Induction-furnace.pptx

AKHILA PRAVALLIKA SURESH RAHUL SRAVYA

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HISTORY Induction furnace works on the principle of induction heating.  The roots of induction heating go back to the very beginning of electromagnetism.  Heating by EDDY CURRENTS or “FOUCAULT CURRENTS” was discovered in 1855 when LEON FOUCAULT found that a significant additional torque was necessary to keep the copper disc rotating when it was placed into the permanent magnetic field and that the disc was heated by the induced currents. 

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At the time there was no mathematical basis for the theoretical description of this effect and it was studied experimentally, mainly in the application to magnetic brakes. This kind of induction was used and continues to be used occasionally for heating of the fast moving strips and rotating cylinders. Recently, this very old phenomenon became the subject of an intensive study for heating of the aIuminium cylinders rotating in the very strong field of a superconductive magnet.

The first practical attempts to use induction currents generated by the alternating field for the heating purposes were made in 1891-92.  Theoretical and experimental studies of eddy current generation started even earlier with main reference to magnetic cores.  A big contribution was made by H.HERTZ, O.HEAVISIDE,J.J THOMSON,J.A. EWING and others.  Studying EM processes in the magnetic cores, sir J. EWING described a coined the word HYSTERESIS. 

It is well known that magnetic field in induction systems may be accurately described by MAXWELL equation.  It is worth to remember that they were not created by Maxwell in the present elegant form.  Modification of these equations must be credited to OLIVER HEAVISIDE, a brilliant scientist. 

After several attempts of scientists, the Induction furnace was designed by professor E. NORTHRUP .  He not only built famous channel Induction furnace but also high frequency furnace with mercury arc power supply. 

Definition: An induction furnace is an electric furnace in which the heat is applied by induction heating of metal that uses electric current to melt a metal.  Once molten ,the high frequency magnetic field can also be used to stir the hot metal , which is useful in ensuring that alloying conditions are fully mixed into melt 

Metals which are melted in an Induction furnace include iron and steel, copper, aluminum and precious metals because it is a clean and noncontact process .  It can be used in vacuum or inert atmosphere.  Vacuum furnaces make use of induction heating for production of special steels and other alloys that would oxidize if heated in the presence of air. 

INDUCTION HEATING Induction heating makes use of a transformer effect  Induction heating is a non-contact form of heating  Induction heating is the process of heating an electrically conducting object(usually metals) by electromagnetic induction , through heat generated in the object using eddy currents.  Non-magnetic materials make use of only eddy currents while in magnetic materials, hysteresis losses are in additional. 

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Induction heating relies on the unique characteristics of radio frequency (RF) energy - that portion of the electromagnetic spectrum below infrared and microwave energy. Since heat is transferred to the product via electromagnetic waves, the part never comes into direct contact with any flame, the inductor itself does not get hot (see Figure ), and there is no product contamination. When properly set up, the process becomes very repeatable and controllable. Thus it becomes a noncontact process.

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Principle of induction heating is mainly based on two important well known phenomenon: 1. electromagnetic induction 2. joule heating effect

Electro-Magnetic induction The energy transfer to the object to be heated occurs by means of electromagnetic induction.  Any electrically conductive material placed in a variable magnetic field is the site of induced electric currents , called eddy currents which will eventually lead to joule heating. 

JOULE HEATING Joule heating also known as OHMIC HEATING and RESISTIVE HEATING, is the process by which the passage of an electric current through a conductor releases heat.  The amount of heat produced is proportional to the square of the current multiplied by electrical resistance of the wire. Q ∝ i²R 

FACTORS CONTROLLING INDUCED HEAT IN THE DISC: 1. High coil current(I).  2. Larger no. of coil turns(N).  3. High frequency supply(f).  4. Close spacing between coil and charge(d).  5. The disc may be of magnetic material(μ).  6. Higher electrical resistivity of the disc(ρ). Heat induced ∝ all the above factors. 

INDUCTIONHEATING Hysteresis loss ∝ f  Eddy current losses ∝ f²  The frequency of AC depends on the object size, material type , coupling and penetration depth.  Depth of penetration of induced current into the disc  d≈½Π√(ρ×10^9/fμ) 

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Operating frequencies range from utility frequency (50 or 60 Hz) to 400 kHz or higher, usually depending on the material being melted, the capacity (volume) of the furnace and the melting speed required. Generally, the smaller the volume of the metal, the higher the frequency of the furnace used; this is due to the skin depth which is a measure of the distance an alternating current can penetrate beneath the surface of a conductor. For the same conductivity, the higher frequencies have a shallow skin depth—that is less penetration into the melt. Lower frequencies can generate stirring or turbulence in the metal.

CURIE TEMPERATURE: The temperature at which alignments of domain become random and material lose its magnetism.  At higher frequency, the heating due to hysteresis become very small as compared to eddy currents , this is due to higher temperature attained by the material at which it lose its domain alignment , thus become un magnetised.  At higher frequency, eddy current losses also do not follow f² law as frequency is increased higher and higher. 

FEATURES An electric induction furnace requires an electric coil to produce the charge. This heating coil is eventually replaced.  The crucible in which the metal is placed is made of stronger materials that can resist the required heat and the electric coil itself cooled by a water system so that it does not overheat or melt.  The advantage of induction furnace is a clean energy efficient and well controllable melting process compared to most other means of metal melting. 

The induction furnace can ranges in size from a small furnace used for very precise alloys only about a kilogram in weight to much larger furnaces made to mass produce clean metal for many different applications.  Foundries use this type of furnace and now also more iron foundries are replacing cupolas with induction furnaces to melt cast iron as the former emit lots of dust and other pollutants. 

CONSTRUCTION 

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There are many different designs for the electric induction furnace but they are centered around basic idea. The electric coil is placed around or inside of crucible , which holds the metal to be melted often this crucible is divided into 2 different parts

The lower section holds the melt in its purest form ,the metal as the manufacturers desire it , while the higher section is used to remove the slag , the contaminants that rise to the surface of the melt.  Crucibles may also be equipped with strong lids to lessen how much air has access to the melting metal until it is poured out making a pure melt . 

Types of induction furnace There are two main types of induction furnace , coreless and channel CORELESS INDUCTION FURNACE:  The flux produced in by the primary winding sets up eddy current in the charge which flow concentrically in the primary winding.  These currents heats up the charge to the melting point and provide stirring action to the charge. 

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Since the frequency of supply is very high the skin effect in the primary coil increases the effective resistance of the coil and hence copper losses tends to be high and additional cooling is necessary. The coil is therefore made of hollow copper conductor through which cooling water can be circulated.

ADVANTAGES OF CORELESS: They are fast in operation.  If the frequency is high, the lower magnetic flux density due to absence of iron core for a given primary current would be compensated for and the need for heavy iron core can be eliminated.  They are used for steel production.  It is very widely employed for various industrial activities like soldering , brazing , hardening and annealing drying paints etc. 

Core type induction furnace 

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Furnace having core with secondary short circuited in coil form around the furnace and primary connected to supply. It operates at a frequency of order 10 Hz. If current density exceeds about 500A/sq.cm , the current around the melt interacts with the alternating magnetic field and completely interrupts the secondary side. This is known as PINCH EFFECT.

This furnace is inconvenient where different types of charges are to be melted.  PRACTICAL EXAMPLE OF CORE TYPE FURNACE: AJAX-WYATT FURNACE: This is normally used for melting and refining brass and non ferrous metals.  DISADVANTAGE OF CORETYPE: the necessity of magnetic yoke , the short circuited secondary formed by the charge and the PINCH EFFECT, made the core type less popular. 

ADVANTAGES OF INDUCTION FURNACE: Higher yield. The absence of combustion sources reduces oxidation losses that can be significant in production economics.  Faster Startup. Full power from power supply is available instantaneously; thus reducing the time to reach working temperature.  Flexibility. No molten metal is necessary to start medium frequency coreless induction melting equipment. 

Natural Stirring. Medium frequency units can give a strong stirring action resulting in a homogeneous melt.  Cleaner Melting. No bi-product are produced due to cleaner melting environment.  Compact Installation. High malting rates can be obtained from small furnaces.  Energy Conservation. Overall energy efficiency in induction melting ranges from 55 to 75 percent, and is significantly better than combustion processes. 

DISADVANTAGES The one major draw back to induction furnace usage in a foundry is the lack of refining capacity; charge materials must be clean of oxidation products and of known composition, and some alloying elements may be lost due to oxidation.  Removal of s & p is limited, so selection of charges with less impurity is required. 

APPLICATIONS The large scale application of electromagnetic induction has become very commonplace in today’s manufacturing industries. Many components are heated in very large ovens or furnaces to several thousand degrees Celsius.  A simple induction furnace where the object being heated is placed inside a copper coil that carries a high frequency AC current and cooling water within the tube 

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They range in use from the small, domestic furnaces used for heating, to annealing (heat treating) metal parts, hardening metals, cap sealing, heat shrink fitting and wire stripping. Even the de-gassing of cathode ray tube components is done by induction heating. The induction furnace is also used to heat small items to weld the parts together in router bits (used in woodworking).

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