Compartmentalized immersion heater containing electrically insulating ceramic powder

Abstract

An immersion heater (100) configured to be in contact with a material to be heated (90), in particular a molten non-ferrous metal, comprising: - a sheath (120) delimiting a first compartment (150) at least partially filled with electrically insulating powder (130), said sheath being configured to be in contact with the material to be heated, - a wall (140) disposed inside the first compartment (150) and delimiting at least a second compartment (145), the latter being able to remain empty of solid material or to be able to be at least partially filled with an inert material, - a plurality of heating elements (115, 116) disposed in said electrically insulating ceramic powder (130), between said sheath (120) and said wall (140). Figure for the abbreviation: [Fig 1]

Description

Title of the invention: Compartmentalized immersion heater comprising electrically insulating ceramic powder TECHNICAL FIELD OF THE INVENTION

[0001] The present invention falls within the field of foundry equipment and relates mainly to the foundry of non-ferrous metals having a relatively low melting point, typically below 1100 °C.

[0002] The invention relates to an electric immersion heater for maintaining a previously molten metal bath in a liquid state or for melting a batch of solid metal, in particular alloys or unalloyed metals of aluminum, magnesium, or zinc. PRIOR TECHNOLOGY

[0003] In the context of foundry equipment, an electric immersion heater is an electrical device which includes a heating zone intended to be brought into contact with a bath of liquid metal to be heated, and a non-heating zone allowing the connection between the heating zone and an electrical power supply box.

[0004] In a foundry workshop, the most common use of an immersion heater is to maintain the temperature of molten metal, previously melted using a gas furnace, for example. As an example, this temperature maintenance can be carried out in a liquid metal processing ladle during degassing or filtration of the liquid metal. According to a more recent application designed by the applicant company, an immersion heater can be used to melt metal in its solid state and optionally to maintain the temperature of the resulting molten metal.

[0005] Immersion heaters for heating a liquid metal bath have been known for decades, for example from GB 1 027 163, FR 2 720 888. They generally comprise one or more heating elements placed in a cylindrical sheath that separates them from the bath they are to heat. As taught by FR 2 699 038, the sheath can be made of an inert ceramic material, for example silicon nitride, boron nitride, SiAlON, or silicon carbide. The heating element can be made of a heating ceramic, for example SiC, as is known from WO 2005 / 060314, or graphite, as described in FR 2 559 886 and FR 2 622 382.

[0006] Immersion heaters are also known for heating a bath of liquid metal, the tubular sheath of which comprises a plurality of metallic heating elements embedded in an electrofused magnesia powder, which is an electrical insulator and has suitable thermal conductivity. Immersion heaters that are ac- Currently on the market, they use this principle.

[0007] In a context of electrification of industrial methods and more particularly of foundry methods, especially with a view to limiting the use of more traditional heating methods based on the use of fossil fuels, there is a need for electrical devices providing electrical power, and therefore a heating capacity, greater than that offered by current devices.

[0008] To meet this need, several currently available immersion heaters can be used together in the same bath. Depending on the heating capacity required for a furnace, as many immersion heaters as necessary are used. Since the lifespan of the sheaths in a molten metal bath is limited, these immersion heaters must be replaced regularly. Increasing their number leads to an increase in both investment and maintenance costs. An alternative to using multiple immersion heaters would be to use heaters with greater capacity and size. However, increasing the size of the immersion heaters presents certain technical and economic challenges. OBJECTS OF THE INVENTION

[0009] The applicant company found that it was not possible to produce a high-power immersion heater at a reasonable price. This increase in power is costly due to the materials used in the heater's structure. As mentioned above, an immersion heater typically consists of an inert tubular sheath surrounding a core containing electrically insulating ceramic powder, for example, electrofused magnesia, in which the heater's heating elements and the electrical supply lines necessary for their operation are embedded. The electrically insulating ceramic powder has a very high production cost.Therefore, increasing the volume of the immersion heater to accommodate more heating elements, or larger and more powerful ones, results in a cost, along with the necessary electrically insulating ceramic powder, that is too high. An alternative approach is thus sought to reduce the cost of immersion heaters without compromising their quality and lifespan.

[0010] Thus, the present invention aims to overcome the limitations described above and in particular to provide an immersion heater offering increased heating capacity, while limiting the cost associated with this increase in power.

[0011] To this end, according to a first object, the invention relates to an immersion heater configured to be brought into contact with a material to be heated, in particular a molten non-ferrous metal, which comprises: - a sheath delimiting a first compartment at least partially filled with electrically insulating ceramic powder, said sheath being configured to be in contact with the material to be heated, - a partition arranged inside said first compartment and delimiting at least a second compartment, the latter being able to remain empty of solid matter or to be at least partially filled with an inert material, - a plurality of heating elements arranged in the electrically insulating ceramic powder, between said sheath and said wall.

[0012] Thanks to these arrangements, a part of the first compartment, delimited by the sheath, and which, in immersion heaters known in the prior art, is usually filled with electrically insulating ceramic powder, instead comprises an inert material or a gas whose cost is much lower than the cost of the electrically insulating ceramic powder.

[0013] In embodiments, the inert material filling at least part of the second compartment delimited by the wall comprises high temperature resistance fibers, for example polycrystalline mullite fibers or silico-aluminous fibers.

[0014] In some embodiments, the second compartment delimited by the wall remains empty of solid matter. Advantageously, the second compartment delimited by the wall is filled at least partially with an inert gas, which may be argon or another noble gas.

[0015] In some embodiments, the electrically insulating ceramic powder is selected from zinc oxide powder, alumina powder, magnesium oxide (magnesia) powder, or boron nitride powder. It advantageously exhibits good thermal conductivity.

[0016] In embodiments, the wall disposed inside the sheath and delimiting a compartment filled at least in part with an insulating material is formed at least in part of a ceramic material.

[0017] Thanks to these arrangements, the wall can be formed of a comparatively low cost material compared to the cost of electrically insulating ceramic powder, while exhibiting sufficient mechanical strength and thermal resistance characteristics.

[0018] In some embodiments, the wall is formed of a high-performance alloy, preferably a nickel-based alloy exhibiting low expansion at 1100°C. For example, the wall is formed of an alloy selected from the Inconel alloys or the CMSX single-crystal alloys.

[0019] A high-performance alloy or superalloy is defined as a metallic alloy exhibiting excellent mechanical strength and good resistance to machining. high temperature, good surface stability as well as good resistance to corrosion and oxidation.

[0020] In some embodiments, the sheath has a circular cross-section. Advantageously, the sheath has an outside diameter greater than or equal to 50 millimeters, preferably greater than 75 mm, more preferably greater than 95 mm, and even more preferably greater than 115 mm.

[0021] Thanks to these arrangements, the immersion heater has sufficient dimensions to accommodate more heating elements or larger heating elements.

[0022] In some embodiments, the wall is a tube with a circular cross-section. Advantageously, the center of the sheath with a circular cross-section and the center of the wall with a circular cross-section are substantially identical, so that the first compartment, formed between the sheath and the wall, and intended to be filled at least in part with electrically insulating ceramic powder, has an annular cross-section.

[0023] Thanks to these arrangements, the heating elements can be positioned in said first annular compartment. The heating elements thus positioned are close to the sheath, thereby allowing better heat diffusion towards the sheath and indirectly towards the material to be heated.

[0024] In embodiments, the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis passing through the center of the sheath and in which the immersion heater has at least 15 turns, preferably at least 20 turns, preferably at least 25 turns, preferably at least 30 turns. For example, the immersion heater has 36 turns.

[0025] A coil is a plurality of heating resistors supplied by the same current supplies and forming a substantially linear assembly arranged longitudinally in the immersion heater, preferably near the sheath.

[0026] As detailed previously, the immersion heater of the invention is particularly distinguished from known immersion heaters in that the diameter of the first compartment delimited by the sheath can be increased to accommodate more heating elements or larger heating elements. Thus, advantageously, the immersion heater comprises 16, 24, 32, or 36 turns.

[0027] In embodiments, the heating elements are arranged in substantially straight coils arranged substantially parallel to an axis running through the center of the sheath and in which said coils are positioned near the inner perimeter of the sheath.

[0028] Thanks to these arrangements, the coils thus positioned are close to the sheath, thus allowing better heat diffusion towards the sheath and indirectly towards the material to be heated.

[0029] In embodiments, the heating elements comprise electrical resistances comprising molybdenum or an alloy selected from an iron-chromium-aluminum alloy (FeCrAl), a nickel alloy and a chromium alloy.

[0030] Apart from the heating elements and the external electrical connection elements, the device according to the invention advantageously has a cylindrical shape and a coaxial and symmetrical structure with respect to the central axis. Other embodiments can also be envisaged in which the device has a different shape and / or structure, for example, an orthogonal cross-section or a cross-section of variable diameter.

[0031] A second object of the invention is represented by a method for manufacturing an immersion heater according to any one of the embodiments of the invention, wherein a sheath, a tubular wall, a plurality of heating elements, and an electrically insulating powder are supplied; said tubular wall is placed inside said sheath; the heating elements are introduced into the first compartment between the sheath and the wall; and said first compartment is then filled with electrically insulating powder in stages, at least some stages, and preferably each stage, of filling being followed by at least one stage of compaction. BRIEF DESCRIPTION OF THE FIGURES

[0032] Other advantages, purposes and particular features of the invention will become apparent from the following non-limiting description of at least one particular embodiment of the immersion heater that is the subject of the present invention, with reference to the accompanying drawings, in which:

[0033] [Fig. 1] schematically represents, in axial cross-sectional view, a first embodiment of an immersion heater according to the invention,

[0034] [Fig.2]] represents, schematically and in cross-sectional view, the thermo diver, the subject of the first embodiment,

[0035] The reference numbers mentioned in the figures refer to: 90 a liquid to be heated 80, 81 an arrow representing a heat flow 100 an immersion heater 110 a means of supplying electrical current 115, 116 heating elements 120 a sheath 130 electrically insulating ceramic powder 140 the tubular ceramic wall 145 a second compartment 150 a first compartment DETAILED DESCRIPTION OF THE INVENTION

[0036] The present description is given by way of non-limiting grammar, each feature of an embodiment being able to be advantageously combined with any other feature of any other embodiment.

[0037] Figures 1 and 2 show a first particular embodiment of an immersion heater 100 according to the invention. In [Fig. 1], an axial cross-sectional view of the immersion heater is shown. In [Fig. 2], a transverse cross-sectional view of the heating part of the immersion heater 100, along the section plane AA, visible in [Fig. 1], is shown.

[0038] The immersion heater 100 is an electric immersion heater comprising a heating zone 101 and a non-heating zone 102 that connects the heating zone to a support for the immersion heater 100, which is connected to an electrical power supply 110. The immersion heater 100 is intended to be positioned in a tank (not shown) containing metal to be heated. The heating zone 102 of the immersion heater 100 is intended to be in contact with the metal, whether it is molten metal to be kept at a temperature in order to maintain it in a liquid state or solid metal to be heated.

[0039] The heating zone 101 of the immersion heater 100 comprises a plurality of heating elements, 115, 116, brought to a high temperature by the passage of an electric current; this is a resistive heating system. As will be explained in greater detail below, each heating element comprises an electrical conductor, which is in contact with an electrically insulating material 130 capable of transmitting its heat to a sheath 120 (outer sheath) forming the outermost layer of the body of the immersion heater 100, which in turn transmits the heat to the metal or alloy to be melted or kept in a liquid state.

[0040] The heat transfer mechanism from the immersion heater's sheath to the metal in the tank is most efficient, occurring by conduction, when the heater is immersed in liquid metal or when its sheath is in contact with solid metal over a significant portion of its surface. Outside these areas of direct contact, heat transfer between the sheath and the metal in the tank is radiative and / or convective. The heat transfer from the heating element 115 to the sheath 120 and then to the material to be heated 90 is represented by arrow 80 in Figures 1 and 2.

[0041] The immersion heater 100 is, for example, configured to maintain in a liquid state a metal 90 in liquid form comprising a non-ferrous metal or an alloy based on a non-ferrous metal which has a melting point low enough to be compatible with the use of an electric immersion heater. In particular, the metal Non-ferrous metal can be any metal or alloy with a melting point not exceeding approximately 1100°C. The base metal can, for example, be chosen from aluminium, zinc, magnesium, copper, tin, lead, lithium, and silver.

[0042] The sheath 120 of the immersion heater 100 is preferably made of boron nitride or silicon nitride. These refractory materials have high heat resistance and are chemically inert, in addition to offering satisfactory mechanical strength. The sheath 120 delimits a first compartment 150 inside which the heating elements 115, 116 of the immersion heater 100 are arranged.

[0043] Said first compartment 150 is at least partially filled with electrically insulating ceramic powder 130. The electrically insulating ceramic powder 130 is a material selected for its electrical insulating properties, allowing direct contact with the electrical conductors of the heating element, and for its thermal conductivity. Furthermore, the electrically insulating ceramic powder 130 must exhibit excellent temperature resistance and must not degrade over time, thus allowing the immersion heater to have an effective lifespan typically ranging from one to several tens of months of continuous use. It advantageously exhibits good thermal conductivity.

[0044] According to one embodiment, the electrically insulating ceramic powder 130 is an electrofused magnesia (magnesium oxide: MgO) powder. The electrofused magnesia powder is produced by melting magnesium oxide at a temperature exceeding 3,000°C. In other embodiments, the electrically insulating ceramic powder is selected from zinc oxide (ZnO) powder, alumina (Al₂O₃) powder, or boron nitride powder.

[0045] Inside the first compartment 150 delimited by the sheath 120, the immersion heater 100 comprises at least one wall 140 delimiting a second, closed compartment 145, separate from the first compartment. The function of this second compartment is to reduce the quantity of electrically insulating ceramic powder 130 required to manufacture the immersion heater 100 and thus reduce its manufacturing cost. The wall 140 is typically a tubular wall, and its axial cross-section is advantageously circular.

[0046] The wall 140 is made of a material with sufficient mechanical and thermal resistance characteristics, without requiring good thermal conductivity. Preferably, the wall 140 is made of ceramic, for example alumina. According to other embodiments, the wall 140 is made of mullite or cordierite.

[0047] In a preferred embodiment, the second compartment 145 delimited by the wall 140 remains empty, i.e., it contains gas, which may be air. Preferably, the air is expelled in order to be replaced by an inert gas, which is unlikely to to oxidize the wall, which may be, for example, argon or another noble gas. In another embodiment, said second compartment 145 is at least partially filled with an inert material in particulate form, to exert a counter-pressure. For example, said inert material comprises high-temperature resistant fibers, for example, polycrystalline mullite fibers or silico-aluminous fibers. According to another variant, said inert material is alumina powder.

[0048] The sheath 120 typically has a circular cross-section. Advantageously, the outer diameter of the sheath 120 is at least 55 millimeters (hereinafter abbreviated as "mm"), preferably at least 75 mm. In an alternative embodiment, the diameter of the sheath 120 is greater than 95 mm or greater than 115 mm. Increasing the diameter of the sheath 120 allows for a greater number of heating elements 115, 116 to be placed inside the immersion heater 100, thereby increasing its heating capacity.

[0049] Furthermore, the wall 140 is also a tube with a circular cross-section, forming a second compartment 145 with a round cross-section. The first compartment 150, formed on one side by the sheath 120 and on the other by the wall 140, thus has an advantageous annular shape for the placement of the heating elements 115, 116, as detailed in the following paragraph. Preferably, the tube forming the wall 140 has a constant diameter along the entire height of the immersion heater.

[0050] Such alumina tubes are common products that exist on the market for other uses; therefore, their price is significantly lower than that of the quantity of electrofused magnesia powder (for example) saved by inserting such a tube into the device according to the invention.

[0051] The immersion heater 100 shown in Figures 1 and 2 comprises sixteen heating elements, of which only heating elements 115 and 116 are labeled in Figures 1 and 2. In other embodiments, the immersion heater 100 comprises at least 20 heating elements, preferably at least 30. Preferably, and as illustrated in Figures 1 and 2, the heating elements are positioned near the inner periphery of the sheath 120 to allow for better heat transfer to the sheath. In other words, if we consider a cross-section of the immersion heater 100 as illustrated in [Fig. 2], the heating elements 115 and 116 are arranged in a concentric circle with the circle formed by the sheath 120. Any other arrangement and any other number of heating elements may be adopted without departing from the scope of the present invention.

[0052] Preferably, the heating elements are arranged in substantially straight coils arranged substantially parallel to an axis running through the center of the sheath 120 and said coils are positioned near the inner perimeter of the sheath 120.

[0053] Preferably, the heating elements 115, 116 comprise resistors electrical components containing molybdenum or an alloy selected from an iron-chromium-aluminium (FeCrAl) alloy, a nickel-chromium alloy.

[0054] According to a particular embodiment (not illustrated), the tubular wall 140 forms a compartment with a rectangular or polygonal cross-section, for example octagonal or hexagonal.

[0055] According to a particular embodiment (not illustrated), the second compartment 145 formed by the wall 140 does not have the same dimensions over the entire height of the immersion heater 100. For example, the second compartment 140 at a cross-section in the non-heating part of the immersion heater 100 has a smaller surface area than the surface area of ​​the second compartment 140 at a cross-section in the non-heating part of the immersion heater 100.

[0056] According to a particular embodiment (not illustrated), the immersion heater comprises a first compartment at least partially filled with electrically insulating ceramic powder and a plurality of walls arranged inside the first compartment, each of said walls delimiting a compartment that may remain empty of solid material or that may be at least partially filled with an insulating material

[0057] We describe here a method for manufacturing the immersion heater according to the invention. The inventors realized that good thermal contact between the electrical conductors and the sheath 120 depends on the density of the bed of electrically insulating ceramic powder with which these conductors are in contact. According to the invention, a sheath 120, a tubular wall 140, a plurality of heating elements 115, 116, and an electrically insulating powder 130 are supplied; said tubular wall 140 is placed inside said sheath 120; the electrical conductors are introduced into the first compartment 150 between the sheath 120 and the wall 140; and then said first compartment 150 is filled with electrically insulating ceramic powder in stages, at least some (and preferably each) filling stages being followed by at least one tamping stage.

[0058] In a first embodiment, during this compaction step, the powder is pressed using an annular or ring-shaped compaction tool, capable of being inserted from above into the annular space formed by the first compartment 150. Thus, the said first compartment is progressively filled with a dense and compacted bed of electrically insulating ceramic powder, which ensures excellent thermal contact with the heating elements 115,116.

[0059] In a first variant of this process, each step of introducing the electrically insulating ceramic powder, or at least one of these steps, and preferably at least the last, is followed by a compaction step by vibrating the sheath 150 and / or the tube 140.

[0060] In another variant of this method, said compaction tool is made as a vibrocompaction tool.

[0061] These two variants can be combined.

Claims

Claims

1. Immersion heater (100) configured to be brought into contact with a material to be heated (90), in particular a molten non-ferrous metal, characterized in that it comprises: - a sheath (120) delimiting a first compartment (150) at least partly filled with electrically insulating ceramic powder (130), said sheath being configured to be brought into contact with the material to be heated, - a wall (140) arranged inside the first compartment (150) and delimiting at least one second compartment (145), the latter being able to remain empty of solid material or being able to be filled at least partly with an inert material, - a plurality of heating elements (115, 116) arranged in said electrically insulating ceramic powder (130), between said sheath (120) and said wall (140).

2. Immersion heater (100) according to claim 1, wherein the wall (140) disposed inside the sheath (120) and delimiting a second compartment (145) filled at least in part with an insulating material is formed at least in part from ceramic material.

3. An immersion heater according to one of claims 1 or 2, wherein said electrically insulating ceramic powder (130) is selected from a zinc oxide powder, an alumina powder, an electrofused magnesia powder, or a boron nitride powder.

4. Immersion heater (100) according to one of claims 1 to 3, in which said inert material at least partially filling the second compartment (145) delimited by the wall (140) comprises fibers with high temperature resistance, for example polycrystalline mullite fibers or silico-alumina fibers.

5. Immersion heater (100) according to one of claims 1 to 3, in which the second compartment (145) delimited by the wall (140) remains empty of solid material and comprises an inert gas, preferably argon or another noble gas.

6. Immersion heater (100) according to one of claims 1 to 5, in which the sheath (120) has a circular section.

7. Immersion heater (100) according to claim 6, in which the sheath (120) has an external diameter greater than or equal to 50 millimeters, preferably greater than 75 mm, more preferably greater than 95 mm, and even more preferably greater than 115 mm.

8. Immersion heater (100) according to one of claims 1 to 7, in which the wall (140) is a tube of circular section.

9. Immersion heater (100) according to one of claims 6 or 7 and according to claim 8, in which the center of the sheath (120) of circular section and the center of the wall (140) of circular section are substantially identical, so that the first compartment formed between the sheath (120) and the wall (140) and intended to be filled at least in part with electrically insulating ceramic powder has an annular section.

10. Immersion heater (100) according to one of claims 1 to 9, in which the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis running through the center of the sheath (120) and in which the immersion heater comprises at least 15 turns, preferably at least 20 turns, preferably at least 20 turns, preferably at least 25 turns, preferably at least 30 turns.

11. Immersion heater (100) according to one of claims 1 to 10, in which the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis running through the center of the sheath (120) and in which said turns are positioned close to the inner periphery of the sheath (120).

12. A method of manufacturing an immersion heater according to any one of claims 1 to 11, wherein a sheath (120), a tubular wall (140), a plurality of heating elements (115, 116) and an electrically insulating ceramic powder (130) are provided, said tubular wall (140) is placed inside said sheath (120), the heating elements (115, 116) are introduced into the first compartment (150) between the sheath (120) and the wall (140), and then said first compartment (150) is filled with electrically insulating ceramic powder in stages, at least some stages, and preferably each stage, of filling being followed by at least one tamping stage.

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