Patent Publication Number: US-2016238323-A1

Title: Plate fin heat exchangers and methods for manufacturing same

Description:
TECHNICAL FIELD 
     The present generally concerns heat exchangers and more particularly to a method of fabricating lightweight plate fin heat exchangers for use with corrosive cooling fluids such as deionized water. 
     BACKGROUND 
     Plate fin heat exchangers are generally well known in the prior art, where the heat exchanger design uses plates and finned chambers to transfer heat between fluids. The design is effectively known for its efficient construction and relatively high ratio of heat transfer surface area to volume. 
     The plate fin heat exchanger is widely used in many industries, including aerospace and automotive applications because of its compact size and lightweight properties, and also in cryogenic systems where heat transfer between fluids with small temperature differences is possible. 
     A plate fin heat exchanger is generally made of layers of corrugated sheets separated by flat metal plates to create a series of finned chambers. Separate hot and cold fluid streams flow through alternating layers of the heat exchanger and are enclosed at the edges by side bars. Heat is transferred from one fluid stream through the separator plate and fin interface, then through the next set of fins into the adjacent fluid. The fins also serve to increase the structural integrity of the heat exchanger and allow it to withstand elevated pressures while providing an extended surface area for heat transfer. 
     Typically, aluminum alloy plate fin heat exchangers have been used for weight critical applications, and stainless steel or nickel alloy constructions are more commonly found in the chemical industry, especially when corrosive fluids are present. In some aerospace applications, titanium plate fin heat exchangers are used. 
     One major disadvantage of plate fin heat exchangers is that they are prone to fouling due to their small flow channels, and this is especially the case for aluminum alloy heat exchangers due to material compatibility issues with the cooling fluid selected. For these applications, deionized water cannot be used as a coolant and must be substituted with a cooling fluid that incorporates corrosion inhibitors. 
     Although extremely corrosive to most materials, deionized water as a cooling fluid offers significant advantages due to its chemical and electrical properties which make it an optimal choice for cooling when the liquid circuit contains micro-channels or when sensitive electronics are involved. 
     Because of its pure nature and extremely low concentration of ions, deionized water has important performance attributes. First, it eliminates the possibility of mineral deposits within the heat exchanger that would block cooling flow and therefore degrade cooling efficiency. Second, it also eliminates the risk of electrical arcing due to static charge buildup from the circulating coolant, which might damage sensitive electronics in components being cooled within the cooling loop. 
     Deionized water is also an advantage because of its higher specific heat capacity (c p ) when compared to other cooling fluids. For example, at 20° C. and constant pressure of 1 atmosphere, the specific heat for deionized water is 4182 J/kg_K, where a similar cooling fluid with corrosion inhibitors such as a mixture of 50/50 Water-Ethylene Glycol (WEG) is approximately 3384 J/kg_K, almost 20% lower. This parameter not only affects the heat exchanger&#39;s size, but also the coolant flow rate necessary to reject a specific amount of heat and therefore the size, capacity and power of the coolant pump required. 
     Stainless steel or nickel alloy plate fin heat exchangers can be implemented where corrosive cooling fluids (i.e. deionized water) are required, but these constructions are much heavier that aluminum alloy designs and are not favorable for weight critical applications such as aerospace. 
     Titanium plate fin heat exchangers can also be used when corrosive cooling fluids are present, and they offer some weight advantage over stainless steel and nickel alloy constructions (30-50% less weight), but they are very expensive and still do not offer the same level of thermal performance when compared to aluminum alloys. 
     A number of different attempts have been made to manufacture a aluminum alloy heat exchanger for use with deionized water including for example U.S. Pat. No. 5,561,981 to Quisenberry et al for “Heat Exchanger for Thermoelectric Cooling Device” on Oct. 8 th , 1996. In this embodiment, a heat absorption assembly for use with a thermoelectric cooling device is discussed, which comprises a thermal transfer plate having a Teflon coated, serpentine fluid conduit therein facilitating the flow of a variety of cooling fluids (including deionized water) to be cooled therethrough without contamination thereof. Although this patent addresses the problem of using a cooling fluid such as deionized water in an aluminum alloy heat exchanger, it has a number of significant drawbacks when considering plate fin heat exchangers specifically. Because a plate fin heat exchanger design incorporates micro-channels for the coolant flow, a Teflon coating would not be practical since it would be very difficult to coat such small channel features without the risk of blockage. Further, the thermal conductivity of Teflon itself is very low (0.25 W/m_K) when compared to a typical aluminum alloy used for heat exchangers (202 W/m_K), and it would therefore act as a thermal insulator even if the Teflon coating was thin, thereby reducing the overall thermal efficiency of the heat exchanger. 
     Another example is provided in U.S. Pat. No. 8,741,445 B2 to Bannai et al. for “Aluminum or aluminum alloy material having surface treatment coating film, and method for treating a surface thereof” on Jun. 3 rd , 2014. In this embodiment, a heat exchanger of an aluminum or aluminum alloy material having a surface treatment coating film on a surface of a substrate formed by an aluminum or aluminum alloy is discussed. In particular, the invention relates to a heat exchanger used by incorporating the aluminum or aluminum alloy material in automobiles or the like, and to a surface treatment method therefor. Although this patent addresses the problem of aluminum oxidation within a heat exchanger, it does not specifically target the use of corrosive cooling fluids such as deionized water within the liquid cooling loop, and it is more focused on protecting the cooling fins as opposed to the internal micro-channels of a plate fin heat exchanger. Further, it is questionable whether the surface treatment applied would be practical and effective for the internal cooling loop of the heat exchanger since it would be applied as a post processing step after the heat exchanger is constructed, bringing into question whether the entire internal surface within the plate fin heat exchanger would be covered, thereby leaving areas of exposed aluminum which would oxidize. 
     U.S. patent application Ser. No. 11/852,721 from Ohwaki et al. for “Aluminum alloy with high seawater corrosion resistance and plate-fin heat exchanger” on filing date Sep. 10 th , 2007, describes an aluminum alloy for use in a plate fin heat exchanger having a heat transfer portion with seawater as a coolant including an organic phosphonic acid underlying coating disposed on the surface of the aluminum alloy and a fluorocarbon resin coating disposed on the organic phosphonic acid underlying coating, the fluorocarbon resin coating having an average thickness of 1 to 100_m after drying, where the aluminum alloy has improved durability of coating adhesion and excellent seawater corrosion resistance. Similarly, although this patent addresses the problem of aluminum oxidation within the liquid cooling loop of a plate fin heat exchanger, it does not specifically address the extremely corrosive nature of deionized water. For example, the very lack of ions makes this coolant unusually corrosive and it is sometimes referred to as the “universal solvent,” making it one of the most aggressive solvents known. In fact, to a varying degree, it will dissolve everything to which it is exposed. With that in mind, aluminum coatings that may be acceptable for other “corrosive” cooling fluids will likely not be practical for use with deionized water. 
     Thus, there is a need for an improved method of fabricating a low cost, lightweight, high performance plate fin heat exchanger where corrosive cooling fluids such as deionized water can be used. 
     BRIEF SUMMARY 
     We have designed a low cost method for producing lightweight, high efficiency plate fin heat exchangers for operation with corrosive cooling fluids such as deionized water. The method involves a hybrid, laminar construction where lightweight materials with a specific geometry, having a relatively high thermal conductivity and compatible with deionized water, such as flexible graphite and stainless steel film, are chosen for the liquid cooling loop, and materials with very high thermal conductivity, such as aluminum or carbon foam, are chosen for the cooling fins. 
     Components within the liquid cooling loop are fabricated via cutting through a sheet of thermally conductive material compatible with corrosive cooling fluids such as deionized water, and then finishing the cut sheet. 
     Components within the cooling fin section are fabricated with corrugated, thermally conductive material which is typical to plate fin heat exchangers. Alternate fin arrangements are possible including a square profile, triangular profile, or semi-circle profile. Further, other cooling “fin” mechanisms are possible such as carbon foam. 
     The hybrid subassemblies are adhesively bonded together, then stacked in an alternating manner and compressed between an upper and lower endplate. 
     Unlike the examples described above, our method produces a practical, thermally efficient, lightweight, corrosion resistant, plate fin heat exchanger compatible with corrosive cooling fluids such as deionized water. It requires only die cutting cooling flow channels and/or manifolds within the liquid cooling section, and then finishing the parts by pressing. Our method cuts all cooling flow channels and/or manifolds in one step, and the “finishing” step does not require careful part alignment. Furthermore, our method only requires one die per part. The liquid cooling and cooling fin subassemblies are stacked in an alternating fashion and compressed between an upper and lower endplate. 
     Accordingly, there is provided a plate fin heat exchanger subassembly comprising: 
     first, second and third plates in intimate contact with each other, the first plate being sandwiched between the second and third plates, each of the plates having a first at least one fluid opening and a second at least one fluid opening located at the ends of each plate, the first plate having at least one elongate fluid channel extending between the first and second fluid openings; 
     a first fluid passageway for a first heat exchanger fluid extending through the first and second fluid openings in each plate and along the elongate channel in the first plate; and 
     at least one fin having first and second thermal transfer surfaces, the first thermal transfer surface being in intimate contact with the second plate, second fluid passageways for a second heat exchanger fluid being located on each side of the fin. 
     In one example, the first, second and third plates each include a first set of a plurality of fluid openings and a second set of a plurality of fluid openings located at the ends of each plate. The first plate includes a plurality of elongate fluid channels extending between the first and second set of fluid openings, the channels being disposed substantially parallel to each other. 
     In one example, the subassembly includes a plurality of interconnected fins having first and second heat transfer surfaces, the first heat transfer surface of each fin being in intimate contact with the second plate so as to create a plurality of second fluid passageways for the second heat exchanger fluid to flow therealong. First and second manifold spacers are located at either end of the plurality of interconnected fins, the first and second manifold spacers having respectively first and second set of fluid openings therein, the fluid openings of the manifold spacers being in fluid communication with the fluid openings in the first, second and third plates. 
     In one example, the first plate includes a first and second set of fluid openings, the channels being disposed substantially parallel to each other and located at each end of the first plate, and a plurality of elongate fluid channels connecting the first and second set of openings for the first heat exchanger fluid; a plurality of interconnected fins having first and second heat transfer surfaces, the first heat transfer surface of each fin being in intimate contact with the second plate so as to create a plurality of second fluid passageways for the second heat exchanger fluid to flow therealong; and the first heat exchanger fluid flowing along the elongate channels in the first plate flowing substantially orthogonal to the second heat exchanger fluid flowing along the second fluid passageways. 
     In another example, the first heat exchanger fluid is a corrosive coolant fluid. The corrosive coolant fluid is de-ionized water. 
     In one example, the second heat exchanger fluid is air. 
     In one example, the interconnected fins include first and second extensions at each end, the extensions having therein a plurality of fluid openings therein, the openings being in fluid communication with the fluid openings in the first, second and third plates. 
     In one example, the fluid flow channels are disposed in a serpentine flow field pattern. 
     In another example, embossed tabs are located between the serpentine channels to maintain channel spacing. The first plate includes first and second serpentine manifold openings, each opening located in a tab extending away from the first plate. 
     In one example, the fin is made of rectangular or square corrugated aluminum. 
     In another example, the fin is made of semicircular corrugated aluminum. 
     In another example, the subassembly, according to claim  1 , the fin is made of triangulated corrugated aluminum. The aluminum fin is anodized. 
     In yet another example, the second and third plates are separator plates. 
     Accordingly in another aspect, there is provided a plate fin heat exchanger, the heat exchanger comprising: 
     a lower compression end plate having a first heat exchanger fluid inlet connected thereto; 
     an upper compression end plate, a first heat exchanger fluid outlet connected thereto; and 
     two stacked heat exchanger subassemblies located between the lower and upper compression end plates, each subassembly having: 
     first, second and third plates in intimate contact with each other, the first plate being sandwiched between the second and third plates, each of the plates having a first and at least one fluid opening and a second and at least one fluid opening located at the ends of each plate, the first plate having at least one elongate fluid channel extending between the first and second fluid openings; 
     a first fluid passageway for a first heat exchanger fluid extending through the first and second fluid openings in each plate and along the elongate channel in the first plate; 
     a plurality of interconnected fins having first and second heat transfer surfaces, the first heat transfer surface of each fin being in intimate contact with the second plate so as to create a plurality of second fluid passageways for the second heat exchanger fluid to flow therealong; and 
     between the two subassemblies, at least one manifold spacer body is located at each end of the interconnected fins, the manifold spacer body being located to permit a two-pass heat exchanger configuration. 
     In one example, the manifold spacer body is located in a central portion of a stacked heat exchanger. 
     In one example, each subassembly includes first and second manifold spacers which are located at either end of the plurality of interconnected fins, the first and second manifold spacers having respectively a first and second set of fluid openings therein, the fluid openings of the manifold spacers being in fluid communication with the fluid openings in the first, second and third plates. 
     In one example, the heat exchanger includes a plurality of stacked heat exchanger subassemblies. 
     Accordingly, there is provided a method for producing lightweight, high efficiency plate fin heat exchangers for operation with corrosive cooling fluids such as deionized water, the method comprising: 
     cutting through a thermally conductive, corrosion resistant sheet to create therein at least one opening for a heat exchanger fluid. 
     The method, as described above, further comprising: 
     finishing the cut sheet by pressing it between two rigid, flat plates. 
     In one example, the rigid, flat plates each include a non-stick coating. 
     The method, as described above, further comprising: 
     finishing the cut sheet by pressing it between two parallel rollers. 
     In one example, the parallel rollers each include a non-stick coating. 
     In another example, the cutting step is carried out using a die having at least one blade. The die has two blades. The die is a rule die, flexible die or solid engraved die. The two blades of the die are located side-by-side. 
     In another example, the cut plate includes at least one coolant flow opening. The cut plate includes a plurality of coolant flow openings. At least one coolant inlet manifold opening and at least one coolant outlet manifold opening located at the ends of the coolant flow openings and in communication therewith. 
     In another example, the cut plate is a coolant flow field plate. 
     In another example, the cut plate includes a plurality of coolant inlet manifold openings and a plurality of coolant outlet manifold openings. 
     In yet another example, the cut plate is a separator plate. 
     In one example, the thermally conductive sheet is flexible graphite. 
     In another example, the thermally conductive sheet is stainless steel film. 
     According to another aspect, there is provided a method for producing coolant flow field plates with complex cooling flow field geometries, the method comprising: 
     cutting through a thermally conductive, corrosion resistant sheet to create therein at least one opening for a cooling fluid; and 
     embossing the sheet to create therein at least one support for the at least one opening for a cooling fluid. 
     The method, as described above, further comprising: 
     finishing the cut sheet by pressing it between two rigid, flat plates. 
     In one example, the rigid, flat plates each include a non-stick coating. 
     The method, as described above, further comprising: 
     finishing the cut/embossed sheet by pressing it between two parallel rollers. 
     In one example, the parallel rollers each include a non-stick coating. 
     In another example, the cutting step is carried out using a die having at least one blade. The die has two blades. The die is a rule die, flexible die or solid engraved die. The two blades of the die are located side-by-side. 
     In another example, the embossing step is carried out using a die having at least one embossing feature. The die has two embossing features. The die is a rule die, flexible die or solid engraved die. 
     In another example, the cutting step and the embossing step are carried out simultaneously using a die having at least one blade and one embossing feature. The die has two blades and one embossing feature. The die has two blades and two embossing features. The die is a rule die, flexible die or solid engraved die. The two blades of the die are located side-by-side. 
     In another example, the cut/embossed plate includes at least one coolant flow opening. The cut/embossed plate includes a plurality of coolant flow openings. At least one coolant inlet manifold opening and at least one coolant outlet manifold opening located at the ends of the coolant flow openings and in communication therewith. 
     In another example, the cut/embossed plate is a coolant flow field plate. 
     In another example, the cut/embossed plate includes a plurality of coolant inlet manifold openings and a plurality of coolant outlet manifold openings. 
     In yet another example, the cut/embossed plate is a separator plate. 
     In one example, the thermally conductive sheet is flexible graphite. 
     In another example, the thermally conductive sheet is stainless steel film. 
     According to another aspect, there is provided a method for producing a plate fin heat exchanger comprising: 
     providing a cooling flow field plate subassembly having a cooling flow field plate and cooling flow field plate separators located on either side; and 
     locating a manifold spacer at each end of the cooling flow field plate subassembly; 
     locating a corrugated cooling fin or other cooling means between the manifold spacers; and 
     stacking the above repeating unit and compressing it between two end plates for form a plate fin heat exchanger. 
     In another example, the step of compressing the repeating unit creates a plurality of axial coolant flow channels between the coolant flow field plate subassemblies. 
     In another example, the cooling flow field plate subassembly is sealed internally with an adhesive perimeter seal. 
     In another example, the manifold spacer is sealed with an adhesive to the cooling flow field plate subassembly. 
     In another example, the cooling fin is made of rectangular or square corrugated aluminum. 
     In another example, the cooling fin is made of semicircular corrugated aluminum. 
     In another example, the cooling fin is made of triangulated corrugated aluminum. 
     In another example, the aluminum fin is anodized to provide an electrically non-conductive coating to prevent galvanic corrosion when in direct contact with the stainless steel coolant flow field plate separator. 
     In another example, the stainless steel coolant flow field separator is passivated to add corrosion protection from the deionized water. 
     In another example, the cooling fin is made of carbon foam. 
     In another example, prior to the compressing step, a silicone gasket is placed between the repeating unit manifold spacers to provide sealing at this interface. 
     In another example, a coolant flow redirection plate is added at midpoint to form a dual pass heat exchanger. 
     Accordingly, there is provided a plate fin heat exchanger subassembly subunit comprising: 
     first, second and third plates in intimate contact with each other, the first plate being sandwiched between the second and third plates, each of the plates having a first at least one fluid opening and a second at least one fluid opening located at the ends of each plate, the first plate having at least one elongate fluid channel extending between the first and second fluid openings; 
     a first fluid passageway for a first heat exchanger fluid extending through the first and second fluid openings in each plate and along the elongate channel in the first plate; and 
     at least one fin having first and second thermal transfer surfaces, the first thermal transfer surface being in intimate contact with the second plate, second fluid passageways for a second heat exchanger fluid being located on each side of the fin, the second thermal transfer surface being in intimate contact with another plate located on top of the at least one fin, the other plate having a first at least one fluid opening and a second at least one fluid opening located at the ends of the plate, thereby creating a set of second fluid passageways, which are disposed substantially parallel to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of that described herein will become more apparent from the following description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a perspective view of a plate fin heat exchanger assembly; 
         FIG. 2  is a perspective, exploded view of a plate fin heat exchanger repeating unit subassembly comprising cooling fin, manifold spacers, coolant flow field plate and coolant flow field plate separators; 
         FIG. 3  is a perspective view of a plate fin heat exchanger repeating unit subassembly in an assembled state; 
         FIG. 4  is a perspective view of a plate fin heat exchanger midpoint subassembly to enable a two-pass coolant flow configuration; 
         FIG. 5  is a top view of a coolant flow field plate with straight channels; 
         FIG. 6  is a top view of a coolant flow field plate separator; 
         FIG. 7  is a front view of a rectangular cooling fin configuration; 
         FIG. 8  is a front view of a triangular cooling fin configuration; 
         FIG. 9  is a front view of a carbon foam cooling fin configuration; 
         FIG. 10  is a perspective view of another example of plate fin heat exchanger repeating unit subassembly where the cooling fins are positioned at a 45 degree angle; 
         FIG. 11  is a perspective top view a plate fin heat exchanger repeating unit subassembly where the cooling fins are positioned at a 45 degree angle; 
         FIG. 12  is a perspective top view of a coolant flow field plate where the cooling fins are positioned at a 45 degree angle; 
         FIG. 13  is a perspective top view of a coolant flow field separator where the cooling fins are positioned at a 45 degree angle; 
         FIG. 14  is a perspective view of another example of a coolant flow field plate where a serpentine flow field pattern is employed, showing embossed tabs to maintain flow channel spacing; 
         FIG. 15  is a top view of the same example of coolant flow field plate where a serpentine flow field pattern is employed, showing embossed tabs to maintain flow channel spacing; and 
         FIG. 16  is a diagrammatic representation of the heat exchanger showing (arrows) heat exchanger fluid flow. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     Unless otherwise specified, the following definitions apply: 
     The singular forms “a”, “an” and “the” include corresponding plural references unless the context clearly dictates otherwise. 
     As used herein, the term “comprising” is intended to mean that the list of elements following the word “comprising” are required or mandatory but that other elements are optional and may or may not be present. 
     As used herein, the term “consisting of” is intended to mean including and limited to whatever follows the phrase “consisting of”. Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory and that no other elements may be present. 
     As used herein, the term “coolant flow field plate” or “fluid flow field plate” is intended to mean a plate that is made from a suitable thermally conductive material. The material is typically substantially fluid impermeable, that is, it is impermeable to the fluids typically found in heat exchanger applications. The coolant flow field plates can be made of the following materials: graphitic carbon impregnated with a resin or subject to pyrolytic impregnation; flexible graphite; metallic material such as stainless steel, aluminum, nickel alloy, or titanium alloy; carbon-carbon composites; carbon-polymer composites; or the like. Flexible graphite, also known as expanded graphite, is one example of a suitable material that is compressible and, for the purposes of this discovery, easily cut through and embossed. 
     As used herein, the term “fluid” is intended to mean liquid or gas. In particular, the term fluid refers to the liquids or gases typically used as coolants in heat exchanger applications. In this context, the inventors have described the coolant as first and second heat exchanger fluids, where the first heat exchanger fluid is de-ionized water and the second heat exchanger fluid is air. In the examples described below, the first heat exchanger fluid and the second heat exchanger fluid flow orthogonal to each other. 
     Referring now to  FIG. 1  and  FIG. 16 , in which a plate fin heat exchanger assembly is shown generally at  10 . The plate fin heat exchanger assembly  10  comprises a lower compression end plate  12 , which incorporates an inlet  14  for the first heat exchanger fluid, and an upper compression end plate  16  which incorporates an outlet  18  for the first heat exchanger fluid. Compressed between the lower compression end plate  12  and upper compression end plate  16  is a plurality of plate fin heat exchanger repeating unit subassemblies  20 , and the midpoint plate fin heat exchanger subassembly  22 , which facilitates a two-pass coolant flow configuration. A method of compressing the upper and lower endplates,  12  and  16 , respectively, includes several thin, welded stainless steel straps or integrated tie rods (not shown). Warm liquid coolant enters at coolant fluid inlet  14 , travels longitudinally across the heat exchanger twice (two-pass configuration), and then exits cooler at coolant fluid outlet  18 . The second heat exchanger fluid, cool incoming cooling air,  24  enters the plate fin heat exchanger assembly  10  over the entire frontal area and warm exhaust cooling air  26  exits over the entire rearward area. Heat is therefore transferred from the first heat exchanger fluid liquid coolant entering at  14  and exiting at  18 , to incoming and exhaust cooling air, the second heat exchanger fluid at  24  and  26 , respectively. Still referring to  FIG. 1 , the cooling air (the second heat exchanger fluid) flows along a second and third fluid passageway created between sandwiched fins on either side of each fin. Referring to  FIG. 16 , the first fluid passageways (shown by the arrows) are shown schematically as moving vertically on either side of the stacked heat exchanger  10  and horizontally. The vertical movement illustrates movement of the first heat exchanger fluid through the manifold openings and the plate opening described below, whereas the horizontal movement illustrates flow of the first heat exchanger fluid through the elongate fluid channels in the first plate. The second heat exchanger fluid flows orthogonally to the first heat exchanger fluid. 
     Referring now to  FIG. 2 , in which an exploded view of a plate fin heat exchanger repeating unit subassembly is shown at  20 , comprising lower coolant flow field plate separator  30  (a third plate), coolant flow field plate  40  (a first plate), upper coolant flow field plate separator  50  (a second plate), manifold spacer  60  and cooling fin  70 . Coolant flow field plate  40  is located between lower coolant flow field plate separator  30  and upper coolant flow field plate separator  50 , where intimate contact is made between the bottom face of coolant flow field plate  40  and top face of lower coolant flow field plate separator  30  to maintain good thermal conductivity at the interface. An adhesive perimeter seal (not shown) is applied to ensure no external leakage occurs. Similarly, intimate contact is made between the top face of coolant flow field plate  40  and bottom face of upper coolant flow field plate separator  50  to ensure good thermal conductivity at this interface. An adhesive perimeter seal (not shown) is also incorporated. Manifold spacer  60  is located above upper coolant flow field plate separator  50  where it is adhesively bonded at each end of the top face. Cooling fin  70  is placed in intimate contact with the top face and center section of upper coolant flow field plate separator  50  and is held in place by compression forces. A first plurality of liquid coolant (first heat exchanger fluid) manifold openings  32  and  34  are shown at each end of lower coolant flow field plate separator  30 . A second plurality of liquid coolant manifold openings  42  and  46  are shown at each end of coolant flow field plate  40 , with an elongate coolant flow field channel opening  44  joining each individual set. A third plurality of liquid coolant manifold openings  52  and  54  are shown at each end of upper coolant flow field plate separator  50 . A fourth plurality of liquid coolant manifold openings  62  and  64  are shown at manifold spacer  60 . When plate fin heat exchanger repeating unit subassembly  20  is assembled, manifold openings  32 ,  42 ,  52  and  62  align to make a continuous passage. Similarly, manifold openings  34 ,  46 ,  54 , and  64  also align to make a continuous passage. In general, heat transfer within the plate fin heat exchanger unit subassembly  20  is facilitated as follows. Warm liquid coolant enters through liquid coolant manifold openings  32  and passes to coolant manifold opening  42 . At this point, a proportion of liquid coolant travels down coolant flow field channel opening  44 , and a proportion continues on to coolant manifold opening  52 , through coolant manifold opening  62 , and then on to the adjacent plate fin heat exchanger repeating unit subassembly  20  located above. Liquid coolant passing down coolant flow field channel opening  44  passes to coolant manifold opening  46  and is mixed with liquid coolant entering at coolant manifold opening  34  entering from the adjacent plate fin heat exchanger repeating unit subassembly  20  located below. The exiting liquid coolant then passes through coolant manifold openings  54  and  64 , and then on to the adjacent plate fin heat exchanger repeating unit subassembly  20  located above. Heat from the liquid coolant travelling down coolant flow field channel opening  44  is transferred to upper coolant flow field plate separator  50  which is in intimate contact above. The heat is then transferred via conduction into cooling fin  70  which is in intimate contact with upper coolant flow field plate separator  50 . Cool incoming cooling air (the second heat exchanger fluid)  24  enters at the frontal face of cooling fin  70 , where heat is transferred to the cooling air via convection. Warm exhaust cooling air  26  exits at the rearward face of cooling fin  70 . In a similar manner, heat is also transferred to lower coolant flow field plate separator  30  located below which is also in intimate contact with coolant flow field plate  40 , and then into the cooling fin  70  of the adjacent plate fin heat exchanger unit subassembly  20 . 
     Referring now to  FIG. 3 , in which a view of a fully assembled plate fin heat exchanger repeating unit subassembly  20  is shown, illustrating how the identified parts are in intimate contact with each other. 
     Referring now to  FIG. 4 , in which a view of the midpoint plate fin heat exchanger subassembly  22  is shown. Effectively, this subassembly is identical to plate fin heat exchanger repeating unit subassembly  20 , except that manifold spacer  60  with coolant manifold openings  62 , is replaced with manifold spacer  66  which does not have any coolant manifold openings. Manifold spacer  66  facilitates a two-pass heat exchanger configuration allowing the liquid coolant (first heat exchanger fluid) to pass longitudinally through the bottom half of plate fin heat exchanger assembly  10 , continue upwards through coolant manifold openings  64 , and then pass longitudinally through the upper half of plate fin heat exchanger assembly  10 . 
     Referring now to  FIG. 5 , where a general perspective top view of an individual coolant flow field plate  40  is shown. The plate includes at least one elongate coolant flow field channel opening  44 . In the example shown, a plurality of elongate coolant flow field channel openings  44  are cut through the plate  40  and extend parallel to each other along the central portion of the plate  40 . Each elongate coolant flow field channel opening  44  includes coolant manifold openings  42  and  46 , which are located at each end of the elongate coolant flow field channel opening  44 . The coolant flow field plate  40  also includes a peripheral area  49 , which forms a boundary around the elongate coolant flow field openings  44 . An adhesive perimeter seal (not shown) is applied around peripheral area  49  to eliminate external liquid coolant leakage. 
     Referring now to  FIG. 6 , where a general perspective top view of an upper coolant flow field plate separator  50  is shown, indicating coolant manifold openings  52  and  54 . Since lower coolant flow field plate separator  30  is effectively identical to upper coolant flow field plate separator  50 , this FIG. is also an accurate representation of this component. 
     Referring now to  FIG. 7 , where a frontal, section view of cooling fin  70  is shown indicating a rectangular cooling fin pattern. Horizontal flat sections  72  maintain intimate contact with adjacent plate fin heat exchanger repeating unit subassembly  20 , namely the bottom face of lower coolant flow field plate separator  30 . Horizontal flat sections  74  maintain intimate contact with the top face of upper coolant flow field plate separator  50 , within the same plate fin heat exchanger repeating unit subassembly  20 . Horizontal flat sections  72  and  74  are spaced accordingly to mechanically support adjacent coolant flow field separators  30  and  50 , while providing the optimum fin spacing to reject the maximum amount of heat while minimizing the pressure drop of cooling air passing through airflow channels  78 . Vertical cooling fin sections  76  also provide mechanical support allowing overall heat exchanger axial compression and are again sized accordingly to provide the optimum cooling fin surface area and reduced pressure drop of cooling air (second heat exchanger fluid) passing through airflow channels  78 . The fins described herein each have a first and a second thermal transfer surface  79 ,  81  which lie in intimate contact with the separator plates described herein. 
     Referring now to  FIG. 8 , where a frontal, section view of an alternative cooling fin  70  is shown at  80 , indicating a triangular cooling fin pattern, which is a folded cooling fin similar to the previous rectangular version, but compressed longitudinally to form a triangle. This configuration provides more mechanical support for adjacent coolant flow field separators  30  and  50 . 
     Referring now to  FIG. 9 , where a frontal, section view of another alternative cooling fin  70  is shown at  90 , indicating a cooling fin configuration which incorporates carbon foam of a specific porosity. This configuration also provides more mechanical support for adjacent coolant flow field separators  30  and  50 , but possibly at the expense of higher cooling airflow restriction. 
     Referring now to  FIG. 10 , in which a view of a fully assembled plate fin heat exchanger repeating unit subassembly  200  is shown, illustrating an alternative embodiment of a plate fin heat exchanger where the cooling fins are positioned at a 45 degree angle, and comprising lower coolant flow field plate separator  230 , coolant flow field plate  240 , upper coolant flow field plate separator  250 , manifold spacer  260  and cooling fin  270 . Effectively, the construction of subassembly  200  is very similar in nature to plate fin heat exchanger repeating unit subassembly  20 , previously defined. 
     Referring now to  FIG. 11 , in which a perspective top view of a fully assembled plate fin heat exchanger repeating unit subassembly  200  is shown, again illustrating an alternative embodiment of a plate fin heat exchanger where the cooling fins are positioned at a 45 degree angle. 
     Referring now to  FIG. 12 , where a general perspective top view of an individual coolant flow field plate  240  is shown. The plate includes at least one elongate coolant flow field channel opening  244 . In the example shown, a plurality of elongate coolant flow field channel openings  244  are cut through the plate  240  and extend parallel to each other along the central portion of the plate  240 . Each elongate coolant flow field channel opening  244  includes coolant manifold openings  242  and  246 , which are located at each end of the elongate coolant flow field channel opening  244 . The coolant flow field plate  240  also includes a peripheral area  249 , which forms a boundary around the elongate coolant flow field openings  244 . An adhesive perimeter seal (not shown) is applied around peripheral area  249  to eliminate external liquid coolant leakage. 
     Referring now to  FIG. 13 , where a general perspective top view of an upper coolant flow field plate separator  250  is shown, indicating coolant manifold openings  252  and  254 . Since lower coolant flow field plate separator  230  is effectively identical to upper coolant flow field plate separator  250 , this FIG. is also an accurate representation of this component. 
     Referring now to  FIG. 14 , where an isometric view of another example of a coolant flow field plate is shown at  300  illustrating a serpentine flow field pattern and showing embossed tabs to maintain flow channel spacing. The plate includes at least one serpentine coolant flow field channel opening  302 . In the example shown, a plurality of serpentine coolant flow field channel openings  302  are cut through the plate  300  and extend in a serpentine manner throughout the central portion of plate  300 . The serpentine coolant flow field channel openings  302  include coolant manifold openings  308  and  310 , which are located at the ends of the serpentine coolant flow field channel openings  302 . The coolant flow field plate  300  also includes landings  304  which are located between the coolant flow field channel openings  302 . At certain locations down the length of the coolant flow field channels  302 , one or more coolant flow field channel supports  306  are placed to maintain the correct spacing of coolant flow field channel  302  due to the complex, unsupported nature of the serpentine flow field design. A peripheral area  312  forms a boundary around the serpentine coolant flow field openings  302  where an adhesive perimeter seal (not shown) is applied to eliminate external liquid coolant leakage. 
     Referring now to  FIG. 15  where a perspective top view of the same example of coolant flow field plate  300  is shown, indicating a serpentine flow field pattern and showing embossed tabs  306  to maintain the correct spacing of coolant flow field openings  302 . 
     The plate fin heat exchangers described herein are particularly well suited for use in liquid cooled fuel cell systems for unmanned aerial vehicle (UAV) applications, which require very lightweight fuel cell systems with high energy density. Other uses for the lightweight plate fin heat exchangers include liquid cooled fuel cell systems for auxiliary power units (APUs) and small mobile applications such as scooters. Indeed, the plate fin heat exchangers may be useful in many other liquid cooled fuel cell applications such as automotive, stationary and portable power. 
     Other applications that require the use of deionized water as a coolant are found in industries such as lasers, medical equipment, laboratory instrumentation, pharmaceutical production, cosmetics, food processing, semiconductor manufacturing, plating and chemical processing to name a few, and in some cases the plate fin heat exchangers described herein may be particularly well suited where stainless steel or titanium heat exchangers are not practical. 
     Manufacturing Process—Prototype Level 
     Flexible graphite is used to produce the lower coolant flow field plate separator  30 , coolant flow field plate  40  and upper coolant flow field plate separator  50 . Alternatively, the lower coolant flow field plate separator  30  and upper coolant flow field plate separator  50  may be produced from thin, stainless steel film. 
     Flexible dies used in the cutting and embossing process, available from many die manufacturers, are typically used for label cutting and embossing applications and generally can fabricate hundreds of thousands of plates. The flexible die design is dependent on feature geometry and material thickness. 
     Typically, for the lower coolant flow field plate separator  30 , a 0.015″ thick flexible graphite sheet or a 0.001″ stainless steel film is used. 
     Typically, for the coolant flow field plate  40 , a 0.040″ thick flexible graphite sheet is used. 
     Typically, for the upper coolant flow field plate separator  50 , a 0.015″ thick flexible graphite sheet or a 0.001″ stainless steel film is used. 
     Typically, for the manifold spacer  60 , most lightweight plastics or elastomers are suitable of varying thickness, depending on the corresponding cooling fin height. 
     Typically, for the cooling fin  70 , folded aluminum is used varying from 0.004″ to 0.006″ in thickness. 
     Cutting 
     The lower coolant flow field plate separator  30 , coolant flow field plate  40  and upper coolant flow field plate separator  50  are individually cut through using their respective flat, flexible dies using a manual, reciprocal hydraulic press. 
     The press cutting force varies from 10,000 lbs. to 60,000 lbs., which is monitored with a pressure gauge, and which depends on the number and spacing of die features. Thus, a tightly packed die with many features requires a greater cutting force. 
     Once cut through, the plates  30 ,  40  and  50  are removed from the die with suboptimal feature definition, part deformation and jagged edges where the die cutter penetrated the flexible graphite or thin, stainless steel material. The scrap material that is removed during the cutting can be recycled. The dies are designed and selected in such that they cut the specific flow field openings and manifold openings in one cutting step. 
     Cutting and Embossing 
     Similarly, the serpentine coolant flow field plate  300  is individually cut through and embossed using a flat, flexible die and a manual, reciprocal hydraulic press. 
     In this case, the cutting force varies from 10,000 lbs. to 200,000 lbs., which is monitored with a pressure gauge and which depends on the number and spacing of die and embossing features. Thus, a tightly packed die with many embossing features requires a greater cutting force. 
     Once cut through and embossed, the serpentine coolant flow field plate  300  is removed from the die with suboptimal feature definition, part deformation and jagged edges where the die cutter penetrated the flexible graphite material. The scrap material that is removed during the cutting can be recycled. The dies are designed and selected in such that they cut the specific flow field openings and manifold openings in the plates, as well as emboss the coolant flow field channel supports, as illustrated in  FIGS. 14 and 15 , in one manufacturing step. 
     Finishing 
     After cutting through, or cutting through and embossing, each plate is then pressed between two flat, rigid, parallel plates in the same manual hydraulic press to improve feature tolerance, eliminate undesired deformation caused by the die, and to “flatten” rough, jagged edges left by the cutting process. 
     A thin layer of Teflon is the applied to the pressing fixture on either side of the plates to improve surface finish and to eliminate “sticking”. The cut through plates  30 ,  40  and  50  are then ready for plate fin heat exchanger assembly. In a similar manner, the cut through and embossed plate  300  is also ready for plate fin heat exchanger assembly. 
     Manufacturing Process—Production Level 
     For higher volume manufacturing, rotary die cutting is used for increased throughput. Rotary flexible dies are available from many die manufacturers. Cylindrical flexible dies are mounted on a magnetic cylinder and mate with a cylindrical anvil, where each die can use the same magnetic cylinder to reduce cost. Rotary die cutting equipment for the label making industry is used. Automated reciprocal die cutting systems using flat dies with automatic material feed and part removal is also appropriate. 
     Flexible graphite material (available in rolls) is automatically fed into the equipment. Typically, 3000 plates per hour are potentially possible using this manufacturing method. 
     Cutting 
     The lower coolant flow field plate separator  30 , coolant flow field plate  40  and upper coolant flow field plate separator  50  are individually cut through using their respective rotary, flexible dies and rotary die cutting equipment. The distance between the rotary die and anvil is adjusted to achieve optimal part cutting. An automated scrap removal system removes residual flexible graphite for recycling. 
     Cutting and Embossing 
     Similarly, the serpentine coolant flow field plate  300  is individually cut through and embossed using a rotary, flexible die and rotary die cutting equipment. The distance between the rotary die and anvil is adjusted to achieve optimal part cutting and embossing. An automated scrap removal system removes residual flexible graphite for recycling. 
     A plate handling system, which is typically a conveyor, groups and transports the cut through plates to the “finishing” area. 
     Finishing 
     Each cut through, or cut through and embossed plate is automatically fed into a rotary flattening system which comprises of two parallel rollers with Teflon coating and adjustable spacing. The finished plates are automatically removed from the rollers via conveyor and transported to their respective part bins. The plates are then ready for plate fin heat exchanger assembly. 
     Alternatives 
     For the case where the lower coolant flow field plate separator  30  and upper coolant flow field plate separator  50  are constructed with thin stainless steel film, the mating aluminum cooling fin  70  would be anodized or another suitable coating applied to prevent galvanic corrosion between the two dissimilar metals by electrically isolating the two parts, while not creating a significant thermal barrier at the interface. Further, the stainless steel parts would also be passivated to provide a protective layer against the corrosive nature of deionized water. 
     A unitary body of the lower coolant flow field plate separator  30 , coolant flow field plate  40  and upper coolant flow field plate separator  50  is also contemplated where the components are fused together with a method other than adhesive, such as mechanical bonding under force. 
     The “finishing” stage of the part fabrication could be used to increase the density of the flexible graphite and therefore improve mechanical and thermal properties (i.e. a 0.050″ thick cut part could be pressed down to 0.040″). 
     The plates can be fabricated with a high volume manufacturing process (reciprocal or rotary die-cutting commonly used in label making) therefore reducing overall part cost. 
     Parts can be fabricated using very low cost tooling (flat or cylindrical flexible dies). Moreover, flexible graphite or stainless steel raw material is inexpensive and is available in various forms and thicknesses. 
     Flexible graphite has a typical density of 1.12 g/cc resulting in very lightweight components. Further, plate fin heat exchanger plates fabricated via die-cutting have reduced mass because material is removed for flow channels and manifolds. 
     Coolant flow field channel depth may be changed easily by changing the thickness of flexible graphite sheet and using same die. For example, if more cooling is required for a specific application, a thicker cooling flow field plate can be substituted allowing higher cooling flows and heat removal. 
     The number of plate fin heat exchanger unit subassemblies  20  is easily adjusted to increase or reduce the heat exchanger&#39;s capacity, depending on cooling requirements. 
     Several options are available to mechanically compress lower compression end plate  12  and upper compression end plate  16  to maintain good sealing and thermal conductively between components, including thin, low profile stainless steel straps, axial tie rods, etc. The compression system would also incorporate a means to allow expansion and contraction of the plate fin heat exchanger while maintaining a constant compression force, such as Belleville washers or the like. 
     Due to the nature of die cutting and the flexibility to cut virtually any shape, the construction of the plate fin heat exchanger can basically take on any form, which is an advantage for heat exchanger packaging when trying to optimize heat rejection with limited volume available, or in the case where the packaging space is not the typical shape of a conventional heat exchanger design. 
     In the case where the plate fin heat exchanger has similar cooling flow characteristics to that of a fuel cell stack, the tooling required to die cut parts such as the lower coolant flow field plate separator  30 , coolant flow field plate  40  and upper coolant flow field plate separator  50 , can also be used to fabricate fluid flow field plates for the fuel cell stack, thereby reducing overall tooling costs for a liquid cooled fuel cell system. 
     To further improve sealing at the perimeter interfaces of the lower coolant flow field plate separator  30 , coolant flow field plate  40  and upper coolant flow field plate separator  50 , a raised perimeter sealing feature can be pressed into the coolant flow field separators  30  and  50  if fabricated with thin stainless steel film, adding stiffness to this part and providing a local sealing point with the flexible graphite at this interface, thereby compressing and deforming the flexible graphite and improving sealing characteristics. 
     Other Embodiments 
     From the foregoing description, it will be apparent to one of ordinary skill in the art that variations and modifications may be made to the embodiments described herein to adapt it to various usages and conditions.