Mineral insulated cable, method for manufacturing mineral insulated cable, and method and system for heating a material

Abstract

The mineral-insulated cable includes an elongated core made of a conductive ceramic-based material with a negative temperature coefficient. The elongated core is positioned on the central axis of the mineral-insulated cable and is surrounded by an electrically insulating layer made of a mineral material. The electrically insulating layer is electrically conductive to the conductive ceramic-based material. A metallic outer sheath is concentrically wrapped around the electrically insulating layer. Electrical current can be passed through the elongated core at high voltage to generate up to 15 kW of heat per meter of cable.

Description

[Technical Field]

[0001] In one aspect, the present invention relates to a mineral insulated cable. In another aspect, the present invention relates to a method for manufacturing a mineral insulated cable. In yet another aspect, the present invention relates to a method for heating a material using the mineral insulated cable and / or the mineral insulated cable manufactured according to the method. In yet another aspect, the present invention relates to a system for heating a material using the mineral insulated cable and / or the mineral insulated cable manufactured according to the method. [Background technology]

[0002] U.S. Patent No. 10,119,366 describes an inorganic insulated heater cable having a temperature limited heater as the heating element. A "temperature limited heater" generally refers to a heater that regulates (e.g., reduces) heat output above a specified temperature without the use of external controls such as a temperature controller, power regulator, rectifier, or other device.

[0003] The cable includes a conductive core circumferentially surrounded by thin concentric conductive layers, with another concentric layer of ferromagnetic conductor separating the thin concentric conductive layers from the conductive core. The thin concentric conductive layers are surrounded by relatively thick concentric layers of electrical insulators, including inorganic insulators such as MgO, and an outer metallic jacket. The conductive core and thin concentric conductive layers are made of a non-ferromagnetic material (e.g., copper or a copper alloy). At temperatures below the Curie temperature and / or phase transition temperature range of the ferromagnetic material, the magnetic properties of the ferromagnetic material confine most of the current flow to the thin concentric conductive layers. Therefore, the thin concentric conductive layers provide most of the cable's resistive heat output at temperatures below the Curie temperature and / or phase transition temperature range. The thin concentric conductive layers may have a cross-sectional area approximately one-half or one-third of the cross-sectional area of ​​the conductive core, such that the inner conductor provides the desired amount of heat output and desired turndown ratio.

[0004] The temperature limited heaters described above operate with high frequency alternating current (AC) or modulated direct current (DC) power, which is required to generate skin effect currents in the ferromagnetic conductor. The "turndown ratio" of a temperature limited heater is the ratio of its maximum resistance below the Curie temperature to its minimum resistance above the Curie temperature for a given AC or modulated DC current. However, this heater cable only operates with AC or modulated DC, suffers from reactive power losses, and its overall behavior is frequency dependent. Additionally, a ferromagnetic core is required. Summary of the Invention

[0005] According to one aspect of the present invention, there is provided a mineral insulated cable, the mineral insulated cable comprising: an elongated core on a central axis of a mineral insulated cable; an electrically insulating layer concentrically surrounding the elongated core, the electrically insulating layer comprising an inorganic material; a metallic outer sheath concentrically surrounding the electrically insulating layer; The elongated core comprises an electrically conductive ceramic-based material having a negative temperature coefficient.

[0006] According to a second aspect of the present invention, there is provided a method of manufacturing a mineral insulated cable, the method comprising: selecting an electrically conductive ceramic-based material, a metallic outer sheath, and an electrically insulating inorganic material; - manufacturing an intermediate assembly, providing an elongated core comprising an electrically conductive ceramic-based material; - placing an elongated core on the central axis of a mineral insulated cable; - disposing an electrically insulating layer comprising an inorganic material concentrically surrounding an elongated core; - disposing a metallic outer sheath concentrically surrounding the electrically insulating layer; - subjecting the intermediate assembly to a diameter reduction including the steps of machining and heat treating.

[0007] The mineral insulated cable may be employed in a method for heating a substance, - passing an electric current through an elongated core in a direction along a central axis; - bringing the substance to be heated into heat exchange contact with a mineral insulated cable; and -Transfer of heat from an elongated core to a substance.

[0008] Finally, the mineral insulated cable may be included in a system for heating a substance, the system comprising: a container for holding a substance to be heated, in which a mineral insulated cable is placed; a current supply electrically connected to the elongated core of the mineral insulated cable, the current supply arranged to pass an electric current through the elongated core in a direction along the central axis. [Brief explanation of the drawings]

[0009] The drawings depict, by way of example only, one or more implementations consistent with the present teachings, by way of illustration and not by way of limitation. In the drawings, like reference numbers refer to the same or similar elements. [Figure 1] 1 shows a schematic cross-sectional view of a mineral insulated cable according to one embodiment of the present invention. [Figure 2] 2 shows a schematic cross-sectional view of a mineral insulated cable according to another embodiment of the present invention. [Figure 3] 2 shows a schematic cross-sectional view of a mineral insulated cable according to yet another embodiment of the present invention. [Figure 4] FIG. 1 shows a schematic cross-sectional view of a heater vessel containing a mineral insulated cable. DETAILED DESCRIPTION OF THE INVENTION

[0010] Those skilled in the art will readily appreciate that while the detailed description of the present invention has been presented with reference to one or more embodiments each having a particular combination of features and means, many of those features and means may be equally or similarly applied independently in other embodiments or combinations.

[0011] The present disclosure provides an inorganic insulated cable including an elongated core including an electrically conductive ceramic-based material having a negative temperature coefficient (NTC). The elongated core is disposed on a central axis of the inorganic insulated cable and is surrounded by an electrically insulating layer including an inorganic material. The electrically conductive ceramic-based material is electrically conductive relative to the electrically insulating layer. A metallic outer sheath is concentrically wrapped around the electrically insulating layer.

[0012] The negative temperature coefficient causes the resistivity of the conductive ceramic-based material to decrease with increasing temperature. This results in a self-regulating, inorganically insulated heating cable. The heat dissipated per unit length in any section of the cable by current is proportional to the local resistivity of the elongated core in that section of the cable. This means that hot spots in the cable are mitigated. This mitigation also works locally in the cable section because the resistance, and therefore the heat dissipation rate, also decreases locally with increasing (local) temperature in that section. Depending on whether the cable is powered by current control or voltage control, the total current through the heater cable may remain the same or increase slightly (due to a slight decrease in the overall series resistance of the cable as the resistance in the local section drops); therefore, heat continues to be dissipated in the remaining sections of the cable, even if there is local turndown in one or more sections.

[0013] Avoiding global or localized overheating of the cable (local hot spots) has many advantages, one of which is avoiding damage to the electrical insulation layer surrounding the elongated core by ensuring that its insulating properties are not impaired by overheating.

[0014] The resistive drop is independent of the current frequency and advantageously operates with a DC current so that reactive power losses can be avoided and all power can be used to heat the substance.

[0015] The conductive ceramic-based material can be selected from a variety of materials and does not need to be ferromagnetic. Preferably, the conductive ceramic-based material includes oxides such as magnesia, silica, alumina, beryllia, zirconia, or mixed oxides such as spinel. Preferably, these ceramic materials may contain small amounts of alkali oxides, such as, but not limited to, LiO, NaO, and KO. Amounts of about 1 mol% to about 10 mol% are known to reduce the resistivity of pure metal oxides by several orders of magnitude.

[0016] The conductivity of ceramic materials can be increased to a desired level (typically to a resistivity of less than 100 μΩ·m at 20°C) by incorporating conductive materials (e.g., carbon or metals such as copper, aluminum, iron, tin, or nickel). Such materials typically exhibit a negative temperature coefficient of resistivity, meaning that the resistivity decreases with increasing temperature. The conductive material can take the form of conductive particles dispersed in a matrix of another ceramic material. Thus, these conductive particles can include carbon particles and / or metal particles and / or their compounds.

[0017] The resistivity of conductive ceramic-based materials can range from 1 μΩ·m to 100 μΩ·m at 20°C. Resistivity can decrease by an order of magnitude at high temperatures, depending on the material selected. In some embodiments, conductive ceramic-based materials can exhibit NTC thermistor behavior similar to certain semiconductor resistors. Such behavior is characterized by a relatively sharp temperature transition in resistivity. As used herein, the term ceramic-based material refers to a final material comprising a ceramic material and conductive particles dispersed therein. The term ceramic material refers to a matrix material.

[0018] The conductive material can be agglomerated into a rod. One method of agglomerating the material includes sintering. The rod, or several rods arranged in series, can, in one group of embodiments, constitute the elongated core and can be in direct contact with the electrically insulating layer.

[0019] However, in another group of embodiments, the elongated core may further comprise a tube having a bore surrounded by a cylindrical wall, the wall surrounding the electrically conductive ceramic-based material. In another group of this embodiment, the electrically conductive ceramic-based material is maintained within the bore of the tube. In such cases, the electrically conductive ceramic-based material and the electrically insulating layer are separated by the wall of the tube. Employing a tube filled with a semiconducting filler material has manufacturing advantages. An advantage of this group of embodiments is that the electrically conductive ceramic-based material does not need to be agglomerated into a rod (although it can nevertheless be agglomerated if desired). Instead, the tube can be filled with a powder of the electrically conductive ceramic-based material.

[0020] Another advantage of this group of embodiments is that the tube also provides the opportunity to tailor the electrical behavior of the elongated core to the specifications required for a particular heating application. In particular, the tube itself can be an electrically resistive tube, whereby the cylindrical wall is made of a resistive material. A conductive ceramic-based material preferably extends through the bore in electrical contact with the wall along a substantial length of the resistive tube. By combining the electrically resistive tube with the conductive ceramic-based material, current flowing longitudinally through the elongated core, i.e., along the bore, sequentially distributes (in accordance with Kirchhoff's law, without wishing to be limited by theory) flow through the tube wall and through the conductive ceramic-based material filling the bore, minimizing the overall resistance to flow through the elongated core. The resulting combined local electrical conductance at any section along the length of the elongated core is determined by the sum of the conductance of the tube wall and the conductive ceramic-based material at that section. The temperature dependence of resistivity is the net result of the temperature dependence of the conductive ceramic-based material and the temperature dependence of the tube. Because the conductive ceramic-based material is in electrical contact with the wall along a substantial length of the resistive tube, current is continuously redistributed throughout the tube and the conductive ceramic-based material as it flows along the length of the elongated core.

[0021] The inorganic material of the electrically insulating layer always has a much higher resistivity than the conductive ceramic-based material in the elongated core, which allows the inorganic material to act as an electrical insulator even at high temperatures.

[0022] In another group of embodiments, the conductive ceramic-based material of the elongated core may be provided as a concentric layer around an insulating core. The resistivity of the insulating core is generally lower than that of the conductive ceramic-based material, preferably at least 1000 times lower. The insulating core may be made of the same material as the electrically insulating layer concentrically surrounding the elongated core. It is also possible for the insulating core to be made from a ceramic matrix material used for the conductive ceramic-based material. This group of embodiments may be combined with the group of embodiments described above, where the elongated core further comprises a tube having a bore surrounded by a cylindrical wall. In such a case, the elongated core comprises (from the inside to the outside) an insulating core, concentric layers of conductive ceramic-based material, and a tube.

[0023] Referring now to Figure 1, there is shown a cross-sectional view of a first embodiment of a mineral insulated heater cable as proposed herein. At its center is an elongated core 10 on a central axis A. The elongated core 10 comprises an electrically conductive ceramic-based material 14. The elongated core 10 is surrounded by an electrically insulating layer 16 that concentrically encases the elongated core 10. A metallic outer sheath 18 is concentrically wrapped around the electrically insulating layer 16.

[0024] The metallic outer sheath 18 is preferably made of a material that is chemically resistant and mechanically robust at the operating temperature and in contact with the heated substance. Alloys that can be used in the cable's desired operating temperature range include, but are not limited to, 304 stainless steel, 310 stainless steel, Incoloy® 800, and Inconel® 600 (Inco Alloys International, Huntington, W.Va., USA). The metallic outer sheath 18 may be coated with one or more protective coating layers. The thickness of the metallic outer sheath 18 may need to be sufficient to withstand high-temperature, corrosive environments for 3 to 10 years. The thickness may range from approximately 1 mm to approximately 3.5 mm. Larger or smaller thicknesses may be used to meet the requirements of a particular application.

[0025] The electrical insulation layer 16 can be made of a variety of materials, particularly inorganic materials. Suitable materials include, but are not limited to, MgO, alumina, zirconia, BeO, different chemical variations of spinel, and combinations thereof. MgO can provide good thermal conductivity and electrical insulation properties. Desirable electrical insulation properties include low leakage current and high dielectric strength. Low leakage current reduces the likelihood of thermal breakdown, while high dielectric strength reduces the likelihood of arcing across the insulator. Thermal breakdown can occur when leakage current gradually increases the temperature of the insulator and also causes arcing across the insulator. The thickness of the electrical insulation layer 16 is primarily a function of the maximum desired breakdown voltage between the core 10 and the outer sheath 18 and the insulating properties of the layer. However, for certain high-voltage applications (e.g., potential differences up to 10 kV) and 85% compressed MgO as the insulation layer 16, the thickness may need to be up to 25 mm. For most applications, the thickness range of the electrically insulating layer 16 is from about 4 mm to 25 mm, preferably from about 9 mm to 25 mm.

[0026] The conductive ceramic-based material 14 comprises a ceramic matrix, preferably an oxide matrix such as an MgO matrix, with conductive particles dispersed therein. The resulting material typically has a negative temperature coefficient of resistivity. The resistivity of the conductive ceramic-based material can range from 1 μΩ·m to 100 μΩ·m at 20°C. The diameter of the elongated core 10 can range from about 10 mm to about 40 mm.

[0027] 2 shows a second embodiment, which differs from the first embodiment in that the elongated core 10 comprises a tube 12 surrounding an electrically conductive ceramic-based material 14. The tube 12 has a cylindrical wall made of a metallic material.

[0028] The tube can be a resistive tube made of a material having a resistivity of at least 0.05 μΩ·m at 20°C, preferably at least 0.1 μΩ·m, 0.3 μΩ·m, or 0.5 μΩ·m at 20°C. Higher values ​​may be preferred, typically ranging up to about 5 μΩ·m at 20°C. In certain embodiments, the material is made of a resistive metal alloy, such as a nickel-chromium alloy. The wall thickness of the tube 12 relative to the bore diameter and the resistivity of the selected metal material are selected so that the resistance of the tube 12 (derived from Ohm's Law) is such that the tube 12 is electrically and structurally stable for the desired power dissipation per unit length, the cable length, and / or the maximum voltage allowed for the core material. Wall thicknesses typically range from 0.5 mm to about 2.5 mm. However, the preferred thickness is influenced by the material's resistivity (e.g., resistivity at 20°C) combined with the desired heat output, cable length, and available drive voltage; therefore, larger or smaller thicknesses may be used depending on the design parameters. The inner diameter of the resistance tube 12 can be selected, for example, in the range of about 3.5 mm to about 38 mm, preferably 5 mm to 38 mm, and more preferably 10 mm to 38 mm. The preferred diameter depends on the desired resistance reduction amount in the elongated core and the type of semiconductive material filled therein, and in some cases may be outside the above range.

[0029] The conductive ceramic material 14 filling the bore is preferably in electrical contact with the wall of the tube 12 along a substantial length. Due to the negative temperature coefficient of the elongated core, as the temperature of any section of the cable increases, the effective core resistance in that section decreases because more current can now pass through the conductive ceramic material 14 rather than through the wall of the tube 12. This has a limiting effect on the amount of heat that can be generated. The resistivity of the tube 12 can be at least 0.05 μΩ·m at 20°C, which includes almost all but the best conductors of metals. Examples include tungsten and iron, or alloys such as brass, copper-nickel, and nickel-chromium ("nichrome"). By selecting a material with a slightly higher resistivity, it is possible to achieve the desired heat output without significantly reducing the wall thickness. Preferably, the material is selected to have a resistivity of at least 0.3 μΩ·m at 20°C. This includes metals such as constantan. Most preferably, the resistivity is at least 0.5 μΩ·m at 20°C. This includes high resistance alloys such as nichrome and other alloys typically found in electrical resistance heating devices. The resistivity of the tube 12 is preferably less than 5 μΩ·m at 20° C., more preferably less than 2 μΩ·m at 20° C., and most preferably less than 1 μΩ·m at 20° C. This maintains the voltage drop per unit length of the cable at a desired level.

[0030] Figure 3 shows a third embodiment, which differs from the first embodiment in that the elongated core 10 includes an insulating core 15 surrounded by a conductive ceramic-based material 14. This embodiment can also be combined with the embodiment of Figure 2. The insulating core allows for localization of the conductive ceramic-based material 14 where it is most effective, on the periphery (due to the conductive skin effect).

[0031] In each of the above embodiments, the overall outer diameter of the mineral insulated cable may suitably range from about 25 to about 60 mm. In some embodiments, the mineral insulated cable may be capable of delivering greater than 7 kW / m of cable length, e.g., up to 15 kW / m, at a core temperature ranging from 600°C to 850°C, preferably 700°C to 850°C, and a temperature differential between the core and sheath of 250°C to 400°C.

[0032] The mineral-insulated cable described above can be manufactured according to certain known manufacturing methods for conventional metal-insulated cables, but note that the elongated core, as described herein, is a conductive ceramic-based core rather than a monolithic resistance wire. Once the elongated core is available, the cable can be manufactured by placing the elongated core on the central axis of the mineral-insulated cable, disposing a concentric electrical insulation layer around the elongated core, and disposing a concentric metallic outer sheath around the electrical insulation layer. This intermediate assembly then undergoes diameter reduction, which involves alternating machining and heat treatment processes. This results in compression of the ceramic material in the insulation layer. The target compression is determined by the cable's desired breakdown voltage. Roughly speaking, the target compression is typically 85% or greater, with 100% compression being equal to the density of the crystalline material. Typically, a diameter reduction of 10% to 30% is sufficient to achieve the target compression.

[0033] Further details can be found, for example, in U.S. Patent No. 10,119,366, which describes the manufacturing process in detail. See also Chapter 16 of Electric Cables Handbook / BICC Cables (3rd Edition, edited by G.G. Moore, Blackwell Science Ltd., 1997). In a typical manufacturing process used to make (form) mineral-insulated cables, the metal sheath of the cable begins as a strip of conductive material (e.g., stainless steel). The strip is formed (wound longitudinally) into a partial cylinder, and an electrical insulator block (e.g., magnesium oxide block) is inserted into the partial cylinder-shaped sheath. The inserted block can be a partial cylinder block, such as a half-cylinder block. Following block insertion, an elongated core is placed within the partial cylinder and inside the half-cylinder block. Once the electrical insulator block and elongated core are in place, the portion of the sheath, including the block and elongated core, can be formed into a complete cylinder around the block and elongated core. The longitudinal edges of the strip may be welded together to close the cylinder, forming a mineral insulated cable having an elongated core and electrical insulator blocks inside a sheath. The process of inserting blocks and closing the sheath cylinder may be repeated along the length of the sheath to form intermediate assemblies of any desired length.

[0034] After the intermediate assembly is formed, further steps can be taken to reduce the voids and / or porosity within the assembly and increase the breakdown voltage. The intermediate assembly can be moved through a progressive reduction system (cold work system) to reduce the voids within the assembly. One example of a progressive reduction system is a roller system. In a roller system, the intermediate assembly can progress through multiple horizontal and vertical rollers by alternating between horizontal and vertical rollers. The rollers can progressively reduce the size of the intermediate assembly into the final mineral-insulated cable. Alternatively, this reduction can be achieved in a pultrusion table drawing process, in which the intermediate assembly is pulled through a succession of drawing dies.

[0035] The mineral-insulated cable assembly is preferably heat-treated (annealed) during the reduction process. Without being bound by theory, it is believed that heat-treating (annealing) the assembly helps restore the mechanical properties of the metals used in the mineral-insulated cable. Heat-treating (annealing) the cable can be described as a heat treatment that relieves stress and returns a material (e.g., a metal alloy material) to its natural state (e.g., the state of the alloy material before any cold working or heat treatment of the alloy material). For example, when austenitic stainless steel is cold-worked, it can become stronger, but due to the brittleness of austenitic stainless steel, it can also become more brittle to the point where further cold working can fracture the material. The strength of the annealed material and the strength that can be achieved by cold working before failure can vary depending on the material being processed.

[0036] In some embodiments, heat treatment allows for further reduction (cold working) of the mineral insulated cable. For example, a mineral insulated cable assembly can be heat treated to reduce stresses in the metal within the assembly after cold working and to improve the cold working (progressive reduction) characteristics of the metal.

[0037] Metal alloys in inorganic insulated cables (e.g., stainless steel used as the sheath (or outer conductor)) may need to be rapidly quenched after heat treatment. Rather than allowing the components to form crystals, the metal alloy may be rapidly quenched to solidify the alloy while the components are still in solution, which may not contribute as needed to the mechanical properties of the metal alloy. During quenching, the metal sheath may first cool, and then heat is more gradually transferred from the interior of the cable through the sheath. Thus, the metal sheath contracts, contracting and squeezing the electrical insulation (e.g., MgO), further compressing the electrical insulation.

[0038] As the electrical insulators and elongated core cool, they may shrink, leaving small voids, which may relieve pressure from, for example, seams between electrical insulator blocks within the mineral-insulated cable assembly. Small voids or seams can contribute to increased pore volume and / or porosity in the electrical insulator, which can adversely affect breakdown voltage. For example, heat treatment can reduce the breakdown voltage by about 50% or more for typical heat treatments of the metals used in the mineral-insulated cables described herein. Such a reduction in breakdown voltage may cause short circuits or other electrical breakdowns when the mineral-insulated cable is used at medium to high voltages (e.g., voltages of about 5 kV or greater). A final reduction (cold working) of the mineral-insulated cable after heat treatment may be applied to restore the breakdown voltage to an acceptable value for long heater lengths. However, the final reduction should preferably not be as large as the previous reduction to avoid straining the metals in the cable assembly beyond acceptable limits or excessive strain. Excessive reduction in the final reduction may require additional heat treatment to restore mechanical properties to the metal in the mineral insulated cable. Thus, the final reduction (cold working) step may reduce the cross-sectional area of ​​the mineral insulated cable sufficiently to compress the electrical insulation, reduce or substantially eliminate voids in the electrical insulation (e.g., reduce pore volume and / or porosity), and restore the breakdown voltage characteristics of the electrical insulation to a desired level.

[0039] In one group of embodiments, the elongated core consists essentially of an electrically conductive ceramic-based material agglomerated into a rod. This may be accomplished by mixing ceramic powder with conductive particles in a liquid binder to form an uncured matrix. The uncured matrix may then be formed into an uncured rod by extrusion or other suitable technique. The uncured rod may then be heated to agglomerate the ceramic powder and conductive particles, thereby dispersing the conductive particles in a matrix of ceramic material. The heating may sinter the particulates within the rod.

[0040] In another group of embodiments, a conductive ceramic-based material is disposed within the bore of the tube, and manufacturing the elongated core may include filling the bore of said tube with the conductive ceramic-based material.

[0041] There are several options for achieving the filling of the elongated core with the conductive ceramic-based material. Three examples will be briefly described. In the first example, a tube is provided that is preferentially arranged vertically, and the bore of the tube is filled from the top with powder of the conductive ceramic-based material. Vibration and / or ramming may be applied to more effectively fill the powder into the bore. The elongated core thus provided has a predetermined length.

[0042] A second example of producing an elongated core is similar to the above, except that instead of a powder, a macroscopically consolidated block (e.g., a cylindrical block) of conductive ceramic-based material is inserted into the bore of the tube. In this example, the tube can be oriented horizontally. Preferably, the macroscopically consolidated block fits snugly inside the bore. Small gaps are acceptable, as they can disappear in the subsequent reduction process. The elongated core thus provided has a predetermined length.

[0043] A third example is a semi-continuous process in which a tube is provided in the form of a formable strip (preferably a metal strip or a processable polymer) and then formed around a macroscopic consolidated block (e.g., a cylindrical block) of conductive ceramic-based material, similar to how a metal sheath is formed around an inorganic insulating material as described above. The tube may optionally be sealed by the contacting long edges, for example, by welding, although in some embodiments, sealing is not required. The resulting elongated core produced by this example may be of indefinite length.

[0044] Macroscopic consolidation of the conductive ceramic-based material in the second and third examples may be achieved by sintering.

[0045] The mineral-insulated cables described herein and / or manufactured as described herein can be used to heat a substance. FIG. 4 shows a schematic diagram of an example of a system for heating a substance employing a heat exchanger generally modeled after a tube-and-shell heat exchanger. The system includes a vessel 20 for holding the substance to be heated. The vessel 20 may suitably include an inlet 22 and an outlet 24 for allowing the substance 28 to enter and exit the vessel 20, also typically referred to as the shell side of the heat exchanger. A mineral-insulated cable 5 may be placed within the vessel in place of a heating tube, or may be routed through the vessel inside a conduit. Those skilled in the art will recognize that many variations and possibilities exist.

[0046] In the illustrated embodiment, multiple parallel-arranged cables 5 are shown, each in a single-pass configuration. Those skilled in the art will appreciate that many variations are possible, including applying 180° U-bends to create multiple passes with one cable. Several baffles 26 may be provided to better distribute material throughout all of the cables 5. A current supply 25 is electrically connected to the elongated core 10 of the mineral-insulated cable 5. Only one connection pole is shown schematically in FIG. 2 . Those skilled in the art will appreciate that many variations are possible for the return connection. In use, the material 28 to be heated is in heat exchange contact with the mineral-insulated cable 5, while current flows through the elongated core 10, resistively heating the cable 5. Heat is transferred from the cable 5 to the material 28. Local overheating of the cable 5 (at hot spots) is avoided by providing a self-regulating local resistance reduction anywhere within the cable 5, as described herein. Heat exchange contact can be achieved through direct physical contact or through indirect contact via one or more other intermediate materials.

[0047] A typical vessel 20, as shown in FIG. 4, may be cylindrical in shape, have a diameter typically between 2 m and 5 m, and have a length typically between 10 and 30 m. However, depending on requirements, the vessel may have different shapes and / or sizes outside of these typical ranges. In some embodiments, to achieve high heat loads, the total length of the cable provided within the vessel may be several kilometers, in some instances up to 10 km. Heat loads may exceed 10 MW.

[0048] In a preferred embodiment, the material is heated using electricity generated by renewable power generation, such as wind or solar power, and in the event of a temporary outage of the renewable power generation, heat can be extracted from the material. The material to be heated can be, for example, a molten salt. Molten salts are a commonly proposed solution for energy storage. Typical choices include eutectic mixtures to lower the melting point of the molten salt, but the present invention is not limited by any particular choice of salt or mixture.

[0049] The heating vessels described above are examples of mineral-insulated cable applications for process heating. The cable may be immersed in and / or completely surrounded by the flowing material to be heated. Mineral-insulated cable may also be applied to heating pipes, vessels, and the like by electric trace heating, whereby the mineral-insulated cable extends in physical contact with the outside of the pipe or vessel (or the like). Mineral-insulated cable may be packed with the pipe or vessel under a layer of insulation.

[0050] Those skilled in the art will appreciate that the present invention can be implemented in various ways without departing from the scope of the appended claims.

Claims

1. An inorganic insulated cable, an elongated core on the central axis of said mineral insulated cable; an electrically insulating layer concentrically surrounding said elongated core, said insulating layer comprising an inorganic material; a metallic outer sheath concentrically surrounding said electrically insulating layer; An inorganic insulated cable, wherein the elongated core comprises a conductive ceramic-based material having a negative temperature coefficient.

2. 10. The mineral insulated cable of claim 1, wherein the conductive ceramic-based material is agglomerated into rods.

3. 3. The mineral insulated cable of claim 1, wherein the elongated core further comprises a tube having a bore surrounded by a cylindrical wall, the wall surrounding the conductive ceramic-based material, thereby maintaining the conductive ceramic-based material within the bore of the tube.

4. 4. The mineral insulated cable of claim 3, further comprising: said tube being a resistance tube, whereby said cylindrical wall is made of a resistance material, whereby said conductive ceramic-based material extends through said bore in electrical contact with said wall along a substantial length of said resistance tube.

5. 5. The mineral insulated cable of claim 4, wherein the resistive material has a resistivity of at least 0.05 μΩ·m at 20°C.

6. 6. The mineral insulated cable of claim 5, wherein the resistivity is less than 5 μΩ·m at 20°C.

7. 7. The mineral insulated cable according to claim 3, wherein the cylindrical wall is made of metal.

8. 8. A mineral insulated cable according to any one of claims 1 to 7, wherein the conductive ceramic-based material of the elongated core is provided as a concentric layer around an insulating core.

9. 9. The mineral insulated cable according to claim 1, wherein the conductive ceramic-based material comprises conductive particles dispersed in a matrix of ceramic material.

10. 10. The mineral insulated cable of claim 9, wherein the conductive particles comprise carbon particles and / or metal particles.

11. 1. A method for manufacturing a mineral insulated cable, comprising: - selecting an electrically conductive ceramic-based material, a metallic outer sheath, and an electrically insulating inorganic material; - manufacturing an intermediate assembly, - providing an elongated core comprising said conductive ceramic-based material; - placing said elongated core on the central axis of said mineral insulated cable; - disposing an electrically insulating layer comprising said inorganic material concentrically surrounding said elongated core; - disposing the metallic outer sheath concentrically around the electrically insulating layer; - subjecting said intermediate assembly to a diameter reduction including the steps of machining and heat treatment.

12. providing the elongated core, - mixing ceramic powder and conductive particles with a liquid binder to form an uncured substrate; - extruding the uncured substrate into an uncured rod; - heating the uncured rod to agglomerate the ceramic powder and the conductive particles, whereby the conductive particles are dispersed in a matrix of ceramic material.

13. 13. The method of claim 11 or 12, wherein providing the elongated core comprises filling a bore of a tube with the electrically conductive ceramic-based material.

14. 1. A method of heating a substance, comprising: - providing a mineral insulated cable according to any one of claims 1 to 10 and / or a mineral insulated cable manufactured according to any one of claims 11 to 13, - passing an electric current through said elongate core in a direction along said central axis; - bringing a substance to be heated into heat exchange contact with said mineral insulated cable; - transferring heat from said elongated core to said substance.

15. 1. A system for heating a substance, comprising: a container for holding the substance to be heated; - an uninsulated cable according to any one of claims 1 to 10 and / or a mineral insulated cable manufactured according to any one of claims 11 to 13, placed in said container; a current supply electrically connected to the elongated core of the mineral insulated cable, the current supply being arranged to pass an electric current through the elongated core in a direction along the central axis.

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