ELECTRIC GAS HEATER

Abstract

An electric gas heater (2) comprises a housing (4), a number of thin tubes (16) arranged in a bundle (18) inside the housing (4), an insulation member (20) configured for supporting the number of thin tubes (16) separated from each other and electrically insulated from each other. Individual tubes (16) of the number of thin tubes (16) are of an electric resistance material, and the insulation member (20) comprises a fibrous material.

Claims

1. An electric gas heater, comprising: a housing, a number of thin tubes arranged in a bundle inside the housing, an insulation member configured for supporting the number of thin tubes separated from each other and electrically insulated from each other, electrical conductors configured for connecting the number of thin tubes with an external electric power supply, and inside the housing an inlet chamber upstream of the number of thin tubes and an outlet chamber downstream of the number of thin tubes, wherein a gas flow path extends from the inlet chamber via insides of the number of thin tubes to the outlet chamber, and wherein individual tubes of the number of thin tubes are of an aluminium oxide forming electric resistance material or of a molybdenum based alloy.

2. The electric gas heater according to claim 1, wherein the insulation member seals the inlet chamber from the outlet chamber so that the gas flow path constitutes a main flow path for gas from the inlet chamber to the outlet chamber.

3. The electric gas heater according to claim 1, wherein the insulation member comprises a fibrous material.

4. The electric gas heater according to claim 3, wherein the fibrous material comprises a vacuum formed fibrous material.

5. The electric gas heater according to claim 4, wherein in the vacuum formed fibrous material the fibres are bound to each other via a binder.

6. The electric gas heater according to claim 1, wherein the aluminium oxide forming electric resistance material is an iron-chromium-aluminium (FeCrAl) alloy comprising at least 3 wt % aluminium.

7. The electric gas heater according to claim 1, wherein the insulation member extends along≥50% of a length of the thin tubes.

8. The electric gas heater according to claim 1, wherein the number of thin tubes are configured to be electrically heated up to a temperature of 1250° C.

9. The electric gas heater according to claim 1, wherein the number of thin tubes are configured to be electrically heated up to a temperature of 1300° C.

10. The electric gas heater according to claim 1, wherein the housing forms a pressure vessel.

11. The electric gas heater according to claim 1, wherein the housing comprises a sealable opening sized such that the number of thin tubes arranged in a bundle are extractable out of the housing as one unit via the opening.

12. The electric gas heater according to claim 1, wherein individual thin tubes of the bundle are arranged for an energy transfer of up to 100 W/cm.sup.3.

13. The electric gas heater according to claim 1, wherein individual thin tubes of the bundle have an inner diameter within a range of 7-30 mm and a wall thickness within a range of 1-3 mm.

14. The electric gas heater according to claim 1, wherein individual thin tubes of the thin tubes arranged in the bundle are arranged with outer diameters of adjacent thin tubes within a range of 10-30 mm from each other.

15. A method for heating a gas in an electric gas heater according to claim 1, comprising steps of: supplying a gas to the inlet chamber whereby the gas is conducted along the gas flow path via the insides of the number of thin tubes to the outlet chamber, supplying an electric current to the number of thin tubes in order to heat the number of thin tubes, continue with conducting the gas along the gas flow path via the insides of the number of thin tubes to the outlet chamber, and leading the gas from the outlet chamber.

16. The electric gas heater according to claim 1, wherein the insulation member extends along≥90% of the length of the thin tubes.

17. The electric gas heater according to claim 1, wherein the number of thin tubes are configured to be electrically heated up to a temperature within a range of 900-1250° C.

18. The electric gas heater according to claim 1, wherein individual thin tubes of the bundle have an inner diameter within a range of 9-20 mm and a wall thickness within a range of 1.5-2.5 mm.

19. The electric gas heater according to claim 1, wherein individual thin tubes of the bundle are arranged for an energy transfer of up to within a range of 40-70 W/cm.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Various aspects and/or embodiments of the invention, including its particular features and advantages, will be readily understood from the example embodiments discussed in the following detailed description and the accompanying drawings, in which:

[0044] FIGS. 1a and 1b illustrate two views of an electric gas heater,

[0045] FIG. 2 illustrates a cross sectional view through an electric gas heater,

[0046] FIGS. 3a-3c illustrate embodiments of tubes of an electric gas heater and their arrangement in a bundle inside a housing of a gas heater, and

[0047] FIG. 4 illustrates a method for heating a gas in an electric gas heater.

DETAILED DESCRIPTION

[0048] Aspects and/or embodiments of the invention will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity.

[0049] FIGS. 1a and 1b illustrate two views of an electric gas heater 2 according to embodiments.

[0050] The electric gas heater 2 comprises a housing 4. The gas to be heated flows through the housing 4, from an inlet 6 to an outlet 8. The heater 2 further comprises a number of thin tubes arranged in a bundle inside the housing 4, see further below with reference to FIGS. 2-3c. Electrical conductors 10 are provided for connecting the number of thin tubes with an external electric power supply.

[0051] In the illustrated embodiments, the housing 4 has a tubular shape, the inlet 6 connects radially to the housing 4, and the outlet 8 extends axially from the housing 4. The inlet 6 is provided in an inlet pipe 12 extending radially from the housing 4. The outlet 8 is provided by the tubular housing 4. A lid 14 closes the housing 4 in an axial direction on the inlet side of the housing 4. The electrical conductors 10 extend through the lid 14 in an axial direction of the tubular housing 4.

[0052] However, the invention is not limited to the illustrated embodiments. The housing of the gas heater may have any suitable shape for accommodating a bundle of tubes, connecting electrical conductors to the tubes, and permitting a gas flow through the housing. For instance, the inlet may be arranged axially in the same manner as the outlet 8 shown in FIGS. 1a and 1b, instead of radially and/or the outlet may be arranged radially.

[0053] FIG. 2 illustrates a cross sectional view through the electric gas heater 2 along line II— II in FIG. 1b.

[0054] In FIG. 2, the number of thin tubes 16 arranged in a bundle 18 inside the housing 4 are clearly shown. The tubes 16 are of an electric resistance material or a molybdenum based alloy. The tubes 16 are directly heated by electric current supplied to the tubes 16 via the electrical conductors 10. When flowing through the electric resistance material or the molybdenum based alloy, the electric current causes the electric resistance material or the molybdenum based alloy to heat up.

[0055] The gas heater 2 comprises an insulation member 20 configured for supporting the number of thin tubes 16 separated from each other and electrically insulated from each other. The insulation member 20 also supports the tubes 16 within the housing 4.

[0056] In the illustrated embodiments, the insulation member 20 extends substantially along the entire axial length of the bundle 18 of tubes 16. For instance, the insulation member 20 may extend along at least 90% of the length of the tubes 16, such as along the entire lengths of the tubes 16. In this manner, the thin tubes 16 may be securely supported and electrically heated up to high temperatures, at which the strength of the tubes 16 is reduced. Moreover, in this manner the insulation member 20 may insulate an inside of the housing 4 against the heat from the tubes 16.

[0057] The insulation member 20 may comprise a number of individual members 20′. In the illustrated embodiments, the individual members 20′ are arranged side-by-side abutting against each other. Alternatively, the individual members 20′ may be arranged at a distance from each other, or only some of the individual members 20′ may abut against each other and some are arranged at a distance from each other. Thus, according to some embodiments, the insulation member 20, as formed by the individual members 20′, may extend along at least 50% of the length of the tubes 16.

[0058] The insulation member 20 supporting the tubes 16 in this manner provides for the gas heater 2 to be positioned in any required position within an industrial plant.

[0059] The insulation member 20 may comprise a fibrous material. The fibrous material provides a comparatively lightweight insulation member 20 for the tubes 16. This may be of importance when the insulation member 20 extends along a substantial portion of the length of the tubes 16, such as e.g. at least 50% of the length of the tubes 16 or even along>90% of the length of the tubes 16.

[0060] The fibrous material may comprise a vacuum formed fibrous material.

[0061] The vacuum forming process for producing a vacuum formed fibrous material, as such, is known and is therefore not described.

[0062] According to one embodiment, the vacuum formed fibrous material after manufacturing may be fully cured, i.e. all fibres of the fibrous material are bound to each other via a binder and no portion of the insulation member 20 contains free fibres. In this manner, it may be ensured that the insulation member 20 is able to support the tubes 16. Also, the holes for the tubes 16 may be easily drilled through the insulation member 20 or individual members 20′.

[0063] According to one embodiment, the vacuum formed fibrous material may comprises>40% Al.sub.2O.sub.3 fibres with SiO.sub.2 fibres in balance and binder residue. Alternatively, the vacuum formed fibrous material may comprises>50% Al.sub.2O.sub.3 fibres with SiO.sub.2 fibres in balance and binder residue, the vacuum formed fibrous material may comprises>60% Al.sub.2O.sub.3 fibres with SiO.sub.2 fibres in balance and binder residue.

[0064] The vacuum formed fibrous material may be a comparatively lightweight material. A density of the vacuum formed fibrous material may be >250 kg/m.sup.3 in order to provide an insulation member configured to support the number of thin tubes 16 therein as well as in the housing 4 of the heater 2. An upper end of a density range may be approximately 500 kg/m.sup.3.

[0065] As mentioned above, the insulation member 20 may comprise a compact (non-fibrous) refractory material e.g., when a higher weight of the gas heater 2 may be accepted.

[0066] Inside the housing 4 there is arranged an inlet chamber 22 upstream of the number of thin tubes 16 and an outlet chamber 24 arranged downstream of the number of thin tubes 16. A gas flow path extends from the inlet chamber 22 via insides of the tubes 16 to the outlet chamber 24. In FIG. 2, the gas flow path is indicated with broad arrows in the inlet and outlet chambers 22, 24 and with narrow arrows in some of the tubes 16.

[0067] The inlet chamber 22 may be considered to form a manifold for distributing a collective gas stream to the individual tubes 16. Similarly, the outlet chamber 24 may be considered to form a manifold for converging the distributed gas streams in the tubes 16 back into one collective gas stream. Accordingly, in the gas flow path extending from the inlet chamber 22 to the outlet chamber 24, distributed gas flow paths are provided via the insides of the tubes 16. In the distributed gas flow paths of the insides of the tubes 16, the gas is heated.

[0068] According to some embodiments, the inlets of the respective individual tubes 16 may be provided with flow restrictions. That is, an upstream portion of each tube 16 may have a reduced inner diameter in comparison with downstream portions of the tube 16. Namely, at high gas flow, the gas flow is evenly distributed between the individual tubes 16 irrespectively of whether the tubes 16 are provided with flow restrictions or not. However, at low gas flow such flow restrictions may contribute to an even distribution of the gas flow between the individual tubes 16 of the bundle 18 from the inlet chamber 22 into the tubes 16. Thus, in a gas heater wherein during use the gas flow varies over a larger flow range, such flow restrictions may be advantageous.

[0069] The insulation member 20 seals the inlet chamber 22 from the outlet chamber 24 to the extent that the gas flow path constitutes a main flow path for gas from the inlet chamber 22 to the outlet chamber 24. Accordingly, the insulation member 20 may not provide a gas tight seal between the inlet and outlet chambers 22, 24. However, the insulation member 20 does provide a sufficiently high pressure drop, i.e. gas flow resistance, such that the gas flowing from the inlet chamber 22 to the outlet chamber 24 will mainly flow through the insides of the tubes 16 instead of outside them. For instance, at least 90% of the gas may flow through the insides of the tubes 16 from the inlet chamber 22 to the outlet chamber 24. A certain flow of gas along the outsides of the tubes 16 may be permitted since also along the outsides of the tubes 16, the gas may be heated. However, along an inner surface of the housing 4 any gas flow should be prevented by the insulation member 20 since there the gas will not be heated. A bad seal along the inside of the housing 4 would permit a portion of the gas to escape unheated along the inside from the inlet chamber 22 to the outlet chamber 24.

[0070] Insides of the inlet and outlet chambers 22, 24 may be provided with protective elements 26, 26′. The protective elements 26, 26′ may be arranged adjacent to the housing 4 in order to protect the housing 4 from the warm gas in the inlet and outlet chambers 22, 24. According to some embodiments, the protective members 26, 26′ may comprise a fibrous material of the same kind as the insulation member 20.

[0071] In this context, it may be mentioned that the gas heater 2 is suited to elevate the temperature of already hot gas. For instance, the gas flowing into the inlet chamber 22 may have a temperature within a range of 300-900° C.

[0072] FIGS. 3a-3c illustrate embodiments of tubes 16 of a gas heater 2 and their arrangement in a bundle 18 inside a housing 4 of a gas heater 2. The gas heater 2 may be a gas heater 2 as discussed above with reference to FIGS. 1a-2.

[0073] FIG. 3a shows a view into an inlet chamber 22 of the heater 2. FIG. 3b shows a partial view into the inlet chamber 22. FIG. 3c shows two tubes 16.

[0074] The gas heater 2 comprises a number of thin tubes 16. Mentioned purely as an example, the number of tubes 16 may be e.g. 50 to 500 tubes, such as 200-300 tubes. The individual tubes 16 of the number of thin tubes 16 are of an electric resistance material or a molybdenum based alloy and are supported and electrically insulated from each other by an insulation member.

[0075] The tubes 16 are electrically connected to each other at their end portions via electrically conductive connectors 28. The connectors 28 provide parallel connections between some of the tubes 16 and serial connections between some of the tubes 16. Depending on the voltage connected to the electrical conductors 10 and the electrical resistivity of the individual tubes 16, a suitable configuration of parallel and serial connection between the tubes 16 may be provided.

[0076] Via the electrical conductors 10, the tubes 16 are directly or indirectly connected to mains power. For instance, the tubes 16 may be connected to each other in such a manner that mains power at 400 V may be supplied to the tubes 10 via the electrical conductors 10.

[0077] The thin tubes 16 have a small diameter and a thin wall thickness.

[0078] According to embodiments, individual thin tubes 16 of the bundle 18 may have an inner diameter within a range of 7-30 mm, such as 9-20 mm and a wall thickness within a range of 1-3 mm, such as 1.5-2.5 mm. In this manner, good heat transfer to the gas to be heated may be achieved in the tubes 16 without too large a pressure drop along each of the individual tubes 16.

[0079] Due to the insulation member 20 supporting the individual thin tubes 16, the tubes 16 may be of such weak dimensions as exemplified above, even when the lengths of the tubes 16 are long. In particular, this may be the case in embodiments wherein the insulation member 20, as formed by the individual members 20′, extend along at least 50% of the length of the tubes 16. Mentioned purely as an example, the length of the individual tubes 16 may be within a range of 0.5-2.5 m, or within a range of 1-2 m.

[0080] The electric resistance material is a material that forms at least one heat resistant oxide. As discussed above, the electric resistance material is an aluminium oxide (i.e. alumina) forming alloy.

[0081] According to one example, the alumina forming alloy is a FeCrAl alloy comprising at least 3 wt % aluminium. Thus, the tubes 16 may be configured to be electrically heated up to a temperature of 1250° C. while maintaining a practical operational lifespan of the tubes 16.

[0082] It is believed that the bundle 18 of tubes 16 may be configured for an energy transfer up to 5 MW/m.sup.3 or even higher, according to one embodiment, the energy transfer is within the range of 2 to 5 MW/m.sup.3. Namely, the herein discussed electric gas heater 2 provides a space efficient transfer of energy/heat from the tubes 16 of the bundle 18 to the gas to be heat. The space efficiency may be achieved due to the arrangement of the number of thin tubes 16 being supported separated from each other by the insulation member 20. It should be noted that only the volume of the bundle 18 of tubes 16 is included in these energy transfer figures. The volume of the inlet and outlet chambers 22, 24 is excluded.

[0083] Purely mentioned as examples, a larger gas heater may be one designed for 5-10 MW with a volume of the bundle 18 of approximately 1.5-2.0 m.sup.3, wherein the bundle 18 may comprise several hundreds of tubes 16, which may be arranged within a range of 20-30 mm from each other. A comparatively smaller gas heater may be designed for 0.5-1 MW with a volume of the bundle 18 of approximately 0.2 m.sup.3, wherein the tubes 16 within the bundle 18 may be arranged within a range of 10-20 mm from each other. The above-mentioned arrangements of the tubes 16 from each other relates to ranges of distances between the outer diameters of adjacent tubes 16 in the bundle 18.

[0084] Within the above discussed distance ranges of 20-30 mm and 10-20 mm, respectively, electric discharge and/or short circuit between the individual tubes 16 is avoided. The voltage applied to the tubes 16 is relevant in the context of the distance between the tubes 16. Generally, the higher the power rating for a gas heater, the higher a voltage is applied to the tubes 16. Thus, the distance range between the tubes 16 is larger for higher power rated gas heaters 2 than for lower power rated gas heaters 2.

[0085] According to embodiments, individual tubes 16 of the thin tubes 16 arranged in the bundle 18 may be arranged with outer diameters of adjacent tubes 16 within a range of 10-30 mm from each other.

[0086] According to embodiments, individual tubes 16 of the bundle 18 may be arranged for an energy transfer of up to 70 W/cm.sup.3, or up to 100 W/cm.sup.3, or up to within a range of 40-70 W/cm.sup.3, or up to within a range of 30-60 W/cm.sup.3.

[0087] An efficient transfer of energy/heat from the individual thin tubes 16 to the gas to be heated is achieved in the gas heater 2. The thin tubes 16 may suitable be of the dimension discussed above. Energy transfer in the upper range 100 W/cm.sup.3 may come at the cost of a high pressure drop of the gas as it flows through the tubes 16 and may be achieved for some gases, such as hydrogen, and/or under specific operating conditions, which may include one or more of operation under high pressure and/or with a lower outlet temperature e.g. 600 degrees Celsius. More reasonable pressure drop may be achieved at the energy transfer figures within the ranges 40-70 W/cm.sup.3 and 30-60 W/cm.sup.3. Also these energy transfer figures depend on the gas to be heated and the conditions under which the gas heater 2 is operated.

[0088] A different manner of specifying the energy transfer would be to define the energy transfer per area on an inside of the thin tubes 16. For instance, the FIG. 60 W/cm.sup.3 would correspond to approximately 15 W/cm.sup.2 in embodiments of the gas heater 2.

[0089] The following non-limiting examples relate to gas heaters 2 operated at atmospheric pressure provided with tubes 16 having outer and inner diameters of 17.15 and 12.53 mm and arranged with a centre-to-centre distance of 35 mm. A surface temperature of the tubes 16 of 1250 degrees Celsius is provided and a maximum pressure drop of 100 mBar is allow. The gas heated is air with an inlet temperature of 20 degrees Celsius.

[0090] In an embodiment of the above, the gas heater 2 is designed with an outlet temperature of 600 degrees Celsius, an energy transfer of approximately 18 W/cm.sup.2 may be achieved. If instead an outlet temperature of 1100 degrees Celsius is provided by the gas heater, only a lower energy transfer of approximately 3 W/cm.sup.2 may be achieved. Operating the gas heater 2 under pressure and/or permitting a higher pressure drop will improve these energy transfer figures.

[0091] It is thus, easily foreseeable that energy transfer figures within a range of 2-20 W/cm.sup.2 may be achieved in the gas heater 2 when operated with air under atmospheric pressure.

[0092] The housing 4 may form a pressure vessel. Instead of the individual tubes being able to withstand a pressure difference between their insides and outsides, the housing 4 is devised to withstand a pressure difference between its inside and its outside. Depending on the relevant pressure levels, temperature levels, and type of gas being heated, the housing 4 may comprise low carbon unalloyed, low alloyed, alloyed, or stainless steel, which are suitable for forming a pressure vessel. Moreover, in embodiments wherein the housing forms a pressure vessel, the gas heater 2 may be directly connected to, and utilised in, industrial processes wherein the gas to be heated is pressurised.

[0093] Mentioned purely as an example, the pressure vessel may be designed to withstand a gas pressure inside the housing 4 within a range of 10-15 bar, or even up to 30 or 40 bar, depending on the industrial process wherein the heater 2 is used.

[0094] Some examples of industrial processes where the gas heater 2 with pressure vessel properties may be utilised are: [0095] Energy storage by means of the heated gas heating a bed of metal or ceramic pellets or beds comprising natural material such as rocks, volcanic rocks, the bed providing a counter pressure to the gas being heated in the gas heater. [0096] Direct reduction of iron pellets with hydrogen or natural gas to produce direct reduction iron, DRI. In this process the high gas temperature achieved in the gas heater 2 may be particularly useful. Gas heated to temperatures within a range of 1000-1100° C. or higher such as up to 1250° C. or up to 1300° C. is advantageous in the direct reduction process. With the present gas heater 2 this is achieved in a gas heater 2 of compact format with high energy density, which is able to heat high gas flows. In the gas heater 2 this is possible due to the use of aluminium oxide forming electric resistance material in the directly electrically heated thin tubes 16, which also lends the gas heater 2 few components and the compact format. [0097] Various chemical processes such e.g. Fischer-Tropsch synthesis.

[0098] Use of the heater 2 is not limited to these example processes. Moreover, the heater 2 may be utilised for heating non-pressurised or low pressure gas.

[0099] The gas heater 2 is particularly suited for heating large gas flows. Even the above exemplified gas heater being provided with a bundle 18 of tubes 16 having a volume of 0.2 m.sup.3 may heat a gas flow of 400-500 m.sup.3/hour to temperatures within a range of 900-1250° C. Further, the above exemplified gas heater being provided with a bundle 18 of tubes 16 having a volume of 1.5-2.0 m.sup.3 may heat a gas flow of up to 3000 m.sup.3/hour to temperatures within a range of 900-1250° C. Even larger flows, such as 15000-20000 m.sup.3/hour are foreseen to be heated in larger versions of the gas heater.

[0100] For easy mounting, replacement, and servicing of the tubes 16, the housing 4 may comprise a sealable opening sized such that the tubes 16 arranged in a bundle 18 may be extracted out of the housing 4 as one unit via the opening. In the exemplified embodiments, see e.g. FIGS. 1a and 2, the opening of the housing 4 is covered by the lid 14 during use of the gas heater 2. The lid 14 seals the opening and is removably attached to the tubular portion of the housing 4, e.g. via nuts and bolts.

[0101] If the housing 4 form a pressure vessel and comprises one or more lids 14, as in the exemplified embodiments, see e.g. FIG. 1a, the lid 14 must close the housing 4 in a manner to fulfil the requirements of a pressure vessel.

[0102] FIG. 4 illustrates a method 100 for heating a gas in an electric gas heater 2 according to any one of aspects and/or embodiments discussed herein, such as e.g. the gas heater 2 discussed above with reference to FIGS. 1a-3c. Accordingly, in the following reference is also made to FIGS. 1a-3c.

[0103] The method 100 for heating a gas in an electric gas heater 2 comprises the steps of: [0104] supplying 102 a gas to the inlet chamber 22 whereby the gas is conducted along the gas flow path via the insides of the number of thin tubes 16 to the outlet chamber 24, [0105] supplying 104 an electric current to the number of thin tubes 16 in order to heat the number of thin tubes 16, [0106] continue with conducting 106 the gas along the gas flow path via the insides of the number of thin tubes 16 to the outlet chamber 24, and [0107] leading 108 the gas from the outlet chamber 24.

[0108] The gas will start to flow as soon as it is supplied to the inlet chamber and thereby it will be conducted along the gas flow path via the bundle of this tubes.

[0109] The method 100 may be utilised for heating gas in an industrial process.

[0110] According to embodiments of the method 100, the gas may for example but not limited thereto to air, hydrogen, nitrogen, carbon dioxide, synthesis gas, or pyrolysis gases. In this manner, a suitable gas for a relevant industrial process may be heated in the gas heater 2.

[0111] According to embodiments of the method 100, the step 102 of supplying a gas to the inlet chamber 22 may comprise supplying the gas at a temperature within a range of 300-900° C. to the inlet chamber 22. In this manner, the property of the gas heater 2 to elevate already hot gas to even higher temperatures may be utilised in an industrial process.

[0112] It is to be understood that the foregoing is illustrative of various example embodiments and that the invention is defined only by the appended claims. A person skilled in the art will realize 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 invention, as defined by the appended claims.