Abstract:
A method and system is disclosed in which an air to air heat exchanger keeps two air streams separate through the use of metal fins. Edges of alternating fins are folded so that the end is in the proximity of the adjacent fin and the remaining gap is sealed. The fin stack is held together by interlocking features or through the use of a tube, in which case the fin stacks may be stamped and stacked on alignment stakes during production using the same holes as the tubes. The heat exchanger may have extended use in removing moisture and heat from a hot humid air stream. The heat exchanger has utility in heat recovery ventilation systems, clothes dryer heat recovery or other air to air heat exchanger processes where the pressure difference between the airstreams is less than approximately 5000 Pa.

Description:
PRIORITY STATEMENT UNDER 35 U.S.C. §119 &amp; 37 C.F.R. §1.78 
       [0001]    This non-provisional application claims priority based upon prior U.S. Provisional Patent Application Ser. No. 62/130,063 filed Mar. 9, 2015 in the name of Jeremy Rice entitled “Compact Stacked Fin Heat Exchanger,” the disclosure of which is incorporated herein in its entirety by reference as if fully set forth herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Air to air heat exchangers are used in many heat transfer and heat recovery applications. In applications where heat is desired to be transferred between two separated airstreams without the transfer of moisture, metal foil, often aluminum, is used as the boundary to prevent mixing between the two airstreams. As depicted in  FIG. 1 , at the edge of the fins  103 , it is necessary to block airflow  100  from entering the gap between one pair of fins  103 , while allowing the same stream of airflow  101  to enter the neighboring gap. This alternating pattern of allowing airflow to enter only pre-determined channels keeps a first airflow stream  101  and second airflow stream  102  separate, and allows heat to be transferred via the metal foil  103 . 
         [0003]    One method that is often used to prevent air  100  from entering a predetermined gap is rolling  104  the edges of adjacent fins  103 ,  106  to create a mechanical seal preventing airflow from one airstream  100  to enter that particular channel. While this method is used quite frequently, the formation of the rolled edge  104  is a time consuming process, since the edge can&#39;t be formed with a simple up/down progressive stamping machine, which can add to the product cost. Additionally, the fin pitch  105  has to be relatively large to allow for the roll  104  to be formed. The lower manufacturing limits on this pitch is around 2.0 mm, but more typically heat exchangers are designed for fin pitches of 4 mm to 8 mm fin pitch. The result of the higher fin pitch and time it takes to create the rolled edge  104  is the heat exchanger&#39;s volumes are relatively large for the airflow and heat exchanged. The heat exchangers tend to be long in the direction of the airflow paths  101 ,  102  which leads to higher pressure losses. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention enables tight fin pitches, down to approximately 1.0 mm, for air to air heat exchangers, with air streams separated by metal foil. These fin pitches can enable twice the fin density and twice the heat transfer coefficient, leading to a quadrupling of the heat transfer that may be exchanged in the same volume. The formation of the fin stack and the sealing of the alternating channels, is separated, which allows for high manufacturing process efficiency. 
         [0005]    In the present invention, there are two major process steps, the fin stack formation and the sealing of the edges with a spray coating process. In the fin stack formation step, the stack is mechanically held together by interlocking fins, in which each fin interlocks with the adjacent fin. Alternatively, the fin stack can be held together by several rods or tubes that pass through the interior of the fins with an interference fit. The fins can either press fit onto the rods, or, alternatively, the tubes can be expanded. In either case, the interference fit maintains the fin spacing. After the mechanical forming process, the edge of every other fin is formed into a 90 degree edge that is intended to touch the neighboring fin, and thus block air from entering the channels adjacent to this 90 degree formation. The coating process helps seal the remaining gaps on the edge that remain as a result of tolerances on the creation of the formed edge. Additionally, the sealing helps bond the formation and neighboring fin together, thus strengthening the fins. 
         [0006]    The foregoing has outlined rather broadly certain aspects of the present invention in order that the detailed description of the invention that follows may better be understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a schematic drawing of a rolled edge design in accordance with prior art; 
           [0009]      FIG. 2  is a top perspective view of the heat exchanger and airstreams in one embodiment of the present invention; 
           [0010]      FIG. 3  is a close-up view of the top front corer of the heat exchanger and airstreams of the foregoing embodiment; 
           [0011]      FIG. 4  is a schematic drawing depicting one embodiment of the edge sealing of stacked fins by a coating process; 
           [0012]      FIG. 5  is a schematic drawing depicting a fin stack before the coating process; 
           [0013]      FIG. 6  is a schematic drawing depicting the fin stack of  FIG. 5  after the coating process is complete; 
           [0014]      FIG. 7  is a schematic drawing depicting a fin stack held in place by interlocking fins; 
           [0015]      FIG. 8  is a schematic drawing depicting a fin stack held in place by a tube; 
           [0016]      FIG. 9  is a schematic drawing depicting a lap joint sealing of the stacked fins by a coating process; 
           [0017]      FIG. 10  is a schematic drawing depicting a fin stacking method onto a rotating table with a single fin type; 
           [0018]      FIG. 11  is a schematic drawing depicting a fin; and 
           [0019]      FIG. 12  is a schematic drawing depicting a fin stacking method utilizing two stamping presses. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention is directed to a compact stacked fin heat exchanger. The configuration and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of contexts other than a stacked fin heat exchanger. Accordingly, the specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
         [0021]    A representation of one embodiment of the present invention is presented in  FIG. 2 . Two separate airstreams enter the heat exchanger  107  and transfer heat without mixing. A first airstream  101  has a relatively high temperature when entering the heat exchanger  107 , and releases heat to a second airstream  102  that has a relatively low temperature when entering the heat exchanger  107 . As the first airstream  101  releases heat, its temperature reduces, while the second airstream  102  increases in temperature as it picks up heat. The first airstream  101  and the second airstream  102  are separated by a series of fins  108 , as is represented in a close-up depiction of one embodiment of the invention in  FIG. 3 . The first airstream  101  is blocked by a series of alternating folded edges  109  on every other fin  108 , preventing the first airstream  101  from mixing with the second airstream  102 . The second airstream  102  is blocked by a second series of alternating folded edges  110  preventing the second airstream  102  from mixing with the first airstream  101 . 
         [0022]    A schematic of the folded edged  109  is represented in  FIG. 4 . The edge  109  is part of a fin or plate  114  that keeps the first airstream  101  and second airstream  102  separate. The edge  109  is created with a 90 degree bend from the fin  114 , and nearly touches the neighboring fin  113 . Since the fin  114  is made from a mechanical process, the edge  109  and the neighboring fin  113  do not come into perfect contact, therefore a small gap  112  is created. Since the first airstream  101  and the second airstream  102  are desired to not mix, the gap  112  must be sealed. A secondary coating process, such as spray paint or powder coating, may be applied to the surface. This coating process creates a film  111  that can penetrate and seal the gap  112 , thus preventing the first airstream  101  and the second airstream  102  from mixing. 
         [0023]    It may be required to apply the coating in multiple passes to ensure the coating penetrates the gaps. If a powder coating process is used, a thicker coating may be used than conventional liquid coatings, without running. The powder coating process will need to be cured so that the particles (powder) can melt and bond to the base surface. Since the coating bonds to the base surface (fin), it has a beneficial effect of strengthening the fin&#39;s edge. Since the metal fin is thin (0.1 mm to 0.5 mm thick), the stiffness of the edge increases significantly as a result of bonding two flat fins with a 90 degree connection. This stiffening is similar to an I-beam element used to strengthen structures. 
         [0024]    A view of a fin stack with edges  109  formed by a progressive stamping process is represented in  FIG. 5 . Since the fins may be created by a thin piece of metal foil ranging from 0.1 mm to 0.5 mm thick, the strength of the fin is limited. Due to the fin&#39;s limited strength, the fin is subject to deformation, thus creating a gap  112  of varying size. In an effort to minimize this gap  112 , a spacer feature  115  can be added to the gaps in which an airstream  101  is allowed to flow through. This spacer  115  limits the span in which the edge  109  is not reinforced, and helps keep the gap  112  within a tolerance that may be sealed by the coating process. A view of the fin stack after the coating process is presented in  FIG. 6 . In this image, there are no more visible gaps  112  indicating that a seal was formed, and mixing can be prevented between the two airstreams. 
         [0025]    The benefits of having a simple 90 degree edge  109  with a separate coating  111  vs a rolled edge  104  is that the metal formation process is readily attainable by a progressive stamping process, whereby, each fin  114  may be created independently from the previous fin  114  and stacked. Additionally, the fins  114  may have a tighter fin pitch, down to 1.0 mm, versus a rolled edge, which has a lower bound fin pitch of approximately 2.0 mm. The tighter fin pitch enables twice the fin area per unit length as well as twice the heat transfer coefficient from airstream  101  to fin  114 , since the distance from the bulk airstream temperature to the fin  114  is half. The combined effect of the increased fin area and heat transfer coefficient is that the same total amount of heat may be transferred with approximately 25% of the heat exchanger length. 
         [0026]    The final issue that must be addressed is maintaining the fin stack&#39;s form prior to the coating process. This issue may be addressed in multiple ways. In a first method, represented in  FIG. 7 , the fin stack may be held together by interlocking features  116 . These features keep the fins  114  from being pulled apart as well as being pressed together. The interlocking feature can be used in lieu of or in conjunction with the spacers  115 . The interlocking features must be present on opposing sides of the fin, to ensure the fin stack is secure in all three dimensions. 
         [0027]    In a second method, as represented in  FIG. 8 , the fin stack may be held in place by a tube  120  or tubes. The tube and fins can be forced together with an interference fit, which may be created by two techniques. The first technique, the cut-out in the fin, which the tube penetrates, may have an internal diameter that is smaller than the outer diameter of the tube. The fin may be pressed onto the tube, creating a friction or interference fit between the tube and the fins. The process may be repeated to create the fin stack. In a second technique, the diameter of the cut-out in the fin may be larger than the outer diameter of the tube. The fin stack may first be loosely stacked onto the tube (or tubes). Once the stack has the desired amount of fins, the stack may be compressed with a controlled amount of force to get a close fit on the fin&#39;s edges  109 , but not too much force to deform the fins. Finally, the tube may be expanded until the outer diameter of the tube is larger than the cut-out diameter in the fin, thus creating an interference fit. 
         [0028]    The shape of the fins, and thus the heat exchanger, is highly customizable by the methods set forth herein. Rectangular, hexagonal, circular and many other shapes may be used. Additionally, the airstreams may be designed to flow in a cross-flow or counter-flow pattern, depending on the application and constraints. Additionally, the fin pitch on each airstream may be different. This feature may be useful in situations where one air stream has a high humidity level or even a higher volumetric flow with respect to a second air stream, in which the heat transfer requirements are not balanced. In the case with an air stream with a high humidity level, condensation is likely to occur when that airstream is cooled. Since the sealing process is performed at relatively low temperatures, fins with a hydrophilic coating may be used, so that the condensed water does not form droplets on the fins. Additionally, this feature gives flexibility in designing the heat exchanger, where one air stream may have a relatively large cross-section for entering air, the second may have a smaller cross-section. Each application will require its own optimization. 
         [0029]    The fins described so far are generally flat and thin. The general construction of the heat exchanger may be kept the same, with features added to the fins to enhance the heat transfer, as well as increase the structural integrity of the fins. Some of these features include ribs or waves, than can be implemented in a variety of manners. Embossing features may be used that are tall enough so that they touch the neighboring fin in the middle of the channel. These features can extend the allowable pressure difference between the air streams. Since the foil is relatively thin, it is expected that the useful pressure difference between airstreams is limited by approximately 5000 Pa. 
         [0030]    Another embodiment is presented in  FIG. 9 , which includes a first air stream  101  and second air stream  102 , which are separated by plates  117  and  118 . At the entrance, a lap joint  119  prevents the first airstream  101  from mixing with the second airstream  102 . A coating  111  may be applied to the face of the heat exchanger, thus creating a seal in the lap joint  119  between a first plate  117  folded under and a second plate  118  folder over. In some embodiments, the fold on the second/outer plate  118  is shorter than the fold on the first/inner plate  117 , thereby exposing a portion  121  of the outer surface of the inner fin  117  for the coating to land. 
         [0031]    In the method where the plates of a heat exchanger are integrally held together by a tube or tubes  120 , the plates may be stamped on a stamping press  201  and stacked onto the alignment stakes  121  which are held in place on a table  203 , as represented in  FIG. 10 . The alignment stakes go into the same holes  210  in the plate as the tubes  120  but can have a smaller diameter than the tubes, or a pointed tip, allowing for easier placement of the fins. The sheet metal flows  202  from a roll, to the press  201  and then onto the table  203 . The tubes  120  can replace the stakes  121  during another process and then can be expanded to form an interference fit with the plates. In this process, a single plate type is stacked onto the stakes, and the table  203  is rotated in 90 degree alternating clockwise and counterclockwise directions, in the time between the placement of each individual plate. 
         [0032]    One embodiment of a plate utilized in this process is represented in  FIG. 11 . The main surface  209  of the plate is generally square, however, a first direction  204  is shorter than a second direction  205  by two times the plate thickness plus stamping and stacking tolerances. The first set of edges  206  on the opposing ends of the first direction  204  are folded upward, while the second set of edges  207  on the opposing ends of the third direction  205  are folded downward. The second set of edges  207  are shorter than the first set of edges  206 . The second set of edges  207  form the outer surface of the lap joint  119 , while the first set of edges  206  form the inner surface of the lap joint  119 . The plate has holes  210  that must align with the stakes  121  during the stacking process. The location of the holes  210  must align to the same location, on a fixed reference plane, when the plate is rotated by 90 degrees. The plates are often a metal foil, therefore, embossings  216  may be used to maintain a desired spacing, add strength and increase the heat transfer effect on the fins. 
         [0033]    Alternately, hexagonally shaped plates may be used for air streams that are desired to flow counter to each other rather than cross each other. A similar fin stacking and table rotating approach may be used for a hexagonal fin, however the table must be rotated 180 degrees between the stacking of the plates versus 90 degrees. 
         [0034]    There may be situations where a single fin type cannot be used to obtain the desired geometry, such as rectangular cross-flow heat exchangers and varying fin pitch heat exchangers. In these situations, two presses may be used to create a single heat exchanger, as presented in  FIG. 12 . The first fin type can flow  213  from a metal coil through a first press  211  onto a stacking table  215 . A second fin type can flow  214  from a second metal coil through a second press  212  onto the same stacking table  215 . The presses have to be timed to stack fins in an alternating sequence from the first press  211  followed by the second press  212 . 
         [0035]    In an effort to speed production of these heat exchangers, the stacking table  215  may allow for stacking of two heat exchangers in parallel, and rotate in 180 degree increments, to allow for both presses to run continuously. 
         [0036]    While the present system and method has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions “in one embodiment” or “in another embodiment” are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms “a”, “an” and “the” mean “one or more” unless expressly specified otherwise. The term “connected” means “communicatively connected” unless otherwise defined. 
         [0037]    When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be readily apparent that a single embodiment may be substituted for that one device. 
         [0038]    In light of the wide variety of methods for constructing stacked fin heat exchangers known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto. 
         [0039]    None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims.