Patent Publication Number: US-10317143-B2

Title: Heat exchanger and method of making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. patent application Ser. No. 15/317,451, filed Dec. 9, 2016, which is a National Stage Entry of International Patent Application No. PCT/US2015/037587, filed Jun. 25, 2015, which claims priority to U.S. Provisional Patent Application No. 62/018,947, filed Jun. 30, 2014, the entire contents of all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to heat exchangers, and specifically relates to compact heat exchangers for heating and/or cooling a high-pressure fluid. 
     BACKGROUND 
     Heat exchangers are used to transfer thermal energy between two (or more) fluids while maintaining isolation between the fluids. Such devices typically operate by providing discrete channels or fluid flow paths for each of the fluids. Thermal energy from the hotter of the fluids is convectively transferred to the channels or flow paths through which that fluid is directed, is transferred (typically by thermal conduction) to the channels of flow paths through which the cooler of the fluids is directed, and is convectively transferred to that fluid. 
     Certain challenges are known to result when one of the fluids is at an elevated pressure. The elevated fluid pressure acting on the walls of channels through which the pressurized fluid is directed frequently mandates the use of channels that are rather small in size, in order to maintain acceptably low levels of mechanical stress. However, such small channel sizes also reduce the amount of surface area available to achieve the desired heat transfer, leading to increases in the length and/or number of such channels in order to meet the performance demands. Such increases lead to increased cost, size, and manufacturing complexity, and can be especially challenging in application where compact heat exchangers are desirable. Such applications, by way of example only, include refrigeration systems, fuel heating for combustion engines, vaporizers for fuel cell systems, Rankine cycle waste heat recovery evaporators, and others. 
     SUMMARY 
     According to some embodiments of the invention, a heat exchanger for transferring heat from a hot gas to a fluid includes a casing defining an internal volume of the heat exchanger, with a hot gas flow path extending through the casing from a hot gas inlet to a hot gas outlet. A fluid inlet and a fluid outlet are joined to the casing, and a plurality of fluid conduits extend through the internal volume between the fluid inlet and the fluid outlet. Each of the fluid conduits defines a hydraulically separate and continuous flow path between the fluid inlet and the fluid outlet. 
     In some embodiments, the flow paths defined by the fluid conduits are non-planar. In some such embodiments each of those flow paths is in the shape of a helix over at least a majority of the length of the flow path. In some embodiments the casing defines a longitudinal axis, and each of the non-planar flow defines a helical axis that is parallel to, and offset from, the longitudinal axis. 
     In some embodiments, at least the casing, the fluid inlet, the fluid outlet, and the fluid conduits are joined together in a common brazing process. In some embodiments casing is constructed of multiple parts that are joined in a common brazing operation with the fluid inlet, the fluid outlet, and the fluid conduits. In some embodiments the heat exchanger includes extended surfaces arranged along the hot gas flow path and joined to the fluid conduits. 
     According to another embodiment of the invention, a heat exchanger for transferring heat from a hot gas to a fluid includes two or more corrugated fin structures defining hot gas flow channels extending in a generally linear first direction, and a fluid conduit with an outer wall that is at least partially bonded to at least two of the corrugated fin structures. The fluid conduit defines a plurality of sequentially arranged flow passes for the fluid traveling through the fluid conduit. Each of the flow passes is arranged to direct the fluid in a direction that is generally perpendicular to the first direction. In some such embodiments the flow passes are oriented at an angle of inclination to the first direction that is no more than two degrees. 
     In some embodiments the heat exchanger includes a first fin structure arranged between a second and a third fin structure. Sequential flow passes are alternatingly arranged between the first and second fin structures, and the first and third fin structures. In other embodiments the heat exchanger includes a first corrugated fin structure formed into an annular shape bounded by a first inner diameter and a first outer diameter, and a second corrugated fin structure formed into an annular shape bounded by a second inner diameter and a second outer diameter, with the second outer diameter being smaller than the first inner diameter. The sequentially arranged flow passes are arranged between the second outer diameter and the first inner diameter. In some such embodiments the fluid conduit is one of several fluid conduits providing hydraulically parallel circuits for the fluid, and each one has an outer wall joined to the fin structures. In some embodiments each of the fluid conduits defines a helical flow path. 
     According to another embodiment of the invention, a fluid connection for a heat exchanger includes a connector body with a brazeable outer surface, a fluid manifold located within the connector body, and an externally accessible port connection fluidly coupled to the manifold. Flow conduit access channels extend between the outer surface of the connector and the manifold, and a braze alloy chamber at least partially intersects each of the access channels between the outer surface and the manifold. 
     According to another embodiment of the invention, a method of making a heat exchanger includes arranging flow conduits within a heat exchanger casing, extending an end of each conduit through an aperture in the wall of the casing, inserting the ends into a connector body, and, in a common brazing operation, joining the flow conduits to the connector body and joining the connector body to the casing. In some embodiments the method includes performing a leak test on the joints between the fluid conduits and the connector body after brazing and, if a leak path is found, placing additional braze paste into the braze alloy chamber and re-brazing the heat exchanger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a heat exchanger according to an embodiment of the invention. 
         FIG. 2  is a perspective view showing select portions of the heat exchanger of  FIG. 1 . 
         FIGS. 3A, 3B and 3C  are perspective views showing the heat exchanger of  FIGS. 1-2  in progressive stages of assembly. 
         FIG. 4  is a perspective view of a heat exchanger according to another embodiment of the invention. 
         FIG. 5  is a perspective view showing select portions of the heat exchanger of  FIG. 4 . 
         FIG. 6  is another perspective view showing select portions of the heat exchanger of  FIG. 4 . 
         FIG. 7  is a plan view showing select portions of the heat exchanger of  FIG. 4 . 
         FIG. 8  is a partial, sectioned, perspective view of the heat exchanger of  FIG. 4 . 
         FIG. 9  is a partial section view of the heat exchanger of  FIG. 4 . 
         FIG. 10  is a partial perspective view showing select portions of the heat exchanger of  FIG. 4 . 
         FIG. 11  is another partial section view of the heat exchanger of  FIG. 4 . 
         FIG. 12  is a plan view showing portions of a heat exchanger according to another embodiment of the invention. 
         FIG. 13  is a perspective view showing select portions of the heat exchanger of  FIG. 12 . 
         FIG. 14  is an exploded perspective view of components to be used in some embodiments of the heat exchanger of  FIG. 4 . 
         FIG. 15  is a partial section view of the components of  FIG. 14 . 
     
    
    
     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 accompanying 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. 
     A heat exchanger  1  according to one embodiment of the invention is illustrated in  FIGS. 1-3 . The heat exchanger  1  is configured to enable the transfer of thermal energy from a hot gas to a fluid. In some preferable embodiments the fluid enters the heat exchanger  1  as a pressurized liquid and is vaporized or, in some cases, partially vaporized as it passes through the heat exchanger  1  by heat received from the hot gas concurrently passing through the heat exchanger  1 . In other embodiments the fluid enters the heat exchanger  1  as a pressurized liquid and exits the heat exchanger  1  as a heated liquid. In still other embodiments the fluid enters the heat exchanger  1  as a low pressure liquid or as a gas. 
     The heat exchanger  1  includes a casing  10  that bounds an internal volume of the heat exchanger  1 . A hot gas inlet  11  and a hot gas outlet  12  are provided in the casing  10 , and a hot gas flow path extends through the heat exchanger  1  between the hot gas inlet  11  and the hot gas outlet  12 . In the embodiment of  FIG. 1 , the hot gas inlet  11  and the hot gas outlet  12  are shown as being at flange mounts arranged at opposite ends of the casing  10 . However, it should be appreciated that other arrangements of the hot gas inlet and outlet may be equally suitable or more suitable, depending upon the application wherein the heat exchanger  1  is used. 
     The exemplary casing  10  is constructed of several discrete pieces that are joined together to define the internal volume of the heat exchanger  1 . Inlet and outlet diffusers  14  join the inlet  11  and the outlet  12  to a substantially rectangular center portion of the casing  10  wherein the heat transfer between the hot gas and the fluid occurs. The substantially rectangular center portion of the casing  10  is constructed of a top plate  18 , a bottom plate  17 , side plates  19  (only one is visible in  FIG. 1 , but it should be understood that a similar side plate  19  is located on the opposite side of the heat exchanger  1 ), and corner posts  15 ,  16 . Two fluid inlet/outlet ports  13  are joined to the casing  10  to allow for the fluid to enter and exit the heat exchanger  1 , one of the inlet/outlet ports  13  functioning as an inlet and the other as an outlet. 
       FIG. 2  illustrates the heat exchanger  1  with certain portions of the casing removed in order to facilitate the description of internal details of the heat exchanger  1 . Certain aspects of the illustrated embodiment will now be explained with reference to that figure, as well as with reference to  FIGS. 3A-C  depicting the heat exchanger  1  at various stages of assembly and construction. 
     The fluid to be heated by the hot gas is conveyed through the heat exchanger  1  by way of several fluid conduits  2  that extend through the internal volume of the casing  10 . Three such fluid conduits  2  are shown in the embodiment of  FIG. 2 , but it should be understood that the number of fluid conduits  2  can be increased or decreased depending upon the needs of the application. An individual one of the fluid conduits  2  is shown in  FIG. 3A , and is characterized by a continuous conduit wall  7  extending between spaced apart ends  4  and defining a non-planar flow path for the fluid passing through the conduit. The conduit wall  7  of the exemplary embodiment has a cross-section that is of an annular shape in order to provide a design well-suited to elevated pressure operation, but it should be understood that other cross-sectional shapes might alternatively be employed. Each flow conduit  2  defines a plurality of flow passes  5  arranged to allow the fluid to flow therethrough in serial fashion. The flow passes  5  are alternatingly arranged in two spaced apart parallel planes, with arcuately shaped bend sections  6  joining successive flow passes  2 , thereby creating the non-planar flow path. 
     Corrugated fin structures  3  are additionally provided in the heat exchanger  1 , and are joined to the fluid conduits  2  for both structural stability and improved heat transfer. Each of the corrugated fin structures  3  includes alternating crests and troughs joined by flanks, and can be constructed by forming a continuous sheet of metal through a fin rolling process. Although not shown, surface enhancement features such as louvers, lances, bumps, and the like can optionally be provided on the flanks of the corrugated fin structures to further improve heat transfer. Each of the corrugated fin structures defines a series of hot gas flow channels  8  extending in a longitudinal direction of the heat exchanger  1 . 
     The spacing between those ones of the flow passes  5  of a given fluid conduit  2  arranged in one common plane, and those ones of the flow passes  5  of that fluid conduit  2  arranged in the other common plane, can be optimized to allow for the insertion of one of the corrugated fin structures  3  within that spacing, with the outer wall  7  of the fluid conduit  2  touching or almost touching both the crests and troughs of the corrugated fin structure  3 , as shown in  FIG. 3B . Such flow conduit and corrugated fin structure combinations can be arranged into a stack, with additional corrugated fin structures  3  arranged between adjacent ones of the combinations, as well as above and below the stack. The entire stack can be joined together to form a monolithic heat exchanger core by, for example, brazing. As a result of such joining, the outer wall  7  of each flow pass  5  is joined to the crests of one corrugated fin structure  3  and the troughs of another. Generally speaking, where there are N fluid flow conduits in a heat exchanger according to such an embodiment of the invention, there are (2N+1) corrugated fin structures. 
     The corner posts  15  and  16  are spaced apart so as to substantially block the bypass of hot gas around the hot gas flow channels  8 , as well as to provide a space for the bend sections  6  of the fluid conduits  2 . Solid corner posts  16  are arranged at two of the opposing corners of the core, while corner posts  15  containing a fluid manifold (not shown) are arranged at the other two opposing corners. Flow conduit connection holes  23  corresponding to the ends  4  of the fluid conduits  2  are provided in each of the corner post  15 , and the ends  4  of the fluid conduits  2  are received therein and are joined to the corner posts  15  in order to provide sealed flow channels for the fluid through the internal volume of the heat exchanger  1 . 
     Alignment apertures  20  are provided in the top plate  18  and the bottom plate  17  in order to allow for ease of assembly of the heat exchanger  1 . The apertures  20  are sized and located to correspond to protrusions  21  and  22  provided at ends of the corner posts  15  and  16 . Hollow protrusions  22  are provided at one end of each of the corner posts  15 , that one end corresponding to the fluid port  13  for that corner post  15  (the top plate  18  end in the embodiment of  FIG. 1 ). Solid protrusions  21  are provided at the opposing end of the corner posts  15 , and at either ends of the corner posts  16 . While the solid protrusions  21  need not extend beyond the surface of the top plate  18  or the bottom plate  17 , it can be preferable for the hollow protrusions  22  to be longer in order to facilitate the assembly of the port  13  to that protrusion  22 . The hollow protrusions  22  allow for fluid communication between the manifold located within the corner post  15  and the fluid port  13 . 
     In some preferable embodiments, at least that portion of the heat exchanger  1  shown in  FIG. 2  is joined together in a common brazing operation. Generally speaking, a brazing operation typically includes heating assembled metal components to a temperature that is near to, but less than, the melting temperature of the metal. A braze alloy with a lower melting temperature than the base metal, having been applied to the assembly prior to such heating in those areas where joints between the various components are desired, is caused to melt at the elevated temperature and flows to wet the metal surfaces at the joint locations. Upon cooling of the assembly, the liquefied braze alloy solidifies, creating metallurgical joints at those wetted locations. Various braze alloy compositions are known for use with different base metals such as steels, aluminum, copper, and alloys of the same. The braze alloy can be provided in various forms, for example as a clad layer on one or more of the parts, as a paste, as a spray, as a separate thin sheet, or in some other form, again varying with the base metal to be brazed. As used herein, the term “common brazing operation” means that joints between the indicated components are made within the same brazing operation. 
     In at least some embodiments, the heat exchanger  1  is constructed of austenitic stainless steel material and is brazed using a Nickel-Chromium brazing alloy. Very thin sheets of such braze alloy are assembled between the fluid conduit wall  7  and the crests or troughs of the corrugated fin structures  3 . Braze alloy in a paste form is applied at the flow conduit connection holes  23  and at the alignment protrusions  21  extending through the alignment apertures  20  of the bottom plate  17 . Upon heating of the assembly to the brazing temperature, the braze alloy reflows to create braze joints as previously described. The braze alloy provided between the fluid conduits  2  and the corrugated fin structures  3  flows by capillary action to additionally form joints between adjacent passes  5  of the fluid conduits  2 , providing a more rigid and robust structure. Additional components of the heat exchanger  1  can be assembled after brazing. For example, the top plate  18 , side plates  19 , and diffusers  14  can be welded into place. The fluid inlet and outlet fittings  13  can be provided as two-part fittings, with one part welded in place to the top plate  18  and the other part joined by mechanical threads. In some embodiments at least some of these additional parts can, however, be joined in the brazing operation. 
     A heat exchanger  101  according to another embodiment of the invention is depicted in  FIG. 4 . The heat exchanger  101  provides certain advantages over the heat exchanger  1  in that it is more amenable to joining all of the parts in a common brazing operation. The heat exchanger  101  again includes a casing  110  defining an internal volume therein for the hot gas to pass through, with a hot gas inlet  111  arranged at one end of the casing  110  and a hot gas outlet  112  arranged at an opposing end of the casing  110 . In certain embodiments (for example, when it is desirable for the hot gas to traverse an even number of passes through the heat exchanger) the hot gas inlet  111  and hot gas outlet  112  can be arranged at a common end of the heat exchanger. In still other embodiments the hot gas inlet and/or outlet are arranged at a location on the casing  110  other than an end. 
     The heat exchanger  101  further includes two ports  113  joined to the casing  110 . A fluid connection is provided between the ports  113  as will be described in more detail later, so that one of the ports  113  can serve as a fluid inlet and the other of the ports  113  can serve as a fluid outlet. Depending upon the requirements of the application, the heat exchanger  101  can be operated in a counter-flow mode of operation by having that one of the fluid ports  113  located nearest to the hot gas outlet  112  serve as the fluid inlet, or in a concurrent-flow operation by having that one of the fluid ports  113  located nearest to the hot gas inlet  111  serve as the fluid inlet. 
     The casing  110  of the heat exchanger  101   101  includes a centrally located casing cylinder  124  joined to diffusers  114  at either end. Fluid connections  130  are joined to the diffusers  114  in order to provide the fluid ports  113 . 
     Fluid conduits  102  extend between the fluid connections  130  to provide a plurality of fluid flow paths through the heat exchanger  101  for a fluid to be heated by the hot gas passing therethrough. As best seen in  FIG. 5 , the fluid conduits  102  again define non-planar flow paths for the fluid through the internal volume of the casing  110 . In the exemplary embodiment three such fluid conduits  102  are provided, but it should be understood that more or fewer such fluid conduits  102  can be used as determined by the needs of the application. 
     The multiple flow conduits  102  are wound together into a cylindrical shape, so that each of the flow conduits  102  defines a helical flow path through a substantial portion of the casing cylinder  124 . In so doing, each complete 360° convolution of a fluid conduit  102  defines a flow pass  105  for the fluid oriented substantially in cross-flow to the hot gas traveling through the heat exchanger  101 . In other words, as the hot gas flow is traveling in a longitudinal direction generally parallel to the axis of the casing cylinder  124 , the fluid traversing any flow pass  105  is traveling in a direction that is always generally perpendicular to that longitudinal direction. 
     In many applications, particularly those wherein the fluid traveling along the fluid conduits  102  is at an elevated pressure, it is desirable to have a flow channel that is small in size, thereby minimizing the structural loads imposed on the fluid conduit  102  by the fluid pressure. Such structural loading can be further minimized by providing flow channels that are circular in cross-section, so that the tube wall  106  is an annular shape in cross-section. Whether the flow channel is circular in cross-section or not, the size of the channel can be quantified by its hydraulic diameter, calculated as four times the flow area divided by the wetted perimeter, and having units of length. For a circular channel the hydraulic diameter is equal to the actual diameter, whereas for non-circular channels the hydraulic diameter is the diameter of a circular channel that exhibits an equivalent ratio of flow area to wetted perimeter. In some preferable embodiments of the invention the fluid conduits  102  have a hydraulic diameter that is no greater than one millimeter. 
     However, oftentimes in conflict with the desire to minimize the size of the channels for pressure resistance purposes is the desire to maximize the surface area of the channel wall in order to facilitate the transfer of heat to the fluid passing through the channel. As the channel size is reduced, maintaining channel surface area requires that the length of the channel be increased. It can be problematic, though, to increase substantially the channel length within a fixed volume. The non-planar fluid conduits of the heat exchanger  101  provide a solution to that problem by enabling flow channels of rather small cross-section, but substantial length. Each flow pass  105  occupies only a small portion of the length of the heat exchanger  101  in the longitudinal direction, and many such flow channels can be provided in series with one another for each of the flow conduits  102  in order to enable the requisite long channel length. Furthermore, adjacent ones of the flow channels  105  can be placed directly alongside one another for compactness without blocking the flow of the hot gas over the surfaces of the fluid conduit walls  106 . 
     The design of the heat exchanger  101  provides flexibility in adjusting the pressure drop by allowing for the total number of flow passes  105  (e.g. the total length available divided by the outer dimension of the fluid conduit wall  106 ) to be distributed amongst multiple fluid conduits  102  without impacting the total surface area available for heat transfer. Increasing the number of such fluid conduits  102  decreases both the length of each conduit and the fluid velocity in the conduits, and will therefore lead to a dramatic reduction in the pressure drop incurred. The maximum number of flow passes  105  can be attained by having adjacent ones of the flow passes in direct contact with one another, as best seen in  FIG. 7 . This compact arrangement allows for each of the flow passes  105  to be arranged in substantially cross-flow orientation to the flow of exhaust gas, which is traveling in the direction indicated by the arrow  109  (i.e. in the longitudinal direction of the heat exchanger  101 ). As the fluid traverses one of the flow passes  105 , the instantaneous direction of fluid flow through the conduit  102  is approximately perpendicular to the direction of the hot gas flow, although it will vary slightly from a truly perpendicular arrangement due to the angle of inclination, θ. In some preferable embodiments the angle of inclination θ is no greater than two degrees. 
     One potential shortcoming of the wound together flow conduits  102  as depicted in  FIG. 5  is that a portion of the outer surfaces of the tube walls  106  is not available to the flow of hot gas for convective heat transfer, that portion of the tube wall instead being in intimate contact with the tube wall  106  of another flow conduit  102 . In order to address the potentially deleterious effect on heat transfer that could result, it can be advantageous to provide a corrugated fin structure  103   a  within an annulus located radially outward of the cylinder formed by the fluid conduits  102 , and a corrugated fin structure  103   b  within an annulus located radially inward of that cylinder. The corrugated fin structures  103   a,b  can initially be formed as planar structures similar to the corrugated fin structures  3  of the embodiment of  FIG. 2 , and can subsequently be formed into an annular shape. Crests of the corrugated fin structures  103   b , and troughs of the corrugated fin structure  103   a , can be bonded to the tube walls  106  in order to provide decreased resistance to heat transfer so that the corrugated fin structures  103   a, b  can effectively operate as extended heat transfer surfaces for the hot gas. As before, each of the corrugated fin structures defines a series of hot gas flow channels  108  extending in a longitudinal direction (i.e. the direction indicated by the arrow  109 ) of the heat exchanger  101 . 
     In one embodiment of the invention, the components of the heat exchanger  101  are assembled and joined to form a completed heat exchanger  101  in one brazing operation. This common brazing operation creates the requisite joint between the components of the casing  110 , between the fluid conduits  102  and the fluid connections  130 , and between the fluid conduits  102  and the corrugated fin structures  103   a,b  (if present). 
     To assemble the heat exchanger  101 , the corrugated fin structure  103   a  is formed into an annular shape and inserted into the casing cylinder  124 . Resizing of the corrugated fin structure  103   a  can optionally be performed after the insertion by mechanically re-sizing the internal diameter of the annular shape with a cylinder having a slight interference fit with the corrugated fin structure  103   a . Such a re-sizing operation creates a more uniform internal diameter of the corrugated fin structure  103   a , as well as slightly flattening the troughs of the corrugations to increase the surface area available for joints between the corrugated fin structure  103   a  and the fluid conduits  102 . 
     The fluid conduits  102 , having been wound into the cylindrical shape shown in  FIG. 5 , are inserted into the center of the corrugated fin structure  103   a . Braze alloy can be placed between the corrugated fin structure  103   a  and the fluid conduits  102  as a thin sheet inserted prior to, or concomitant with, the insertion of the fluid conduits  102 . Alternatively, the braze alloy can be applied as a spray or a paste onto the troughs of the corrugated fin structure  103   a , or onto the outer surfaces of the tube walls  106 , or both. In some embodiments having compatible metal alloys, the braze alloy can be applied as a clad layer onto some of the metal surfaces. 
     The corrugated fin structure  103   b  is formed into an annular shape and is inserted into the center of the cylinder formed by the fluid conduits  102 . Braze alloy can be inserted between the crests of the corrugated fin structure  103   b  and the fluid conduits  102  in a similar manner as was described for the corrugated fin structure  103   a . A central core  128  is inserted into the center of the corrugated fin structure  103   b , and can be sized to have a slight interference fit with the corrugated fin structure  103   b  so that the crests of the corrugated fin structure  103   b  are pressed tightly against the fluid conduits  102 . The central core  128  can be a solid cylinder, or a hollow cylinder with caps on one or both ends. 
     In some embodiments it can be preferable to select the specific alloy compositions of the various components to ensure better bonding between components during brazing. The casing cylinder  124 , for example, can be constructed of an alloy having a slightly lower coefficient of thermal expansion than that of the internal components. As the assembly is heated to the brazing temperature, the internal components will thermally expand by a greater percentage than will the casing cylinder  124 , thereby ensuring that tight contact is maintained between the components intended to be joined by the braze alloy. As one non-limiting example, the casing cylinder  124  can be constructed of grade  409  ferritic stainless steel while the internal components (e.g. the corrugated fin structures  103   a  and  103   b , the fluid conduits  102 , and the center core  128 ) are constructed of grade  316  stainless steel, which has a coefficient of thermal expansion that is approximately one and a half times that of grade  409  stainless steel. 
     Connection of the ends  104  of the fluid conduits  102  to the fluid connectors  130  in a brazing operation can be especially problematic. The small internal size of the fluid conduits  102  makes them especially prone to clogging by braze alloy when the braze alloy is liquefied at braze temperature. In some embodiments of the invention, the fluid connectors  130  have been designed with specific features to prevent such clogging and allow for the fluid conduits  102  to be economically joined to the fluid connectors  130  in a common brazing operation with the other components to be joined. 
     With specific reference to  FIGS. 8 and 9 , the fluid connections  130  as depicted include a connector body  135  having a brazeable outer surface. The connector body  135  can, for example, be constructed of a similar alloy as the rest of the casing  110 . Within the connector body  135  is located a fluid manifold  131  in connection with the fluid port  113  that functions as either the inlet or the outlet for the fluid flow. The fluid manifold serves either to distribute the fluid to the plurality of fluid conduits  102  (in the case where the fluid connector  130  provides the fluid inlet port) or to receive the fluid from the plurality of fluid conduits  102  (in the case where the fluid connector  130  provides the fluid outlet port). Multiple flow conduit access channels  133 , each corresponding to one of the plurality of fluid conduits  102 , extend from an outer surface of the connector body  135  to the fluid manifold  131 . The flow conduit access channels  133  are sized to be slightly larger than the outer dimensions of the tube walls  106  so that a braze alloy can flow by capillary action during brazing to fill the clearance void, thereby joining the tube walls  106  to the connector body  135 . In some preferable embodiments both the tube walls  106  of the fluid conduits  102  and the flow conduit access channels  133  are circular in cross-section for ease of assembly and to promote a uniform braze joint. 
     A braze alloy chamber  132  is further provided within the connector body  135 . The braze alloy chamber partially intersects each of the flow conduit access channels  133  at a location between the outer surface of the connector body  135  and the manifold  131 . An externally accessible opening  134  of the braze alloy chamber  132  is provided on an external surface of the connector body  135 . While the exemplary embodiment places the opening  134  on a different external surface of the connector body  135  than that surface which is intersected by the flow conduit access channels  133 , in some alternative embodiments they can be the same external surface. It is preferable, however, that the opening  134  of the braze alloy chamber  132  be accessible after assembly of the connector  130  to the casing  110 . 
     During assembly of the heat exchanger  101 , and preferably prior to a common brazing operation for the components of the heat exchanger  101 , the diffusers  114  are assembled to the casing cylinder  124 . As best seen in  FIG. 9 , the casing cylinder  124  has flared ends sized to receive an end of a diffuser  114 . Preferably some clearance is provided between the flared end and the diffuser  114  so that braze alloy (which can, for example, be applied in paste form at the joint) can wick by capillary action into that clearance gap to provide a metallurgical joint between the components. In assembling the diffuser  114  to the cylinder  124 , ends  104  of the fluid conduits  102  can be made to pass through an aperture  126  of the casing  110 , provided in this case within the diffuser  114 . 
     The fluid connector  130  can be assembled to the casing  110  by inserting the ends  104  of the fluid conduits  102 , having been made accessible by passing through the aperture  126  so as to be external to the casing  110 , into the corresponding flow conduit access channels  133  so that the ends  104  reside within the manifold  131 . Coincident therewith, outer surfaces of the connector body  135  are disposed near to or against corresponding surfaces  127  of the casing  110 . The corresponding surfaces  127  of the exemplary embodiment are provided by a depression formed into the diffuser  114 . Braze alloy is applied between those surfaces so that the connector  130  can be joined to the casing  110  in the common brazing operation, thereby additionally closing off the aperture  126  from the external environment to prevent leakage of the hot gas through the aperture  126  during operation. 
     Prior to the common brazing operation, a braze alloy paste is dispensed into the braze alloy chamber  132  through the opening  134 . The braze alloy paste is preferably dispensed after assembly of the fluid conduits  102  to the fluid connector  130 , in order to avoid clogging of the open ends  104  with paste during the insertion of the fluid conduits  102  into the fluid connector  130 . As best seen in  FIG. 9 , the braze alloy chamber  132  is located so as to prevent it from being blocked by the inserted fluid conduits  102 . The flow conduit access channels  133  are arranged so that the centroidal axes of all such channels  133  are aligned in a plane. The braze alloy chamber  132  extends parallel to, but offset from, that plane to ensure that the chamber  132  is not completely blocked along the entirety of its length, even though the chamber  132  is smaller in cross-section than the flow conduit access channels  133 . This enables the braze alloy chamber  132  to be kept to a small enough internal volume so as to avoid an excess of braze alloy, which could otherwise result in clogging of the fluid conduits  102 . 
     In some embodiments of the invention, the heat exchanger  101  is fabricated using a single common brazing operation as previously described, and after brazing the heat exchanger  101  is tested for leaks along the fluid flow path between the inlet and outlet ports  113 . As the only joints created along that fluid flow path are those between the fluid connections  130  and the fluid conduits  102 , in the event of a leak path being indicated by the leak test, the heat exchanger  101  can be repaired by introducing additional braze alloy paste (for example, a braze alloy paste having a slightly lower melting point than the braze alloy paste originally used) into the braze alloy chambers  132  and re-brazing the heat exchanger  101 . In the case where no leak path is indicated during the leak testing, the braze alloy manifold opening  134  can be permanently sealed (by, for example, welding) to further seal the fluid flow path against eventual leakage. Such a process can be especially beneficial when the fluid intended to be circulated along that flow path presents a danger if leakage occurs. 
     In some preferable embodiments of the invention, the fluid conduits  102  of the heat exchanger  101  are provided with a compliant portion  125  between the flow passes  105  and one or both of the fluid connections  130 , as shown in  FIG. 10 . The compliant portion  125  can be provided by having the length of the fluid conduits  102  extending between the corrugated fin structures  103   a,b  and the fluid connection  130  be substantially greater than the actual distance therebetween. In some embodiments the compliant portion  125  can be provided as an additional extension of the helical profile beyond the region where the fluid conduits  102  are bonded to the corrugated fin structures. Such a compliant portion  125  can prevent excessive stresses on the braze joints between the fluid conduits  102  and the fluid connector  130  as a result of thermal cycling events, for example. 
     In some embodiments of the invention, the integrity of the braze joints between the corrugated fin structures  103   a,b  and the tube walls  106  can be improved by the addition of thin metallic shims  129  arranged between the tube walls  106  and the corrugated fin structures  103   a,b  as shown in  FIG. 11 . The presence of the shims  129  can prevent the loss of braze alloy to the crevices between adjacent passes  105  of the fluid conduits  102 , which could result in insufficient braze alloy remaining for the bonding of the corrugated fin structures  103   a,b  and the tube walls  106 . The metallic shims  129  can be formed into a cylindrical shape prior to insertion, and braze alloy can be provided on either side of each shim  129  as a separate sheet, spray, coating, clad layer, or other form. During the brazing operation, the corrugated fin structures  103   a,b  and the tube walls  106  and the metallic shims  129  are brazed together to form a bonded unit. As a further benefit, the metallic shims can partially conform to the surfaces of the tube wall  106 , thereby reducing the thermal resistance through the bonded joint by providing additional lateral heat spreading. 
     An alternative embodiment of a heat exchanger  201  according to the present invention is depicted in  FIGS. 12 and 13 . The heat exchanger  201  again uses helically wound flow conduits  202 , but avoids the use of corrugated fin structures. An advantage of such a design can be found in reduced manufacturing complexity and material costs, although at the expense of reduced heat transfer per unit volume resulting from the lack of extended heat transfer surfaces for the hot gas. In contrast to the embodiment of  FIGS. 4-7 , the flow conduits  202  of the heat exchanger  201  are displaced relative to one another such that no two of the helix axes are coincident. As best seen in  FIG. 12 , the fluid conduits  202  can be arranged to fill the inner volume of a casing cylinder  210  (similar to the casing cylinder  110  of the previously described embodiment). Such an arrangement exposes essentially the entirety of the outer surface of the fluid conduits  202  to the gas flow passing through the heat exchanger  201 , and provides a plurality of flow channels for the hot gas between the overlapping coils of the fluid conduits  202 . Rods  240  extend through the helical coils in order to maintain the relative arrangement of the fluid conduits  202 . Each such rod  240  is located internally of two of the helixes defined by fluid conduits  202  and externally of the other two of the helixes, so that the positioning of the four fluid conduits  202  is maintained. While the exemplary embodiment of  FIGS. 12 and 13  has four fluid conduits  202 , it should be understood that more or fewer such conduits can be provided. In general, when rods  240  are present, the rods  240  are preferably arranged so that each rod  240  is located interior to at least two of the helices and exterior to at least one of the helices. 
     The outer casing  210  of the heat exchanger  201  can in general be of a similar design to the outer casing  110  of the heat exchanger  101 , including for example diffusers  114  and fluid connections  130 . The lack of corrugated fin structures within the heat exchanger  201  avoids the need to create internal braze joints other than the joints between the ends of the fluid conduits  202  and the fluid connections  130 . This allows for the entire fluid conduits  202  to be compliant, enabling a structurally robust design. 
     An alternative construction for the central core  128  of the embodiment of  FIGS. 4-6  is depicted in  FIGS. 14-15 , and is identified as  128 ′. As shown in the exploded perspective view of  FIG. 14 , the central core  128 ′ includes a metallic sleeve  301  having a generally cylindrical form, with both ends of the sleeve  301  being open. A slit  302  extends longitudinally along the length of the sleeve  301 . By way of example, the sleeve  301  and slit  302  could be formed by sawing or otherwise slitting a tube, or by forming a flat sheet into a cylindrical form without joining the free edges, thereby resulting in the formation of the slit  302 . Preferably the outer diameter of the sleeve  301  is slightly less than the inner diameter formed by the troughs of the corrugated fin structure  103   b , so that the sleeve  301  is easily inserted into the central portion of the heat exchanger during assembly. 
     Once the sleeve  301  has been so inserted, end caps  303  are inserted into the open ends of the sleeve  301  to diametrically expand the sleeve  301 . This diametrical expansion disposes the core  128 ′ against the troughs of the corrugated fin structure  103   b , thereby ensuring good contact between surfaces to be brazed. The end caps  303  can be provided with a series of ramped steps  304  along their periphery, as best seen in the partial cross-sectional view of  FIG. 15 . As the end caps  303  are inserted, the ramped steps  304  progressively expand the slit sleeve  301  in the radial direction. Friction between the inwardly facing surface of the sleeve  301  and the steps  304  can ensure that the end caps  303  are retained within the sleeve  301  during the brazing process. 
     In some embodiments, the ramped steps  304  can be replace with a continuous cone-shaped surface having an angle that is sufficiently small so as to allow for retention of the end caps  303  by frictional forces. Alternatively, or in addition, the positioning of the end caps  303  can be maintained through the use of one or more mechanical fasteners. By way of example, a bolt can be inserted through holes provided in each of the end caps  303  and a nut can be fastened to a threaded end of the bolt to maintain the positioning of the end caps after insertion. In some such embodiments the bolt can be constructed of a material having a lower thermal coefficient of expansion than the sleeve so that the end caps are drawn further into the sleeve during the brazing process, thereby further expanding the sleeve to ensure that contact is maintained between parts to be joined. In other alternative embodiments, the end caps can be designed to extend over a substantial portion of the length of the sleeve  301  and can be provided with ramped surfaces that engage and function as a wedge to enlarge the sleeve  301  in the radial direction. 
     Various alternatives to the certain features and elements of the present invention are described with reference to specific embodiments of the present invention. With the exception of features, elements, and manners of operation that are mutually exclusive of or are inconsistent with each embodiment described above, it should be noted that the alternative features, elements, and manners of operation described with reference to one particular embodiment are applicable to the other embodiments. 
     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 in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention.