Patent Publication Number: US-8123109-B2

Title: Method for making brazed heat exchanger and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of copending application Ser. No. 11/824,263 filed Jun. 29, 2007, which is a Division of copending application Ser. No. 10/449,173 filed May 30, 2003, the contents of which are hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an improved method for making a metal heat exchanger with high heat transfer efficiency. Specifically, this disclosure relates to an improved method for making a brazed heat exchanger containing enhanced boiling surfaces. 
     BACKGROUND 
     Two designs of heat exchanger are presently in general use for reboiler-condensers in cryogenic, refinery and chemical applications. One type of heat exchanger in current use is a vertical shell and tube heat exchanger. To achieve a sufficiently high degree of heat transfer at relatively low temperature differences with this design, enhanced boiling layers (EBL) are used. An EBL typically has a structure comprising a multitude of pores that provide boiling nucleation sites to facilitate boiling. An EBL is applied to the inside of the tubes, and longitudinal flutes are provided on the outside of the tubes to facilitate heat transfer. 
     Enhanced boiling layers were first proposed for heat exchangers in U.S. Pat. No. 3,384,154. This patent discloses mixing metal powder in a plastic binder in solvent and applying the slurry to a base metal surface. The coated metal is subjected to a reducing atmosphere and heated to a temperature for sufficient time so that the metal particles sinter together and to the base metal surface. U.S. Pat. No. 3,457,990 discloses an enhanced boiling surface with reentrant grooves mechanically or chemically formed therein. 
     Other methods of applying EBLs have been disclosed. GB 2 034 355 discloses applying an organic foam layer to a metal heat transfer member and plating the foam with metal such as copper first by electroless, then by electrodeposition. U.S. Pat. No. 4,258,783 discloses mechanically forming indentations in a heat transfer surface and then electrodepositing metal on the pitted surface. GB 2 062 207 discloses applying metal particles to a metal base by powder flame spraying. EP 303 493 discloses spraying a mixture of metal and plastic material onto a base metal by flame or plasma spraying. U.S. Pat. Nos. 4,767,497 and 4,846,267 disclose heat treating an aluminum alloy plate to produce a precipitate followed by chemically etching away the precipitate to leave a pitted surface. EP 112 782 discloses applying a mixture of brazing alloy and spherical particles to a metallic wall and heating the coated wall to melt the brazing material. 
     A common heat exchanger used in cryogenic, refinery and chemical applications is the plate-fin brazed aluminum heat exchanger fabricated by disposing corrugated aluminum sheets between aluminum parting sheets or walls to form a plurality of fluid passages. The sheets are either clad with an aluminum brazing layer or a layer of brazing foil is inserted between the surfaces to be bonded. When heated to a predetermined temperature for a predetermined period of time, the brazing foil or cladding melts and forms a metallurgical bond with the adjacent sheets. The resulting heat exchanger contains numerous passages consisting of alternate layers of closely spaced fins. A typical arrangement of alternate layers of passages each containing fins with a density of 6 to 10 fins/cm (15 to 25 fins/inch), and a fin height of 0.5 to 1 cm (0.2 to 0.4 inch). In a common application, a first series of alternating passages carry vapor for condensing, while a second series of alternating passages carry a liquid for boiling. Typical brazed aluminum heat exchangers must be able to withstand 2068 to 2758 kPa (300 to 400 psia). 
     Patents proposing replacing fins with an enhanced boiling layer in the boiling passages of a brazed heat exchanger include U.S. Pat. Nos. 5,868,199; 4,715,431 and 4,715,433. These patents propose to stack aluminum sheets each with an EBL applied on one side to define boiling channels and with fins on the other side of the aluminum sheets to define condensing channels. Layers of brazing material are disposed between bonding surfaces in the stack, and the stack is subjected to heating over a period of time to obtain a brazed heat exchange core. Such brazed aluminum heat exchangers described in these patents have not been commercialized because EBLs are typically brazed at 565° to 593° C. (1050° to 1100° F.) while the subsequent brazing of the metal components together occur at around 593° to 621° C. (1100° to 1150° F.). Maintaining the integrity and effectiveness of the EBL, particularly the porous structure provided by the mutually bonded metal particles, during the second hotter heat treatment to effect brazing has been difficult. This difficulty accounts for the lack of commercially available brazed heat exchangers with EBL in the boiling passages. 
     SUMMARY 
     We provide an improved method for making a brazed metal heat exchanger and the resulting apparatus. An enhanced boiling layer (EBL) is provided on the walls of the boiling passages. The melting temperature of the brazing material is lower than the melting temperature of the metal particles in the enhanced boiling layer. In an embodiment, the metal in the enhanced boiling layer and/or the brazing layer is an alloy of a first metal and a second metal which alloy has a lower melting temperature than that of the first metal. Different second metals can be used in the EBL and in the brazing material so long as the second metal provides an alloy with a lower melting temperature. In an embodiment, the concentration of the second metal in the brazing material is greater than in the EBL. Hence, we have found that even when the brazing temperature gets within about 8.3 Celsius degrees (15 Fahrenheit degrees) of the melting point of the metal in the EBL for an extended period of time, the EBL unexpectedly retains its porosity, and thus its effectiveness. In an embodiment, the condensing passages contain fins to facilitate heat transfer. 
     We also provide a metal heat exchanger with EBLs in the boiling passages with undiminished heat transfer capability despite being subjected to brazing temperature during manufacturing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of three heat exchangers. 
         FIG. 2  is a perspective view of the core of a heat exchanger in  FIG. 1  with layers broken away to reveal internals. 
         FIG. 3  is a perspective view of the core of the heat exchanger in  FIG. 1  but taken from a different perspective than  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Our methods can be used to construct any configuration of heat exchanger by brazing including shell and tube but may be most appropriately applied to plate exchangers. The boiling and cooling passages of the heat exchangers may be oriented to provide cross flow, counter-current flow or cocurrent flow. Moreover, the heat exchanger may be applied in the context of cryogenic air separation, hydrocarbon processing or any other process that relies on boiling to effect heat exchange. Several types of metals can be used for construction of heat exchangers. Aluminum is the most widely used metal for brazed heat exchangers. Aluminum is suitable for cryogenic applications because it resists embrittlement at lower temperatures. Steel or copper may be used for heating or cooling fluids that may be corrosive to aluminum. For purposes of illustration, our structures will be described with respect to a counter current, aluminum, plate heat exchanger useful in the context of cryogenic air separation. 
       FIG. 1  shows a train of typical plate heat exchangers  10  used in cryogenic air separation. The heat exchangers  10  have alternating boiling passages  12  and cooling passages  14  provided in a core  20 . A liquid such as liquid oxygen is delivered by conduits  16  to manifolds  18  and distributed to the boiling passages  12 . Delivery of liquid to the boiling passages  12  by means other than the conduits  16  or the manifolds  18  underneath the core  20  is contemplated such as by thermosiphoning at the bottom of the boiling passages  12 . Moreover, liquid may be delivered to the boiling passages  12  from the side or from the top of the core  20 , perhaps through a distribution network that may comprise distributor fins. The liquid boils in the boiling passages  12 , thereby indirectly withdrawing heat conducted from the cooling passages  14 . Gaseous oxygen from the boiling passages  12  are collected such as by headers  22  and removed through a conduit  24 . Collection of gases from the boiling passages  12  by means other than the conduits  24  or the headers  22  above the core  20  is contemplated such as may be provided in a thermosiphoning arrangement. Moreover, gases may be collected from the boiling passages  12  from the side or from the top of the core  20 , perhaps through a collection network that may comprise collection fins. A fluid such as gaseous nitrogen is delivered by conduits  26  to manifolds  28  and distributed to the cooling passages  14 . Delivery by means other than by the conduits  26  or the manifolds  28  is also contemplated. A liquid or gas can be cooled in the cooling passages  14 . Moreover, if a gas is delivered to the cooling passages  14 , it may be cooled to such extent to effect a phase change with or without temperature change depending on the needs of the process. Heat conducted across the walls between the cooling passages  14  and the boiling passages  12  to support the boiling in the boiling passages  12  cools the fluid in the cooling passages  14 , thereby condensing the nitrogen gas in the case of air separation. Fluid such as liquefied nitrogen from the cooling passages  14  is collected such as by headers  30  and removed through conduits  32 . Collection of cooled fluid from the cooling passages  14  by means other than the headers  30  and the conduits  32  is contemplated. Moreover, the delivery and collection manifolds and conduits shown in the embodiment in  FIG. 1  may be modified and remain within the scope of our disclosure. 
       FIG. 2  shows the core  20  of one of the heat exchangers  10  with parts broken away to reveal internals. A cap sheet  40  is disposed on both ends of the core  20  to define the last channel on each end. Part of the cap sheet  40  illustrated in  FIG. 2  is broken away to reveal the boiling passage  12 . Vertical spacer bars or spacer members  42  are disposed between opposing edges of the cap sheet  40  and a metal wall  44  with a boiling side  44   a  covered with an enhanced boiling layer (EBL)  46 . The EBL  46  comprises thermoconductive particles bonded to the boiling side  44   a  and to each other to form a texture of pores in which nucleate boiling sites are provided. The thermoconductive particles are metal particles in an embodiment. Hence, the boiling passage  12  is defined by an inner surface of the cap sheet  40 , inner edges of the vertical spacer bars  42  and the boiling side of the metal wall  44 . Outer vertical margins  48  of the boiling side  44   a  are devoid of the EBL  46  to provide a bonding surface. Vapor leaves the boiling passages  12  through boiling outlets  49 , which may be collected by the boiling headers  22 , shown in the embodiment of  FIG. 1 . Moreover, it is contemplated that the boiling passages  12  may contain fins to further facilitate heat transfer. Behind the broken away metal wall  44  and the vertical spacer bars  42  is the cooling passage  14  including primary fins  52  comprising a corrugated sheet of a primary fin stock  54 . The primary fins  52  extend laterally between inner edges of the vertical spacer bars  42  at opposite ends of the cooling passage  14 . Distributor fins  56  comprising a distributor fin stock  58  or being integral with the primary fin stock  54  are disposed in an inclined configuration to evenly distribute cooling fluid from cooling inlets  50  along the tops of the channels provided by the primary fins  52 . In the embodiment of  FIG. 2 , cooling fluid is received into cooling inlets  50  which may come from the cooling manifold  28  as shown in the embodiment of  FIG. 1 . Another type of distribution configuration with or without fins may be used to distribute cooling fluid. In another embodiment, the cooling inlets  50  may be considered the tops of the channels provided by the primary fins  52 . For purposes of illustrating the tops of the primary fins  52 , only one set of the distributor fins  56  is shown in  FIG. 2 . Cooling outlets  64  which may be defined by collection fins  66  allow cooled fluid to exit the core  20 . In the embodiment of  FIG. 2 , cooling fluid exits through cooling outlets  64  which may enter into the cooling header  30  in the embodiment of  FIG. 1 . Horizontal spacer bars  60  seal the top and the bottom of the cooling passages  14 . The spacer bars  42 ,  60  and the fins  52 ,  56 ,  66  space a cooling side  44   b  (the opposite side) of the metal wall  44  from the cooling side  44   b  of the adjacent metal wall  44 . In an embodiment, no horizontal spacer bars  60  are provided in the boiling passages  12  to permit entry and exit of fluid to and from the boiling passages  12 , respectively. Hence, the vertical spacer bars  42  are sandwiched between opposite ends of each pair of the adjacent metal walls  44 , while the horizontal spacer bars  60  are sandwiched only between the adjacent cooling sides  44   b . However, if the fins  52 ,  56 ,  66  are arranged and bonded appropriately to withstand operating pressure, it is contemplated that spacer bars  42 ,  60  can be omitted between the cooling sides  44   b  in the cooling passage  14 . Hence, the fins  52 ,  56 ,  66  would provide the spacing function. The walls  44  have an alternating orientation. Except when adjacent to the cap sheet  40 , the cooling side  44   b  of the metal wall  44  is always facing the cooling side  44   b  of an adjacent wall, and the boiling side  44   a  of a wall is always facing the boiling side  44   a  of the adjacent metal wall  44 . It is also contemplated in embodiments that the cooling passages  14  include no fins and that the boiling passages  12  be equipped with fins. 
       FIG. 3  shows the core  20  of  FIG. 2  but from a perspective that shows the bottom of the core  20 . All elements in  FIG. 2  that are visible in  FIG. 3  are referenced with numerals. Additionally, boiling inlets  51  to the boiling passages  12  are shown. In an embodiment, the boiling inlets  51  may receive boiling liquid from boiling manifolds  18  ( FIG. 1 ). Moreover, the bottom of the cap sheet  40  and the first metal wall  44  are broken away to reveal collection fins  66  from a third fin stock  68 . The collection fins  66  comprising the third fin stock  68  or being integral with the primary fin stock  54  are disposed in an inclined configuration to evenly collect cooling fluid from cooling outlets  64  along the bottoms of the channels provided by the primary fins  52 . Another type of collection configuration with or without fins may be used to collect cooling fluid. In another embodiment, the cooling outlets  64  may be considered the bottoms of the channels provided by the primary fins  52 . For purposes of illustrating the bottoms of the primary fins  52 , only one set of the collection fins  66  is shown in  FIG. 3 . 
     The EBL is added to the boiling side by any of the methods known in the art, such as by applying a slurry, flame spraying, plasma spraying or by electrodeposition. However, it is critical that the subsequent brazing step not diminish the heat exchange efficiency of the EBL once applied. In an embodiment, the melting point of the EBL is higher than the melting point of the brazing metal. The relative melting points of the brazing metal and EBL may be obtained by alloying a second metal with a first metal that has the effect of providing a melting point of the alloy that is lower than the melting point of the first metal. The concentration of the second metal may be higher in the brazing metal than in the EBL material, so that the EBL has a higher melting point that can withstand the brazing step without loss of structural integrity. In brazed aluminum heat exchangers, aluminum is the first metal and silicon, manganese, magnesium or alloys thereof may be the second metal. In brazed steel heat exchangers, nickel may be the first metal and phosphorous may be the second metal. In brazed copper heat exchangers, copper may be the first metal and phosphorous may be the second metal. 
     In the case of copper being the first metal used to provide the EBL and the brazing material, brazing occurs at about 100° C. (180° F.) below the melting temperature of copper or at about 960° C. (1760° F.). In the case of aluminum being the first metal, brazing occurs at about 49° to 54° C. (120° to 130° F.) below its melting temperature of about 649° C. (1200° F.). If nickel is the first metal, the brazing step in the furnace will take place at a temperature of about 1037° C. (1900° F.) which is 38° C. (100° F.) below the melting temperature of steel. At these temperatures, the second metal lowers the melting point of the alloy with the first metal. The liquefied brazing metal flows and diffuses into the base metal and forms a metallurgical bond. By alloying more of the second metal with the first metal in the braze material than in the EBL material, the EBL once applied will be able to withstand the subsequent lower temperature brazing heat treatment. 
     It is also contemplated that sintering may be used to form the EBL instead of brazing. In sintering, the metal is heated to the point of molecular agitation and diffuses over a relatively long period of time into an adjacent metal to form metallurgical bonds. Sintering may be used to provide the EBL with brazing at a lower temperature to bond the components of the heat exchanger together. 
     In an embodiment, the first step of applying the EBL is applying a polymer binder to the boiling side of the metal wall. A metal powder which may comprise the first metal and the second metal are then sprinkled onto the plastic binder. The metal wall with metal powder bound by the plastic thereto is blanketed with an inert atmosphere such as nitrogen and the temperature is raised to a brazing temperature for sufficient time to effect metallurgical bonds between the metal powder particles to each other and to the boiling side of the metal wall. The plastic binder decomposes under heat and evaporates. The circulating inert gas diminishes formation of an oxide film and also purges the decomposition gases from the binder material. The bonded metal powder forms a highly porous, three-dimensional matrix that provides the EBL with nucleate boiling sites. 
     Appropriate plastic binders include polyisobutylene, polymethylcellulose having a viscosity of at least 4000 cps and sold commercially as METHOCEL and polystyrene having a molecular weight of 90,000. The binder may be dissolved in an appropriate solvent such as kerosene or carbon tetrachloride for polyisobutylene and polymethylcellulose binders and xylene or toluene for polystyrene binder. The boiling side should be cleaned to be free of grease, oil or oxide to obtain proper bonding of the EBL thereto. Before applying the plastic solution, the boiling side may be flushed with the plastic solution to facilitate wetting, thereby obtaining a more even distribution of plastic binder. The plastic solution may be applied to the boiling side in a way that will achieve a uniform layer such as by spraying, dipping, brushing or paint rolling. After application, the layer is air dried either during or after the application of the metal powder to evaporate away most of the solvent. A solid, self-supporting layer of metal powder and binder is left in place on the metal wall by the binder. 
     The metal powder comprising the first and second metal are mixed with a flux. Upon heating, the flux melts and draws oxides from the metal which could inhibit the bonding of the metal particles to each other and to the boiling side. The flux may be a mineral salt such as commercially available potassium aluminum fluoride, which is a mixture of KAlF 4  and KAlF 6 . Other fluxes may be suitable. 
     The core  20  of the heat exchanger  10  is assembled by stacking layers of components. If the brazing of the core  20  will not be performed in a vacuum furnace, each component should be coated with flux before stacking A suitable way to coat components with flux components is to mix the flux with denatured alcohol in 1:1 volumetric ratio and brush or spray the flux solution onto the component before stacking The order of stacking will be described with the side shown in  FIGS. 2 and 3  on the bottom. The cap sheet  40  is placed on the bottom of a stacking surface with the outer surface of the cap sheet  40  down. A layer of brazing foil is layered at least on the two vertical margins  48  of an inner surface of the cap sheet  40  or perhaps over the whole inner surface of the cap sheet  40 . The vertical spacer bars  42  are stacked on the vertical margins  48  of the inner surface of the cap sheet  40 . The brazing foil may be provided only at the vertical margins  48  of the cap sheet  40  because only the vertical spacer bars  42  will be brazed to the inner surface of the cap sheet  40  that is defining the boiling passage  12  in this case. Typically, no horizontal spacer bars  60  are stacked in the boiling passage  12 . However, in an embodiment, if the cap sheet  40  is defining the cooling passage  14 , the horizontal spacer bars  60  should be stacked on and brazed to the cap sheet  40 . A layer of brazing foil is stacked on top of the vertical spacer bars  42 . Strips of the brazing foil may be placed just over the vertical spacer bars  42 . The metal wall  44  with the EBL  46  on the boiling side  44   a  facing downwardly toward the cap sheet  40  and the cooling side  44   b  facing upwardly is stacked on top of the vertical spacer bars  42 . The vertical margins  48  of the boiling side  44   a  which are devoid of the EBL  46  will rest on the brazing foil on top of the vertical spacer bars  42 . A layer of brazing foil is laid on top of the cooling side  44   b  of the metal wall  44 . The primary fin stock  54  comprising the primary fins  52 , the distributor fin stock  58  comprising the distributor fins  56 , the collection fin stock  68  comprising the collection fins  66  and the horizontal spacer bars  60  and the vertical spacer bars  42  are all stacked on top of the layer of brazing foil laid on top of the cooling side  44   b  of the metal wall  44 . A layer of brazing foil is laid upon the primary fin stock  54 , the distributor fin stock  58 , the collection fin stock  68  comprising the collection fins  66  and the spacer bars  42 ,  60 . Next, another metal wall  44  with the cooling side  44   b  facing downwardly and the boiling side  44   a  facing upwardly is laid upon the layer of brazing foil. On the top of the metal wall  44 , strips of brazing foil are laid down just in the vertical margins  48  of the boiling side  44   a  outside of the EBL  46 . The vertical spacer bars  42  are laid down on top of the strips of brazing foil in the vertical margins  48 . Strips of brazing foil are laid on top of the vertical spacer bars  42 . An additional metal wall  44  with the boiling side  44   a  facing downwardly is stacked on top with the vertical margins  48  mating with the strips of brazing material on top of the vertical spacer bars  42 . The rest of the core  20  of the heat exchanger  10  is stacked as previously described until the cap sheet  40  is stacked on the top of the stack. It is also contemplated that both sides of the primary fin stock  54 , the spacer bars  42 ,  60  and/or the cooling side  44   b  of the metal wall  44  may be integrally clad with a layer of brazing material. This would obviate the need for adding layers of brazing foil in the stack constituting the core  20 . However, if just the fin stock  54 ,  58 ,  68  and/or the spacer bars  42 ,  60  can be obtained with brazed material clad on both sides, the use of brazing foil may be obviated. 
     After the core  20  is fully stacked it is inserted into a furnace with an atmosphere of inert gas and heated so that the center  20  of the core attains an elevated temperature. After remaining at the elevated temperature for a period of time, it is allowed to cool. The elevated temperature is above the melting temperature of the brazing material and below the melting temperature of the EBL  46  material upon application and the melting temperature of the base metal. In an embodiment, the elevated temperature may be below the melting temperature of the EBL  46  material after application. In a controlled atmosphere brazing environment, Aluminum Alloy 4047 may be used for the brazing material in which case the elevated brazing temperature would be approximately 607° to about 618° C. (1125° to 1145° F.). Aluminum alloy designations given herein will be pursuant to the convention of alloys used by those of ordinary skill in the art of aluminum brazing. The brazing material melts and forms a metallurgical bond with adjacent metal members to provide a robust metal heat exchanger core. The EBL  46  maintains its highly porous structural integrity. Residues of flux on the surface of the core  20  may remain but will typically wash out without affecting operation. 
     After brazing the core  20  together, the manifolds  18 ,  28  and the headers  22 ,  30  are welded to the core  20  as shown in the embodiment in  FIG. 1 . The conduits  16 ,  24 ,  26 ,  32  are all affixed to the appropriate manifold  18 ,  28  or the header  22 ,  30 . Other delivery, distribution, collection and recovery equipment than shown in the embodiment of  FIG. 1  may be used. 
     Alternatively, one or both of the brazing steps may take place in a vacuum oven. Flux becomes unnecessary and a lower temperature is typically used for brazing. However, in the vacuum brazing process, it takes longer for the core to reach the brazing temperature, after which, cooling is allowed. If the stacked core is brazed in a vacuum environment, Aluminum Alloy 4104 may be used for brazing material in which case the elevated brazing temperature would be approximately 582° to about 593° C. (1080° to 1100° F.). 
     It is important, for purposes of this invention, that the EBL be able to withstand the final brazing heat treatment. In a brazed aluminum heat exchanger, brazing material, whether it be powder, foil or cladding may comprise a eutectic alloy of at least about 80 wt-% aluminum and about 10 to about 15 wt-% silicon. In an embodiment, the eutectic alloy comprises about 11 to about 13 wt-% silicon and at least about 85 wt-% aluminum. In a further embodiment, the brazing eutectic alloy may be Aluminum Alloy 4047 and comprise about 12 wt-% silicon and about 88 wt-% aluminum. Other components of the core  20 , such as the walls, the fin stock and the spacer bars may comprise Aluminum Alloy 3003 which comprises a highly proportioned aluminum alloy of as low as about 98 wt-% aluminum and as high as about 2 wt-% manganese. Small amounts of magnesium and iron may also be present in Aluminum Alloy 3003. The term “highly proportioned” means greater than 90 wt-%. Other components comprising substantially pure aluminum or highly proportioned aluminum alloys may be suitable. In vacuum brazing applications, about 1 to 2 wt-% of magnesium may be provided in the highly proportioned aluminum alloy. The material comprising the EBL may comprise about 0.5 to about 1.5 wt-% silicon and at least about 95 wt-% substantially pure aluminum or highly proportioned aluminum alloy. In an embodiment, the EBL may comprise about 5 to about 11 wt-% brazing material and at least about 85 wt-% substantially pure aluminum or highly proportioned aluminum alloy. In an embodiment, the EBL comprises at least about 90 wt-% pure or highly proportioned aluminum and a eutectic alloy including about 11 to about 13 wt-% silicon and at least about 85 wt-% aluminum. In an embodiment, the eutectic alloy in powder form is mixed with powdered substantially pure or highly proportioned aluminum. To prevent oxidation of the aluminum in nonvacuum brazing ovens, a flux comprising about 5 to about 10 wt-% of a powdered mineral salt should be included in the EBL material upon application. 
     While not wishing to be bound to any particular theory, we believe that upon heating, a powdered EBL material mixture described above, the brazing eutectic alloy powder melts and wets the solid, unmelted substantially aluminum powder, thereby forming an alloy. We believe that after application, the resulting alloy in the EBL melts at a higher temperature than the brazing eutectic alloy by virtue of the lower concentration of the silicon metal in the aluminum alloy. The EBL is then able to withstand brazing temperatures associated with bonding the stacked heat exchanger core that are perilously close to the temperature at which the EBL material was initially brazed without loss of performance. 
     If the EBL is sintered, pure Aluminum Alloy 3003 powder may be sintered at about 1185° F. (641° C.). Brazing foil comprising the eutectic of silicon and aluminum mentioned above may be used to bond the core together at a brazing temperature of about 604° to 613° C. (1120° to 1135° F.) under a controlled inert atmosphere and a brazing temperature of about 566° to 596° C. (1050° to 1105° F.) in a vacuum environment. 
     EXAMPLE I 
     An enhanced boiling powder was obtained by mixing 83.6 wt-% Aluminum Alloy 3003 powder, 8.4 wt-% brazing flux comprising potassium aluminum fluoride and 8.0 wt-% Aluminum Alloy 4047 brazing powder. An adhesive comprising 38 wt-% polyisobutylene sold as CS-200 A3 by Clifton Adhesives and 62 wt-% VARSOL light kerosene solvent was mixed and brushed onto three tubular walls comprising Aluminum Alloy 3003. The enhanced boiling powder was then sprinkled onto the adhesive and heated under nitrogen in a small furnace. Each coated tubular wall was heated to 621° C. (1150° F.) for nine minutes. The adhesive and solvent evaporated off, leaving an EBL of about 0.3 to 0.4 millimeters (10 to 15 mils) thick. The resulting EBL had a highly porous structure and was determined to have boiling heat transfer coefficients above 204,418 kJ/hr/m 2 K (10,000 BTU/hr/ft 2 ° F.). 
     EXAMPLE II 
     Two metal tubular walls were coated with the adhesive and the enhanced boiling powder as explained in Example I. Each tubular wall was heated in a controlled nitrogen atmosphere to a brazing temperature of 623° C. (1153° F.) in a closed retort at about atmospheric pressure and then allowed to cool. 
     A first tubular metal wall was heated and cooled over a period of 48 minutes. The first tubular metal wall was tested and determined to have a heat transfer coefficient of above 204,418 kJ/hr/m 2 K (10,000 BTU/hr/ft 2 /° F.), which is more than adequate for a surface with an EBL. The first tubular metal wall was then subjected to a second furnacing to simulate vacuum brazing of an entire heat exchanger core by heating it to a temperature of 593° C. (1100° F.) and allowing it to reside at that temperature over a twenty-four hour period before cooling. Visual inspection revealed that the quality of the EBL was not impacted. The first tubular metal wall was again tested and determined to have a heat transfer coefficient of above 204,418 kJ/hr/m 2 K (10,000 BTU/hr/ft 2 /° F.). 
     A second tubular metal wall was heated and cooled over a period of 36 minutes. The second tubular metal wall was tested and determined to have a heat transfer coefficient of above 204,418 kJ/hr/m 2 K (10,000 BTU/hr/ft 2 /° F.), which is adequate for a surface with an EBL. The second tubular metal wall was then subjected to a second furnacing to simulate controlled atmosphere brazing of an entire heat exchanger core by heating it to a temperature of 613° C. (1135° F.) and allowing it to reside at that temperature over a two hour period under nitrogen at atmospheric pressure before cooling. Visual inspection revealed that the quality of the EBL was not impacted. The second tubular metal wall was again tested and determined to have a boiling heat transfer coefficient of above 204,418 kJ/hr/m 2 K (10,000 BTU/hr/ft 2 ° F.). After heating the EBL to a temperature of 8.3 Celsius degrees (15 Fahrenheit degrees) from the brazing temperature of the EBL, the structure of the EBL withstood the heat treatment without noticeable loss to structure or performance.