EP0020215A1 - Induction reheating furnace with travelling field - Google Patents

Abstract

Electric oven with variable geometry, for induction heating of metallic products, in which the heating device is a flat inductor with sliding magnetic field, constituting a large movable side wall of the oven and equipped on its active face with a refractory lining. According to the invention, the oven comprises means for thermal protection of the inductor (11) constituted by a cooling screen (20) interposed between the active face (18) of the inductor and the refractory lining (19), and in which circulates a cooling fluid. The invention not only provides effective thermal protection of the inductor, but also improves the overall heating efficiency of the furnace.

Description

The present invention relates to an electric furnace with variable geometry intended for the induction heating of metallic products, in particular of large format, such as steel slabs which are placed on edge inside the furnace.

An oven of this type is described in French Patent No. 2,339,316. The heating device is a flat inductor with a sliding magnetic field, constituting the essential element of at least one large movable side wall of the oven. This inductor comprises electrical conductors with bars which are housed in notches formed in a magnetic yoke on the face facing the interior of the furnace and hereinafter referred to as "active face of the inductor".

One of the essential requirements is to ensure that the inductor is kept at an acceptable temperature level in the face of the thermal load originating from both the internal heating and the radiation of the product in the oven.

According to the aforementioned patent, the thermal protection of the inductor is obtained by the combined action of a refractory coating equipping the active face of the inductor and a circulation of cooling fluid in the notches.

Such a solution is not without drawbacks if we consider for example that, in the case of a sliding field inductor, the air gap plays a crucial role for the electrical efficiency of the oven, and that in this respect the The thickness of the refractory lining should be as small as possible.

A similar observation relates to the cooling in the notches, in particular when, in accordance with the embodiment described in the aforementioned patent, the electrical conductors are "Roebel" type bars intended to facilitate the passage of the necessary high intensities. In this case, in fact, the possibilities of cooling by internal circulation of fluid are on the other hand more limited than for the more conventional tubular conductors which are commonly used for heating by solenoid, or by any other type of field inductor. magnetic stationary.

According to a preferred embodiment described in the aforementioned patent, the thermal protection of the inductor is supplemented by a circulation of cooling fluid in channels parallel to the bars, placed at the head of notches. Here again, certain difficulties may appear: difficulties of practical realization first, because it is not easy to install such channels in the notches of the cylinder head; mechanical strength difficulties then due to phenomena of vibration of the inductor during the passage of the alternating current; electrical difficulties finally because these channels, arranged parallel to the busbars, can become the seat of parasitic eddy currents.

The object of the present invention is to improve the cooling efficiency of the inductor without having the aforementioned drawbacks.

To this end, the subject of the invention is an electric furnace with variable geometry intended for the induction heating of metallic products, in particular of large format, such as steel slabs, arranged on edge inside the furnace and in which the heating device is a plane inductor with a sliding magnetic field constituting at least one large movable side wall of the oven and equipped on its active face with a refractory lining, oven comprising means for cooling the inductor with internal circulation of a cooling fluid and characterized in that these means consist of an independent cooling screen, interposed between the active face of the inductor and the refractory lining and having a composite structure having a plurality of successive electrical discontinuities in the direction of the primary current in the inductor.

As can be understood, the present invention therefore consists in replacing the cooling channels incorporated in the magnetic yoke of the inductor by a cooling screen independent of the yoke, masking the active face of the inductor and sandwiched between the latter and the refractory lining.

However, arrangements must be made to make this screen cool, as transparent as possible to the magnetic field. In this regard, it is advantageous to use, for the manufacture of the screen, a material having a low electrical conductivity and a relative magnetic permeability close to the unit. However, for obvious reasons of mechanical strength, it is hardly conceivable at the present time to use a material other than a metal, and in particular steel, so that one can fear some difficulties in plan of the magnetic flux transmission through the screen.

In order to overcome these difficulties, and thus allow the cooling device to be a "heat screen" without constituting a "magnetic screen", a characteristic of the invention consists in providing this screen with a composite structure of so as to present electrical discontinuities one after the other in a parallel direction to the conductor bars of the inductor.

In this way, in fact, the eddy currents which tend, as we know, to form in a direction parallel to that of the primary current of the inductor, will see their possibility of development in the screen considerably reduced and, with it, the absorption of the magnetic field through it.

One possible technology consists in making the walls of the screen in alternating metallic and non-metallic strips, oriented perpendicular to the conductive bars of the inductor.

A preferred embodiment, in accordance with the invention, resides in that the cooling screen consists of metal tubes placed side by side in close proximity but without contact with each other, so as to provide between two consecutive tubes a electrical insulation space, said tubes being oriented perpendicular to the busbars.

Of course, these provisions can be advantageously supplemented by others, consisting, depending on the above-mentioned indications, of an appropriate choice of the metal from which the screen is made, for example a non-magnetic stainless steel.

In a particularly advantageous manner, this screen can be produced from elementary units which are assembled on the oven. This solution allows prefabricating standard size units, the number of which will be used will depend on the size of the oven intended to receive them.

• - la figure 1 est une section verticale perpendiculaire au plan des grandes faces de l'inducteur,
• - la figure 2 est une vue en section horizontale passant par une encoche de la culasse magnétique,
• - la figure 3 est une vue de face d'une unité élémentaire constitutive de l'écran refroidissant selon l'invention,
• - la figure 4 est une vue de profil de l'unité représentée sur la figure 3.

The invention will be well understood and other aspects and advantages will emerge more clearly in the light of the description which follows, given with reference to the accompanying drawing plates in which:

• FIG. 1 is a vertical section perpendicular to the plane of the large faces of the inductor,
• FIG. 2 is a view in horizontal section passing through a notch in the magnetic yoke,
• FIG. 3 is a front view of an elementary unit constituting the cooling screen according to the invention,
• - Figure 4 is a side view of the unit shown in Figure 3.

In all the figures, the same elements are designated by identical references.

In Figures 1 and 2, there is shown at 1 a steel slab arranged on edge inside the heating oven 2 by means of a support base 3. In order to be able to adapt the geometry of the oven to the format of the .brame, the large side walls 4, placed opposite the large faces of the slab, are movable in translation on a raceway 5 by means of wheels 6. The displacements of the walls are controlled by jacks shown in 7, 7 '. The closing of the heating enclosure is completed laterally by two small refractory partitions 8, at the upper part by a refractory cover 9 and the sealing of the enclosure at its lower part is ensured by compression of an elastic seal 10 under the action of the base 3.

As can be clearly seen in FIG. 1, the large side walls 4 of the furnace are essentially constituted by a plane electromagnetic inductor 11. This is formed from a magnetic yoke 12, laminated vertically and having, on the side of the slab, horizontal notches in which are housed bars 13 of the Roebel type for the passage of electric current.

These bars are connected in a known manner to a polyphase power supply, not shown (for example three-phase), so as to generate in the enclosure of the furnace a mobile magnetic field, sliding vertically.

The conductive bars 13 are more visible in FIG. 2 where their curved ends 14 are also shown more generally called "coil heads" and allowing the series connection of the bars connected to the same phase of the power supply.

It is reported that to absorb the vibration energy due to the passage of alternating current, the magnetic yoke 12 is fragmented into several elementary blocks separated by intermediate plates 15 of relatively soft metal, such as aluminum, the assembly being held by clamping rods 16. On the other hand, in order to block the bars 13, longitudinal elastic shims 17 are placed at the head of each notch. If necessary, a more detailed description of the technology of the inductor and the furnace in general can be found with reference to patent No. 2,339,316 and to its first certificate of addition No. 2,354,015.

As can be seen, the inductor is equipped on its active face, designated at 18, with a refractory lining 19 with interposition between the two, with a cooling screen 20 of composite structure according to the invention, and of which a shape preferred embodiment but not limiting is described in more detail below.

The screen 20 is constituted by a plurality of parallel metal tubes 21 arranged side by side in close proximity but without mutual contact so as to define therebetween electrical insulation spaces 22. These tubes 21 are oriented perpendicular to the busbars 13, that is to say vertically in the example described. To give the assembly additional rigidity, the spaces 22 are preferably filled with an appropriate material, insulating from heat and electricity, for example a material commercially available under the designation "Syndanio", composed a mixture of mineral stone and asbestos agglomerated under high pressure.

The purpose of this composite structure of the screen 20 and its orientation with respect to the busbars is, as already mentioned, to disadvantage the development of undesirable eddy currents.

Of course, the screen 20 is part of a cooling circuit (not shown) to allow internal circulation of a cooling fluid, for example water, and which will be considered as such in the following.

In accordance with a preferred variant, the screen 20 is produced by assembling identical elementary cooling units 23, a copy of which is clearly visible in FIGS. 3 and 4.

It can be seen that each unit 23 comprises an inlet pipe 24 and an outlet pipe 25 for the internal circulation of the cooling water.

To this end, the tubes 21 belonging to the same unit 23 can be mounted in parallel or preferably in series as is the case in FIG. 3. In this case, the unit 23 constitutes a cooling coil in which the water circulates successively in each of the tubes 21 which allows a better distribution of the absorption of the heat flow.

As can be seen, the tubes communicate with each other at their end by means of two transverse conduits, respectively upper 26 and lower 27. This particular technology has the advantage of being able to pass from a parallel mounting of the tubes to a mounting in series without requiring significant modifications. In fact, as clearly shown in FIG. 3, to go from a parallel mounting to a serial mounting, it suffices to obstruct the transverse pipes 26 and 27 in suitable places, that is to say between two tubes 21 neighboring and distant from a space corresponding to a pair of tubes, the closure points being offset by one unit between the two lines 26 and 27.

These closures can be carried out simply, as shown in the figure by means of pellets 28 placed in the conduits after having made in the latter semi-annular cutouts 29 in the appropriate places.

Note that the inlet and outlet pipes 24 and 25 are located on the same side of the screen, which has the advantage of greater practical simplicity in terms of the overall architecture of the oven.

We will now give some figures, absolutely non-limiting, on the structure of the cooling unit 23.

The non-magnetic stainless steel tubes 21 have an external diameter of 15 mm and a thickness of 2 mm. In this regard, it is preferable, for electromagnetic reasons, to choose the smallest possible tube thickness, compatible with the requirements of mechanical strength of the screen, which can be improved, as we have seen, by placing in the spaces 22 of the insulating shims.

These spaces 22 are of small width: 2 mm in the example described. This value can be further reduced, the only condition being that the tubes are not in contact with each other in order to avoid electrical continuity in the transverse direction. The length of the tubes is preferably at least equal to the height of the active region of the inductor. This arrangement has the additional advantage of placing the transverse lines 26 and 27 outside the air gap and thus preventing them from becoming the seat of eddy currents.

The units 23 are fixed to the furnace using two identical flanges extending the screen vertically on either side.

• - le fait que l'écran refroidissant est un organe autonome rapporté sur le four, donc structurellement indépendant de l'inducteur, ce qui permet une grande simplicité de conception et de réalisation ainsi qu'une grande souplesse de maintenance.
• - l'efficacité de l'écran pour la protection thermique de l'inducteur est telle que l'épaisseur du revêtement réfractaire peut être substantiellement réduite par rapport à celle du four connu décrit dans le brevet n° 2 339 316. On peut ainsi passer d'une épaisseur voisine de 10 cm à environ 3 cm. Il en résulte compte tenu de l'épaisseur de l'écran (1,5 cm environ), une réduction de l'entrefer de l'ordre de 5 cm, et ceci grâce à la possibilité d'adapter la géométrie de l'enceinte du four à celle des produits à chauffer.

In Figures 3 and 4, only the upper flange 30 has been shown. As can be seen, this flange consists of an L-shaped section, the small branch 31 of which is welded to the transverse pipe and the large branch 32 of which, placed in the plane of the internal surface of the screen, has two spacers in U, 33 and 34. The upper spacer 34 is pierced with orifices 35 for the passage of fixing bolts shown at 36 in FIG. 1. In addition to the effectiveness of the thermal protection of the inductor, the invention has many other advantages, among which we can cite:

• - The fact that the cooling screen is an autonomous member attached to the furnace, therefore structurally independent of the inductor, which allows great simplicity of design and construction as well as great flexibility of maintenance.
• - The efficiency of the screen for the thermal protection of the inductor is such that the thickness of the refractory lining can be substantially reduced compared to that of the known oven described in patent No. 2,339,316. with a thickness close to 10 cm to approximately 3 cm. This results, taking into account the thickness of the screen (approximately 1.5 cm), a reduction in the air gap of the order of 5 cm, and this thanks to the possibility of adapting the geometry of the oven enclosure to that of the products to be heated.

• - une amélioration du cos φ de l'installation. A cet égard, l'expérience a montré que le cos φ du four connu décrit dans le brevet précité était légèrement inférieur à celui d'un chauffage par un inducteur classique à champ magnétique stationnaire. Grâce à l'invention, le cos devient désormais supérieur, ce qui implique une économie, par exemple au niveau des condensateurs électriques ainsi qu'au niveau des câbles d'amenée du courant qui peuvent être moins gros.
• - une amélioration du rendement électrique de chauffage, surtout sensible dans le cas, conforme à l'invention, d'un chauffage par inducteur à champ magnétique mobile.
• - une amélioration du rendement de chauffage global du four, car l'amélioration du rendement électrique de chauffage prime sur les pertes thermiques, légèrement accrues, dues à la réduction de l'épaisseur du revêtement réfractaire.
• - enfin, un avantage très particulier au cas d'un inducteur à champ glissant, à savoir la réduction, voire la suppression, du déséquilibrage entre phases électriques, responsable de ce que l'on appelle en électrotechnique "les effets d'extrémités" ou plus précisément les "effets de longueur finie" propres aux moteurs linéaires à induction. En effet, le fait de pouvoir réduire l'entrefer permet de réduire corrélativement le pas polaire de l'inducteur tout en conservant constante la valeur du rapport entre le champ magnétique créé au voisinage de l'inducteur et le champ magnétique agissant sur le produit à chauffer, autrement dit en conservant constant l'affaiblissement relatif du champ magnétique depuis l'inducteur jusque sur la surface du produit.

Reducing the air gap has other advantages which can be listed as follows:

• - an improvement in the cos φ of the installation. In this regard, experience has shown that the cos φ of the known oven described in the aforementioned patent was slightly lower than that of heating by a conventional inductor with stationary magnetic field. Thanks to the invention, the cos now becomes higher, which implies a saving, for example at the level of the electric capacitors as well as at the level of the current supply cables which can be smaller.
• an improvement in the electrical heating efficiency, especially noticeable in the case, in accordance with the invention, of heating by inductor with a mobile magnetic field.
• an improvement in the overall heating efficiency of the furnace, since the improvement in the electrical heating efficiency takes precedence over the slightly increased thermal losses due to the reduction in the thickness of the refractory lining.
• - finally, a very particular advantage in the case of a sliding field inductor, namely the reduction, or even the elimination, of the imbalance between electrical phases, responsible for what is called in electrical engineering "the effects of extremities" or more precisely the "finite length effects" specific to linear induction motors. Indeed, the fact of being able to reduce the air gap makes it possible to correlatively reduce the pole pitch of the inductor while keeping the value of the ratio between the magnetic field created in the vicinity of the inductor and the magnetic field acting on the product constant. heating, in other words keeping the relative weakening of the magnetic field constant from the inductor to the surface of the product

It is therefore possible, therefore, to increase the number of pairs of magnetic poles per phase of the power supply, and therefore to correspondingly reduce the electrical asymmetry between the input end and the output end of the magnetic field. , for example between the lower and upper ends of the inductor when the mobile magnetic field slides vertically from bottom to top.

Claims (6)

1 °) Electric oven with variable geometry intended for the induction heating of metallic products and in which the heating device is a plane inductor with sliding magnetic field constituting at least one large movable side wall of the oven and equipped on its active face with a coating refractory, furnace comprising means for cooling the inductor by internal circulation of a cooling fluid and characterized in that said means consist of an independent cooling screen, interposed between the active face of the inductor and the refractory lining, and having a composite structure having a plurality of consecutive electrical discontinuities in the direction of the primary current in the inductor. 2) An electric oven according to claim 1, characterized in that the cooling screen is constituted by a plurality of parallel metal tubes arranged side by side, in close proximity but without mutual contact so as to define spaces between them. electrical insulation, said tubes being oriented perpendicular to the direction of the primary current in the inductor. 3 °) Electric oven according to claim 2, characterized in that the cooling screen is formed by assembling identical elementary units each comprising a plurality of tubes. 4 °) Electric oven according to claim 3, characterized in that the tubes belonging to the same elementary unit are connected in series so as to constitute a cooling coil, and in that the elementary units are mounted in parallel with each other. 5 °) Electric oven according to claim 4, characterized in that the tubes are connected in series at their ends. 6 °) Electric oven according to claim 2, characterized in that the electrical insulation spaces are occupied by a material insulating from electricity and giving the assembly additional mechanical rigidity.

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