FR2898460A1 - Electrical immersion heater for molten metal bath, comprises electrically insulating-thermally conducting refractory core having helicoid grooves on a heated part of the heater and longitudinal grooves on a non-heated part of the heater - Google Patents

Abstract

The electrical immersion heater for molten metal bath, comprises electrically insulating-thermally conducting refractory core (9) having helicoid grooves on the heated part of immersion heater and longitudinal grooves on the non-heated part, a metallic resistance, an electrical wire, an external refractory sheath material, and a boron nitride powder transfer unit. The metallic resistance is placed in helicoidal grooves of core at the heated part of the heater, and the electrical wire is placed in the longitudinal grooves of core in the non-heated part of the heater. The electrical immersion heater for molten metal bath, comprises electrically insulating-thermally conducting refractory core (9) having helicoid grooves on the heated part of immersion heater and longitudinal grooves on the non-heated part, a metallic resistance, an electrical wire, an external refractory sheath material, and a boron nitride powder transfer unit. The metallic resistance is placed in helicoidal grooves of core at the heated part of the heater, and the electrical wire is placed in the longitudinal grooves of core in the non-heated part of the heater. The boron nitride powder transfer unit allows to ensure a thermal transfer between refractory core having heating element (8) and the sheath material. The refractory core has two helicoid grooves, which allows a single phase electric supply, and has three helicoid grooves with a common point at an end of heated part of the heater to allow a triphased electric supply (star system). The refractory core is made up of ceramics obtained by processing, pressing or molding. The refractory core is metallic but coated with a refractory deposition. The electrically insulating core ensures an insulation of resistance wire to the effects of thermal cycles and increased temperature level. The longitudinal grooves have an upper section on the heated part to increase the section of resistant wire and to reduce thermal heating at the non-heated part. The material constituting the wire in non-heated part of the heater is different from the wire in the heated part of the heater. The core comprises a hole (13) in its longitudinal axis for fixing a thermocouple (14) to measure a temperature of the heated part. The refractory core is in a form of an application tube, where the external sheath of immersion heater is of large diameter.

Description

Electric immersion heater for liquid metal baths Domaine de

  The present invention relates to electric immersion heaters for maintaining a molten metal bath. By liquid metal, we mean all pure metals (aluminum, magnesium, zinc, lead, tin ...) but also all their alloys whose melting temperature is generally between 200 and 1000 C.

  BACKGROUND OF THE INVENTION Electric immersion heaters are already widely used in industry to maintain constant temperature melting furnaces. They are mostly in the form of an axisymmetric element placed vertically in the oven. They are usually suspended from the top wall of the oven. The immersion heater comprises a heating zone which must be immersed in the product to be heated and a non-heating zone for connecting the heating zone and the power supply box. The lower part of the immersion heater being immersed in the liquid, it is necessarily closed and sealed, therefore all the electrical connections are made by the upper part of the immersion heater. The heating zone has an internal heating element carried at high temperature by the passage of an electric current. This internal heating element transmits its heat to a sheath constituting the body of the immersion heater, which in turn transmits heat to the molten metal or alloy. The main characteristics of this type of chiller concern the high operating temperature (for example of the order of 800 C at the sheath in the case of a liquid aluminum bath), the high power density at the level of the heating zone and the strong chemical aggressivity of molten metal baths (especially near the free interface of molten materials). This aggressiveness requires the use of specific materials for producing the sheath: silicon carbide, sialon, silicon nitride, carbon / carbon composite, graphite, alumina, boron nitride, etc. For example, the French application 272088 describes the case of aluminum nitride used either as a solid material for producing the sheath, or as a coating on a standard substrate. The Australian patent AU-B-52441/86 uses the same concept by seeking to apply a protective coating having the same coefficient of expansion as the metal constituting the sheath which makes it possible to withstand the constraints of differential expansions. The heating elements are generally made in the form of a resistive wire (French applications FR-A-2623043, 2641930), a carbon element placed in an inert atmosphere to prevent its oxidation at high temperature (French applications FR-A-2864416, 2622382, 2559886, European Patent EP0510701A2), a semiconductor-type material placed between two electrodes (US Pat. No. 4,039,737). The materials must be able to withstand a continuous operating temperature of the order of 1000 to 1200 C at the resistive wire. This is the reason why one finds, in the resistances of the metallic type, alloys based on nickel-chromium, tungsten or molybdenum. Resistors based on silicon carbide or molybdenum bisilicide can be used under an oxidizing atmosphere (air) while carbon or carbon-based composites require operation under a neutral or reducing atmosphere. However, the use of non-metallic resistors generally requires the use of an electrical transformer to adapt the characteristics of the main network to those of the resistor. This point is all the more crucial as some heating elements exhibit resistance variations as a function of temperature but also during aging (in particular silicon carbides).

  SUMMARY OF THE INVENTION The present invention relates to an elongated immersion heater for heating liquid metal baths while supporting the constraints related to these baths (high operating temperature, resistance to thermal shock, corrosion resistance, service life significant).

  Another object of the invention is to use a metal-type electrical resistor allowing a direct power supply of the immersion heater from the main network without the use of a transformer.

  Another object of the invention is to be able to propose a connection allowing a direct supply of the immersion heater under a three-phase network.

  Another object of the invention is to ensure a sufficient life and reliability allowing the use of the immersion heater in an industrial setting.

  For this purpose, the present invention provides for the introduction of a metallic electrical resistance on a refractory core. Two zones can then be considered: - a heating zone on which the resistance is helically wound.

  This resistance may consist of a single wire for a supply under a single-phase voltage or three son connected to a common point for supplying a three-phase voltage. In this case the 3 wires describe three helicoids nested in each other. a cold zone on which the wire is placed in the longitudinal axis of the immersion heater and for which the wire has a lower ohmic value than in the cold zone. This cold zone makes it possible to make the electrical connection between the external electrical network and the heating zone. The material constituting the core must be refractory in order to withstand the maximum temperature of the metal son during operation, electrical insulation in order not to put the metal son in short circuit and thermal conductor to promote the cooling of the metal son constituting the resistance. The assembly thus formed is placed in a ceramic sheath or metal with protective coating. This sheath is then placed in the molten metal in order to maintain the latter in temperature. Since the outer diameter of the refractory core equipped with the resistive wires is smaller than the inside diameter of the sheath, it is necessary to place a thermally conductive and electrical insulating material between the core and the sheath. This material must also support the operating temperature of the resistive wires and be compatible with the different materials used. It can, for example, be obtained by compacting a refractory and thermally conductive powder. It is essential to ensure a good thermal transfer by conduction between the son and the sheath otherwise heat exchange will take place essentially by radiation with consequent a sharp increase in the temperature of the wire which can lead to its destruction (overheating, melting ... ). Furthermore, this heat transfer must be stable over time to ensure a sufficient life of the product. Thermal transfer must not be degraded by thermal cycling and differential expansion differences between different materials. The refractory core may be drilled longitudinally at its center over a portion of its length to receive a thermocouple for a temperature measurement of the heating element. This measurement can be used for temperature regulation of the molten metal bath or for overheating of the immersion heater. The upper part of the immersion heater must then be sealed to prevent moisture or other pollution from entering the immersion heater. This closure may advantageously be performed with a silicone or acrylic sealant to obtain a good seal with the sheath and the current supply son while having a good temperature resistance.

Brief description of the drawings

  These and other objects, features and advantages of the present invention will be set forth in detail in the following description of particular embodiments given in a non-limiting manner in relation to the attached figures, in which: FIG. longitudinal sectional view of an exemplary conventional embodiment of an immersion heater in the case of a single-phase supply; Figure 2 gives a sectional view of the immersion heater along the line II-II of Figure 1; Figure 3 gives a sectional view of the immersion heater along the line IV-IV of Figure 1; Figure 4 is a perspective view of the refractory core on the heating portion in the case of a single-phase supply; Figure 5 gives a perspective view of the refractory core on the heating part in the case of a three-phase power supply; Figure 6 gives a perspective view of the refractory core at the transition zone in the case of a single-phase power supply; Figure 7 gives a perspective view of the immersion heater at the transition between heating zone and cold zone in the case of a single-phase power supply.

Detailed Description 10

  Figure 1 shows, by way of example, a section of an immersion heater in the case where the latter is powered by a single-phase source. This immersion heater is intended to be mounted on an oven or a ladle containing the molten metal 6. This metal 6 preferably has a melting point of between 200 ° C. and 1000 ° C. It is usually aluminum, magnesium, lead or zinc and their respective alloys. The immersion heater is usually mounted on the top wall of the furnace 5, but side or bottom side implantations are also permitted. The immersion heater has two zones along its length: a heating zone (hot part) Lc which must necessarily be immersed in the molten metal 6, otherwise the immersion heater will be destroyed. The minimum distance between the free liquid surface and the top of the heating zone should be 3 to 6 cm. a non-heating zone (cold part) Lf making it possible to make the connection between the heating zone Lc and the electric power supply 7. The temperature along this cold zone Lf varies from the operating temperature to the ambient temperature. The power dissipated by the immersion heater on this cold zone Lf must be as low as possible in order to avoid overheating of the immersion heater when the latter is in the air or in an insulating sleeve such as for example when crossing the wall. .

  The heating element 8 of the immersion heater consists of a core 9 of refractory material supporting the resistive wire 10 and the current supply wires 16. This core 9 has two zones along its length: an area on which grooves are made helical. This zone corresponds to the heating zone Lc of the immersion heater. an area on which straight grooves are made. This zone corresponds to the cold zone Lf of the immersion heater. According to a preferred embodiment of the invention, this core 9 is made of ceramic having good resistance to thermal shock and good resistance to temperature such as boron nitride, alumina, aluminum nitride , silicon carbide ... In the case of a bath of molten aluminum, the bath temperature being about 750 C, the temperature of the heating wire 10 will be of the order of 1000 to 1050 C. The material chosen must therefore support this level of temperature continuously. Various methods can be envisaged for producing this core (for example machining, molding or isostatic pressing). The process will be selected depending on the material and depending on the number of pieces to be made. As the heating wires 10 are wound on the core 9, it is necessary for this core to be electrically insulating in order to avoid short circuits between the wires 10.

  According to another preferred embodiment of the invention it is possible to use a refractory metal core provided to apply a deposit on this core in order to achieve electrical insulation for usual voltages of a few hundred volts. This deposit may, for example, be based on alumina, silica, aluminum nitride or silicon carbide. The coefficient of expansion of the material constituting the deposit must be close to that of the material constituting the core in order to avoid the appearance of cracks or cracks over time which could harm the electrical insulation of the resistive wires 10 and therefore adversely affect the life of the immersion heater. The resistive wire 10 is placed in the helical grooves of the core 9. This resistive wire 30 is preferably of a metallic nature, for example based on nickel / chromium, tungsten or chromium alloy.

  The current feed wire 16 is placed in the straight grooves of the core 9. This current feed wire is also preferably of a metallic nature (chromium, nickel, copper, etc.). The heating element 8 thus constituted is placed in a sheath 11 partially immersed in the molten metal 6. This sheath must withstand temperatures close to 1000 C, be a good thermal conductor to ensure a good heat transfer between the element heating 8 and the molten metal 6 and be compatible with molten metal baths in which it is dipped. By way of example, the materials constituting this sheath may be compounds based on: silicon carbide, sialon, silicon nitride, alumina, carbon / carbon composite, graphite, boron nitride, etc. Furthermore this sheath must not not be porous to avoid penetration of the alloy into the immersion heater. Since the outer diameter of the heating element 8 is smaller than the internal diameter of the sheath 11, it is necessary to fill the annular space thus created by a refractory material 12, electrical insulator and good thermal conductor to ensure the heat transfer between the resistive son 10 and the sheath 11. This material 12 may advantageously be obtained in the form of a compacted ceramic powder, allowing intimate contact with the resistive son 10 and with the sheath 11. For example, this material 12 can be obtained from a powder of boron nitride, aluminum nitride or magnesia. The refractory core 9 may be pierced longitudinally by a hole 13 allowing the passage of a thermocouple 14. This thermocouple 14 may be preferably positioned at the upper portion of the heating zone Lc. The temperature measurement can be used, for example, to regulate the bath in temperature or to protect the immersion heater from possible overheating, especially during the opening of the oven or when the level of liquid metal is lowered below the heating zone. The upper part of the immersion heater comprises a closure 15 making it possible to seal the heating element relative to the external environment in order to protect it against any pollution (dust, moisture) which could reduce its service life. Given the environment of the immersion heater and its position above the oven, the temperature can reach a hundred degree. It is therefore advisable to use a silicone or acrylic mastic type material to ensure this seal. The connection between the main electricity network 7 and the current leads 14 is carried out according to the rules of the art with standard electric wires connected to the leads 14 via sleeve 16.

  Figure 2 gives a sectional view along the line II-II of Figure 1. This cut is made at the heating zone Lc of the immersion heater. The core 9 has helical grooves 20 inside which the resistive wire 10 is placed. The assembly is placed in the sheath 11. The annular clearance is filled with the refractory material 12 (electrical insulator and thermal conductor). By way of example, the inside diameter of the sheath 11 is 16 mm, the diameter of the core 9 is 13 mm and the heating wires have a diameter of 0.5 mm. The annular clearance filled by the refractory material 12 thus has a radius of 1.5 mm. The use of a compacted powder is particularly relevant since it allows intimate contact with the sheath 11 on the one hand, but also with the resistive wire 10: the compacted powder, used as a refractory material 12, makes it possible to ensure a good thermal contact between the resistive wire 10 and the core 9 at the helical grooves 20. 20 This good contact makes it possible to reduce the hot spots at the level of the wire and thus to ensure a sufficient lifetime and a reliability allowing the use of the immersion heater in an industrial setting.

  FIG. 3 gives a sectional view along the line IV-IV of FIG. 1. This section is thus made at the level of the cold length Lf of the immersion heater. The core 9 has straight grooves 21 inside which are placed the current leads 16 for making the electrical connection between the resistive wire 10 of the heating zone Lc and the outside of the immersion heater. The wire 16 may be of the same nature as the wire 10. However, the thermal power dissipated on the cold zone must be as low as possible because the immersion heater is not immersed in the metal. Therefore, the electrical resistance of the current lead 16 should be as low as possible. It is therefore preferable to have a wire 16 of a different nature from that of the wire 10, for example copper or molybdenum (metals that are good electrical conductors), or a wire having a section much greater than that of the wire 10 (for example a diameter of 2 mm). These two parameters can be conjugated in order to minimize the resistance of the supply wires 16.

  FIG. 4 gives a perspective view of the core 9 at the level of the heating length in the case of a single-phase power supply. The core 9 comprises two helical grooves 20a and 20b receiving the resistive wires 10. The dimensions of the grooves 20 (pitch, depth, width) can be adjusted to the dimensions of the wires 10 in order to adjust the performance of the immersion heater to the electrical characteristics of the source and the needs of the user. The resistive wire 10 is placed in the grooves. At the end of the core 9, the resistive wires placed in each groove 20a and 20b are connected to ensure electrical continuity.

  Figure 5 gives a perspective view of the core 9 at the cold length in the case of a three-phase power supply. The core 9 comprises three helical grooves 20a, 20b and 20c receiving the resistive wires 10. These three independent grooves, the dimensions of which are also adjusted according to the performances required of the immersion heater, make it possible to receive the resistive wires 10. In order to realize a Star-type assembly under a three-phase network, the 3 son placed in the three helical grooves 20 are interconnected at the end of the core 9 (for example by shrinking or welding).

  Figure 6 gives a perspective view of the core 9 at the cold zone in the case of a single-phase power supply. This core comprises two straight grooves 21a and 21b for receiving the current supply wires 16. These two grooves 21 are judiciously distributed over the periphery of the core 9 so as to be aligned with the helical grooves 20 when the hot zones and cold are connected. The same principle is used for the links between the two zones in the case of a three-phase supply. 10

Claims (9)

claims   1. Electrical immersion heater, especially for molten metal bath, characterized in that it contains: - a core, refractory, electrical insulator and thermal conductor having helically shaped grooves on the heating portion of the immersion heater, of longitudinal shape on the in the non-heating part, a metal resistor is placed in the helical grooves of the core at the heating zone of the immersion heater; current feed wires placed in the longitudinal grooves of the core at the cold zone; of the immersion heater. an outer sheath made of refractory material, inert with respect to molten metals, a transfer means, preferably a compacted boron nitride powder, making it possible to ensure the heat transfer between the refractory core comprising the element heating and outer sheath.   2. Immersion heater according to claim 1, characterized in that the refractory core comprises two helical grooves for a single-phase electrical supply.   3. Immersion heater according to claim 1, characterized in that the refractory core comprises three helical grooves with a common point at the end of the heating zone allowing a three-phase power supply (star assembly).   4. Thermoplongeur according to claims 1 to 3, characterized in that the refractory core is made of ceramic obtained by machining, pressing or molding.   5. Immersion heater according to claims 1 to 3, characterized in that the refractory core is of a metallic nature but coated with a refractory deposit and electrical insulator to ensure the isolation of the resistive son even under the effect of thermal cycles and high temperature levels.   6. Immersion heater according to claims 1 to 5, characterized in that the longitudinal grooves present on the non-heating portion of the core, have a section greater than those encountered on the heating portion, in order to increase the section of the resistive son and thus reducing the thermal heating at the level of the non-heating zone.   7. Immersion heater according to claims 1 to 6, characterized in that the material constituting the son in the cold zone is of different nature from that constituting the son of the heating zone.   8. Immersion heater according to claims 1 to 7, characterized in that the core has a hole in its longitudinal axis for the establishment of a thermocouple for measuring the temperature of the heating zone. 30   9. Immersion heater according to claims 1 to 7, characterized in that the refractory core is in the form of a tube for applications where the outer sheath of the immersion heater is of large diameter. 5