Electric heaters and electric heating systems

Abstract

An electric heater (2) for a fluid, comprising: a plurality of electrically conductive tubes (4), the plurality of electrically conductive tubes (4) being arranged in a tube bundle (10) for resistive heating, the tube bundle (10) having an axial extension (12); and an electrical connector (6) being arranged between the tubes (4). The electric heater (2) is configured to allow a fluid to flow through the tube bundle (10) in direct contact with the inner and outer surfaces of the tubes (4). Each of the tubes (4) has an outer diameter in the range of 6 to 40 mm and has an outer diameter to wall thickness ratio in the range of 5 to 15 or in the range of 7-12. The ratio between the total cross-sectional surface area between the tubes (4) and the total cross-sectional area inside the tubes (4) is in the range of -10% to +30% or in the range of -5% to +25% of the ratio between the outer diameter and the inner diameter of one tube (4).

Description

Technical Field

[0001] The present invention relates to an electric heater for a fluid and an electric heating system for a fluid. Background Art

[0002] Electric heaters and electric heating systems are known which comprise electric heating tubes through which a fluid to be heated is conducted.

[0003] US Pat. No. 4,233,494 discloses a flow-through heater for fluids, particularly an air heater for regenerating a carbon dioxide adsorber in an air rectification system. Air is pumped from an upper chamber in a cylindrical housing through Ni-Cr steel heating tubes arranged in parallel groups to a lower chamber connected to the carbon dioxide adsorber. The tube groups are suspended at their upper ends from respective Al2O3 ceramic retainer plates, which are seated on flanges protruding into corresponding openings in a carrier plate, which is removably secured to the interior of the housing. The tubes in each group are connected in series to a voltage source.

[0004] Another alternative, as disclosed in US Pat. No. 927,173, is to direct the fluid to be heated not only through the tubes but also over the exterior of the tubes. US Pat. No. 927,173 discloses an electric heater having a resistive element in the form of nickel tubes. Within the housing, a plurality of thin-walled nickel tubes are mounted with insulation within transverse sheet metal walls. Air to be heated flows from the heater's air inlet through the nickel tubes and the exterior of the nickel tubes to the air outlet. Summary of the Invention

[0005] While known electric heaters utilizing electric heating tubes, such as those disclosed in US 4,233,494 and US 927,173, provide efficient heating of fluids, there remains a need for more effective and efficient fluid heaters.

[0006] It would be advantageous to achieve an efficient electric heater for a fluid and / or an effective and efficient electric heating system for a fluid. In particular, it would be desirable to provide a compact electric heater for a fluid. To better address one or more of these issues, an electric fluid heater for a fluid and / or an electric heating system for a fluid is provided.

[0007] According to one aspect of the present invention, this is achieved by an electric heater for a fluid, the electric heater comprising: a plurality of electrically conductive tubes arranged in a tube bundle for resistive heating, the tube bundle having an axial extension; an electrical connector arranged between tubes in the plurality of electrically conductive tubes; and an electrical conductor configured to connect the plurality of tubes to an external power source. The electric heater is configured to cause the fluid to flow through the tube bundle parallel to the axial extension and in direct contact with the inner and outer surfaces of the tubes. The outer diameter of each tube is in the range of 6-40 mm, and the ratio of the outer diameter to the wall thickness of each tube is in the range of 5-15, or in the range of 7-12. In a cross section of the tube bundle perpendicular to the axial extension, the ratio of the total cross-sectional surface area between the tubes to the total cross-sectional area of the tube interior is in the range of -10% to +30%, or in the range of -5% to +25%, of the ratio between the outer diameter and the inner diameter of one of the tubes.

[0008] Because the electric heater is configured to cause the fluid to be heated to flow through the tube bundle in direct contact with both the inner and outer surfaces of the tubes, and because the tubes have an outer diameter in the range of 6 to 40 mm and a ratio of outer diameter to wall thickness in the range of 5 to 15, or in the range of 7 to 12, the tubes are thin-walled. This thin-walled tube type, when electrically heated, provides for uniform temperature distribution between the radially inner and radially outer portions of the tube wall, while also providing sufficient mechanical stability for this fluid heater design. Furthermore, because the ratio of the total cross-sectional surface area between the tubes to the total cross-sectional area of the tube interiors is in the range of -10% to +30% or -5% to +25% of the ratio between the outer diameter and inner diameter of one of the tubes, a substantially uniform distribution of fluid (mass) flow is provided through the interiors and along the exteriors of the tubes in the tube bundle. Consequently, the available fluid-contacting heating surface of the tubes is efficiently utilized to heat the fluid flowing through the electric heater. The electric heater provides a high fluid heating capacity per unit volume of heater, i.e., the electric heater is compact.

[0009] More specifically, the electric heater of the present invention ensures that the flow area inside the tubes of the tube bundle is substantially equal to the flow area between the tubes. This ensures that heat is transferred substantially equally to the fluid passing through the tubes and the fluid passing between the tubes. Consequently, when fluid flows through the tube bundle of the electric heater, both the fluid flow inside the tubes and the fluid flow outside the tubes experience a uniform temperature increase. Combined with the defined range of tube diameters and the ratio of outer diameter to wall thickness, significant temperature differences between the fluid flowing inside the tubes and the fluid flowing outside the tubes toward the outlet end of the tube bundle are avoided. This means that stresses in the tube walls caused by temperature differences between the outside and inside of the tubes, namely, thermal stresses caused by radial temperature differences, are avoided.

[0010] Furthermore, the pressure drop of the fluid flow outside the heating tube remains substantially equal to the pressure drop of the fluid flow inside. The pressure drop is primarily determined by the friction between the fluid and the tube surface. Friction increases with decreasing density due to the increase in temperature along the axial direction, which causes the fluid velocity to accelerate with a quadratic dependence. By dividing the mass flow rate by adjusting the tube spacing in proportion to the amount of heat transferred (primarily proportional to the heated surface area of the heating tube), an equal temperature increase of the fluid inside and outside the tube is achieved, and this can be achieved by an arrangement in the aforementioned ratio.

[0011] Typically, electric heaters can be designed for high mass flow rates and high temperatures with power ratings in the megawatt range. The electric heaters can be used as such, or they can form part of a fluid flow heating system configured to heat, for example, air, hydrogen, hydrocarbons, and other gases or liquids to high temperatures, such as 600°C or even higher, and configured for large-scale heating processes in the megawatt range. For these purposes, in particular, the electric heaters include directly electrically heated thin-walled tubes, particularly arranged in bundles with respect to the cross-sectional area formed on the inside and outside of the tubes and with respect to the inner and outer diameters of the tubes, to form a compact, effective, and efficient heating system. Alternatively, the directly electrically heated tubes can be referred to as active tubes.

[0012] An electric heater for a fluid is alternatively referred to herein as an electric heater or simply a heater. An electric heater can be used to heat a fluid, such as a gas in an industrial process. The heater can, for example, be used in an industrial process. The fluid heated in the heater can be an energy carrier in the industrial process, and / or the fluid can be used as a heat source in the industrial process, and / or the fluid can be a process fluid used in the industrial process.

[0013] The electric heater provides directly electrically heated tubes, and thereby directly energized tubes, without any additional heating elements or insulating material. In other words, the interior of the thin tubes does not have any heat generating components extending through the thin tubes. Consequently, there are no active internal elements or insulating layers arranged in the tubes, such as wire heating elements or mineral insulated heating elements. Thus, a basis is provided for an uncomplicated design of the electric heater, in particular a tube bundle, and makes it suitable for large power ratings. The electrical energy supplied to the heater during use of the heater is efficiently converted into heat, which is transferred to the fluid to be heated in the heater without any thermal barriers which would reduce the effectiveness of the heater (i.e. the ability to provide a high outlet temperature) and / or would reduce its efficiency (i.e. would provide higher heat losses, lower power density and increased pressure drop).

[0014] The conductive tube may have a resistivity between 0.05 Ω·m and 5 μΩ·m. Such resistivity may be provided by the materials discussed below.

[0015] Thus, the relevant materials and their resistivities are significantly different from those of ceramic catalyst supports, which have very different resistivities and would not be suitable for building large heaters or providing the required surface loading within the typical available line voltages described later.

[0016] When an electric current flows through the conductive tubes, the tubes heat up. The tubes can be made of a material formed from aluminum oxide. The aluminum oxide forms a protective layer, allowing the tubes to withstand high temperatures and other harsh environmental or fluid conditions, and thus allowing the heating of various gases to high temperatures. These tubes can also be made of alternative materials, for example, when the fluid to be heated is a non-oxidizing fluid, such as hydrogen or nitrogen.

[0017] The electric heater has a simple construction, requiring only a few distinct components. Although the tube bundle of the electric heater may include hundreds or even thousands of individual tubes, the tubes may be of a single type or a limited number of different types. This results in a heater that is particularly reliable in operation during use in industrial processes.

[0018] The electric heater can be arranged at any desired and therefore suitable location where the fluid is to be heated, suitably at a location where the fluid to be heated can be guided through the tube bundle, for example in a conduit, pipe, duct or housing. The electric heater can be arranged in the housing, optionally together with one or more further electric heaters, to form an electric heating system as described below.

[0019] Herein, the plurality of conductive tubes may alternatively be referred to as a plurality of tubes or simply as tubes.

[0020] In the tube bundle, the tubes of the plurality of electrically conductive tubes may be arranged parallel to each other or substantially parallel to each other and at a distance from each other.

[0021] Electrical connectors are disposed between tubes of the plurality of electrically conductive tubes and connected to the tubes so as to allow current from an external power source to flow through the tubes of the tube bundle in series, parallel, or a combination of series and parallel.

[0022] The external power supply may comprise mains power, or may be connected to mains power via a transformer for adjusting the voltage of the current supplied to the electric heater.

[0023] The outside diameter to wall thickness ratio is also referred to as the standard dimension ratio SDR. Thus, the SDR is defined as the ratio of the outside diameter of a tube to the wall thickness of the tube. Tubes having an SDR in the range of 5-15 or in the range of 7-12 as defined herein are generally considered to be thin-walled tubes. In the present context, tubes in the larger SDR range of 5 to 15 provide a sufficiently large heat transfer surface, i.e. a sufficiently large surface on the inside and outside of the tube to provide effective heat transfer to the fluid to be heated, and to provide the above-mentioned uniform temperature distribution between the radially inner and outer parts of the tube wall. Tubes in the smaller SDR range of 7-12 provide not only a sufficiently large heat transfer surface and a uniform temperature distribution between the radially inner and outer parts of the tube wall, but also provide sufficient strength and electrical resistance for tubes made of the relevant materials, such as the materials exemplified below, i.e. the materials in each tube are utilized in an optimal manner.

[0024] The aforementioned ratio between the total cross-sectional surface area between the tubes and the total cross-sectional area inside the tubes is within a specific range of the ratio between the outer diameter and the inner diameter of one of the tubes, which allows for a substantially uniform distribution of the flow rate (volume or mass per unit time) of the fluid in the tube bundle along the heat transfer surface provided by the interior of the tubes and along the heat transfer surface provided by the exterior of the tubes. Thus, the available fluid-contacting heat transfer surface on both the interior and exterior of the tubes is effectively utilized to heat the fluid flowing through the electric heater, while stresses in the tube walls caused by radial temperature differences are avoided, or at least kept to a minimum. Furthermore, the temperature rise of the fluid flowing inside and outside the tubes remains equal or substantially equal. Thus, in steady-state operation of the heater, the ratio between the fluid flowing inside the tubes and the fluid flowing outside the tubes remains constant at various flow rates through the heater because the pressure drops across both the interior and exterior of the tubes are equal or substantially equal.

[0025] Regarding the aforementioned ratio of the total cross-sectional surface area between the tubes to the total cross-sectional area within the tubes, this ratio, within a relatively high range of -10% to +30% of the ratio between the outer diameter and the inner diameter of one of the tubes, provides sufficiently good heat transfer characteristics through the heater. Specifically, within this range, the advantages discussed above of uniform distribution of heat transfer between the interior and exterior of the tubes, as well as the associated low thermal stresses and uniform pressure drop at variable flow rates, are exhibited. In practice, this range primarily relates to the manufacturing tolerances applied to the heater and potentially necessary components, such as electrical connectors, spacing elements, and support elements between or within the tubes. This range is characterized by a very uniform distribution of heat transfer between the interior and exterior of the tubes and the absence, or at least substantial absence, of thermal stresses in the radial direction of the tubes.

[0026] With respect to the aforementioned ratio of the total cross-sectional surface area between the tubes to the total cross-sectional area inside the tubes, this ratio being in the relatively low range of -5% to +25% of the ratio between the outer diameter and the inner diameter of one of the tubes, very good heat transfer characteristics are provided by the heater. This can be achieved, for example, when electrical connectors, spacer elements, and support elements, for example, between the tubes or inside the tubes, are optimized for low pressure drop.

[0027] According to an embodiment, each tube of the plurality of conductive tubes may be arranged at an angle within a range of 0 to 15 degrees or 0 to 5 degrees to an adjacent tube. In this manner, the tubes may be arranged parallel or substantially parallel to each other in the tube bundle, or the tubes may be arranged obliquely to each other in the tube bundle.

[0028] For example, the tube bundle may have a certain taper due to the angles between the individual tubes.

[0029] Alternatively, some of the individual tubes may be arranged so that the angles therebetween point in a first direction, and some of the individual tubes may be arranged so that the angles therebetween point in an opposite second direction, such that along the axial extension of the tube bundle the tube bundle has a substantially similar cross-section perpendicular to the axial extension.

[0030] The angles between the individual tubes in the tube bundle can be used to mechanically stabilize the tubes within the tube bundle and the tube bundle within the shell, reactor, or pipeline. For example, such stabilization can be utilized in embodiments where the tube bundle is arranged such that the axial extension has a vertical component (e.g., the axial extension extends vertically) during use of the heater.

[0031] In embodiments where each tube of the plurality of electrically conductive tubes is arranged at an angle >0 degrees to an adjacent tube, the above-mentioned ratio between the total cross-sectional surface area between the tubes and the total cross-sectional area of the interior of the tubes (which ratio is within a specific range of the ratio between the outer diameter and the inner diameter of one of the tubes) can be achieved according to at least one of the following: an average value over the entire extension of the tube bundle, an average value at a middle portion of the tube bundle viewed along the extension of the tube bundle, and / or an average value over the entire tube bundle.

[0032] According to embodiments, the electric heater may include spacer elements arranged between the tubes to support the tubes in the tube bundle. The spacer elements may be non-conductive. The spacer elements may be arranged with a gap along the axial extension that is less than 40% of the total axial length of the tube bundle. This stabilizes the tubes within the tube bundle and the tube bundle within the housing or pipe.

[0033] The spacer elements are arranged along the axial extension at intervals less than 40% of the total length, meaning that at least two spacer elements are arranged between two adjacent tubes, viewed along the axial extension. For example, two or three spacer elements may be arranged along the axial extension of a single tube. Depending on the total length of the tube bundle, the number of spacer elements along a single tube may be greater than three.

[0034] The spacer elements may abut against the tubes to support them.Each spacer element may abut against all tubes surrounding the associated spacer element.

[0035] The spacer element may have any suitable shape, such as tubular, cylindrical, spherical, box-shaped, and the like.

[0036] According to an embodiment, the tubes may comprise structural elements configured for positive locking engagement with the spacer elements. In this way, it may be ensured that the spacer elements remain positioned within the tube bundle when the tubes are subjected to thermal movements.

[0037] The term form-locking engagement relates to a engagement or connection which utilizes mating elements to prevent, in particular, an axial displacement of the spacer element relative to the tube or tubes.

[0038] According to an embodiment, the electric heater may comprise a support element arranged inside the tube for mechanically supporting the tube.

[0039] Because the tubes are thin, i.e., have a high SDR, and they are heated to a temperature at which they lose at least some of their inherent stability, the support element can ensure that the thin tubes maintain sufficient dimensional stability so that they do not collapse at the softening temperature of the relevant material. The support element can be made of the same or similar material as the tubes to improve voltage distribution, for example, by equalizing the electrical potential of each tube at the portion where the support element is disposed. Alternatively, the support element can be made of a material with a higher softening temperature, such as a ceramic material.

[0040] Depending on the embodiment, the tubes may be composed of a conductive material for active resistive heating or more than one conductive material for active resistive heating, wherein the conductive material may be the same or different in all tubes and may be selected from the group consisting of an iron-chromium-aluminum alloy (FeCrAl alloy) (i.e., an alloy that will form an aluminum oxide layer on the exterior of the product), a nickel-based alloy, a tungsten-based alloy, or a molybdenum-based alloy. In this way, the heater can operate at high temperatures and, depending on the flow rate of the fluid, high fluid temperatures can be achieved. High fluid temperatures are useful in many industrial processes.

[0041] According to an embodiment, the plurality of electrically conductive tubes may be configured to be electrically heated to a temperature up to 1300° C., such as up to a temperature in the range of 600-1300° C. In this way, the fluid to be heated may be heated to a high temperature usable in industrial processes.

[0042] Such a temperature or temperature range may be applied to a heater in which the conductive material is selected from any of the alloys mentioned above.

[0043] According to some embodiments, such a temperature or temperature range may be applied to a heater in which the conductive material is a FeCrAl alloy or a nickel-based alloy.

[0044] According to an embodiment, the plurality of electrically conductive tubes may be configured to be electrically heated to a temperature of up to 2050°C, such as up to a temperature in the range of 800-2050°C.

[0045] According to some embodiments, such a temperature or temperature range may be applied in heaters where the conductive material is a tungsten-based alloy or a molybdenum-based alloy.

[0046] According to another aspect, an electric heating system for a fluid is provided, comprising a housing and at least one electric heater according to any of the aspects and / or embodiments discussed herein. The at least one electric heater is disposed in the housing. A flow path for the fluid to be heated extends through the tube bundle and along the axial extension.

[0047] In this way, an electric heating system to be installed in an industrial plant can be provided. The electric heating system provides the above-mentioned advantages of electric heaters.

[0048] The housing not only protects the electric heater but can also guide the fluid to flow toward and / or through the tube bundle of the electric heater. Additionally, the housing can be designed as a pressure vessel.

[0049] Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Various aspects and / or embodiments of the present invention, including specific features and advantages thereof, will be readily understood from the example embodiments discussed in the following detailed description and accompanying drawings, in which:

[0051] Figure 1a shows an isometric view of an electric heater for a fluid according to an embodiment,

[0052] Figure 1b Shown Figure 1b Isometric view of the end of an electric heater.

[0053] Figure 2 Schematically showing a cross-sectional view of three tubes of an electric heater,

[0054] Figure 3 schematically shows a side view of the end of an electric heater,

[0055] Figure 4 Two tubes of an electric heater according to an embodiment are shown,

[0056] Figure 5 An embodiment of an electric heating system for a fluid is schematically shown, and

[0057] Figure 6 The tubes of a heater according to an embodiment are schematically shown. DETAILED DESCRIPTION

[0058] Aspects and / or embodiments of the present invention will now be described more fully. Like reference numerals refer to like elements throughout. For the sake of brevity and / or clarity, well-known functions or configurations may not necessarily be described in detail.

[0059] Figure 1a Shown is an isometric view of an electric heater 2 for a fluid according to an embodiment. Figure 1b An isometric view of the end of the electric heater 2 is shown.

[0060] The electric heater 2 may form part of an electric heating system, as described below with reference to Figure 5 Electric heater system discussed.

[0061] The electric heater 2 includes a plurality of conductive tubes 4 , an electrical connector 6 and an electrical conductor 8 .

[0062] The tubes 4 are arranged in a tube bundle 10 for resistance heating. The tube bundle 10 has an axial extension 12. Electrical connectors 6 are arranged between the tubes 4 and are configured to conduct electrical current between the tubes 4. Two electrical conductors 8 are provided, configured to connect the tubes 4 to an external power source 13. Each electrical conductor 8 is connected to a single tube 4 among the tubes 4. However, although not shown in the figure, more than two electrical conductors may be present.

[0063] In use of the electric heater 2 , electric current flows to / from the tubes 4 via the electrical conductors 8 and between the tubes 4 via the electrical connectors 6 .

[0064] Thus, the electric heater 2 provides direct electrical heating of the tube 4 .

[0065] The axial extension 12 of the tube bundle 10 may be a straight line drawn through the midpoint of the cross-sectional area of the tube bundle at each opposing end portion 9 , 11 of the tube bundle 10 .

[0066] The electric heater 2 is configured to cause the fluid to be heated to flow through the tube bundle 10 parallel to the axial extension 12 and in direct contact with the inner and outer surfaces of the tubes 4 .

[0067] The tube bundle 10 has a hot end portion 9 (ie, at the fluid outlet end of the tube bundle 10 ) and a cold end portion 11 (ie, at the fluid inlet end of the tube bundle 10 ). Figure 1b The cold end portion 11 is shown in FIG. The hot end portion 9 is Figure 3 and will be discussed further below.

[0068] The terms hot and cold are relative terms, meaning that during use of the electric heater 2, the cold end portion 11 is typically colder than the hot end portion 9 because the fluid is heated along the axial extension 12 of the heater from the fluid inlet to the fluid outlet. The temperature at the cold end portion 11 may depend on the temperature of the fluid entering the heater 2. Therefore, during use of the heater 2, the cold end portion 11 does not necessarily have a low temperature.

[0069] Suitably, the electrical conductor 8 may be arranged at the cold end portion 11 of the tube bundle 10 .

[0070] The tubes 4 are sized and positioned relative to each other so as to achieve a high heat flux per volume of the heater 2 to the fluid. Figure 2 .

[0071] The electric heater 2 comprises spacer elements 14 which are arranged between the tubes 4 to support the tubes 4 in the tube bundle 10. The spacer elements 14 are non-conductive.

[0072] The spacer elements 14 can abut against the tubes 4 to support the tubes 4 relative to each other in the tube bundle 10. Each spacer element 14 can abut against all adjacently arranged tubes 4. That is, adjacent tubes 4 in the tube bundle 10 abut against the same spacer element 14.

[0073] In the embodiment shown, each spacing element 14 abuts three adjacent tubes 4 within the tube bundle 10, with the exception of the outermost spacing elements 14, which each abut two tubes 14. Depending on how the tubes are arranged within the tube bundle, a spacing element may abut more than three tubes.

[0074] The spacer elements 14 thus ensure that the tubes 4 are securely positioned relative to one another within the tube bundle 10 while ensuring electrical insulation between the tubes 4 , ie preventing electrical short circuits between the tubes 4 .

[0075] At least two spacer elements 14 are arranged aligned in respective gaps along the axial extension 12 between two adjacently arranged tubes 4. Such aligned spacer elements 14 are arranged along the axial extension 12 with a gap i.

[0076] According to some embodiments, the spacer elements 14 may be arranged along the axial extension 12 with a gap i that is less than 40% of the total length L of the tube bundle 10 along the axial extension 12 .

[0077] At least two aligned spacer elements 14 are arranged between two adjacent tubes 4. The number of aligned spacer elements 14 arranged between two tubes 4 may depend on the total length L of the tube bundle 10. The longer the tubes 4 and the tube bundle 10 are, the more aligned spacer elements 14 may be provided between adjacent tubes 4.

[0078] Furthermore, since the tubes 10 are less stable at the hot end portion 9 than at the cold end portion 11 , the spacer elements 14 may be arranged closer to each other at the hot end portion 9 of the tube bundle 10 than at the cold end portion 11 of the tube bundle 10 .

[0079] In the embodiment shown, the spacer element 14 comprises a tubular element arranged along the tube 4. In this way, during use of the electric heater 2, fluid flowing outside the tube 4 can flow through the tubular element of the spacer element 14. Therefore, the spacer element 14 provides a lower flow resistance than a solid spacer element.

[0080] The spacer element 14 is shown as a tubular element having a circular cross section. Alternatively, tubular elements having other cross sections may be used, such as oval, square, etc.

[0081] Each of the tubular spacer elements 14 may have an extension along the axial extension 12 in the range of 1%-30% of the total length L of the tube bundle 10 .

[0082] According to an alternative embodiment, a solid spacer element can be used. In such an embodiment, the fluid flowing outside the tube will flow around the solid spacer element. Such an element preferably has a low pressure drop, such as that achieved by a spherical or torpedo shape.

[0083] The tubes 4 comprise structural elements 16 which are configured for positive locking engagement with the spacer elements 14. The positive locking engagement ensures that the spacer elements 14 remain positioned relative to the tubes 4 within the tube bundle 10.

[0084] exist Figure 1b In the embodiment of , the structural elements which are in positive locking engagement with the spacing element 14 are not visible. Figure 1b The structural elements 16 visible in FIG. 1 may be used in other arrangements of tubes 4 , for example when more tubes 4 are to be added to the tube bundle 10 .

[0085] This form-locking engagement can already be utilized when assembling the tube bundle 10 during production of the heater 2. Thus, during production of the heater 2, the spacer elements 14 and the tubes 4 can be fixedly positioned relative to each other.

[0086] Furthermore, during use of the heater 2, it is ensured that the spacer element 14 remains positioned within the tube bundle 10. In particular, the form-locking engagement ensures the relative position between the spacer element 14 and the tubes 4 when the tubes 4 are subjected to thermal movements such as expansion and contraction.

[0087] In the embodiment shown, the structural elements 16 are projections that protrude radially from the tubes 4. The spacer elements 14 are provided with appropriately sized holes or recesses into which the projections extend. Since the tubes 4 abut against the spacer elements 14 in the tube bundle 10 and the projections extend into the holes, a positive locking engagement between the spacer elements 14 and the tubes 4 is ensured.

[0088] The structural element 16 may take any other suitable form and may include one or more of a pin, a plate, a groove, a recess, and / or a disc segment configured to retain the spacer element 14 in its axial position within the tube bundle. The spacer element 14 may include suitable elements and / or features and / or be shaped in a suitable manner for positive locking engagement with the one or more of the pin, plate, groove, recess, and / or disc segment.

[0089] Electrical connectors 6 arranged between the tubes 4 are arranged at two opposite axial ends 9, 11 of the tube bundle 10. The tubes 4 can be connected in series via the electrical connectors 6.

[0090] The electrical connector 6 may be welded to the tube 4. The electrical connector 6 may be made of the same material as the tube 4, or may be made of a different material than the tube 4.

[0091] The electrical connector 6 may be arranged at the respective outermost end of the axial end portions 9 , 11 as seen along the axial extension 12 .

[0092] The electrical connectors 6 may take any suitable form, provided they provide an electrical connection between the tubes 4 that is dimensioned for the associated current. The electrical connectors 6 may comprise one or more of a plate, a pin, a clamp, or a rod. The electrical connectors 6 may be welded, brazed, screwed, or clamped to the associated tubes 4.

[0093] Another option may be to refer to Figure 6 The electrical connector is implemented as discussed.

[0094] The plurality of electrically conductive tubes 4 may be an even number of tubes 4 . An electrical conductor 8 configured to connect the plurality of tubes 4 to an external power source 13 may be arranged at one axial end 11 of the tube bundle 10 .

[0095] Therefore, when the tubes 4 are connected in series, the currents flowing through the tubes 4 during use of the heater 2 have a vector sum equal to zero.

[0096] The external power supply 13 may be an AC or DC power supply. The preferred design may be connected directly to a line voltage such as 115-990V, but in some heaters a range of 50 to 20.000V may also be suitable.

[0097] The tubes 4 can be composed of a non-ceramic material. The tubes 4 can be composed of a conductive material for active resistive heating or more than one conductive material for active resistive heating. The conductive material can be the same or different in all tubes 4 and can be selected from the group consisting of an iron-chromium-aluminum alloy (FeCrAl alloy), a nickel-based alloy, a tungsten-based alloy, or a molybdenum-based alloy.

[0098] Thus, the heater 2 can be operated at a high temperature.See further example embodiments discussed below.

[0099] Such a tube 4 may be manufactured by conventional methods, such as rolling, or by additive manufacturing techniques using powders.

[0100] According to some embodiments, the pipe 4 can be designed and arranged to provide a heating rate of 20-200 kW / m 2 or 30-120kW / m 2 Specific energy transfer within a range of . Thus, a compact heater structure can be provided. See further example embodiments discussed below.

[0101] The heat transfer surfaces used in the heater 2 include the inner and outer surfaces of the tubes 4 .

[0102] During use of the heater 2 , the plurality of electrically conductive tubes 4 may be configured to be electrically heated to a temperature of up to 1300° C., such as up to a temperature in the range of 600-1300° C.

[0103] For molybdenum-based alloys or tungsten-based alloys, higher tube temperatures of more than 2000°C can be achieved.

[0104] Figure 6 The four adjacent conductive tubes 4 of the heater according to the embodiment are schematically shown. The heater may be the one described above with reference to Figure 1a and Figure 1b Heater 2 discussed.

[0105] Likewise, the tubes 4 are arranged together with further tubes to form a tube bundle.

[0106] In these embodiments, the electrical connectors 6 between the tubes 4 4 are provided by utilizing one or more tube blanks having a length that is significantly longer than the axial extension of the tube bundle in which the tubes 4 are arranged.

[0107] During the manufacture of the tubes 4, the tube blank is bent to a length corresponding to the desired tube length in the tube bundle. The resulting bend 5 of the tube blank is provided with a through hole 7, for example by drilling, to allow fluid to flow through the interior of the tube 4 during use of the heater. The bend 5 will serve as an electrical connector 6 between the tubes 4 in the tube bundle.

[0108] This may provide an efficient way of connecting the tubes 4 using the bends 5 of the tubes as connectors 6. These connectors 6 may have the same outer and / or inner diameter as the tubes 4.

[0109] As described above, the connector 6 can be manufactured by bending (eg, hot bending) a tube blank and machining a hole in the bent portion 5 .

[0110] An alternative way of providing electrical connectors between the tubes would be to pre-form, by casting or additive manufacturing, a bent tube section corresponding to the bend 5 described above and to weld the pre-formed bent tube section to the ends of the respective tubes. The four tubes provided in this way would look similar to Figure 6 Four tubes 4 are shown.

[0111] Figure 2 Schematically shows Figure 1a and Figure 1b Shown is a cross-sectional view of three adjacent tubes 4 of a heater 2. The three tubes 4 form part of a bundle 10 of tubes 4.

[0112] The tubes 4 are sized and positioned relative to each other so as to provide a high heat transfer per volume of fluid to the heater 2, ie to achieve a compact heater 2 and a high outlet temperature.

[0113] First, each tube 4 has an outer diameter Do in the range of 6-40 mm and a wall thickness Wt. The outer diameter to wall thickness ratio Do / Wt=SDR, i.e., standard dimension ratio, is in the range of 5-15 or in the range of 7-12. Therefore, the tube 4 is a thin-walled tube that provides a large heat transfer surface and a uniform temperature distribution between the radial inner and outer tube surfaces.

[0114] Secondly, in the cross section of the tube bundle 10 perpendicular to the axial extension 12, i.e. Figure 2 As shown, the ratio between the total cross-sectional surface area A between the tubes 4 and the total cross-sectional area B inside the tubes 4 is in the range of -10% to +30% or -5% to +25% of the ratio between the outer diameter Do and the inner diameter Di of one of the tubes 4.

[0115] The resulting ratio ' (including the corresponding range) can be expressed by the following equations (1) and (2), where x represents a factor describing the previously mentioned range and ratio represents the nominal ratio:

[0116]

[0117] One way to arrange the tubes 4 relative to each other in the tube bundle 10 is in an equilateral triangle arrangement. The center-to-center distance between the tubes 4 in the equilateral triangle arrangement can be expressed as the spacing in equation (3):

[0118]

[0119] The spacing according to equation (3) provides the opportunity to manufacture the heater bundle in a rational and economical manner while further compensating for tolerances introduced by the manufacturing process. Furthermore, the ratio-dependent spacing provides room for optimizing heat transfer and pressure drop with respect to the connector 6 and spacer 14, with respect to the range of variation. This results in a customized system specific to the customer's requirements while retaining its unique design.

[0120] Herein, the centre-to-centre distances of the tubes 4 in an equilateral triangle arrangement are given for a number of example heaters.

[0121] Each tube 4 of the plurality of tubes 4 may have the same outer diameter Do and the same SDR. Alternatively, the tubes 4 may have substantially the same outer diameter Do and substantially the same SDR, such as Do and SDR each varying within a range of ±5%.

[0122] An example of a heater that implements the above expression A / B = Do / Di has a tube outer diameter Do of 17.15 mm and a tube inner diameter Di of 12.53 mm, and an SDR of 7.424, which results in Do / Di = 1.369. With a center-to-center spacing of 21.48 mm between the tubes, the cross-sectional area A of the tube exterior is 84.37 mm. 2 , the cross-sectional area B inside the tube is 61.62mm 2 Therefore, A / B = 1.369 = Do / Di.

[0123] exist Figure 2 , the cross-sectional area A outside the tubes 4 is indicated by the dot-dash lines between the tubes 4. The cross-sectional area B inside the tubes 4 is indicated by vertical shading. The relationship between the cross-sectional areas A / B outside and inside the tubes 4 is schematically illustrated with reference to only three tubes 4 in the tube bundle 10. In the case of more than three tubes 4 in the tube bundle 10, this relationship can be illustrated in a similar manner.

[0124] The relationship between the cross-sectional area A between the tubes 4 (i.e., the cross-sectional area of the exterior of the tubes 4 within the tube bundle 10) and the cross-sectional area B of the interior of the tubes 4 (defined relative to the outer and inner diameters of the tubes 4) is such that the flow rate of the fluid (volume or mass per unit time) is substantially evenly distributed along the heat transfer surface provided by the interior of the tubes 4 in the tube bundle 10 and along the heat transfer surface provided by the exterior of the tubes 4. Thus, the available fluid-contacting heat transfer surface on both the interior and exterior of the tubes 4 is efficiently used to heat the fluid flowing through the electric heater while avoiding stresses in the tube walls of the tubes 4 caused by radial temperature differences.

[0125] Figure 3 Schematically shows Figure 1a 、 Figure 1b and Figure 2 A side view of the hot end portion 9 of the heater 2 is shown in FIG.

[0126] Likewise, the tubes 4 , the electrical connectors 6 between the tubes 4 , and the support elements 14 between the tubes 4 are shown.

[0127] The electric heater includes a support element 18 arranged within the tube 4. The support element 18 mechanically supports the tube 4. In particular, the tube 4 benefits from this mechanical support within the tube 4 when the electric heater is operated at such high temperatures that the tube 4 loses at least some of its inherent stability.

[0128] The support elements 18 may comprise the same material as the tubes 4. In this case, the support elements 18 of each tube 4 may be formed as one piece with the associated tube 4. Alternatively, the support elements 18 may be positioned in the tubes 4 after they have been formed.

[0129] Another alternative may be that the support element 18 comprises a non-conductive, heat-resistant material, such as a ceramic material. Such a support element 18 may be positioned inside the tube 4, for example when the electric heater is assembled.

[0130] The support element 18 is formed so that, during use of the heater, the fluid flow through the interior of the tube 4 is affected as little as possible. Suitably, the member 20 of the support element 18 extends axially along the axial extension of the associated tube 4. In the circumferential direction, such a member 20 has only a smaller extension.

[0131] In the embodiment shown, the stent element 18 is provided with three radially extending members 20. However, the stent element may have fewer or more than three radially extending members or any other shape as long as the stent element prevents collapse of the tube 4 and provides low additional pressure drop.

[0132] The use of support elements 18 may be a balancing act between the pressure drop they add to the fluid flow within tube 4 and the support they provide to tube 4 .

[0133] According to an embodiment, the support element 18 can be arranged within the last 40% of the total length L of the tube 4, as seen in the flow direction of the fluid to be heated. In other words, the support element 18 can be arranged at or near the hot end portion 9 of the tube 4, and accordingly, this hot end portion 9 can be supported when the tube 4 may lose some of its inherent stability during use of the heater.

[0134] The total length L of the tube 4 is Figure 1a and may correspond substantially to the total length L of the tube bundle 10. Arranging the support elements 18 within the last 40% of the total length L of the tubes 4 encompasses arranging the support elements 18 within the last 30% or the last 20% or even less.

[0135] The length of each support element 18 may be 40% or less of the total length L of the associated tube 4. Alternatively, each support element 18 may be shorter than the total length L of the associated tube 4, for example, less than 10% of the total length or less than 5% of the total length. One or more such short support elements 18 may be arranged within the last 40% of the total length of the associated tube 4.

[0136] Figure 4 Two tubes 4 of an electric heater according to an embodiment are shown.

[0137] This heater is similar in many respects to Figures 1a to 3 Therefore, the following will mainly discuss the differences.

[0138] Likewise, the heater comprises a plurality of electrically conductive tubes 4 arranged in a tube bundle 10, electrical connectors 6 between the tubes 4 and support elements 14 between the tubes 4. The tube bundle 10 has an axial extension 12.

[0139] According to an embodiment, each tube 4 of the plurality of conductive tubes 4 may be arranged to form an angle α with an adjacent tube 4 within a range of 0-15 degrees, 0-10 degrees, or 0-5 degrees.

[0140] exist Figures 1a to 3 In the above embodiment, the tubes 4 in the plurality of conductive tubes 4 are arranged parallel to each other and parallel to the axial extension of the tube bundle 10. That is, in Figures 1a to 3 In the embodiment of FIG. 5 , the angle α is 0 degrees.

[0141] exist Figure 4 In an embodiment, the angle α between adjacent tubes 4 is greater than 0 degrees, for example in the range of 0.5-15 degrees. In other words, at least some of the tubes 4 are arranged in a bundle 10 of these tubes 4 to be inclined relative to each other.

[0142] For example, when the heater is arranged so that the axial extension 12 of the tube bundle 10 extends vertically or is arranged to have a vertical component, the individual tubes 4 are stabilized relative to each other within the tube bundle because the tubes 4 are arranged at an angle α>0 degrees relative to each other (such as in the range of 0.5-15 degrees).

[0143] An angle α>0 degrees between the tubes 4 can be achieved by arranging spacer elements 14 of different sizes at opposite ends of the tube bundle 10 .

[0144] According to some embodiments, an angle α between the tubes 4 > 0 degrees provides a conical or spherical shape for the tube bundle, i.e., the cross-section of the tube bundle at one end portion is larger than the cross-section at the other end portion. With the tube diameter being the same along the length of the tube bundle, the conical shape requires that the cross-sectional area between the tubes 4 decreases towards one end of the tube bundle, suitably towards the outlet end of the tube bundle during use of the heater. This decrease in cross-sectional area means that the fluid velocity will increase towards the outlet end of the tube bundle. The average velocity change at a constant angle from the cold end portion 11 (inlet) to the hot end portion 9 (outlet) remains constant, so that the pressure drop will remain consistent inside and outside the tubes and still provide the verification function of the present invention.

[0145] At the outlet end, the tube temperature is inherently higher than at the inlet end. Compared to a tube bundle in which the tubes are arranged parallel to one another, the increased fluid velocity toward the outlet end has the effect of increasing heat transfer, thereby cooling the tubes 4 more and reducing the temperature at the outlet end. This reduced temperature reduces tube 4 aging, i.e., increases the service life of the tubes 4.

[0146] exist Figure 4 In FIG. 1 , another alternative electrical connection scheme is also shown. This alternative electrical connection scheme is not limited to Figure 4 Instead, it can be applied in Figures 1a to 3 Similarly, the application in Figures 1a to 3 The electrical connector 6 in the embodiment can be used in Figure 4 In the embodiment.

[0147] The electric heater comprises another set of electrical connectors 6' arranged between the tubes 4 at a distance from the axial end 22 of the tube bundle 10. Together with the electrical connectors 6, this set of electrical connectors 6' forms a parallel connection between the tubes 4 at the axial end 9 of the tube bundle 10.

[0148] In this way, the electrically parallel-connected sections of tubes 4 are subjected to only a portion of the current as the remaining sections of tubes 4 not connected in parallel. Consequently, the surface load on the parallel-connected sections of tubes 4 is reduced compared to the remaining sections of tubes 4. In other words, the heating and power transfer per surface area at the axial ends 9 are reduced. This can be advantageous at the hot end 9 of the tube bundle 10, as the temperature at this end is lower compared to tube bundles without electrically parallel-connected sections of tubes. Consequently, aging of the tubes 4 is reduced.

[0149] Figure 5 An embodiment of an electric heating system 30 for a fluid is schematically shown. Figure 5 A cross section through the electric heating system 30 is shown. Figures 1a to 4 and Figure 6 .

[0150] The electric heating system 30 includes a housing 32 and at least one electric heater 2 according to any of the aspects and / or embodiments discussed herein, such as described above with reference to FIG. Figures 1a to 4 and Figure 6 Any heater discussed.

[0151] The at least one electric heater 2 is arranged in a housing 32. A flow path 34 for the fluid to be heated extends through the tube bundle 10 formed by the plurality of tubes 4 of the at least one heater 2 and along the axial extension 12 of the tube bundle 10. Optionally, the housing 32 may comprise insulating or refractory material to insulate against the high temperatures inside the heating system 30 and also to provide suitable electrical insulation.

[0152] Each heater 2 is supported by one or more support arrangements (not shown) within a housing 32. Furthermore, one or more flow restricting members (not shown) may be provided within the housing 32 to surround each heater to ensure that the fluid to be heated will flow along the flow path 34 through the tube bundle 10 of each heater 2.

[0153] exist Figure 5 In FIG, two heaters 2 are visible. Optionally, a third heater and further heaters can be arranged in the housing 32. The flow path 34 is indicated by a broad arrow.

[0154] When the two or more heaters 2 are arranged in parallel, as in the illustrated embodiment, the flow paths 34 extend in parallel through the heaters 2 .

[0155] Thus, the electric heating system 30 can include at least two electric heaters 2 according to any of the aspects and / or embodiments discussed herein. The flow path 34 can extend in parallel through the at least two electric heaters 2, and the at least two electric heaters 2 can be electrically connected in parallel, in a star connection, in a delta connection, or in a combination thereof. In this way, an efficient electric heating system 30 with a high fluid flow capacity can be provided. The electric heating system 30 can be efficiently connected to an external power source 13 via electrical conductors (not shown) of each heater 2.

[0156] Each of the star connection and the delta connection requires at least three electric heaters 2 in the heating system 30 .

[0157] Inside the housing 32, an inlet chamber 36 upstream of the heater 2 and an outlet chamber 38 downstream of the heater 2 may be arranged. The flow path 34 extends from the inlet chamber 36 via the heater 2 to the outlet chamber 38.

[0158] The inlet chamber 36 can be considered to form a manifold for distributing the converging fluid stream to the various heaters 2 and their respective bundles 10 of tubes 4. Similarly, the outlet chamber 38 can be considered to form a manifold for converging the distributed fluid streams in the heaters 2 back into a single converging fluid stream. Along the flow path 34 in the heater 2, the fluid is heated as it flows along the inner and outer surfaces of the tubes 4.

[0159] The electric heater 2 according to aspects and / or embodiments discussed herein and / or the electric heating system 30 according to aspects and / or embodiments discussed herein may be used to heat a fluid, such as a gas, in an industrial process.

[0160] For example, the heated fluid may be used in an industrial process. The fluid heated in the heater 2 / system 30 may be an energy carrier in the industrial process and / or the fluid may be used as a heat source in the industrial process and / or the heated fluid may be a process fluid in the industrial process.

[0161] In the following, three exemplary heating implementations and exemplary embodiments of the electric heater 2 or electric heating system 30 will be discussed. Figures 1a to 6 .

[0162] Due to the high SDR tubes discussed above and the relationship between the quotient of the outer and inner cross-sectional areas of the tubes and the quotient of the outer and inner diameters of the tubes, the heater 2 / system 30 of the present invention provides a high surface to fluid flow ratio. Thus, the heater 2 / system 30 can be designed for minimal size while taking into account the fluid flow characteristics.

[0163] By using the above materials, the tube 4 can withstand temperatures up to 1400° C. or 2050° C. However, lower temperatures, such as 1300° C. or temperatures in the range of 600-1300° C. discussed above, can increase the life expectancy of the heater 2 / system 30 .

[0164] Examples 1-4 demonstrate how different flow characteristics and boundary conditions affect the design of the heater 2 / system 30. Various fluids and tube diameters are considered and their performance is shown.

[0165] Example 1

[0166] Example 1 involves heated dry air. Many common industrial processes require heated air. The heater 2 / system 30 of Example 1 includes 6,120 tubes 4, each with an outer diameter (Do) of 17.15 mm, a SDR of 7.4, and a center-to-center distance of 22 mm, arranged in an equilateral triangle. The tubes 4 are 0.9 m long and arranged in a 1.55 m x 1.68 m duct. The heater 2 / system 30 is designed for 51 MW of power.

[0167] 101kW / m² supplied by heater 2 / system 30 2 The surface load of heater 2 / system 30 is 1200°C. Heater 2 / system 30 ensures high energy density, thus allowing for compact dimensions of heater 2 / system 30. In this example, dry air with an inlet temperature of 45°C and a mass flow rate of 76 kg / s is heated to an outlet temperature of 800°C. The maximum temperature of tube 4 of heater 2 / system 30 is 1200°C.

[0168] Example 2

[0169] Example 2 involves a hydrogen mixture consisting of 95% H₂ and 5% N₂. In Example 2, the outer diameter of the heater tube used was smaller than in Example 1, but the surface loading was similar. Furthermore, due to the smaller tube diameter and the properties of the gas, the size of the heater 2 / system 30 was reduced compared to Example 1. However, a higher outlet temperature was still achieved.

[0170] The heater 2 / system 30 of Example 2 includes 20,286 tubes 4, each having an outer diameter (Do) of 6 mm, an SDR of 6, and a center-to-center distance of 7.50 mm, arranged in an equilateral triangle. The tubes 4 are 0.85 m long and arranged in a 1.0 m x 0.99 m duct. The heater 2 / system 30 is designed for 51 MW of power.

[0171] 100 kW / m² is provided by the correspondingly compact heater 2 / system 30 2In this example, a gas containing 95% H2 and 5% N2 at an inlet temperature of 45°C and a mass flow rate of 1.45 kg / s is heated to an outlet temperature of 1000°C. The maximum temperature of the tube 4 of the heater 2 / system 30 is 1200°C.

[0172] Example 3

[0173] Example 3 involves carbon dioxide. Heated carbon dioxide is another gaseous fluid commonly used in industrial processes. By using 40 mm tubing, high gas outlet temperatures can be achieved at higher mass flow rates with reasonable heater 2 / system 30 dimensions.

[0174] Accordingly, the heater 2 / system 30 of Example 3 includes 1320 tubes 4, each having an outer diameter (Do) of 40 mm, an SDR of 13.3 mm, and a center-to-center distance of 52 mm, arranged in an equilateral triangle. The tubes 4 are 1.7 m long and arranged in a 1.7 m x 1.9 m duct. The heater 2 / system 30 is designed for 51 MW of power.

[0175] Provides 106kW / m in heater 2 / system 30 2 In this example, carbon dioxide with an inlet temperature of 45°C and a mass flow rate of 145 kg / s is heated to an outlet temperature of 1100°C. The maximum temperature of the tube 4 of the heater 2 / system 30 is 1200°C.

[0176] Example 4

[0177] Example 4 involves heating a gas mixture consisting of 95% H₂ and 5% N₂. Superheating large quantities of low-density gases will play a significant role in the decarbonization industry. Molybdenum heating elements allow temperatures exceeding 1100°C. Using a tube with an outer diameter of 15 mm, Heater 2 is capable of superheating the mixture to 1300°C.

[0178] Heater 2 / system 30 of Example 4 includes 14,520 tubes, each with an outer diameter (Do) of 15 mm, an SDR of 6, and a center-to-center distance of 18.5 mm, arranged in an equilateral triangle. The tubes 4 are 2.2 m long and arranged in a 2.1 m x 2.1 m duct. Heater 2 / system 30 is designed for 100 MW of power.

[0179] Provides 40kW / m in heater 2 / system 30 2 In this example, a hydrogen / nitrogen mixture with an inlet temperature of 700°C and a mass flow rate of 16.7 kg / s is heated to an outlet temperature of 1300°C. The maximum temperature of the tube 4 of the heater 2 / system 30 is 1380°C.

[0180] It should be understood that the foregoing is a description of various example embodiments and that the present invention is limited only by the appended claims. Those skilled in the art will recognize that the example embodiments may be modified and that different features of the example embodiments may be combined to create embodiments other than those described herein without departing from the scope of the present invention as defined by the appended claims.

Claims

1. An electric heater (2) for a fluid, the electric heater (2) comprising: A plurality of conductive tubes (4) arranged in a tube bundle (10) for resistive heating, the tube bundle (10) having an axial extension (12); an electrical connector (6) arranged between the conductive tubes (4) of the plurality of conductive tubes (4); and an electrical conductor (8) configured to connect the plurality of conductive tubes (4) to an external power source (13), wherein The electric heater (2) is configured to cause a fluid to flow through the tube bundle (10) parallel to the axial extension (12) and in direct contact with the inner and outer surfaces of the conductive tube (4), wherein The outer diameter (Do) of each of the conductive tubes (4) is in the range of 6 to 40 mm, and the outer diameter to wall thickness ratio (SDR) of each of the conductive tubes (4) is in the range of 5 to 15, and wherein In a cross section of the tube bundle (10) perpendicular to the axial extension (12), a ratio between a total cross-sectional surface area (A) between the conductive tubes (4) and a total cross-sectional area (B) inside the conductive tubes (4) is within a range of 90% to 130% of a ratio between the outer diameter (Do) and the inner diameter (Di) of one of the conductive tubes (4).

2. The electric heater (2) according to claim 1, wherein: Each conductive tube (4) of the plurality of conductive tubes (4) is arranged to form an angle (α) within a range of 0 to 15 degrees with an adjacent conductive tube (4).

3. The electric heater (2) according to claim 1 or 2, comprising spacer elements (14) arranged between the conductive tubes (4) to support the conductive tubes (4) in the tube bundle (10), wherein the spacer elements (14) are non-conductive, and wherein the spacer elements (14) are arranged with a gap (i) along the axial extension (12), and the gap (i) is less than 40% of the total length (L) of the tube bundle (10) along the axial extension (12).

4. The electric heater (2) according to claim 3, wherein: The spacer element (14) comprises a tubular element arranged along the conductive tube (4).

5. The electric heater (2) according to claim 3, wherein: The conductive tube (4) comprises a structural element (16) configured for positive locking engagement with the spacing element (14).

6. The electric heater (2) according to any one of claims 1-2, wherein: The electrical connectors (6) arranged between the conductive tubes (4) are arranged at two opposite axial ends (9, 11) of the tube bundle (10), and wherein the conductive tubes (4) are connected in series via the electrical connectors (6).

7. The electric heater (2) according to claim 6, wherein: The plurality of conductive tubes (4) are an even number of tubes (4), and wherein the electrical conductor (8) configured to connect the plurality of conductive tubes (4) to an external power source (13) is arranged at one axial end (11) of the tube bundle (10).

8. The electric heater (2) according to any one of claims 1-2, comprising a support element (18), the support element (18) being arranged inside the conductive tube (4) for mechanically supporting the conductive tube (4).

9. The electric heater (2) according to claim 8, wherein: The support element (18) is arranged within the last 40% of the total length of the conductive tube (4) as seen in the flow direction of the fluid to be heated.

10. The electric heater (2) according to any one of claims 1-2, wherein: The conductive pipe (4) is designed and arranged to conduct 20-200kW / m 2 Specific energy transfer within a range.

11. The electric heater (2) according to any one of claims 1 to 2, wherein: The conductive tube (4) is composed of a conductive material for active resistance heating, or is composed of more than one conductive material for active resistance heating, and wherein the conductive material is the same or different in all the conductive tubes (4) and is selected from the group consisting of: iron-chromium-aluminum alloy (FeCrAl alloy), nickel-based alloy, tungsten-based alloy or molybdenum-based alloy.

12. The electric heater (2) according to any one of claims 1-2, comprising another set of electrical connectors (6'), which are arranged between the conductive tubes (4) and at a certain distance from the axial end (22) of the tube bundle (10), wherein the another set of electrical connectors (6') together with the electrical connectors (6) form a parallel connection between the conductive tubes (4) at the axial end (9) of the tube bundle (10).

13. The electric heater (2) according to any one of claims 1-2, wherein: The plurality of electrically conductive tubes (4) are configured to be electrically heated to a temperature of up to 1300°C.

14. The electric heater (2) according to any one of claims 1-2, wherein: The plurality of electrically conductive tubes (4) are configured to be electrically heated to a temperature of up to 2050°C.

15. The electric heater (2) according to any one of claims 1-2, wherein: The ratio between the total cross-sectional surface area (A) between the conductive tubes (4) and the total cross-sectional area (B) inside the conductive tubes (4) is within a range of 95% to 125% of the ratio between the outer diameter (Do) and the inner diameter (Di) of one of the conductive tubes (4).

16. The electric heater (2) according to claim 1 or 2, wherein: The plurality of electrically conductive tubes (4) are configured to be electrically heated to a temperature in the range of up to 600-1300°C.

17. The electric heater (2) according to claim 1 or 2, wherein: The plurality of electrically conductive tubes (4) are configured to be electrically heated to a temperature in the range of up to 800-2050°C.

18. The electric heater (2) according to claim 1, wherein The outer diameter to wall thickness ratio (SDR) of each of the conductive tubes (4) is in the range of 7 to 12.

19. The electric heater (2) according to claim 2, wherein: Each conductive tube (4) of the plurality of conductive tubes (4) is arranged to form an angle (α) within a range of 0 to 5 degrees with an adjacent conductive tube (4).

20. The electric heater (2) according to claim 10, wherein: The conductive pipe (4) is designed and arranged to conduct 30 to 120 kW / m 2 Specific energy transfer within a range.

21. An electric heating system (30) for a fluid, comprising a housing (32) and at least one electric heater (2) according to any one of claims 1 to 20, wherein: The at least one electric heater (2) is arranged in the housing (32), and wherein a flow path (34) for the fluid to be heated extends through the tube bundle (10) and along the axial extension (12).

22. The electric heating system (30) according to claim 21, comprising at least two electric heaters (2) according to any one of claims 1-20, wherein: The flow path (34) extends in parallel through the at least two electric heaters (2), and wherein the at least two electric heaters (2) are electrically connected in parallel, in a star configuration, in a delta configuration, or in a combination thereof.

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