US20070261837A1

US20070261837A1 - Compact high temperature heat exchanger, such as a recuperator

Info

Publication number: US20070261837A1
Application number: US11/565,382
Authority: US
Inventors: Jeroen Valensa; Mark Voss
Original assignee: Modine Manufacturing Co
Current assignee: Modine Manufacturing Co
Priority date: 2005-12-01
Filing date: 2006-11-30
Publication date: 2007-11-15
Also published as: FR2896866A1; DE102006056807A1; JP2007155321A

Abstract

A heat exchanger for transferring heat energy between a first working fluid and a second working fluid. The heat exchanger can include a first sheet contoured to define a plurality of first fins and having an upper end and a lower end, a second sheet contoured to define a plurality of second fins and being positioned between the upper end of the first sheet and the lower end of the first sheet, and a housing formed from a third sheet and at least partially enclosing the first sheet and the second sheet.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 [U.S.C. § 119 to U.S. Provisional Patent Application No. 60/741,537, filed on Dec. 1, 2005, the contents of which is fully incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to heat exchangers, and more specifically to heat exchangers (e.g., recuperators, exhaust gas waste heat recovery systems, and the like) that can be compact and/or can operate at high temperatures.

SUMMARY

Compact recuperators can be used in a number of applications, such as, for example, in microturbines and high temperature fuel cells. In these and other applications, system efficiency can be optimized by heating a low temperature incoming air stream to a temperature closer to a desired process operating temperature via the transfer of thermal energy from a high temperature waste stream of exhaust gas or air. The recuperator can be a heat exchanger which allows for the efficient transfer of heat energy from the hot stream to the cold stream while maintaining isolation of the two streams.
In operation, compact recuperatores can be exposed to operating temperatures above 750° C. at a hot end, and to near-ambient temperatures (i.e., less than 100° C.) at a cold end. The combination of high temperatures and large temperature gradients with thin section materials has led to the use of materials that exhibit high-temperature strength and corrosion resistance at elevated temperatures, such as high nickel content alloys. Such materials are relatively expensive, which increases the demand for recuperator designs that minimize the amount of material required to attain the desired heat transfer capability. At the same time, the recuperator designs often minimize the thermal stress induced by temperature gradients throughout the device.
In some embodiments, the invention provides a heat exchanger for transferring heat energy between a first working fluid and a second working fluid. The heat exchanger can include a pair of first fins formed from a single sheet and at least partially defining a flow path for the first working fluid and a second fin positioned between the pair of first fins and at least partially defining a flow path for the second working fluid. The flow path of the first working fluid can be separated from the flow path of the second working fluid.
In other embodiments, the present invention provides a heat exchanger including a first corrugated sheet having a pair of peaks and separating a flow path for the first working fluid from a flow path for the second working fluid, and a second corrugated sheet positioned between the pair of peaks of the first corrugated sheet.
In still other embodiments, the invention provides a heat exchanger including a first sheet contoured to define a plurality of first fins and having an upper end and a lower end, a second sheet contoured to define a plurality of second fins and being positioned between the upper end of the first sheet and the lower end of the first sheet, and a housing formed from a third sheet and at least partially enclosing the first sheet and the second sheet.
The present invention also provides a method of assembling a heat exchanger, including the acts of corrugating a first sheet to define a pair of peaks and to at least partially define a flow path for a first working fluid and a flow path for a second working fluid, corrugating a second sheet, and nesting the second sheet in the first sheet between the pair of peaks and along the flow path for the second working fluid.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially exploded, perspective view of a portion of a heat exchanger according to some embodiments of the present invention;
FIG. 2 is a partially broken, perspective view of the heat exchanger shown in FIG. 1;
FIG. 3 is a partially exploded, perspective view of the heat exchanger shown in FIGS. 1 and 2 and including a housing;
FIG. 4 is a perspective view of the heat exchanger shown in FIGS. 1-3; and
FIG. 5 is an end view of the heat exchanger shown in FIGS. 1-4.

DETAILED DESCRIPTION

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,” and “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.
Also, it is to be understood that phraseology and terminology used herein with reference to device or element orientation (such as, for example, terms like “central,” “upper,” “lower,” “front,” “rear,” and the like) are only used to simplify description of the present invention, and do not alone indicate or imply that the device or element referred to must have a particular orientation. In addition, terms such as “first”, “second,” and “third” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance.
FIGS. 1-5 illustrate a heat exchanger 10 (e.g., a recuperator) for use in a microturbine, and/or a high-temperature fuel cell. In other embodiments, the heat exchanger 10 can be included in a vehicle having an internal combustion engine and can operate as an exhaust gas waste heat recovery system, or alternatively, as a portion of an exhaust gas waste heat recovery system. In yet other embodiments, the heat exchanger 10 can be included and/or used with oxide fuel cell-based auxiliary power units installed in vehicles (e.g., recreational vehicles, commercial or industrial vehicles, and the like). In still other embodiments, the heat exchanger 10 can be used in other applications (e.g., non-vehicular applications), such as, for example, in electronics cooling, industrial equipment, building heating and air-conditioning, and the like.
In some embodiments, the heat exchanger 10 can provide high heat transfer effectiveness with minimal size and weight, a low-cost construction due to a minimization of scrap material, easily accessible joints and connection points for simplified repair and location of leaks and other failures during or after manufacture, and/or a thermally unconstrained design. In other embodiments, the heat exchanger 10 can include one or more additional features or advantages not specified herein.
In the illustrated embodiment of FIGS. 1-5, the heat exchanger 10 transfers heat energy from a high temperature first working fluid (e.g., exhaust gas, water, CO₂, an organic refrigerant, R12, R245fa, air, and the like) to a lower temperature second working fluid (e.g., exhaust gas, water, CO₂, an organic refrigerant, R12, R245fa, air, and the like).
As shown in FIGS. 1-5, the heat exchanger 10 can include a core 60 having a first sheet or separator sheet 12 and a number of second sheets 16. In the illustrated embodiment of FIG. 2, the core 60 includes twenty-one second sheets 16 and a single first sheet 12. In other embodiments, the core 60 can include two or more first sheets 12 and one, two, three, or more second sheets 16.
In the illustrated embodiment of FIGS. 1-5, the first sheet 12 is contoured and includes a number of fins, peaks, or corrugations 22 and a number of channels 14 positioned between the peaks 22. In addition, each of the second sheets 16 is contoured and includes a number of fins, peaks, or corrugations 26 and a number of channels 28.
In the illustrated embodiment, each of the peaks 22 of the first sheet 12 and each of the peaks 26 of the second sheets 16 has a substantially rounded end, and each of the channels 14 of the first sheet 12 and each of the channels 28 of the second sheets 16 and has a similarly rounded cross section. In other embodiments, the peaks 22 and/or channels 14 of the first sheet 12 and/or the peaks 26 and the channels 28 of the second sheets 16 can be pointed, squared, or irregularly shaped. In yet other embodiments, the first sheet 12 and/or the second sheets 16 can have a substantially saw-tooth shape. In still other embodiments, adjacent peaks 22 and/or channels 14 of the first sheet 12 and/or adjacent peaks 26 and/or channels 28 of the second sheets 16 can have different shapes.
In some embodiments, the peaks 22 and channels 14 can be formed by folding or corrugating the first sheet 12, and the peaks 26 and the channels 28 can be formed by folding or corrugating the second sheets 16. In other embodiments, the first sheet 12 and/or the second sheets 16 can be cast or molded in a desired shape.
The first sheet 12 and/or the second sheets 16 can be manufactured from one or more materials suitable for operation in a high-temperature heat exchanger. In some embodiments, the first sheet 12 and the second sheets 16 can be manufactured from a high nickel content alloy. In some embodiments, the first sheet 12 and the second sheets 16 can be manufactured from materials having substantially the same or substantially similar coefficients of thermal expansion. In other embodiments, the first sheet 12 and/or the second sheets 16 can be manufactured from other materials (e.g., aluminum, iron, and other metals, composite material, and the like).
As shown in FIGS. 1-5, the second sheets 16 can be positioned in channels 1]4 of the first sheet 12 between upper and lower ends of the peaks 22 of the first sheet 12. In some embodiments, the second sheets 16 can be positioned in the channels 14 between alternating pairs of peaks 22 of the first sheet 12. In other embodiments, the second sheets 16 can be positioned at regular or irregular intervals between pairs of peaks 22 of the first sheet 12.
In the illustrated embodiment of FIGS. 1-5, the second sheets 16 are thinner than the first sheet 12. In other embodiments, each of the second sheets 16 can have a different thickness. Alternatively or in addition, one or more of the second sheets 16 can have a thickness that is greater than or substantially equal to the thickness of the first sheet 12, depending upon the flow characteristics (e.g., flow rate, temperature, pressure, etc.) of the first working fluid and/or the second working fluid, the particular first working fluid selected, the mass flow rate of the first working fluid through the heat exchanger 10, the particular second working fluid selected, the mass flow rate of the second working fluid through the heat exchanger I 0, and the like.
In some embodiments, surface convolutions can be formed along the inner or outer surface of one or more of the second sheets 16. In some such embodiments, the surface convolutions can include louvers, offset lances, bumps, channels, recesses, ribs, etc. In other embodiments, surface convolutions can be formed along the inner or outer surfaces of the first sheet 12. In these embodiments, the surface convolutions can increase the rigidity or strength of the core 60, improve the rate of heat transfer between the first working fluid and the second working fluid, and/or improve the efficiency of the heat exchanger 10. By improving the efficiency of the heat exchanger 10, the inclusion of such surface convolutions can also or alternatively ensure effective operation of the heat exchanger 10 while minimizing the amount of material required to manufacture the heat exchanger 10.
As shown in FIGS. 1 and 2, the first sheet 12 and the second sheets 16 can be sized such that the corrugation height h of the second sheets 16 is substantially similar to the space d between adjacent peaks 22 of the first sheet 12 (i.e., the width of the channels 14 of the first sheet 12). In this manner, the second sheets 16 can be securely nested between adjacent peaks 22 of the first sheet 12.
In some embodiments, such as the illustrated embodiment of FIGS. 1-5, the width W of the first sheet 12 can be greater than the width w of the second sheets 16 such that an unfinned section 20, 21 is located on either side of each second sheet 16 within each channel 14 of the first sheet 12. In other embodiments, the width W of the first sheet 12 can be less than or substantially equal to the width w of one or more of the second sheets 16 such that no unfinned sections are located along one or more of the channels 14 of the first sheet 12. In these embodiments, the second sheets 16 can provide structural support and rigidity to the core 60. In embodiments in which the pressure of one of the first working fluid and the second working fluid is significantly greater than the pressure of the other of the first working fluid and the second working fluid, the additional structural support and rigidity provided by the second sheets 16 can prevent or reduce wear or damage to the core 60. In other embodiments, unfinned sections 20, 21 are located at only one end of one or more of the channels 14 of the first sheet 12.
In some embodiments, one or more of the second sheets 16 can be secured to the first sheet 12 between adjacent peaks 22 of the first sheet 12. In some such embodiments, the peaks 26 of the second sheets 16 are connected (e.g., welded, soldered, brazed, etc.) to the first sheet 12. In other embodiments, the second sheets 16 can be supported in the channels 14 for movement relative to the first sheet 12, or alternatively, the second sheets 16 can be connected to the first sheet 12 in another manner, such as, for example, by an interference fit, adhesive or cohesive bonding material, fasteners, etc.
As shown in FIGS. 1-5, the heat exchanger 10 can also or alternatively include a housing or enclosure 50. In some embodiments, the housing 50 can be formed from a third sheet 30, which can be wrapped around two or three sides of the core 60. Alternatively or in addition, the third sheet 30 can include tabs 32, which can be bent or shaped to extend across one or more of the channels 14 (e.g., the upwardly opening channels 14 shown in FIG. 5) of the first sheet 12. As shown in FIGS. 1-5, the tabs 32 can extend across opposite ends of alternating channels 14 of the first sheet 12. In other embodiments, the tabs 32 can extend across one or both ends of adjacent channels 14. In some embodiments having tabs 32, the tabs 32 can be secured (e.g., welded, soldered, brazed, etc.) to the first sheet 12 to seal or substantially seal the ends of the channels 14. In other embodiments, the tabs 32 can be connected to the first sheet 12 in another manner, such as, for example, by an interference fit, adhesive or cohesive bonding material, fasteners, etc.
In some embodiments, such as the illustrated embodiment of FIGS. 1-5, a surface 36 of the third sheet 30 can be secured to the peaks 22 of the first sheet 12. Alternatively or in addition, ends of the third sheet 30 can define flanges 40, which can extend outwardly across side walls 42 of the core 60. In the illustrated embodiment of FIGS. 1-5, the flanges 40 extend across front and rear peaks 22 of the first sheet 12 so that the third sheet 30 at least partially encloses the core 60.
In some embodiments, such as the illustrated embodiment of FIGS. 1-5, the heat exchanger 10 can include a first flow path (represented by arrows 62 in FIG. 4) for the first working fluid and a second flow path (represented by arrows 64 in FIG. 3) for the second working fluid. In the illustrated embodiment of FIGS. 1-5, the heat exchanger 10 is configured as a single-pass heat exchanger with the first working fluid traveling along the first flow path 62 through at least one of a number of channels 14 (i.e., the downwardly opening channels 14 shown in FIGS. 1-5) defined by the first sheet 12 and with the second working fluid traveling along the second flow path 64 through at least one of a number of other channels 14 (i.e., the upwardly opening channels 14 shown in FIGS. 1-5) defined by the first sheet 12. In these embodiments, the first sheet 12 can separate the first and second flow paths 62, 64 and can prevent mixing of the first and second working fluids.
In other embodiments, the heat exchanger 10 can be configured as a multi-pass heat exchanger with the first working fluid traveling in a first pass through one or more of the channels 14 and then traveling in a second pass through one or more different channels 14 in a direction opposite to the flow direction of the first working fluid in the first pass. In these embodiments, the second working fluid can travel along the second flow path 64 through at least one of a number of other channels 14.
In yet other embodiments, the heat exchanger 10 can be configured as a multi-pass heat exchanger with the second working fluid traveling in a first pass through one or more of the channels 14 and then traveling in a second pass through one or more different channels 14 in a direction opposite to the flow direction of the second working fluid in the first pass. In these embodiments, the first working fluid can travel along the first flow path 62 through at least one of a number of other channels 14.
In still other embodiments, the heat exchanger 10 can be configured as a multi-pass heat exchanger wherein the first working fluid makes more than two consecutive passes through the heat exchanger 10. In other embodiments, the heat exchanger 10 can be configured as a multi-pass heat exchanger wherein the second working fluid makes more than two consecutive passes through the heat exchanger 10.
As shown in FIGS. 1-5, the first and second flow paths 62, 64 can be counter-directional. In other embodiments, the first and second flow paths 62, 64 can be substantially parallel.
In embodiments such as the illustrated embodiment of FIGS. 1-5 having a first flow path 62 and a second flow path 64, the housing 50 can define a first inlet 41 and a first outlet 43 for the first flow path 62. As shown in FIGS. 1-5, the housing 50 can also or alternatively define a second inlet 44 for the second flow path 64 and a second outlet 45 for the second flow path 64.
In the illustrated embodiment of FIGS. 1-5, connectors 56, 58 are secured to the housing 50 around the second inlet 44 and the second outlet 45, respectively. In some embodiments, the housing 50 can also include connectors located adjacent to the first inlet 41 and/or the first outlet 43 to prevent the first or second working fluids from bypassing the core 60 or a portion of the core 60. In some embodiments, the connectors 56, 58 provide the only points of contact between the core 60 and the housing 50, thereby minimizing or preventing unwanted heat transfer and ensuring that the core 60 remains thermally unconstrained. In addition, in some embodiments, the housing 50 and/or the core 60 can include insulation and/or refractory material to further minimize and/or prevent unwanted heat transfer.
As shown in FIGS. 1-5, during operation of the heat exchanger 10, the first working fluid can travel along the first flow path 62 through the first inlet 41 into an inlet of one or more of the channels 14 (i.e., one or more of the downwardly opening channels 14 shown in FIGS. 1-5) adjacent to the first side 52 of the first sheet 12, through the second sheet 16 positioned in the channel(s) 14, and out through the first outlet 43 adjacent to the second side 54 of the first sheet 12.
As also shown in FIGS. 1-5, the second working fluid can travel along the second flow path 64 through the second inlet 44 into one or more of the channels 14 (i.e., one or more of the upwardly opening channels 14 shown in FIGS. 1-5), a first unfinned section 20 of the channel(s) 14, the second sheet 16 positioned in the channel(s) 14, the second unfinned section 21 of the channel(s) 14, and out the second outlet 45.
In the illustrated embodiment of FIGS. 1-5, the temperature of the first working fluid at the first inlet 41 is greater than the temperature of the second working fluid at the second inlet 44, and heat energy is transferred from the first working fluid to the second working fluid. Because the first flow path 62 is substantially linear, impingement of the flow of the first working fluid through the core 60 can be minimized.
During assembly, the first sheet 12 and one or more second sheets 16 are contoured. The second sheets 16 are then inserted into the channels 14 defined between the peaks 22 formed by the first sheet 12. The third sheet 30 is then fitted around the first sheet 12 to at least partially enclose the first sheet 12 and the second sheets 16. In some embodiments, the second sheets 16 and the third sheet 30 can be secured to the first sheet 12 in a single operation (e.g., welded, soldered, brazed, etc.). In some such embodiments, the tabs 32 can also or alternatively be secured to the ends 52, 54 of the first sheet 12 during the same operation.
In embodiments, such as the illustrated embodiment, in which the core 60 is formed from a single first sheet 12 and one or more second sheets 16 and in which the housing 50 is formed from a third sheet 30, material usage can be minimized. In some such embodiments, little or no scrap is generated during manufacture of the heat exchanger 10.
In the illustrated embodiment of FIGS. 1-5, the only connection points required to prevent cross-leakage between the first flow path 62 and the second flow path 64 are located between the edges of the tabs 32 and the contoured surface of the first sheet 12. In some such embodiments, these connection points are readily accessible during the manufacture of the heat exchanger 10, thereby simplifying the identification and maintenance of leaks in the heat exchanger 10.
Various features and advantages of the invention are set forth in the following claims.

Claims

1. A heat exchanger for transferring heat energy between a first working fluid and a second working fluid, the heat exchanger comprising:

a pair of first fins formed from a single sheet and at least partially defining a flow path for the first working fluid; and

a second fin positioned between the pair of first fins and at least partially defining a flow path for the second working fluid, the flow path of the first working fluid being separated from the flow path of the second working fluid.

2. The heat exchanger of claim 1, wherein a flow direction of the first working fluid through the heat exchanger is substantially counter to a flow direction of the second working fluid.

3. The heat exchanger of claim 1, wherein the second fin is formed from a corrugated sheet.

4. The heat exchanger of claim 3, wherein the pair of first fins are corrugations formed along the sheet.

5. The heat exchanger of claim 1, wherein an end of the second fin is secured to one of the pair of first fins and an other end of the second fin is secured to an other of the pair of first fins.

6. The heat exchanger of claim 1, wherein a pair of second fins are positioned along the flow path of the second working fluid, the second fin being one of the pair of second fins.

7. The heat exchanger of claim 6, wherein the pair of second fins are formed from a single sheet.

8. The heat exchanger of claim 7, wherein the pair of second fins are corrugations formed along the sheet.

9. The heat exchanger of claim 7, wherein the pair of first fins have an upper end and a lower end, and wherein the second sheet is positioned between the upper end of the pair of fins and the lower end of the pair of first fins.

10. The heat exchanger of claim 1, wherein the pair of first fins are formed from a first corrugated sheet, wherein the second fin is formed from a second corrugated sheet, and wherein a housing is formed from a third sheet, the housing at least partially enclosing the first sheet and the second sheet.

11. The heat exchanger of claim 10, wherein the first sheet and the third sheet substantially enclose the flow path for the first working fluid, and wherein the first sheet, the second sheet, and the third sheet together at least partially enclose the flow path for the second working fluid.

12. The heat exchanger of claim 10, wherein the second sheet is nested in the first sheet between the pair of first fins.

13. A heat exchanger for transferring heat energy between a first working fluid and a second working fluid, the heat exchanger comprising:

a first corrugated sheet having a pair of peaks and separating a flow path for the first working fluid from a flow path for the second working fluid; and

a second corrugated sheet positioned between the pair of peaks of the first corrugated sheet.

14. The heat exchanger of claim 13, wherein a flow direction of the first working fluid along the flow path is substantially counter to a flow direction of the second working fluid along the flow path.

15. The heat exchanger of claim 13, wherein the second sheet has a pair of peaks, and wherein the pair of peaks of the second sheet are secured to one of the pair of peaks of the first sheet.

16. The heat exchanger of claim 13, wherein a side of the second sheet is secured to one of the pair of peaks of the first sheet and an other side of the second sheet is secured to an other of the pair of peaks of the first sheet.

17. The heat exchanger of claim 13, wherein the first sheet has a lower end, wherein the pair of peaks of the first sheet provide an upper end, and wherein the second sheet is positioned between the upper end and the lower end of the first sheet.

18. The heat exchanger of claim 13, wherein a housing is formed from a third sheet, the housing at least partially enclosing the first sheet and the second sheet.

19. The heat exchanger of claim 18, wherein the first sheet and the third sheet substantially enclose the flow path for the first working fluid, and wherein the first sheet, the second sheet, and the third sheet together at least partially enclose the flow path for the second working fluid.

20. The heat exchanger of claim 13, wherein the second sheet is nested in the first sheet between the pair of peaks of the first sheet.

21. A method of assembling a heat exchanger, the method comprising the acts of:

corrugating a first sheet to define a pair of peaks and to at least partially define a flow path for a first working fluid and a flow path for a second working fluid;

corrugating a second sheet; and

nesting the second sheet in the first sheet between the pair of peaks and along the flow path for the second working fluid.

22. The method of claim 21, wherein nesting the second sheet between the pair of peaks includes securing an end of the second sheet to one of the pair of the peaks of the first sheet and securing an other end of the second sheet to an other of the pair of peaks of the first sheet.

23. The method of claim 21, further comprising securing a third sheet to the peaks of the first sheet to at least partially enclose the first flow path.

24. The method of claim 23, wherein securing the third sheet to the peaks of the first sheet includes at least partially enclosing the second flow path between the peaks of the first sheet and the third sheet.

25. The method of claim 21, wherein corrugating the first sheet includes defining a second pair of peaks, and further comprising corrugating a third sheet and nesting the third sheet in the first sheet between the second pair of peaks.

26. The method of claim 21, wherein nesting the second sheet between the pair of peaks includes positioning the second sheet between an upper end of the first sheet and a lower end of the first sheet.

27. A heat exchanger for transferring heat energy between a first working fluid and a second working fluid, the heat exchanger comprising:

a first sheet contoured to define a plurality of first fins and having an upper end and a lower end;

a second sheet contoured to define a plurality of second fins and being positioned between the upper end of the first sheet and the lower end of the first sheet; and

a housing formed from a third sheet and at least partially enclosing the first sheet and the second sheet.

28. The heat exchanger of claim 27, wherein the housing defines an inlet and an outlet for a Mow path for the first working fluid and defines an inlet and an outlet for a flow path for the second working fluid.

29. The heat exchanger of claim 28, wherein the second sheet is positioned along the flow path of the second working fluid.

30. The heat exchanger of claim 28, wherein a flow direction of the first working fluid is substantially counter to a flow direction of the second working fluid.

31. The heat exchanger of claim 27, wherein the second sheet is nested in the first sheet between two of the plurality of first fins.

32. The heat exchanger of claim 27, wherein the plurality of first fins are corrugations formed along the first sheet.

33. The heat exchanger of claim 27, wherein a side of the first sheet is secured to one of the plurality of first fins and an other side of the second sheet is secured to an other of the plurality of first fins.

34. The heat exchanger of claim 27, wherein the plurality of second fins are corrugations formed along the second sheet.