WO2010027763A2

WO2010027763A2 - Flow bypass sealing apparatus for annular heat exchanger and method of making the same

Info

Publication number: WO2010027763A2
Application number: PCT/US2009/054838
Authority: WO
Inventors: Paul G. Anderson; Robert P. Morgan; Fern L. Tuchowski; Michael J. Reinke; Matthew C. Kennedy; Jeroen Valensa; Michael S. Eklund; William David Lyle; Martin Perry
Original assignee: Modine Manufacturing Company
Priority date: 2008-08-25
Filing date: 2009-08-25
Publication date: 2010-03-11
Also published as: WO2010027763A3

Abstract

A heat exchanger includes a pair of substantially concentric tubes defining a first flow path between the tubes, and a second flow path being defined by one of an inner one of the pair of tubes and an outer one of the pair of tubes. A fin is supported between the concentric tubes along the first flow path and surrounding an outer perimeter of the inner one of the pair of tubes, a first side of the fin being secured to one of the pair of tubes. A sleeve extends around a second side of the fin. An elastic element is positioned between the sleeve and another of the pair of tubes for biasing the sleeve into engagement with the second side of the fin and substantially sealing a bypass around the fin between the other of the pair of tubes and the sleeve.

Description

FLOW BYPASS SEALING APPARATUS FOR ANNULAR HEAT EXCHANGER AND METHOD OF MAKING THE SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application Serial No. 61/091,545, filed August 25, 2008, the entire contents of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to heat exchangers in general and in more particular applications, to recuperative heat exchangers which find many uses in industry, including in fuel cell systems.

SUMMARY

[0003] One of the enabling technologies for the commercial success of fuel cell systems, especially high temperature fuel cell systems such as, for example, solid oxide fuel cell (SOFC) systems and molten carbonate fuel cell (MCFC) systems, is heat exchanger technology to efficiently recapture the waste heat of the products leaving the fuel cells, e.g. exhaust stream, in order to preheat the anode and cathode feed streams. Although several types of heat exchangers to accomplish such a task are known to those skilled in the art, one especially promising type is an annular heat exchanger, wherein two flow streams exchange heat while flowing through annuli located on either side of a metal cylindrical wall. A solid oxide fuel cell system incorporating such heat exchangers is described in pending United States Patent Publication No. 2008/0038622 to Valensa et. al., which contents are hereby incorporated by reference in their entirety.

[0004] The heat exchangers described in the above patent application provide annular flow channels for the heat-exchanging flows, each of said channels being bounded on one side by the inner or outer wall surface of the metal cylinder through which heat is transferred and bounded on the other side by the inner or outer wall of another cylinder. As further described, the transfer of heat between the fluid and the heat transferring cylinder wall can be aided by heat transfer surface area enhancement features, such as convoluted fin surfaces bonded to said cylinder wall for example.

[0005] A requirement for optimizing the rate of heat transfer between the two fluids in such a heat exchanger is ensuring that the flows pass through the heat exchange channels formed by such heat transfer surface area enhancement features, and ensuring that there is no substantial bypassing of the flows around those channels. One way to accomplish this may be by bonding the surface area enhancement features located on the external wall of the heat transferring cylinder to the inner wall of the other cylinder bounding the flow on that side, and likewise doing the same with the features located on the inner wall of the heat transferring cylinder and the outer wall of the other cylinder bounding the flow on that side. However, this could lead to undesirable heat transfer occurring between one or both of the two flows and heat sources or sinks in thermal contact with those other cylinders.

[0006] An alternate approach would be to assemble the heat exchanger such that a minimal gap exists between the heat transfer surface area enhancement features and the walls of the bounding cylinders. This approach ensures that there is no substantial bypassing of flow around the heat exchange channels, but still provides a substantial resistance to any undesirable heat transfer occurring through the walls of the bounding cylinder. In practice, however, this approach has been found to be difficult to achieve. Especially as the diameters of such heat exchangers increase, the need for assembly clearances between the surface enhancement features and the bounding cylinders, and the potential for cylinders to deviate from a perfectly cylindrical profile, can lead to substantial gaps between the surface area enhancement features and the bounding cylinders. Even more troubling, the magnitudes of these substantial gaps can be highly varied in their location along the circumference of the heat exchanger, which can lead to locally high bypass flow in certain regions of the heat exchanger. Since the portion of the flow that bypasses the heat exchange channels tends to experience less transfer of heat than the portion of the flow that does not, the result can be a non-uniform temperature and flow rate distribution along the circumference as the flow exits the heat exchanger, resulting in a sub-optimally performing heat exchanger. [0007] In light of these issues, a reliable means for sealing off bypass flow paths in this type of heat exchanger is desirable. Such a design that also simplifies assembly and disassembly is also desirable.

[0008] In some embodiments, the present invention provides a heat exchanger including a heat conducting cylinder, a plurality of heat transfer surface enhancement features bonded to one surface of the cylinder, a bounding cylinder concentric to the heat conducting cylinder in order to bound a flow passing through the heat exchanger to an annular flowpath, a thin wrap located between the heat conducting cylinder and the bounding cylinder and at least partially bonded to the heat transfer surface area enhancement features to further bound the flow to a plurality of heat exchanging flow channels formed by the heat transfer surface area enhancement features, and a compliant spring member located between the thin wrap and the bounding cylinder in order to block the flow from bypassing the heat exchanging flow channels through a gap between the thin wrap and the bounding cylinder.

[0009] In some embodiments, the compliant spring member includes at least one domed portion to locally adjust to the variation in height of the gap between the wrap and the bounding cylinder.

[0010] In some embodiments, the compliant spring member can be partially bonded to one of either the wrap or the bounding cylinder. The compliant spring member can also or alternatively be constructed from a single strip of material, and can include a plurality of slits to facilitate the assembly of the spring member to the wrap or the bounding cylinder.

[0011] In some embodiments, the space underneath the domed portion of the compliant spring member is at least partially filled with a compliant material in order to further block bypass flow. The compliant spring member can also or alternatively be constructed of a material that exhibits a greater thermal expansion coefficient than does the material to which it is bonded.

[0012] In one embodiment, the invention provides a heat exchanger. The heat exchanger includes a pair of substantially concentric tubes defining a first flow path between the tubes, a second flow path being defined by one of an inner one of the pair of tubes and an outer one of the pair of tubes. A fin is supported between the concentric tubes along the first flow path and surrounding an outer perimeter of the inner one of the pair of tubes, a first side of the fin being secured to one of the pair of tubes. A sleeve extends around a second side of the fin. An elastic element is positioned between the sleeve and an other of the pair of tubes for biasing the sleeve into engagement with the second side of the fin and substantially sealing a bypass around the fin between the other of the pair of tubes and the sleeve.

[0013] In another embodiment, the invention provides a recuperative heat exchanger for a fuel cell system, the fuel cell system including a fuel cell stack. The heat exchanger includes a fin supported between a pair of concentric tubes and being secured to one of the pair of tubes. The tubes define an exhaust flow path and a feed stream flow path, the fin facilitating recuperative heat transfer from the exhaust flow to the feed stream. A sleeve is positioned around a free side of the fin for movement with the free side of the fin relative to the one of the pair of tubes to accommodate thermal expansion of the one of the pair of tubes.

[0014] The present invention also provides a heat exchanger including a pair of substantially concentric tubes defining a first flow path between the tubes. A second flow path can be defined by one of an inner one of the pair of tubes and an outer one of the pair of tubes. The heat exchanger can also include a sleeve located between the concentric tubes along the first flow path and a fin structure supported between the sleeve and one of the pair of tubes along the first flow path. The fin structure and the sleeve and the one of the pair of tubes can define a heat transfer portion of the first flow path. An elastic element can be positioned between the sleeve and another of the pair of tubes and substantially seal a bypass around the heat transferring portion of the first flow path.

[0015] In some embodiments, the present invention provides a heat exchanger for a fuel cell system. The fuel cell system can include a fuel stack configured to receive a feed stream and discharge an exhaust. The heat exchanger can include a fin structure supported between a pair of concentric tubes and secured to one of the pair of tubes. The tubes can define an exhaust flow path and a feed stream flow path. The fin structure can facilitate heat transfer from an exhaust traveling along the exhaust flow path to a feed stream traveling along the feed stream flow path. The heat exchanger can include a sleeve supported by the fin structure along one of the exhaust flow path and the feed stream flow path. [0016] The present invention also provides a method of making a heat exchanger including the acts of providing a first tube having a first diameter and a second tube having a second diameter smaller than the first diameter, securing a fin structure to one of the inner surface of the first tube and the outer surface of the second tube, securing a first surface of a sleeve to the fin structure, securing an elastic element to one of a second surface of the sleeve and the other of the inner surface of the first tube and the outer surface of the second tube, and placing the second tube inside the first tube.

[0017] In yet another embodiment, the invention provides a method of forming a heat exchanger. The method includes aligning a pair of substantially concentric tubes to define a first flow path between the tubes and a second flow path by one of an inner one of the pair of tubes and an outer one of the pair of tubes. The method also includes inserting a fin between the concentric tubes along the first flow path and around an outer perimeter of the inner one of the pair of tubes, securing a first side of the fin to one of the pair of tubes, positioning a sleeve around a second side of the fin, and positioning an elastic element between the sleeve and an other of the pair of tubes to bias the sleeve into engagement with the second side of the fin and substantially seal a bypass around the fin between the other of the pair of tubes and the sleeve.

[0018] In yet another embodiment, the invention provides a method of forming a recuperative heat exchanger for a fuel cell system. The method comprises the acts of positioning a fin between a pair of concentric tubes that define an exhaust flow path and a feed stream flow path, the fin for facilitating recuperative heat transfer from the exhaust flow to the feed stream, securing the fin to one of the pair of tubes, and positioning a sleeve around a free side of the fin for movement with the free side of the fin relative to the one of the pair of tubes to accommodate thermal expansion of the one of the pair of tubes.

[0019] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Fig. 1 is a sectional view of a portion of a heat exchanger to be used with the present invention; [0021] Fig. 2 is an enlarged detail view of the section A-A from Fig. 1 ;

[0022] Fig. 3 is an isometric view of a portion of a heat exchanger embodying the present invention;

[0023] Fig. 4 is a partially sectioned portion of the heat exchanger embodying the present invention shown in Fig. 3;

[0024] Fig. 5 is a plan view of a portion of a compliant spring member according to the present invention;

[0025] Fig. 6 is a section view of a portion of a heat exchanger embodying the present invention, with details removed in order to show certain aspects of the invention with greater clarity;

[0026] Fig. 7 is a section view similar to Fig. 7 showing another embodiment of the present invention;

[0027] Fig. 8 is a section view similar to Fig. 7 showing another embodiment of the present invention;

[0028] Fig. 9 is a side view of a compliant spring member according to the present invention in a pre-assembled condition; and

[0029] Fig. 10 is a plan view of a portion of a compliant spring member according to another embodiment of the present invention.

[0030] Fig. 11 is a schematic view of a fuel cell system including a heat exchanger of Fig. 1.

DETAILED DESCRIPTION

[0031] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings.

[0032] A style of heat exchanger that can derive benefit from the present invention is depicted in Fig. 1, and certain aspects of this heat exchanger style are shown in greater detail in Fig. 2. The heat exchanger 1 as shown in Figs. 1 and 2 comprises a first annular flow channel bounded by an outermost cylinder 10, or tube, and an intermediate cylinder 2, or tube, and a second annular flow channel bounded by the intermediate cylinder 2 and an innermost cylinder 13, or tube. The heat exchanger 1 further comprises a first heat transfer surface enhancement feature 4, or fin, located in the first annular flow channel and bonded to the outermost surface of the intermediate cylinder 2. The heat exchanger 1 additionally comprises a second heat transfer surface enhancement feature 3 located in the second annular flow channel and bonded to the innermost surface of the intermediate cylinder 2. During operation of the heat exchanger, the heat transfer surface enhancement features 3 and 4 provide for the efficient transfer of heat through the intermediate cylinder 2 between a first fluid flowing through the first annular flow channel and a second fluid flowing through the second annular flow channel. Although the heat transfer surface enhancement features 3 and 4 as depicted in Figs. 1 and 2 are of a convoluted fin structure, it should be readily apparent to those skilled in the art that various other styles of heat transfer surface enhancement features known in the art of heat transfer could be used. In addition, various methods by which the surface enhancement features might be bonded to the intermediate cylinder, including but not limited to brazing, soldering, welding, and swaging, should be readily apparent to those skilled in the art.

[0033] Although the heat exchanger 1 as shown in Figs. 1 and 2 appears to provide a zero clearance fit between the extended surface features 3 and 4 and the bounding cylinders 13 and 10 respectively, such a construction might not be practical in certain applications. For example, the dimensional tolerances on the cylinder diameters required to achieve such a zero clearance might not be achievable at a reasonable cost. Additionally, maintaining a perfectly cylindrical shape profile might be difficult, especially for larger cylinders. Consequently, it may be necessary in certain applications to provide for a substantial nominal clearance between the extended surface features 3 and 4 and the bounding cylinders 13 and 10 respectively in the design of such a heat exchanger.

[0034] It has been found that as the nominal gap between the extended surface features 3 and 4 and the bounding cylinders 13 and 10 respectively increases from the zero clearance condition, a substantial decrease in heat exchanger performance can be observed. This decrease in performance is largely due to a portion of the flow in each annular flow channel flowing through the gap between the extended surface features and the corresponding bounding cylinder. Furthermore, as the gap varies along the circumference of the flow channel, substantial circumferential flow maldistribution can occur, with a relatively greater rate of flow bypass occurring in regions where the gap is relatively large.

[0035] A relevant portion of a heat exchanger 1 embodying the present invention is shown in Fig. 3, and a partial section view of the same heat exchanger 1 is shown in Fig. 4 to illustrate certain aspects of the embodiment with greater clarity. In the depicted embodiment, a thin wrap 5, or sleeve, is at least partially bonded to the outermost crests of the surface enhancement feature 4. The depicted embodiment further comprises a compliant spring member 6, or elastic element, attached to the outer surface of the wrap 5. The compliant spring member 6 comprises two cylindrical sections 8a and 8b that are at least partially bonded to the outer surface of the wrap 5, said sections being separated by a domed section 9. The term "domed" in this context is meant to denote that a cross-section of the section 9 in a plane passing through the axis of the cylindrical heat exchanger 1 has a domed, or arc-shaped, profile. In some embodiments, the domed profile may include a portion of a substantially circular shape.

[0036] In one embodiment, the sections 8a and 8b of the compliant spring member 6 are bonded to the wrap 5 by brazing. In other embodiments, the sections 8a and 8b of the compliant spring member 6 are bonded to the wrap 5 by a plurality of spot welds.

[0037] As further shown in Figs. 3 and 4, a preferred embodiment of the present invention may include a plurality of slits 7 in the domed section 9 in order to allow each portion of the domed section 9 between two adjacent slits 7 to be radially compressed independently of the neighboring portions. The slits 7 extend in a direction substantially parallel to the direction of flow through the first and second annular flow channels. In the absence of such slits, the bending stiffness of the domed section 9 makes forming the compliant spring member 6 into the cylindrical shape required for bonding to the wrap 5 very difficult. The spacing distance between the slits 7 can be adjusted in order to facilitate the assembly of the compliant spring member 6 to the wrap 5. These features may be more easily understood with reference to Fig. 5, which shows a plan view of a portion of the compliant spring member 6 according to the embodiment of the present invention shown in Figs. 3 and 4.

[0038] Fig. 6 shows a partial sectional view of an embodiment of the present invention with certain aspects removed in order to illustrate the intended operation of the invention with greater clarity. The scales of certain features in Fig. 6 have been exaggerated in order to more clearly illustrate certain aspects of the presented embodiment. As shown in Fig. 6, the wrap 5 subdivides the annular flow channel bounded by cylinders 2 and 10 into an inner annular flow channel bounded by cylinders 2 and 5, and an outer annular flow channel bounded by cylinders 5 and 10. Upon assembly, the radially outermost portion of the domed section 9 of the compliant spring member 6 can be compressed inward by the outer cylindrical wall 10, so that the compliant spring member 6 blocks the undesirable bypass flow through the outer annular flow channel bounded by cylinders 5 and 10 and forces the flow to pass through the inner annular flow channel bounded by cylinders 2 and 5 containing the heat transfer surface enhancement feature 4. Because the plurality of slits 7 in the domed section 9 allow each portion of the domed section 9 between two adjacent slits 7 to be radially compressed independently of the neighboring portions, variability in the height of the outer annular flow channel bounded by cylinders 5 and 10 can be accommodated.

[0039] In another embodiment of the invention, the compliant spring member 6 may be constructed of a material that exhibits a greater thermal expansion coefficient than does the material of the wrap 5. Because the ends 8a and 8b are constrained to move along with the wrap 5, the increased thermal expansion of the spring member material 6 at operating temperatures would result in a radially outward movement of the domed section 9 (in the absence of a preexisting contact with the bounding cylinder 10). This would thereby enable the heat exchanger to be assembled with reduced or no interference between the domed section 9 and the bounding cylinder 10, while still enabling the sealing off of the bypass flow annulus at operating conditions, thus simplifying both assembly and disassembly of the heat exchanger 1.

[0040] It should be understood that the presence of the plurality of slits 7 can allow for some bypass flow to pass through the flow annulus bounded by cylinder 10 and wrap 5 in certain embodiments of the invention. In one embodiment, the width and spacing of the plurality of slits 7 is selected so that a circumferentially uniform relationship between bypass flow rate and pressure drop is established that is independent of the variation in the gap between the wrap and the bounding cylinder, thereby preventing a circumferential maldistribution of the bypass flow. In certain applications some amount of bypass flow may be tolerable, provided that the flow is not maldistributed around the circumference.

[0041] In one embodiment of the invention depicted in Fig. 7, bypass flow through the plurality of slits 7 is prevented by including a compressible material 11 , or deformable element, between the domed section 9 and the wrap 5. For example, compressible material 11 may be a strip of refractory blanket material. In another embodiment depicted in Fig. 8, the compressible material may be replaced with a braided ceramic sleeving material 12 that can be partially flattened into an oval shape having a rounded cross-sectional shape upon assembly.

[0042] It should be understood that while the described embodiments have described blocking the bypass flow in the annular flow channel bounded by an intermediate cylinder 2 and an outer cylinder 10, analogous embodiments can exist wherein the bypass flow in the annular flow channel bounded by an intermediate cylinder 2 and an inner cylinder 13 is so blocked. Furthermore, some embodiments of the invention could include such flow blocking means in the annular flow channels on both sides of the intermediate cylinder 2.

[0043] It may be preferable in some embodiments to bond the sections 8a and 8b of a compliant spring member 6 to the inner surface of the outermost bounding cylinder 10. It may further be preferable in some embodiments to bond the sections 8a and 8b of a compliant spring member 6 to the outer surface of the innermost bounding cylinder 13.

[0044] It has been contemplated by the inventors that in certain embodiments of the invention it may be preferable for the compliant spring member 6 to comprise additional domed sections 9, each pair of domed sections 9 separated by an additional cylindrical section 8. In some embodiments comprising multiple domed sections it may be preferable to stagger the alignment of the slits 7 in successive domed sections 9.

[0045] In some embodiments of the invention a compliant spring member 6 for sealing off bypass flow is formed by bending the long ends 8a and 8b of a thin metallic strip 6 by a predetermined angle θ (shown in Fig. 9), so that the center section 9 assumes a domed profile when the ends 8a and 8b are aligned to be coplanar. The metallic strip 6 is bent along a first bend line parallel to a first edge and located between the first edge and a centerline of the metallic strip 6 parallel to the first edge by an angle of between 0 and 90 degrees to form a first bent tab. A second edge of the metallic strip 6 parallel to and opposite the first edge is bent along a second bend line parallel to the second edge and located between the second edge and the centerline by the angle to form a second bent tab. The act of securing the compliant spring member 6 occurs after the acts of bending the first edge and bending the second edge and includes forming the domed profile in the metallic strip 6 between the first and second bent tabs to define the compliant spring member 6.

[0046] In some embodiments of the invention, one or more flexible strips 14a, 14b are woven through the plurality of slits 7 in a basket weave pattern in order to more fully block fluid flow through the slits 7. These strips 14a, 14b are preferably fashioned from a thin, highly flexible material, such as for example a ceramic cloth material, or woven ceramic. As shown in Fig. 10, in some embodiments it may be advantageous to include multiple strips 14a, 14b, wherein the weaving pattern through the slits of a first strip 14a is offset from the weaving pattern through the slits of a second strip 14b.

[0047] In some exemplary embodiments, one or more heat exchangers according to the present invention may be of benefit in a high temperature fuel cell system, such as the fuel cell system 15 of FIG. 11. The fuel cell system 15 comprises a fuel cell stack 16 having a cathode side 17 and an anode side 18, the fuel cell stack being configured so that the cathode side 17 receives a cathode feed 19 (comprising, for example, air) and discharges a cathode exhaust 21. The fuel cell stack 16 is further configured so that the anode side 18 receives an anode feed 20 (comprising, for example, hydrogen and/or a gaseous hydrocarbon) and discharges an anode exhaust 21.

[0048] In many cases it is highly preferable for the fuel cell stack 16 to operate nearly isothermally at an elevated temperature, and as a result it may be highly desirable to recuperate the heat from the exhaust streams in order to preheat the feed streams. In the fuel cell system 15, this recuperation of heat is accomplished through the use of a first heat exchanger 23 to transfer heat from the cathode exhaust 21 to the cathode feed 19, a second heat exchanger 24 to transfer heat from the anode exhaust 22 to the anode feed 20, and a third heat exchanger 25 to transfer additional heat from the anode exhaust 22 to the cathode feed 19. While some fuel cell systems may make use of all three heat exchangers 23, 24, and 25, it should be understood that each heat exchanger may be capable of operating independently of any of the other heat exchangers, and that some fuel cell systems might not include one or more of these heat exchangers. A heat exchanger according to the present invention may be especially well suited for use as one or more of the heat exchangers 23, 24 and 25.

[0049] It should be understood that the embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes are possible.

Claims

1. A heat exchanger comprising: a pair of substantially concentric tubes defining a first flow path between the tubes, a second flow path being defined by one of an inner one of the pair of tubes and an outer one of the pair of tubes; a sleeve located between the concentric tubes along the first flow path; a fin structure supported between the sleeve and one of the pair of concentric tubes along the first flow path, the fin structure and the sleeve and the one of the pair of tubes defining a heat transfer portion of the first flow path; and an elastic element positioned between the sleeve and an other of the pair of tubes and substantially sealing a bypass around the heat transferring portion of the first flow path.

2. The heat exchanger of claim 1, wherein the elastic element is formed from metal.

3. The heat exchanger of claim 1 , wherein the elastic element is bonded to at least one of the sleeve and the other of the pair of tubes.

4. The heat exchanger of claim 1, wherein the elastic element includes at least two slits opening between opposite sides of the elastic element and extending in a direction substantially parallel to the first flow path.

5. The heat exchanger of claim 4, further comprising a strip woven around the elastic element and through the slits.

6. The heat exchanger of claim 5, wherein the strip is formed from woven ceramic.

7. The heat exchanger of claim 5, wherein the strip is a first strip, and further comprising a second strip woven around the elastic element through the slits in a different woven pattern than the first strip.

8. The heat exchanger of claim 1 , wherein together the elastic element and the sleeve define an interior space, and wherein the interior space is at least partially filled with a deformable element.

9. The heat exchanger of claim 8, wherein the deformable element includes a braided ceramic sleeve.

10. The heat exchanger of claim 8, wherein the deformable element has a rounded cross-sectional shape.

11. The heat exchanger of claim 1 , wherein the elastic element has a greater thermal expansion coefficient than the sleeve.

12. The heat exchanger of claim 1, wherein the elastic element has an arcuate cross sectioned shape with opposite ends engaging one of the sleeve and the other of the pair of tubes and the radially-outermost portion of the arc-shaped strip engaging an other of the sleeve and the other of the pair of tubes.

13. The heat exchanger of claim 1, wherein a surface of the fin structure is secured to the one of the pair of tubes.

14. A heat exchanger for a fuel cell system, the fuel cell system including a fuel stack configured to receive a feed stream and discharge an exhaust, the heat exchanger comprising: a fin structure supported between a pair of concentric tubes and being secured to one of the pair of tubes, the tubes defining an exhaust flow path and a feed stream flow path, the fin structure facilitating heat transfer from an exhaust traveling along the exhaust flow path to a feed stream traveling along the feed stream flow path; a sleeve supported by the fin structure along one of the exhaust flow path and the feed stream flow path; and an elastic element positioned between the sleeve and an other of the pair of tubes and substantially preventing the one of the exhaust flow and the feed stream flow from bypassing the fin structure.

15. The heat exchanger of claim 14, wherein the elastic element is formed from a metal.

16. The heat exchanger of claim 14, wherein the elastic element includes a domed portion to locally vary the size of the bypass.

17. The heat exchanger of claim 14, wherein the elastic element is bonded to at least one of the sleeve and the other of the pair of tubes by one of brazing, welding, soldering, swaging and cohesive or adhesive bonding materials.

18. The heat exchanger of claim 14, wherein the elastic element includes at least two slits opening between opposite sides of the elastic element and extending in a direction substantially parallel to one of the exhaust flow path and the feed stream flow path.

19. The heat exchanger of claim 18, further comprising a strip woven around the elastic element and through the slits to substantially seal the slits.

20. The heat exchanger of claim 19, wherein the strip is formed from woven ceramic.

21. The heat exchanger of claim 20, wherein the strip is a first strip, and further comprising a second strip woven around the elastic element through the slits in a different woven pattern than the first strip.

22. The heat exchanger of claim 14, wherein together the elastic element and the sleeve define an interior space, and wherein the interior space is at least partially filled with a deformable element.

23. The heat exchanger of claim 22, wherein the deformable element includes a braided ceramic sleeve.

24. The heat exchanger of claim 14, wherein the elastic element has a greater thermal expansion coefficient than the sleeve.

25. The heat exchanger of claim 14, wherein the elastic element is formed by an arc- shaped strip with opposite ends of the arc-shaped strip engaging one of the sleeve and the other of the pair of tubes and the radially-outermost portion of the arc-shaped strip engaging an other of the sleeve and the other of the pair of tubes.

26. A method of making a heat exchanger, the method comprising the acts of: providing a first tube having a first diameter and a second tube having a second diameter smaller than the first diameter; securing a fin structure to one of the inner surface of the first tube and the outer surface of the second tube; securing a first surface of a sleeve to the fin structure; securing an elastic element to one of a second surface of the sleeve and the other of the inner surface of the first tube and the outer surface of the second tube; and placing the second tube inside the first tube.

27. The method of claim 26, wherein the act of placing the second tube inside the first tube causes a deformation of the elastic element.

28. The method of claim 26, wherein the act of securing the fin structure further includes brazing.

29. The method of claim 28, wherein the act of securing a first surface of the sleeve comprises securing the first surface of the sleeve to the fin structure.

30. The method of claim 29, wherein the act of securing the first surface of the sleeve includes brazing.

31. The method of claim 26, wherein the act of securing the elastic element further comprises at least partially filling an interior space between the elastic element and the one of the second surface of the sleeve and the other of the inner surface of the first tube and the outer surface of the second tube with a deformable element.

32. The method of claim 26, further comprising the acts of: bending a first edge of a metallic strip along a first bend line parallel to the first edge and located between the first edge and a centerline of the strip parallel to the first edge by an angle of between 0 and 90 degrees to form a first bent tab; and bending a second edge of the metallic strip parallel to and opposite the first edge along a second bend line parallel to the second edge and located between the second edge and the centerline by the angle to form a second bent tab; wherein the act of securing the elastic element occurs after the acts of bending the first edge and bending the second edge and includes forming a domed profile in the metallic strip between the first and second bent tabs to define the elastic element.