CROSS-REFERENCE TO RELATED PATENT APPLICATIONS j0O l] The present application claims the benefit of United States Provisional Patent Application Serial No, 61/588,086 filed January 18, 2012 and United States Provisional Patent Application Serial No. 61 /732,751 filed December 3, 2012 the entireties of which are hereby incorporated by reference.

FIELD OF THE INVENTION

J0002] The present invention relates generally to thermal electric power generating plants, and more specifically to air cooled condenser finned tubes and related method for fabricating the same.

BACKGROUND OF THE INVENTION

[0003] Rejection of waste heat in a Rankine cycle used in thermal electric power generation plants via Dry Cooling techniques instead of Wet Cooling is an inherently more

environmentally friendly option. Indeed, governmental restriction o water consumption for industrial use, especiall to condense waste steam in power plants, has emerged as a growing worldwide trend. Driven by the increasing scarcity of water, power plant designers have been turning to heat exchangers in the form of air cooled condensers in lieu of the conventional "water cooled" condensers. The air cooled condensers (ACCs) consist of inclined tube bundles arranged in an array of "cells ¹ ' wher ein the power gen eration plant turbine exhaust steam flows inside the tubes and is condensed by the flow of cooling air in a cross flow arrangement delivered by axial fans located generally underneath. The steam therefore undergoes a phase change from gas to liquid between the tube inlet and outlet. To minimize the '"parasitic power" (energy needed to run the fans), the dr cooling industry has steadily evolved from using multi-row finned tube bundles to a single tube ro over the past 70 years.

[0004] Finned tubes have been proposed for ACCs. These fumed tubes are generally comprised of aluminum cl added carbon steel tabes with brazed aluminum fins, aluminized carbon steel tubes with brazed aluminum fins, and stainless steel tubes with laser welded stainless steel fins.

jOOOSj The above mentioned tube configurations have several disadvantages that are limiting the widespread application of ACCs, such as: (1) carbon steel tubes are subject to flow accelerated corrosion issues that are being exacerbated by the high cycles and fast starts of the latest generation of power plants; (2) contamination of condensate (deleterious iron carry

- i - o ver) by corrosion of ihe carbon steel tubing and associated additional water treatment required to address the more stringent water chemistry requirements of modern power plants; and (3) the high capital cost associated with stainless steel tubes with, laser welded stainless steel fins.

}0006| An improved tube construction and fabrication process is desired,

SUM ARY OF THE INVENTION

|Θ007| The present disclosure provides an improved finned tube assembly and a method for bonding an aluminum fin to an uncoated bare steel tube. In one embodiment, the method employs a flux, mixture comprising powdered flux and an oil based carrier. In a preferred, embodiment, water is not used in the flux mixture. Advantageously, the meihod

advantageously eliminates the need to first provide an aluminum clad layer (or otherwise aluniiui/.ed surface) on the outer surface of the tube for bonding the tube to the fin before beginning the brazing process, eliminates drying of fluxed tubes, and reduces the deleterious inter/metallic layer (e.g. FeAB) between the dissimilar metals which is formed during brazing. The latter is beneficial because FeAB is relatively brittle so that it is desirable to minimize the thickness of this layer to avoid joint fracture. The method, according to the present disclosure provides long term corrosion protection of the external tube surface after brazing. The method is applicable to tubes constructed from carbon steels, erritic stainless steels, austenitic stainless steels, and other sieei alloys,

jOOOS] in one preferred embodiment, the steel core tube is stainless steel. The stainless steel, core tube provides a unique solution to the flow accelerated corrosion and iron transport issues that currently plague the power plant air cooled condenser industry. This invention particularly addresses the more stringent water chemistry requirements and cyclic power plant loading scenarios that exist today.

[Θ0Ο ] The present disclosure further provides a heat exchanger of the air cooled condenser (ACQ type having high efficiency, lower manufacturing costs, and longer life than heretofore known air cooled condensers. .Both the method and heat exchanger according to the present disclosure al low for maintaining cost effective manufacturing.

}OO10] According to one embodiment of the present invention, a tube assembly for a heat exchanger includes a bare steel tube and at least one set of aluminum fins bonded directly to an exposed outer surface of the bare steel tube by a brazing filler metal comprised of aluminum. In one embodiment, the steel tube is made of stainless steel In ano ther embodiment, the steel tube is made of low carbon steel. The set of aluminum fins has a serpentine configuration comprising peaks and ^' valleys. In a certain embodiment, the steel lube has an oblong cross-sectional shape.

jOil 11 According to another embodiment of the present Invention, a finned tube brazing preassembly for heat processing in a brazing fti nace is provided. The preassemblv includes a bare steel tube having an exposed outer surface, a set of aluminum fins, a fluoride based flux and oil based carrier mixture disposed between the bare steel tube and the set of aluminum fins, and a brazing filler metal comprising aluminum. The brazing filler is disposed proximate to the set of aluminum fins and the ilox and oil based carrier inixiure for bonding the .fins to the tube. The brazing filler metal forms a brazed bond between the bare steel tube and. set of aluminum fins when heat processed, in the brazing furnace., i one embodiment, the oil based carrier is vanishing oil. In one embodiment, the flux and oil based carrier mixture is applied to the exposed outer surface 124 of tube 102 at a rate of about 25 g/ni2 flux and about 35 g ni2 oil based carrier which may be vanishing oil. In various embodiments, the bare steel tube is preferably stainless steel or low carbon steel.

[00:12} According to another embodiment of the present invention, an air cooled condenser sized for industrial and commercial application is provided. The ai cooled condenser includes an inlet steam distribution header for conveying steam, a condensate outlet header for conveying condensate, and an array of tube bundles. The tube bundles each comprise a plurality of finned tube assemblies having a bare steel tube with an exposed outer surface and a se of aluminum fins brazed directly onto the tube by a brazing filler metal. The steel, tubes are spaced apart by the aluminum fins. The steel tubes further have an inlet end fluidly coupled to the inlet steam distribution header and an outlet end finally coupied to the outlet header. A forced draft fan is provided and arranged t blow air through the tube bundles. In various embodiments, the bare steel tube is preferably stainless steel or low carbon steel. |0013 A method for forming a tube assembly for an air cooled condenser is provided. The method includes the steps of: providing a bare steel tube hav ing an exposed exterior surface of steel; providing an aluminum fin; applying a flux and oil based carrier mixture onto the exposed exterior surface of the steel tube; providing a brazing filler metal; bringing into mutual contact the bare steel tube, aluminum fin, flux and oil based carrier inixiure, and brazing filler metal wherein the bare steel tube, aluminum fin, flux and oil based carrier mixture, and brazing filler metal collectively define a finned tube brazing preassembly; loading the finned tube brazing preassembly into a brazing furnace; and heating the finned tube brazing preassembly to a temperature sufficient to melt the brazing filler metal and bond the alumimim fin directly onto the bare steel tube. In various embodiments, the bare steel tube is preferably stainless steel or low carbon steel.

(0914] A method for condensing steam using an air cooled condenser according to the present disclosure is also provided. The method includes: providing a» air cooled condenser comprising an array of tube bundles, an inlet steam distribution header conveying steam, a condensate outlet header conveying condensate, and a forced draft fan blowing air through the tube bundles; the tube bundles each comprising a plurality of firmed tube assemblies having a bare steel tube with an exposed outer surface and a set of aluminum fins brazed directl onto the tube with a brazing filler metal , the tubes having an in!et end flu idly coupled to the inlet steam distribution header and an outlet end fluklly coupled to the outlet header: flowing steam through the inlet steam distribution header; receiving steam through the inlet end of each tube; condensing the steam in each tube between the inlet and outlet ends;

passing liquefied water condensa te through the outlet end of each, tube; and collecting the condensate in the condensate outlet header. In various embodiments, the bare steel tube is preferably stainless steel or low carbon steel.

(001.5] A flux mixture suitable for brazing aluminum fins onto a bare steel tube is provided. In one embodiment, the flux mixftsre includes a flux powder and an oil based carrier. In one embodiment, the oil based carrier is preferably an aliphatic hydrocarbon, and more preferably a vanishing oil. The flux powder and oil based carrier form a flux gel or paste suitable for application to an air cooled condenser tube or other structure.

BRIEF DESCRIPTION OF THE DRAWINGS

(0016] FIG. I A is a perspective view of an air cooled condenser system according to one embodimen t of the present disclosure.

[0017} FIG. IB is schematic flow diagram of a Rankine cycle and components for a thermal power generating plant.

[0018] FIG. 2A is a perspective view of a finned tube assembly used in the air cooled condenser of FIG . 1.

(0019] FIG. 2B is a. transverse cross-sectional v iew of the tube assembly of FIG. 2B;

[0020] FIG. 2C is a transverse cross-sectional view of an alternative embodiment of a tube assembly usable in the air cooled condenser of FIG. 1 .

[0021} FIG. 2D is a transverse cross-sectional view of another alternative embodiment of a tube assembly usable in the air cooled condenser of FIG. 1. [0022] FIG, 2E is a transverse cross-sectional view of another alternative embodiment of a tube assembly usable in the air cooled condenser of FIG. 1.

(0923] FIG. 2f is a transverse cross-sectional view of another alternative embodiment of a tube assembly usable in the air cooled condenser of FIG. 1

|0624] FIG. 3 is an exploded perspective view of the finned tube assembly of FIG. 2A,

[0025] FIG. 4 is an exploded perspective view of a first embodiment of a finned tube preassembly for forming a tube assembly usable in the air cooled condenser of FIG. 1.

[0026] FIG, 5 is an exploded perspective view of a second embodiment of a finned tube preassembly for forming a tube assembly usable in the air cooled condenser of FIG. i .

[0027] FIG. 6 is an exploded perspective view of a third embodiment of a finned tube preassembly for forming a in be assembly usable in the air cooled condenser of FIG. i .

[0028] FIG. 7 is a perspective view of a sev eral tube assembly of FIG. 2 brazed together io form a portion of a tube bundle usable in die air cooled condenser of FIG. i .

[0029] All drawings are schematic and not necessarily to scale.

DETAILED DESCRIPTION OF THE DRAWINGS

[0030] The features and benefits of the invention ar illustraied and described herein by reference to exemplary embodiments. This description of exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not. intended in any way to limit the scope of the present invention.

Relative terras such as "lower," "upper," "horimntal," ^' "vertical,", "above," "below," "up," "down," "top" and "'bottom" as well as derivative thereof (e.g., "horizontally,"

"downwardly," "upwardly," etc.} should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation- Terms such as "attached, ^'* "affixed," "connected," "coupled," "interconnected," and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Accordingly, the disclosure expressly should not he limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features. [063J j FIG, 1.A depicts a heat exchanger in the form of an air cooled condenser (ACQ system 20 as used in a thermal electric power generation plant for converting low pressure steam into l iquid ("condensate"). Air cooled condenser system 20 includes an air cooled condenser 22 and exhaust steam supply 30 which in one embodiment is fluidly connected to the steam exhaust from the turbine of a turbine-generator set 25 (see PIG. I B) as will be known to those skilled in the art. in the present embodiment being described, the fluid is initially lo w pressure turbine exhaust steam (vapor phase of water) upstream of the air cooled condenser and liquid condensate (condensed water) downstream of the air cooled condenser.

[0032] In one embodiment, the steam supply 30 includes a main steam duct 32 which is fluidly coupled to a piping distribution manifold 34 that branches into a plurality of risers 36 and distribution headers 38 for conveying inlet steam into the air cooled condenser 22, as shown. Risers 36 may be generally vertically oriented and distribution headers 38 may be generally horizontally oriented. Each set of risers 36 and distribution headers 38 supply steam to an array of condenser tube bundles 100 comprised of a plurality of individual finned tubes 102. Tubes 102 each have inlet ends 126a fluidly coupled to one of the distribution headers 38 to receive water in the steam phase and outlet ends 126b fluidly coupled to a condensate outlet header 24 which collects the condensed steam or condensate (liquid phase water) from the tubes.

|0633] With additional reference to FIG. I B showing a schematic diagram of a conventional Rankine cycle of a thermal electric power generation plant, the outlet headers 24 are fluidly connected to condensate return, piping 26 to route the liquid condensate back to a condensate return pump 28 which pumps the condensate to the steam generator ("boiler") feed system. The condensate fTeedwatef at this stage in cycle) is generally pumped through one or more feedwater heaters 21 to pre-heat the feedwater. Feedwater pumps 29 pump the feedwater to a steam generator 23 (e.g. nuclear or fossil fuel fired) where the liquid feedwater is evaporated and converted back to steam. The steam flows through, a turbine-generator set 25 which produces electricity in a known manner. The pressure of the steam drops as it flows through the turbine converting thermal and kinetic energy into electric energy. The low pressure steam at the outlet: of the turbine is collected and returned to the main steam duct 32 to complete the flow path back to the air cooled condenser system 20.

10034] Referring back to FIG. 1 A, the air cooled condenser 22 further includes a support structure 40 to elevate the tube bundles 100 above the ground so that air may be blown vertically up through the tube bundles from below in one possible embodiment by an air moving system comprised of a plurality of forced draft, fans 60 {fan blade shown in FIG. 1 A). The fans 60 are each mounted OH a fan deck platform 50 supported by support structure 40. In one preferred embodiment, the fan deck platform 50 and tube bundles 100 are elevated vertically above the ground by a distance thai is at least as great as the height of the tube bundles (defined as being measured from fee distribution header vertically to fee outlet header 24. The support structure 40 may include columns 44 arid cross-bracing as required to support the weight of the tube bundles 1 0, fans 1.02, risers 36, distribution headers 38, and outlet headers 24, as we ^' ll as to laterally stiffen the structure to compensate for wind loads. In some embodiments, wind walls 44 may be provided around the tube bundles 1.02 to counter the effects of prevailing winds which may adversely affect normal upwards and outwards airflow through the tube bundles 100 from the forced draft, fan. 60.

J003S The air cooled condenser 22 may be configured such that a single steam distribution header 38 feeds a pair of spaced apart tube bundles 102. In one embodiment,, the tube bundles 1.00 in each pair may be arranged at. an angl to each other as shown forming a generally tent-like triangular configuration with a fan 60 disposed between and at the bottom or below the tube bundles. Each tube bundle 1 0 has a separate outlet header 24 disposed near and supported fay fee fan deck platform 50. The outlet headers 24 may be spaced apart on opposing sides of the fen 60 in one non-limiting arrangemen t . The tube bundles 100 may be disposed at an suitable angle to each other.

[0036! FIG- 2A depicts an exemplary finned tube assembly 1 4 of tube bundle 100 which includes a longitudinally-extending elongated tube 102 and two sets of cooling fins i .10 bonded to the tube by a unique brazing method according to the present disclosure, as further described herein. A plurality of these tube assemblies 104 are essentially stacked and arranged together in adjacent, parallel relationship forming the tube bundles 10O(see, e.g. FIG. ?), In one embodiment, tube bundle 1 0 is comprised of a single row of adjacent tube assemblies 104 each fluidly connected between a distribution header 38 and an outlet header .24 (see, e.g. FIG. 1 A). In a preferred embodiment, as best shown in FIG. ? _? a single set of fins 1 10 is disposed between each tube 102 which are laterally spaced apart by the fins.

[0037] FIG . 3 depicts an. exploded view of a finned tube assembly 104 prior to brazing, which may be defined as a fumed tube preassembly.

]0038j Referring to FIGS. 2, 3 and 7, tube 1.02 has an inner surface 122 that forms a longitudinal internal .flow conduit or passagewa 120 and. an exposed outer surface 124 on which the two sets of fins 1 10 are bonded, as further described herein. Internal passageway 120 extends from an inlet end 1 6a which is fluidly connected to distribution header 38 to an opposin outlet end 126b which is fluidly connected to outlet header 24, The interna! passageway 12 is in .fluid communication with both the distribution header 38 and outlet header 24. Passageway 120 is configured and dimensioned for transporting a steam-liquid water phase mixture through the tubes 102 of the air cooled condenser 22. internal passageway 120 is a sealed flo conduit which in operation with fins i 10 performs the function of removing heat from the turbine exhaust fluid which enters inlet end 1 6a of tube 102 in a steam phase from distribution header 38, condenses in flowing through the tube via heat transfer, and leaves the outlet end 126b in the liquid phase ("condensate") which is collected in the outlet header 24.

10039] Tube 102 (and the resulting interna! passageway 120) preferably may have a transverse cross-section that is non-circular and may be generally described as oblong, elliptical, or ovoid in shape. In the illustrated preferred embodiment, tube J 02 generally comprises opposing top and bottom substantially .flat walls 130a, 130b that are connected by iatera! walls 132a, ! 32b. la one embodiment, flat walls 130a and 130b are oriented parallel to each other. Flat walls 130a-b each have a width Wl that is larger than height H2 of lateral sections 132a-b as further shown, for example, in FIG. 2B. Flat wails 130a, 130b correspondingly define respective flat exposed outer surfaces 124 on which fins 110 are bonded as further described herein,

(00403 FIGS. 2B-F show several examples of possible embodimen ts of tubes 102 hav ing a non-circular transverse cross-sections and flat wails 130a, 130b in accordance with the present disclosure, which are suitable for employ ing the fm-to-tube bonding process described herein. The tubes 102 may each be formed as a single unitary monolithic structure (e.g. by extruding) in cross-section as shown in FIGS 2F and 4-6, or be comprised of two or more configured tube wall segments that are joined together at joints by a suitable fabrication means used in the art to form a sealed flow conduit such, as seam welding, brazing, crimping, or other techniques suitable to provide a leak-proof tube construction.

(0041] It will be appreciated thai embodiments of the in vention are not limited to any particular type of tube construction and the tube 102 can take on a wide vari ety of non- circular transverse cross-sectional shapes. For example, the top and bottom flat walls 130a, 130b may have an outwardly convex transverse cross-section being arcuately curved away from the longitudinal axis LA of die tube t resist deformation in partial or full vacuum conditions inside the tube,

(0042] Referring to FIG, 2 A, tubes 102 ma be configured and dimensioned for industrial or commercial applicaiion in an air cooled condenser system used in a thermal power generation plant to cool and condense exhaust steam from the turbine, in such applications, tubes .1 2 extend a longitudinal length LI which in some embodiments may be between about 10 to 60 feel. The width W! the tube 1 2 may be in a range between about 4 to 1 inches. The thickness of the tube wall is preferably sufficient to promote good heat transfer and support the weight of the tube and fins 1 10. in one embodiment, for example, the tube wall thickness Tl (e.g. walls 130a, 130b and 132a, 132b measured in transverse cross-section as shown in FIG. 2A) may be about 0.035 to 0.12 inches. In one embodiment, the wall thickness Ti is about 0.050 inches. Of course, the invention is not so limited and the longitudinal length LI , width W L and wall thickness can be any desired measurement. Moreover, while the tube 1 2 is exemplified as extending along a linear longitudinal axis, the tube 102, in other embodiments, can include curves, bends and/or angles in one or more orthogonal directions. 10043 The tube 102 dimensions can be optimized for varying market conditions based on materials used. For example, a tube width W i of 9.25 inches (235 mm) by a height 02 of 0.79 inches (20 mm) with a 0.039 inch ( 1 mm) wall thickness Tl have been, determined feasible with SS409 material. The accompanying AL3003 fin is 8,5 inches (215 mm) long (measured longitudinally along the longitudinal axis LA), 0,83 inches (21 mm) high HI, and 0.01 inches (0.25 mm) thick (sheet thickness) placed at a fin pitch of 0.09 inches (2.31 mm). j0044] For application in an air cooled condenser suitable for an industrial use such as in a power generation plant, tube 102 is preferably constructed of steel. Any suitable steel having appropriate heat transfer properties for a given application may be used, in one preferred embodiment, the steel may be stainless steel for corrosion resistance. Non-limiting examples of suitable stainless steels are Grade 409SS or Grade 3Cr 12 stainless. Other suitable ferrittc or austenilic stainless steels may be used.

| u045] In a preferred embodiment , tubes 102 are constructed of bare steel having an exposed outer surface 124 on which fms 1 1 are directly bonded, has a metallurgical composition of steel composition. In one embodiment, tube Ϊ 2 therefore has a homogeneous metallurgical composition comprised uniformly of steel from end to end and in transverse cross- section between the inner surface 122 and exposed outer surface 124.

[Θ646] Tubes 102, and in particular exposed outer surface 124 on top and bottom Hat walls 130a, 130b to which the fms 1 10 are bonded, are preferably free of any coating, cladding, surface chemistry modification, impregnation, or other application, which, incorporate another material other tha steel such as particularly metals, alloys, or compositions containing aluminum. As further described herein, the im-io-tube bond is advantageously formed on hare steel without the aid and expense of first applying an aluminum coating on or aluminizing the exposed outer surface 124. 1Θ047] Referring to 2A-F and 3, fins .1 10 will be described in greater detail Each set of fins 1 10 is preferably formed of a corrugated sheet of material having a high coefficient of thermal conductivity, such as aluminum in a preferred, embodiment. The metal sheet is originally flat and then shaped by a suitable fabrication technique to form the corrugations. The corrugated sheets 20A, 208 can be of any length. Either a single or a plurality of the corrugated sheets can be used to cover substantially the entire longitudinal length LI of a flat wall 130a or 130b of the finned tube assembly 104. in other embodiments, comigated sheets of material may cover less than the entire length L I or only intermittent portions of the Oat walls 130a, 1.30b.

(0048] An aluminum sheet usable for forming fins 1 10 according to the present disclosure is a fiat element which may be made from aluminum alloy in the I xxx, 3xxx„ 5xxx or 6xxx families as designated by the Aluminum Association, which is adapted and suitable for heat absorption and discharge to cooling medium flowing past the sheet. In one embodiment, without limitation, exemplary corrugated fins 1 10 may be formed from of sheets of A! 3003 material hav ing a thickness of about 0.010 inches.

[0949]

(0950] Each of the sets of fins 1 1 has a generally serpentine configuration as shown in FIGS. 2-7 (inclusive ofFIGS. 2A-F) comprising a pluralit of undulating and alternating peaks 131. and valleys .133. Lateral airflow passages are formed in the gaps between the peaks and valleys for airflow generally perpendicular to the length Ll of the tube and longitudinal axis L A (see FIG. 2 A). The peaks 1 31 define mounting base areas on opposing top and bottom sides of fins 1 10 for bonding to tubes 102. The tips of the peaks 131 form laterally extending ridges disposed perpendicular to the longitudinal length Ll and longitudinal axis LA of tubes 1.02 which are bonded to the tube 1.02 during the brazing process. Except for the two outermost tubes 102 in a tube bundle 100, the ridges are configured to abuttingly contact the exposed outer surfaces 124 on top and bottom flat walls 130a, 130b of adjacent tubes for bonding to the wails in the manner described herein.

(0051 j In one embodiment as shown in FIG, 3, the fin i it ⁾ to tube 102 joint ma be made by an interrupted fin edge having a square saw tooth configuration. The contact surfaces between the fin and the bare exposed outer tube surface 124 on. top and. bottom flat walls 1 0a, 5.30b is made of narrow metal, strips of fin punctuated by narrow vertically extending slits 134 formed in the fin. Slits 134 extend perpendicular to outer surface 1 4 and fiat walls 130a, 130b in the embodiment shown. Slits 134 preferably may be evenly spaced apart as shown, or alternatively have unequal spacing. Slits 1 4 are formed in the peaks 131 of the fin 1 10 and extend partially down/up along the height HI of the fin (see FIG. 2B defining height, dimension). Using this saw tooth configuration, heat produced during the brazing process advantageously does not cause excessive surface deformation in the tube.. This unique fin base design creates a controlled yield zone in the base of the fin ( i.e. where peaks 131 abut flat wails 130a, 130b) to accommodate the differential thermal expansion rates of the aluminum fin and steel tube. This feature significantly mitigates deformation of the tube during the post braze cool down by allowing the fin to contract more than the parent tube.

[O052j In other embodiments, the edges of the fins 1 .1 at the peaks i 3 i may be laterally continuous without interruption, as shown for example in FIG. 2A.

[Θ ^β 53] According to an aspect of the present invention, a process or method for bonding an aluminum fin to an uncoated bare steel tube is provided. In a preferred embodiment, the bonding method is brazing. An overview of components, materials,, pre-bra/.ing assembly steps, and furnace brazing process will first be described.

[0054) Referring to FIG, 3 for general reference, the method for bonding aluminum fins 1 10 to bare steel tubes .102 comprises essentially at least the following general steps (to be further explained herein): (1) providing at least one first structural component in the form of a bare steel tube 1 2 which in this embodiment is stainless steel, oil based, carrier brazing flux 140 gel or paste which preferably con tains a vanishing oil, brazing filler metal 150 in one of three physical delivery formats as shown in FIGS. 4-6 and further described herein, and at least one other second structural component in the form of an aluminum fin i 1.0; (2) bringing these components into physical, contact; (3) heating these components in a brazing furnace to a temperature between about 577 C and 610 C, preferably between the temperatures of about 585 C and 600 C; and (4) subsequently holding this temperature range for about two to six minutes, preferably about three to five minutes, wherein a brazed bond, occurs on at least one point of contact between the tubes and fins in which the braze filler metal is used as a bonding agent.

[1)055] The method according to the invention is based on the finding that the overall time the braze filler metal is at brazing temperature may be significantly reduced, i.e., b at least 10%, if the flat outer surface 124 of the tube 102 is not coated or clad with aluminum or another material from a previous operation prior to brazing. This reduction, of total time at or above the brazing temperature reduces the formation of mtermetaUics (Fe A13) formed between the dissimilar materials. The method is also less costly because the finned tube assembly 104 does not have to be dried (eliminate water) before brazing.

- Π - [0056} Upon heating of the fins l it ⁾ and tube 102 brought into abutting contact with, each other, the braze -filler metal and brazing substrates melt together irt a single step, it being provided according to the invention that the oil based carrier braze flux 140 gel and brazing filler metal 150 deli vered as an addition to the flux brazing gel (FIG. 5} or as a foil sheet (FIG. 6) or as a clad layer roiled onto the aluminum fin (FIG. 4} is then used as a brazing material. This offers the advantage that an aluminum clad material has not been placed through a previous heating cycle before brazing. This reduces cost of manufacture and reduces the negative impact of intermetaSIic formation because the cladding and brazing process is the same step. There is also power consumption savings on the whole which is accompanied by lower costs.

J0057 In die method according to the invention, when the brazing filler metal 150 is supplied in the form of a foil sheet 152, as further described herein, the foil sheet is in abutting contact with outer surface 124 of the tube 102, thereby when the foil sheet melts during the brazing process, the external surface of the tube is imparted with enhanced corrosion protection from the alumimtm-siiicoa layer. In one representative example, without limitation, an aluminum silicon coating having a thickness of about 25 microns may be deposited on the steel tube 1 2 by the brazing process.

10058] In one preferred and present embodiment being discussed, tube 102 is stainless steel . The brazing method according to the present invention can be applied to both ierritic and austenitic stainless steel tubes.

[0059} As noted above, FIGS. 4-6 show three possible approaches for introducing the brazing filler -metal 150 into the brazing process. These three figures each depict an exploded view of a finned tube assembly 1 4 prior to brazing with components and products used during the brazing process to bond the fins 1. 1.0 to the steel tube 102. Accordingly, FIGS. 4-6 depict the un-wsed components used to braze and form a permanently bonded finned tube assembly, which may be defined herein as a finned tube brazing preassembiy . In ail three filler metal 150 delivery mechanisms described herein, the aluminum or aluminum silicon filler metal is provided proximate to the bonding site between the aluminum fins 1 10 and the exposed outer surface 124 of the steel tube 102 for brazing the fins to the tube.

1 _. 0060] The brazing .filler metal .150 preferably has a preponderance of aluminum, as much as 85 weight % or more, where the remaining proportion is predominantl silicon. Accordingly, a preferred brazing filler metal is aluminum silicon (AISi), In some embodiments, the brazing filler metal may contain about 6-12% silicon. Zinc may be added to the brazin filler metal alloy to lower the melting temperature, thereby allowing the brazing to take place at a lower temperature range (540 C to 590 C).

{0961] Referring to FIG. 4, the brazing filler metal 150 may be provided as clad layers hoi rolled or otherwise bonded onto an aluminum sheet which forms a cladded fin 110. The aluminum fin 1 1 , typically aa3003, is cSadded with art AISi brazing alloy consisting of about 6 to 12% silicon. The addition of silicon promotes brazing by reducing the melting temperature of the alloy, decreasing the surface tension and thereby increasing the wettabilit of the alloy in addition to minimizing the intermetallic alloy (e.g. FeAB) layer thickness. The thickness of the AISi clad layer on the fin sheet metal is between about 10% and 20% of the total thickness of the fin 1 10, and preferably about 15%.

|0t*62 In one possible embodiment, fin 1 Ϊ0 may therefore be constructed as a three-layer composite having an aa3003 aluminum core with brazing filler metal 1.50 cladded on each side. In one exemplary embodiment, a suitable cladded. fin composite construction is aa4343/aa3003/aa4343. The aa4343 cladding is an AISi composition having a silicon content of about 6.8-8.20 %. A representative non-limiting thickness for fin 1 10 constructed in this manner is about 0.012 inches. Other suitable thicknesses of the fin and cladding may be provided.

J0M3| The foregoing resulting t ube assembly 104 prior to brazing and bonding of the fms 110 onto tube 1.02 is shown in FIG. 4. Tube 102 is bare steel (i.e. oncoated and not aluminized in any manner), and preferably stainless steel in this embodiment. Flux 140 is applied between the cladded fins 1 10 and flat outer surfaces 124 on top and bottom flat, walls 130a, 130b, The assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

[0064] Referring to FIG. 5, the brazing filler et l 150 may alternatively be provided as an additive mixed with the flux 1 0. A powder based filler metal such as aluminum powder may be used, in one embodiment, a powdered AISi brazing alloy is used, such as without: limitation aa 4343 (6.8-8.2% Si), aa 045 (9-1.1% Si), or aa 4047 (1 1 -13% Si) which are suitable, is added to the flux 140 and beneficially increases the exterior corrosion protection of the stainless steel. Preferably, the brazing alloy used for the .filler metal 150 is aa 4045 or 4047, and more preferably 4045 in some embodiments dependent upon, the brazing oven temperature profile used. This is particularly advantageous for heat exchangers that are located in aggressive environments such as those in salt air or in the vicinity of chemical plants whose emissions attack most corrosion-prone metals. Specimens subjected to a prolonged ASTM b~ 117 salt spray test (750 hours) are used to confirm corrosion resistance in marine air environment.

[00 5] The foregoing resulting tube assembly 104 prior to brazing and bonding of the fins 1 10 onto tube 102 is shown in FIG. 5. Tube 102 is bare steei (i.e. unc-oated and not alurakvized in any manner), and preferably stainless steel in this embodiment. Fins 110 are uncladded and formed as a single layer sheet of aluminum (e.g. aa 3003) as described herein. Flux 140 is applied between the uncladded fins 110 and flat outer surfaces 124 on top and bottom flat wails 130a, 130b. The assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

(0066] Referring to FIG. 6, the brazing filler metal 150 ma alternatively be provided in the form of a sheet of brazing foil 152. in one embodiment, the foil may be an AISi material such as without limitation as an example aa 4045. Foils 152 having a representative sheet thickness of about 0. 10 to 0.15 inches may be used, in one embodiment, the sheet thickness of foil 152 used may be about 0, 15 inches.

[Θ067] The foregoing resulting tube assembly 1 4 prior to brazing and bonding of the fins 1 .10 onto tube 1 2 is shown in FIG. 6. Tube 1 2 is bare steel (i.e. uncoated and not aluminized in any manner), and preferably stainless steel in this embodiment. Brazing foil 152 is placed against the peaks 131 of the fins 1 10. Flux 140 is applied between the foil 512 and flat outer surfaces 124 on top and bottom flat walls 130a, 130b, The assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

[Θΰ68| The fin and the tube asseinbly 104 according to FIGS. 4-6 described above are brazed together within a controlled atmosphere brazing furnace at a temperature suitable to form a bond between the fin and tube. Any suitable commercially available brazing furnace may be used to braze the fumed tube assemblies 104 formed, according to the present disclosure. { ^' 0969] A suitable brazing flux such as a fluoride based flux with a. cesium or lithium additive, is preferably utilized to sequester the negati ve effects of the chromium and nickel compounds within the stainless steel parent material. Cesium and or lithium additives to fluoride based fluxes bind and retard the negative effec ts of chromium and nickel at brazing temperatures. This practice -requires a very specific time vs. temperature brazing cycle that is both shorter in duration and lower in temperature. This approach farther enhances the braze joint strength and toughness by reducing the mtermetalhe layer (e.g. FeA ) thickness within the braze joint |0Θ?0] Suitable cesium and lithium fluxes are commercially available under the brand name NOCOLO ® from So ^' lvay Fluor GmbH of Hannover, Germany. Advantageously, this eliminates the current general industrial practice of requiring either a roller clad or alumifl.iir.ed layer on the parent tube 1.02 materia] to enable using alura unn-io-aiuniiniioi braze processes. This will reduce labor and material costs while improving the heat transfer rate.

}0§?!J The inventors have discovered that mixing an. oil-based additive to the flux admixture instead of water for a carrier as conventionally used in the art to prepare a spreadable flux paste or gel from a powdered flux product produces improved brazing performance and adhesion between aluminum fins and bare steel tubes in the brazing furnace, in one preferred embodiment, a suitable oil-based carrier is an aliphatic hydrocarbon such as without limitation vanishing oil or lubricant. This oil-based carrier advantageously evaporates during processing and therefore does not interfere with the brazing.

J0072 A suitable non-aqueous oil based carrier is Evaplube brand vanishing oil which is commercially a vailable from General Chemical Corporation of Brighton, Michigan. In one embodiment, Evap-Lube 2200 has been used. This product is in a liquid oil form and has a specific gravity of 0.75 Ϊ -0.768 (water ~ i .O), boiling point of 340-376 degrees F, vapor pressure at 68 degrees F of 0.5 mmHg, evaporation rate of 0.1 6, and is 100% volatile by volume.

10673] To prepare suitable spreadable flux mixtures comprised of flux powder (e.g.

NOCOLOK® flux) and an oil based carrier (e.g. Evap-Lube 2200), the relative amounts of each used preferably may be in the ranges of about 40-65% by weight vanishing oil to about 60-35 % by weight flux, and more preferably about 48-58% by weight vanishing oil to about 52-42 % by weight flux, in one representative embodiment, without limitation, about 3% by weight vanishing oil may be used with the remaining weight percentage (47%) of product in the mixture being flu or fiitx with additional additives.

|0 ^§ ?4J The foregoing oil based carrier and powdered flux mixtures produce a very viscous flux mixture {similar to a gel or wail paper paste in consistency and viscosity) that is readily spreadable on the tubes 102 in preparation for brazing. Advantageously, for the present brazing application, the Evap-Lube 2200 vanishing oil evaporates readily leaving little or no residual oils, and therefore does not in terfere with the formation of a brazed bond between the fins 110 and bare steel tube 102. The oil based carrier and fluoride based flux brazing gel or paste is an admixture of halid.es including, but not. limited to. potassium, aluminum fluoride, cesium aluminum fluoride, and lithium aluminum fluoride,

j007S| A suitable representative application rate of the flux and oil based carrier mixture may be about 25 g/rrf flux to 35 g/nr of vanishing oil. [0676] In alternati ve em odimeals, a long chain alcohol may be added to further extend and improve the ^readability of die flux-oil based carrier mi ure which may be used for longer lengths of bare steel tubes 102 to foe prepared for brazing. In certain embodiments, the long chain alcohol ma be glycol including hexylene glycol and propylene glycol. Glycol or another long chain alcohol may be added to the flux and oil based carrier mixture in amount from about and including 25% by weight or less in some embodiments, or alternatively in a range of 1-25% b weight in other embodiments, in one embodiment, if glycol or another long chain alcohol is added to the flux mixture, the weight percentage of the oil based carrier used is preferably reduced proportionately while maintaining the same weight percentage of fl x power in the mixture to provide optimum brazing performance and bonding,

f 0077{ In using the vanishing oil and fluoride based flux brazing mixture gel to prepare a bra/.e filler metal delivery system in which the filter metal 150 is mixed directly into the flux 140 as shown in FIG. 5 and described above, the flux mixture comprises NOCOLO -® flux, Evaphibe vanishing oil (e.g. Evap-Lube 2200), and powdered aluminum, in various

embodiments, the aluminum content of the flux 140 gel/paste may be in the range of about 10-50% A.1 powder by weight. In one representative example, for illustration, approximately 60 g/ HT of ahmimtm powder may be added which may be AiSi in some embodiments. To make a an aluminum preparation having a paste-like consistency for mixing with the flux gel, approximately 90 g/srf of Evaphibe ma added to that amount of aluminum powder.

Approximately 25 g m2 NOCOLOK® flux and about 35 g/m2 Evap-Lube 2200 are used in the oil based carrier flux gel mixture, as described above. Adding up all of the foregoing constituents, the aluminum powder is therefore about 30% of the total (2K>g m2) filler metal- flux gel mixture by weight in this example when combined to form a flux gel or paste that is applied to the bare tube surfaces.

f 0 ?8 In one embodiment, the alumimun particle size of the aluminum or AISi power may be without limitation about 5-10 microns,

f0t>7*>] An exemplary method for bonding an aluminum fin 1 10 to a bare steel tube 102 will now be described based on the foregoing parameters and materials,

[Θ08Θ] The method generally begins by first providing a preasserably of individual components as shown in either FIGS. 4, 5, or 6 which have been describe above. Essentially, a bare steel tube 102 is provided and sets of aluminum fins 1 1 which comprise the main parts that are to be brazed and bonded together. Tube 102 may be stainless steel m this example such as Type 409SS. Fins 1 10 may be aa3003 aluminum. O08J j Tube 102 is initially cleaned using a suitable cleaner to remove drawing oils and grime in preparing the outer surface 124 of the tube for receiving flux 1.40 which may be provided in a gel or paste form in. the present embodiment. Water based cleaners may be used, and alternatively in other possible embodiments acetone may be used, ideally, the outer surface 124 of tube 102 along top and bottom flat walls 130a, 130b where fins 1 10 will be bonded should be thoroughly clean of contaminants that might adversely affect the formation of a good brazed joint between the tube and fms.

[0082} Next, the oil based carrier flux 140 mixture bracing gel or paste is applied to tubes 102. The flux 140 is applied to the outer surface .124 of tube 102 along top and bottom flat walls 130a, 130b (see, e.g. FIGS. 4-6) before the fins 1.1.0 are placed against in surface contact with the tube surfaces and flux, in the embodiment of FIG, 5, the flux 140 will contain the AISi filler metal 150 as already described herein, in the embodiments of FIGS. 4 and 6, the flu will generally not contain any filler metal 1 50 which is provided by other ways described herein such as by being clad onto the fins 1 10 (FIG. 4) or provided in the form of separate sheets of foil (FIG. 6).

]0083j The method next continues by bringing the tube 1 2 with flux 140 applied and fins 1 1 into surface contact with each other and forming the preassemb!y shown in F GS, 4 and 5. With respect to FIG. 6, the AISi filler metal foil 152 is placed on the flux 140 preferably after it is applied to tube 102, and then the tins are brought into surface contact with the foil adhered to the tube by the gel or paste like flux.

[0084) The foregoing assembled but unbrazed finned tube assemblies 104 as shown in FIGS . 4-6 are held together by any suitable means such as clamping in preparatio for processing in the brazing furnace.

[0085] The tube assembly 1.04 is next loaded into a bracing- furnace, heated, to a. suitable brazing temperature and held at that temperature for a sufficient period of time to form a permanent bond between the aluminum fins 110 and the tube 102, as already described herein. The bonded tube assembiy 104 is then cooled and removed from the brazing furnace.

[0086) In an alternative method for bonding fins 1 1 to tube ] 02 and forming a completed tube assembly, the brazing process may be applied to half-tube segments comprised of one set of fins 1 1 and one of the fiat wall .130a or 130b (see, e.g. FIG. 3). For example, a first set of fins i 1 may be brazed onto flat, wall 130a, and a. second set of fins may be brazed onto flat wall 130b. Then, the two brazed half tubes may be joined together by a suitable method such as welding to produc the completely tube assembly 104 shown in FIG. 2A, This

- Π - fabrication technique allows gravity to assist the flow of the braze material into the braze joint.

(0987] According to another embodiment, a tube assembly 104 comprised of a bare carbon steel tube 102 and fins I 10 may be fabricated in according with the foregoing method. In one embodiment, low carbon steel having a wall thickness T! of about 0,060 inches may be used. In another embodiment, a low carbon steel having a. chrome (Cr) content of 0.1 -0.25% may be used with a wall thickness T I of 0.060 inches. The construction may use a. brazing filler metal 150 in the form of foil 152 shown in FIG, 6 made of aa4045 aluminum with a sheet thickness of about 0.015 inches. The flux 140 may be a OCOLOK® and Evaplube mixture as described herein, and in. some possible embodiments an aluminum or AtSt filler in the form of flakes or powder may be added to the flux nnxture. A water based cleaner is preferred to prepare the tube 102 for brazing that removes rust, oils, and other surface contaminants from outer surface 124 of the tube; however, other suitable cleaning soiuiions may be used. Preferably, the flux is applied immediately after cleaning to prevent reoccurrence of oxide formation on the tube, in some embodiments, a binder may be added to the flux mixture to dry the flux for handling.

(0100} While the invention has been described and illustrated in sufficient detail that those skilled in this art can readily make and use .if various alternatives, modifications, and improvements should become readily apparent without departing from the spirit and scope of the invention.