AU6838294A - Modular induction furnace - Google Patents

Description

MODULAR INDUCTION FURNACE

TECHNICAL FIELD

This invention relates to electric induction furnaces of the kind known as submerged-resistor furnaces or channel induction furnaces.

BACKGROUND ART

Such furnaces may be used to melt non-ferrous metals and metallic alloys. Typically they comprise a refractory hearth or pot adapted to hold a charge of metal to be melted and a so-called inductor.

The inductor is essentially a transformer comprising an endless ferro-magnetic core, a primary winding sleeved on said core, and a secondary circuit linking said core and comprising a loop of molten metal in a secondary passageway defined by a refractory body. The secondary passageway is generally U-shaped and opens at its ends into the interior of the furnace pot, so that the secondary circuit is completed through the pool of metal in the pot. At start up it is necessary to charge the passageway and pot with sufficient molten metal to establish the secondary circuit. Once started, it is desirable for the furnace to run continuously for an extended period, during which further liquid or solid metal is charged into the pot from time to time.

Many modern furnaces have more than one secondary passageway with the several passageways disposed in a symmetrical array. Usually there are as many primary windings as there are secondary passageways respectively closely associated therewith. Frequently the respective passageways share a common central leg, thus if there are two secondary circuits the passageway may be W shaped.

It will be obvious from the foregoing that when the inductor is energised by applying an alternating voltage to the or each primary winding, a secondary current will be induced to flow in the molten metal constituting the, or each, secondary circuit. Each secondary circuit has some electrical resistance so that heat is generated by the secondary current, which heat is relied upon to heat and melt the metal in the furnace.

On the other hand it is by no means obvious as to what causes the observed flow, at a substantial rate, of molten metal in the secondary passageway or passageways, which flow is essential to the rapid transfer of heat from the inductor to the metal in the pot and the efficient operation of the furnace. Standard texts skirt the issue by using vague generalities. For example, one well known Electrical Engineering handbook of which we are aware states that "the hot metal rises, and cooler metal from the main chamber takes its place", but this inadequately explains the steady uni-directional circulation which occurs. Another respected text states it is the "electro-dynamic force of the current in the secondary circuit" which causes the circulation, which is not very meaningful. More recent explanations have postulated second order effects involving eddy currents in the secondary circuit, especially in the pool of metal in the pot itself, and/or leakage fluxes. It has been suggested that such effects may trigger flow in one direction which is subsequently maintained by thermal convection currents. In any event, the fundamental reasons for the pumping effect, which causes uni-directional metal flow through the secondary passageway, and which is a feature of successful furnace designs, are not well understood. Thus, induction furnace designs are largely empirical, and, having regard to this, and to the cost of induction furnaces and the plants they serve, design changes in induction furnaces of the kind under discussion are made with caution and usually represent very small departures from established practice.

In particular, in prior known designs the secondary passageways have always been disposed below the liquid level in the furnace pot and have extended downwardly from their open ends, through which they communicate with the interior of the pot. Possibly this has been so because of a perceived benefit from thermal convection.

Also, it has been customary for the inductor to be bolted to the bottom or sides of the furnace pot to form a unitary structure therewith. Thus, a conventional inductor comprises a refractory body of sufficient strength to withstand the considerable hydrostatic head of molten metal in the lower parts of the secondary passageway, and to assist in maintaining the integrity of the furnace pot having regard to the fact that the pot has openings formed in it to provide communication between the interior of the pot and the passageway within the refractory body. Furthermore, the ferro-magnetic core and the primary winding or windings are normally housed within the refractory body. Thus prior known inductors have been complex, massive and expensive items. DISCLOSURE OF INVENTION

A major deficiency of prior known inductors is the tendency for cracks to form in the refractory body allowing molten metal to leak from the furnace or reach a primary winding, with consequent destruction or at least electrical failure thereof. In the event that such a failure requires the replacement of a prior known bolt-on inductor, then the level of liquid metal in the furnace pot has to be lowered to below the entrances of the secondary passageway or passageways. This is a troublesome and dangerous operation. It must be remembered that if a lot of metal is left in the pot, and as a result, or for some other reason, the replacement of a faulty inductor takes longer than intended, the residual metal in the secondary passageways of other inductors may freeze. In so doing the metal contracts and may destroy a sound inductor or inductors in its or their entirety.

The repair of such a failure is not only expensive in its own right but also may incur much greater costs because of the down time it may cause in a whole manufacturing plant. Indeed, where continuity of production is important it is usual to provide an entire stand-by furnace, notwithstanding the capital cost involved, merely to guard against a minor crack developing in the refractory brickwork of an inductor of an operating furnace.

An object of the present invention is to alleviate the above- indicated deficiencies of prior known furnaces and to provide a furnace of enhanced reliability.

The invention is founded on the appreciation that the pumping effect is dependent on the shape of the secondary passageway, and in particular on the shapes of the end parts of the passageway communicating with the pool of molten metal in the pot. If those end parts differ, so that the flow patterns in the circulating metal near the ends of the passageway also differ, it has been found that pressure differentials created at the ends of the passageway are such that it becomes unnecessary for the passageway to be below the liquid level in the pot while to achieve circulation of molten metal around the secondary circuit at velocities of some metres per second.

This enables a secondary passageway to be provided that is defined wholly, or for the most part, by a rigid, tubular duct that may be supported from, or independently of, the pot above or adjacent the pot with its ends dipping into the pool of metal in the pot, so long as means to evacuate the tubular duct, so as to draw metal into the passageway from said pool at start-up, are also provided. This, in turn, enables the inductor structure to be relatively lightly constructed; it enables the inductor to be of modular form, in that said tubular duct on the one hand and the core and primary winding on the other hand may be provided as separable, independently replaceable units, and in that the inductor as a whole and the pot may likewise be separable, independently replaceable units; it enables a reliable pot devoid of secondary passageway entrance holes to be used; and it enables, for example, one spare tubular duct to be retained for servicing one or more furnaces in lieu of a complete stand-by furnace.

Therefore, the invention consists in a modular inductor for an induction furnace of the kind comprising a pot having an upper open mouth and which is adapted to hold a pool of molten metal having a maximum liquid level, said inductor comprising duct means defining an open ended, secondary passageway having two end parts and an intermediate part, an endless ferro-magnetic core encircling that part of the duct means defining the intermediate part of the passageway, a primary winding sleeved on the core, and connector means whereby the intermediate part of the passageway may be connected to suction means; said duct means being positionable so that the end parts of the passageway extend into such a pot from above said liquid level to below that level so that, in use, the open ends of the passageway are submerged in the pool of molten metal, and being shaped so that one of the open ends of the passageway has a larger cross-sectional area than that of the other open end.

The enlargement of one end of the passageway relative to the other, the consequent asymmetry of the secondary circuit flow pattern, and the consequent differences in the forces due to the interaction of the secondary flows and the primary field, creates the pumping effect causing molten metal to flow strongly through that passageway. The direction of flow is always into the enlarged end of the passageway and out of the unenlarged end. It is the lack of symmetry between the two duct ends that is the requisite for the creation of metal flow, and in experiments leading to the present invention it was shown that where no asymmetry exists, then no metal flow occurs through the duct means.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the above described invention is described in more detail hereinafter with reference to the accompanying drawings.

Figure 1 is a diagrammatic perspective view of a modular inductor according to the invention. Figure 2 is a view similar to figure 1 of the duct means of the inductor of figure 1.

Figure 3 is a centre plane section of the duct means of figure 2.

BEST MODE OF CARRYING OUT THE INVENTION

The illustrated inductor comprises duct means 4, a ferro-magnetic core 5 and a primary winding 6 sleeved upon the core 5.

The duct means 4 comprise an inverted U shaped, rigid tubular duct, being, in this instance, a ceramic pipe 7 and an asymmetric canoe 8. The pipe 7 and canoe 8 define a secondary passageway having an intermediate part 9 and two end parts 10 and 11 respectively. A spigot 12 extending from the pipe 7 constitutes connector means whereby the intermediate part 9 of the passageway may be connected to suction means, not shown.

The canoe 8 is essentially an open bottomed, asymmetric ceramic shell, united to the pipe 7. In use, the canoe 8 may be at least partly submerged in the pool of molten metal in a furnace pot having an open mouth through which the end parts of the pipe 7 may extend from above. In that event the hollow interior of the canoe 8 is filled with liquid metal, to put the end parts 10 and 11 of the secondary passageway into direct communication with the pool of molten metal. It will be seen, in this instance, that the enlargement of one end of the secondary passageway is created by the shape of the canoe 8.

In other instances the duct means may be no more than a U or other shaped pipe, in which event the passageway enlargement is due to the shape of an end part of the pipe itself. In still other embodiments the pipe ends are attached to a snout, that is to say an elongated body of refractory material defining a passage extending through it. The snout may extend through a side wall of the pot or over the lip of the pot. The upper end of the snout is open above the liquid level to allow connection of a tubular duct with end parts of different cross-sectional area. The lower end of the snout lies under the level of liquid metal in the pot. The snout is inclined to allow the inductor to be positioned away from the congested area above the pot while providing access to the pool of liquid metal in the pot. Thus the snout performs a similar function in relation to an inductor according to the invention as does the conventional throat used in relation to conventional, bolt on inductors in a conventional induction furnace, namely to provide a passage through which liquid metal enters and egresses the secondary passageway.

In any event, the relevant component provides an enlargement of one end of the passageway, and that end only, to provide different flow patterns near the respective entrances to the passageway, thereby optimising the pumping effect of the inductor.

For preference mounting means are used that provide for independent support of the duct means 4, and of the core 5 and primary winding 6. This enables those items to be replaced independently of each other if need be.

The mounting means and the inductor as a whole may be flexible in design, as there is no particular limitation on the shape of the intermediate portion of the tubular duct. Thus, in the illustrated embodiment the core 5 lies in a vertical plane suspended directly above the pot, and the intermediate part 9 of the secondary passageway is an inverted U shape with its crotch portion extending through the core window and its legs extending downwardly through the mouth of the pot.

However it is not essential to the invention for the tubular duct to lie in one plane. For example, it may comprise a horizontally disposed U shaped intermediate part with down turned end parts dipping into the pool in the pot. The lightness and independent mountability of the modular inductors allows a plurality of them to be positioned around the pot at elevations relative to the surface of the pool in the pot to accommodate such design concepts as the need to stir lower corners of the pot, or to direct molten metal directly at a strip being coated, either from above or below.

Multiple duct means, each with its own primary winding and sharing a common core, may be used if required, to obtain the heating and stirring power required in the pot. Alternatively, U shaped parts may be combined by sharing a common inlet passage to form a W shaped passageway. The W shaped part should be energised by two primary coils, one for each side leg of the W, and preferably the cross- sectional area of the centre leg should be twice that of each of the two side channels to maximise the ease of flow.

Normally the pot would be furnished with a lid to cover its mouth.

Such a lid would have clearance holes through it for the end parts the secondary tubular duct. It is preferably made from thermally insulating material, and is then relied upon to protect the primary winding or windings from heat radiated from the surface of the pool.

The pipe 7 is rigid enough to be self-supporting, for example it may be made from the material currently marketed under the trade mark MARSINT. That material is essentially a zirconia stabilised aluminium titanate. Alternatively, an unstabilised aluminium titanate ceramic, such as that marketed under the trade mark TIT, silica glass or other appropriate chemically stable, heat resistant material may be used.

The ceramic pipe of the duct means, especially if constructed from

MASINT or TIT or equivalent, and the suspension of the duct means above the pot, offer many advantages over the prior art. Because the ceramics are prefired there is no need for the long dry-out and preheat schedules required by conventional designs. Because of the superb thermal shock resistance of the preferred ceramics, the ceramic components may be placed in contact with the liquid pot metal much more quickly than for conventional inductors. Because of the location of the inductor and because it is independently mounted, when replacing inductors there is no need to pump out metal from the pot to below the throats of the inductors as in the conventional design which has inductors mounted on the side or bottom of the pot, thus saving time, reducing metal losses during pump-out, and the need for a stand-by pot. The combined advantages mean that an inductor can be changed in an hour compared with the 96 to 140 hours typically required with previous designs.

The spigot 12 is at highest point of the of the pipe 7 and enables a connection to be made to suction means, such as a vacuum pump or other evacuating means. This is required to enable liquid metal to be drawn into the secondary passageway so as to fill it at the time of start up of the furnace. Furthermore some ceramic materials suitable for use as duct material are porous, and when such material is used the evacuating means may be operated from time to time to dispose of accumulated air from within the secondary passageway. The height of a liquid which can be supported by one atmosphere is such that the height, h, atmospheric pressure, p, density of the liquid, p, and acceleration due to gravity, g, are related by:

h=p/pg

For liquid aluminium with a density of 2400 kgm-³, the height is 4.3m, while for liquid zinc with a density of 6600 kgm-³, the height is 1.6m. Other liquid metals and liquid metal mixtures, such as zinc/aluminium and zinc/aluminium/silicon, will vary according to their density. Such heights are therefore those which can be supported by suction means producing a perfect vacuum above the liquid metal in the secondary passageway. These heights represent the maximum possible height obtainable with a vacuum system attached to the duct. In practice, suitable furnaces can conveniently be made with the tubular ducts rising to a height in the range of from 0.5 to 1.5m above the pot's minimum operating liquid level. A typical height of from 0.5 to 1.5m requires only a partial vacuum. For liquid aluminium, the vacuum required for 0.75m is about 0.83 bar, while for zinc it is about 0.52 bar. Other liquids will vary according to their density.

In one embodiment, the vacuum system required to raise and support the liquid in the passageway consists of a liquid ring vacuum pump, capable of operating over the pressure range 0.04 to 1 bar, a 12 L vacuum chamber, two gas inlet valves (the first one manual and the second one automatic) connected to a nitrogen gas supply, level detection means for upper and lower limits, and suitable vacuum hoses and connections and electrical controls. In operation the vacuum pump is operated continuously and the vacuum pressure first adjusted manually using the first gas inlet valve to set the liquid level in the passageway to just below that required, and then the vacuum and hence the liquid height is adjusted by turning the second gas inlet valve on or off automatically to keep the liquid metal between the upper and lower limits. The lower limit must be such that the U or W shaped passageway is completely filled with liquid metal. The vacuum chamber dampens the response of the system, in that the height of the liquid is controlled more by the vacuum pressure in the large chamber rather than by the relatively small volume in the spigot 12 or its equivalent. An inert gas such as nitrogen gas is preferred so as to reduce oxidation of the liquid metal, though air or any other suitable gas may be used.

The enlargement of the end of the secondary passageway may be effected by shaping the tubular duct or canoe, as the case may be, in a variety of ways.

There may be an abrupt transition between the smaller and larger parts of the passageway; alternatively, the transition may be gradual or tapered. Such tapering may or may not extend to the end of the passageway. If it does, the passageway may be said to have an outwardly flared or belled mouth and the term "enlarged cross-sectional area" as applied to a passageway includes passageways having an outwardly flared or belled mouth.

Claims (8)

1. A modular inductor for an induction furnace of the kind comprising a pot having an upper open mouth and which is adapted to hold a pool of molten metal having a maximum liquid level, said inductor comprising duct means defining an open ended, secondary passageway having two end parts and an intermediate part, an endless ferro- magnetic core encircling that part of the duct means defining the intermediate part of the passageway, a primary winding sleeved on the core, and connector means whereby the intermediate part of the passageway may be connected to suction means; said duct means being positionable so that the end parts of the passageway extend into such a pot from above said liquid level to below that level so that, in use, the open ends of the passageway are submerged in the pool of molten metal, and being shaped so that one of the open ends of the passageway has a larger cross-sectional area than that of the other open end.

2. An inductor according to claim 1 wherein the duct means comprise a rigid tubular duct.

3. An inductor according to claim 2 wherein said tubular duct is a ceramic pipe.

4. An inductor according to claim 2 wherein said duct means further comprise a canoe, it being the canoe that defines the open ends of the passageway.

5. An inductor according to claim 1 further comprising a snout through which, in use, said duct means gain access to the pool of molten metal in the pot.

6. An induction heating furnace comprising an inductor according to any one of the preceding claims, a said pot, supporting means for the inductor positioning it so that the end parts of said passageway extend into the pot as aforesaid, and a said suction means connected to said connector means.

7. An induction heating furnace according to claim 6 wherein said suction means comprise a vacuum pump, a vacuum chamber, a gas inlet valve, liquid level detection means, vacuum hoses interconnecting the inductor's connector means, the pump and the chamber, and electrical controls responsive to said detection means and operating said valve to keep said passageway charged with liquid metal.

8. A modular inductor substantially as described herein with reference to the accompanying drawings.