Patent Publication Number: US-2015083382-A1

Title: Heat exchanger

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 61/960,674 filed on Sep. 24, 2013, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This specification relates generally to the field of heat exchangers, and more particularly, to non-adiabatic catalytic heat exchangers. 
     BACKGROUND 
     Heat exchangers in which heat is transferred between a tube-side fluid contained within multiple tubes comprising a bundle of tubes and a shell-side fluid surrounding the bundle of tubes are known. Shell and tube heat exchangers embody such art. 
     Baffles are used in existing shell and tube heat exchangers to cause the shell-side fluid to impinge the outer surface of the tubes in a cross flow pattern and thereby increase the outside film heat transfer coefficient of the tubes relative to equivalent shell-side fluid where the flow is parallel to the axis of the tubes, but cross flow is less efficient than counter flow for the most complete or effective exchange of heat. To the extent that each cross flow pass of shell-side fluid impinges a large length of the tubes, the difference in temperature between the tube-side fluid and shell-side fluid (the ΔT) has a wide range of values across the width of the shell-side pass with respect to distance along the length of the tube bundle. A wide range of ΔT introduces inefficiency relative to counter current flow. Baffles may cause the shell-side fluid to impinge surfaces that are not primary heat transfer surfaces to undergo virtually 180° changes of direction, such that the considerable pressure drop in the shell-side fluid from the change of direction does not directly impinge a primary heat transfer surface to break down a boundary layer impeding heat transfer or thereby provide a useful heat transfer function. 
     It is desirable for changes of fluid flow direction to be useful in impinging only heat transfer surfaces to improve the film heat transfer coefficient in exchange for the associated pressure drop. The inherently non-uniform velocity across the cross section of the flow path of a fluid undergoing changes of flow direction through wide cross sectional paths as in shell-side fluid passing between successive baffles introduces further inefficiency in that the higher velocity fluid improves heat transfer at disproportionately greater expense of pressure drop than the lower velocity fluid. It is therefore desirable that flow through the shell-side fluid be at uniform velocity in all locations and that changes of fluid flow direction only occur as a result of impingement onto heat transfer surfaces, and preferably primary heat transfer surfaces. 
     Helical baffles having a common helicoid surface through which multiple tubes project are known. Shell-side baffles in the shape of a single helicoid of the approximate transverse cross section as the tube bundle are known for creating a helical shell-side fluid path through a tube bundle, the helical path being laterally bounded by the shell, resulting in the deficiency of forcing changes of direction of shell-side fluid flow via surfaces that are not primary heat transfer surfaces and of creating cross flow rather than countercurrent flow. Because the baffle forms an acute angle to the tube wall in the direction of the inlet on one side of a given tube and forms an acute angle to the tube wall in the direction of the outlet on the other side of the given tube, the fluid passing between the tubes is only directed to impinge the tube wall from one direction as opposed to the distinct, multiple oblique fins associated with individual tubes in a bundle as disclosed herein to cause fluid passing between the tubes to impinge the tube wall on multiple sides and from multiple directions. Helical baffles cause uneven heat transfer around the tube perimeter and normally undesirably limit or render impractical the maintenance of heat exchangers, such as for the reduction or removal of fouling on the outside surfaces of a bundle of tubes. 
     Both longitudinal and transverse externally finned tubes for increasing the effective surface area of the outside of a tube are known. Fins provide additional or extended surface area for tube outside film heat transfer at the expense of masking primary heat transfer surface area and reducing the fluid velocity and film heat transfer coefficient of those primary heat transfer surfaces. These types of fins replace primary heat transfer surface area having a limited film heat transfer coefficient with area intimately contacting high thermal conductivity solids having much greater surface area than the primary heat transfer surface itself. Longitudinal and transverse fins are advantageously used in systems where the heat transfer through the fin material exceeds the heat transfer through the film layer of the fluid at the primary heat transfer surface, such as with copper or aluminum fins contacting a low conductivity fluid such as a gas. In such cases, the fins can have high aspect ratio of length (distance from primary heat transfer surface) to fin thickness. Conventional fins, whether transverse or longitudinal, are less effective if they must be composed of lower conductivity material, such as but not limited to iron or nickel alloys including carbon steel, stainless steel, Inconel® or plastic, wood, and the like, or when the thermal conductivity of the shell-side fluid is high such as with liquids. Applications at elevated temperature or in corrosive conditions may necessitate the use of fin or tube materials of much lower conductivity than Al or Cu, requiring less effective fins and/or thicker low aspect ratio fins. The material consumption, fabrication cost, and weight to be supported by tube sheets for finned tubes of these relatively lower conductivity materials are disadvantageous. Further, fins often require expensive joining to the tubes for good solid state thermal conductivity and make cleaning of the heat transfer surfaces difficult or impractical. 
       FIG. 1A  shows an exemplary tube  101  with a wall  102 , an external surface  103 , a tube axis  104 , an external fin and a fin surface  105  in the form of a ring (shown as a cross-hatched area). A reference plane  106  (shown as a second cross-hatched area) intersects the tube axis  104  along the line of the axis. The intersection between the reference plane  106  and the fin surface  105  is a line  107 , which line is perpendicular to the tube axis  104 , defining the external fin  105  as a transverse external fin. 
       FIG. 1B  shows a tube  101  with a wall  102 , external surface  103 , tube axis  104  and external fin and fin surface  105  in the form of a helicoid (shown as a cross-hatched area). Reference plane  106  (shown as a second cross-hatched area) intersects the tube axis  104  along the line of the axis. The intersection between the reference plane  106  and the fin surface  105  is line  107  which line is perpendicular to the tube axis  104 , defining the external fin  105  as a transverse external fin. 
       FIG. 1C  shows a tube  101  with wall  102 , external surface  103 , tube axis  104  and an external fin and fin surface  108  (shown as a cross-hatched area). Reference plane  106  (shown as a second cross-hatched area) intersects the tube axis  104  along the line of the axis. The defining intersection between the reference plane  106  and the fin surface is a surface  109 , which surface coincides with the fin surface  108 , defining the external fin as a longitudinal external fin. The plane of the fin surface shown in  FIG. 1C  is normal to the tube wall  102  at the fin&#39;s intersection with the tube wall. 
       FIG. 1D  shows a tube  101  with wall  102 , external surface  103 , tube axis  104  and an external fin and fin surface  108  (shown as a cross-hatched area). Reference plane  106  (shown as a second cross-hatched area) intersects the tube axis  104  along the line of the axis. The defining intersection between the reference plane  106  and the fin surface is line  110 , which line  110  is parallel to the tube axis  104 , defining the external fin  108  as a longitudinal external fin. Although the plane of the fin surface  108  shown in  FIG. 1D  intersects the tube wall  102  at an oblique angle to the wall, the defining intersection defines the fin in  FIG. 1D  as an external longitudinal fin and not an oblique fin. 
     SUMMARY 
     In accordance with an embodiment, a heat exchanger is provided. The heat exchanger includes a packing comprising at least one external oblique fin having a surface, and a plurality of tubes, each tube having an inlet, an outlet, a wall, an outer surface, and an axis. The plurality of tubes are substantially parallel and separate from each other. 
     In another embodiment, the packing includes a plurality of fins arranged in a plurality of columns. Each column includes a plurality of external oblique fins associated with a respective tube disposed along at least part of a length of the respective tube, and first fins in a first column have substantially the same first orientation. The first orientation of the first fins in the first column is different from a second orientation of second fins in a second column. 
     In another embodiment, each tube includes a reference plane. A defining intersection is an intersection between the reference plane and a surface of the at least one external oblique fin. An angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 90°. 
     In another embodiment, the angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 45°. 
     In another embodiment, the heat exchanger further includes at least one of a plurality of transitional members, a plurality of supports and a plurality of skids. 
     In another embodiment, the heat exchanger further includes a helicoid transition member. 
     In another embodiment, the heat exchanger is a tube and shell type heat exchanger. 
     In accordance with another embodiment, a heat exchanger is provided. The heat exchanger includes a packing and a plurality of tubes. Each tube has an inlet, an outlet, a wall, and an axis. The plurality of tubes are substantially parallel to each other and separate from each other. The packing includes one or more external fins having a helicoid shape and having an axis of rotation that does not coincide with the tube axis. 
     In another embodiment, the axis of rotation is external to each of the plurality of tubes. 
     In another embodiment, the packing includes first external helicoidal fins twisted in a first direction and second external helicoidal fins twisted in a second direction opposite the first direction. 
     In accordance with another embodiment, a heat exchanger is provided. The heat exchanger includes a plurality of tubes, each tube having an inlet, an outlet, a wall, an outer surface, and an axis. The heat exchanger also includes a packing comprising a plurality of fins arranged in a plurality of columns, each column including at least one external oblique fin associated with a respective tube disposed along at least part of a length of the respective tube. 
     In one embodiment, the heat exchanger also includes a first fin in a first column having a first orientation, and a second fin in a second column having a second orientation different from the first orientation. 
     In another embodiment, each tube has a reference plane, wherein a defining intersection is an intersection between the reference plane and a surface of the at least one external oblique fin. The angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 90°. In another embodiment, the angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 45°. 
     In another embodiment, the heat exchanger includes at least one of a plurality of transitional members, a plurality of supports and a plurality of skids. 
     In another embodiment, the heat exchanger includes a helicoid transition member. 
     In another embodiment, the heat exchanger is a tube and shell type heat exchanger. 
     In another embodiment, the plurality of tubes are substantially parallel and separate from each other. 
     These and other advantages of the present disclosure will be apparent to those of ordinary skill in the art by reference to the following Detailed Description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an orthogonal view of a first external transverse fin. 
         FIG. 1B  shows an orthogonal view of a second external transverse fin. 
         FIG. 1C  shows an orthogonal view of a first external longitudinal fin. 
         FIG. 1D  shows an orthogonal view of a second external longitudinal fin. 
         FIG. 2  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 3  shows a longitudinal cross section of the heat exchanger of  FIG. 2 . 
         FIG. 4  shows an orthogonal view of the heat exchanger of  FIG. 2 . 
         FIG. 5A  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 5B  shows a transverse projection of a heat exchanger in accordance with another embodiment. 
         FIG. 5C  shows a transverse projection of a heat exchanger in accordance with another embodiment. 
         FIG. 6  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 7  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 8  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 9  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 10A  shows a transverse projection of a heat exchanger in accordance with an embodiment. 
         FIG. 10B  shows a longitudinal cross section of the heat exchanger of  FIG. 10A . 
         FIG. 10C  shows a longitudinal cross section of the heat exchanger of  FIG. 10A . 
         FIG. 11A  shows a longitudinal cutaway view of a shell and tube heat exchanger in accordance with an embodiment. 
         FIG. 11B  is a transverse projection of the central section of the heat exchanger of  FIG. 11A . 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description discloses various exemplary embodiments and features of the invention. These exemplary embodiments and features are not intended to be limiting. 
     In accordance with an embodiment, a heat exchanger is provided containing a packing and a plurality of tubes (sometimes referred to herein as a bundle of tubes), wherein the individual tubes have an inlet, an outlet, a wall, an outer surface, an axis, and a reference plane, wherein the tubes are substantially parallel to each other and are separated from each other, and wherein the packing contains at least one external oblique fin. The packing may additionally contain transitional members, supports, struts, skids, or helicoids, and may include multiple and preferably identical sub-assemblies, which sub-assemblies may be individually inserted into or removed from the bundle of tubes for maintenance purposes. 
     The fins may be constructed of low thermal conductivity materials. The packing may or may not be permanently joined to the tubes. The fins are proximate the tube walls and may or may not contact the tube walls. The fins are preferably are less than 0.25 tube diameters from the tubes, and more preferably are less than 0.1 tube diameters from the tubes. The fins may be flat or curved sheets, and may be rigid or non-rigid. The fins are preferably substantially impervious. 
     The design of the packing causes shell-side fluid passing between the tubes to be deflected in its flow direction primarily by primary heat transfer surfaces and not by the fins themselves or by a shell or housing of the tube bundle such as to exploit all such deflections for breaking down boundary layers at primary heat transfer surfaces for the improvement of film heat transfer coefficients of the primary heat transfer surfaces. Generally helical flow patterns are created, the angle of which is determined by the relative values of the heat exchanger being small, being light weight, and having low pressure drop. The shell-side fluid is further directed to impinge tubes from multiple directions as opposed to impinging tubes from one lateral direction as is known with shell-side baffles. 
     In a heat exchanger in which heat is transferred between a first fluid within a plurality of tubes or a bundle of tubes and a second fluid surrounding the tubes, it is desirable that the heat transfer coefficient be high, the pressure drop of the respective fluids, gross consumption and net finished weight of materials of construction and fabrication cost be minimal, and that the materials of construction be durable in the given application environment. 
     Systems, methods, and apparatus described herein offer numerous advantages. For example, systems, methods, and apparatus described herein provide a heat exchanger having a tube or a plurality (bundle) of tubes that will cause a fluid to impinge greater portions of the outside surface of the tube or of the bundle of tubes in such a way that the ratio of the outside film heat transfer coefficient divided by the pressure drop is higher than in other known art for equivalent applications. Systems, methods, and apparatus described herein also reduce the material consumption, finished weight, and fabrication cost of heat exchangers containing bundled tubes. Certain systems, methods, and apparatus described herein provide a heat exchanger packing that is installable, removable, and maintainable, both in fouling and non-fouling conditions. Systems, methods, and apparatus described herein also provide fins of less expensive materials. Other advantages will become clear to one reasonably skilled in the art upon reading the disclosure set forth herein. 
     For the purposes of this invention, the following terms shall have the indicated meanings: 
     A tube includes tubes, pipes, channels and conduits having an inlet, an outlet, an axis, a length, a reference plane, and a lateral wall enclosing a volume, such wall having an outer surface. The cross section of a tube may be round, triangular, square or any other shape. 
     A baffle is a common sheet that completely engulfs, or is penetrated by, multiple tubes. 
     A fin is a flat or curved sheet having a thickness wherein the sheet is associated with a tube for extending the surface area of the tube or for directing the flow of a fluid through or around a tube. A sheet may be associated with a tube either by way of being joined to the tube or by being proximate to the tube and in fixed orientation or position with respect to the tube during use or operation. A fin may be proximate multiple tubes, but only completely engulfs at most one tube as opposed to a baffle. 
     An external fin is a fin that either extends from the external surface of a tube or directs the flow of fluid outside or around the associated tube. 
     A tube axis is a line running longitudinally along the tube&#39;s length at the tube&#39;s mid-transverse cross section. 
     A reference plane is a plane that includes and is aligned with the tube axis such that the intersection of the reference plane and the tube axis is the tube axis, as opposed to a plane that does not intersect the tube axis or that intersects the tube axis at a single point. 
     A fin surface is the surface coinciding with the mid thickness of a fin. Fins are classified herein by the shape of the intersection between a reference plane and the fin surface, wherein said intersection is referred to as the defining intersection. 
     A transverse fin is a fin for which the defining intersection is a line that is perpendicular to the tube axis.  FIGS. 1A and 1B  illustrate examples of transverse fins. 
     A longitudinal fin is a fin for which the defining intersection is one of the fin surface itself or a line that is parallel to the tube axis.  FIG. 1C  illustrates a longitudinal fin in which the defining intersection is the fin surface, and  FIG. 1D  illustrates a longitudinal fin in which the defining intersection is a line parallel to the tube axis. 
     An oblique fin is a fin for which the defining intersection is a line that is oblique to the tube axis. A fin for which the defining intersection is a curved line is defined as the straight line passing through the ends or extremities of the curved line, such as in the case of a helicoidal fin rotating about an axis that is external to the tube or otherwise does not coincide with the tube axis.  FIGS. 2 to 11  illustrate examples of an oblique fin. 
     A fin for which reference planes within one or more arcs about the tube axis create defining intersections that define the fin as a transverse fin and reference planes within one or more arcs about the tube axis create defining intersections that define the fin as an oblique fin is defined herein as being of the type represented by the larger cumulative angles of arcs. For example, a flat fin at an oblique angle to a tube encompassing a 360° arc about the tube that creates defining intersections that define the fin as transverse at two diametrically opposite reference planes and creates defining intersections that define the fin as oblique for all other angles over two arcs of about 180° each is defined as oblique. All of the exemplary fins illustrated in the present disclosure fall within arcs of only 120° of reference planes about their respectively associated tubes in which arcs, all reference planes and defining intersections define the fins as oblique and not transverse or longitudinal. 
     Elements depicted in the Drawings with corresponding numeral references in more than one figure are corresponding elements. 
       FIG. 2  shows a transverse projection of a heat exchanger in accordance with an embodiment. Heat exchanger  111  includes a packing  1  that engulfs and lies between a plurality of vertical tubes  2 , the tubes having walls  3 , outer surfaces  4 , and axes  18 . Packing  1  includes external oblique fins, for example, in columns  5 ,  6 , and  7  (shown as cross hatched areas). Fins or columns of fins  5 ,  6 , and  7  are differentiated from each other by their respective orientations denoted by the different directions of their respective cross hatching. The tubes  2  are separated from each other and have substantially parallel axes. Each of the respective fins  5 ,  6 , and  7  are at their lowest elevations at their respective curved edges  8  and slope upward from those curved edges in the direction of their respective cross hatch lines as is depicted in  FIGS. 3 and 4 . Fluid flows through packing  1  from top to bottom. Lines A-A 1 , B-B 1 , and C-C 1  shown in  FIG. 2  represent longitudinal cross sections of heat exchanger  111 . 
     In various embodiments, the tube and fin components are constructed of materials that are compatible with the specific application such as in strength and resistance to corrosion and erosion. The tubes preferably have high thermal conductivity. In various embodiments, high temperature and corrosion resistance metal alloys may be used, including, but not limited to, low and high alloy steels such as stainless steels and nickel alloys such as Inconel®. In some embodiments, the packing does not predominantly rely on solid state thermal conduction through the fins, but instead enhances convection through the boundary layers surrounding the tubes; packings composed of low cost materials of construction of low thermal conductivity and/or high corrosion resistance such as polymers, refractory, glass, wood, cardboard, cloth and the like may be used. The fins may be only as thick as is necessary for structural purposes and may or may not be rigid. The fins may or may not be attached to the tubes. The gap between the fins and the tubes is preferably as small as possible. A fin may or may not contact the tube; contact between the fin and the tubes is not necessary. The tubes may be of any cross sectional shape. The shape of the fins may be altered to accommodate different tube packing patterns, tube spacing, tube diameters, tube shapes, prescribed ratios of heat transfer coefficient to pressure drop, and different fluid properties. It is preferred that the horizontal projections of the fins as depicted in  FIG. 2  substantially occlude all vacancies between the tubes, although within any particular horizontal or transverse cross section, the space between the tubes preferably has as high a void content as possible. The transverse projection of fins  5 ,  6 , and  7  may overlap, and the fins may have flat or curved surfaces. The fins may be made of expanded metal, but are preferably solid, flat impermeable sheets without perforations. 
       FIG. 3  shows longitudinal cross section A-A 1  of the heat exchanger  111  shown in  FIG. 2 . Packing  1  engulfs and lies between vertical tubes  2 , the tubes having walls  3 , outer surfaces  4 , and axes  18 , and the packing consists of oblique external fins  5  in a plurality of columns  19  of fins. 
     Similarly, cross section B-B 1  and cross section C-C 1  (indicated in  FIG. 2  but not shown) have similar configurations. The multiple tubes are aligned vertically such that the respective tubes have a vertical wall and a vertical axis  18 . Reference plane  9  (shown as cross hatched area in  FIG. 3 ) intersects multiple tube axes along the length of their respective axes. The defining intersection between the reference plane and each fin  5  is a line represented by line  10 . Line  10  is oblique to the axis, defining each fin  5  as an oblique fin. 
     All fins  5 ,  6 , and  7  of  FIG. 2  extend from a first tube at their curved edge, which edge is joined to or proximate the first tube, to an adjacent second tube at an oblique angle to the axis of the first or associated tube. The upward angle between a fin and the axis of the first or associated tube is preferably the same for all fins and is greater than 0° and less than 90°, and preferably greater than 0° and less than 45°. Greater angles within the range are preferable in applications where the weight or material cost of the heat exchanger is relatively more important than the shell-side pressure drop, and smaller angles are preferable in applications where the pressure drop of the shell-side fluid is relatively more important than the size or cost of the heat exchanger. Optimal vertical distances between consecutive fins in a given column may be calculated or experimentally determined by one skilled in the art. It should be understood that the heat exchanger consisting of tubes and packing may be in any orientation, such as with horizontal tube axes. Vertical tubes have the advantage of lower strength requirements of the fins to support the tubes or maintain tube spacing, permitting the fins to be constructed of thinner sheets. Preferably, the fins in different columns associated with a common tube are at substantially identical elevations, spacings and numbers to the fins in the other associated columns of fins as opposed to the fins of the respective columns being staggered with respect to each other. 
       FIG. 4  shows an orthogonal view of the heat exchanger  111  of  FIG. 2 . Packing  1  engulfs and lies between vertical tubes  2  which tubes have inlets  22  and outlets  23  and comprises external oblique fin types  5 ,  6 , and  7 , which fins are arranged in their respective columns and are inclined at oblique angles to the vertical and to the tube axes (not shown). The lowest portions of the fins are along their respective curved edges  8  and the fins rise in elevation in the directions of the respective cross hatch lines drawn on them. Fins  5 ,  6 , and  7  are not joined to each other along entire edges, but are only proximate or joined to each other at points along their edges. The fins abut or may be joined to the tubes along the curved edges  8  of the respective fins. Fluid flows vertically downward through the embodiment illustrated in  FIGS. 3 and 4  such that the fins induce the fluid to impinge the tubes over as much of the tubes&#39; outer surfaces as possible and to be deflected primarily by primary heat transfer surfaces, which are the tube wall surfaces. The fins are designed as to deflect the fluid minimally in its flow direction so as to induce minimal pressure drop. In this embodiment, the fins are flat sheets, and are all at the same oblique angle to the vertical and to the tube axes (not shown). A plurality of fins are shown arranged in columns of fin types  5 ,  6 , and  7 , respectively along the lengths of the tubes. 
       FIG. 5A  shows a transverse projection of a heat exchanger in accordance with an embodiment.  FIG. 5B  shows a transverse projection of a heat exchanger in accordance with another embodiment.  FIGS. 5A and 5B  have the same center-to-center tube spacing, but are different from each other with respect to the diameters of the vertical tubes  2 . Each tube has a wall  3 , outer surface  4 , and axis  18 , in each of the respective figures. A single sub-assembly  20  shown in each figure includes a column of oblique external fins  5 , a column of oblique external fins  6 , and a column of multiple oblique external fins  7 . Numbered fins differ from each other in orientation. The lowest elevations of each of the fins are along their curved edges  8 , and the fins rise in elevation from their respective curved edges to their corners or apexes  16  opposite those curved edges. Fins  5 ,  6 , and  7  are proximate each other at their corners  11  and are welded to vertical rod  12 , which rod extends along the entire length of the sub-assembly, joining all fins  5 ,  6 ,  7 , within their respective columns. Optionally, sub-assembly  20  also contains vertical support rods  13  and  14  which rods join fins  5  to each other, fins  6  to each other, and fins  7  to each other within their respective columns. Also optionally, a sub-assembly may contain multiple support rods  15  joining vertical rods  13  and  14  at multiple elevations within a given column. Multiple rods  15  in a given column of rods  15  may be at different angles to the axis from each other. For example, consecutive rods  15  in a given column may be alternately horizontal and at an oblique angle the horizontal, such as lying along a surface of fins in their respective columns, to buttress the sub-assembly from distortion and to cause the sub-assembly to be a structurally sound unit which may be removed from or inserted into a bundle of tubes to permit cleaning or other maintenance. Additional struts or supports (not shown) may be added. In one embodiment the sub-assemblies are provided with skids or bearing surfaces near the tubes to cause the sub-assemblies to slide into and out of tube bundles without the fins being deformed or damaged. Rods  13  and  14  may function as skids such that a small or minimal clearance is provided between the tubes and fin edges  8  as one skilled in the art may design in a variety of ways. Sub-assembly  20  includes all shown fins, rods, and skids and excludes the tubes. The tubes of  FIG. 5A  are larger in diameter, but of the same center to center spacing, as the tubes in  FIG. 5B . The shapes and sizes of the fins are accordingly adapted to the tube diameters and spacing as shown. 
     Although the tubes in the examples are arranged in hexagonal close packing or triangular arrays, other tube patterns may be used, such as square arrays of tubes, etc. The tube spacing and patterns may be varied at the edges of a tube bundle from those within a tube bundle. The tubes may additionally enclose other tubes. 
       FIG. 5C  shows a transverse projection of a heat exchanger in accordance with an embodiment. Heat exchanger  112  includes packing  1 , which engulfs and lies between a plurality of vertical tubes  2  and includes three sub-assemblies  20  similar to those of  FIG. 5B . The sub-assemblies are distinguished from each other only by the direction of the cross hatching marking their respective locations, but are otherwise identical to each other. Each sub-assembly  20  contains a plurality of external oblique fins of types  5 ,  6 , and  7  arranged in a plurality of columns, wherein all fins rise in elevation from their respective curved edges  8  to their corners or apexes  16  opposite the curved edges. The orientations of the fins cause fluid flowing vertically downward or into the drawing through packing  1  to generally flow with transverse vector components according to dotted arrows  17 . It can be seen that the flow directions of the three dotted arrows form a clockwise helical rotation of the fluid within a given sub-assembly and form a counter clockwise helical rotation for fluid with respect to three adjacent fins of three different sub-assemblies. 
       FIG. 6  shows a transverse projection of a heat exchanger in accordance with another embodiment. Heat exchanger  113  includes packing  1 , which engulfs and lies between a plurality of vertical tubes  2 , which tubes have walls  3 , outer surfaces  4 , and axes  18 . Packing  1  includes a plurality of oblique external fin types  5 ,  6 , and  7 , arranged in columns, wherein the respectively numbered fins differ from each other in orientation. Packing  1  also includes a plurality of transitional members  31 ,  32 ,  33 ,  34 ,  35 , and  36 , arranged in a plurality of columns, which members are sheets wherein the numbered transitional members differ from each other in orientation. All fins and members are at the same oblique angle to the tube axes, for example, and their steepest downward slopes and greatest angles to the vertical tube axes are in the respective directions of dotted arrows  37  and  38 , respectively. 
     The packing around the tubes consists of multiple sub-assemblies of fins and members and rods (not shown), supports (not shown) and skids (not shown) surrounded by triangular groups of three tubes in an arrangement similar to that of the sub-assemblies shown in  FIGS. 5A and 5B . Sub-assemblies may contain any repeating number and configuration of columns of fins and transition members, which combination of columns of fins and members forms a repeating pattern or group constituting all components of the packing. For example, a sub-assembly may include one fin and any two adjacent transition members to the fin, where the associated fin and member are in identical orientation to each other in each sub-assembly. As another example, a sub-assembly may include two fins and four transition members, preferably adjacent to each other. As a further example, a sub-assembly may include three fins and six transition members, preferably adjacent to each other. The same combination of three fins and six transition members in the same relation to each other makes up each sub-assembly. Multiple sub-assemblies of identically oriented components are combined to constitute an entire packing. Because neighboring fins and members are in planes at different orientations, they are not joined to each other along extended lengths of their edges, but are only proximate each other at points along their edges or corners. 
     Shell-side fluid flows vertically downward through the packing such that its path consists of counterclockwise helical paths between certain groupings of three tubes as indicated with reference to members  31 ,  32 , and  33  and clockwise helical paths between certain other groupings of three tubes as indicated with reference to members  34 ,  35 , and  36 . The relative areas of the fins and of the members may be altered such that fins have a broader projected area at the expense of the members or the members have a broader projected area at the expense of the fins, as compared to those shown in  FIG. 6 , or such that the projections of the fins and members overlap. 
       FIG. 7  shows a transverse projection of a heat exchanger in accordance with another embodiment. Heat exchanger  114  includes packing  1 , which engulfs and lies between a plurality of vertical tubes  2 , which tubes have walls  3 , outer surfaces  4 , and axes  18 . Packing  1  also includes a plurality of external oblique fin types  5 ,  6 , and  7  arranged in columns, wherein the respectively numbered fin types differ from each other in orientation. Packing  1  also includes helically wound sheets  42  and  43  (both shown as cross hatched areas) with straight lines  46  representing a transverse cross section of the sheets  42  and  43 . Fluid flowing vertically downward flows through columns of fins with transverse vector components denoted by dotted arrows  41 , flows through helicoids  42  in a counterclockwise helical pattern denoted by dotted arrows  44 , and flows through helicoids  43  in a clockwise helical pattern denoted by dotted arrows  45 . The opposite flow rotations are induced by the shell-side fluid passing through helicoids  42  and  43 , where helicoids  42  and  43  are twisted in opposite directions to each other. Fins extend laterally from one tube to another as shown and have curved or shaped edges  8  where they abut or are proximate those respective tubes. Sub-assemblies containing columns of fins and sub-assemblies containing helicoids or alternatively sub-assemblies containing both fins and helicoids in combination may be constructed such that the sub-assemblies can be installed into and removed from the array of multiple tubes by sliding the sub-assemblies longitudinally between the tubes. 
       FIG. 8  shows a transverse projection of a heat exchanger in accordance with another embodiment. Heat exchanger  115  includes packing  1 , which engulfs and lies between a plurality of vertical tubes  2 , which tubes have walls  3  shown as dotted areas, outer surfaces  4 , and axes  18 . Packing  1  also includes a plurality of external oblique fin types  5 ,  6 , and  7 , arranged in a plurality of columns. The respectively numbered fin types differ from each other in orientation. Helically wound sheets  42  and  43  (both shown as cross hatched areas) with straight lines  46  representing a transverse cross section of the sheets  42  and  43 . The cross sections of the example helicoids are truncated to form triangles instead of having a circular cross sectional area. Shell-side fluid flowing vertically downward flows through columns of fins with transverse vector components denoted by dotted arrows  41 , flows through helicoids  42  in a counterclockwise helical pattern denoted by dotted arrows  44  and flows through helicoids  43  in a clockwise helical pattern denoted by dotted arrows  45 . Fins extend laterally from a first tube to a second tube as shown from their curved edges  8  to their opposite apexes  16 , where the apex is at the fin&#39;s highest elevation and the curved edge is at the fin&#39;s lowest elevation. First sub-assemblies containing columns of fins and second sub-assemblies containing columns of helicoids or alternatively sub-assemblies containing both fins and helicoids in combination are constructed such that the sub-assemblies can be installed into and removed from the array of tubes. Each of the sub-assemblies is preferably identical to each other or is at most of two types of sub-assembly, wherein the first type contains helicoids  42  and the second type contains helicoids  43 . 
       FIG. 9  shows a transverse projection of a heat exchanger in accordance with another embodiment. Heat exchanger  116  includes packing  1 , which engulfs and lies between a plurality of vertical tubes  2 , which tubes have walls  3  shown as cross hatched areas, outer surfaces  4 , and axes  18 . The packing contains helically wound sheets  42  and  43 . The transverse projection of helically wound sheet  42  is shown as areas with diamond patterned or crossing diagonal lines, and the transverse projection of helically wound sheet  43  is shown as dotted areas. Shell-side fluid flowing vertically downward flows through helicoids  42  in a counterclockwise helical pattern denoted by dotted arrows  44  and flows through helicoids  43  in a clockwise helical pattern denoted by dotted arrows  45 . Lines  46  denote the intersections of sheets  42  and  43  with a particular common transverse cross section of packing  1 , though helicoids  42  and  43  may rotate in or out of phase with each other. Helicoidal shapes  42  and  43  are preferably of the same curvature and angle to the axis of the tubes or of the helicoids and are preferably in phase with each other as denoted in  FIG. 9 . The transverse projections of sheets  42  and  43  preferably abut each other in such a manner as to close all gaps between the projections of the edges of adjacent helicoidal sheets without overlapping. Preferably, projections of sheets  42  are next to projections of sheets  43  and are not next to projections of other sheets  42  as in the alternating pattern of sheets  42  and  43  shown in  FIG. 9 . Sub-assemblies containing one or more helicoids are constructed such that the sub-assemblies can be installed into and removed from the array of tubes. The sub-assemblies preferably contain multiple axial rods  47 , shown with respect to one helicoid  43 , along the length of the sub-assemblies to stiffen the helicoidal sheets and to function as skids to slide against the tubes during insertion or removal of sub-assemblies from the tube bundle. Each of the two types of sub-assembly containing helicoids  42  and  43 , respectively, is preferably identical to each sub-assembly of its respective type. 
       FIG. 10A  shows a transverse projection of a heat exchanger in accordance with another embodiment. Heat exchanger  117  includes packing  1 , which engulfs and surrounds a plurality of substantially vertical tubes  2  having walls  3 , outer surfaces  4 , and axes  18 . Packing  1  also includes alternating columns of external oblique fins of types  51  (shown as areas with vertical lines), and  52  (shown as areas with horizontal lines). Packing  1  also includes longitudinal radial walls  53  separating columns  51  from columns  52 , and contains regions  54 . Fins  51  rise in elevation from their curved inner edges  8 , which edge  8  is proximate the outer surface of the associated tube, to their outer curved edges  55 . Fins  52  rise in elevation from their curved outer edges  55  to their inner curved edges  8 , which edge  8  is proximate the outer surface of the associated tube. Lines A-A and B-B in  FIG. 10A  represent longitudinal cross sections of heat exchanger  117 . Regions  54  extend along the length of the packing and may contain a void space, an impermeable solid object, a static mixer, or any other structure. 
       FIG. 10B  shows longitudinal cross-section A-A of  FIG. 10A . Column  51  includes a plurality of oblique fins  56 , which abut outer surface  4  of tube wall  3  of tube  2  and extend to region  54 . Longitudinal radial wall  53 , which is not in the cross section A-A, is shown in relation to the tube wall and fins of column  51 , showing a gap  58  between surface  4  and wall  53  to permit fluid flowing downward through column  51  and impinging surface  4  to return from surface  4  via an adjacent column  52 , shown in  FIG. 10C . 
       FIG. 10C  shows longitudinal section B-B of  FIG. 10A . Column  52  includes a plurality of fins  57 , which abut outer surface  4  of tube wall  3  of tube  2  and extend to region  54 . Longitudinal radial wall  53 , which is not in the cross section B-B, is shown in relation to the tube wall and fins of column  52 , showing gap  58  between surface  4  and wall  53  to permit fluid flowing downward through column  52  of  FIG. 10B  toward region  54  to return from region  54  via an adjacent column  51 . 
       FIG. 11A  shows a longitudinal cutaway view of a shell and tube heat exchanger in accordance with an embodiment. Heat exchanger  30  comprises a shell  21 , a plurality of tubes  2  shown as dotted areas, tube sheets  22 , a tube side inlet  23  and outlet  24 , a shell side inlet  25  and outlet  26 , and a central section  27  shown as a cross-hatched area, which section contains the plurality of tubes within a packing. The tubes extend longitudinally beyond the packing at each end to permit inlet and outlet shell side gas streams to flow transversely or laterally between the tube extensions beyond the ends of the packing and so feed into and out of the ends of the packing unimpeded by the packing itself. 
       FIG. 11B  is a transverse projection of the central section  27  of the heat exchanger  30  of  FIG. 11A . Packing  1  engulfs and lies between a plurality of parallel tubes  2  (shown as dotted areas), the tubes having walls (not shown), outer surfaces (not shown), and axes (not shown). The packing  1  also contains columns of oblique fins of types  5 ,  6 , and  7 , which fin types differ from each other only in orientation. The tubes and packing in this embodiment are similar to those shown in  FIGS. 2 ,  3  and  4 . The packing  1  also contains an area  28  (shown as a cross hatched area) which is the space between the part of the heat exchanger, consisting of the tubes and packing, and at least one of the heat exchanger&#39;s shell shown in  FIG. 11A , a shroud (not shown), internal lagging (not shown), or insulation (not shown). Area  28  may be filled with impermeable material from end to end of the packing, or area  28  may be fitted with a hollow structure bounded by impermeable walls or sheets on its inner and outer transverse sides and across one or more transverse cross sections such as to block shell side fluid from bypassing the part of the heat exchanger containing the tubes and packing. Area  28  may alternatively be fitted with tubes of different shapes or cross sectional dimensions and/or with fins of different cross sections, such as truncated or extended fins and the like to enable effective heat exchange within section  28 . 
     Thus, in accordance with an embodiment, a heat exchanger is provided. The heat exchanger includes a packing comprising at least one external oblique fin having a surface, and a plurality of tubes, each tube having an inlet, an outlet, a wall, an outer surface, an axis, and a reference plane. The plurality of tubes are substantially parallel and separate from each other. 
     In another embodiment, the packing includes a plurality of fins arranged in a plurality of columns. Each column includes a plurality of external oblique fins associated with a respective tube disposed along at least part of a length of the respective tube, and first fins in a first column have substantially the same first orientation. The first orientation of the first fins in the first column is different from a second orientation of second fins in a second column. 
     In another embodiment, a defining intersection is an intersection between the reference plane and a surface of the at least one external oblique fin. An angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 90°. 
     In another embodiment, the angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 45°. 
     In another embodiment, the heat exchanger further includes at least one of a plurality of transitional members, a plurality of supports and a plurality of skids. 
     In another embodiment, the heat exchanger further includes a helicoid transition member. 
     In another embodiment, the heat exchanger is a tube and shell type heat exchanger. 
     In accordance with another embodiment, a heat exchanger is provided. The heat exchanger includes a packing and a plurality of tubes. Each tube has an inlet, an outlet, a wall, and an axis. The plurality of tubes are substantially parallel to each other and separate from each other. The packing includes one or more external fins having a helicoid shape and having an axis of rotation that does not coincide with the tube axis. 
     In another embodiment, the axis of rotation is external to the plurality of tubes. 
     In another embodiment, the packing includes first external helicoidal fins twisted in a first direction and second external helicoidal fins twisted in a second direction opposite the first direction. 
     In accordance with another embodiment, a heat exchanger is provided. The heat exchanger includes a plurality of tubes, each tube having an inlet, an outlet, a wall, an outer surface, and an axis. The heat exchanger also includes a packing comprising a plurality of fins arranged in a plurality of columns, each column including at least one external oblique fin associated with a respective tube disposed along at least part of a length of the respective tube. 
     In one embodiment, the heat exchanger also includes a first fin in a first column having a first orientation, and a second fin in a second column having a second orientation different from the first orientation. 
     In another embodiment, each tube has a reference plane, wherein a defining intersection is an intersection between the reference plane and a surface of the at least one external oblique fin. The angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 90°. In another embodiment, the angle between the defining intersection and the tube axis in the direction of the inlet is greater than 0° and less than 45°. 
     In another embodiment, the heat exchanger includes at least one of a plurality of transitional members, a plurality of supports and a plurality of skids. 
     In another embodiment, the heat exchanger includes a helicoid transition member. 
     In another embodiment, the heat exchanger is a tube and shell type heat exchanger. 
     In another embodiment, the plurality of tubes are substantially parallel and separate from each other. 
     The present description anticipates that one skilled in the art will be able to calculate or experimentally determine optimal angles, shapes, spacings, overlaps, thicknesses, supports, skids, and materials of construction of the listed components and of divisions of the described components into sub-components, and such common and often necessary adjustments are within the scope of the present invention. 
     Although the present invention has been described in terms of certain preferred embodiments, various features of separate embodiments can be combined to form additional embodiments not expressly described. Moreover, other embodiments apparent to those of ordinary skill in the art after reading this disclosure are also within the scope of this invention. Furthermore, not all of the features, aspects and advantages are necessarily required to practice the present invention. Thus, while the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the heat exchanger or process illustrated may be made by those of ordinary skill in the technology without departing from the spirit of the invention. The inventions may be embodied in other specific forms not explicitly described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. Thus, scope of the invention is indicated by the following claims rather than by the foregoing description.