operation V-Shaped portion must be kept of charge in order to
maintain continuity of the secondary circuit.
9.
This furnace can be operated at power frequency.
10.
Its operating power factor is about 0.8 to o.85 and with normal supply
frequency its efficiency is about 75%.
ADVANTAGES
1.
Suitable for continuous operation.
2.
It is widely used for melting and refining brass and of other non-ferrous
metals and alloys.
HIGH
FREQUENCY CORELESS FURNACE
This
furnace consists of three main parts (i) Primary winding (ii) the refractory
contained and (iii) frame which includes supports and a tilting mechanism.
1.The
main feature of this furnace is that it contains no heavy iron core with
results there is no continuous path for the magnetic flux.
2.
Its construction is simple and its weight is less compared to other furnaces.
3.
Due to less weight it is conveniently tilted for pouring.
4.
This furnace is mainly operated at high frequency
5.
Skin effect exists in this furnace
6.
The primary winding coils are made of hollow tubes and are cooled by
circulation of water through them.
7.
Convenient shape of crucible may be used.
8.
Generally frequency in the range of 500 to 1000 Hz is employed for large furnaces
of up -to 5 tons,
9.
Frequency of 105 are employed for smaller laboratory furnaces for
melting a small quantity of finely divided metals.
10.
For efficient operation, the ratio between the diametre (d) of the charge and
the current penetration (t) should be nearly 8 i.e. d/t = 8.
OPERATION
1.
The charge is put into the crucible and primary is connected to the ac source.
2.
The magnetic flux produced by the primary winding setup eddy currents in the
charge
which,
tend to flow concentrically, and are sufficient to heat the charge to melting
point and also setup electromagnetic forces which produces stirring action.
3.
The magnetic coupling between the primary and secondary winding is low, hence
the furnace power factor is lies between 0.1 and 0.3.
4.
Static capacitors are used to improve the power factor.
5.
By increase the charge diametre d, efficient operation can be achieved even at
low frequencies. This is the reason lower frequencies are used for larger
furnaces and vice versa.
ADVANTAGES
1.
Low erection cost.
2.
Low operating and maintenance cost.
3.
Charging and pouring is simple.
4.
Suitable for intermittent operation.
5.
Simple power control devices can be employed.
6.
Suitable for precious metal melting.
7.
Less melting time.
8.
Free from dirt, smoke and noises.
9.
Fast in operation.
10.
Possible for accurate temperature control.
DISADVANTAGES
1.
Its initial cost is very high 3 to 4 times that of any other type.
2.
It is only economical in sizes up to 4-5 tons.
APPLICATIONS
It
is highly suitable for the production of highest grade alloy steel and there is
no contamination of the product by impurities.
Low frequency furnaces are Direct core, Indirect core and Vertical core (or) Ajax Wyatt furnace.
DIRECT CORE TYPE
In this furnace, the charge forms the short-circuited secondary which is magnetically coupled to the primary by an iron core.
CONSTRUCTION
1. The furnace consists of a circular hearth in the form of a trough (a concave shape with an open top) which contains the charge to be melted in the form of an annular (shaped like a ring) ring.
2. The metal ring quite large in diametre is magnetically inter-linked with an electrical winding.
3. The ring is energized from an a.c. source.
4. This furnace is like a transformer in which the charge to be heated forms a single turn short circuited secondary and is magnetically coupled to the primary by an iron core.
5. When there is no molten metal in the ring, the secondary becomes open-circuited thereby cutting off the secondary current.
6. So, to start the furnace, molten metal has to be poured in the annular.
7. The magnetic coupling between the primary and secondary is very poor, it results in high leakage reactance and low power factor.
8. To minimize the leakage reactance, low primary frequency of the order of 10 Hz is used.
9. For producing low frequencies a special Motor-Generator set is required which increases cost.
10. If the current density increases beyond limit, it produces ‘PINCH EFFECT’ which can cause temporary interruption of the secondary circuit.
DISADVANTAGES
1. This furnace cannot function if the secondary circuit is not closed.
2. The crucible used is of very odd shape and size. This is not convenient from metallurgical point of view.
3. This furnace is not suitable for intermittent services.
PINCH EFFECT
The current density of the charge is to be limited by 480-500 Amps per centimetre square. High current density will produce high electro-magnetic force in the molten metal and that will cause adjacent particles or molecules of the molten metal carrying current in the same direction to repel each other since similar charges repel each others. This repulsion may cause interruption of the secondary circuit". This effect is known as pinch effect.
INDIRECT CORE TYPE INDUCTION FURNACE
In this type of furnace, a suitable element is heated by induction which, in turn, transfer the heat to the charge by radiation.
CONSTRUCTION
1. In this furnace, the secondary winding is formed by the walls of a metal cylinder.
2. An iron core links the primary and secondary winding.
3. When primary winding is connected to a.c. supply secondary current is induced in the metal container by transformer action which heats up the container.
4. Heat produced, due to induced currents, is transferred to the charge by radiation.
5. The part AB of the magnetic circuit situated inside the oven chamber consists of a special alloy which loses its magnetic properties at a particular temperature but regains them when cooled back to the same temperature.
6. If the oven temperature attains the critical temperature, the reluctance of the magnetic circuit increases highly there by cutting off the supply of heat.
7. The bar AB is detachable and can be replaced by other bars having different critical temperature.
9. It is similar to the resistance furnace but with a poor power factor about 0.7.
10. The temperature of furnace can be very easily controlled and a temperature up-to 1000 degree centigrade can be obtained.
The
power supply for the electric arc furnace is low-voltage and high current type.
1.
Power consumption of the arc furnace is very high.
2.
The secondary voltage (arc voltage) is of the order of 50 V- 150V.
3.
Heating effect is proportional to the square of the current, therefore to
achieve higher temperature heavy currents are essential.
4.
The secondary current will be of several hundred or thousand amperes for
melting the metals.
5.
Insulation and safety point of view the maximum secondary voltage is limited to
275V (Line to line open circuit voltage)
6.
The use of low voltage and high current the electrodes are kept very near to
the charge as the arc length is small.
7.
To take care of electro-mechanical and thermal stresses a special type of
transformer with high ruggedness and robustness is required.
8.
The transformer used with arc furnace is oil immersed type.
9.
Both core and shell types are used but shell type is preferred because the
secondary leads can be brought out.
10.
In the primary side of the transformer small amount of current is handled so
tappings should be provided for controlling secondary voltage.
11.
It is desirable to arrange the furnace and transformer in such way that the
secondary leads are of shorter length, so that inductance of the leads is
reduced.
12.
To minimize the skin effect the shape of the leads to be taken into account.
13.
The leads from the transformer to the furnace are to carry heavy currents so
leads consisting of rectangular strips spaced a few mm apart are mostly
employed as current carrying conductors.
14.
A typical specification for a 3-phase arc furnace transformer includes an
extended primary winding with taps there in for the secondary voltage range
235-220-205-190-175-160 volts with primary connected in delta.
15.
This voltage range is extended by changing the connection of the primary
winding from delta to star giving 58% voltage from each tap. [1/1.732 = 0.577 x
100 = 58%]
FURNACE
- A
furnace is a device used for high-temperature heating. The name derives from
Greek word 'fornax', which means oven.
ARC
When
a high voltage is applied across the air gap, the air in the gap gets ionized under
the influence of electrostatic forces and becomes a conducting medium. Current
flows in the form of a continuous spark, called the arc. Very high voltage is
required to establish an arc across an air gap but to maintain an arc, small
voltage may be sufficient.
PRINCIPLE
OF ARC FURNACE
An Arc can also be produced by short
circuiting the two electrodes momentarily and then withdrawing them back. In
this method of striking an arc, high voltage is not required. Arc drawn between
two electrodes produces heat and has a temperature between 1000 degree
centigrade and 1500 degree centigrade depending on the material of the
electrode used.
TYPES
OF ELECTRODE
1.
Carbon electrodes are made of anthracite coal and coke, which are used
with small furnaces for manufacture of ferro-alloys, Aluminium and Calcium
carbide, Phosphorous etc.
2.
Graphite electrodes are obtained by heating the carbon electrode to a
very high temperature.
3.
Self-baking electrodes are made of special paste, whose consumption
depends upon the type of process for which they are used, contained in thin steel
cylinder. The flow of current produces heat and the paste is baked and formed
into an electrode which are employed in ferro alloys and electrochemical
furnaces and in electrolytic production of aluminum.
PROPERTIES
OF ELECTRODES
Most
commonly used electrodes are carbon and graphite electrodes with a size of
diameter of 18 cm to 27 cm and are possessing the following properties.
1. Good electrical conductivity
2. Insolubility,
3. Infusibility
4. Chemical
inertness
5. Mechanical strength and
6. Resistance to thermal shock.
[Thermal
shock is a variation in temperature which causes tension in a material]
COMPARISON OF CARBON AND GRAPHITE ELECTRODE
1.Carbon
electrodes are amorphous (having no definite form or distinct shape).
2.Graphite
electrodes are obtained by heating carbon electrodes to very high temperature,
hence the impurities in the carbon electrodes are volatilized (chemistry- make
volatile; cause to pass off in a vapor).
3.Specific
resistance is lower in graphite electrodes than that of carbon, hence the size
of the graphite electrode will be half of that of carbon for the same
resistance, this leads to easy replacement and lighter control mechanism can be
used compared to carbon.
4.Size
of carbon electrode is higher than that of graphite electrode for same
conductivity and therefore larger area of charge is in contact with the
electrode this results in uniform distribution of heat.
5.The
life of the refractory lining will be affected due to the arc being brought
near to the side of the furnace, because of bigger size of carbon electrode.
6.When
temperature exceeds 600 degree centigrade the oxidation of electrodes start and
consumption of carbon electrodes begin.
7.Amount
of graphite electrode consumed is about half that of carbon for the same work.
8.Carbon
electrodes are cheap and cost less than one half as much for same weight as
graphite electrodes.
TYPES
OF ARC FURNACES
There
are three types of arc furnaces namely (i) Direct arc furnace (ii) Indirect arc
furnace and (iii) Submerged arc furnace.
DIRECT
ARC FURNACE
As the arc in direct contact with the
charge, and heat is produced by the current flowing through the charge itself,
the direct arc furnace gives high temperature.
CONSTRUCTION
1.
This furnace consists of circular steel casting lined inside with refractory
material
2.
The cover is removable and a spare is usually kept for rapid replacement.
3.
The holes are provided on the cover through which the electrodes are inserted.
4.
The electrodes may be of carbon or graphite.
5.
Automatic regulators are used to maintain the desired length to be inserted in
the furnace.
6.
The arc chamber consists of a suitable acid (ground ganister) or basic
(magnesite mix) refactory lining supported on a metal frame work.
7.
The acid process is mainly for making steel castings
8.
This furnace is suitable for making alloy steels such as stainless and
high-speed steel (High-speed steel (HSS or HS) is a subset of tool steels,
commonly used in tool bits and cutting tools)
9.Ttwo
electrodes are sufficient for single phase and d.c supply with this furnace
operation.
10.
The voltage between steel and electrodes may be of 40-145V.
11.
Three phase supply given to the electrodes spaced at the corners of an equilateral
triangle; the charge forms the star point.
12.
In three phase furnace the voltage applied varies between 6.6 kV – 20kV.
13.
The arc is controlled by varying the input voltage or by varying the arc length
or by arc resistance.
14.
Its operating power factor is 0.8 lagging and for one ton furnace, power
required is 200 kW and the energy consumed is 1 MWh per ton.
15.
The capacity of this furnaces is between 5 – 10 tons.
ADVANTAGE
Since
the arc current flows through the charge, the stirring action is inherent due
to the electromagnetic force setup by the current which is the salient feature
of this furnace. This results in uniform heating of the charge.
DISADVANTAGES
Cost of this furnace is very high, due to this it is restricted to refining
than melting.
APPLICATION
The
common application of this type of furnace is to produce high speed steel.
INDIRECT
ARC FURNACE
It is generally in cylindrical shape and
arc formed between the electrodes above the charge and heat is transmitted to
the charge solely by radiation.
CONSTRUCTION
1.
The arc is produced by bringing the electrodes into solid contact and then
withdrawing them.
2.
Heat developed is lower than that of direct arc furnace.
3.
The arc is struck between the charge and the electrodes which are projected
from the top of the furnace.
4.
The electrodes are projecting through the chamber from each end and along the
horizontal axis.
5.
The heat from the arc and the hot refractory lining is transferred to the top
of the layer of the charge by radiation.
6.
In this furnace current does not flow through the charge, so there is no
stirring action.
7.
This furnace requires racking mechanism that is the main reason why it is in
cylindrical shape.
8.
Racking mechanism must be used for thorough mixing of the charge.
9.
While racking the molten metal in contact with the refractory lining will take
some of its heat thus preventing it from high temperature.
10.
The charge in this furnace is not heated only by radiation from the arc between
electrode tips but also by conduction from the heated refractory, during
racking action; so the efficiency of this furnace is high. Sometimes it also
called as racking furnace.
In
this furnace input power is regulated by adjusting the arc length by moving the
electrodes.
SUPPLY
1. Suitable
for single and three phase supply system.
2. Capacity
of this furnace varies from 0.25 ton to 3 tons.
3. The
operating power factor is 0.85 lagging.
ADVANTAGES
1.
Flexible in operation.
2.
Melting is rapid and takes place in a complete closed chamber resulting in
small heat losses and low power consumption.
3.
Metal losses due to oxidation and volatilization are quite low.
4.
Overall cost of production of molten metal per ton is low.
5.
Sound casting in thin and intricate (having many complexly arranged elements;
elaborate) designs can be produced.
DISADVANTAGES
1.
Rocking mechanism is required and
2. Costly than direct arc furnace
APPLICATIONS
1.
This furnace is employed in iron foundries where relatively small quantity of
metal is required intermittently.
2.
Mainly used for melting non-ferrous metals
SUBMERGED ARC FURNACE
In this furnace arc is formed in between electrodes and hearth
electrodes.
CONSTRUCTION
1.
It is cylindrical in shape and carbon electrodes are commonly used.
2.
In this furnace hearth is lined of magnesite which becomes comparatively good
electrical conductor when hot.
3.
Sometimes conduction hearth is also used as an electrode in this furnace.
4.
The number of electrodes depends upon the supply i.e. single or three phase
supply in which bottom conductor is connected to neutral.
5.
The holes are provided on the cover through which the electrodes are inserted.
6.
The arc current from the top electrode passes through the arc to the charge and
returns through electrodes at the bottom of the charge.
7.
By varying the distance between the electrodes or supply voltage, power can be
controlled.
8.
Charge between the electrodes acts as a resistance providing better heat
distribution and better mixing of charge is naturally obtained in this furnace.
9.
Under short-circuited condition the current is limited by the charge.
10.
Its operating power factor is 0.8 lagging.
APPLICATIONS
It
is suitable for manufacturing of Ferro-alloy like Ferro chrome and Ferro-
manganese.
JEAN BERNARD LÉON FOUCAULT – French physicist who determined the speed of light and showed that it travels slower in water than in air; invented the Foucault pendulum and the gyroscope.
Foucault was born on 18th September 1819. He studied medicine, which he abandoned in favour of physics due to a blood phobia.
He first directed his attention to the improvement of Louis Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy.
In 1850, he did an experiment using the Fizeau–Foucault apparatus to measure the speed of light.
In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis (diurnal motion).
Foucault achieved the demonstration by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon, Paris. The experiment caused a sensation in both the learned and popular worlds, and "Foucault pendulums" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the gyroscope as a conceptually simpler experimental proof.
In September 1855 he discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current or "Foucault currents" induced in the metal.
He died on 11th February 1868
He lived for 48 years in this planet but his name is one of the 72 names inscribed on the EIFFEL TOWER.
CHOICE OF FREQUENCY
Low temperature heating of metal, annealing – 50 – 500 HZ.
Melting, deep heat penetration – 500 HZ – 10 KHz
Surface heating of metals – 10 KHz – 200 KHZ
Surface hardening – 100 KHZ – 500 KHZ
Heating metal pieces, wire and metal strips – 400 – 1000 KHZ
Dielectric heating – 1MHZ – 50 MHZ.
EDDY CURRENT – An electric current in a conducting material that results from induction by a moving or varying magnetic field. “Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. The heat can be harnessed for induction heating.”
THE EDDY CURRENT LOSS - Eddy current are those which are produced or induced in the masses of metals, whenever these metals are moved in magnetic field, or the magnetic field moves through the metals. The direction of the eddy currents is always in opposite direction to the cause (motion) to produce them. It is the loss of power i.e. I2R due to eddy currents, and it is expressed in watts, which causes the output of the machine to decrease.
Pe = Kef2Bm2 watts
HYSTERESIS LOSS - The term "hysteresis" is derived from an ancient Greek word meaning "deficiency" or "lagging behind". It was coined around 1890 by Sir James Alfred Ewing to describe the behaviour of magnetic materials. It is the loss of power due to hysteresis, and it is expressed in watts or kilowatts. This actually raises the temperature of the part where the magnetic reversal occurs. Hysteresis cannot be avoided but can be minimized by selecting proper materials which has lesser hysteresis constant. Wh = KhfBm2 watts
At high frequency the heating due to hysteresis becomes very small as compared to eddy currents.
PRINCIPLE OF EDDY CURRENT HEATING
1. It works on the principle of electromagnetic induction.
2. A finite value of diametre and thickness of a metal disc surrounded by a copper coil in which an alternating current is flowing.
3. We find that a secondary current I is caused to circulate around the outer surface of the disc.
4.In transformer the electrical energy is available in secondary while in induction heating it is used to heat the charge itself which acts as a short circuited secondary.
5. The high frequency current carrying coil is known as heater coil or work coil. The material which is to be heated is known as the charge or load.
6.The current flows on the outer surface of the metal disc and in so doing, heats this surface. The heat energy is transferred to the metal at an extremely rapid rate, much faster than conventional methods of heating metal.
7.The heat is generated within the metal without any physical contact between the source of electrical energy and the metal being heated.
8. The medium of energy transmission, the magnetic field, can penetrate any non-metallic substance placed between the heating coil and the material being heated.
9.The process employed is referred to as high frequency eddy current heating.
10. The heat in the disc can be increased by increasing the coil current, increasing the number of turns, frequency of supply, close spacing between the coil and work, using higher permeability magnetic material and higher electrical resistivity of the disc.
Eddy current loss is responsible for the production of heat although hysteresis loss also contributes to some extent in the case of magnetic material. As the eddy current loss is proportional to Bm^2 F^2, this loss can be controlled by controlling Bm and the supply frequency f.
The depth of penetration ‘d’ of eddy currents into the charge is given by:
D = [1/2∏] √ρx 10^9/μf cm, ρ – specific resistance of molten charge in ohm-cm
f – frequency in Hz , μ – permeability of the charge
This shows higher the frequency of supply the lower the depth of penetration.
The depth is directly proportional to 1/√f. The supply frequency is usually employed between 10kHZ to 40kHZ
ADVANTAGES
1. It is quick, clean and convenient method.
2. Heat produced in the body to be heated up directly, hence wastage is less.
3. Eddy current heating can easily take in vacuum or special atmosphere.
4. Temperature control is easy. 5. Heat can be made to penetrate into the metal surface to any desired depth.
APPLICATIONS
It is used for surface hardening, annealing, soldering, welding, drying paints, melting of precious metals, sterilization of surgical instruments and gorging of bolts and rivet heads.
PROBLEM
– 01 – In the case of hardening of a steel, the depth of
penetration is 1.5mm. The relative permeability is unity and resistivity of steel
is 5 x 10^-7 ohm-metre. Determine the necessary frequency required.
PROBLEM
– 02 – A slab of insulating material 150 cm^2 in area and 1
cm thick is to be heated by dielectric heating. The power required is 400 W at
30 MHz. Material has relative permittivity of 5 and power factor of 0.05.
Absolute permittivity = 8.854 x 10^-12 F/m. Determine the necessary voltage.
PROBLEM
– 03 – A piece of an insulating material is to be heated by
dielectric heating. The size is 10cm x 10cm x 3cm. a frequency of 20 MHz is
used and power absorbed is 400W. Calculate the voltage necessary for heating
and current that flows in the material. The material has a relative
permittivity of 5 and power factor of 0.05. If the voltage were limited to
1800V, what will be the frequency to get the same loss?
PROBLEM
– 04 – A plywood board 0.5m x 0.25m x 0.02m is to be heated
from 15 degree centigrade to 150 degree centigrade in 10 minutes by dielectric
heating employing a frequency of 30MHz. Determine the power required in the
heating process. Assume specific heat of wood 1500 J/Kg degree centigrade;
weight of wood 500kg/m^3 and efficiency of process 60 percent.
PROBLEM
– 05 – A piece of plastic material of size 4cm x 2 cm x 1cm
is heated by being placed between two electrodes, each having an area of 20cm x
2 cm and the distance of separation being 1.6 cm. The frequency of voltage
impressed across electrodes is 20 MHz. if the power consumed is 80W, find:
