Patent Application
20220178584
The present invention relates to an electric heater to heat a flow of a fluid and in particular although not exclusively to an electric heater having a jacket block to accommodate a heating element including fins to positionally stabilise and centralise the heating element.
Electric heaters for heating gases to high temperatures typically include a tube adapted for the through-flow of the gas and an electrical heating element (positioned within the tube) to heat the flowing gas.
Conventionally, relatively fine wires are wound in a spiral configuration within the tube such that the heating effect is achieved by passing current through the wires as the gas flows through the tube. Accordingly, the effectiveness of the conversion of the electrical energy into heat (via the heating wire) depends for example on the available electrical voltage applied and the resistance of the wire. In particular, this conversion to heat energy is dependent on a maximum temperature achievable by the wire, the flow resistance and the surface area available for heat exchange. Typically, maximum gas temperatures that may be achieved by conventional electric process heaters may be of the order or around 700 to 900° C. However, the higher the temperature the greater the tendency for fracture and failure of the wire.
More recently, EP 2926623 discloses an electric flow heater in which the heating wire has a larger cross sectional surface area to provide a desired cross-sectional ratio with the tubular bore through which the rod extends. A single heating element extends through multiple bores (formed within elongate tubular elements) via a plurality of bent (or looped) ends. Gas heating temperatures of up to 1200° C. are described.
Whilst conventional electric flow heaters may be capable of achieving high temperatures of the order of 1100° C., high gas speeds and large pressure differentials cause tension and movement or vibration of the heating element and the surrounding tubes or jacket blocks through which the element extends. The heating element is susceptible to mechanical impact and stress which inevitably results in breakage. This phenomenon is even more pronounced when the elongate tube (jacket block) is orientated vertically where gravitational forces further contribute to the stresses and physical demands on the heating element.
Moreover, to maximise efficiency, the heating element is typically passed through multiple bores within a surrounding elongate jacket block. The element, having U-shaped bent axial end sections, emerges from and returns into adjacent bores at each axial end of the jacket block. Small positional deviation of the heating element, for example resultant from localised temperature variations within the bores, causes displacement of the bent end sections that can lead to the element (at the bend ends) contacting itself. This in turn causes a short circuit and failure of the electric heater. The risk of short circuiting is even greater for some of more recent heaters due to their very fine tolerances. Accordingly, what is required is an electric fluid flow heater that addresses these problems.
It is an objective of the present invention to provide an electric fluid flow heater which will, independently of the shape of a bore or a channel, provide excellent heating of a fluid, such as a gas. Furthermore, it is an objective of the present invention to provide a flow heater comprising straight rods which will elongate in lengthwise direction (of the fluid flow) which will provide the fluid flow heater with a controlled and efficient heat transfer without being dependent of an annular gap. Additionally, it is an objective to provide a flow heater having a low pressure drop while keeping a uniform heat transfer to the fluid.
It is a further objective of the present invention to provide an electric fluid flow heater to heat a fluid and in particular a gas (gas-phase medium) capable of achieving modest to high temperatures of the order of 700° C., such as up to 900° C., such as up to 1100° C. and potentially up to 1300° C. with minimised physical stress, fatigue and damage to the heating element so as to enhance the heater service lifetime.
It is a further objective to stabilise and centralise the heating element extending within at least one jacket element (that in turn forms an elongate jacket block) such that independent movement of the heating element relative to the jacket element of jacket block is minimised and preferably eliminated.
It is a further specific objective to stabilise the heating element positioned within the jacket block to avoid the heating element contacting itself at bent axial end sections that extend axially beyond the jacket block and longitudinal bores or channels extending through the jacket block.
It is a yet further specific objective to increase the efficiency of thermal transfer between the gas flowing through the jacket element (jacket block and elongate bores or channels) and the heating element to maximise efficiency of the electric heater.
Accordingly, an electric fluid flow heater is provided in which the heating element (at least partially accommodated within a jacket element and/or jacket block), is positionally stabilised internally within the heating element (jacket block) by a set of fins which are both stabilising and centering. Such fins project radially into each bore or channel (through which the heating element passes) so as to sit around the heating element and prevent lateral movement within each respective bore. Additionally, the fins will provide a homogenous temperature distribution because each channel or bore will be centred.
Accordingly, at least three stabilisation fins are provided internally within each bore or channel being the minimum number to achieve sufficient stabilisation and in particular centering of the heating element on the axial centre of each bore or channel. Accordingly, the heating element is prevented from radial (lateral) deviation within each bore or channel which in turn prevents the U-shaped looped ends of the heating element (that extend axially beyond the jacket element, jacket block) from contacting one another that would otherwise provide a short circuit and failure of the electric heater.
The present electric fluid flow heater will due to the fins have more free flow cross sectional area in the fluid flow heaters jacket element and/or jacket block, that typically have a square outer shape, this means that the more area within the electric fluid flow heater can be used for heating of the fluid and also indirect heating of the flow will be possible as heat will be transferred by radiation. Additionally, the fins will also provide for a proper centering of the jacket element and/or jacket block, leading to less exposure to failure, especially in cyclic operation.
According to a first aspect of the present invention that is provided an electric heater to heat a flow of a fluid comprising: at least one axially elongate jacket element defining an axially elongate jacket block having first and second lengthwise ends; a plurality of longitudinal bores or channels extending internally through the jacket block and being open at each of the respective first and second lengthwise ends each of the bores or channels defined by an internal facing surface of the at least one jacket element; at least one heating element extending axially and in particular extending axially straight through the bores or channels and having respective bent axial end sections such that the at least one heating element emerges from and returns into adjacent or neighbouring bores or channels at one or both the respective first and second lengthwise ends, the at least one heating element and the jacket block forming a heating assembly; characterised by: at least three fins projecting radially inward from the at least one jacket element towards the at least one heating element within each of the bores or channels. Thus, the present electric heater will be free-shape meaning that due to the fins, the bores or channels may have any shape and thereby fulfilling the cross sectional area ratio limits as described below.
According to one embodiment, the axially elongated jacket element may be a rod or a wire. According to another embodiment, the axially elongated jacket element has an axially straight alignment of at least 75% of its length. According to another embodiment, the whole elongated jacket element has a straight alignment.
The stabilisation and centering fins and in particular the cross sectional profile of the jacket element that defines each bore is adapted to maximise the efficiency of thermal energy transfer from the heating element to the flowing gas. This is achieved via the internal shape profile of each bore or channel that may be considered to comprise ‘lobes’ that are, in part, defined by the centering fins. In particular, in a cross sectional plane of the jacket element/jacket block, an internal facing surface of the bore or channel is distanced from an external facing surface of the heating element by a sufficient separation distance to enable the flow of a sufficient volume of gas evenly and uniformly around the heating element. According to one embodiment, the surface area ratio which is defined as the cross sectional area of the heating element divided by the cross sectional area of the bore or channel or cavity is in the rage of from 0.12 to 0.72.
The centering fins therefore provide directing or channeling of the flow of gas uniformly around the heating element. This is advantageous to avoid localised temperature differentials axially along and radially around the heating element that otherwise lead to stress and in particular bending or distortion of the heating element. As will be appreciated, this in turn provides or contributes to lateral displacement of the U-shaped end sections and increases the risk of short circuiting.
Moreover, the available free-flow surface area around the heating element will provide for a uniform and controlled heating and cooling along the cross sectional area of the heating element.
Optionally in a cross sectional plane through the jacket block a radial separation distance between the internal facing surface of each bore or channel and an external facing surface of the at least one heating element is non-uniform between each of the fins in a circumferential direction around the at least one heating element. Preferably, in said cross sectional plane, said separation distance is at a maximum at a position centrally between adjacent fins in a circumferential direction. Such an arrangement is advantageous to maximise the capacity and rate of gas flow through the bores or channels and avoid undesirable localised heating.
Reference within this specification to ‘fins’ and stabilising fins' and ‘centering fins’ and stabilising and centering fins' encompass ribs, ridges, splines and projections extending radially inwardly from the body of the jacket element/jacket block towards the heating element and may change its shape in axial direction or even partly disappear.
The fins may be linear or may be curved or bent in their longitudinal direction. Optionally the fins may be helical or part helical along their length. Such an arrangement may assist the control of the flow of gas through the heating assembly and reduce a tendency for localised heating variation along and around the heating element.
Reference within this specification to ‘at least one axially elongate jacket element’ and ‘axially elongate jacket block’ encompass a cover, a sleeve and other jacket-type elements having a length that is greater than a corresponding width or thickness so as to be ‘elongate’ in an axial direction of the heater. Reference to such ‘elongate’ elements and blocks encompasses bodies that are substantially continuously solid between their respective lengthwise ends and that do not comprise gaps, voids, spacings or other separations or between the lengthwise ends.
Preferably, the elongate jacket elements and elongate jacket blocks are substantially straight/linear bodies comprising at least one respective internal bore or channel to receive straight or linear sections of heating element. Accordingly, the present jacket elements and jacket blocks is configured to substantially encase surround, cover, house or contain the straight/linear sections of the heating element substantially along the length of the straight/linear sections between U-shaped bent or curved end sections of the heating element. Accordingly, it is preferred that the bent or curved sections of the heating element project only from and are not covered or housed by the heating element/jacket block.
Accordingly, reference within this specification to ‘jacket’ element and ‘jacket’ block encompass respective hollow bodies to contain, house, surround or jacket a heating element substantially continuously between the bent or curved end sections of the heating element that project from the respective lengthwise ends of the jacket element/block.
The effect of elongate jacket element and jacket block having a corresponding axially elongate internal bore or channel is to maximise the efficiency of thermal energy transfer between the heating element and the fluid flowing through the bore or channel in close confinement around the heating element. The lengthwise elongate configuration of the heating element and block provides that the flowing fluid is appropriately contained within the bore or channel around the heating element substantially the full length of the straight/linear section of heating element.
Within this specification, reference to the respective first and second lengthwise ends of a heating element that emerge from the bores or channels within the elongate heating element/jacket block, may be considered to be distinguished from the straight/linear sections of heating element that are housed continuously within the bore or channel of the element/block. As will be appreciated, almost all of the thermal transfer between heating element and fluid occurs within the elongate bore(s) or channel(s).
Preferably, the at least one jacket element (jacket block) comprises a non-electrically conducting material. Optionally, the jacket element (jacket block) is formed exclusively from a refractory or a ceramic material. Optionally, the jacket element may comprise a core material that is at least partially surrounded or encased by a refractory or a ceramic (i.e., non-electrically conducting) material formed as a coating at the external region of the jacket element and within each elongate bore or channel. Accordingly, the jacket element (jacket block) is configured to be heat resistant and electrically insulating.
Preferably, the heater comprises a plurality of the jacket elements assembled together as a unitary body and at least partially surrounded by spacers (that extend between the surrounding casing and the jacket block). The jacket elements are assembled and bound together as an assembly optionally via the spacers and/or other support members positioned at different regions along the length of the jacket block so as to positionally secure the jacket block relative to the casing and other components of the electric heater.
Optionally, the casing (alternatively termed a sheath) comprises a generally hollow cylindrical or hollow cuboidal shape encapsulating the heating assembly. Preferably, the spacers are attached to a radially inner surface of the casing. Optionally, the spacers may be welded to the inner surface of the casing for ease of manufacturing and to impart a structural strength to the heater. Accordingly, the spacers may be considered to form part of the casing.
Reference within this specification to ‘heating element’ encompasses relatively thin wires and larger cross sectional heating elements. Such a heating element, rod, bar or wire may comprise an iron-chromium-aluminium (Fe—Cr—Al) alloy or a nickel-chromium-iron (Ni—Cr—Fe) alloy but other suitable alloys or materials could also be used. In many practical cases the maximum internal spacing between the heating element and the internal facing surface that defines each bore or channel is from 0.5 to 20 mm, but may also fall within a broader range between 0.2 mm and 50 mm. Optionally, in particular a thicker heating element could in turn comprise a bundle of individual rods or wires which are optionally intertwined or twisted together. With such embodiments, the above-mentioned internal spacing is defined by the internal spacing between the bundle of rods or wires relative to regions of the internal facing surface that defines each longitudinal bore or channel being furthest separated from the heating element.
Optionally, a width of each of the fins in a circumferential direction decreases in a direction towards the at least one heating element. Optionally, each of the fins may be generally wedged shaped in a cross sectional plane of the bores or channels. Optionally, in the cross sectional plane of the bores or channels, each fin may comprise a polygonal, rectangular, square, triangular or semi-circular cross sectional shape profile.
Preferably, in said cross sectional plane the internal facing surface comprises curved regions and linear or planar regions. In particular, the internal facing surface of the bores or channels between the fins in a circumferential direction is not continuously curved. The bores or channels positioned in a circumferential direction between the stabilising fins project radially outward beyond an imaginary circle centred on and extending around the heating element (positioned within each bore or channel). Relative to this imaginary circle, each channel may be considered to be enlarged having a greater cross sectional area (by extending radially outward beyond the imaginary circle) so as to increase the available volume for the through-flow of gas. This configuration is beneficial to both enhance the energy transfer and heating capacity of the subject invention whilst reducing localised temperature deviation along the length of the heating element.
Optionally, the linear or planar regions of the internal facing surface represent regions of an imaginary polygon surrounding the at least one heating element, the imaginary polygon being interrupted in the circumferential direction by the fins. Optionally, in the circumferential direction, the curved regions are located at the position centrally between the adjacent fins and flanked at either side by the respective linear of planar regions. Preferably, in said cross sectional plane, a shape of the internal facing surface between the fins in a circumferential direction is non-continuously curved. Preferably, in said cross sectional plane, a shape of the internal facing surface between the fins in a circumferential direction is not formed exclusively by an arc of a circle having a radius larger than a radius of the at least one heating element. The leaf or petal shaped configuration of the bores or channels around the heating element controls the flow of gas and prevents undesirable localised heating variations.
Preferably, the fins extend over a majority of a length of each bore or channel between the first and second lengthwise ends. Preferably, the fins extend over a full length of each of the bores or channels between the first and second lengthwise ends. Preferably, each of the fins comprise the same depth in a radial direction towards the at least one heating element. Optionally, each of the fins project radially inward from a planar region of the internal facing surface. Preferably, in the cross sectional plane, each of the fins comprise a wedge shape profile with a thinnest part of each wedge positioned radially closest to the at least one heating element.
Optionally, the heater may comprise three, four, five or six fins projecting radially inward at each respective bore or channel. However, it will be possible to have more than six fins as the number of fins will depend on the design. Thus, as will be appreciated, any number of fins may be provided to achieve the desired directing of the flow of gas flowing through the heater, to centralise the heating element and to prevent undesirable temperature gradients around the heating element. The fins may be linear along their length or may be bent, curved or follow a non-linear direction along the length of the bores or channels. Optionally, the fins may be helical or part helical along their length between the respective ends of the heating element/jacket block.
Optionally, the electric heater may further comprise a casing positioned to at least partially surround the heating assembly and the casing comprises an outer sheath and a plurality of spacers extending radially between the outer sheath and the jacket block. Optionally, the spacers comprise a part-disc shaped member having a central aperture through which a part of the jacket block extends. Preferably, the heater may further comprise a plurality of the jacket elements assembled together as a unitary body and at least partially surrounded by the spacers. Optionally, the elongate jacket block may comprise a single elongate jacket element having the plurality of longitudinal bores or channels extending through the jacket block.
A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:
Referring to
A heating assembly indicated generally by reference 5 is mounted within chamber 4 and is formed from a plurality of lengthwise elongate jacket elements 6 assembled and held together to form a lengthwise elongate jacket block 7. Each elongate jacket element 6 comprises a lengthwise extending longitudinal internal bore or channel 8 extending the full length of each jacket element 6 so as to be open at a first and second axial end 7a, 7b of the jacket block 7. The jacket element 6 and jacket block 7 are formed as hollow bodies in which the solid mass and volume extends continuously between the first and second axial ends 7a, 7b. That is, the jacket elements 6 and jacket blocks 7 are not discontinuous between respective ends 7a, 7b. Such an arrangement is advantageous to maximise the extent and efficiency of thermal energy transfer within the respective jacket elements 6 as explained in further detail herein.
Jacket block 7 is mounted in position (within casing 2) via a pair of disc-shaped spacers 9a, 9b positioned in a lengthwise direction towards each jacket block axial end 7a, 7b. Sheath 3 and spacers 9a, 9b may be formed from metal such that spacers 9a, 9b are secured to an internal facing surface 3b of sheath 3 via welding. Each spacer 9a, 9b comprises a central aperture 10 having a rectangular shape profile and dimensioned to accommodate jacket block 7 that also comprises an external generally cuboidal shape profile. Accordingly, jacket block 7 is mounted within each spacer aperture 10 so as to be suspended within chamber 4 and spatially separated from sleeve internal facing surface 3b.
A heating element indicated generally by reference 11 is formed as an elongate wire (or rod) having respective ends 11d, 11e projecting generally from one of the axial ends of jacket block 7. Ends 11d, 11e are illustrated in
Referring to
As will be appreciated, the dimensions of the heating element 11 and bores or channels 8 are carefully controlled to achieve a desired small separation gap between an inward facing surface 13 of each bore or channel 8 and an external surface 24 of heating element 11. Such an arrangement is advantageous to maximise the effectiveness and efficiency of heat energy transfer from element 11 to a gas phase medium initially introduced into the chamber 4 at position 14a to then flow through each of the bore or channel 8 and exit from the heating assembly 5 at position 14b. This effectiveness and efficiency of heat energy transfer is also provided, in turn, by the heating elements 6 extending continuously lengthwise (axially) between respective ends 7a, 7b. In particular, heating element 11 is entirely and continuously housed, covered and contained by the elongate jacket elements 6 between ends 7a, 7b. When the electric heater 1 is suspended vertically in use, undesirable contact between the bent end sections 11a, 11b and the end faces 6c, and in particular the annular edges that define the entry and exit end of each bore or channel 8, contribute to fatigue and damage to the heating element 11 and a corresponding reduction in the service lifetime of the heater 1.
Advantageously and referring to
According to the specific implementation of
A set of gas-flow channels 40 are defined between each fin 25 in the circumferential direction around heating element. Each channel 40 is defined, in part, by the tapered side faces 32 of each fin 25, the transition faces 30, the planar faces 31, the arcuate corner surfaces 30 and the external surface 24 of heating element 11. The generally square or rectangular cross sectional profile of each bore or channel 8 (notwithstanding the presence of fins 25 and the rounding of the corners 29) serves to maximise the cross sectional surface area for the through flow of gas around heating element 11. This is important to maximise the energy transfer between heating element 11 and the flowing gas. This shape profile in addition to the presence of stabilising fins 25 is beneficial to control and direct the flow of the gas around the heating element 11 to avoid undesirable differential heating that would otherwise lead to bending and distortion of the heating element 11 in use. Stabilising fins 25 also provide a means of preventing large positional shifts of the heating element 11 within each bore or channel 8 as indicated. In the cross sectional plane of
As will be appreciated, whilst the subject invention is described with reference to a collection of heating elements 6 assembled together as a unitary body, the jacket block 7 may comprise a single body having a plurality of internal bores or channels 8 each provided with a shape profile and stabilisation fins 25 as described. The single jacket block 7 according to any such further implementations may be positionally stabilised within casing 2 via corresponding stabilising spaces 9a, 9b having appropriately sized apertures 10.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19164790.8 | Mar 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/058015 | 3/23/2020 | WO | 00 |
