Heat exchanger with a bond and a method for manufacturing the same

A heat exchanger having a first heat transfer tube with a first primary straight part and a first secondary straight part is provided. The heat exchanger includes a first primary bond part and a first secondary bond part. The first primary bond part is welded to the first secondary bond part to form a first primary bond that bonds the first primary straight part and the first secondary straight part of the first heat transfer tube. The first primary bond limits a first primary aperture and a first secondary aperture formed by the holes of the bond parts, wherein the straight parts of the first heat transfer tube extend through the first primary bond via the apertures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Application, filed under 35 U.S.C. § 371, of International Application No. PCT/FI2019/050363, filed May 9, 2019, which international application claims priority to and the benefit of Finland Application No. 20185466, filed May 21, 2018; the contents of both of which as are hereby incorporated by reference in their entireties.

BACKGROUND

Related Field

The invention relates to methods for manufacturing tube heat exchangers. The invention relates to heat exchangers particularly suitable for fluidized bed boilers. The invention relates to heat exchangers suitable for circulating fluidized bed boilers. The invention relates to fluidized bed heat exchangers. The invention relates to a heat exchanger for a loopseal of a circulating fluidized bed boiler. The invention relates to particle coolers.

Description of Related Art

A fluidized bed heat exchanger is known from U.S. Pat. No. 9,371,987. A heat transfer tube of the fluidized bed heat exchanger comprises straight parts and curved parts, whereby the heat transfer tube is configured to meander. Long tubes are not mechanically rigid, whereby they need to be mechanically supported in use. In the prior art document, walls of a space isolated from the fluidized bed provide for mechanical support for the tubes. In the alternative, the tubes could be supported to a wall of a furnace. From the document U.S. Pat. No. 8,141,502 it is known to support the tubes from beneath over substantially their whole length.

However, a structure wherein the walls support the tubes is hard to manufacture. The wall supporting the tubes may be provided with suitable apertures for the tubes. However, in such a manufacturing method, the tube needs to be assembled from multiple pieces; at least the straight parts and the curved parts, which are welded together. Welding, even if a well-known process, is somewhat burdensome, since the heat transfer tube needs to withstand a pressure of the order of 120 bar and a temperature of the order of 600° C.

BRIEF SUMMARY

The present invention aims at providing a mechanical support for heat transfer tubes of a heat exchanger, which support can be easily manufactured. The support, i.e. a bond, is disclosed in the description. A heat exchanger with such a bond is disclosed in an independent claim. A method for manufacturing such a heat exchanger is disclosed in an independent claim. The bond is suitable for use with a heat transfer tube or tubes that are bent at some locations. The bond is suitable for use with a heat transfer tube or tubes that need not be assembled or further assembled.

To illustrate different views of the embodiments, three orthogonal directions Sx, Sy, and Sz are indicated in the figures. Preferably, in use, the direction Sz is substantially vertical and upwards. In this way, the direction Sz is substantially reverse to gravity. A direction ShinFIG.10refers to a horizontal direction, which is perpendicular to Sz.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

FIG.1ashows a circulating fluidized bed boiler1in a side view. The circulating fluidized bed boiler1comprises a furnace50, a cyclone40, which is a means40for separating bed material from flue gas, and a loopseal5. the loopseal5is configured to receive bed material from the cyclone40. InFIG.1a, a flue gas channel is indicated by the reference number20. Flue gas is expelled from the furnace50via the flue gas channel20.

FIG.1bshows a bubbling fluidized bed boiler1in a side view. The bubbling fluidized bed boiler1comprises a furnace50, and a flue gas channel20.

Typically, the fluidized bed boiler1(bubbling or circulating) comprises flue gas heat exchangers26,28within the flue gas channel20. The flue gas heat exchangers26,28are configured to recover heat from flue gases. Some of the flue gas heat exchangers may be superheaters26configured to superheat steam by recovering heat from flue gas. Some of the heat exchangers may be economizers28configured to heat and/or boil water by recovering heat from flue gas.

In a circulating fluidized bed boiler (FIG.1a), bed material is conveyed from an upper part of the furnace50to the cyclone40in order to separate the bed material from gases. From the cyclone40, the bed material falls through a channel60to a loopseal5. In the loopseal5, a layer of bed material is formed. The bed material is returned from the loopseal5to the furnace50via a pipeline15. In the loopseal5, the walls51of the loopseal5limit a volume V into which a fluidized bed of the circulating bed material is arranged. In a bubbling fluidized bed boiler (FIG.1b), the bed material is fluidized in the furnace50. Thus, the walls51of the furnace50limit a volume V into which a fluidized bed of the bed material is arranged.

In general, a fluidized bed boiler1comprises piping for heat transfer medium. In use, the heat transfer medium circulates in the piping and becomes heated by heat exchangers, in particular the flue gas heat exchangers26,28and the fluidized bed heat exchanger10. The piping forms a circulation for heat transfer medium. In the circulation, the same heat transfer medium may flow in between the flue gas heat exchangers26,28and the fluidized bed heat exchanger10. Typically the circulation is formed such that the heat exchange medium is first heated in the economizers28and thereafter in the superheaters26. Moreover, after the superheaters26, the heat exchange medium is heated in the fluidized bed heat exchanger10. Thereafter, the medium (e.g. superheated steam) is typically conveyed to a steam turbine.

The present invention relates in particular to a structure of a heat exchanger and a method for manufacturing such a heat exchanger. In a preferably use, the heat exchanger is arranged in a fluidized bed, such as in the loopseal5of a circulating fluidized bed boiler or in the furnace of a bubbling fluidized bed boiler. In general, a heat exchanger comprises a number of tubes, in which a first heat transfer medium, such as water and/or steam, is configured to flow. Outside the tubes, second heat transfer medium, such as bed material, is configured to flow, whereby heat is transferred from the second heat transfer medium to the first heat transfer medium through a wall of the tube. The heat exchanger10, which, when installed in a fluidized bed, forms a fluidized bed heat exchanger10, can be manufactured a as a part of a boiler or as a spare part for the boiler. Thus, an embodiment concerns a heat exchanger10. In addition, an embodiment concerns fluidized bed boiler1.

In this description, the following terms are used:

A heat transfer tube refers to a tube. The heat transfer tube may be made from only one substantially homogeneous material, e.g. metal, such as steel. When considered feasible a heat transfer tube may is referred to as a “plain heat transfer tube” to distinct from a “coaxial heat transfer tube”. A plain heat transfer tube may consist of some metal, since metals in general conduct heat well.

A coaxial heat transfer tube refers to an arrangement of heat transfer tubes, in which a laterally outermost heat transfer tube encircles an inner heat transfer tube. A coaxial heat transfer tube is an arrangement of heat transfer tubes (typically only two heat transfer tubes) that are mutually coaxial.

A straight part refers to such a part of a heat transfer tube (plain tube or coaxial tube), that has been obtained from a tube manufacturer, and has not been bent. Commonly, tube manufacturers supply straight rigid tubes. In terms of a radius of curvature, a radius of curvature rs(seeFIG.2a) of a central line of the straight part is at least 1 meter (1 m). A radius of curvature rsof a straight part may be infinite or substantially infinite.

A curved part refers to such a part of a heat transfer tube (plain or coaxial), that has been bent. In terms of a radius of curvature, a radius of curvature rc(seeFIG.2a) of a central line of the curved part is less than 1 meter (1 m). Preferably, a radius of curvature rcof a curved part is at least three times a diameter of the heat transfer tube.

FIG.2ashows a heat transfer tube100, i.e. a first heat transfer tube100in a side view. A heat exchanger10of the present invention comprises a first heat transfer tube100. As indicated inFIG.2a, the first heat transfer tube100comprises a first primary straight part101, a first primary curved part102, a first secondary straight part103, a first secondary curved part104, a first tertiary straight part105, and also a further (i.e. tertiary) curved part106and a further (i.e. quaternary) straight part107. At least a curved part is left in between two straight parts of the tube100in the direction of extension of the tube such that the straight parts of the first heat transfer tube100extend parallel in a first plane P in a longitudinal direction dl. InFIG.2a, the direction of flow of heat transfer medium within the tube100in the first primary straight part101is reverse to the direction of flow of heat transfer medium within the tube in the first secondary straight part103. This is also reverse to the direction of flow of heat transfer medium within the tube in the first tertiary straight part105.FIG.2ashows also a distributor header142configured to feed heat transfer medium into the first heat transfer tube100and optionally into other heat transfer tubes of the heat exchanger10.FIG.2ashows also a collector header144configured to collect heat transfer medium from the first heat transfer tube100, and optionally from other heat transfer tubes of the heat exchanger10.

A heat exchanger10may be modular, i.e. insertable into e.g. a boiler1and removable therefrom. For reasons of handling such a heat exchanger10, the heat transfer tube100is preferably mechanically supported. For this reason, a heat exchanger10is equipped with a first primary bond530as shown inFIG.2b. Preferably, the heat exchanger10is equipped also with a first secondary bond540as shown inFIG.2b. A distance dbondis left in between the first primary bond530and the first secondary bond in the longitudinal direction dl, as indicated inFIG.11a. The distance dbondmay be e.g. at least 50 cm, such as at least 1 m. A sufficiently large distance improves the mechanical stability of the heat exchanger.

The first primary bond530and the first secondary bond540may be manufactured following the principles presented later in this application. They may be structurally identical. The first primary bond530binds at least two parts of at least one heat transfer tube together to support the heat transfer tube(s). In an embodiment, the first primary bond530is supported or configured to be supported to a supportive structure of a boiler. For example, the first primary bond530may be supported, e.g. connected, to a floor or a beam of a boiler in the space V. In an embodiment, the first primary bond530supported or configured to be supported to a supportive structure underneath the heat transfer tube(s)100,200. In such an embodiment, the bond530should bear a part of the weight of the heat transfer tubes.

The first primary bond530comprises a first primary bond part510and a first secondary bond part520.FIG.3ashows the sectional view IIIa-IIIa ofFIG.2b. Thus, in typical use,FIG.3ais a top view of the first primary bond530and the first primary straight part101of the tube100.

Referring toFIG.3a, the first primary bond part510comprises a first primary surface511. In an embodiment, the whole first primary surface511is planar. In the embodiment ofFIG.3a, the first primary surface511faces to the longitudinal direction dlof the first straight parts (101,103) of the tube100. However, as indicated inFIG.4b, this is not necessary. The first primary bond part510comprises a first secondary surface512opposite the first primary surface511. In an embodiment, the whole first secondary surface512is planar. InFIG.3a, the first secondary surface512faces to a direction −dl, which is reverse to the longitudinal direction dl. The first primary bond part510comprises a first tertiary surface513. As will be discussed later, the first tertiary surface513may be manufactured by cutting. Thus, an angle between a normal of the first tertiary surface513and a normal of the first primary surface511depends on how the first primary bond part510has been manufactured, e.g. cut from a plate.

The first primary bond part510and the first straight parts (101,103) of the tube100are arranged with respect to each other in such a way that a part the first tertiary surface513faces towards the first primary straight part101and a part the first tertiary surface513faces towards the first secondary straight part103. In particular, surfaces of the holes514,515will face the straight parts101,103, as detailed below. This has the effect that the parts of the tube100can be fitted to the holes514,515. Thus, at least a part of the first tertiary surface513faces in a direction of a normal N of the first plane P. The first tertiary surface513connects the first primary surface511and the first secondary surface512. Preferably, at each point of the first tertiary surface513, a tangential direction of the first tertiary surface513is a direction within the plane P, as indicated inFIG.3a. However, apart from the surfaces of the holes514,515, the first tertiary surface may be arranged at a different angle relative to the plane P (not shown). Still, as indicated inFIGS.4aand4b, preferably, all planar parts of first tertiary surface513faces in a direction of a normal N of the first plane P. Preferably also, at all points, the first tertiary surface513has a normal that belongs to a plane, of which normal is unidirectional with the longitudinal direction dl(seeFIGS.4aand4b).

Referring now toFIGS.3band3c, a first primary hole514is arranged on the on the first tertiary surface513. The first primary hole514is configured to receive a part of the first primary straight part101. Thus, the shape of the first primary hole514is adapted, i.e. fitted, to the outer surface of the first primary straight part101. In this way, the first primary bond part510limits, on the first tertiary surface513, a first primary hole514extending through the first primary bond part510from the first primary surface511to the first secondary surface512in the longitudinal direction dl. Moreover, a part of the first primary straight part101is arranged into the first primary hole514, as indicated inFIG.3b. The first primary hole514forms a part of a first primary aperture533.

In a similar manner, a first secondary hole515is arranged on the on the first tertiary surface513. The first secondary hole515is configured to receive a part of the first secondary straight part103. Thus, the shape of the first secondary hole515is adapted, i.e. fitted, to the outer surface of the first secondary straight part103. In this way, the first primary bond part510limits, on the first tertiary surface513, a first secondary hole515extending through the first primary bond part510from the first primary surface511to the first secondary surface512in the longitudinal direction dl. Moreover, a part of the first secondary straight part103is arranged into the first secondary hole515, as indicated inFIG.3b. The first secondary hole515forms a part of a first secondary aperture534.

The holes514and515, and also524,525, which will be defined later, are indentations on the surface513(or523), defining apertures of the bond530for receiving a part of a heat transfer tube; in particular a part of a straight part thereof. The shape(s) of the hole(s) is/are adapted, i.e. fitted, to the corresponding part(s) of a tube or tubes in such a way, that in use, essentially no gap is left in between the tertiary surface513,523and an outer surface of the tube. For reasons of manufacturing tolerance, a gap having a width of at most 0.5 mm may be left at some points in between a surface of a hole (514,515,524,525) and an outer surface of a part (101,103) of a tube100. Thus, even ifFIG.3bshows a gap in between the tube parts (101,103) and the holes (514,515,524,525) for reasons of presentation, preferably no such gap is present in the heat exchanger. A small gap or no gap at all improves the wear resistance of the tube100, since in such case movements between the bond parts510,520and the tube parts101,103are reduced, which reduces wear of the tube100or tubes100,200,300,400.

In order to bind the first straight parts (101,103) together, the first primary bond part510extends from the first secondary hole515to the first primary hole514.

Referring toFIG.3a, the first secondary bond part520comprises a second primary surface521. In an embodiment, the whole second primary surface521is planar. InFIG.3athe second primary surface521faces to the longitudinal direction dlof the first straight parts (101,103) of the tube100. However, as indicated above, this is not necessary. The first secondary bond part520comprises a second secondary surface522opposite the second primary surface521. In an embodiment, the whole second secondary surface522is planar. Thus, inFIG.3a, the second secondary surface522faces to a direction −dl, which is reverse to the longitudinal direction dl. The first secondary bond part520comprises a second tertiary surface523. The first secondary bond part520and the first straight parts (101,103) of the tube100are arranged with respect to each other in such a way that at least parts of the second tertiary surface523face towards the first straight parts (101,103). At least parts of the second tertiary surface523also face in a direction of a normal N of the first plane P. The second tertiary surface523connects the second primary surface521and the second secondary surface522. Preferably, at each point of the second tertiary surface523, a tangential direction of the second tertiary surface523is a direction within the plane P, as indicated inFIG.3a. As for the first tertiary surface513, preferably, all planar parts of the second tertiary surface523faces in a direction of a normal N of the first plane P. Preferably also, at all points, the second tertiary surface523has a normal that belongs to a plane, of which normal is unidirectional with the longitudinal direction dl(seeFIGS.4aand4b).

Referring now toFIGS.3band3c, a second primary hole524is arranged on the on the second tertiary surface523. The second primary hole524is configured to receive a part of the first primary straight part101. Thus, the shape of the second primary hole524is adapted to the outer surface of the first primary straight part101. In this way, the first secondary bond part520limits, on the second tertiary surface523, a second primary hole524extending through the first secondary bond part520from the second primary surface521to the second secondary surface522in the longitudinal direction di. Moreover, a part of the first primary straight part101is arranged into the second primary hole524, as indicated inFIG.3b. The second primary hole524forms a part of the first primary aperture533.

In a similar manner, a second secondary hole525is arranged on the on the second tertiary surface523. The second secondary hole525is configured to receive a part of the first secondary straight part103. Thus, the shape of the second secondary hole525is adapted to the outer surface of the first secondary straight part103. In this way, the first secondary bond part520limits, on the second tertiary surface523, a second secondary hole525extending through the first secondary bond part520from the second primary surface521to the second secondary surface522in the longitudinal direction di. Moreover, a part of the first secondary straight part103is arranged into the second secondary hole525, as indicated inFIG.3b. The second secondary hole525forms a part of a first secondary aperture534.

In order to bind the first straight parts (101,103) together, the first secondary bond part520extends from the second secondary hole525to the second primary hole524.

In the heat exchanger10, the first primary bond part510has been welded to the first secondary bond part520to form a first primary bond530that bonds the parts of the first heat exchanger tube100. When welded together, the first primary hole514and the second primary hole524in combination form the first primary aperture533of the first primary bond530, through which the first primary straight part in particular101of the heat transfer tube100extends. A shape of the first primary aperture533is adapted to a shape of an outer surface of the straight part101of the heat transfer tube100. In a similar manner, the first secondary hole515and the second secondary hole525in combination form a first secondary aperture534of the first primary bond530, through which the straight part103of the heat transfer tube100extends. A shape of the first secondary aperture534is adapted to a shape of the outer surface of the straight part103of the heat transfer tube100. In this way, in an embodiment, a curved part (e.g.102) of the first heat transfer tube100does not extend through the first primary bond530. In this way, in an embodiment, a curved part (e.g.102) of the first heat transfer tube100does not extend within the bond530.

As indicated above and inFIG.4a, in the embodiment, the first primary surface511has a normal N511that is parallel with the longitudinal direction dlof the first straight parts (101,103) of the tube100. Such a structure may be manufactured e.g. by forming the first tertiary surface513by cutting from a plate500in a direction of a normal on the plate500. However, the first tertiary surface513may be cut at a different angle. In addition or alternatively, if the first tertiary surface513is cut by using a fluid jet, the first tertiary surface513is not perpendicular to the main surface501of the plate500(seeFIG.12). Referring toFIG.4b, in such a case, the first primary surface511has a normal N511that forms an angle ϕ with the longitudinal direction dlof the first straight parts (101,103) of the tube100. However, preferably the normal N511of the first primary surface511is substantially parallel with the longitudinal direction dlof the first straight parts (101,103) of the tube100. More specifically in an embodiment, [i] the surface normal N511is parallel with the longitudinal direction dlor [ii] the surface normal N511forms an angle ϕ with the longitudinal direction dl, wherein the angle ϕ is less than 45 degrees, such as less than 30 degrees or less than 15 degrees, preferably less than 5 degrees. A small angle makes it easier to assemble the bond530.

As shown inFIG.2b, preferably the heat exchanger10comprises a first secondary bond540. The first secondary bond540may be manufactured in a similar manner as the first primary bond530. Also the first secondary bond540is configured to bind together at least the first primary straight part101and the first secondary straight part103. InFIG.2b, the first secondary bond540binds also the first tertiary straight part103and the first quaternary straight part107together.

When manufacturing such a heat exchanger10, a first heat transfer tube100as detailed above and/or below is arranged available. The tube100may be manufactured e.g. by bending or the tube100may be e.g. bought. The first primary bond part510and the first secondary bond part520may be cut from a plate500, as indicated inFIG.12. The plate500has a thickness tp. The thickness tpis oriented in a direction dtpof thickness tpof the plate500, as shown inFIG.12. Cutting lines are shown inFIG.12in grey colour. When cut through the lines, the first primary bond part510and the first secondary bond part520are formed. These parts are shown inFIG.12. As indicated above, the cutting lines may extend through the plate500in a direction of the thickness tpof the plate500, or the cutting lines may be arranged at an angle relative to the direction of the thickness tp. Naturally, it would be possible to cut the first primary bond part510from the plate500and the first secondary bond part520from another plate.

Initially the plate500has a main surface501, which has a surface normal that is parallel to direction dtpof thickness of the plate500. Typically, the main surface501of the plate501is planar. In addition, typically a surface opposite to the main surface501is also planar. Since the bond530needs to have sufficient mechanical strength, the first primary bond part510is cut from the plate500such that a part of the main surface501forms either the first primary surface511or the first secondary surface512. At least a part of the first tertiary surface513is formed by said cutting. In an embodiment, the resulting first tertiary surface faces in a direction that is perpendicular or substantially perpendicular to the direction dtpof thickness of the plate500. The term “substantially perpendicular” may refer to an angle of (at most 90 degrees and) more than 45 degrees, such as more than 60 degrees or more than 75 degrees, preferably more than 85 degrees; in line with the aforementioned angle ϕ. While forming at least a part of the first tertiary surface513, also the first primary hole514and the first secondary hole515are formed by the cutting. As a result, the method comprises forming a first primary hole514that is configured to receive a part of the first primary straight part101of the first heat transfer tube100. The shape of the hole514is adapted to the surface of the part101as discussed above. Moreover, method comprises forming a first secondary hole515that is configured to receive a part of the first secondary straight part103of the first heat transfer tube100. The shape of the hole515is adapted to the surface of the part103as discussed above.

The first secondary bond part520is cut from the plate500(or a second plate) in a similar manner. The first secondary bond part520is cut from the plate500such that a part of the main surface501(or a main surface of the second plate) forms either the second primary surface521or the second secondary surface522. Moreover, at least a part of the second tertiary surface523is formed by said cutting. In an embodiment, the resulting second tertiary surface faces in a direction that is perpendicular or substantially perpendicular to the direction dtpof thickness of the plate500or the second plate. The term “substantially perpendicular” may refer to an angle of (at most 90 degrees and) more than 45 degrees, such as more than 60 degrees or more than 75 degrees, preferably more than 85 degrees; in line with the aforementioned angle ϕ. While forming at least a part of the second tertiary surface523, also the second primary hole524and the second secondary hole525are formed by the cutting. As a result, the method comprises forming a second primary hole524that is configured to receive a part of the first primary straight part101of the first heat transfer tube100. The shape of the hole524is adapted to the surface of the part101as discussed above. Moreover, method comprises forming a second secondary hole525that is configured to receive a part of the first secondary straight part103of the first heat transfer tube100. The shape of the hole515is adapted to the surface of the part103as discussed above.

After forming said holes514,515,524,525, parts of the straight parts101,103are arranged in the holes as indicated inFIG.3b. Consequently, in an embodiment the tube100and the parts510,520are arranged in such a way that a direction of thickness of the first primary bond part510and the longitudinal direction dlare parallel or form the aforementioned angle ϕ. Moreover, a direction of thickness of the first secondary bond part520and the longitudinal direction dlare parallel or form an angle of e.g. less than 45 degrees in line with what has indicated for the angle ϕ. As indicated inFIG.12, during cutting, the direction of thickness of the first primary bond part510is parallel to the direction dtpof the plate500. Moreover, during cutting, the direction of thickness of the first secondary bond part520is parallel to the direction dtpof the plate500of the second plate.

Thereafter, the first primary bond part510is welded to the first secondary bond part520to form the first primary bond530. The first primary bond530bonds at least the straight parts (101,103) of the first heat exchanger tube100together. A first secondary bond540may be manufactured in a similar manner.

The plate500may be cut by using a laser. In addition or alternatively, the plate500may be cut by using a fluid jet, e.g. a liquid jet or a gas jet. The effect of a fluid jet may be improved by using abrasive particles, such as sand. Using a fluid jet for cutting may have the effect that the tertiary surface513is not perpendicular to the first surface511.

Preferably, the plate500comprises weldable metal having a melting point of at least 1000° C. Such metals are typically mechanically strong. Examples of suitable such metals include steel, such as austenitic steel. In a heat exchanger10, the bond parts510,520comprise such a material as discussed for the plate500.

To have a sufficient mechanical stability, preferably, the thickness tpof the plate is from 15 mm to 40 mm. Correspondingly and with reference toFIG.5a, in an embodiment of the heat exchanger10, the first primary bond part510has a first primary thickness tl1in a direction of a normal N511of the first primary surface511, wherein the first primary thickness tl1is from 15 mm to 40 mm. When the first primary bond part510is made from a plate500, the first primary thickness tl1is constant. Moreover, the first secondary bond part520has a second primary thickness tl2in a direction of a normal N521of the second primary surface521, wherein the second primary thickness tl2is from 15 mm to 40 mm. When the first secondary bond part520is made from the plate500or another plate, the second primary thickness tl2is constant. When the bond parts510,520are made from the same plate500, the first primary thickness tl1equals the second primary thickness tl2. However, as indicated above, the parts510,520need not be made of the same plate500.

The mechanical stability can be affected also by selecting the thickness and other dimensions of the bond parts510,520. However, if the bond parts510,520are large, different parts of heat transfer tubes must be are arranged far away from each other, whereby the size of the heat exchanger10increases. Typically, a ratio of the surface area of the heat transfer tubes100,200,300,100b,200bto the volume of the heat exchanger10is maximized for good heat recovery. From point of view of these considerations, the aforementioned thickness has been found particularly suitable, in particular, when the bond530comprises steel.

As for the other dimensions of the bond parts510,520, also the other dimensions should be reasonable large to have the mechanical supportive function and reasonably small for a compact heat exchanger. In particular, in some uses, the bond(s)530and/or540are used to mechanically support the tube(s)100,200,300,400from below and against gravitational forces of the tube(s). Therefore, a thin, e.g. plate-like, bond would not provide sufficient support. However, if the bond(s)530,540are used to hang the tubes, a thinner bond or bonds could suffice. Thus, and with reference toFIG.5, in a preferable embodiment, in between the first primary hole514and the first secondary hole515, the first primary bond part510has a first secondary thickness tt1in a direction that is perpendicular to a normal N511of the first primary surface511and forms a minimum angle with a normal N of the first plane P. It is noted that all directions of the first primary surface511are perpendicular to the normal N511. Moreover, each one of the directions of the first primary surface511forms an angle (optionally zero degrees) with a normal N of the first plane P. Thus, only one direction of the first primary surface511form a minimum angle (optionally zero degrees) with a normal N of the first plane P. The directions of thicknesses are clarified inFIGS.4band5a.

The first secondary thickness tt1needs not be constant, but may depend e.g. on the level (e.g. height) of measuring the thickness; as indicated e.g. inFIG.3c. From point of view of mechanical support, a minimum tt1,minof the first secondary thickness tt1, i.e. a minimum first secondary thickness tt1,minmay determine the supportive capability of the bond530. Therefore, in an embodiment the minimum first secondary thickness tt1,minis from 10 mm to 50 mm, preferably from 15 mm to 50 mm. The first secondary thickness tt1may depend on location, as seen fromFIG.3c. Thus, in an embodiment, in between the first primary hole514and the first secondary hole515, the first primary bond part510has only such first secondary thicknesses tt1that are from 10 mm to 50 mm. In other words, a maximum tt1,maxof the first secondary thickness tt1may be at most 50 mm. This helps to keep the manufacturing costs at reasonable level and the heat exchanger reasonably small.

In a similar manner, in a preferable embodiment, in between the second primary hole524and the second secondary hole525, the first secondary bond part520has a second secondary thickness tt2in a direction that is perpendicular to a normal N521of the second primary surface521and forms a minimum angle with a normal N of the first plane P. The second secondary thickness tt2has a minimum value, minimum second secondary thickness tt2,minand a maximum value tt2,max. In an embodiment, the minimum second secondary thickness tt2,minis from 10 mm to 50 mm, preferably from 15 mm to 50 mm. The second secondary thickness tt2may depend on location. Thus, in an embodiment, in between the second primary hole524and the second secondary hole525, the first secondary bond part520has only such secondary thicknesses tt2that are from 10 mm to 50 mm. In other words, a maximum of the second secondary thickness tt2,maxmay be at most 50 mm.

Regardless of whether the bond parts510,520are made from the same plate500or from different plates, the second thicknesses (tt1, tt2) may be different from each other. However, preferably the second primary thickness tt1equals the second secondary thickness tt2at least locally, i.e. at a certain location (e.g. level in the Sz direction, seeFIG.3c). This prevents the bond530,540from warping in use; or at least diminishes the tendency or warping in a hot environment.

The bond parts510,520are most preferably welded together as indicated inFIGS.3a,3b,3c,4a,4b,5a, and6a. Preferably, the heat exchanger10comprises a first welding joint531that joins the second tertiary surface523to the first primary surface511(seeFIG.4a) or the first secondary surface512(seeFIG.3a). Preferably, the heat exchanger10comprises a second welding joint532that joins the first tertiary surface513to the second secondary surface522(seeFIG.4a) or the second primary surface521(seeFIG.3a). The welding joints531,532are evidence of welding. Thus, an embodiment of the method comprises welding the second tertiary surface523to the first primary surface511or the first secondary surface512. Furthermore, an embodiment of the method comprises welding the first tertiary surface513to the second secondary surface522or the second primary surface521. It has been found that welding is this manner also prevents the bond530,540from warping in use; or at least diminishes the tendency or warping in a hot environment.

In order to have sufficiently strong joint in between the bond parts510,520, the welding joins531,532should be sufficiently long. Therefore, and with reference toFIG.3c, in an embodiment the first welding joint531extends in a direction dextthat is a direction within the first plane P and perpendicular to the longitudinal direction dl. In an embodiment the first welding joint531extends in between the first primary straight part101and the first secondary straight part103. Preferably the first welding joint531extends in this direction, and optionally also in the aforementioned location, at least 5 cm. In a similar manner, in an embodiment, the second welding joint532extends in the direction dext. In an embodiment, the second welding joint532extends in between the first primary straight part101and the first secondary straight part103. Preferably the second welding joint532extends in this direction, and optionally also in the aforementioned location, at least 5 cm. In a preferable embodiment, the welding joints531,532do not extend fully to either of the straight parts101,103of the tube100. Correspondingly, preferably, a distance of at least 1 mm is left in between the first welding joint531and both the straight parts101,103and a distance of at least 1 mm is left in between the second welding joint532and both the straight parts101,103. This has the effect that the welding of the bond parts510,520together does not affect the mechanical properties, in particular the capability of withstanding high pressure, of the heat transfer tube100.

As indicated above, such welding diminishes warping of the bond. In addition to the measures of the bond parts and the type of welding, the tendency of warping can be affected by the relative positioning of the first primary bond part510and the second primary bond part520. Referring toFIG.5a, a length ltotof the first primary bond530in a direction of a normal N511of the first primary surface511is defined by the first primary thickness tl1, the second primary thickness tl2, and the overlapping distance do. Mathematically: ltot=tl1+tl2−do. Typically the normals N511and N512of the primary surfaces511,521are unidirectional.

When there is at least partial overlap, as inFIGS.5aand5b, a part of the first tertiary surface513faces a part of the second tertiary surface523. If there was a full overlap, as inFIG.5b, the overlapping distance dowould equal ltot. In such a case, the aforementioned surfaces would not be welded to each other. In contrast, in such a case, e.g. the surface512would be welded to the surface522and the surface511would be welded to the surface521(seeFIG.4afor the surfaces). However, such welding would be harder to perform in a reliable manner. Thus, at least a risk of warping, would be increased. Moreover, it has been found that, if welded in this manner warping of the bond530would occur. Warping of the bond530is use may be a result of thermal expansion of the bond parts510,520and the tube(s)100,200,300,400.

If there was no overlap, as inFIG.5c, the overlapping distance dowould be zero, and correspondingly the thickness ltotof the bond530would be the sum of thicknesses of its parts. Moreover, as indicated inFIG.5c, not even a part of the first tertiary surface513faces a part of the second tertiary surface523. In such a case, the aforementioned surfaces would be welded to each other. This solution is as easily manufacturable as the one ofFIG.5a. However, it has been found that the warping of the bond is minimized by a partial overlap (FIG.5a), and is the worst by a full overlap (FIG.5b). Therefore, preferably the overlapping distance dois not zero. In other words, preferably a total thickness ttotof the first primary bond530in the longitudinal direction dlis less than the sum tl1+tl2of the thicknesses tl1, tl2of the first bond parts510,520in the direction of the normal N511of the first primary surface511. Preferably, the overlapping distance dois from 10% to 90% of the smaller of the thicknesses tl1and tl2of the bond parts510,520. More preferably, the overlapping distance dois from 25% to 75%, such as from 33% to 66% of the smaller of the thicknesses tit and tit of the bond parts510,520.

As indicated above, the movement of the tube parts101,103relative to the bond530is diminished primarily by tight fitting of the tube parts101,103to the apertures533,534formed by the holes514,515,524,525. Moreover, secondarily, the movement can be further diminished by providing stoppers onto some of the surfaces of the tube parts. For this reason, and with reference toFIGS.6ato6c, in an embodiment the first primary straight part101of the tube100is equipped with a first primary stopper131and a first secondary stopper132. As indicated inFIG.6afor the tube part101, at least part of the first primary bond530is left in between the first primary stopper131and the first secondary stopper132. In this way, the stoppers131,132prevent the movement of the tube part101relative to the bond530; at least when the stoppers131,132are arranged in such a way that the first primary stopper131contacts the first primary bond530and/or the first secondary stopper132contacts the first primary bond530. In a similar manner the first secondary straight part103can be locked to the bond530. Thus, in an embodiment, the first secondary straight part103of the tube100is equipped with a second primary stopper133and a second secondary stopper134, as seen fromFIGS.6band6c. At least part of the first primary bond530is left in between the second primary stopper133and the second secondary stopper134. However, all the straight parts of the tube(s) need not to be equipped with stopper. Thus, e.g. the first secondary straight part103need not be locked to the bond530by the stoppers134,134. In case the heat exchanger comprises a second heat transfer tube200, at least one of its straight parts may be equipped with stoppers (not shown).

Referring toFIG.3c, the first primary bond part510has a first quaternary surface514that forms an angle with the first primary surface511. This angle may be, but need not be, straight. The first quaternary surface514also forms an angle with the first tertiary surface513. This angle may be, but need not be, straight. The first quaternary surface514may be planar. The first quaternary surface514also faces away from the first heat transfer tube100. In a similar manner, the first secondary bond part520has a second quaternary surface524that forms an angle with the second primary surface521and with the second tertiary surface523and faces away from the first heat transfer tube100. In an embodiment, the quaternary surfaces514and524are not welded together. This has the effect, that when a bridge551,552(seeFIG.11c) is used to connect two different bonds together, the bond530can be more easily fixed to a bridge551,552from an end of the bond530, when an end of the bond530is free from a welding joint. For example a bridge551,552may be equipped with holes configured to receive the bond530, in particular the quaternary surfaces514,524.

Referring toFIGS.2band3b, in an embodiment, the first heat transfer tube100comprises one or more other straight parts (105,107), including e.g. the first tertiary straight part105. In an embodiment, the first primary straight part101, the first secondary straight part103, and the other straight parts (105,107) extend parallel in the first plane P in the longitudinal direction dl. Preferably the tube100is designed in such a way that both the distributor header142and the collector header144are left on the same side of the heat exchanger10, e.g. on the same side of the tubes (100,200,300,400), as indicated e.g. inFIG.11b. Referring toFIG.3b, in such a case, the first tertiary surface513of the first primary bond part510is provided with one or more other holes516,517extending through the first primary bond part510in the longitudinal direction dl. Moreover, a part or parts of the other straight part or parts105,107is/are arranged into the other hole or holes516,517of the first primary bond part510. In a similar manner, the second tertiary surface523of the first secondary bond part520is provided with one or more other holes526,527extending through the first secondary bond part520in the longitudinal direction dl. Moreover, a part or parts of the other straight part or parts105,107is/are arranged into the other hole or holes526,527of the first secondary bond part520. The other holes form other apertures in manner discussed e.g. for the first primary aperture533, and a straight part of one of the other straight parts (105,107) extend through one of these other apertures.

However, it may be feasible to use more than one heat transfer tubes side by side in such a way that the same bond530is used to bond more than one heat transfer tubes. With reference toFIG.8a, in an embodiment, the heat exchanger10comprises a second heat transfer tube200. The second heat transfer tube200comprises a second primary straight part201, a second primary curved part202, and a second secondary straight part203. Also the second straight parts201,203extend mutually in parallel and in the first plane P, wherein also the first straight parts101,103extend. Moreover, the second straight parts201,203extend in parallel with the first straight parts101,103in the longitudinal direction dl. Referring toFIG.8b, when such tubes100,200are used, the first tertiary surface513of the first primary bond part510is provided with two or more other holes516,517extending through the first primary bond part510in the longitudinal direction dl. Moreover, a part of the second primary straight part201is arranged in one (516) of the other holes516,517and a part of the second secondary straight part203is arranged in another one (517) of the other holes516,517of the first primary bond part510. In a similar manner, the second tertiary surface523of the first secondary bond part520is provided with one or more other holes526,527extending through the first secondary bond part520in the longitudinal direction dl. Moreover, a part of the second primary straight part201is arranged in one (526) of the other holes526,527and a part of the second secondary straight part203is arranged in another one (527) of the other holes526,527of the first secondary bond part520. The other holes516,517,526,527are adapted to the surface of the second heat transfer tube200as detailed in connection with the holes514,515,524,525and the first heat transfer tube. The other holes form other apertures in manner discussed e.g. for the first primary aperture533, and straight parts of the other tube200extend through these other apertures.

Referring toFIGS.9aand9b, it may be feasible to bind even more heat transfer tubes with one bond530. Thus, in an embodiment, the heat exchanger10comprises the first100and the second200heat transfer tubes as discussed above, and further comprises a third heat transfer tube300. The third heat transfer tube300comprises a third primary straight part301, a third primary curved part302, and a third secondary straight part303. Also the third straight parts301,303extend mutually in parallel and in the first plane P, wherein also the first and second straight parts101,103,201,203extend. Moreover, the third straight parts301,303extend in parallel with the first and second straight parts101,103,201,203in the longitudinal direction di. A bond530can be used to bind the straight parts101,201,301,103,203,303of the three tubes100,200,300in the manner discussed above for one or two tubes.

Referring toFIG.9b, in an embodiment, the heat exchanger10comprises the first100, second200, and third300heat transfer tubes as discussed above, and further comprises a fourth heat transfer tube400. The fourth heat transfer tube400comprises a fourth primary straight part401, a fourth primary curved part402, and a fourth secondary straight part403. Also the fourth straight parts401,403extend mutually in parallel and in the first plane P, wherein also the first, second, and third straight parts101,103,201,203,301,303extend. Moreover, the fourth straight parts401,403extend in parallel with the first, second, and third straight parts101,103,201,203,301,303in the longitudinal direction dl. A bond530can be used to bind the straight parts101,201,301,401,103,203,303,403of the four tubes100,200,300,400in the manner discussed above for one or two tubes.

The number Ntubeof heat transfer tubes, of which straight parts extend in a plane P can be one, as indicated inFIG.2a, two, as indicated inFIG.8a, three, as indicated inFIG.9a, four, as indicated inFIG.9b, five (not shown), six (not shown) or more than six (not shown). Preferably the number Ntubeof such heat transfer tubes is at least two, two, at least three, three or four. Preferably such a number of tubes and their curved parts are used that the tertiary surface513,523of each one of the bond parts510,520is provided with from 8 to 24 holes, such as from 12 to 18 holes, and a part of a straight part of a heat transfer tube is provided in each one of the holes. Also preferably, the tertiary surface513,523of each one of the bond parts510,520is provided with an even number (i.e. an integer multiple of two) of holes, and a part of a straight part of a heat transfer tube is provided in each one of the holes.

Referring now toFIG.7a, the first primary straight part101of the first heat transfer tube100may comprise a first primary straight part111of a first inner heat transfer tube110and a first primary straight part121of a first outer refractory120. Optionally, the first primary straight part101may comprise some thermally insulating material140in between the inner heat transfer tube110and the outer refractory120. Referring toFIG.7b, also the first primary curved part102of the first heat transfer tube100may comprise a first primary curved part112of the first inner heat transfer tube110and a first primary curved part122of the first outer refractory120. Optionally, the first primary curved part101may comprise some thermally insulating material140in between the inner heat transfer tube110and the outer refractory120. This has the beneficial effect as disclosed in the prior art publication U.S. Pat. No. 9,371,987. In a similar manner, any or all of the second heat transfer tube200, the third heat transfer tube300, and the fourth heat transfer tube400may comprise an inner tube and outer refractory.

Having an outer refractory120has the further beneficial effect that, in use, the temperature of an outer surface of the refractory120is much higher than a temperature of a heat transfer tube100, if it consisted of a plain heat transfer tube. Moreover, the temperature of the first primary bond530is also high in use. Thus, having an outer refractory diminishes temperature difference, in use, between the bond530and the outer surface of the tube100. This improves the fitting in between the bond530and the tube100also in use. Moreover, this also diminishes warping of the bond530in use.

A heat exchanger10typically comprises a first heat transfer tube arrangement comprising the first100(and optionally also second, third, fourth, fifth and sixth200,300,400) heat transfer tubes that extend in the same plane P; and the first primary bond530, and optionally the first secondary bond540binding these tubes together. Referring toFIGS.10,11band11c, a heat exchanger10typically comprises a second heat transfer tube arrangement comprising at least a secondary first heat transfer tube100bextending in a second plane P′, which is parallel to the first plane P. The second heat transfer tube arrangement may further comprise a secondary second heat transfer tube200bextending in the second plane P′, optionally also a secondary third heat transfer tube300bextending in the second plane P′, and optionally also a secondary fourth heat transfer tube400b(and optionally also secondary fifth and secondary sixth heat transfer tubes) extending in the second plane P′. Referring toFIGS.10and11c, the tubes of the second heat transfer tube arrangement may be joined using a second primary bond530b; and optionally also a second secondary bond540b. As indicated inFIG.10, the secondary first heat transfer tube100bcomprises straight parts101b,103bsimilar to the first heat transfer tube100. In a similar manner the secondary second heat transfer tube200bcomprises straight parts201b,203b, the secondary third heat transfer tube300bcomprises straight parts301b,303b, and the secondary fourth heat transfer tube400bcomprises straight parts401b,403b. In a similar manner, a third heat transfer tube arrangement may be joined using a third primary bond530c(seeFIG.11c).

The second heat transfer tube arrangement (100b,200b,300b,400b) is supported by the second primary bond530bin a same manner as the first heat transfer tube arrangement (100,200,300,400) is supported by the first primary bond530. Thus, in an embodiment, the heat exchanger10comprises a second primary bond530bthat is configured to support at least two other straight parts (101b,103b,201b,203b,301b,303b,401b,403b) of at least one other heat transfer tube (100b,200b,300b,400b), i.e. a secondary heat transfer tube. Moreover, the other at least two straight parts of the secondary heat transfer tube or tubes (100b,200b,300b,400b) extend parallel with each other in a second plane P′. The second plane P′ is parallel to the first plane P. Moreover, the second plane P′ is arranged at a distance from the first plane P.

Referring toFIG.11c, in an embodiment the first primary bond530is connected to the second primary bond530bby a first bridge551. In the embodiment ofFIG.11c, an end of the first primary bond530is fixed to the first bridge551and an end of the second primary bond530bis fixed to the first bridge551. Moreover, all the heat transfer tubes100,200,100b,200bof the heat exchanger10are arranged on a same side of the first bridge551. For example inFIG.11c, all the heat transfer tubes are arranged above the first bridge551. In addition, the embodiment ofFIG.11ccomprises a second bridge552. Another end of the first primary bond530is fixed to the second bridge552and another end of the second primary bond530bis fixed to the second bridge552. Moreover, all the heat transfer tubes100,200,100b,200bof the heat exchanger10are arranged in between the first bridge551and the second bridge552. The purpose of the bridges551,552is to bind the primary bonds530,530btogether. The bridges551,552provide for mechanical support for the tubes100,200,100b,200bin a direction of a normal N of the first plane P; which, in use, may be horizontal. The bonds530,530bprovide for mechanical support for the tubes100,200,100b,200bin another direction, which, in use, may be vertical.

Referring toFIG.11c, in an embodiment, at least at one point in between [i] two straight parts of a tube that are separated by a curved part of the tube or [ii] two straight parts different tubes, in a direction of the first plane P, the second primary bond530bcontacts the first primary bond530to form a mechanical locking537between the first primary bond530and the second primary bond530b. This helps to lock the tubes to each other also in a direction of a normal N of the first plane P and also in a central part of the heat exchanger. This is particularly feasible in case of a modular heat exchanger. In the mechanical locking537a surface of the first primary bond530that faces away from the first heat transfer tube100in a direction of a normal N of the first plane P contacts a surface of the second primary bond530bthat faces away from the secondary first heat transfer tube100bin a direction of a normal of the second plane P′. The mechanical locking537can, in some tube configurations, be made with a central bridging element553. The central bridging element553can be fixed to both the first primary bond530and the second primary bond530b. The mechanical locking (537,553) is made at a point that is left in a direction within the first plane P and perpendicular to the longitudinal direction dl(i.e. the direction dextofFIG.3c) in between [i] two straight parts of a tube that are separated by a curved part of the tube or [ii] two straight parts different tubes.

Moreover, preferably in some regions, a gap538(i.e. a distance) is left in between the second primary bond530band the first primary bond530, as indicated by the reference numeral538inFIG.11c. As indicated inFIG.11c, such a gap538is left in between the second primary bond530band the first primary bond530in a direction of a normal N of the first plane P.

The heat exchanger100needs not comprise the first bridge551, since a central bridging element553(i.e. a first central bridging element533) can be used for the purpose of binding the bonds530,530btogether near an end of the bonds. The heat exchanger100needs not comprise the second bridge552, since a central bridging element553(i.e. a second central bridging element) can be used for the purpose of binding the bonds530,530btogether near another end of the bonds. However, preferably to elements (bridges and/or central bridging elements) are used to connect the bonds530,530btogether.

Referring toFIG.10, from the point of view of modularity, it is beneficial that the heat exchanger10has substantially rectangular shape. More precisely, in an embodiment, the heat exchanger10comprises the distributor header142and the collector header144such that the distributor header142extends in a first direction dfeed. From the point of view of modularity, it is beneficial that the normal N of the first plane P is parallel to the first direction dfeedor forms an angle α of at most 60 degrees with the first direction dfeed. It is also preferable, that the first plane P is, in use substantially vertical. Thus, in an embodiment, a normal N of the first plane P is configured to be horizontal in use to form an angle β of at most 45 degrees with a horizontal direction Shin use. At least in such a case, the heat exchanger forms a modular assembly that can be inserted into the space V and removed therefrom via an opening in the wall51. Such procedure is described in more detail in an international patent application PCT/FI2016/050760.

Preferably the heat exchanger10is used in a fluidized bed boiler as a fluidized bed heat exchanger10. More preferably the fluidized bed heat exchanger10is used in a loopseal5of a circulating fluidized bed boiler. Thus, in an embodiment, the fluidized bed boiler1comprises means40for separating bed material from flue gas. Referring toFIG.1a, in an embodiment, the fluidized bed boiler1comprises a cyclone40for separating bed material from flue gas. The fluidized bed boiler comprises a loopseal5configured to receive bed material from the means40for separating bed material from flue gas (e.g. from the cyclone). Moreover, at least a part of the fluidized bed heat exchanger10is arranged in the loopseal5. Referring toFIGS.2b,2c,7a,7b, and8, for example, the distributor header142and to collector header144may be arranged outside the loopseal. However, at least most of the heat transfer tubes (100,200) or heat transfer tubes (110,120,210,220) are arranged in to the loopseal as indicated above. For example, in an embodiment, at least 90% of the heat transfer tubes (100,200) or heat transfer tubes (110,120,210,220) of the fluidized bed heat exchanger10, as measured lengthwise, are arranged in the loopseal5as indicated above.