Compartment immersion heater containing electrically insulating ceramic powder

Abstract

The invention relates to an immersion heater (100) configured to be in contact with a material (90) to be heated, in particular a molten non-ferrous metal, comprising:-a sheath (120) defining a first compartment (150) at least partially filled with an electrically insulating powder (130), the sheath being configured to be in contact with the material to be heated; -a partition wall (140) arranged inside the first compartment (150) and defining at least one second compartment (145) at least partially filled with an inert material; -a plurality of heating elements (115, 116) arranged in the electrically insulating ceramic powder (130) between the sheath (120) and the partition wall (140).

Description

Technical Field

[0001] This invention belongs to the field of casting equipment and mainly relates to the casting of non-ferrous metals with relatively low melting points (usually below 1100°C).

[0002] The present 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, particularly alloys or non-alloys of aluminum, magnesium or zinc. Background Technology

[0003] In the context of casting equipment, an electric immersion heater is an electrical device that includes a heating zone designed to contact a bath of liquid metal to be heated, and a non-heating zone that allows connection between the heating zone and a power supply box.

[0004] In foundries, the most common use of immersion heaters is to maintain the temperature of molten metal, for example, by first melting the metal using a gas-fired furnace. As an example, this temperature maintenance can be performed in molten metal handling ladles during degassing or filtration of liquid metal. According to a more recent application designed by the applicant company, immersion heaters can be used to melt solid metal and potentially maintain the temperature of the resulting molten metal.

[0005] Immersion heaters designed for heating baths of liquid metal have been known for decades, for example from GB 1027 163 and FR 2 720 888; they typically comprise one or more heating elements housed in a cylindrical sheath that separates them from the bath of metal to be heated. As taught in FR 2 699 038, the sheath can be made of an inert ceramic material, such as silicon nitride, boron nitride, SiAlON, or silicon carbide. The heating elements can be made in the form of heating ceramics, for example, SiC as known from WO 2005 / 060314, or graphite as described in FR 2 559 886 and FR 2 622 382.

[0006] Immersion heaters for heating liquid metal baths are also known, whose tubular sheath includes multiple metal heating elements embedded in fused magnesium oxide powder, which is an electrical insulator and has suitable thermal conductivity. This principle is currently used in commercially available immersion heaters.

[0007] In the context of electrification of industrial methods, particularly casting methods, and especially in order to limit the use of more traditional heating methods based on fossil fuels, there is a need for electrical equipment that can provide electricity and therefore requires greater heating capacity than existing equipment.

[0008] To meet this need, multiple currently available immersion heaters can be used together in the same metal bath. The required number of immersion heaters is determined by the furnace's heating capacity. Since the lifespan of the jacket in the liquid metal bath is limited, these immersion heaters must be replaced periodically. Increasing the number of immersion heaters leads to increased investment and maintenance costs. An alternative to increasing the number of immersion heaters is to provide immersion heaters with larger capacity and size. However, increasing the size of immersion heaters presents certain technical and economic difficulties. Summary of the Invention

[0009] The applicant company has found it impossible to produce high-power immersion heaters at a reasonable price. In fact, such an increase in power is prohibitively expensive due to the materials used in the immersion heater structure. As described above, immersion heaters typically consist of an inert tubular sheath surrounding a core containing electrically insulating ceramic powder (e.g., fused magnesium oxide), within which the heating elements of the immersion heater and the electrical supply lines required for their operation are embedded. Electrically insulating ceramic powder has a very high production cost. Therefore, increasing the volume of the immersion heater to accommodate more or larger and more powerful heating elements would result in excessively high costs associated with the required electrically insulating ceramic powder. Therefore, we are seeking alternatives to reduce the cost of immersion heaters without compromising their quality and lifespan.

[0010] Therefore, the present invention aims to overcome the above-mentioned limitations, and in particular to provide an immersion heater that provides increased heating capacity while limiting the cost associated with the increase in power.

[0011] Therefore, according to the first objective, the present invention relates to an immersion heater configured to contact a material to be heated (particularly molten non-ferrous metal), comprising:

[0012] A sheath defining a first compartment, the first compartment being at least partially filled with electrically insulating ceramic powder, the sheath being configured to contact the material to be heated.

[0013] A partition wall, arranged inside the first compartment and defining at least one second compartment, the second compartment being at least partially filled with an inert material.

[0014] - Multiple heating elements are arranged in the electrically insulating ceramic powder, located between the sheath and the partition wall.

[0015] As a result of these arrangements, a portion of the first compartment, defined by the sheath, which in immersion heaters known in the art is typically filled with electrically insulating ceramic powder, instead contains an inert material or gas, the cost of which is far less than that of the electrically insulating ceramic powder.

[0016] According to the invention, the second compartment is at least partially filled with an inert material. The inventors discovered that implementing an immersion heater with no solid material in the second compartment leads to thermal convection. This thermal convection occurs because the temperature difference between the bottom of the immersion heater (where its resistance is between 800°C and 1100°C) and the top of the immersion heater causes airflow. Hot air, having a lower density and pressure than cold air, rises in the pipe. Adding an inert material, particularly a refractory ceramic insulator, to the second compartment helps limit this thermal convection effect.

[0017] In some embodiments, the inert material filling at least a portion of the second compartment defined by the partition wall is a refractory ceramic insulating material. The material may be flexible, machined, or vacuum-formed.

[0018] In some embodiments, the inert material filling at least a portion of the second compartment defined by the partition wall comprises high-temperature resistant fibers. For example, these high-temperature resistant fibers are polycrystalline mullite fibers or aluminosilicate fibers.

[0019] In some embodiments, the second compartment defined by the partition wall is partially filled with an inert material and partially free of solid material. Advantageously, the second compartment defined by the partition wall is at least partially filled with an inert gas, which may be argon or another inert gas.

[0020] In some embodiments, the second compartment is filled with at least 30% by volume of inert material, and the remaining volume of the second compartment remains free of solid material. In other embodiments, the second compartment is filled with at least 40%, 60%, 80%, or 90% inert material.

[0021] An electric immersion heater includes a heating zone and a non-heating zone, the non-heating zone allowing a connection to be established between the heating zone and the support structure of the immersion heater connected to a power source. The heating zone of the immersion heater is located at the bottom of the immersion heater; it contains multiple heating elements, typically resistors, which reach high temperatures through the passage of electric current.

[0022] Advantageously, the portion of the second compartment corresponding in height to the heating zone of the immersion heater is filled with an inert material, while the portion of the second compartment corresponding in height to the non-heating zone of the immersion heater remains free of solid material.

[0023] These arrangements enable the prevention of heat convection while limiting the amount of inert material required to manufacture immersion heaters.

[0024] In some embodiments, the immersion heater includes at least one thermocouple-type temperature sensor. Advantageously, at least one thermocouple is positioned in the first compartment of the immersion heater, more preferably in the heating zone of the immersion heater.

[0025] Therefore, at least one thermocouple is arranged in electrically insulating ceramic powder. Advantageously, the temperature sensor is protected by a thermocouple sheath.

[0026] In some embodiments, the electrically insulating ceramic powder is selected from zinc oxide powder, aluminum oxide powder, magnesium oxide (MgO) powder, or boron nitride powder. Advantageously, it exhibits good thermal conductivity.

[0027] In some embodiments, the partition walls located inside the sheath and defining a compartment at least partially filled with insulating material are at least partially formed of ceramic material.

[0028] Because of these arrangements, the partitions can be formed from materials that are relatively inexpensive compared to electrically insulating ceramic powder, while exhibiting sufficient mechanical strength and thermal resistance properties.

[0029] In some embodiments, the partition is formed of a high-performance alloy, preferably a nickel-based alloy that exhibits low expansion at 1100°C. For example, the partition is formed of an alloy selected from chromium-nickel-iron alloys or CMSX single-crystal alloys.

[0030] High-performance alloys or high-temperature alloys are defined as metallic alloys that exhibit excellent mechanical strength and good creep resistance, good surface stability, and good resistance to corrosion and oxidation at high temperatures.

[0031] In some embodiments, the sheath has a circular cross-section. Advantageously, the outer diameter of the sheath is greater than or equal to 50 mm, preferably greater than 75 mm, more preferably greater than 95 mm, and even more preferably greater than 115 mm.

[0032] Because of these arrangements, immersion heaters have sufficient size to accommodate more or larger heating elements.

[0033] In some embodiments, the partition is a tube with a circular cross-section. Advantageously, the center of the circular cross-section sheath and the center of the circular cross-section partition are substantially the same, such that the first compartment formed between the sheath and the partition and intended to be at least partially filled with electrically insulating ceramic powder has an annular cross-section.

[0034] Because of these arrangements, the heating element can be placed in the first compartment of the annular cross-section. The heating element, positioned in this way, is close to the sheath, thus allowing for better heat diffusion into the sheath and indirectly into the material to be heated.

[0035] In some embodiments, the heating element is arranged as substantially straight turns, which are arranged substantially parallel to an axis passing through the center of the sheath, and wherein 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.

[0036] A turn consists of multiple heating resistors powered by the same current leads and forming a basic linear assembly, which is arranged longitudinally in an immersion heater, preferably close to the sheath.

[0037] As detailed above, the immersion heater of the present invention differs from known immersion heaters in that the diameter of the first compartment defined by the sheath can be increased to accommodate more or larger heating elements. Therefore, advantageously, the immersion heater has 16, 24, 32, or 36 turns.

[0038] In some embodiments, the heating element is arranged as a substantially straight coil, which is arranged substantially parallel to an axis passing through the center of the sheath, and wherein the coil is positioned near the inner circumference of the sheath.

[0039] Because of these arrangements, the turns positioned in this way are close to the sheath, thus allowing for better heat diffusion to the sheath and indirectly to the material being heated.

[0040] In some embodiments, the heating element includes a resistor comprising molybdenum or an alloy selected from iron-chromium-aluminum (FeCrAl), nickel alloys, and chromium alloys.

[0041] In addition to the heating element and the electrical connection elements to the outside, the device according to the invention advantageously has a cylindrical shape and a coaxial and symmetrical structure relative to the central axis. Other embodiments are also contemplated, in which the device has different shapes and / or structures, such as orthogonal sections or sections with variable diameters.

[0042] A second object of the present invention is a method for manufacturing an immersion heater according to any embodiment of the present invention, wherein a sheath, a tubular partition, a plurality of heating elements and an electrically insulating powder are provided, the tubular partition is placed inside the sheath, the heating elements are introduced into a first compartment between the sheath and the partition, and then the first compartment is filled in stages with the electrically insulating powder, at least some of the filling stages, preferably each filling stage is followed by at least one compaction stage. Attached Figure Description

[0043] Other advantages, objects, and specific features of the invention will become apparent from the following non-limiting description of at least one specific embodiment of the immersion heater, which is the subject of the invention, with reference to the accompanying drawings, wherein:

[0044] Figure 1 schematically shows a schematic axial cross-sectional view of a first embodiment of the immersion heater according to the present invention.

[0045] Figure 2 schematically shows a schematic cross-sectional view of the immersion heater, which is the subject of the first embodiment.

[0046] The reference numerals in the attached figures refer to:

[0047] 90 Liquid to be heated

[0048] Arrows 80 and 81 indicate heat flow.

[0049] 100 Immersion Heater

[0050] 110 Current supply device

[0051] 115, 116 Heating elements

[0052] 120 protective sleeve

[0053] 130 Electrically insulating ceramic powder

[0054] 140 tubular septa

[0055] 145 Second compartment

[0056] 150 First compartment Detailed Implementation

[0057] This description is given as a non-limiting illustration, and each feature of the implementation can be advantageously combined with any other feature of any other implementation.

[0058] Figures 1 and 2 illustrate a first specific embodiment of the immersion heater 100 according to the present invention. Figure 1 shows an axial cross-sectional view of the immersion heater. Figure 2 shows a cross-sectional view of the heating portion of the immersion heater 100 according to the cross-sectional plane AA visible in Figure 1.

[0059] The immersion heater 100 is an electrically immersed heater comprising a heating zone 101 and a non-heating zone 102, the non-heating zone 102 allowing a connection to be established between the heating zone and a support for the immersion heater 100 connected to an electrical supply device 110. The immersion heater 100 is intended to be positioned in a container (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 held at a temperature to maintain its liquid state or solid metal to be heated.

[0060] The heating zone 101 of the immersion heater 100 includes a plurality of heating elements 115, 116, which reach high temperatures by the passage of an electric current; this is a type of resistance heating. As will be explained in more detail below, each heating element has an electrical conductor that contacts an electrically insulating material 130 capable of transferring its heat to a sheath 120 (outer sheath) forming the outermost layer of the body of the immersion heater 100, which in turn transfers heat to the metal or alloy to be melted or kept in a liquid state.

[0061] When an immersion heater is submerged in liquid metal or when its sheath is in contact with solid metal with a significant portion of its surface, the heat transfer mechanism from the heater sheath to the metal in the tank is most efficient through conduction. Outside these areas of direct contact, heat transfer between the sheath and the metal contained in the tank is radiative and / or convective. In Figures 1 and 2, heat transfer from the heating element 115 to the sheath 120 and then to the material to be heated 90 is indicated by arrow 80.

[0062] For example, the immersion heater 100 is configured to hold a liquid metal 90 comprising a non-ferrous metal or a non-ferrous metal-based alloy in liquid form, said non-ferrous metal or alloy having a sufficiently low melting point to be compatible with the use of the electric immersion heater. Specifically, the non-ferrous metal can be any metal or alloy with a melting point not exceeding about 1100°C. Base metals can be selected, for example, from aluminum, zinc, magnesium, copper, tin, lead, lithium, and silver.

[0063] The sheath 120 of the immersion heater 100 is preferably formed of boron nitride or silicon nitride. These refractory materials have high heat resistance and chemical inertness, and also provide satisfactory mechanical strength. The sheath 120 defines a first compartment 150 in which the heating elements 115, 116 of the immersion heater 100 are arranged.

[0064] The 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 its thermal conductivity. Furthermore, the electrically insulating ceramic powder 130 must possess excellent temperature resistance; it must not degrade over time, allowing the effective lifespan of the immersion heater to typically range from one month to several tens of months of continuous use. Advantageously, the electrically insulating ceramic powder exhibits good thermal conductivity.

[0065] According to one embodiment, the electrically insulating ceramic powder 130 is fused magnesium oxide (MgO) powder. The fused magnesium oxide powder is prepared by melting magnesium oxide at a temperature exceeding 3000°C. In other embodiments, the electrically insulating ceramic powder is selected from zinc oxide (ZnO) powder, aluminum oxide (Al2O3) powder, or boron nitride powder.

[0066] Within a first compartment 150 defined by a sheath 120, the immersion heater 100 has at least one partition wall 140 defining a second enclosed compartment 145 distinct from the first compartment. This second compartment reduces the amount of electrically insulating ceramic powder 130 required to manufacture the immersion heater 100, thereby lowering its manufacturing cost. The partition wall 140 is typically tubular, with an advantageously circular axial cross-section.

[0067] The partition 140 is made of a material with sufficient mechanical strength and thermal resistance characteristics, without requiring good thermal conductivity. Preferably, the partition 140 is formed of ceramic, such as alumina. According to other embodiments, the partition 140 is formed of mullite or cordierite. According to other embodiments, the partition is formed of a high-performance alloy, preferably a nickel-based alloy exhibiting low expansion at 1100°C. For example, the partition is formed of an alloy selected from chromium-nickel-iron alloys or CMSX single-crystal alloys.

[0068] In one embodiment, a second compartment 145 defined by partition 140 retains a portion free of solid material. The portion of the second compartment free of solid material contains a gas, which may be air. Preferably, the air is vented to replace it with an inert gas unlikely to oxidize the partition, such as argon or another inert gas.

[0069] The second compartment 145 is at least partially filled with an inert material in granular form. The inert material contained in the second compartment 145 helps prevent heat convection. The inert material also exerts counter-pressure on the partition wall 140.

[0070] For example, the inert material includes high-temperature resistant fibers, such as polycrystalline mullite fibers or aluminosilicate fibers. According to another variation, the inert material is alumina powder.

[0071] The sheath 120 typically has a circular cross-section. Advantageously, the outer diameter of the sheath 120 is at least 55 mm (hereinafter referred to as "mm"), preferably at least 75 mm. According to alternative embodiments, 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 the placement of a greater number of heating elements 115, 116 inside the immersion heater 100, thereby increasing its thermal power.

[0072] Furthermore, the partition wall 140 is also a tube with a circular cross-section, forming a second compartment 145 with a circular cross-section. The first compartment 150 is formed on one side by a sheath 120 and on the other side by the partition wall 140, thus having an annular shape, which is advantageous for the placement of the heating elements 115, 116, as detailed in the following paragraphs. Preferably, the tube forming the partition wall 140 has a constant diameter over the entire height of the immersion heater.

[0073] Such alumina tubes are common products available on the market for other applications; therefore, their price is significantly lower than the cost of the amount of fused magnesium oxide powder (e.g.) saved by inserting such tubes into the device according to the invention.

[0074] The immersion heater 100 shown in Figures 1 and 2 has 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 includes at least 20 heating elements, preferably at least 30 heating elements. Preferably, and as shown in Figures 1 and 2, the heating elements are positioned near the inner circumference of the sheath 120 to allow for better heat transfer to the sheath. In other words, if we consider the cross-section of the immersion heater 100 as shown in Figure 2, the heating elements 115 and 116 are arranged concentrically with the circle formed by the sheath 120. Any other arrangement and any other number of heating elements may be used without departing from the scope of the invention.

[0075] Preferably, the heating element is arranged as a substantially straight coil, which is arranged substantially parallel to an axis passing through the center of the sheath 120, and the coil is positioned near the inner circumference of the sheath 120.

[0076] Preferably, the heating elements 115 and 116 include resistors containing molybdenum or alloys selected from iron-chromium-aluminum alloys (FeCrAl) and nickel-chromium alloys.

[0077] According to a particular embodiment (not shown), the tubular partition 140 forms a compartment with a rectangular or polygonal (e.g., octagonal or hexagonal) cross-section.

[0078] According to a particular embodiment (not shown), the second compartment 145 formed by the partition wall 140 does not have the same size over the entire height of the immersion heater 100. For example, the surface area of ​​the second compartment 140 at the cross-sectional level of the non-heated portion of the immersion heater 100 is smaller than the surface area of ​​the second compartment 140 at the cross-sectional level of the non-heated portion of the immersion heater 100.

[0079] According to a particular embodiment (not shown), the immersion heater includes a first compartment at least partially filled with electrically insulating ceramic powder and a plurality of partitions disposed inside the first compartment, each of the partitions defining a compartment that can be kept free of solid material or can be at least partially filled with insulating material.

[0080] We hereby describe a method for manufacturing an immersion heater according to the invention. The inventors recognized that good thermal contact between the electrical conductors and the sheath 120 depends on the density of the bed of electrically insulating ceramic powder in contact with these conductors. According to the invention, a sheath 120, a tubular partition 140, a plurality of heating elements 115, 116, and electrically insulating powder 130 are provided. The tubular partition 140 is placed inside the sheath 120. The electrical conductors are introduced into a first compartment 150 between the sheath 120 and the partition 140. The first compartment 150 is then filled in stages with electrically insulating ceramic powder, with at least some (preferably each) filling stages followed by at least one compaction stage.

[0081] In the first embodiment, during the compaction step, a compaction tool with an annular shape or cross-section is used to press the powder, the tool being insertable from above into the annular space formed by the first compartment 150. Thus, the first compartment is gradually filled with a dense and compacted bed of electrically insulating ceramic powder, which ensures excellent thermal contact with the heating elements 115, 116.

[0082] In a first variation of the method, each step of introducing the electrically insulating ceramic powder (or at least one of these steps, preferably at least the last step) is followed by a compaction step through a vibrating sheath 150 and / or tube 140.

[0083] In another variation of the method, the compaction tool is made into a vibratory compaction tool.

[0084] These two variations can be combined.

Claims

1. Immersion heater (100) configured to be in contact with a material (90) to be heated, in particular a molten non-ferrous metal, characterized in that, The immersion heater (100) comprises: - a sheath (120) defining a first compartment (150) at least partially filled with an electrically insulating ceramic powder (130), the sheath being configured to be in contact with the material to be heated, - a partition wall (140) arranged inside the first compartment (150) and defining at least one second compartment (145) at least partially filled with an inert material, - a plurality of heating elements (115, 116) arranged in the electrically insulating ceramic powder (130) between the sheath (120) and the partition wall (140).

2. The immersion heater (100) of claim 1, wherein, The partition wall (140) arranged inside the sheath (120) and defining a second compartment (145) at least partially filled with an insulating material is at least partially formed of a ceramic material.

3. The immersion heater of claim 1 or 2, wherein, The electrically insulating ceramic powder (130) is chosen among a zinc oxide powder, an aluminum oxide powder, an electrically fused magnesium oxide powder or a boron nitride powder.

4. The immersion heater (100) according to any one of claims 1 to 3, wherein, The inert material filling at least part of the second compartment (145) defined by the partition wall (140) comprises high-temperature resistant fibers, such as polycrystalline mullite fibers or silica alumina fibers.

5. The immersion heater (100) according to any one of claims 1 to 4, wherein, The second compartment (145) defined by the partition wall (140) is partially filled with an inert material and remains partially free of solid material.

6. The immersion heater (100) of claim 5, wherein, The immersion heater comprises a heating zone (101) housing the heating elements and a non-heating zone (102), and wherein the second compartment (145) corresponding in height to the heating zone (101) of the immersion heater is partially filled with an inert material, whereas the second compartment (145) corresponding in height to the non-heating zone (102) of the immersion heater remains partially free of solid material.

7. The immersion heater (100) according to any one of claims 1 to 6, wherein, The sheath (120) has a circular cross-section.

8. The immersion heater (100) of claim 7, wherein, The sheath (120) has an outer diameter greater than or equal to 50 mm, preferably greater than 75 mm, more preferably greater than 95 mm, even more preferably greater than 115 mm.

9. The immersion heater (100) according to any one of claims 1 to 8, wherein, The partition wall (140) is a tube having a circular cross-section.

10. The immersion heater (100) according to any one of claims 7 or 8 and according to claim 9, wherein The center of the sheath (120) having a circular cross-section and the center of the partition wall (140) having a circular cross-section are substantially identical, so that the first compartment formed between the sheath (120) and the partition wall (140) and intended to be at least partially filled with an electrically insulating ceramic powder has an annular cross-section.

11. The immersion heater (100) according to any one of claims 1 to 10, wherein, The heating elements are arranged in substantially straight turns arranged substantially parallel to an axis passing through the center of the sheath (120), and wherein 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.

12. The immersion heater (100) according to any one of claims 1 to 11, wherein, The heating elements are arranged in substantially straight turns arranged substantially parallel to an axis passing through the center of the sheath (120), and wherein the turns are positioned near the inner periphery of the sheath (120).

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

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