Abstract:
Shell-and-tube heat exchangers that utilize one or more foam heat transfer units engaged with the tubes to enhance the heat transfer between first and second fluids. The foam of the heat transfer units can be any thermally conductive foam material that enhances heat transfer, for example graphite foam. These shell-and-tube heat exchangers are highly efficient, inexpensive to build, and corrosion resistant. The described heat exchangers can be used in a variety of applications, including but not limited to, low thermal driving force applications, power generation applications, and non-power generation applications such as refrigeration and cryogenics. The foam heat transfer units can be made from any thermally conductive foam material including, but not limited to, graphite foam or metal foam. In an embodiment, the heat exchanger utilizes tubes that are twisted around a central foam heat transfer unit.

Description:
[0001]    This application claims the benefit of U.S. Provisional Applicant Ser. No. 61/439,564, filed on Feb. 4, 2011, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This disclosure relates to heat exchangers in general, and, more particularly, to heat exchangers, including but not limited to shell-and-tube heat exchangers, employing heat conducting foam material. 
       BACKGROUND 
       [0003]    Heat exchangers are used in many different types of systems for transferring heat between fluids in single phase, binary or two-phase applications. An example of a commonly used heat exchanger is a shell-and-tube heat exchanger. Generally, a shell-and-tube heat exchanger includes multiple tubes placed between two tube sheets and encapsulated in a shell. A first fluid is passed through the tubes and a second fluid is passed through the shell such that it flows past the tubes separated from the first fluid. Heat energy is transferred between the first fluid and second fluid through the walls of the tubes. 
         [0004]    A shell-and-tube heat exchanger is considered the primary heat exchanger in industrial heat transfer applications since they are economical to build and operate. However, shell-and-tube heat exchangers are not generally known for having high heat transfer efficiency. 
       SUMMARY 
       [0005]    Shell-and-tube heat exchangers are described that utilize one or more foam heat transfer units engaged with the tubes to enhance the heat transfer between first and second fluids. The foam of the heat transfer units can be any thermally conductive foam material that enhances heat transfer, for example graphite foam. The shell-and-tube heat exchangers described herein are highly efficient, inexpensive to build, and corrosion resistant. The described heat exchangers can be used in a variety of applications, including but not limited to, low thermal driving force applications, power generation applications, and non-power generation applications such as refrigeration and cryogenics. The foam heat transfer units can be made from any thermally conductive foam material including, but not limited to, graphite foam or metal foam. 
         [0006]    In one embodiment, a heat exchanger includes a tube having a central axis and an outer surface. A heat transfer unit is connected to and in thermal contact with the outer surface of the tube, with the heat transfer unit having a heat transfer surface extending substantially radially from the outer surface of the tube. The heat transfer unit includes graphite foam. For example, the heat transfer can consist essentially of, or consist of, graphite foam. 
         [0007]    In another embodiment, a heat exchanger includes a tube bundle having a central axis and a plurality of tubes for conveying a first fluid. A first tube sheet and a second tube sheet are provided, and each of the tubes includes a first end joined to the first tube sheet in a manner to prevent fluid leakage between the first end and the first tube sheet and a second end joined to the second tube sheet in a manner to prevent fluid leakage between the second end and the second tube sheet. A heat transfer unit is connected to and in thermal contact with the tubes, with the heat transfer unit consisting essentially of graphite foam. 
         [0008]    One suitable method for connecting the tubes and the tube sheets is friction-stir-welding (FSW). The use of FSW is particularly beneficial in heat exchanger applications subject to corrosive service, since the FSW process eliminates seams, no dissimilar metals are used and, in the case of saltwater environments, no galvanic cell is created. 
         [0009]    In another embodiment, the heat transfer unit is in the form of a generally radiused and wedge-shaped, planar body that consists essentially of foam material, for example graphite foam. 
         [0010]    The body includes first and second opposite major surfaces, a support rod hole or cut-out extending through the body from the first major surface to the second major surface, an arcuate radially outer edge connected to linear side edges at opposite ends of the outer edge, and at least two tube contact surfaces opposite the radially outer edge. In other embodiments, the heat transfer units can be a combination of radiused and triangular or square shaped to fit in the pitch space between tubes. All of the heat transfer units described herein can be used by themselves or together in various combinations that one finds suitable to increase the heat transfer efficiency of the heat exchanger. 
         [0011]    In an embodiment, the tubes can be twisted around a foam heat transfer unit. In addition, each tube can be twisted around its own axis to further increase heat transfer efficiency. 
         [0012]    The tubes of the shell-and-tube heat exchangers described herein can be arranged in numerous patterns and pitches, including but not limited to, an equilateral triangular pattern defining a triangular pitch between tubes, a square pattern defining a square pitch between tubes, and a staggered square pattern defining a square or diamond pitch between tubes. 
         [0013]    The shell-and-tube heat exchangers described herein can also be configured to have any desired flow configuration, including but not limited to, cross-flow, counter-current flow, and co-current flow. In addition, the tubes can have any desired tube layout/configuration including, but not limited to, single pass and multi-pass. Further, the shell, tubes, tube sheets, and other components of the described heat exchangers can be made of any materials suitable for the desired application of the heat exchanger including, but not limited to, metals such as aluminum, titanium, copper and bronze, steels such as carbon steel and high alloy stainless steels, and non-metals such as plastics, fiber-reinforced plastics, thermally enhanced polymers, and thermoplastics. 
     
    
     
       DRAWINGS 
         [0014]      FIG. 1  shows a conventional shell-and-tube heat exchanger. 
           [0015]      FIG. 2  is an exploded view of an improved shell-and-tube heat exchanger described herein. 
           [0016]      FIG. 3  illustrates a tube bundle for the shell-and-tube heat exchanger of  FIG. 2 . 
           [0017]      FIG. 4  is a partial view of the tube bundle of  FIG. 3 . 
           [0018]      FIG. 5  illustrates a foam heat transfer unit used with the tube bundle of  FIGS. 2-4 . 
           [0019]      FIGS. 6A-E  illustrate an exemplary process of forming the heat transfer unit of  FIG. 5 . 
           [0020]      FIG. 7  illustrates another example of a foam heat transfer unit useable with the tube bundle. 
           [0021]      FIG. 8  illustrates still another example of a foam heat transfer unit. 
           [0022]      FIG. 9  illustrates still another example of a foam heat transfer unit. 
           [0023]      FIG. 10A  is a cross-sectional view of a tube bundle with another example of a foam heat transfer unit. 
           [0024]      FIGS. 10B and 10C  illustrate additional examples of tube patterns for tube bundles. 
           [0025]      FIG. 11  illustrates an example of an improved shell-and-tube heat exchanger that employs twisted tubes together with a foam heat transfer unit. 
           [0026]      FIG. 12  is a cross-sectional view of the shell-and-tube heat exchanger of  FIG. 11 . 
           [0027]      FIG. 13  is a cross-sectional view of another implementation of twisted tubes and foam heat transfer units. 
           [0028]      FIG. 14  illustrates details of the portion within the triangle in  FIG. 13 . 
           [0029]      FIG. 15  illustrates details of the portion within the hexagon in  FIG. 13 . 
           [0030]      FIG. 16  is a cross-sectional view of an improved shell-and-tube heat exchanger that employs an additional example of foam heat transfer units. 
           [0031]      FIGS. 17A-F  illustrate examples of patterns formed by different configurations of foam heat transfer units. 
           [0032]      FIG. 18  shows an example of a plate that can be used to strengthen a heat transfer unit. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows a conventional shell-and-tube heat exchanger  10  that is configured to exchange heat between a first fluid and a second fluid in a single-pass, primarily counter-flow (the two fluids flow primarily in opposite directions) arrangement. The heat exchanger  10  has tubes  12 , a tube sheet  14  at each end of the tubes, baffles  16 , an input plenum  18  for a first fluid, an output plenum  20  for the first fluid, a shell  22 , an inlet  24  to the input plenum for the first fluid, and an outlet  26  from the output plenum for the first fluid. In addition, the shell  22  includes an inlet  28  for a second fluid and an outlet  30  for the second fluid. 
         [0034]    The first fluid and the second fluid are at different temperatures. For example, the first fluid can be at a lower temperature than the second fluid so that the second fluid is cooled by the first fluid. 
         [0035]    During operation, the first fluid enters through the inlet  24  and is distributed by the manifold or plenum  18  to the tubes  12  whose ends are in communication with the plenum  18 . The first fluid flows through the tubes  12  to the second end of the tubes and into the output plenum  20  and then through the outlet  26 . At the same time, the second fluid is introduced into the shell  22  through the inlet  28 . The second fluid flows around and past the tubes  12  in contact with the outer surfaces thereof, exchanging heat with the first fluid flowing through the tubes  12 . The baffles  16  help increase the flow path length of the second fluid, thereby increasing the interaction and residence time between the second fluid in the shell-side and the walls of tubes. The second fluid ultimately exits through the outlet  30 . 
         [0036]    Turning to  FIGS. 2-4 , an improved shell-and-tube heat exchanger  50  is illustrated. The heat exchanger is illustrated as a single-pass, primarily counter-flow (the two fluids flow primarily in opposite directions) arrangement. However, it is to be realized that the heat exchanger  50  could also be configured as a multi-pass system, as well as for cross-flow (the two fluids flow primarily generally perpendicular to one another), co-current flow (the fluids primarily flow in the same directions), or the two fluids flow can flow at any angle therebetween. 
         [0037]    The heat exchanger  50  includes a shell  52  and a tube bundle  54  that is configured to be disposable in the shell  52 . In the illustrated embodiment, the shell  52  includes an axial inlet  56  at a first end for introducing a first fluid and an axial outlet  58  at the opposite second end for the first fluid. In addition, the shell includes a radial inlet  60  near the first end for introducing a second fluid and a radial outlet  62  near the second end for the second fluid. 
         [0038]    The shell  52  is configured to enclose the tube bundle  54  and constrain the second fluid to flow along the surfaces of tubes in the tube bundle. The shell  52  can be made of any material that is suitably resistant to corrosion or other effects from contact with the type of second fluid being used, as well as be suitable for the environment in which the heat exchanger  50  is used. For example, the shell can be made of a metal including, but not limited to, steel or aluminum, or from a non-metal material including, but not limited to, a plastic or fiber-reinforced plastic. 
         [0039]    The tube bundle  54  extends substantially the length of the shell and includes a plurality of hollow tubes  64  for conveying the first fluid through the heat exchanger  50 . The tubes  64  are fixed at a first end  66  to a first tube sheet  68  and fixed at a second end  70  to a second tube sheet  72 . As would be understood by a person of ordinary skill in the art, the tube sheets  68 ,  72  are sized to fit within the ends of the shell  52  with a relatively close fit between the outer surfaces of the tube sheets and the inner surface of the shell. When the tube bundle  54  is installed inside the shell  52 , the tube sheets of the tube bundle and the shell collectively define an interior chamber that contains the tubes  64  of the tube bundle. The radial inlet  60  and radial outlet  62  for the second fluid are in fluid communication with the interior chamber. Due to the closeness of the fit and/or through additional sealing, leakage of the second fluid from the interior chamber of the shell past the interface between the outer surfaces of the tube sheets  68 ,  72  and the inner surface of the shell is prevented. 
         [0040]    As shown in  FIG. 3 , the ends of the tubes  64  penetrate through the tube sheets  68 ,  72  via holes in the tube sheets so that inlets/outlets of the tubes are provided on the sides of the tube sheets facing away from the interior chamber of the shell. The ends of the tubes  64  may be attached to the tube sheets in any manner to prevent fluid leakage between the tubes  64  and the holes through the tube sheets. In one example, the ends of the tubes are attached to the tube sheets by FSW. The use of FSW is particularly beneficial where the heat exchanger is used in an environment where it is subject to corrosion, since the FSW process eliminates seams, no dissimilar metals are used and, in the case of saltwater environments, no galvanic cell is created. 
         [0041]    FSW is a known method for joining elements of the same material. Immense friction is provided to the elements such that the immediate vicinity of the joining area is heated to temperatures below the melting point. This softens the adjoining sections, but because the material remains in a solid state, the original material properties are retained. Movement or stirring along the weld line forces the softened material from the elements towards the trailing edge, causing the adjacent regions to fuse, thereby forming a weld. FSW reduces or eliminates galvanic corrosion due to contact between dissimilar metals at end joints. Furthermore, the resultant weld retains the material properties of the material of the joined sections. Further information on FSW is disclosed in U.S. Patent Application Publication Number 2009/0308582, titled Heat Exchanger, filed on Jun. 15, 2009, which is incorporated herein by reference. 
         [0042]    The tubes  64  and the tube sheets  68 ,  72  are preferably made of the same material, such as, for example, aluminum, aluminum alloy, or marine-grade aluminum alloy. Aluminum and most of its alloys, as well as high alloy stainless steels and titanium, are amenable to the use of the FSW joining technique. The tubes and tube sheets can also be made from other materials such as metals including, but not limited to, high alloy stainless steels, carbon steels, titanium, copper, and bronze, and non-metal materials including, but not limited to, thermally enhanced polymers or thermoset plastics. 
         [0043]    Other joining techniques can be used to secure the tubes and the tube sheets, such as expansion, press-fit, brazing, bonding, and welding (such as fusion welding and lap welding), depending upon the application and needs of the heat exchanger and the user. 
         [0044]    In the example illustrated in  FIGS. 2-4 , the tubes  64  are substantially round when viewed in cross-section and substantially linear from the end  66  to the end  70 . However, the shape of the tubes, when viewed in cross-section, can be square or rectangular, triangular, oval shaped, or any other shape, and combinations thereof. In addition, the tubes need not be linear from end to end, but can instead be curved, helical, and other shape deviating from linear. A total of seven tubes  64  are illustrated in this example. However, it is to be realized that a smaller or larger number of tubes can be provided. 
         [0045]    It is preferred that the tubes be made of a material, such as a metal like aluminum, that permits extrusion or other seamless formation of the tubes. By eliminating seams from the tubes, corrosion is minimized. 
         [0046]    The tube bundle  54  also includes a baffle assembly  80  integrated therewith. In the illustrated embodiment, the baffle assembly  80  is formed by a plurality of discrete (i.e. separate) heat transfer units  82  that are connected to each other so that the baffle assembly  80  has a substantially helix-shape that extends along the majority of the length of the tube bundle  54  around the longitudinal axis of the tube bundle. More preferably the helix-shaped baffle assembly  80  formed by the heat transfer units  82  extends substantially the entire axial length of the tube bundle. 
         [0047]    The baffle assembly  80  increases the interaction time between the second fluid in the interior chamber of the shell and the walls of the tubes  64 . Further, as described further below, the heat transfer units  82  forming the baffle assembly are made of material that is thermally conductive, so that the baffle assembly  80  effectively increases the amount of surface area for thermal contact between the tubes and the second fluid. In addition, the substantially helix-shaped baffle assembly  80  substantially reduces or even eliminates dead spots in the interior chamber of the shell. The helix-shaped baffle assembly  80  can reduce pressure drop, reduce flow restriction of the fluid, and reduce the required force of pumping, yet at the same time provide directional changes of the second fluid to increase interaction between the second fluid and the tubes. Thus, the baffle assembly  80  provides the heat exchanger  50  with greater overall heat transfer efficiency between the second fluid and the tubes. 
         [0048]    In an embodiment, the heat transfer units  82  can be strengthened by the use of solid or perforated plates, made from a thermally conductive material such as aluminum, affixed to the heat transfer units  82 . The plates can be affixed to the units  82  in a periodic pattern along the helix, or they can be affixed to the units in any arrangement one finds provides a suitable strengthening function. The plates can be used to assist in the assembly of the tube bundle and the heat exchanger, and can assist with minimizing the pressure drop on the shell-side flow.  FIG. 18  shows an example of such a plate. 
         [0049]    Referring to  FIG. 5  together with  FIGS. 2-4 , each heat transfer unit  82  comprises a generally wedge-shaped, planar body  84  having a generally triangular or pie-shape that has radiused inner surfaces to fit the curvature of the outer surfaces of the tubes. As described further below, the unit  82  includes a foam material such as graphite foam or metal foam. Preferably, the unit  82  consists essentially of the foam material, and more preferably consists of the foam material. 
         [0050]    The body  84  includes a first major surface  86  and a second major surface  88  opposite the first major surface. In the illustrated embodiment, the major surfaces  86 ,  88  are substantially planar. However, one or more of the major surfaces  86 ,  88  need not be planar and could have contours or be shaped in a manner to facilitate fluid flow across or past the unit  82 . Fin patterns shown in  FIGS. 17A-17F  could be used to enhance flow and heat transfer over the major surfaces  86 ,  88 . The fins could extend substantially perpendicular to the surfaces  86 ,  88 . Alternatively, certain edges of the body  84  could have fin patterns shown in  FIG. 17A  thru  17 F to enhance flow and heat transfer from the edges of the heat transfer unit. A support rod hole  90  extends through the body  84  from the first major surface  86  to the second major surface  88  for receipt of a support rod described below. In another embodiment, an open-ended slot is used instead of the hole  90  to receive the support rod. Therefore, any opening, such as a hole or slot, could be used to receive the support rod. 
         [0051]    The perimeter of the body  84  is defined by an arcuate radially outer edge  92  connected to linear side edges  94 ,  96  at opposite ends of the outer edge. The side edges  94 ,  96  converge toward a common center  98  which is removed during formation of the unit  82 . The side edges  94 ,  96  terminate at radiused tube contact surfaces  100 ,  102 , respectively, that are positioned on the body  84  opposite the radially outer edge  92 . 
         [0052]    Each of the contact surfaces  100 ,  102  is configured to connect to an outer surface of one of the tubes  64  for establishing thermal contact between the heat transfer unit  82  and the tubes. To maximize thermal contact, the contact surfaces  100 ,  102  are configured to match the outer surface of the tubes  64 . In the illustrated embodiment, the contact surfaces  100 ,  102  are curved, arcuate, or radiused to generally match a portion of the outer surface of the tubes  64 . However, the contact surfaces  100 ,  102  can have any shape that corresponds to the shape of the tubes, for example square or rectangular, triangular, oval, or any other shape, and combinations thereof. 
         [0053]    The body  84  also includes a finger section  104  that in use extends between the two tubes  64  engaged with the contact surfaces  100 ,  102 . The finger section  104  includes linear edges  106 ,  108  that extend from the contact surfaces  100 ,  102  and that terminate at a third tube contact surface  110  that is configured to contact an outer surface of a third tube  64  for establishing thermal contact with the third tube. The contact surface  110  is configured to match the outer surface of the third tube. In the illustrated embodiment, the contact surface is slightly curved or arcuate to generally match a portion of the outer surface of the third tube. However, the contact surface  110  can have any shape that corresponds to the shape of the third tube, for example square or rectangular, triangular, oval, or any other shape, and combinations thereof. In certain embodiments, for example where contact between the body  84  and a third tube is not desired or where there is insufficient space between the tubes for the finger section to extend through, the finger section  104  can be eliminated. 
         [0054]      FIGS. 3 and 4  show the heat transfer units  82  mounted in position on the tube bundle  54 . As shown in  FIG. 3 , a plurality of support rods  120  are mounted at one end thereof to the tube sheet  72  and extend substantially parallel to the tubes  64 . The opposite ends of the support rods  120  are unsupported and not fixed to the tube sheet  68 . In another embodiment, the opposite ends of the support rods are also fixed to the tube sheet  68 . In the illustrated embodiment, four support rods  120  are provided and are evenly spaced around the tube bundle  54 . However, a larger or smaller number of support rods  10  can be used based in part on the size of the heat transfer units  82  that are used. 
         [0055]    The heat transfer units  82  are mounted on the tube bundle  54  with the outer edges  92  thereof facing radially outward. A support rod  120  extends through the hole  90  or other opening and the tube contact surfaces  100 ,  102 ,  110  are in thermal contact with outer surfaces of three separate tubes  64 . When in thermal contact with the tubes, the major surfaces  86 ,  88  form heat transfer surfaces that extend substantially radially from the outer surfaces of the tubes. As used herein, “in thermal contact” includes direct or indirect contact between the tube contact surfaces and the tubes to permit transfer of thermal energy between the tube contact surfaces and the tubes. Indirect contact between the tube contact surfaces and the tubes could result from the presence of, for example, an adhesive or other material between the tube contact surfaces and the surfaces of the tubes. When a hole is used, the hole  90  is preferably sized such that a relatively tight friction fit is provided with the support rod  120  to prevent axial movement of the heat transfer unit on the rod. If desired, fixation of the heat transfer unit  82  on the rod  120  can be supplemented by fixation means, for example an adhesive between the hole  90  and the rod. Instead of the hole, a slot can be formed that receives the support rod which can be secured via a friction fit or bonded using an adhesive. 
         [0056]    If adhesive bonding is used, the adhesive can be thermally conductive. The thermal conductivity of the adhesive can be increased by incorporating ligaments of highly conductive graphite foam, with the ligaments in contact with the surfaces heat transfer unit(s) and the tubes, and the adhesive forming a matrix around the ligaments to keep the ligaments in intimate contact with the tubes and heat transfer units. The ligaments will also enhance bonding strength by increasing resistance to shear, peel and tensile loads. 
         [0057]    As best seen in  FIG. 4 , the heat transfer units  82  are arranged in a helical manner to form the baffle assembly  80 . Each heat transfer unit is axially and rotationally offset from an adjacent heat transfer unit with a small overlap region  122  between each pair of adjacent heat transfer units. Because of the overlap regions  122 , the baffle assembly formed by the heat transfer units is substantially continuous along the length of the tube bundle  54 . The amount of overlap provided in the region  122  can vary based on the size and depth or thickness of the heat transfer units. In the overlap regions  122  the adjacent heat transfer units can be secured together. For example, the heat transfer units  82  can be frictionally engaged in the overlap regions so that friction maintains the relative rotational positions of the heat transfer units. Alternatively, an adhesive or other fixation technique can be provided at the overlap regions to fix the relative rotational positions of the heat transfer units. 
         [0058]    The periodicity of the helix can be changed by altering the angle of rotation of the heat transfer units. For example, the helix can have an angle of 30 degrees, 60 degrees, 90 degrees, 120 degrees, 150 degrees, 180 degrees and other angles. A person having ordinary skill in the art can determine the desired angles of rotation depending upon, for example, the desired performance of the heat exchanger. 
         [0059]    In addition, as discussed above, a metal plate ( FIG. 18 ) can be used to strengthen the foam heat transfer units  82  and assist in fabrication of the tube bundle. The support plate can also be embedded within the foam heat transfer unit  82  during formation of the heat transfer units  82 . The metal plate secures the positioning of the tubes in a fixed pattern as an alternating baffle that travels in a helical pattern down the tube axes. The metal plate can be used to overlap two or more foam pieces to provide strength of the graphite core assembly. 
         [0060]    When the tube bundle is installed in the shell  52 , the heat transfer units  82  are also sized such that the radially outer edges  92  thereof are positioned closely adjacent to, or in contact with, the interior surface of the shell to minimize or prevent the second fluid flowing in the shell from flowing between the radially outer edges  92  and the interior surface. This forces the majority of the fluid to flow past the tubes  64  in a generally spiral flow path defined by the heat transfer units  82 . In some embodiments, the heat transfer units  82  need not overlap, but can instead be sized and mounted so as to have gaps between adjacent heat transfer units to permit some of the fluid to flow axially between the adjacent heat transfer units. 
         [0061]    The unit  82  (as well as the heat transfer units described below) includes, consists essentially of, or consists entirely of, a foam material such as graphite foam or metal foam. The term foam material is used herein to describe a material having closed cells, open cells, coarse porous reticulated structure, and/or combinations thereof. Examples of metal foam include, but are not limited to, aluminum foam, titanium foam, bronze foam or copper foam. In an embodiment, the foam material does not include metal such as aluminum, titanium, bronze or copper. 
         [0062]    In one embodiment, the foam material is preferably graphite foam having an open porous structure. Graphite foam is advantageous because graphite foam has high thermal conductivity, a mass that is significantly less than metal foam materials, and has advantageous physical properties, such as being able to absorb vibrations (e.g. sound). Graphite foam can be configured in a wide range of geometries based on application needs and/or heat transfer requirements. Graphite foam can be used in exemplary applications such as power electronics cooling, transpiration, evaporative cooling, radiators, space radiators, EMI shielding, thermal and acoustic signature management, and battery cooling. 
         [0063]      FIGS. 6A-E  depict an exemplary process of how the heat transfer units  82  can be made. It is to be realized that this process is exemplary only and that other processes can be used. The heat transfer units  82  can be made by a process that stamps a foam material into a plurality of the wedge-shaped bodies  84 .  FIG. 6A  shows a die  128  for simultaneously punching a plurality of the bodies  84  from a circular foam substrate  130  ( FIG. 6D ). In  FIG. 6B , the foam substrate is shown as stamped by the die.  FIG. 6C  shows the stamped material being pulled up and transitioned with the press to force the foam from the die.  FIGS. 6D and 6E  show the foam pressed out of the die  128 , creating a plurality of the wedge-shaped bodies  84 . In the illustrated example, five wedge-shaped bodies  84  are formed with each stamping sequence. However, a smaller or larger number of bodies  84  can be formed if desired. A clover-leaf shaped remainder  132  is left at the center of the substrate  130  which can be discarded. 
         [0064]      FIGS. 6D and 6E  show the bodies  84  without the holes  90 . The holes  90  could be formed directly by the die  128 . Alternatively, if the die does not form the holes, the holes can be created in the bodies  84  after the stamping process through a separate machining process. 
         [0065]      FIG. 7  shows another embodiment of a foam heat transfer unit  150  disposed on a tube  64  of a tube bundle of a shell-and-tube heat exchanger. The heat transfer unit  150  comprises a generally cylindrical body with a central passage through which the tube  64  extends. The heat transfer unit  150  is in thermal contact with, directly or indirectly, the outer surface of the tube  64 . The body of the heat transfer unit  150  includes opposite end surfaces  152  that form heat transfer surfaces extending substantially radially from the outer surface of the tube. The heat transfer unit  150  can be fixed on the tube to maintain the axial position thereof in any suitable manner, for example by a friction fit or by using an adhesive. Axially extending channels  154  are formed in the body that extend between the end surfaces  152 . The channels  154  are evenly circumferentially spaced from one another around the body. In the illustrated embodiment, four channels  154  are shown, although a smaller or larger number of channels  154  can be used. 
         [0066]    In  FIG. 7 , a pair of the heat transfer units  150  are shown disposed on the tube  64 , spaced from each other with an axial gap between the heat transfer units. The two heat transfer units are rotated, for example, approximately 45 degrees relative to each other. However, the rotational angle between the heat transfer units can be more or less than 45 degrees, with the angle chosen based on, for example, the number of grooves and the spacing of the heat transfer units on the tube  64 . 
         [0067]    As shown by the arrows in  FIG. 7  representing the flow of fluid, a fluid flowing through the channel  154  impacts the surface of the adjacent heat transfer unit between the channels  154  causing the fluid to change direction in order to flow into the channels  154  of the adjacent heat transfer unit  150 . Additional heat transfer units  150  can be disposed along the entire length of the tube  64 , spaced from each other and rotated relative to a preceding heat transfer unit, similar to that shown in  FIG. 7 . 
         [0068]      FIG. 8  shows an embodiment of a foam heat transfer unit  160  disposed around the tube  64  of a tube bundle of a shell-and-tube heat exchanger. The heat transfer unit  160  is configured as a cylindrical sleeve with at least one end surface  162  that forms a heat transfer surface extending substantially radially from the outer surface of the tube. The heat transfer unit  160  can extend along any length of the tube, and preferably extends along substantially the entire length of the tube. The heat transfer unit  160  can be fixed on the tube to maintain the axial position thereof in any suitable manner, for example by a friction fit or by using an adhesive. In another embodiment, the heat transfer unit  160  is formed by two or more semi-circular sections that are fixed to the outer surface of the tube to form a sleeve. In addition, the sections can be spaced from one another to form one or more grooves between the sections that extend along the axis of the tube  64 . 
         [0069]    With each of the heat transfer units  150 ,  160 , they can be used by themselves, with each other, or with the heat transfer units  82 . In addition, when the heat transfer units  150 ,  160  are mounted on the tubes  64 , the outer surfaces of the heat transfer units  150 ,  160  preferably are in thermal contact with, directly or indirectly, the outer surfaces of the heat transfer units  150 ,  160  of one or more adjacent tubes  64 . 
         [0070]      FIG. 9  shows an embodiment of a portion of a tube bundle  170  of a shell-and-tube heat exchanger with a plurality of tubes  172  similar in function to the tubes  64 . A plurality of identical foam heat transfer units  174  are illustrated as being engaged with the tubes  172  and spaced along the length of the tubes. The heat transfer units  174  have bodies that are constructed as cradles or frames so that each heat transfer unit  174  is configured to engage with a plurality of the tubes  172 . In particular, the body of each heat transfer unit  174  is formed with a pair of outer tube contact surfaces  176   a ,  176   b  and three inner tube contact surfaces  178   a ,  178   b ,  178   c . However, the heat transfer units  174  can be configured to engage with more or less tubes as well. Each heat transfer unit  174  also includes generally planar end surfaces that form heat transfer surfaces extending substantially radially from the outer surface of the tubes. 
         [0071]      FIG. 9  shows a first set of the heat transfer units on one side of the tubes  172  with the outer contact surfaces  176   a ,  176   b  facing upward, and a second set of the heat transfer units on the opposite side of the tubes  172  with the outer contact surfaces  176   a ,  176   b  facing downward. The first set of heat transfer units is axially or longitudinally offset from the heat transfer units of the second set. In the embodiment illustrated in  FIG. 9 , seven tubes  172  can be engaged with the heat transfer units  174 , including two tubes engaged with the tube contact surfaces  176   a ,  176   b  of the upper set, two tubes engaged with the tube contact surfaces  176   a ,  176   b  of the lower set, and three tubes engaged with the inner tube contact surfaces  178   a ,  178   b ,  178   c  of the upper and lower set. It is to be realized that the heat transfer units  174  can be configured to engage with a larger or smaller number of tubes. 
         [0072]    Depending upon the layout of the heat transfer units  174 , the heat transfer units can create offsets, spirals or other flow patterns, in either counter, co-current or cross-flow arrangements.  FIGS. 17A-F  illustrate examples of patterns formed by different configurations of the foam heat transfer units  174  from  FIG. 9 . For example, as shown in  FIG. 17A , the heat transfer units can be arranged into a baffled “offset” configuration.  FIG. 17B  shows the heat transfer units arranged disposed in an offset configuration. When viewed from the top, each of the heat transfer units may have the shape of, but not limited to, square, rectangular, circular, elliptical, triangular, diamond, or any combination thereof.  FIG. 17C  shows the heat transfer units arranged into a triangular-wave configuration. Other types of wave configurations, such as for example, square waves, sinusoidal waves, sawtooth waves, and/or combinations thereof are also possible.  FIG. 17D  shows the heat transfer units arranged into an offset chevron configuration.  FIG. 17E  shows the heat transfer units arranged into a large helical spiral.  FIG. 17F  shows the heat transfer units arranged into a wavy arrangement or individual helical spirals. 
         [0073]      FIG. 10A  shows another embodiment of a tube bundle that has a plurality of tubes  190  arranged with an equilateral triangular pitch (i.e. the space between the tubes is generally an equilateral triangle).  FIG. 10B  shows tubes  190  of a tube bundle arranged with a square pitch, while  FIG. 10C  shows tubes  190  of a tube bundle arranged with a staggered square pitch. 
         [0074]    In  FIGS. 10A-C , foam heat transfer units  192  are shaped to fit in the pitch space between the tubes. For example, as shown in  FIG. 10A , foam heat transfer units  192  are disposed between the tubes  190  and have surfaces that are in thermal contact with the tubes. Each of the heat transfer units  192  comprises a generally triangular body, that can be radiused to the curvature of the tubes, with a generally triangular cross-section, and with the three surfaces of the triangular body in thermal contact with, directly or indirectly, three separate tubes  190 . 
         [0075]    The heat transfer units  192  may be arranged as required for heat transfer efficiency and/or providing directional flow of the fluid outside the tubes  190 . For example, the heat transfer units  192  can be arranged in any configuration to mimic a helix, multiple helix, offset baffle, offset blocks, or other patterns as shown in  FIGS. 17A-F . 
         [0076]    A person of ordinary skill in the art would realize that the tubes can be arranged with other pitch shapes between the tubes, and that the foam heat transfer units can have other corresponding shapes as well. 
         [0077]    With reference to  FIGS. 11 and 12 , another embodiment of a shell-and-tube heat exchanger  200  is illustrated that employs a tube bundle that includes twisted tubes  202  together with a foam heat transfer unit  204 . This embodiment has a number of advantages, including strengthening the tube core, eliminating the need for baffles, minimizing vibrations, and enhancing heat transfer on both the tube side (i.e. on the helical tubes) and on the shell side (the foam heat transfer unit). 
         [0078]    The heat exchanger  200  includes a shell  206  that has axial inlets and outlets at each end for a first fluid to flow into and out of the tubes  202 . Tubes sheets, similar to the tube sheets  68 ,  72  would be provided at each end of the tube bundle, would be attached to each tube  202 , and would fit within and close off the ends of the shell  206 . The shell also includes a radial inlet  208  and a radial outlet  210  for a second fluid. 
         [0079]    In this embodiment, the tubes  202  are twisted helically around the foam heat transfer unit  204  along the length of the heat transfer unit  204 . The heat transfer unit  204  comprises a central, solid body of foam such that at any cross-section of the tube bundle, the foam body forms a heat transfer surface extending substantially radially from the outer surface of the tube(s). In  FIG. 11 , the heat transfer unit  204  is represented by the dashed line extending the length of the shell  206 . The dashed line is not intended to imply that the heat transfer unit  204  is broken into sections or is discontinuous (although it is possible that the heat transfer unit  204  could be broken into separate section or made discontinuous if desired). The helical arrangement of tubes  202  enhances heat flow between the fluid flowing in the tubes and the fluid flowing in the shell outside of the tubes, by breaking up boundary layers inside and/or outside the tubes and combining axial and radial flow of the fluid along and around the outer surface of the tubes. In addition, the use of a baffle can be eliminated if desired. Further, the tubes  202  could be twisted about their own axes as well. 
         [0080]    Although  FIGS. 11 and 12  show six tubes  202 , a smaller or larger number of tubes can be used. For example, as discussed further below with respect to  FIGS. 13-15 , three tubes can be helically wound around a central, solid heat transfer unit. 
         [0081]      FIG. 13  is a cross-sectional view of another embodiment of a tube bundle that contains many axial tubes  222  disposed in a shell  224 . Two different implementations of the twisted or helical tube concept are illustrated. The triangle  226  in  FIG. 13  illustrates three tubes  228  helically twisted about a central, solid body foam heat transfer unit  230 . This is illustrated more fully in  FIG. 14  which additionally shows an optional sleeve  232  disposed around the assembly formed by the tubes  228  and the heat transfer unit  230  to form a tube-within-a-tube construction. The heat transfer unit  230  comprises a central, solid body of foam such that at any cross-section, the foam body forms a heat transfer surface extending substantially radially from the outer surface of the tube(s). In  FIG. 14 , the heat transfer unit  230  is represented by the dashed line extending the length of the sleeve  232 . The dashed line is not intended to imply that the heat transfer unit  230  is broken into sections or is discontinuous (although it is possible that the heat transfer unit  230  could be broken into separate section or made discontinuous if desired). 
         [0082]    Returning to  FIG. 13 , a hexagonal arrangement  240  of the twisted tube concept is illustrated and shown more fully in  FIG. 15 . In the hexagonal arrangement  240 , a tube within a tube concept is provided similar to the single arrangement shown in  FIG. 14 , wherein a hexagonal pattern of six tubes-within-tubes assemblies  242  are used. Each assembly  242  includes a plurality of tubes  244 , for example three tubes, helically twisted about a central, solid body foam heat transfer unit  246 , with the tubes  244  and the heat transfer unit  246  disposed within a larger fluid carrying tube  248 . So the first fluid flows within the tubes  244  as well as within the tubes  248  in contact with the outside surfaces of the tubes  244 . 
         [0083]    This twisted tube concept can be used by itself or in combination with any of the embodiments previously described herein. For example,  FIG. 9  shows an arrangement similar to  FIG. 14 , with a plurality of the tubes  228  twisted helically around the heat transfer unit  230 , and the tubes  228  and unit  230  disposed inside one of the tubes  172  to function together with the heat transfer units  174  at increasing the effectiveness of the heat exchanger. 
         [0084]    The heat transfer units  204 ,  230  have been described above as being solid bodies. However, the heat transfer units  204 ,  230  need not be solid. Instead, the heat transfer units  204 ,  230  can function as fluid carrying fluid distribution tubes which would be useful for creating a baffle-less design in a spray evaporator. For example, with reference to  FIG. 12 , the heat transfer unit  204  can carry a fluid and be configured to spray the fluid outward as shown by the arrows onto the surfaces of the tubes  202 . The sprayed fluid exchanges heat with the tube surfaces, causing some or all of the sprayed fluid to change phase into a vapor. Likewise, as illustrated by the arrows in  FIGS. 13 and 14 , the heat transfer unit  230  can be configured to spray fluid outward onto the tubes. One can also alternate foam and spray tubes too in various configurations. 
         [0085]      FIG. 16  illustrates another embodiment of a shell-and-tube heat exchanger that uses rectangular blocks of foam heat transfer units  300  that are in thermal contact with, directly or indirectly, a plurality of axial tubes  302 . The blocks would extend some or all of the axial length of the tubes  302 . The blocks form a staggered diagonal baffle arrangement which is useful in applications where the second fluid flows in a cross-flow direction relative to the flow of the first fluid through the tubes  302 . However, other heat transfer unit configurations and arrangements, as well as other flow patterns, are possible. 
         [0086]    All of the shell-and-tube heat exchangers described herein operate as follows. A first fluid is introduced into one axial end of the tubes of the tube bundles, with the fluid flowing through the tubes to an outlet end where the first fluid exits the heat exchanger. The tubes can be single pass or multi-pass. Simultaneously, a second fluid is introduced into the shell. The second fluid can flow counter to the first fluid, in the same direction as the first fluid, or in a cross-flow direction relative to the flow direction of the first fluid. As the second fluid flows through the shell, it contacts the outer surfaces of the tubes and/or the surfaces of the heat transfer units. Because the first fluid flows within the tubes, separated from the second fluid, heat is exchanged between the first and second fluids. 
         [0087]    Depending upon the application, the first fluid can be at a higher temperature than the second fluid, in which case heat is transferred from the first fluid to the second fluid via the tubes and the heat transfer units. Alternatively, the second fluid can be at a higher temperature than the first fluid, in which case heat is transferred from the second fluid to the first fluid via the tubes and the heat transfer units. 
         [0088]    The first and second fluids can be either liquids, gases/vapor or a binary mixture thereof. One example of a first fluid is water, such as sea water, and one example of a second fluid is ammonia in liquid or vapor form, which can be used in an Ocean Thermal Energy Conversion system. 
         [0089]    The examples disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.