THERMAL ISOLATION SCREEN FOR ISOLATING AN ELECTROMAGNETIC INDUCTOR, AND HEAT TREATMENT INSTALLATION COMPRISING SUCH A SCREEN

Abstract

Thermal insulation screen transparent to magnetic flux, intended to isolate an electromagnetic inductor with a transverse or pseudo-transverse field, from the radiation of a heated product (1), the thermal screen being made up of a matrix of blocks (7) made of a thermally insulating material and of a plurality of tubes (8) cooled by the circulation of a fluid, these tubes being imprisoned in said matrix of blocks and the tubes and the blocks being held in place by a support. The screen includes, behind the blocks, heat-conducting means (10) placed so as to intercept the heat flux passing through the blocks, these conducting means being thermally coupled to the cooled tubes (8) for discharging the heat flux into the tubes.

Description

The invention relates to a cooled thermal insulation screen transparent to magnetic flux, intended to isolate an electromagnetic inductor having a transverse or pseudo-transverse field from the radiation of a heated product. The thermal screen is made up of a matrix of blocks made of a thermally insulating material and of a plurality of metal tubes cooled by the circulation of a fluid, these tubes being imprisoned in said matrix of blocks, and the tubes and the matrix of blocks being held in place on a support, which is located on the opposite side from the product to be heated.

The invention relates more particularly, but not exclusively, to a thermal insulation screen for an electromagnetic induction device for heating a continuously running metal strip.

EP 1 349 431 discloses a chamber with a thermal insulation screen and a support consisting of a sheath.

The materials of the support are electrically insulating and consist, for example, of a composite, glass fibres plus epoxy or other resin, glass or ceramic. The support for the thermal screen must not be exposed to excessively high temperatures, typically below 120° C. and preferably below 90° C., depending on the materials used for this support.

The electromagnetic coils for the induction heating are located on the opposite side of the screen from the product to be heated. To obtain good energy efficiency, it is necessary for these coils to be as close as possible to the product to be heated. It is therefore desirable for the thickness of the various walls of the screen to be as small as possible, while still ensuring sufficient thermal protection.

The tendency is to increase the temperature of the heated product, especially for metallurgical reasons, such that the risks of raising the temperature of the support for the screen above the acceptable limit increase substantially.

For better protection of the screen support, it may be conceivable to reduce the separation between the tubes cooled by the circulation of a fluid. However, these tubes are preferably formed by straight segments of coils obtained by bending several times, in opposite directions, a length of tube. The diameter of the tubes used imposes a minimum radius of curvature for bending them, so that it is not possible to reduce the separation between the tubes below a value imposed by acceptable bending of the tubes.

The drawbacks of the prior techniques for a thermal screen stem not only from the technological problems of manufacturing hairpins from tubes bent through 180° with smaller bending radii, but also thermal problems induced by the reduction in the centre-to-centre spacing of the tubes. Such a reduction in centre-to-centre spacing of the tubes results in:

• • increased thermal losses, as additional losses occur in the magnetic field since the length of tubes lying in the field is increased; and
  • lower surface temperature of the blocks, which may affect the heating of the product.

The object of the invention is, above all, to provide a cooled thermal insulation screen that allows better thermal protection of the support for the screen without substantially increasing the thickness of the layer of insulating material, and without having to modify the spacing between the tubes cooled by the circulation of a fluid.

According to the invention, a thermal insulation screen transparent to magnetic flux, intended to isolate an electromagnetic inductor with a transverse or pseudo-transverse field from the radiation of a heated product, the thermal screen being made up of a matrix of blocks made of a thermally insulating material and of a plurality of metal tubes cooled by the circulation of a fluid, these tubes being imprisoned in said matrix of blocks, the tubes and the blocks being held in place by a support, is characterized in that heat-conducting means are provided to the rear of the blocks, these conducting means being thermally coupled to the cooled tubes, said heat-conducting means intercepting the heat flux that passes through the blocks and discharging it into the cooled tubes.

The thermal screen according to the invention protects the support by reducing the operating temperature thereof.

Preferably, the heat-conducting means are also equipped with means for preventing, or reducing, the thermal losses resulting from the induced currents generated by the magnetic flux.

The heat-conducting means may be made of stainless steel, bronze, copper or another suitable material.

The heat-conducting means may be formed by a screen made of a heat-conducting material, especially a metallic material, thermally coupled to the cooled tubes. Advantageously, the screen is formed by metal fins that extend behind the blocks, each fin being in thermal contact with a cooled tube.

The means for preventing, or reducing, the formation of induced currents may comprise slots made in the conducting means. The slots are mainly oriented transversely to the longitudinal direction of the tubes, but they may also be inclined thereto.

Advantageously, the slots open onto the longitudinal edges of the metal screen and stop near the central portion.

The thermal screen is optimized when the induced currents are extremely small. To do this, fins having a sufficiently small area perpendicular to the magnetic field are provided. A good result is obtained by reducing the pitch of the slots, by varying the width of the slot and/or by varying the width of the fin and its thickness.

According to one embodiment, the fins may be produced by lengths of wire that are oriented radially and fixed to the cooled tubes.

Each fin may extend, on each side of a tube, over substantially half the gap between two adjacent tubes.

Each fin may be welded to a corresponding tube. The region of the fin in contact with the tube may have the form of a cradle that follows a portion of the contour of the tube.

The fins may be formed by slots made in an extruded or drawn metal screen/tube assembly made as a single part.

Preferably, the fins extend over the entire length of the tubes in the region of the inductor to be protected from the heat flux.

The invention also relates to a heat treatment installation, especially a furnace, which includes an electromagnetic inductor having a transverse or pseudo-transverse field, for heating a product, characterized in that it includes, for protecting the inductor against the thermal radiation of the heated product, at least one thermal screen as defined above.

The invention consists, apart from the arrangements presented above, of a number of other arrangements that will be explained in greater detail later with regard to exemplary embodiments described with reference to the appended drawings, which are in no way limiting. In these drawings:

FIG. 1 is a diagram of a metal strip heat treatment installation with a thermal insulation screen transparent to magnetic flux according to the invention;

FIG. 2 is a cross section on a larger scale of the thermal insulation screen, with cut-away portions;

FIG. 3 is a plan view with cut-away portions of the thermal insulation screen;

FIG. 4 is a cross section on a larger scale of detail IV of FIG. 1 showing the heat-conducting means according to the invention;

FIG. 5 is an end view of a heat-conducting means;

FIG. 6 is a partial plan view relating to FIG. 5; and

FIG. 7 illustrates, in vertical section, an alternative embodiment of a heat-conducting means.

FIG. 1 of the drawings shows schematically a vertical furnace H of a line for the heat treatment of a metal strip 1, especially a steel strip, which runs continuously at a constant or variable speed. In the example shown, the strip 1 runs vertically between two rollers, a lower roller 2 and an upper roller 3 respectively. This example is not limiting—the strip 1 could run horizontally in a horizontal furnace.

The furnace has generally a prismatic shape of rectangular cross section and its internal volume is isolated from the external atmosphere. The composition of the gas mixture inside the furnace may vary depending on the zones in question. This gas mixture comprises a relatively high proportion of hydrogen, favourable for heat exchange.

The furnace H includes at least one heating section 4 equipped, on each side of the strip 1, with a half-inductor having a transverse or pseudo-transverse flux 5a, 5b. A pair of half-inductors constitutes a heating inductor.

A thermal insulation screen E transparent to the magnetic flux is provided, at the half-inductors 5a, 5b, so as to thermally protect the inductors facing the heated product formed by the strip 1.

As may be seen in FIG. 2, the screen E comprises a support 6 formed from an electrically insulating and especially gas-impermeable material. Advantageously, the support 6 is made of a composite with an epoxy or similar resin.

The minimum dimension k (FIG. 2) of the space between opposed faces of the screen E, along the direction orthogonal to the strip 1, is imposed by considerations for the purpose of avoiding any contact between the strip and the internal surface of the screen. This dimension k may be around twenty centimetres.

The internal faces of the support 6 support and hold in place, by fastening means (not shown), a matrix B of blocks 7 made of thermally insulating material, especially ceramic, and a plurality of tubes 8 cooled by the circulation of a fluid, generally water. The tubes 8 are generally made of metal, but they could be made of another material, for example glass completely transparent to the magnetic field. The tubes 8 are imprisoned in the matrix of blocks 7 and protected against the direct radiation from the strip 1.

The tubes 8 preferably consist of straight segments. According to the example shown in FIG. 3, the tubes 8 are segments of a continuous coil obtained by successively bending a tube through 180° in opposite directions. An inlet and an outlet for the cooling fluid are provided for each coil. Several coils constituting successive panels may be juxtaposed against the internal wall of the support 6. The internal diameter of the tubes 8 is about eight to ten millimetres.

The blocks 7, as may be seen in FIG. 2 and 4, are formed by substantially rectangular parallelepipedal blocks, the two longitudinal faces of which include recesses 7a, 7b in the form of cylindrical portions suitable for following the contour of two successive tubes 8. The blocks 7 may thus be engaged between two straight tubes 8. The blocks 7 bear against one another along the longitudinal direction of the tubes 8. A small transverse space 9 (FIG. 4) may be left between two adjacent blocks 7.

The front face 7c of the blocks 7 is the one directly exposed to the thermal radiation from the strip 1. The rear face 7d of the blocks, which is the face furthest from the strip 1, is at a temperature considerably below that of the face 7c because of the thermal resistance presented by the block 7 and because of the heat extracted by the tubes 8 and the cooling fluid.

In order for the strip 1 to be heated with maximum energy efficiency by electromagnetic induction with a transverse or pseudo-transverse flux, it is desirable to place the inductors as close as possible to the strip 1. When the treatment temperature of the strip 1 increases, there is a risk of the support 6 being exposed to too high a temperature.

To ensure effective thermal protection of the support 6, without the thickness of the insulating blocks 7 being increased and without reducing the distance between successive tubes 8, the invention provides, at the rear face 7d of the blocks, heat-conducting means 10 placed so as to intercept the heat flux Q passing through the blocks 7. These conducting means 10 are thermally coupled to the cooled tubes 8 in order to discharge the heat flux Q into the tubes and into the cooling liquid circulating in the tubes. Furthermore, the conducting means 10 are equipped, as illustrated in FIG. 6, with means 11 for preventing or reducing the formation of induced currents generated by the transverse alternating electromagnetic flux.

Preferably, the conducting means 10 consist of a metal screen S (FIG. 4) of small thickness j especially less than five millimetres. The screen S covers most of the rear faces 7d of the blocks.

The metal screen S is formed by a set of metal fins 12 separated from one another. Associated with each tube 8 are metal fins 12 covering the rear face 7d of the blocks, each fin being in thermal contact with the associated cooled tube 8. The terms “heat sink” or “heat extractor” could be used as synonyms for the term “fin”.

Each fin 12 is formed by a metal strip preferably extending over the entire length of the tube 8. The strip has a projecting central longitudinal portion 12a engaged in a free space 13 between the rear edges of two successive blocks 7. The central portion 12a comes into contact with the wall of the tube 8 to which it is preferably welded. The metal strip extends on either side of the central portion 12a, symmetrically or asymmetrically, with blades 12b, 12c meeting substantially at mid-width of each block 7. Such an arrangement makes it possible to limit the length of the path for discharging the heat into the tube 8 to substantially one half of the width of a block 7. The ends of the blades 12b, 12c of two successive fins are separated by a gap of small width m (FIG. 4), especially less than five millimetres, and preferably less than two millimetres. The rear face 7d of each block is thus practically entirely covered by fins 12.

In the example shown in FIG. 2, the cross section of the thermal screen has a substantially rectangular closed outline. This example is not limiting. The thermal screen may consist, on each side of the strip, of a plate having several layers (insulating blocks, cooled tubes, conducting means). In this case, the cross section of the screen would be a straight segment, as shown in FIG. 4. A plate-shaped heat screen according to the invention is particularly suitable for installations such as those intended in Patent Applications FR 05/06462 and FR 05/06463 filed on 24 Jun. 2005 by the same filing company CELES.

According to the exemplary embodiment shown in FIG. 4, the central portion 12a of the fin has the shape of a dome with its convex side turned towards the tube 8, said dome having oblong openings 14 for welding it.

As a variant, as illustrated in FIG. 7, the projecting central portion 12a has a concavity forming a cradle for following the facing portion of the tube 8 and providing a larger contact area for heat transfer.

According to another embodiment, the fins may be produced by slots made in a tube 8/metal screen S assembly extruded or drawn as a single part.

The means 11 for preventing, or reducing, the formation of induced currents generated by the magnetic flux preferably comprise slots 15 made in the fins 12. The slots are oriented mainly transversely to the longitudinal direction of the fins, as illustrated in FIG. 6, but they may also be inclined thereto. The slots 15, which are parallel to one another, open onto the longitudinal edges of the metal screen S and are closed near the central portion 12a.

The thermal screen is optimized when the induced currents are extremely small. To do this, fins having a sufficiently small area perpendicular to the magnetic field are provided. A good result is obtained by reducing the pitch of the slots, by varying the width of the slot, especially by varying the thickness of a saw cut constituting the slot, and/or by varying the width of the fin and the thickness. According to one example, the slot width is 0.5 mm and the fin width between two slots is adapted to the electrical loss of the currents induced in this fin.

As a variant, the fins may be produced by lengths of wire. The lengths of wire are oriented transversely, radially or obliquely to the cooling tubes 8 and are fixed, especially welded, to the tubes 8, the assembly resembling a comb, the teeth of which would be formed by the lengths of wire. The diameter of the wire may be 1 mm with a gap between each wire of 0.3 mm, this gap constituting the slot 15.

The fin 12, seen from above as illustrated in FIG. 6, has a shape that resembles that of a fishbone.

The presence of the slots 15, which greatly reduce the induced currents generated by the magnetic flux, reduces parasitic heating that would lower the energy efficiency.

Advantageously, the tubes 8 and/or the fins 12 are made of stainless steel. As a variant, the tubes 8 and/or the fins 12 are made of bronze or copper, while other metals may also be envisaged.

The operation results from the foregoing explanations.

The transverse alternating magnetic field heats the strip 1, which radiates heat towards the surfaces that surround this strip. The heat flux Q that passes through the blocks 7 to reach the rear face 7d is predominantly intercepted by the fins 12 and discharged into the tubes 8 and into the cooling fluid. The support 6 is better protected against the heating that would be due to this flux Q.

The invention makes it possible, without the thickness of the heat screen E being substantially increased and without the spacing of the tubes 8 being modified, to provide effective thermal protection of the support 6 for high temperatures of the strip 1, for example 1150° C.

Although the description has been given with regard to a strip 1, the invention applies to other types of metal products, especially steel, copper or aluminium wires or plates. The product may or may not be a running product.

Claims

1. Thermal insulation screen transparent to magnetic flux, intended to isolate an electromagnetic inductor with a transverse or pseudo-transverse field from the radiation of a heated product, the thermal screen being made up of a matrix of blocks made of a thermally insulating material and of a plurality of tubes cooled by the circulation of a fluid, these tubes being imprisoned in said matrix of blocks, the tubes and the blocks being held in place by a support, characterized in that it includes, behind the blocks, heat-conducting means placed so as to intercept the heat flux passing through the blocks, these conducting means being thermally coupled to the cooled tubes for discharging the heat flux into the tubes.

2. Thermal insulation screen according to claim 1, wherein the heat-conducting means are equipped with means for preventing, or reducing, the formation of currents induced by the magnetic flux.

3. Thermal insulation screen according to claim 1, wherein the heat-conducting means are formed by a metal screen thermally coupled to the cooled tubes.

4. Thermal insulation screen according to claim 3, wherein the metal screen is formed by metal fins that extend to the rear of the blocks, each fin being in thermal contact with a cooled tube.

5. Thermal insulation screen according to claim 4, wherein the fins are produced by lengths of metal wire that are fixed to the cooling tubes.

6. Thermal insulation screen according to claim 1, wherein the means for preventing, or reducing, the formation of induced currents generated by the magnetic flux comprise slots made in the conducting means.

7. Thermal insulation screen according to claim 6, wherein the slots are oriented transversely to the longitudinal direction of the tubes.

8. Thermal insulation screen according to the claim 4, wherein the slots open onto the longitudinal edges of the metal screen and are closed near the central portion.

9. Thermal insulation screen according to claim 4, wherein each fin extends on either side of a tube.

10. Thermal insulation screen according to claim 9, wherein each fin extends, on each side of a tube, over substantially half the gap between two adjacent tubes.

11. Thermal insulation screen according to claim 4, wherein each fin is welded to a tube.

12. Thermal insulation screen according to claim 4, wherein the fins are formed by slots made in an extruded or drawn metal screen/tube assembly made as a single part.

13. Thermal insulation screen according to claim 4, wherein the region of the fin adjacent to the tube has the form of a cradle that follows a portion of the contour of the tube.

14. Thermal insulation screen according to claim 4, wherein the fins are provided over the entire length of the tubes, in the region of the inductor to be protected from the heat flux.

15. Heat treatment installation, especially a furnace, which includes an electromagnetic inductor having a transverse or pseudo-transverse field, for heating a product, characterized in that it includes, for protecting the inductor against the thermal radiation of the heated product, at least one thermal screen according to claim 1.