Furnaces Used in Induction Heating | Electrical Engineering

In induction heating effect of currents induced by electromagnetic action in the charge is employed. The heat developed depends on the power drawn by the charge. The power consequently depends upon the voltage and resistance of the charge, because power drawn is equal to V²/R. So to develop heat sufficient to 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. Magnetic materials, therefore, can be easily treated than non-magnetic materials because of their higher permeability.

The various types of induction furnaces are described as follows:

1. Core Type Furnaces:

The core type furnace is just like a transformer having primary connected to the supply and the charge to be heated as secondary.

i. Direct Core Type Induction Furnace:

Direct core type induction furnace consists of an iron core, crucible of some insulating material and primary winding connected to an ac supply. The charge is kept in the crucible, which forms a single turn short-circuited secondary circuit. The current in the charge is very high, of the order of several thousand amperes.

This type of furnace has following drawbacks:

a. As magnetic coupling between the primary and secondary circuit is poor, therefore, leakage reactance is high and power factor is low. This difficulty, however, is overcome by employing supply of frequencies as low as 10 Hz for operation of such furnaces. For obtaining low frequency supply motor-generator set or frequency changer is required, which involves extra cost.

b. If normal frequency supply is employed for operation of such furnaces, the electromagnetic forces cause severe stirring action in the molten metal. Low frequency supply, therefore, is also necessary from this point of view.

c. If the current density exceeds about 5 amps per mm² the pinch effect (formation of bubbles and voids etc.) due to electromagnetic forces may cause complete interruption of the secondary circuit and so of supply.

d. The crucible for the charge is of odd shape and inconvenient from the metallurgical point of view.

e. For functioning of the furnace the closing of the secondary circuit is essential which necessitates the formation of complete ring of the charge around the core. For starting the furnace, either molten metal is poured into the crucible or sufficient molten metal is allowed to remain in the crucible from the previous operation. This is required otherwise, the secondary will remain open circuited and no current will circulate and no heating will take place.

However, once the melting starts additional metal can be added and the furnace tapped periodically. Also in order to close the secondary circuit, an iron ring may be placed in the crucible or the lining may be of graphite. Such furnaces are not suitable for intermittent services or where different types of charges are to be melted.

On account of the drawbacks mentioned above such furnaces have become obsolete nowadays.

ii. Vertical Core Type Induction Furnace:

This is an improved form of furnace described above. The furnace of this type, known as Ajax Wyatt vertical core type furnace employs a vertical channel instead of horizontal one for the charge. The convection currents keep the circulation of molten metal round the V portion.

As V- channel is narrow, so even a small quantity of charge is sufficient to keep the secondary circuit closed. Hence the chances of discontinuity of the circuit are less. Due to pinch effect the adjoining molecules carrying current in same direction will try to repel each other, but because of the weight of the charge they will remain in contact and chances of interruption will be reduced.

The output of the furnace depends upon the type and dimensions of the channels used. In certain furnaces instead V-shaped channels U-shaped channels or rectangular channels are employed.

The inside of the furnace is lined depending upon the charge. Clay lining is used for yellow brass. For red brass and bronze an alloy of magnesia and alumina or corundum having high contents of alumina is employed.

The shell of the furnace is of heavy steel. The top of the furnace is covered with an insulated cover which can be removed for charging. Necessary hydraulic arrangements are usually made for tilting the furnace to take out the molten metal.

Advantages:

1. Highly efficient heat, low operating costs and improved production.

2. Accurate temperature control, uniform castings, reduced metal losses and reduction of rejects.

3. Absence of crucibles.

4. Consistent performance and simple control.

5. Ideal working conditions in a cool atmosphere with no dirt, noise or fuel.

6. Absence of combustion gases resulting in elimination of the most common source of metal contamination.

7. Comparatively high power factor (the average power factor between 0.8 and 0.85) with normal supply frequency since primary and secondary are both on the same central core.

However, it is to be noted that the Vee must be kept full of charge in order to maintain continuity of the secondary circuit. For this reason, this type of furnace is suitable only for continuous operation. These furnaces are widely used for melting and refining of brass and other heavy non-ferrous metals. Its efficiency is about 75 per cent. Standard sizes of these furnaces range 60 to 300 kW, all single phase, 50 Hz in this country for standard voltage up to 600 V.

iii. Tama Furnace:

The main drawback of Ajax Wyatt furnace is that it cannot be used for melting aluminium and its alloys. This is mainly because of tendency of adherence, of any aluminium oxide formed, to the walls of V channel. This makes the operation of furnace impossible any longer. This difficulty, however, can be overcome by using a ring type furnace, known as the Tama furnace.

In this furnace inductor channels are built vertical which facilitates removal of the deposits with a simple and cheap type cleaning tool. Vertical channels can be cleaned without emptying the furnace and without taking long time. The horizontal channel through the base of furnace does not require frequent cleaning. No doubt, it needs emptying of furnace before removal of drain plug but the operation is speedy.

The pf of the Tama furnace is reduced because of its straight sided channels and therefore installation of additional capacitors is required which makes the Tama furnace costlier than the Ajax Wyatt furnace. Stirring action is also set up due to electromagnetic forces in this furnace like Ajax Wyatt furnace.

iv. Indirect Core Type Induction Furnace:

In such a furnace an inductively heated element is made to transfer its heat to the charge by radiation. In this type of furnace the principle of induction has been utilised for providing heat treatment of metallic and other charges in addition to its use for melting metals.

It consists of an iron core linking with the primary winding and secondary also. In this case secondary consists of a metal container forming the walls of the oven proper. Primary winding is connected to the ac supply, inducing currents and heating the metal container. Heat is transmitted to the charge by radiation. It is advantageous in respect of temperature control without use of external control equipment.

It consists of part AB of the magnetic circuit situated in the oven chamber and consisting of a special alloy which loses its magnetic properties at a particular temperature and regains them when cooled to the same temperature. As soon as the oven attains the critical temperature, the reluctance of the magnetic circuit increases many times and the inductive effect correspondingly decreases, thereby cutting off the heat supply. The bar AB is detachable type and can be replaced by others having different critical temperatures between 400 °C and 1,000 °C, according to needs.

From the mode of transmission of heat it will be seen that this furnace is directly in competition with resistance oven; but has comparatively poor power factor (0.8 approximately).

2. Coreless Induction Furnace:

It essentially consists of three main parts:

(i) The primary coil

(ii) The refractory container and

(iii) The frame which includes supports and a tilting mechanism.

The distinctive features of this furnace are the absence of a continuous iron path for the magnetic flux and small quantity of refractory material in comparison with other types of melting furnaces in construction.

Standard preformed crucibles are employed for small furnaces (up to about 200 kg holding capacity). The base and the wall around the crucible are made by ramming granular refractory material. The top of the wall is sealed with refractory cement. The containers of the large furnaces are made in place, the procedure being same as for the smaller furnaces except that a hollow collapsible form is substituted for the crucible to form the receptacle. Acid or basic materials are used as per requirements.

For floor level mounting, the electrical connections are made by knife contacts in order to make the handling of the furnace as a ladle for pouring. For platform mounting the electrical connections are made with flexible cables. This is done as with this arrangement, power can be left on the furnace while pouring—a feature often desirable when a number of castings are poured from one heat. A variation is the lift- coil furnace made in the smaller sizes. The primary coil is lowered and raised over the load crucible.

The charge is put into the crucible and primary winding coil is connected to high frequency ac supply. The flux created by primary winding sets up eddy currents in the charge which tend to flow concentrically with those in the inductor. These eddy currents heat up the charge to its melting point and also set up electromagnetic forces producing stirring action which is essential for obtaining uniform quality of metal.

Because of high frequency employed, which is necessary to induce the required voltage in the secondary, the skin effect produces heat in the primary winding coils. The primary winding coils are, therefore, made from hollow tube and are cooled by circulation of water through it. Insulated supporting structure is employed for such furnaces otherwise stray magnetic field outside the primary will set up emf in it, which will result in circulation of eddy currents in it and so reduction of efficiency.

Standard sizes of coreless induction furnaces for melting non-ferrous metals and alloys range from 50 kg to 500 kg holding capacity.

The choice of frequency of operation plays vital role. It is governed by the factors, material to be heated and thickness of cylinder layer at the outside edge of the crucible. The frequency of the primary current can be ascertained by using penetration formula. Accordingly,

Where, ρ is the resistivity of molten metal in Ω-m, f is supply frequency in Hz and µ is relative permeability (for molten steel µ = 1).

The exact theory also shows that for efficient operation the ratio of radius of piece of material in the charge to the thickness should be greater than 3. If it is taken as 4, then the expression for the frequency, for efficient operation, becomes-

From above Eq. (5.12) it can be seen that by increasing r efficient operation can be obtained with lower frequencies. In most of the modern coreless induction furnaces the frequencies in the range of 500 to 1,000 Hz are used. However, for smaller units for melting small quantities of finely divided metal frequencies up to 100 kHz or even 1,000 kHz are used.

The refractory container makes necessary a large air gap (loose coupling) with consequent low power factor (between 0.1 and 0.3). Static capacitors are, therefore, invariably employed in parallel with such a furnace in order to improve the pf. Since in case of coreless induction furnace operation of does not remain constant, capacitance in the circuit during heat cycle is varied to maintain power factor approximately unity.

The choice of frequency is influenced also by the cost of the capacitors, used for power factor improvement, which decreases with increase of frequency, and the cost of the converting apparatus, which increases with frequency.

Advantages of Coreless Induction Furnaces:

The advantages of a coreless induction furnace over other types are given below:

(i) Low operating cost.

(ii) Low erection cost.

(iii) Automatic stirring action produced by eddy currents.

(iv) Absence of dirt, smoke, noise etc.

(v) Simple charging and pouring.

(vi) Possibility of operating the furnace intermittently, as no time is lost in warming up.

(vii) Precise control of power.

(viii) Less melting time.

(ix) Possibility of employing vacuum heating necessary for precious metal melting.

(x) No contamination of charge and very accurate control of composition so most suitable for production of high grade alloy steels.

The coreless induction furnace is mainly employed as a metal melting unit. An important application of this furnace is the production of carbon free ferrous alloys. Various special uses are: vacuum melting, duplexing steel, heating of charges of non-conducting materials (with or without melting) by the use of conducting crucibles etc. The energy consumption is 600 to 1,000 kWh per tonne of steel.