Patent ID: 12240068

DETAILED DESCRIPTION OF THE DRAWINGS

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

The description of illustrative embodiments according to principles of the present invention 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 of the invention 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 terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives 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 unless explicitly indicated as such. 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. Moreover, the features and benefits of the invention are illustrated by reference to the exemplified embodiments. Accordingly, the invention expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto.

Multiple inventive concepts are described herein and are distinguished from one another using headers in the description that follows. Specifically,FIGS.1-10are relevant to a first inventive concept,FIGS.11-17are relevant to a second inventive concept, andFIGS.18-21are relevant to a third inventive concept. Inventive concept four has no drawings associated therewith. The first through fourth inventive concepts should be considered in isolation from one another. It is possible that there may be conflicting language or terms used in the description of the first through fourth inventive concepts. For example, it is possible that in the description of the first inventive concept a particular term may be used to have one meaning or definition and that in the description of the second inventive concept the same term may be used to have a different meaning or definition. In the event of such conflicting language, reference should be made to the disclosure of the relevant inventive concept being discussed. Similarly, the section of the description describing a particular inventive concept being claimed should be used to interpret claim language when necessary.

I. Inventive Concept 1

With reference toFIGS.1-10, a first inventive concept will be described.

A typical air-cooled condenser unit10is shown inFIG.1and comprises a plurality of inclined tube bundles11arranged in an A-frame structure. A main stem duct12delivers steam from a turbine into a distribution manifold13. The steam passes through the distribution manifold13and into the risers14, where it then flows into the distribution headers15. The distribution headers15deliver the steam into the inclined tube bundles11where thermal energy from the steam is transferred to air flowing on the outside of the inclined tube bundles11. The tube bundles11are positioned atop a fan deck platform16that comprises a plurality of fans17for forcing cooling air to flow adjacent and through the inclined tube bundles11. The fan deck platform may be situated atop a frame18so that cooling air can be drawn upward. A windwall structure19may also be provided.

Referring first toFIGS.2and3concurrently, a finned tube section100A according to an embodiment of the present invention is exemplified. The finned tube section110A extends from a first end115A to a second end116A along a longitudinal axis A-A. In the exemplified embodiment, the finned tube section100A is an elongated tubular structure that is substantially linear and particularly suitable for creating a vertical tube bundle for an air-cooled condenser of a power generation plant. As discussed below, in certain embodiments, a plurality of the finned tube sections100A can be formed and coupled together in axial alignment to form a single finned tube. In one such embodiment, the finned tube sections100A have a length between 4 to 8 feet and are installed vertically in such sections. The invention, however, is not so limited and, in certain embodiment, the finned tube section100A can be formed of a sufficient length such that a single finned tube section100A forms a single finned tube. In such an embodiment, the first end115A of the finned tube section100A will be the top end of the finned tube while the second end116A of the finned tube section100A will be the bottom end of the finned tube (or vice versa). As discussed below, the finned tube section100A is a heat exchange tube in that it effectively transfers thermal energy from a tube-side fluid, such as steam, that is flowing inside of the finned tube section100A to a shell-side fluid, such as air, that is flowing adjacent the finned tube section100A on the exterior thereof.

The finned tube section100A generally comprises a tube110A and a plurality of fins111A extending radially outward from the tube110A. The tube110A comprises an inner surface112A that forms a cavity113A and an outer surface114A from which the plurality of fins111A protrude/extend. The cavity113A extends along a longitudinal axis A-A. In certain embodiments (i.e., embodiment in which an inner tube is not needed), the cavity113A acts as a tube-side fluid path in which the inner surface112A is exposed to the tube-side fluid. In embodiments in which an inner tube is used (described later with respect toFIGS.8-11), the tube110A can be considered an outer tube, the inner surface112A of which is not exposed to the tube-side fluid (such as steam generated in a Rankine power cycle). In the exemplified embodiment, the tube110A has a substantially circular transverse cross-section.

The tube110A also comprises an outer surface114A. The plurality of fins111A protrude radially outward from the outer surface114A of the tube110A. In one embodiment, the finned tube section100A is formed by an extrusion process. As a result, the plurality of fins111A are integral with the tube110A. More specifically, in one such embodiment, both the tube110A and the plurality of fins11A are simultaneously formed in a single extrusion process using a first material, such as an extrudable metal or metal alloy. In one specific embodiment, the finned tube section100A (including both the plurality of fins111A and the tube110A) are formed of a material having a coefficient of thermal conductivity. Suitable materials include, for example, aluminum or aluminum alloy. The utilization of an extruded finned tube section100A allows for the compaction and simplification of the overall heat exchanger, as compared with the state of the art cross flow designs.

While forming the entirety of the finned tube section100A by a single extrusion step is preferred in certain embodiments, the invention is not so limited in other embodiments. In certain other embodiments, the tube110A may be extruded in one step and the fins11A may be extruded subsequently or prior thereto during a separate step, and then subsequently coupled (directly or indirectly) to the tube110A through brazing, welding, thermal fusion, mechanical coupling, or other processes. In still other embodiments, the tube110A and the fins111A can be formed separately by techniques other than extrusion, such as machining, bending, pressing, die-cutting, stamping, and/or combinations thereof.

In the exemplified embodiment, each of the plurality of fins111A extends substantially parallel with the longitudinal axis A-A and covers the entire length of the tube110A, wherein the length is measured from the first end115A to the second end116A. Moreover, each of the plurality of fins111A extends radially outward from the outer surface114A of the tube110A in a linear fashion from a base portion117A to a distal end118A. The base portions117A can be thicker than the remaining portions of the fins11A, thereby promoting stability and conductive heat transfer into the fins111A. In the illustrated embodiment, the fins111A are linear in their longitudinal extension. However, in alternate embodiments, the fins111A may be extruded or otherwise formed with an undulating (wave) geometry to promote heat transfer.

As can best be seen inFIG.3, the plurality of fins111A are arranged about the circumference of the outer surface114A of the tube110A at uniform angular intervals. In the illustrated embodiment, the twenty four (24) fins111A are provided on the tube110A so that an angular interval of approximately 15° exists between adjacent fins111A. Of course, the exact number of fins111A, along with the angular spacing between them can vary as needed. For example, depending on the diameter of the tube110A and the heat duty demand, the number and height of the radial fins111A can be selected. The tube110A can have as many radial fins111A as the state of the art extrusion technology will allow. In one exemplary embodiment, providing 24 fins111A on a 1.5 inch nominal ID tube110A, wherein each fin111A is 1.5 inch high has been determined to be feasible. A larger number of fins may be achieved if a larger size tube is selected.

Referring now toFIG.4, the formation of a finned tube200using a plurality of the finned tube sections100A-B according to an embodiment of the present invention will be described.FIG.4illustrates three of these finned tubes200, which are identical for the formation and structural purposes described herein, despite their different functionality when incorporated into a tube bundle. The arrows indicating steam flow in the finned tubes200results from the arrangement shown inFIGS.5-6, which will be described later in this document. For purposes of simplicity, only one of the finned tubes200will be described with the understanding that the discussion is applicable to all of the finned tubes200inFIG.4and/or used to form a tube bundle according to the present invention.

As exemplified, the finned tube200comprises two finned tube sections100A,100B. Finned tube section100A is described above with reference toFIGS.2-3, and is referred to herein as a first finned tube section100A. Finned tube section100B (only a portion of which is shown inFIG.4) is identical to finned tube section100A in all aspects but is either subsequently or previously formed using one of methods discussed above. The finned tube section100B is referred to herein as the second finned tube section100B. Like numbers are used to like parts of the first and second finned tube sections100A,100B with the exception that the suffix “B” is used to denote the parts of the second finned tube section100B rather than the suffix “A,” which is used inFIGS.2-3to describe the first finned tube section100A.

As mentioned above, the finned tube200comprises a first finned tube section100A and a second finned tube section100B arranged in axial alignment. The first finned tube section100A and the second finned tube section100B are aligned adjacent one another so that the longitudinal axes A-A of the first and second finned tube sections100A,100B are substantially aligned and coaxial. When so aligned, the first end115B of the second tube110B of the second finned tube section100B abuts the second end116A of the first tube110A of the first finned tube section100A.

While the first and second finned tube sections100A,100B are aligned so that their longitudinal axes A-A are aligned, the first and second finned tube sections100A,100B (which are adjacent finned tube sections in the finned tube200) are rotated relative to one another so that corresponding ones of their fins,111A,111B are angularly offset from one another. This can improve heat transfer from the tube-side fluid (e.g., steam) to the shell-side fluid (e.g., air). The angular offset, in one embodiment is 1° to 20°. In another embodiment, the angular offset is 5° to 10°.

This concept will be described below with respect to an example to ensure understanding. Assume that the first finned tube section100A was placed in proper alignment and position in an angular/rotational position in which one of its fins111A were angularly located at each of the cardinal points (N, S, E, & W). The second finned tube section100B would then be position in axial alignment with the first finned section100A in an angular/rotational position in which none of its fins111B were located at the cardinal points. Rather, the second finned section100B would be in an angular/rotational position in which one of its fins111B is offset from each of the cardinal points by the angular offsets described above, such as for example 5° to 10°. In alternate embodiments, however, the fins111A,111B of the first and second finned sections100A,100B may be angularly aligned if desired.

Once the first finned tube section100A and second finned tube section100B are aligned and rotationally oriented as described above, the first and second finned tube sections100A,100B are coupled together, thereby forming the finned tube200. The exact technique used to couple, either directly or indirectly, the first finned tube section100A and second finned tube section100B together will depend on the material(s) of which the first finned tube section100A and second finned tube section100B are constructed. Suitable connection techniques include mechanical fastening in which gaskets or other materials can be used achieve a hermetic interface, welding, brazing, thermal fusing, threaded connection, use of a coupler sleeve, a tight-fit connection, and/or combinations thereof. As described below with respect toFIGS.8-10, coupling of the first and second finned tube sections100A,100B can also be accomplished using an inner tube.

While the finned tube200is exemplified as having only two finned tube sections100A,100B, the finned tube200can be formed of more or less finned tube sections100A as desired. In embodiments of the finned tube200in which more than two finned tube sections100A,100B are used, the aforementioned rotational offset can be implemented between each pair of adjacent finned tube sections.

Referring now toFIG.5, an air-cooled condenser600according to an embodiment of the present invention is illustrated. The air-cooled condenser600is a true counter-current/parallel flow air-cooled condenser that, in one embodiment, is constructed with the finned tubes200formed of extruded aluminum or aluminum alloy finned tube section100A,100B in a vertical array (or matrix) configuration.

The air-cooled condenser600generally comprises a shell300and a tube bundle assembly400. The tube bundle assembly400is positioned within an internal cavity301of the shell300. The shell300has an open top end302and an open bottom end303As a result, cool air can flow into the open bottom end302, flow through the internal cavity301where it flows adjacent the finned tubes200and becomes warmed, and exists the shell300as warmed air. A blower304, in the form of a fan or other mechanism capable of inducing air flow, can be provided either above and/or below the tube bundle assembly400. While a single blower304is illustrated, more blowers can be implemented as desired to meet functional demands. In other embodiments, the blower may be omitted.

The tube bundle assembly400generally comprises a tube bundle500formed by a plurality of the finned tubes200, a top header pipe410, a bottom header pipe420, and a plurality of feeder pipes430. Each of the plurality of the finned tubes200of the tube bundle500are oriented in a substantially vertical orientation so that the longitudinal axes A-A (FIG.2) thereof extend substantially vertical. The finned tubes200of the tube bundle500may be arrayed in a triangular, rotated triangular, rectangular or another suitable layout that provides for a uniformly distributed flow area across the tube bundle. In the exemplified embodiment, the finned tubes200of the tube bundles500are arrayed in 3×5 rectangular arrays (seeFIG.6). A desired feature of the tube bundle layout geometry is the ability to make a closely packed bundle of the finned tubes200such that the air flowing axially along the finned tubes200is in close proximity to the finned tubes'200outer surfaces. Factory assembled modules can be delivered and connected into the steam distribution network of a Rankine cycle fluid circuit of a power generation planet, thereby providing an economical heat rejection alternative for small and large scale applications.

Each of the finned tubes200of the tube bundle500is coupled to and fed steam from the top header pipe410, which is in turn operably coupled to a source of steam, such as turbine in a Rankine cycle power generation circuit. Similarly, each of the finned tubes200of the tube bundle500is coupled to the bottom header pipe420so that condensate can gather and be fed back into the Rankine cycle fluid circuit of the power generation plant. In the exemplified embodiment, a top end201of each of the finned tubes200of the tube bundle500is fluidly coupled to the top header pipe410by a separate upper feeder pipe430. Similarly, a bottom end202of each of the finned tubes200of the tube bundle500is fluidly coupled to the bottom header pipe420by a separate lower feeder pipe430. As a result, a hermetic fluid path is formed through the cavity113A (FIG.2) of each of the finned tubes200from the inlet header cavity411of the top header pipe410to the outlet header cavity421of the bottom header pipe420. The top header pipe410is located at an elevation that is greater than the elevation at which the bottom header pipe420is located. The top header pipe410and the upper feeder pipes430can be collectively considered a top network of pipes470while the bottom header pipe420and lower feeder pipes430can be collectively considered a bottom network of pipes480.

The top header pipe410extends along a longitudinal axis B-B (FIG.5) that is substantially horizontal. Similarly, the bottom header pipe420also extends along a longitudinal axis that is substantially horizontal. In other embodiments, the top and bottom header pipes410,420may be inclined.

The top header pipe410is located above the tube bundle500while the bottom header pipe420is located below the tube bundle500. The top and bottom header pipes410,420, however, are specifically designed so as to create minimal impedance and/or obstruction to the vertical flow of air entering and exiting the tube bundle500. In order to accomplish this, each of the top and bottom header pipes410,420is designed to have a transverse cross-section having a major axis AMAJand a minor axis AMIN. Moreover, each of the top and bottom header pipes410,420is oriented so that the minor axis AMINextends substantially perpendicular to the direction of the air flow through the tube bundle500. Thus, in the exemplified embodiment, the minor axis AMINextends substantially horizontal while the major axis AMAJextend substantially vertical. The major axis AMAJhas a length that is larger than the length of the minor axis AMINfor both the top and bottom header pipes410,420. In one such embodiment, the major axis AMAJhas a length that is at least twice the length of the minor axis AMINfor both the top and bottom header pipes. By designing and orienting the transverse cross-sections of the top and bottom header pipes410,420to have the aforementioned major axis AMAJand minor axis AMIN, the top and bottom header pipes410,420achieve two criteria: (1) adequate flow area for the tube side fluid; and (2) maximum opening between the adjacent headers to minimize friction loss to the entering (bottom header) and exiting (top header) air (seeFIG.6also). While not visible from the drawings, each of the horizontal sections of the feeder pipes430may also be designed to have a transverse cross-section comprising a major axis AMAJand a minor axis AMIN, and be oriented, as discussed above and below with respect to the top and bottom headers410,420.

In one embodiment, the top and bottom header pipes210,220(along with the horizontal sections of the feeder pipes430) each have an obround transverse cross-section. The obround shape allows for a large internal flow area for steam while affording ample space for the air to enter and exit the tube bundle500via spaces between the header pipes410,420(and horizontal sections of the feed pipes430). The obround transverse cross section with the flat (long) sides vertical is a preferred arrangement when the tube side fluid is low pressure steam or vapor. As mentioned above, the top header pipe510serves as the inlet for the vapor (exhaust steam) (seeFIG.3for a typical inlet header profile).

As can be seen inFIG.6, the air-cooled condenser can comprises a plurality of tube bundles500housed in separate shells300. In other embodiments, more than one tube bundle500can be housed in a single shell300. All of the inlet header pipes410are preferably manifolded from a single point450of a main steam supply line440. Furthermore, each of the tube bundles500, along with the shell300may be positioned atop a fan deck, which is in turn positioned atop a frame structure (as shown inFIG.1).

Referring back toFIG.5, the up flowing cooling air may be sprayed with a mist of coolant generated by spray nozzles550located within the shell300at a height between the top header pipe410and the bottom header pipe420. The spray nozzles550are operably and fluidly coupled to coolant reservoirs551and further configured to atomize the liquid coolant into a fine mist that is introduced into the air flowing through the tube bundles500. Spaying the mist into the air flow at intermediate height(s) increases the LMTD and promotes heat rejection from the tube side fluid (i.e. the steam). This form of cooling augmentation is unique to this heat exchanger design and results in substantial performance gains of 25 to 30% depending on the ambient conditions. These performance gains can be realized in improved warm weather performance or capital cost reduction and smaller plot area constraints.

Referring now toFIG.7, a housing300suitable for use in the air-cooled condenser600ofFIGS.5and6is illustrated. Depending on the available height, a “chimney” space305above the bundle can be incorporated in the unit to increase the natural draft height. This will reduce the amount of electrical energy required to pump the cooling air through the bundle. In designs where the blower304is located above the tube bundle500, it is possible to provide for additional entry windows310for air to enter the tube bundle500, which will promote increased heat transfer from the tube-side fluid.

Referring now toFIGS.8-10, an alternative construction of the finned tube800is described in which the final finned tube800comprises the finned tube sections100A,100B and an inner tube700. Such an arrangement is particularly useful in power plants where the condensing steam is not permitted to come in contact with aluminum or aluminum alloy of the finned tube sections100A,100B. The finned tube800can be sued in the air-cooled condenser600described above in lieu of or in addition to the finned tubes200.

Referring first toFIG.8, the first and second finned tube sections100A,100B are formed, aligned and oriented as described above with respect toFIGS.2-4. Once this is done, an inner tube700is provided and axially aligned with the cavities113A,113B of the first and second finned tube sections100A,100B along a longitudinal axis C-C. The inner tube700is formed of a material that is different than the material of which the first and second finned tube sections100A,100B are formed. In one embodiment, the inner tube700is formed of a material having a high yield strength, is non-corrosive, and is weldable. A suitable material includes steels, with stainless steel being preferred.

The inner tube700extends along an axis has an outer surface702and inner surface701, which forms cavity703. The inner tube700extends from a bottom end705to a top end704along the longitudinal axis C-C.

Referring now toFIGS.9and10concurrently, the inner tube700is then slid through the cavities113A,113B of the finned tube sections100A,100B. In the exemplified embodiment, the top end704of the inner tube700protrudes slightly from the top end of the first finned tube section100A while the bottom end705of the inner tube700protrudes slightly from the bottom end of the second finned tube section100B (FIG.9). At this stage, the outer diameter of the inner pipe700is smaller than the inner diameter of the tubes110A,110B. As a result, a interstitial space750exists between the outer surface702of the inner tube700and the inner surfaces112A,112B of the tubes110A,110B.

Once the inner tube700is so positioned, the inner tube700is diametrically expanded by applying a force F. Diametric expansion of the inner tube can be achieved by a variety of methods, including hydraulic pressure.

The diametric expansion of the inner tube700continues until the outer surface702of the inner tube700is in substantially conformal surface contact with the inner surfaces112A,112B of the finned tube sections100A,100B, thereby forming the finned tube800. As a result the interstitial space750disappears and there are substantially no gaps and/or voids between the outer surface702of the inner tube700and the inner surfaces112A,112B of the finned tube sections100A,100B. In embodiments using the inner tube700, the tubes110A,110B can be considered outer tubes.

The inner tube700couples the finned tube sections100A,100B together and thus can be used instead of or in conjunction with the other coupling techniques discussed above forFIG.4. When the resulting finned tube800is incorporated into the air-cooled condenser600, the inner tube700can be sued to make the welded joints between the top pipe network470and/or the bottom pipe network460, as shown inFIG.5. Additionally, when the inner tube700is used, the first and second inner tubes100A,100B do not have to be in abutment to effectuate coupling. Because the inner tube700(in contact with the condensing steam) is at a higher temperature than the finned tube sections110A,100B, the risk of the inter-tube interface becoming loose during service is ameliorated.

EXAMPLE

Comparison of a conventional (inclined bundle) air-cooled condenser (FIG.1) and an air-cooled condenser according to the present invention is set forth below in the following table for the performance of the two design concepts:

ConventionalA-FrameHI-PercentACCVACCDifferenceThermal Duty, mmBtu/hr860860—Condensing Pressure, ″HgA2.02.0—Ambient Air Temperature, ° F.6060—Number of Cells Required2012−40%ACC Plot Area (L × W), ft238 × 170240 × 80−53%ACC Height, ft10479−24%Total Extended Heat Transfer8,919,2007,977,250−10%Surface, ft2Total Fan Shaft Power, kW27002700—

The design concepts disclosed herein can be used in a wide variety of coolers that seek to employ air as the cooling medium. Its application to design air cooled condensers to condense exhaust steam in power plants will lead to reduced cost and reduced land area requirement. Additional advantages of the present invention are: (1) modular installation; (2) reduced site construction effort compared to the A-frame design; (3) significantly reduced quantity of structural steel required to erect the system; and (4) ability to reduce fan power consumption by adding an exhaust stack (chimney) to the design.

2. Inventive Concept 2

With reference toFIGS.11A-17, a second inventive concept will be described.

FIG.11Adepicts a heat exchanger in the form of an air cooled condenser (ACC) system1020as used in a thermal electric power generation plant for converting low pressure steam into liquid (“condensate”). Air cooled condenser system1020includes an air cooled condenser1022and exhaust steam supply1030which in one embodiment is fluidly connected to the steam exhaust from the turbine of a turbine-generator set1025(seeFIG.11B) as will be known to those skilled in the art. In the present embodiment being described, the fluid is initially low 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.

In one embodiment, the steam supply1030includes a main steam duct1032which is fluidly coupled to a piping distribution manifold1034that branches into a plurality of risers1036and distribution headers1038for conveying inlet steam into the air cooled condenser1022, as shown. Risers1036may be generally vertically oriented and distribution headers1038may be generally horizontally oriented. Each set of risers1036and distribution headers1038supply steam to an array of condenser tube bundles1100comprised of a plurality of individual finned tubes1102. Tubes1102each have inlet ends1126afluidly coupled to one of the distribution headers1038to receive water in the steam phase and outlet ends1126bfluidly coupled to a condensate outlet header1024which collects the condensed steam or condensate (liquid phase water) from the tubes.

With additional reference toFIG.11Bshowing a schematic diagram of a conventional Rankine cycle of a thermal electric power generation plant, the outlet headers1024are fluidly connected to condensate return piping1026to route the liquid condensate back to a condensate return pump1028which pumps the condensate to the steam generator (“boiler”) feed system. The condensate (“feedwater” at this stage in cycle) is generally pumped through one or more feedwater heaters1021to pre-heat the feedwater. Feedwater pumps1029pump the feedwater to a steam generator1023(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 set1025which 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 duct1032to complete the flow path back to the air cooled condenser system1020.

Referring back toFIG.11A, the air cooled condenser1022further includes a support structure1040to elevate the tube bundles1100above 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 fans1060(fan blade shown in FIG.11A). The fans1060are each mounted on a fan deck platform1050supported by support structure1040. In one preferred embodiment, the fan deck platform1050and tube bundles1100are elevated vertically above the ground by a distance that is at least as great as the height of the tube bundles (defined as being measured from the distribution header vertically to the outlet header1024). The support structure1040may include columns1044and cross-bracing as required to support the weight of the tube bundles1100, fans1102, risers1036, distribution headers1038, and outlet headers1024, as well as to laterally stiffen the structure to compensate for wind loads. In some embodiments, windwalls1044may be provided around the tube bundles1102to counter the effects of prevailing winds which may adversely affect normal upwards and outwards airflow through the tube bundles1100from the forced draft fan1060.

The air cooled condenser1022may be configured such that a single steam distribution header1038feeds a pair of spaced apart tube bundles1102. In one embodiment, the tube bundles1100in each pair may be arranged at an angle to each other as shown forming a generally tent-like triangular configuration with a fan1060disposed between and at the bottom or below the tube bundles. Each tube bundle1100has a separate outlet header1024disposed near and supported by the fan deck platform1050. The outlet headers1024may be spaced apart on opposing sides of the fan1060in one non-limiting arrangement. The tube bundles1100may be disposed at any suitable angle to each other.

FIG.12Adepicts an exemplary finned tube assembly1104of tube bundle1100which includes a longitudinally-extending elongated tube1102and two sets of cooling fins1110bonded to the tube by a unique brazing method according to the present disclosure, as further described herein. A plurality of these tube assemblies1104are essentially stacked and arranged together in adjacent parallel relationship forming the tube bundles1100(see, e.g.FIG.17). In one embodiment, tube bundle1100is comprised of a single row of adjacent tube assemblies1104each fluidly connected between a distribution header1038and an outlet header1024(see, e.g.FIG.11A). In a preferred embodiment, as best shown inFIG.17, a single set of fins1110is disposed between each tube1102which are laterally spaced apart by the fins.

FIG.13depicts an exploded view of a finned tube assembly1104prior to brazing, which may be defined as a finned tube preassembly.

Referring toFIGS.2,3and7, tube1102has an inner surface1122that forms a longitudinal internal flow conduit or passageway1120and an exposed outer surface1124on which the two sets of fins1110are bonded, as further described herein. Internal passageway1120extends from an inlet end1126awhich is fluidly connected to distribution header1038to an opposing outlet end1126bwhich is fluidly connected to outlet header1024. The internal passageway1120is in fluid communication with both the distribution header1038and outlet header1024. Passageway1120is configured and dimensioned for transporting a steam-liquid water phase mixture through the tubes1102of the air cooled condenser1022. Internal passageway1120is a sealed flow conduit which in operation with fins1110performs the function of removing heat from the turbine exhaust fluid which enters inlet end1126aof tube1102in a steam phase from distribution header1038, condenses in flowing through the tube via heat transfer, and leaves the outlet end1126bin the liquid phase (“condensate”) which is collected in the outlet header1024.

Tube1102(and the resulting internal passageway1120) 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, tube1102generally comprises opposing top and bottom substantially flat walls1130a,1130bthat are connected by lateral walls1132a,1132b. In one embodiment, flat walls1130aand1130bare oriented parallel to each other. Flat walls1130a-beach have a width W1that is larger than height H2of lateral sections1132a-bas further shown, for example, inFIG.12B. Flat walls1130a,1130bcorrespondingly define respective flat exposed outer surfaces1124on which fins1110are bonded as further described herein.

FIGS.12B-Fshow several examples of possible embodiments of tubes1102having a non-circular transverse cross-sections and flat walls1130a,1130bin accordance with the present disclosure, which are suitable for employing the fin-to-tube bonding process described herein. The tubes1102may each be formed as a single unitary monolithic structure (e.g. by extruding) in cross-section as shown inFIGS.12F and14-16, 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.

It will be appreciated that embodiments of the invention are not limited to any particular type of tube construction and the tube1102can take on a wide variety of non-circular transverse cross-sectional shapes. For example, the top and bottom flat walls1130a,1130bmay have an outwardly convex transverse cross-section being arcuately curved away from the longitudinal axis LA of the tube to resist deformation in partial or full vacuum conditions inside the tube.

Referring toFIG.12A, tubes1102may be configured and dimensioned for industrial or commercial application 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, tubes1102extend a longitudinal length L1which in some embodiments may be between about 110 to 60 feet. The width W1the tube1102may be in a range between about 4 to 18 inches. The thickness of the tube wall is preferably sufficient to promote good heat transfer and support the weight of the tube and fins1110. In one embodiment, for example, the tube wall thickness T1(e.g. walls1130a,1130band1132a,1132bmeasured in transverse cross-section as shown inFIG.12A) may be about 0.035 to 0.12 inches. In one embodiment, the wall thickness T1is about 0.050 inches. Of course, the invention is not so limited and the longitudinal length L1, width W1, and wall thickness can be any desired measurement. Moreover, while the tube1102is exemplified as extending along a linear longitudinal axis, the tube1102, in other embodiments, can include curves, bends and/or angles in one or more orthogonal directions.

The tube1102dimensions can be optimized for varying market conditions based on materials used. For example, a tube width W1of 9.25 inches (235 mm) by a height H2of 0.79 inches (20 mm) with a 0.039 inch (1 mm) wall thickness T1have 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 H1, and 0.01 inches (0.25 mm) thick (sheet thickness) placed at a fin pitch of 0.09 inches (2.31 mm).

For application in an air cooled condenser suitable for an industrial use such as in a power generation plant, tube1102is 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 3Cr12 stainless. Other suitable ferritic or austenitic stainless steels may be used.

In a preferred embodiment, tubes1102are constructed of bare steel having an exposed outer surface1124on which fins1110are directly bonded has a metallurgical composition of steel composition. In one embodiment, tube1102therefore has a homogeneous metallurgical composition comprised uniformly of steel from end to end and in transverse cross-section between the inner surface1122and exposed outer surface1124.

Tubes1102, and in particular exposed outer surface1124on top and bottom flat walls1130a,1130bto which the fins1110are bonded, are preferably free of any coating, cladding, surface chemistry modification, impregnation, or other application which incorporate another material other than steel such as particularly metals, alloys, or compositions containing aluminum. As further described herein, the fin-to-tube bond is advantageously formed on bare steel without the aid and expense of first applying an aluminum coating on or aluminizing the exposed outer surface1124.

Referring toFIGS.12A-Fand13, fins1110will be described in greater detail. Each set of fins1110is 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 sheets1020A,1020B 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 L1of a flat wall1130aor1130bof the finned tube assembly1104. In other embodiments, corrugated sheets of material may cover less than the entire length L1or only intermittent portions of the flat walls1130a,1130b.

An aluminum sheet usable for forming fins1110according to the present disclosure is a flat element which may be made from aluminum alloy in the 1xxx, 3xxx, 5xxx or 6xxx families as designated by the Aluminum Association, which is adapted and suitable for heat absorption and discharge to a cooling medium flowing past the sheet. In one embodiment, without limitation, exemplary corrugated fins1110may be formed from of sheets of Al 3003 material having a thickness of about 0.010 inches.

Each of the sets of fins1110has a generally serpentine configuration as shown inFIGS.12-17(inclusive ofFIGS.12A-12F) comprising a plurality of undulating and alternating peaks1131and valleys1133. Lateral airflow passages are formed in the gaps between the peaks and valleys for airflow generally perpendicular to the length L1of the tube and longitudinal axis LA (seeFIG.12A). The peaks1131define mounting base areas on opposing top and bottom sides of fins1110for bonding to tubes1102. The tips of the peaks1131form laterally extending ridges disposed perpendicular to the longitudinal length L1and longitudinal axis LA of tubes1102which are bonded to the tube1102during the brazing process. Except for the two outermost tubes1102in a tube bundle1100, the ridges are configured to abuttingly contact the exposed outer surfaces1124on top and bottom flat walls1130a,1130bof adjacent tubes for bonding to the walls in the manner described herein.

In one embodiment as shown inFIG.13, the fin1110to tube1102joint may 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 surface1124on top and bottom flat walls1130a,1130bis made of narrow metal strips of fin punctuated by narrow vertically extending slits1134formed in the fin. Slits1134extend perpendicular to outer surface1124and flat walls1130a,1130bin the embodiment shown. Slits1134preferably may be evenly spaced apart as shown, or alternatively have unequal spacing. Slits1134are formed in the peaks1131of the fin1110and extend partially down/up along the height H1of the fin (seeFIG.12Bdefining 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 peaks1131abut flat walls1130a,1130b) 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.

In other embodiments, the edges of the fins1110at the peaks1131may be laterally continuous without interruption, as shown for example inFIG.12A.

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-brazing assembly steps, and furnace brazing process will first be described.

Referring toFIG.13for general reference, the method for bonding aluminum fins1110to bare steel tubes1102comprises 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 tube1102which in this embodiment is stainless steel, oil based carrier brazing flux1140gel or paste which preferably contains a vanishing oil, brazing filler metal1150in one of three physical delivery formats as shown inFIGS.14-16and further described herein, and at least one other second structural component in the form of an aluminum fin1110; (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.

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., by at least 110%, if the flat outer surface1124of the tube1102is 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 intermetallics (FeAl3) formed between the dissimilar materials. The method is also less costly because the finned tube assembly1104does not have to be dried (eliminate water) before brazing.

Upon heating of the fins1110and tube1102brought into abutting contact with each other, the braze filler metal and brazing substrates melt together in a single step, it being provided according to the invention that the oil based carrier braze flux1140gel and brazing filler metal1150delivered as an addition to the flux brazing gel (FIG.15) or as a foil sheet (FIG.16) or as a clad layer rolled onto the aluminum fin (FIG.14) 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 intermetallic 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.

In the method according to the invention, when the brazing filler metal1150is supplied in the form of a foil sheet1152, as further described herein, the foil sheet is in abutting contact with outer surface1124of the tube1102, thereby when the foil sheet melts during the brazing process, the external surface of the tube is imparted with enhanced corrosion protection from the aluminum-silicon layer. In one representative example, without limitation, an aluminum silicon coating having a thickness of about 25 microns may be deposited on the steel tube1102by the brazing process.

In one preferred and present embodiment being discussed, tube1102is stainless steel. The brazing method according to the present invention can be applied to both ferritic and austenitic stainless steel tubes.

As noted above,FIGS.14-16show three possible approaches for introducing the brazing filler metal1150into the brazing process. These three figures each depict an exploded view of a finned tube assembly1104prior to brazing with components and products used during the brazing process to bond the fins1110to the steel tube1102. Accordingly,FIGS.14-16depict the un-fused components used to braze and form a permanently bonded finned tube assembly, which may be defined herein as a finned tube brazing preassembly. In all three filler metal1150delivery mechanisms described herein, the aluminum or aluminum silicon filler metal is provided proximate to the bonding site between the aluminum fins1110and the exposed outer surface1124of the steel tube1102for brazing the fins to the tube.

The brazing filler metal1150preferably has a preponderance of aluminum, as much as 85 weight % or more, where the remaining proportion is predominantly silicon. Accordingly, a preferred brazing filler metal is aluminum silicon (AlSi). In some embodiments, the brazing filler metal may contain about 6-12% silicon. Zinc may be added to the brazing 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).

Referring toFIG.14, the brazing filler metal1150may be provided as clad layers hot rolled or otherwise bonded onto an aluminum sheet which forms a cladded fin1110. The aluminum fin1110, typically aa3003, is cladded with an AlSi 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 wettability of the alloy in addition to minimizing the intermetallic alloy (e.g. FeAl3) layer thickness. The thickness of the AlSi clad layer on the fin sheet metal is between about 110% and 20% of the total thickness of the fin1110, and preferably about 15%.

In one possible embodiment, fin1110may therefore be constructed as a three-layer composite having an aa3003 aluminum core with brazing filler metal1150cladded on each side. In one exemplary embodiment, a suitable cladded fin composite construction is aa4343/aa3003/aa4343. The aa4343 cladding is an AlSi composition having a silicon content of about 6.8-8.20%. A representative non-limiting thickness for fin1110constructed in this manner is about 0.012 inches. Other suitable thicknesses of the fin and cladding may be provided.

The foregoing resulting tube assembly1104prior to brazing and bonding of the fins1110onto tube1102is shown inFIG.14. Tube1102is bare steel (i.e. uncoated and not aluminized in any manner), and preferably stainless steel in this embodiment. Flux1140is applied between the cladded fins1110and flat outer surfaces1124on top and bottom flat walls1130a,1130b. The assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

Referring toFIG.15, the brazing filler metal1150may alternatively be provided as an additive mixed with the flux1140. A powder based filler metal such as aluminum powder may be used. In one embodiment, a powdered AlSi brazing alloy is used, such as without limitation aa 4343 (6.8-8.2% Si), aa 4045 (9-11% Si), or aa 4047 (11-13% Si) which are suitable, is added to the flux1140and beneficially increases the exterior corrosion protection of the stainless steel. Preferably, the brazing alloy used for the filler metal1150is 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.

The foregoing resulting tube assembly1104prior to brazing and bonding of the fins110onto tube1102is shown inFIG.15. Tube1102is bare steel (i.e. uncoated and not aluminized in any manner), and preferably stainless steel in this embodiment. Fins1110are uncladded and formed as a single layer sheet of aluminum (e.g. aa 3003) as described herein. Flux1140is applied between the uncladded fins1110and flat outer surfaces1124on top and bottom flat walls1130a,1130b. The assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

Referring toFIG.16, the brazing filler metal1150may alternatively be provided in the form of a sheet of brazing foil1152. In one embodiment, the foil may be an AlSi material such as without limitation as an example aa 4045. Foils1152having a representative sheet thickness of about 0.010 to 0.15 inches may be used. In one embodiment, the sheet thickness of foil1152used may be about 0.015 inches.

The foregoing resulting tube assembly1104prior to brazing and bonding of the fins1110onto tube1102is shown inFIG.16. Tube1102is bare steel (i.e. uncoated and not aluminized in any manner), and preferably stainless steel in this embodiment. Brazing foil1152is placed against the peaks1131of the fins1110. Flux1140is applied between the foil1152and flat outer surfaces1124on top and bottom flat walls1130a,1130b. The assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

The fin and the tube assembly1104according toFIGS.14-16described 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 finned tube assemblies1104formed according to the present disclosure.

A suitable brazing flux such as a fluoride based flux with a cesium or lithium additive, is preferably utilized to sequester the negative 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 effects 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 further enhances the braze joint strength and toughness by reducing the intermetallic layer (e.g. FeAl3) thickness within the braze joint

Suitable cesium and lithium fluxes are commercially available under the brand name NOCOLOK® from Solvay Fluor GmbH of Hannover, Germany Advantageously, this eliminates the current general industrial practice of requiring either a roller clad or aluminized layer on the parent tube102material to enable using aluminum-to-aluminum braze processes. This will reduce labor and material costs while improving the heat transfer rate.

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.

A suitable non-aqueous oil based carrier is Evaplube brand vanishing oil which is commercially available 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 0.751-0.768 (water=1.0), boiling point of 340-376 degrees F., vapor pressure at 68 degrees F. of 0.5 mmHg, evaporation rate of 0.16, and is 1100% volatile by volume.

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 53% by weight vanishing oil may be used with the remaining weight percentage (47%) of product in the mixture being flux or flux with additional additives.

The foregoing oil based carrier and powdered flux mixtures produce a very viscous flux mixture (similar to a gel or wall paper paste in consistency and viscosity) that is readily spreadable on the tubes1102in 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 interfere with the formation of a brazed bond between the fins110and bare steel tube1102. The oil based carrier and fluoride based flux brazing gel or paste is an admixture of halides including, but not limited to, potassium aluminum fluoride, cesium aluminum fluoride, and lithium aluminum fluoride.

A suitable representative application rate of the flux and oil based carrier mixture may be about 25 g/m2flux to 35 g/m2of vanishing oil.

In alternative embodiments, a long chain alcohol may be added to further extend and improve the spreadability of the flux-oil based carrier mixture which may be used for longer lengths of bare steel tubes1102to be prepared for brazing. In certain embodiments, the long chain alcohol may 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% by 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 flux power in the mixture to provide optimum brazing performance and bonding.

In using the vanishing oil and fluoride based flux brazing mixture gel to prepare a braze filler metal delivery system in which the filter metal1150is mixed directly into the flux1140as shown inFIG.15and described above, the flux mixture comprises NOCOLOK® flux, Evaplube vanishing oil (e.g. Evap-Lube 2200), and powdered aluminum. In various embodiments, the aluminum content of the flux1140gel/paste may be in the range of about 110-50% Al powder by weight. In one representative example, for illustration, approximately 60 g/m2of aluminum powder may be added which may be AlSi in some embodiments. To make a an aluminum preparation having a paste-like consistency for mixing with the flux gel, approximately 90 g/m2of Evaplube may 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 (210 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.

In one embodiment, the aluminum particle size of the aluminum or AlSi power may be without limitation about 5-10 microns.

An exemplary method for bonding an aluminum fin1110to a bare steel tube1102will now be described based on the foregoing parameters and materials.

The method generally begins by first providing a preassembly of individual components as shown in eitherFIG.14,15, or16which have been describe above. Essentially, a bare steel tube1102is provided and sets of aluminum fins1110which comprise the main parts that are to be brazed and bonded together. Tube1102may be stainless steel in this example such as Type 409SS. Fins1110may be aa3003 aluminum.

Tube1102is initially cleaned using a suitable cleaner to remove drawing oils and grime in preparing the outer surface1124of the tube for receiving flux1140which 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 surface1124of tube1102along top and bottom flat walls1130a,1130bwhere fins1110will be bonded should be thoroughly clean of contaminants that might adversely affect the formation of a good brazed joint between the tube and fins.

Next, the oil based carrier flux1140mixture brazing gel or paste is applied to tubes1102. The flux1140is applied to the outer surface1124of tube1102along top and bottom flat walls1130a,1130b(see, e.g.FIGS.14-16) before the fins1110are placed against in surface contact with the tube surfaces and flux. In the embodiment ofFIG.15, the flux1140will contain the AlSi filler metal1150as already described herein. In the embodiments ofFIGS.14and16, the flux will generally not contain any filler metal1150which is provided by other ways described herein such as by being clad onto the fins1110(FIG.14) or provided in the form of separate sheets of foil (FIG.16).

The method next continues by bringing the tube1102with flux1140applied and fins1110into surface contact with each other and forming the preassembly shown inFIGS.14and15. With respect toFIG.16, the AlSi filler metal foil1152is placed on the flux1140preferably after it is applied to tube1102, and then the fins are brought into surface contact with the foil adhered to the tube by the gel or paste like flux.

The foregoing assembled but unbrazed finned tube assemblies1104as shown inFIGS.14-16are held together by any suitable means such as clamping in preparation for processing in the brazing furnace.

The tube assembly1104is next loaded into a brazing 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 fins1110and the tube1102, as already described herein. The bonded tube assembly1104is then cooled and removed from the brazing furnace.

In an alternative method for bonding fins1110to tube1102and forming a completed tube assembly, the brazing process may be applied to half-tube segments comprised of one set of fins1110and one of the flat wall1130aor1130b(see, e.g.FIG.13). For example, a first set of fins1110may be brazed onto flat wall1130a, and a second set of fins may be brazed onto flat wall1130b. Then, the two brazed half tubes may be joined together by a suitable method such as welding to produce the completely tube assembly1104shown inFIG.12A. This fabrication technique allows gravity to assist the flow of the braze material into the braze joint.

According to another embodiment, a tube assembly1104comprised of a bare carbon steel tube1102and fins1110may be fabricated in according with the foregoing method. In one embodiment, low carbon steel having a wall thickness T1of 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 T1of 0.060 inches. The construction may use a brazing filler metal1150in the form of foil1152shown inFIG.16made of aa4045 aluminum with a sheet thickness of about 0.015 inches. The flux1140may be a NOCOLOK® and Evaplube mixture as described herein, and in some possible embodiments an aluminum or AlSi filler in the form of flakes or powder may be added to the flux mixture. A water based cleaner is preferred to prepare the tube1102for brazing that removes rust, oils, and other surface contaminants from outer surface1124of the tube; however, other suitable cleaning solutions 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.

3. Inventive Concept 3

With reference toFIGS.18-21, a third inventive concept will be described.

While the invention is exemplified inFIGS.18-21as being used to cool spent nuclear fuel that is located within a spent nuclear fuel pool, the invention is not so limited. In other embodiments, the invention can be used to reject waste thermal energy generated by radioactive materials to the ambient air irrespective of the type of radioactive materials being cooled and the type of body of liquid in which the radioactive materials are (or previously were) immersed. In certain embodiments, the pool of liquid can be a reactor pool. In other embodiments, the radioactive materials may be waste, including spent nuclear fuel, high level radioactive waste or low level radioactive waste, and/or non-waste.

Referring first toFIG.18, a cooling system2900for rejecting thermal energy generated by radioactive waste2020to the ambient air2040according to an embodiment of the present invention is schematically illustrated. The cooling system2900generally comprises an air-cooled heat exchanger2100and a heat rejection closed-loop fluid circuit2200that thermally couples the air-cooled heat exchanger2100to the radioactive materials2020, which are immersed in a pool of a liquid2050. As a result of the thermal coupling, thermal energy generated by the radioactive waste2020is transferred to the air-cooled heat exchanger2100(and subsequently to the ambient air2040). Thermal coupling of the air-cooled heat exchanger2100to the radioactive waste2020via the heat rejection closed-loop fluid circuit2200can either be direct thermal coupling or indirect thermal coupling. In the exemplified embodiment, the thermal coupling of the air-cooled heat exchanger2100to the radioactive waste2020via the heat rejection closed-loop fluid circuit2200is accomplished via an indirect thermal coupling that includes an intermediate closed-loop fluid circuit2300. In this embodiment, the intermediate closed-loop fluid circuit2300comprises the pool of liquid2050. In other embodiment, a pool of liquid2050may not be required and the radioactive waste may transfer its thermal energy to a gaseous volume to which the air-cooled heat exchanger2100is thermally coupled.

It should be noted that in certain alternate embodiments of the invention, more than one intermediate closed-loop fluid circuit2300can be included in the cooling system2900that consecutively transfer thermal energy from the radioactive materials2020to the heat rejection closed-loop fluid circuit2200. In such an embodiment, only a first one of the intermediate closed-loop fluid circuits2300will comprise the pool of the liquid2050. Moreover, in certain other alternate embodiments, the intermediate closed-loop fluid circuit2300can be omitted. In such an embodiment, the heat rejection closed-loop fluid circuit2200can include the pool of the liquid2050.

The cooling system2900, in the exemplified embodiment, further comprises an intermediate heat exchanger2310which, as discussed below, transfers thermal energy from the liquid2050to a coolant fluid2101that flows through the heat rejection closed-loop fluid circuit2200. In the exemplified embodiment, the intermediate heat exchanger2310is a tube-and-shell heat exchanger. However, in other embodiments, the intermediate heat exchanger2310can be a plate heat exchanger, a plate and shell heat exchanger, an adiabatic heat exchanger, a plate fin heat exchanger, and a pillow plate heat exchanger.

The system2900further comprises a containment structure2075, which can be in the form of a building or other enclosure. The containment structure2075provides radiation containment as would be appreciated by those skilled in the art. In certain embodiment, the system2900is designed so that the liquid2050, which comes into direct contact with the radioactive waste2020, never exists the containment structure2075. Thus, if a leak were to occur in the intermediate closed-loop fluid circuits2300, the contaminated liquid2050would not be discharged into the surrounding environment. Thus, in the exemplified embodiment, the intermediate heat exchanger2310and the entirety of the intermediate closed-loop fluid circuits2300is located within the containment structure2075. Whether or not containment of the liquid2050within the containment structure is required will depend on whether or not the liquid is contaminated, the type of radioactive waste2020being cooled, and applicable regulations.

As mentioned above, radioactive materials2020are immersed in the pool of the liquid2050, which in the exemplified embodiment is a spent fuel pool. Radioactive materials2020, such as spent nuclear fuel, generate a substantial amount of heat for a considerable amount of time after completion of a useful cycle in a nuclear reactor. Thus, the radioactive materials2020are immersed in the pool of the liquid2050to cool the radioactive materials2020to temperatures suitable for dry storage. In embodiments where the radioactive materials2020are spent nuclear fuel rods, said spent nuclear fuel rods will be supported in the pool of the liquid2050in fuel racks located at the bottom of the pool of liquid2050and resting on the floor. Examples of suitable fuel racks are disclosed in United States Patent Application Publication No. 2008/0260088, entitled Apparatus and Method for Supporting Fuel Assemblies in an Underwater Environment Having Lateral Access Loading, published on Oct. 23, 2008, and United States Patent Application Publication No. 2009/0175404, entitled Apparatus or Supporting Radioactive Fuel Assemblies and Methods of Manufacturing the Same, published on Jul. 9, 2009, the entireties of which are hereby incorporated by reference.

As a result of being immersed in the pool of the liquid2050, thermal energy from the radioactive materials2020is transferred to the pool of the liquid2050, thereby heating the pool of liquid2050and cooling the radioactive materials. However, as the pool of liquid2050heats up over time, thermal energy must be removed from the pool of the liquid2050to maintain the temperature of the pool of the liquid2050within an acceptable range so that adequate cooling of the radioactive materials2020can be continued.

The intermediate closed-loop fluid circuit2300comprises, in operable fluid coupling, the pool of the liquid2050, a tube-side fluid path2320of the intermediate heat exchanger2310, and a hydraulic pump2330. The aforementioned components/paths of the intermediate closed-loop fluid circuit2300are operably and fluidly coupled together using appropriate piping, joints and fittings as is well-known in the art to form a fluid-tight closed-loop through which the liquid2050can flow. The hydraulic pump2330flows the liquid2050through the intermediate closed-loop fluid circuit2300as is known in the art. Of course, valves are provided as necessary and/or desirable along the intermediate closed-loop fluid circuit2300.

In the exemplified embodiment, the tube-side fluid path2320of the intermediate heat exchanger2310comprises a tube-side inlet header2321, a tube-side outlet header2322and interior cavities2324of the heat exchange tubes2325of the intermediate heat exchanger2310. The shell2329of the intermediate heat exchanger2310comprises a tube-side inlet2328for introducing heated liquid2050into the tube-side fluid path2320of the intermediate heat exchanger2310and a tube-side outlet2331for allowing cooled liquid2050to exit the tube-side fluid path2320of the intermediate heat exchanger2310.

Interior cavities2324of the heat exchange tubes2325fluidly couple the tube-side inlet header2321and the tube-side outlet header2322, thereby forming the tube-side fluid path2320of the intermediate heat exchanger2310. The heat exchange tubes2325of the intermediate heat exchanger2310are connected to an inlet tube sheet2326and an outlet tube sheet2327at opposite ends.

The heat rejection closed-loop fluid circuit2200comprises, in operable fluid coupling, a shell-side fluid path2340of the intermediate heat exchanger2310, a tube-side fluid path2110of the air-cooled heat exchanger2100, a fluid coolant reservoir2210and a hydraulic pump2220. The aforementioned components/paths of the heat rejection closed-loop fluid circuit2200are operably and fluidly coupled together using appropriate piping, joints and fittings as is well-known in the art to form a fluid-tight closed-loop through which the coolant fluid2101can flow. The hydraulic pump2220flows the coolant fluid2101through the heat rejection closed-loop fluid circuit2200as is known in the art. Of course, valves are provided as necessary and/or desirable along the heat rejection closed-loop fluid circuit2200. The coolant fluid2101can take on a wide variety of fluids, including both liquids and gases. In one embodiment, the coolant fluid2101is water in liquid phase.

The tube-side fluid path2110of the air-cooled heat exchanger2100comprises, in operable fluid coupling, a coolant fluid inlet header2111, interior cavities2112of a plurality of heat exchange tubes2113, and a coolant fluid outlet header2114. The plurality of heat exchange tubes2113collectively form a tube bundle2115that extends along a substantially vertical longitudinal axis A-A. Furthermore, each of the heat exchange tubes2113of the air-cooled heat exchanger2100are arranged in a substantially vertical orientation. The tube bundle2115further comprises a top tube sheet2116and a bottom tube sheet2117. The heat exchange tubes2113of the air-cooled heat exchanger2100are connected to and extend between the top tube sheet2116and the bottom tube sheet2117.

The air cooled heat exchanger2100further comprises a shell2118that forms a shell cavity2119. The tube bundle2115is positioned within the shell cavity2119. The air cooled heat exchanger2100further comprises a primary air inlet2120, a secondary air inlet2121and an air outlet2122. Each of the primary air inlet2120, the secondary air inlet2121and the air outlet2122form passageway through the shell2118from the shell cavity2119to the ambient air2040. As such, ambient air2040can flow into and/or out of the shell cavity2119via the primary air inlet2120, the secondary air inlet2121and the air outlet2122so that thermal energy can be convectively removed from the exterior surfaces of the heat exchange tubes2113. More specifically, cool ambient air2040flows into the shell cavity2119via the primary air inlet2120and the secondary air inlet2121while warmed ambient air2040flows out of the shell cavity2119via the air outlet2122. As can be seen, the primary air inlet2120is located a first elevation E1, the secondary air outlet2121is located at a second elevation E2and the air outlet2122is located at a third elevation E3. The second elevation E2is greater than the first elevation E1. The third elevation E3is greater than the second elevation E2. In one embodiment, the primary air inlet2120has a greater effective cross-sectional area than the secondary air outlet2121. The invention, however, is not so limited in all embodiments. While not illustrated inFIG.18, the air-cooled heat exchanger2100can comprise a blower (seeFIG.19) to induce air flow through the shell-side fluid path2123of the shell cavity2119. Conceptually, the shell-side fluid path2123of the air-cooled heat exchanger2100is the remaining free volume of the shell cavity2119through which the ambient air2040can flow (after the tube bundle2115and other components are positioned therein).

In other embodiments of the present invention, the air cooled heat exchanger2100may comprise a plurality of secondary air inlets2121. In such instances, the plurality of secondary air inlets2121may be at varying elevations between the first elevation E1and the third elevation E3. Stated another way, in such embodiments the plurality of secondary air inlets2121may be at a plurality of different elevations between the first elevation E1of the primary air inlet2120and the third elevation E3of the air outlet2122. In further embodiments, the secondary air inlet2121may be omitted.

In the exemplified embodiment, the air-cooled heat exchanger2100is a vertical single tube pass counter-current heat exchanger. However, in certain embodiment, multiple pass heat exchangers can be used for either the air-cooled heat exchanger2100and/or the intermediate heat exchanger2310. The heat exchange tubes2325of the intermediate heat exchanger2310and the heat exchange tubes2113of the air-cooled heat exchanger2100are made of made of a highly thermally conductive and corrosion resistant material. Suitable materials include aluminum, copper, and aluminum alloys.

During operation of the system, the hydraulic pumps2330and2210are activated. Activation of the hydraulic pump2330flows liquid2050through the intermediate closed-loop fluid circuit2300while activation of the hydraulic pump2220flows coolant fluid2101through the heat rejection closed-loop fluid circuit2200. As discussed above, the thermal energy generated by the radioactive waste2020is initially transferred to the liquid2050while in the pool. This heated liquid2050flows from the pool and into the tube-side fluid path2320of the intermediate heat exchanger2310. Simultaneously, the coolant fluid2101(which at this stage has been cooled by the air-cooled heat exchanger2100) flows through the shell-side fluid path2340of the intermediate heat exchanger2310. As the heated liquid2050flows through the tube-side fluid path2320of the intermediate heat exchanger2310, thermal energy is transferred from the heated liquid2050to the cool coolant fluid2101that is flowing though the shell-side fluid path2340of the intermediate heat exchanger2310. The cooled liquid2050then exits tube-side path2320of the intermediate heat exchanger2310and is returned back to the pool for further cooling of the radioactive materials2020where it is again heated up and the cycle continues.

The heated coolant fluid2101(which has absorbed the thermal energy from the heated liquid2050) exits the shell-side path2340of the intermediate heat exchanger2310and flows into the top header2111of the air-cooled heat exchanger2100where it is then distributed to the interior cavities2112of the plurality of heat exchange tubes2113. The heated coolant fluid2101flows downward through the plurality of heat exchange tubes2113. As the heated coolant fluid2101flows through the plurality of heat exchange tubes2113, thermal energy from the heated coolant fluid2101is transferred to ambient air2040that is flowing through the shell-side fluid path2123of the air cooled-heat exchanger2100. The ambient air2040enters the primary air inlet2120as cool air. As thermal energy from the coolant fluid2101is transferred to this cool ambient air2040within the shell-side fluid path2123, the ambient air2040becomes warmed and rises naturally within the shell-side fluid path2123and exits the air-cooled heat exchanger2100via the air outlet2122as heated air. Additionally, as the warmed ambient air2040rises within the shell-side fluid path2123, additional cool ambient air2040is drawn into the shell-side fluid path2123via the second air inlet2121. The second air inlet2121also serves as a backup to the primary air inlet2120in the event that the site is flooded and the primary inlet2120becomes submerged in water.

Referring now toFIG.19, a tube-and-shell air-cooled heat exchanger apparatus2500A that is particularly useful as the air-cooled heat exchanger2100for the cooling system2900is illustrated. The tube-and-shell air-cooled heat exchanger apparatus2500A will be described with the understanding that those parts of the tube-and-shell air-cooled heat exchanger apparatus2500A that correspond to the air-cooled hate exchanger2100will be given like reference numbers with the addition of an “A” suffix.

The tube-and-shell air-cooled heat exchanger apparatus2500A generally comprises a tube-and-shell air-cooled heat exchanger2100A and a shroud2160A. The tube-and-shell air-cooled heat exchanger2100A comprises a tube bundle2115A and a shell2118A. The shroud2160A comprises a shroud cavity2161A. The shell2118A comprises a shell cavity2119A. The tube bundle2115A is positioned within the shell cavity2119A and supported therein a substantially vertical orientation along substantially vertical axis A-A. The tube-and-shell air-cooled heat exchanger2100A is positioned within the shroud cavity2161A and supported therein in a substantially vertical orientation along vertical axis A-A. In certain embodiments, the shroud2160A may be omitted. In certain other embodiments, the shroud2160A may be considered the shell of the tube-and-shell air-cooled heat exchanger apparatus2500A while the shell2118A is omitted.

The tube-and-shell air-cooled heat exchanger apparatus2500A comprises a shell-side fluid path2123A and a tube-side fluid path2110A. As mentioned above, the shell-side fluid path2123A can be conceptualized as the free volume of the shell cavity2119that remains after the tube bundle2115A (and other components) is positioned therein. The tube-side fluid path2110A comprises the interior cavities2112A of the plurality of heat exchange tubes2113A along with the coolant fluid inlet header2111A and the coolant fluid outlet header2114A. The coolant2101flows through the tube-side fluid path2110A while the ambient air flows through the shell-side fluid path2123A as discussed above forFIG.18to effectuate transfer of thermal energy from the coolant fluid2101to the ambient air2040.

The tube-and-shell air-cooled heat exchanger apparatus2500A comprises a primary air inlet2120A, a secondary air inlet2121A, and an air outlet2122A. The primary air inlet2120A and the secondary air inlet2122A form passageways from the ambient air2040A outside of the shroud2160A into the shell-side fluid path2123A, thereby allowing cool air to enter the shell-side fluid path2123A from outside of the shroud2160A. The air outlet2122A forms a passageway from the shell-side fluid path2123A to a shroud outlet plenum2162A that circumferentially surrounds a top portion of the shell2118A. A chimney2163A is provided on the shroud2160A that forms a passageway from the shroud outlet plenum2162A to the ambient air2040A outside of the shroud2160A. Thus, as warmed ambient air2040A exits the shell-side fluid path2123A via the air outlet2122A, the warmed ambient air2040A will flow into the shroud outlet plenum2162A, rise therein, and exit the shroud via the passageway of the chimney2163A. In order to induce greater flow of ambient air through the shell-side fluid path2123A of the tube-and-shell air-cooled heat exchanger apparatus2500A, a blower2170A is provided in the chimney2163A. In other embodiments, the blower2170A may be positioned at other suitable locations.

Each of the primary air inlet2120A, the secondary air inlet2121A, and the air outlet2122A extend through the shell2118A and are substantially horizontal. The primary air inlet2120A is formed by one or more conduits that extend through the shroud2160A and to the shell2118A so that all of the incoming cool air flows into the shell-side fluid path2123A and not into the shroud cavity2161A. Similarly, the secondary air inlet2121A is formed by one or more conduits that extend through the shroud2160A and to the shell2118A so that all of the incoming cool air flows into the shell-side fluid path2123A and not into the shroud cavity2161A.

The primary air inlet2120A is located a first elevation E1, the secondary air outlet2121A is located at a second elevation E2and the air outlet2122A is located at a third elevation E3. The second elevation E2is greater than the first elevation E1. The third elevation E3is greater than the second elevation E2. In one embodiment, the primary air inlet2120A has a greater effective cross-sectional area than the secondary air outlet2121A.

The plurality of heat exchange tubes2113A are discontinuously finned tubes. In other words, each of the plurality of heat exchange tubes2113A comprise axial sections that include fins2180A (FIG.20) and axial sections that are free of any fins. In certain alternate embodiments of the invention, a first subset of the heat exchange tubes2113A may be discontinuously finned tubes, a second subset of the heat exchange tubes2113A may be continuously finned along their length, and a third subset of the heat exchange tubes2113A may be free of fins along their entire length.

In the exemplified embodiment, the plurality of heat exchange tubes2113A collectively form the tube bundle2115A. Due their discontinuously finned nature, the tube bundle2115acomprises finned tube sections2151A,2153A and non-finned tube sections2150A,2152A,2154A. The finned tube sections2151A,2153A and the non-finned tube sections2150A,2152A,2154A are in axial alignment and arranged in an alternating manner. In the finned tube sections2151A,2153A of the tube bundle2115A, each of the heat exchange tubes2113A comprise fins2180A that increase thermal energy transfer from the coolant fluid2101A to the ambient air240A by increasing the outer surface area of the tubes2113A. In the non-finned tube sections2150A,2152A,2154A, the plurality of heat exchange tubes2113A are free of any fins.

As can be seen inFIG.19, in the exemplified embodiment, the non-finned tube sections2150A,2152A,1254A are transversely aligned with the primary air inlet2120A, the secondary air inlet2121A, and the air outlet2122A, respectively. By aligning each of the primary air inlet2120A, the secondary air inlet2121A, and the air outlet2122A with one of the non-finned tube sections2150A,2152A,2154A, ambient air2040A can enter and exit the tube bundle2115A more effectively. Stated simply, by omitting (or substantially reducing the number of) the fins in these sections2150A,2152A,2154A, the impedance effect that the fins have on the cross-flow of the ambient air is eliminated and/or minimized. Thus, air flow through the shell-side path2123A is increased. Furthermore, the creation and arrangement of the finned tube sections2151A,2153A and the non-finned tube sections2150A,2152A,2154A on the tube bundle2115A (as discussed above) can create a venturi effect at the secondary air inlet2121A (and potentially at the primary air inlet2120A).

Referring toFIGS.19and21concurrently, it can be seen that providing fins2180A on the finned tube sections2151A,2153A effectively reduces the free transverse cross-sectional area of the shell-side path2123A because the fins2180A occupy additional space of the shell cavity2119A. Thus, from the perspective of the shell-side fluid path2123A, the finned tube sections2151A,2153A create a reduced cross-sectional area, which can be considered a venturi restriction. As a result of the finned section2153A, which is located at an elevation between the secondary air inlet2121A and the air outlet2122A, a venturi is formed that assists in drawings additional cool ambient air2040A into the secondary air inlet2121A. Thus, in the exemplified embodiment, the venturi is created by the fins2180A of the plurality of heat exchange tubes2113A. Each of the fins2180A of the plurality of heat exchange tubes2113A comprise opposing surfaces that extent substantially parallel to the substantially vertical axis A-A.

The shell-side fluid path2123A comprises a first venturi located at an elevation between the primary air inlet2120A and the secondary air inlet2121A. Furthermore, the shell-side fluid path2123A comprises a second venturi located at an elevation between the secondary air inlet2121A and the air outlet2122A. As graphically illustrated inFIG.21, the shell-side fluid path2123A comprises a first free transverse cross-sectional area at the second elevation (i.e. at the secondary air inlet2121A) and a second free transverse cross-sectional area at an elevation between the secondary air inlet2121A and the air outlet2122A, wherein the second free transverse cross-sectional area is less than the first free transverse cross-sectional area. Moreover, the shell-side fluid path2123A comprises a third free transverse cross-sectional area at the third elevation (i.e., at the air outlet2122A), wherein the third free transverse cross-sectional area is greater than the second free transverse cross-sectional area.

In embodiments of the invention where the focus is on existence of a venturi being created in the shell-side fluid path2123A, the venturi can be created in additional ways, such as for example reducing the transverse cross-section of the shell2119A or adding additional flow barriers. In certain other embodiments, a venturi can be created by simply adding more or thicker fins to the desired area of the tube bundle.

Referring now toFIG.20, a transverse cross-section of one of the heat exchange tubes2113A taken along one of the finned tube sections2151A,2153is exemplified. The heat exchange tubes2113A comprise a plurality of fins2180A extending from a tube body2181A. The fins2180A can be formed by extruding a set of axial spines that give the tube2113A a “star burst” cross section. The height of the find2180A is selected to accord with the layout pitch of the tube bundle2115A such that the fins2180A provide a complete cross sectional coverage in the tube bundle2115A so as to promote maximum contact between the turbinated air and the fin surfaces. A candidate shape of the star burst for the square layout pitch is shown inFIG.20. Of course, any number of fin arrangements and patterns can be used in other embodiments of the invention.

The design of the tube-and-shell air-cooled heat exchanger apparatus2500A described above has several parameters for modification to maximize its heat rejection capability for a specific application. The available parameters include tube I.D., number of fins per tube and size/shape of each fin, tube layout pitch, height of the tube bundle, in-tube flow velocity (by using the appropriate size pump) and air flow velocity (by selecting the appropriately sized blower). By an adroit selection of the above design parameters, it is possible to achieve the overall heat transfer coefficient for the bundle in excess of 10 Btu/hr-sq ft-deg F. Scoping calculations show that a 12 ft diameter, 20 ft tall heat bundle can remove as much as 5858 kW from contaminated water @ 140 deg. F. Multiple units can be arrayed in parallel to increase the heat removal capacity to the desired level.

4. Inventive Concept 4

A fourth inventive concept will be described below, and there are no drawings associated with the fourth inventive concept.

As used herein, the term “bonding temperature” refers to the temperature to which a brazing composition must be heated in order to provide suitable adhesion strength between two substrates, e.g., a permanent bond between an aluminum fin and a steel tube.

In some embodiments, the terms “hydrophobic carrier” and “oil based carrier” may be used interchangeably.

In some embodiments, the terms “brazing composition”, “brazing flux” and “flux composition” may be used interchangeably.

Some embodiments of the present invention provide a brazing composition comprising: a metal halide; and from about 40 wt. % to about 65 wt. % of a hydrophobic carrier.

In some embodiments, the metal halide is selected from: potassium fluoride; aluminum fluoride; cesium fluoride; rubidium fluoride; lithium fluoride; sodium fluoride; calcium fluoride; potassium aluminum fluoride; cesium aluminum fluoride; lithium aluminum fluoride; and a combination of two or more thereof. In other embodiments, the metal halide is selected from: potassium aluminum fluoride; cesium aluminum fluoride; lithium aluminum fluoride; and a combination of two or more thereof.

Further embodiments provide a brazing composition further comprising a filler metal. In some embodiments, the filler metal is selected from aluminum, silicon, zinc, an alloy of aluminum and zinc; an alloy of zinc, aluminum and silicon, an alloy of aluminum and silicon; and a combination of two or more thereof.

In some embodiments, the filler metal has an average particle size of from about 1 to about 500 microns. In some embodiments, the filler metal has an average particle size of from 2 to about 100 microns. In other embodiments, the filler metal has an average particle size of from about 3 to about 50 microns. Still further embodiments provide compositions wherein the filler metal has an average particle size of from about 4 to about 25 microns. Yet other embodiments provide a composition wherein the filler metal has an average particle size of from about 5 to about 10 microns.

In some embodiments, the filler metal comprises greater than 50 wt. % aluminum. In other embodiments, the filler metal comprises greater than 85 wt. % aluminum.

In some embodiments, the hydrophobic carrier is a liquid at room temperature. In some embodiments, the hydrophobic carrier comprises a vanishing oil.

In some embodiments, the hydrophobic carrier is present in an amount of from about 48 wt. % to about 58 wt. % of the brazing composition. In some embodiments, the hydrophobic carrier is present in an amount of about 53 wt. % of the brazing composition.

In some embodiments, the brazing composition is substantially acrylate-free. In some embodiments, the brazing composition is acrylate-free.

In some embodiments, the brazing composition has a bonding temperature of from about 550° C. to about 650° C. In some embodiments, the brazing composition has a bonding temperature of from about 575° C. to about 625° C. In some embodiments, the brazing composition has a bonding temperature of from about 585° C. to about 600° C. In some embodiments, the brazing composition has a bonding temperature of about 590° C. In some embodiments, the bonding temperature refers to the bonding temperature of the first component and the second component individually. In some embodiments, the bonding temperature refers to the bonding temperature of the multi-component brazing composition when the first and second components are in intimate contact.

In some embodiments, the filler metal is in the form of a flake or a powder.

In some embodiments, the brazing composition further comprises an additive selected from: an anti-oxidant, an anti-corrosive agent, an anti-foaming agent, a viscosity modifying agent, a plasticizer, a tackifier, a binder, a coupling agent, and a combination of two or more thereof.

In some embodiments, the composition is in the form of a paste or a gel.

Further embodiments provide a multi-component brazing composition comprising: a first component comprising: a metal halide; and a hydrophobic carrier; and a second component comprising a filler metal.

In some embodiments, the metal halide and the filler metal have different average particle sizes. In some embodiments, the filler metal has an average particle size that is greater than the average particle size of the metal halide. In some embodiments, the first component and the second component are present in separate phases. In some embodiments, the separate phases are in intimate contact with one another.

In some embodiments, the first component and the second component are separated prior to use. In some embodiments, the second component comprises a substantially planar substrate. In some embodiments, the substantially planar substrate comprises a foil.

In some embodiments, the substantially planar substrate has a thickness of from about 0.010 to about 0.15 inches. In other embodiments, the substantially planar substrate has a thickness of about 0.15 inches.

In some embodiments, the substantially planar substrate has a dimensional stability sufficient to remain substantially planar after contact with a metal substrate (e.g. a cooling fin). In some embodiments, the substantially planar substrate has a filler metal density of about 60 g/m2. The compositions may reduce the time at which brazing temperature must be maintained during the process by at least 10%, which thereby reduces the formation of intermetallics formed between the fins and the steel tube (dissimilar materials).

In some embodiments, the first component has a metal halide density of about 25 g/m2.

In some embodiments, the methods of the present invention employ a flux mixture comprising a powdered flux and a hydrophobic/oil-based carrier. In some embodiments, the brazing composition is substantially anhydrous. In some embodiments, water is not used in the brazing composition/flux mixture. In some embodiments, the methods described herein: (1) eliminate the need to first provide an aluminum clad layer (or otherwise aluminized surface) on the outer surface of the tube for bonding the tube to the fin before beginning the brazing process; (2) eliminate drying of fluxed tubes; and (3) reduce the deleterious intermetallic layer (e.g. FeAl3) between the dissimilar metals which is formed during brazing. The latter is beneficial because FeAl3 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 methods are applicable to tubes constructed from carbon steels, ferritic stainless steels, austenitic stainless steels, and other steel alloys.

In some embodiments, a brazing composition/flux mixture suitable for brazing aluminum fins onto a bare steel tube is provided. In some embodiments, the flux mixture includes a flux powder comprising a metal halide and a hydrophobic/oil-based carrier. In some embodiments, the oil based carrier comprises an aliphatic hydrocarbon. In some embodiments, 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.

In some embodiments, the tube dimensions can be optimized for varying market conditions based on materials used. For example, a tube width of 9.25 inches (235 mm) by a height of 0.79 inches (20 mm) with a 0.039 inch (1 mm) wall thickness have been determined feasible with SS409 material. The accompanying AL3003 fin is 8.5 inches (215 mm) long, 0.83 inches (21 mm) high, and 0.01 inches (0.25 mm) thick (sheet thickness) placed at a fin pitch of 0.09 inches (2.31 mm).

For application in an air cooled condenser suitable for an industrial use such as in a power generation plant, tube is preferably constructed of steel. Any suitable steel having appropriate heat transfer properties for a given application may be used. In some embodiments, the steel may be stainless steel for corrosion resistance. Non-limiting examples of suitable stainless steels are Grade 409SS or Grade 3Cr12 stainless. Other suitable ferritic or austenitic stainless steels may be used.

An aluminum sheet usable for forming fins according to the present disclosure is a flat element which may be made from aluminum alloy in the 1xxx, 3xxx, 5xxx or 6xxx families as designated by the Aluminum Association, which is adapted and suitable for heat absorption and discharge to a cooling medium flowing past the sheet. In some embodiments, exemplary corrugated fins may be formed from of sheets of Al 3003 material having a thickness of about 0.010 inches.

In some embodiments, the present invention provides a method for bonding a cooling fin to a distributor tube. In some embodiments, the method for bonding a cooling fin to a distributor tube comprises: providing at least one first structural component in the form of a steel tube (e.g., stainless steel), a brazing composition, optionally a filler metal and at least one other second structural component in the form of an aluminum fin; bringing these components into physical contact; heating these components to a temperature between about 577° C. and 610° C., and maintaining this temperature for a time sufficient to form a brazed bond between the steel tube and the cooling fin.

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., by at least 10%, if the flat outer surface of the tube 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 intermetallics (FeAl3) between the dissimilar materials. The method is also less costly because the finned tube assembly does not have to be dried (to eliminate water) before brazing.

In some embodiments, wherein the brazing composition is a multi-component composition, the first component and second component filler melt together in a single step. 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 intermetallic 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.

In those embodiments wherein the filler metal is supplied in the form of a foil sheet, the foil sheet melts during the brazing process and imparts the steel tube with enhanced corrosion protection. In some embodiments, an aluminum silicon coating having a thickness of about 25 microns may be deposited on the steel tube by the brazing process.

In some embodiments, the filler metal has a preponderance of aluminum, as much as 85 weight % or more, where the remaining proportion is predominantly silicon. In some embodiments, the filler metal may contain about 6-12% silicon. Zinc may be added to the filler metal to lower the melting temperature, thereby allowing the brazing to take place at a lower temperature range (540° C. to 590° C.).

In some embodiments, the filler metal is provided as a clad layer hot rolled or otherwise bonded onto an aluminum sheet which forms a cladded fin. In some embodiments, the aluminum fin is cladded with an AlSi brazing alloy consisting of about 6 to 12% silicon. In some embodiments, the addition of silicon promotes brazing by reducing the melting temperature of the alloy, decreasing the surface tension and thereby increasing the wettability of the alloy in addition to minimizing the intermetallic alloy (e.g. FeAl3) layer thickness. In some embodiments, the thickness of the AlSi clad layer on the fin sheet metal is between about 10% and 20% of the total thickness of the fin, and preferably about 15%.

In some embodiments, the cladding is an AlSi composition having a silicon content of about 6.8-8.2%. In some embodiments, the fin has a thickness of about 0.012 inches. Other suitable thicknesses of the fin and cladding may be provided.

In some embodiments, the brazing composition is applied between a cladded fin and one or more flat outer surfaces of a steel tube. In some embodiments, this assembly is clamped together and ready for heating in the brazing furnace to bond the fins to the tube.

In some embodiments, the filler metal is added directly to metal halide and hydrophobic carrier. In some embodiments, a powdered AlSi filler is used, e.g. aa 4343 (6.8-8.2% Si), aa 4045 (9-11% Si), or aa 4047 (11-13% Si); and beneficially increases the exterior corrosion protection of the stainless steel. In some embodiments, the filler metal is aa 4045 or 4047. In other embodiments, the filler metal is 4045. The appropriate filler metal is selected based upon a number of factors including the environment in which the heat exchanger will reside and the particular brazing process used. For example, heat exchangers located in aggressive environments such as those in salt air or in the vicinity of chemical plants are more prone to corrosion. Specimens subjected to a prolonged ASTM b-117 salt spray test (750 hours) are used to confirm corrosion resistance in marine air environment.

In some embodiments, the brazing processes described herein can be carried out in a commercially available brazing furnace.

A suitable brazing composition such as a fluoride based brazing composition with a cesium or lithium additive, is utilized to sequester the negative 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 effects 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 further enhances the braze joint strength and toughness by reducing the intermetallic layer (e.g. FeAl3) thickness within the braze joint

Suitable cesium and lithium fluxes are commercially available under the brand name NOCOLOK® from Solvay Fluor GmbH of Hannover, Germany Advantageously, this eliminates the current general industrial practice of requiring either a roller clad or aluminized layer on the distributor tube.

The inventors have discovered that using a hydrophobic carrier for the metal halide, rather than water, produces improved brazing performance and adhesion between aluminum fins and bare steel tubes in the brazing furnace. In some embodiments, the hydrophobic carrier advantageously evaporates during processing and therefore does not interfere with the brazing.

A suitable non-aqueous hydrophobic carrier is Evap-lube brand vanishing oil which is commercially available from General Chemical Corporation of Brighton, Michigan. This product is in a liquid oil form and has a specific gravity of 0.751-0.768 (water=1.0), boiling point of 340-376 degrees F., vapor pressure at 68 degrees F. of 0.5 mmHg, evaporation rate of 0.16, and is 100% volatile by volume.

To prepare the spreadable brazing compositions described herein, a metal halide powder (e.g. NOCOLOK® flux) and an oil based carrier (e.g. Evap-Lube 2200) are admixed. In some embodiments, the relative amounts of each used preferably may be in the ranges of about 40-65% by weight hydrophobic carrier to about 60-35% by weight metal halide, and more preferably about 48-58% by weight hydrophobic carrier to about 52-42% by weight metal halide. In some embodiments, without limitation, about 53% by weight hydrophobic carrier may be used with the remaining weight percentage (47%) of product in the mixture being metal halide or metal halide with additional additives.

The foregoing oil based carrier and powdered flux mixtures produce a very viscous flux mixture (similar to a gel or wall paper paste in consistency and viscosity) that is readily spreadable on the tubes 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 interfere with the formation of a brazed bond between the fins and bare steel tube.

A suitable representative application rate of the flux and oil based carrier mixture may be about 25 g/m2 flux to 35 g/m2 of vanishing oil.

In alternative embodiments, a long chain alcohol may be added to further extend and improve the spreadability of the brazing compositions described herein which may be used for longer lengths of bare steel tubes to be prepared for brazing. In certain embodiments, the long chain alcohol may be a polyol (e.g. a glycol including hexylene glycol and propylene glycol). Glycol or another long chain alcohol may be added to the brazing composition/flux and hydrophobic/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% by weight in other embodiments. In some embodiments, if a 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 flux powder in the mixture to provide optimum brazing performance and bonding.

In using the vanishing oil and fluoride based flux brazing mixture gel to prepare a braze filler metal delivery system in which the filter metal is mixed directly into the flux, the flux mixture comprises NOCOLOK® flux, Evaplube vanishing oil (e.g. Evap-Lube 2200), and powdered aluminum. In various embodiments, the aluminum content of the flux gel/paste may be in the range of about 10-50% Al powder by weight. In one representative example, for illustration, approximately 60 g/m2 of aluminum powder may be added which may be AlSi in some embodiments. To make an aluminum preparation having a paste-like consistency for mixing with the flux gel, approximately 90 g/m2 of Evap-lube may 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 (210 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.

In one embodiment, the aluminum particle size of the aluminum or AlSi power may be without limitation about 5-10 microns.

In some embodiments, the brazing compositions described herein are applied immediately after the tube is cleaned to prevent reoccurrence of oxide formation on the tube. In some embodiments, a binder may be added to the brazing composition/flux mixture to dry it for handling.

The invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes and are not intended to limit the invention in any manner. Those skilled in the art will readily recognize a variety of noncritical parameters, which can be changed or modified to yield essentially the same results.

EXAMPLES

Example 1

Provided below in Table 1 are exemplary single-component brazing compositions of the present invention.

TABLE 1IIIIIIIVIngredientWt. %Potassium aluminum fluoride31403738Evap-lube 220065534740AlSi35157Propylene glycol12115

Example 2

Provided below in Table 2 are exemplary multi-component brazing compositions of the present invention.

TABLE 2IIIIIIIVIngredientWt. %First ComponentPotassium aluminum fluoride34404045Evap-lube 220065534940Propylene glycol171115Second ComponentAluminum50758488Silicon40211110Zinc10452

As used throughout, ranges are used as shorthand for describing each and every value that is within the range. Any value within the range can be selected as the terminus of the range. In addition, all references cited herein are hereby incorporated by referenced in their entireties. In the event of a conflict in a definition in the present disclosure and that of a cited reference, the present disclosure controls.

While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. Thus, the spirit and scope of the invention should be construed broadly as set forth in the appended claims.