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

Abstract

This mineral-insulated cable includes a core including a resistive tube having a bore surrounded by a cylindrical wall, and a semiconductive filler material filled within the bore. The cylindrical wall is surrounded by an electrically insulating layer including an inorganic material. The cylindrical wall is made of a metallic material having a resistivity of at least 0.05 μΩ·m at 20°C. The semiconductive filler material is in electrical contact with the wall along a substantial length of the resistive tube. The semiconductive filler material has an electric bandgap smaller than the electric bandgap of the inorganic material of the electrically insulating layer. Electric current flows through the core at high voltage, generating 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 the central axis of a mineral insulated cable; an electrically insulating layer concentrically surrounding the elongated core, the insulating layer comprising an inorganic material; a metallic outer sheath concentrically surrounding the electrically insulating layer, and the elongated core a resistance tube having a bore surrounded by a cylindrical wall made of a metallic material having a first resistivity of at least 0.05 μΩ·m at 20°C; and a semiconductive filler disposed within the bore and in electrical contact with the wall along a substantial length of the resistance tube, the semiconductive filler having an electric bandgap smaller than the electric bandgap of the inorganic material of the electrically insulating layer.

[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 a filler material including a semiconductive filler material having an electric band gap smaller than the electric band gap of the inorganic material of the electrically insulating layer, a resistance tube including a cylindrical wall made of a metallic material having a first resistivity of at least 0.05 μΩ m at 20°C, a metallic outer sheath, and an electrically insulating inorganic material; - manufacturing an intermediate assembly; - subjecting said intermediate assembly to a diameter reduction including machining and heat treatment steps; Manufacturing the intermediate assembly includes: - filling a bore of said resistance tube with said filler material in electrical contact with said wall along a substantial length of said resistance tube to provide an elongated core; - placing the elongated core on the central axis of a mineral insulated cable; - disposing an electrically insulating layer comprising the inorganic material concentrically surrounding the elongated core; - disposing a metallic outer sheath concentrically surrounding the electrically insulating layer.

[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; -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 heater vessel containing the mineral insulated cable of FIG. 1. 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 a core including a resistive tube having a bore surrounded by a cylindrical wall, and a semiconductive filler material filled within the bore. The cylindrical wall is surrounded by an electrically insulating layer including an inorganic material. The cylindrical wall is made of a metallic material having a resistivity of at least 0.05 μΩ·m at 20°C. The semiconductive filler material is in electrical contact with the wall along a substantial length of the resistive tube. The semiconductive filler material has an electric bandgap smaller than the electric bandgap of the inorganic material of the electrically insulating layer.

[0012] By combining a resistance tube with a semiconductive filler, a core with a substantially negative temperature coefficient at high temperatures can be achieved. Because the semiconductive filler is in electrical contact with the wall of the resistance tube along a substantial length, current flowing longitudinally through the core, i.e., along the bore, sequentially distributes flow through the walls of the resistance tube and through the semiconductive filler material in the bore (in accordance with Kirchhoff's law, without wishing to be limited by theory), minimizing the overall resistance to flow through the core. The resulting combined local electrical conductance at any section along the length of the core is determined by the sum of the conductance of the walls of the resistance tube and the conductance of the semiconductive filler material at that section.

[0013] Typically, the resistivity of the semiconductive filler at 20°C is much higher than the resistivity of the metallic material of the resistor tube at 20°C. Assuming the resistivity of the semiconductive filler is several orders of magnitude higher than the resistivity of the resistor tube, the current supplied to the core will flow preferentially through the walls of the resistor tube, and thus the effective resistance of the core as a whole will be substantially equal to the resistance of the resistor tube. However, as the temperature increases, more electrons in the semiconductive filler have enough thermal energy to overcome the band gap between the valence band and conduction band of the material and become conduction electrons. This causes a decrease in the resistivity of the semiconductive filler, and therefore an overall decrease in the resistivity of the core, as a higher percentage of the total current is distributed through the semiconductive filler.

[0014] The resistivity of the filler at operating temperatures above a certain high temperature may in some cases be lower than the resistivity of the resistance tube wall at said operating temperature. The inorganic material of the electrical insulating layer always has a much higher resistivity than the semiconductive filler, which allows the inorganic material to function as an electrical insulator even at high temperatures.

[0015] The result is a self-regulating mineral-insulated heating cable. The heat dissipated per unit length in any section of the cable due to electrical current is proportional to the local core resistance in that section of the cable. When the temperature of a particular section of the cable exceeds a certain predetermined elevated temperature (hereinafter, such a section may be referred to as a "hot spot"), the core resistivity in that section decreases, and therefore the heat dissipation rate in that section also decreases. Thus, the local resistance drop in the cable section proportionally reduces the power dissipation in that section, thereby lowering the local temperature at the hot spot to a temperature closer to the design operating temperature. This phenomenon is sometimes referred to as local resistance reduction (LRR). At the hot spot, which is typically a limited section along the length of a mineral-insulated heating cable, the local electrical resistance decreases significantly, thereby proportionally reducing the heat generation rate, thereby lowering the local temperature at the hot spot to a temperature closer to the design operating temperature. The LRR ratio at any point along the length of the cable is defined as the power that would be dissipated in the resistance tube if there was no fill material inside the bore (i.e., if all the current passed through the resistance tube at that point) compared to the reduced power that is actually dissipated (a portion of the total current flows through the semiconductive filler material rather than just through the resistance tube).

[0016] The predetermined elevated temperature is a design parameter that may be based on the requirements of the selected heating application. 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 may increase slightly (due to the slight decrease in the overall series resistance of the cable as the resistance in the local section drops), so that heat continues to be dissipated in the remaining sections of the cable that do not exceed the predetermined temperature.

[0017] Avoiding localized overheating (localized hot spots) in the cable has many advantages, one of which is avoiding damage to the electrical insulation layer surrounding the core by ensuring that the insulating properties are not impaired by overheating. Another advantage of the inherent LRR in hot spots is that the cable can be operated continuously at or as close as possible to the predetermined temperature of the selected heating application without damaging the cable or the material being heated.

[0018] The local resistive drop is independent of the current frequency and advantageously acts with a DC current so that reactive power losses can be avoided and all power can be used to heat the substance.

[0019] Employing a resistor tube filled with semiconductive filler has manufacturing advantages over embodiments in which the core comprises a large resistor surrounded by semiconductive material.

[0020] The semiconductive filler can be selected from a variety of materials and does not need to be ferromagnetic. Preferably, the semiconductive filler material comprises a ceramic semiconductor such as silicon nitride or silicon carbide. A semiconductor is a material that has conductivity between that of a conductor (generally a metal) and a non-conductor or insulator (such as most ceramics). A semiconductor can be a pure element such as silicon or germanium, or a compound such as silicon carbide, silicon nitride, gallium nitride, or iron oxide, or a mixture of two or more pure and / or compound semiconductors. Small amounts of impurities can be added to a pure semiconductor to produce large changes in the conductivity of the material.

[0021] Due to the nonlinearity of the conductivity of semiconducting materials, a negative temperature coefficient may only appear at high temperatures, while at lower temperatures the generally positive coefficient of the resistive tube may be the dominant behavior of the core.By selecting a combination of metallic and semiconducting materials, it is possible to create a heating cable that effectively reduces heat generation locally at high temperatures when hot spots need to be avoided.

[0022] Although the present invention works in principle with any type of resistance tube, the resistance tube is preferably a metallic resistance tube. Certain metals have sufficient resistivity for the purposes of the present invention, and furthermore, metals are relatively easy to form into tubes.

[0023] The resistivity of the resistance tube is at least 0.05 μΩ·m at 20°C. For example, medium- and high-resistivity metals and alloys such as tungsten, iron, constantan, chromium, and nickel-chromium ("nichrome") meet this requirement. By selecting a material with a slightly higher resistivity, it is possible to achieve the desired heat output without making the wall thickness too thin. Preferably, the material is selected to have a resistivity of at least 0.1 μΩ·m at 20°C, more preferably at least 0.3 μΩ·m at 20°C. This includes metals such as constantan and resistive alloys. Most preferably, the resistivity is at least 0.5 μΩ·m at 20°C. This includes high-resistivity alloys such as nichrome and other alloys typically found in electrical resistance heating devices.

[0024] The resistivity of the resistance tube 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 an adequate level of voltage drop per unit length of cable required to achieve sufficient current to generate heat.

[0025] The resistivity of the resistance tube at 20°C may be in the range of 0.05 μΩ·m to 5 μΩ·m, preferably in the range of 0.1 μΩ·m to 5 μΩ·m, more preferably in the range of 0.3 μΩ·m to 5 μΩ·m, and more preferably in the range of 0.5 μΩ·m to 5 μΩ·m.

[0026] Referring now to Figure 1, there is shown a cross-sectional view of an 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 a metallic resistance tube 12 and a semi-conductive filler material 14 filled within the bore of the resistance tube 12. 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.

[0027] 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.

[0028] 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 for 85% compressed MgO as the insulation layer 16, the thickness needs 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.

[0029] The resistance tube 12 has a cylindrical wall made of a metallic material. The material has a resistivity of at least 0.05 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 metallic material is made of a resistive metal alloy, such as a nickel-chromium alloy. The wall thickness of the resistance tube 12 relative to the bore diameter and the resistivity of the selected metallic material are selected so that the resistance (derived from Ohm's Law) of the resistance tube 12 is such that the resistance 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. The wall thickness typically ranges 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) in combination 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 LRR ratio of the core and the type of semiconductive material filled therein, and may be outside the above range in some cases.

[0030] The semiconductive filler material 14 in the bore should be in electrical contact with the wall of the resistance tube 12 along a substantial length. The semiconductive filler material 14 should have an electrical bandgap that ensures that the axial resistance through the filler material is much higher than the axial resistance through the resistance tube 12 up to a specific design temperature. Therefore, under normal operating conditions, heat is generated within the resistance tube 12, and the electrical properties of the core 10 are dominated by the electrical properties of the resistance tube 12. However, the bandgap must be small enough so that the thermal energy of electrons is sufficient to convert to conduction electrons when the operating temperature approaches the specific design temperature. If this occurs in any section of the cable, the core resistance in that section will decrease because current can flow through the filler material 14 rather than the resistance tube 12. This has a limiting effect on the amount of heat that can be generated. The semiconductive fill material may include, but is not limited to, one or more of germanium, silicon, gallium arsenide, gallium phosphide, cadmium sulfide, silicon carbide, gallium nitride, silicon nitride, boron nitride, and several metal oxides, including iron oxide, nickel oxide, and copper oxide. Preferably, the semiconductive fill material is a crystalline powder filled into the bore of the resistor tube. The semiconductive material may be doped, but is preferably undoped to achieve the best LRR ratio of heat generation achievable with undoped material due to employing the greatest possible conductivity contrast of the fill material.

[0031] 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 difference 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 it should be noted that the elongated core, as described herein, is a composite 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] More details can be found, for example, in U.S. Pat. No. 10,119,366, which describes the manufacturing process in detail. rdSee also Chapter 16 of "The Mineral-Insulated Cable" (edition edited by G.G. Moore, Blackwell Science Ltd., 1997). In a typical manufacturing process used to make (form) mineral-insulated cable, 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-cylindrical sheath. The inserted block can be a partial-cylindrical block, such as a half-cylindrical block. Following block insertion, an elongated core is placed within the partial cylinder and inside the half-cylindrical block. Once the electrical insulator block and core are in place, the portion of the sheath, including the block and core, can be formed into a complete cylinder around the block and core. The longitudinal edges of the strip can be welded to close the cylinder, forming a mineral-insulated cable with a core and electrical insulator block inside the sheath. The process of inserting blocks and closing the sheath cylinder can be repeated along the length of the sheath to form an intermediate assembly of the 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] The elongated core is fabricated by filling the bore of the resistor tube with a semiconductive filler material in electrical contact with the wall along a substantial length of the resistor tube. The semiconductive filler material is selected to have an electrical bandgap smaller than the electrical bandgap of the inorganic material of the electrically insulating layer. The resistor tube is made of a metallic material selected to have a resistivity of at least 0.05 μΩ·m at 20°C.

[0040] There are several options for achieving the filling of the elongated core with the filler material. Three examples will be briefly described. The first is to provide a tube of resistive material in a preferentially vertical position and fill the bore of the tube from above with powder of semiconductive filler material. Vibration and / or ramming can be applied to more effectively fill the powder into the bore. The elongated core thus provided has a predetermined length.

[0041] 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 semiconductive filler 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.

[0042] A third example is a semi-continuous process in which the resistive material is provided in the form of a strip and then formed around a macroscopic consolidated block (e.g., a cylindrical block) of semi-conductive filler material, similar to how a metallic sheath is formed around an inorganic insulating material as described above. The resistive tube may optionally be welded by the contacting long edges, although in some embodiments, welding is not required. The resulting elongated core produced by this example may be of indefinite length.

[0043] Macroscopic consolidation of the semiconductive filler material in the second and third examples may be achieved by sintering.

[0044] The mineral-insulated cables described herein and / or manufactured as described herein can be used to heat a substance. FIG. 2 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.

[0045] 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.

[0046] A typical vessel 20, as shown in FIG. 2, 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.

[0047] 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.

[0048] 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.

[0049] 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 a central axis of the 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 a resistance tube having a bore surrounded by a cylindrical wall made of a metallic material having a first resistivity of at least 0.05 μΩ·m at 20° C.; an inorganic insulated cable comprising: a semiconductive filler material disposed within the bore and in electrical contact with the wall along a substantial length of the resistance tube, the semiconductive filler material having an electrical bandgap smaller than the electrical bandgap of the inorganic material of the electrical insulation layer.

2. 2. The mineral insulated cable of claim 1, wherein the second resistivity of the filler at 20°C is higher than the first resistivity of the metallic material at 20°C.

3. 3. The mineral insulated cable of claim 2, wherein the second resistivity at operating temperatures above a predetermined elevated temperature is lower than the first resistivity at said operating temperature.

4. 4. The inorganic insulated cable according to claim 3, wherein the predetermined elevated temperature is within a range of 1° C. to 100° C. higher than the design operating temperature.

5. The inorganic insulated cable according to any one of claims 1 to 4, wherein the filler material comprises a ceramic material.

6. 6. The inorganic insulated cable according to claim 1, wherein the first resistivity is at least 0.5 μΩ·m at 20°C.

7. The inorganic insulated cable according to any one of claims 1 to 6, wherein the first resistivity is less than 5 μΩ·m at 20°C.

8. 1. A method for manufacturing a mineral insulated cable, comprising: selecting a filler material comprising a semiconductive filler material having an electric bandgap smaller than the electric bandgap of the inorganic material of the electrically insulating layer, a resistance tube comprising a cylindrical wall made of a metallic material having a first resistivity of at least 0.05 μΩ·m at 20° C., a metallic outer sheath, and an electrically insulating inorganic material; - manufacturing an intermediate assembly, - filling the bore of the resistance tube with said filler material in electrical contact with said wall along a substantial length of said resistance tube to provide an elongated core; - 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.

9. A method for heating a substance, - providing a mineral insulated cable according to any one of claims 1 to 7 and / or a mineral insulated cable manufactured according to claim 8, - 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.

10. 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 7 and / or a mineral insulated cable manufactured according to claim 8, 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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