Dual layer mineral insulated tube and method for heating a substance

Abstract

Implementations of the disclosed subject matter provide a mineral insulated tube. The tube may include: an elongated core on a central axis of the tube, the elongated core comprises: a resistive hollow tube having a bore surrounded by a cylindrical wall made of a metal material having a first resistivity of at least 0.1 μΩ∙m at 20°C. The tube may further include a semi-conducting layer concentrically enveloping the elongated core, comprising a semi-conducting material in electrical contact with the elongated core, and the semi-conducting material may have a first electric bandgap and a second resistivity. The tube may also include an electrically insulating layer concentrically enveloping the semi-conducting layer including a mineral material that has a second electric bandgap and a metallic outer sheath concentrically enveloping the electrically insulating layer. The first electric bandgap of the semi-conducting material may be less than the second electric bandgap of the mineral material.

Description

DUAL LAYER MINERAL INSULATED TUBE AND METHOD FOR HEATING A SUBSTANCETECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a mineral insulated tube as a heating cable. More specifically, the present invention relates to a method for heating a substance.BACKGROUND

[0002] Currently available process heaters are limited in heat output due to the formation of hotspots caused by the lack of uniformity of heat transfer from the skin of the heating element to the intended fluid or gas or combination of the two. These hotspots are difficult to detect as they can occur is a small segment of the overall heated length of the element. These hot spots result in damage to the material being heated and may also result in premature failure of the heating cable itself. This is a well-known phenomena in heating applications with commercially available heating systems known as fouling or carbonization. As such, there is a need to manage hotspots and control heaters to operate at lower temperatures utilizing a safety factor. This need for control severely limits the heat output (e.g., input to the process) even with complicated control schemes.

[0003] Further, an electric heater (for example a mineral insulated cable heater) may be powered by applying voltage and generating resistive heat flux which is proportional to the square of current and proportional to the resistance of heater core material. As the temperature of the heater element increases, the local resistance (which has a positive temperature coefficient, i.e., the resistance increases with temperature) increases thereby causing a locally higher heat output and higher skin temperatures. This increase in heat output in turn causes the resistance to further increase, eventually leading to thermal run away (e.g., dielectric breakdown and heater failure). This problem of thermal runaway phenomenon results in process inefficiencies andpremature heater failure. The runaway can only be prevented by controlling the overall heat output, which limits heat injection into cooler spots of heat exchanger.

[0004] Commercially available process heaters are constant power which requires regulation and control to prevent thermal runaway (e.g., the heater operating at high temperatures above the dielectric withstand) and to avoid the consequent burnout and failure. Regulating a heater implies reducing the power on the full heated length or circuit length; however, this limits the heat delivery. For high power delivery, safe and reliable operations, and economic controls, a self-regulating heater is needed.

[0005] Further, a supplemental control mechanism is needed to reduce the power / temperature at the local high temperature location or hot spot to prevent heater burnout without requiring an overall reduction in power output or temperature.

[0006] As such, there is a need for self-regulated heating in process heating applications. Self-regulating heater works as the heating element would have a negative temperature coefficient of resistance, which causes the resistance to decrease as the overall temperature increases. As the electrical current in the core of the heater remains constant throughout the entire length of the heater cable, the decrease in local resistance would result in a reduction of power in that local section of the heater cable, thereby mitigating hotspots and the associated thermal runaway challenge described above.

[0007] US patent 10,119,366 describes a mineral insulated heater cable, with a temperature limited heater as the heating member. "Temperature limited heater" generally refers to a heater that regulates heat output (for example, reduces heat output) above a specified temperature without the use of external controls such as temperature controllers, power regulators, rectifiers, or other devices.

[0008] The cable comprises a conductive core circumferentially surrounded by a thin concentric conductive layer, wherein another concentric layer of a ferromagnetic conductor separates the thin concentric conductive layer from the conductive core. The thin concentric conductive layer in turn is surrounded by a relatively thick concentric layer of electrical insulator comprising mineral insulation, such as MgO, and an outer metal jacket. The conductive core andthe thin concentric conductive layer are made from a non-ferromagnetic material (e.g. copper or copper alloy). Below the Curie temperature and / or phase transformation temperature range of the ferromagnetic material, the magnetic properties of the ferromagnetic material confine the majority of the flow of electrical current to the thin concentric conductive layer. Thus, the thin concentric conductive layer provides the majority of the resistive heat output of the cable, below the Curie temperature and / or the phase transformation temperature range. The thin concentric conductive layer may have a cross-sectional area that is around 2 or 3 times less than the cross- sectional area of conductive core, so that the inner conductor provides a desired amount of heat output and a desired turndown ratio.

[0009] The temperature limited heater described above are operated by high frequency AC (alternating current) power or modulated DC (direct current) power, which is required to produce skin effect electricity flow in the ferromagnetic conductor. "Turndown ratio" for the temperature limited heater is the ratio of the highest resistance below the Curie temperature to the lowest resistance above the Curie temperature for a given AC or modulated DC current. However, this heater cable only works with AC or modulated DC, suffers from reactive power losses, and its overall behavior is frequency-dependent. In addition, a ferromagnetic core is required.BRIEF SUMMARY

[0010] According to an embodiment of the disclosed subject matter, a mineral insulated tube may include: an elongated core on a central axis of the tube. The elongated core may include: a resistive hollow tube having a bore surrounded by a cylindrical wall made of a metal material having a first resistivity of at least 0.1 pQ-m at 20°C. The tube may further include a semiconducting layer concentrically enveloping the elongated core, comprising a semi-conducting material in electrical contact with the elongated core, and the semi-conducting material may have a first electric bandgap and a second resistivity. The tube may also include an electrically insulating layer concentrically enveloping the semi-conducting layer including a mineral material that has a second electric bandgap and a metallic outer sheath concentrically enveloping the electrically insulating layer. The first electric bandgap of the semi-conducting material may be less than the second electric bandgap of the mineral material.

[0011] The disclosed subject matter allows for improved efficiency and reduced costs by enabling operation of a process at higher power output while avoiding hotspots and burnout of heating cables. Additional features, advantages, and embodiments of the disclosed subject matter may be set forth or apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary and the following detailed description are examples and are intended to provide further explanation without limiting the scope of the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are included to provide a further understanding of the disclosed subject matter, are incorporated in and constitute a part of this specification. The drawings also illustrate embodiments of the disclosed subject matter and together with the detailed description serve to explain the principles of embodiments of the disclosed subject matter. No attempt is made to show structural details in more detail than may be necessary for a fundamental understanding of the disclosed subject matter and various ways in which it may be practiced.

[0013] FIG. 1 shows cross-sectional view of a mineral insulated tube according to an implementation of the disclosed subject matter.

[0014] FIG. 2 shows a line graph temperature scale according to an implementation of the disclosed subject matter.

[0015] FIG. 3 shows an example side view according to an embodiment of the disclosed subject matter.DETAILED DESCRIPTION

[0016] In general, a problem or disadvantage of the currently available process heaters are that they are limited in heat output due to the formation of hotspots caused by the lack of uniformity of heat transfer from the skin of the heating element to the intended fluid or gas orcombination of the two. These hot spots result in damage to the material being heated and may also result in premature failure of the heating cable itself. This is a well-known phenomena in heating applications with commercially available heating systems known as fouling or carbonization. As such, there is a need to manage hot-spots and control heaters to operate at lower temperatures utilizing a safety factor. This need for control severely limits the heat output (e.g., input to the process) even with complicated control schemes.

[0017] The present invention solves this problem by providing a self-regulating mineral- insulated heating element with a core that has a negative temperature coefficient by virtue of parallel resistance arrangement of a primary and secondary conducting material (e.g., secondary being a semi-conducting material). This design enables the heat flux to locally decrease by itself thereby allowing the overall heating element to operate at higher power output. As the electrical current in the core of the heater remains constant throughout the entire length of the heater cable, the decrease in local resistance would result in a reduction of power in that local section of the heater cable thereby mitigating hotspots and avoiding the associated thermal runaway problem described above. The present invention is achieved using ceramic / mineral materials with a negative thermal coefficient of resistance (NTCR).

[0018] According to an embodiment, the present invention minimizes hotspots and avoids thermal runaway problems by optimizing several design parameters. In an embodiment, a mineral insulated tube may include (1) an elongated core on a central axis of the mineral insulated tube. The elongated core may include (a) a resistive hollow tube having a bore surrounded by a cylindrical wall made of a metal material having a first resistivity of at least 0.1 p -m at 20°C. In general, electrical resistivity is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current.

[0019] The mineral insulated tube may further include (2) a semi-conducting layer concentrically enveloping around the elongated core. The semi-conducting layer may include a semi-conducting material in electrical contact with a wall along a substantial length of the elongated core, and the semi-conducting material may have a first electric bandgap and a second resistivity. It is known that a semiconductor is a material with an intermediate-sized, non-zeroband gap that behaves as an insulator at T=20°C, but allows thermal excitation of electrons into its conduction band at temperatures that are below its melting point.

[0020] The mineral insulated tube may further include (3) an electrically insulating layer concentrically enveloping around the semi-conducting layer. The electrically insulating layer may include a mineral material that has a second electric bandgap.

[0021] The mineral insulated tube may further include (4) a metallic outer sheath concentrically enveloping around the electrically insulating layer.

[0022] The first electric bandgap of the semi-conducting material may be less than the second electric bandgap of the mineral material. For example, the first electric bandgap of the semi-conducting material may be 2eV and the second electric bandgap of the mineral material may be 5 eV.

[0023] According to an embodiment, at 20°C the second resistivity of the semi-conducting material may be higher than the first resistivity of the resistive hollow tube. For example, at 20°C the semi-conducting material may have a second resistivity of le8 p -m and the metal material of the resistive hollow tube may have a first resistivity of 1 p -m.

[0024] In an implementation, there may be multiple temperature factors to consider in order to maximize the operational efficiency of the mineral insulated tube according to the present invention. In an embodiment, the mineral insulated tube may have (1) a designed operating temperature for a specific application and (2) a predetermined elevated temperature. The specific application may have a variable temperature of operation, which may be measured at the frequency of 1 sec using a thermocouple during operation, and when the variable temperature of operation is above the predetermined elevated temperature, the second resistivity may be lower than ten times the first resistivity. According to an implementation, the predetermined elevated temperature may be in the range of from 1°C to 100 °C above the designed operating temperature.

[0025] For example, the mineral insulated tube according to the present invention may be utilized in a specific application such as a process for heating water. When used in this specific application for heating water, the mineral insulated tube may have a designed operatingtemperature of 120°C and a predetermined elevated temperature of 150°C. During the water heating process, the variable temperature of operation in some short segments along the heating element may reach 170°C (e.g., hotspot of 20°C above the predetermined elevated temperature of 150°C). According to the present invention, at these hotspot locations, the second resistivity (e.g., 3.8 pQ-m ) will be lower than ten times the first resistivity (e.g., 0.4 pQ-m).

[0026] In an embodiment, the semi-conducting material may comprise a ceramic material. Examples of a ceramic material may include SiC, MgO, Alumina, BaTiO3, Boron Carbide, Spinels, etc., and other known ceramic materials.

[0027] In another implementation, the first resistivity of the resistive hollow tube at 20°C may be within the range of 0.5 pQ-m to 4 pQ-m.

[0028] According to an embodiment, a method of heating a substance may include providing a mineral insulated tube as described herein and passing an electrical current through the elongated core in a direction along the central axis. A substance to be heated may be brought in contact directly or indirectly with the mineral insulated tube thereby transferring heat from the elongated core to the substance. For example, as used herein directly implies passing the substance to be heated through the resistive hollow tube bore and indirectly implies the substance to be heated may be brought in contact with the metallic outer sheath of the mineral insulated tube.

[0029] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the accompanying drawings, which are described in more detail below. The embodiments disclosed herein are not intended to be exhaustive or limit the invention to the precise form disclosed in the following detailed description. The invention includes any alterations and further modifications in the illustrated devices and described methods and further applications of the principles of the invention as set forth in the claims.

[0030] FIG. 1 shows cross-sectional view of a mineral insulated tube according to an implementation of the disclosed subject matter. As shown in FIG. 1, a mineral insulated tube 5 may include an elongated core 10 on a central axis A of the mineral insulated tube 5. Theelongated core 10 may include (a) a resistive hollow tube 12 having a bore 19 surrounded by a cylindrical wall made of a metal material having a first resistivity of at least 0.1 p -m at 20°C.

[0031] Also shown in FIG. 1, the mineral insulated tube 5 may further include a semiconducting layer 14 concentrically enveloping around the elongated core 10. The semiconducting layer may include a semi-conducting material in electrical contact with a wall along a substantial length of the elongated core 10, and the semi-conducting material may have a first electric bandgap and a second resistivity.

[0032] As shown in FIG. 1, the mineral insulated tube 5 may further include an electrically insulating layer 16 concentrically enveloping around the semi-conducting layer 14. The electrically insulating layer 16 may include a mineral material that has a second electric bandgap.

[0033] Further shown in FIG. 1, the mineral insulated tube 5 may further include a metallic outer sheath 18 concentrically enveloping around the electrically insulating layer 16.

[0034] FIG. 2 shows a line graph temperature scale according to an implementation of the disclosed subject matter. As shown in FIG. 2, the mineral insulated tube may have a designed operating temperature 21 for a specific application and a predetermined elevated temperature 22. The specific application may have a variable temperature of operation 23. When the variable temperature of operation 23 is above the predetermined elevated temperature 22, the second resistivity may be lower than ten times the first resistivity. According to an implementation, the predetermined elevated temperature 22 may be in the range of from 1°C to 100 °C above the designed operating temperature 21.

[0035] FIG. 3 shows an example side view according to an embodiment of the disclosed subject matter. In particular, FIG. 3 shows an example side view of the mineral insulating tube 5 according to the present invention.

[0036] As shown in FIG. 3, a method of heating a substance may include providing a mineral insulated tube 5 and passing an electrical current 25 from a power supply through the elongated core 10 in a direction along the central axis A. A substance 26 to be heated may be brought in contact directly or indirectly with the mineral insulated tube 5 thereby transferring heat from the elongated core to the substance. For example, as used herein directly implies passing thesubstance 26 to be heated through the resistive hollow tube bore 19 and indirectly implies the substance 26 to be heated may be brought in contact with the metallic outer sheath 18 of the mineral insulated tube 5.

[0037] EXAMPLES

[0038] The following Example is based on an embodiment of the disclosed subject matter. A mineral insulated tube was manufactured with Alloy 625 metal hollow tube and surrounded by a layer of 50:50 mixture of Cu and CuO semi-conducting ceramic material. The semiconducting material was further surrounded by a MgO mineral insulating layer encased with an outer Stainless Steel 304 grade sheath.

[0039] At T=20°C, the resistance of metal hollow tube was 6 orders of magnitude less than the resistance of the semi-conducting ceramic material. Applied to a process heating application, with designed operating temperature of T = 400°C and the predetermined elevated temperature of T = 425°C, the variable temperature of operation referred to the process heating application temperature. When the variable temperature of operation, at a specific location along the length of the mineral insulated tube, exceeded the predetermined elevated temperature, the resistance of the semi-conducting material was only 10 times the resistance of the metal hollow tube at that location. More specifically, when the variable temperature of operation, at a specific location (Hotspot; T = 445°C), along the length of the mineral insulated tube, exceeded the predetermined elevated temperature (T = 425°C) , the resistance of the semi-conducting material was only 10 times the resistance of the metal hollow tube at that location. This demonstrates a reduction of resistance of the mineral insulated tube at that location, thereby resulting in a reduction of the variable temperature of operation to a value below the predetermined elevated temperature at the that location. As such, the hotspot (T = 445°C ) is reduced to a temperature within the operating range (T = 400°C-425°C). This feature of the present invention minimizes any over-temperature related degradation, for example, fouling, excessive corrosion, heat transfer fluid decomposition, scaling, etc..

[0040] The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit embodiments of the disclosed subject matter to the precise formsdisclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to explain the principles of embodiments of the disclosed subject matter and their practical applications, to thereby enable others skilled in the art to utilize those embodiments as well as various embodiments with various modifications as may be suited to the particular use contemplated.

Claims

CLAIMS1. A mineral insulated tube, comprising:(1) an elongated core on a central axis of the mineral insulated tube, wherein the elongated core comprises:(a) a resistive hollow tube having a bore surrounded by a cylindrical wall made of a metal material having a first resistivity of at least 0.1 p -m at 20°C;(2) a semi-conducting layer concentrically enveloping around the elongated core, comprising a semi-conducting material in electrical contact with a wall along a substantial length of the elongated core, and wherein the semi-conducting material has a first electric bandgap and a second resistivity;(3) an electrically insulating layer concentrically enveloping around the semiconducting layer, comprising a mineral material that has a second electric bandgap;(4) a metallic outer sheath concentrically enveloping around the electrically insulating layer; wherein the first electric bandgap of the semi-conducting material is less than the second electric bandgap of the mineral material.

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

3. The mineral insulated tube of claim 2, wherein the mineral insulated tube has(1) a designed operating temperature for a specific application and(2) a predetermined elevated temperature, andwherein the specific application has a variable temperature of operation, and when the variable temperature of operation is above the predetermined elevated temperature, the second resistivity is lower than ten times the first resistivity.

4. The mineral insulated tube of claim 3, wherein the predetermined elevated temperature is in the range of from 1°C to 100 °C above the designed operating temperature.

5. The mineral insulated tube according to any one of the preceding claims, wherein the semi-conducting material comprises a ceramic material.

6. The mineral insulated tube of any one of the preceding claims, wherein the first resistivity at 20°C is within the range of 0.5 p -m to 4 pQ-m.

7. A method of heating a substance, comprising: providing a mineral insulated tube according to any one of the preceding claims; passing an electrical current through the elongated core in a direction along the central axis; bringing a substance to be heated in contact directly or indirectly with the mineral insulated tube thereby transferring heat from the elongated core to the substance.

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