Abstract:
An air cooled, finned heat exchanger tube includes a metallic tube through which fluid to be cooled is flowable. The tube has an outer surface on which axially spaced axial indentations are formed over its circumference, and a plurality of axially spaced fins each of which has a main element formed with heat transfer promoting patterns and a base angled with respect to said main element. The base is frictionally and irremovably secured within a corresponding indentation so that said tube outer surface is completely covered.

Description:
FIELD 
     The present invention relates to the field of heat exchangers. More particularly, the invention relates to a fin configuration for air cooled heat exchanger tubes. 
     BACKGROUND 
     In air cooled heat exchangers, and particularly condensers, heat is transferred from the hot fluid that flows inside the tubes to the ambient air by passive or forced air flow on the external side of the heat exchanger tubes. The heat transfer between a naked tube and the air is very poor. To improve the efficiency of heat transfer, the heat transfer area of the tubes has been increased by adding fins to each tube. However, incompatible heat flow patterns from the fin to the air and from the tube to the fin showed that contact between a fin and tube not always led to efficient heat transfer of the heat in the fluid to the air. 
     Finned tubes of air cooled condensers are designed either with plate fins that slide onto the tube and are placed at a desired distance from one another on the tube or by continuously wrapped spiral fins. 
     A major problem in finned tubes that are not made of a single piece is the heat transfer between the tube and the fin. 
     With respect to one prior art fin design whereby the inner diameter of the plate fins is substantially equal to the outer diameter of the tube, the fins are brazed, galvanized, soldered or welded to the tube. Alternatively, the tube is made of relatively soft material which is inflated by a pressure pump. The tube diameter is thereby increased to facilitate fastening of the fin onto the tube. This fin design is used only on relatively small sized heat exchangers because of the cost. 
     In spiral wrapped fins that are usually made of a continuous aluminum ribbon, the fin is either embedded by force into a slot that is preformed on the tube or is wrapped into different types of L-shapes, such as wrap-on, knurled or double L. 
     US 2008/0023180 and US 2010/0155041 disclose finned tubes that are relief structured and that are manufactured by pressing the fin material into grooves formed in the external wall of the tube. The fins, which are substantially parallel to each other, have an annular shape and are substantially perpendicular to the tube. 
     Even though this fin configuration provides a significant increase in the heat transfer coefficient between the fin and air as a result of the relief structure formed on the fin, the point of contact between a fin and tube is small, constituting a limited and unreliable means of heat transmission between the fin and tube. An additional drawback of such a fin design is that the circumference of the tube surface between adjacent fins is bare, and therefore the metallic tube surface is subject to corrosion when exposed to e.g. precipitation, thereby lowering the heat transfer coefficient due to the build up of corrosion or requiring to be made of expensive corrosion resisting materials. 
     It is an object of the present invention to provide heat exchanger tubes having a higher tube-fin heat transfer coefficient than those of the prior art. 
     It is an additional object of the present invention to provide heat exchanger tubes to which fins are connectable by a large area connection that is not labor intensive. 
     It is an additional object of the present invention to provide a heat exchanger that can made from inexpensive tubes, without risk that they will corrode. 
     Other objects and advantages of the invention will become apparent as the description proceeds. 
     SUMMARY 
     The present invention provides an air cooled, finned heat exchanger tube, comprising a metallic tube through which fluid to be cooled is flowable, having an outer surface on which axially extending indentations are formed over its circumference, and a plurality of axially spaced fins each of which having a main element formed with heat transfer promoting patterns and a base angled with respect to said main element, wherein said base is frictionally and irremovably secured within said indentations such that said tube outer surface is completely covered by the base of said fins. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a cross-section view of a fin portion, according to one embodiment of the present invention; 
         FIGS. 2A, 2B and 2C  are respectively cross-sectional, front and plan views of a finned heat exchanger tube that includes the fin portion of  FIG. 1 ; 
         FIG. 3  is a cross-section view of a fin portion, according to another embodiment of the invention; 
         FIGS. 4-6  are a front view of a finned heat exchanger tube according to three embodiments of the invention, respectively; 
         FIG. 7A  is a front view of a finned heat exchanger tube according to another embodiment of the invention; 
         FIG. 7B  is a front view of a preformed ribbon for use in producing the fin of  FIG. 7A ; 
         FIG. 8A  is a front view of a finned heat exchanger tube according to another embodiment of the invention; 
         FIG. 8B  is a front view of a preformed ribbon for use in producing the fin of  FIG. 8A ; 
         FIG. 9  is a cross-section view of a base forming station; 
         FIG. 10  is a cross-section view of a ribbon after exiting from the base forming station of  FIG. 9 ; 
         FIG. 11  is a front view of an indentation forming station; and 
         FIG. 12  is a side view of a fin connecting station. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to an air cooled heat exchanger wherein its thin-walled tubes have fins that are fixed thereto advantageously by an automated large area connection. The fins may be provided with heat transfer promoters for causing enhanced air flow near the fin surface. The heat exchanger can often be used as e.g. a condenser for cooling turbine discharge, for example organic motive fluid circulating in an Organic Rankine Cycle circuit, although other types of air cooled heat exchangers are also within the scope of the invention. Such condensers can be used in geothermal power plants and other power plants such as waste heat power plants or other power units. 
       FIG. 1  illustrates a cross-section view of a fin portion, according to one embodiment of the present invention. A plurality of axially spaced fins, e.g. fins  4 - 7 , are attached to heat exchanger tube  3  through which a fluid to be cooled flows so that the fins are substantially perpendicular to the axis of tube  3 . Each of these fins is formed, and advantageously preformed, with a plurality of depressions  8 , which are depressed with respect to an adjacent planar fin portion  9  to provide an undulated appearance, in order to induce enhanced air flow within the air flowing across the fins and to thereby increase heat dissipation from the fluid flowing in heat exchanger tube  3 . 
     As shown in  FIG. 2 , fin  4  may be made of an annular plate, e.g. made of aluminum and having a thickness ranging from 0.4-1.2 mm, surrounding, and connected to, tube  3 . The plurality of depressions  8   a - c  circumferentially extend throughout the plate and are advantageously concentric, while the radial spacing between adjacent depressions corresponding to the position of a planar portion  9  may differ. 
     Referring back to  FIG. 1 , each fin is also formed with a base  11  that is angled with respect to proximal end  13  thereof, i.e. the most radially inwardly disposed planar fin portion, for contacting, and being connected to, the tube surface by a large area connection. Base  11  is shown to be angled with respect to proximal end  13  by advantageously a right angle, but it will be appreciated that proximal end  13  can be oriented at any other desired angle with respect to base  11 . 
     Base  11  may extend in an opposite direction as the direction to which each undulation  8  is depressed. Advantageously, the length of base  11  is greater than the depression depth of each undulation  8 , and may be sufficiently long to contact the proximal end  13  of the adjacent fin and to cover the entire circumference of the tube surface, thereby preventing the build up of corrosion. Alternatively, the proximal end  13  of one fin may overlap the base  11  of an adjacent fin. 
     For example three depressions having a depth of 1.8 mm and a radial dimension of 2.0 mm are formed in an aluminum ribbon having a width of 20.0 mm, reducing the radial dimension of the fin to 15.875 mm. The base contacting the proximal end of the adjacent fin has an axial dimension of 2.42 mm. 
     In  FIG. 3 , fins  4 ′- 7 ′ are provided with a stepped connection whereby base  11  is connected to tube  3  by a large area connection and an additional surface  15  substantially parallel to base  11  extends to the fin proximal end  13 . Such structure provides an enhanced cover to the entire circumference of the tube surface, thereby providing additional means for preventing the build up of corrosion. 
     The fins may be formed with any other desired pattern. In  FIG. 4 , fin  19  is formed with a plurality of circumferentially and axially spaced dimples  21 , e.g. having a diameter of 4-6 mm. In  FIG. 5 , fin  24  is formed with a plurality of circumferentially spaced circular apertures  27 , e.g. circumferentially spaced by an angle of 30 degrees and having a diameter of one-third to one-fourth the radial dimension of the fin. In  FIG. 6 , fin  28  is formed with three rows of circumferentially spaced small-diameter apertures  29 . 
     The depressions may also be formed with discontinuities. In  FIGS. 7A-B , annular depressions  36  of fin  33  have aligned circumferential discontinuities  38  formed by preformed inclined slots  41  in an aluminum ribbon  40 . In  FIGS. 8A-B , annular depressions  46  of fin  43  have broken circumferential discontinuities  48  formed by preformed vertical slots  51  in an aluminum ribbon  50 . 
     The use of a base in conjunction with the patterned fins of the present invention not only provides added heat transfer for the heat exchanger by virtue of the added heat transfer area connected to the tube surface, but also provides protection against possible corrosion. Vigorous air flow in and in the vicinity of the patterned fins can induces entrainment of rain droplets, or droplets of any other type of precipitation, onto the tube surface. The passage of precipitation derived moisture onto the tube surface is exacerbated by the presence of the preformed patterns, such as apertures  27  of  FIG. 5  or discontinuities  38  of  FIG. 7A , through which precipitation directly falls onto the tube surface, or by the presence of depressions  8  of  FIG. 1  or dimples  21  of  FIG. 4  in which rainwater can accumulate and then drain onto the tube surface. However by forming the fins with a base such that the tube surface is completely covered, precipitation is assured of not reaching the tube surface, thereby preventing potential corrosion. The tube may therefore be made of inexpensive material such as carbon steel by virtue of the corrosion preventing feature afforded by the base, rather than expensive corrosion resisting materials such as stainless steel. 
     The production of a patterned fin with an angled base needs particular care and design since the preformed patterns weaken the tensile strength of a machine fed ribbon of aluminum, or of any other material from which the fins are made, thus rendering a fin assembly operation more prone to failure. Despite the fragility of the preformed aluminum ribbon, the fins are advantageously able to be automatically connected to a tube. 
       FIGS. 9-12  illustrate apparatus for automatically fabricating a tube finned with a base. As the aluminum ribbon is fed, the base is formed and the tube is rotated, allowing the base to be securely connected to the tube surface. 
     With reference to  FIG. 9 , aluminum ribbon  60  is fed to base forming station  62  comprising lower roller  66  and upper roller  67  provided with an angled periphery  69 , for example angled by 20 degrees with respect to the horizontal plane. While ribbon  60  is longitudinally fed when upper roller  67  is being rotated, angled periphery  69  applies a force onto the ribbon to form a base  61  which is angled with respect to elongated portion  63 , as shown in  FIG. 10 . 
     During the formation of the base, a very long tube  72 , e.g. having a length of 20 m, is fed while being spun through an indentation forming station  74  shown in  FIG. 11  prior to connection with the fins. Station  74  comprises two lower rollers  75  and  76  between which tube  72  is fed. Both lower rollers are toothed wheels configured with teeth  79 , or other types of protrusions, on a region of its outer face. Teeth  79  cause shallow axial indentations  86 , e.g. of 0.1 mm depth, to be formed on the circumference of the tube surface. 
     Tube  72  is longitudinally advanced by means of upper roller  83 , which is raised and lowered by a piston driven mechanism  89  as well as lower rollers  75  and  76 . Upper roller  83  and lower rollers  75  and  76  may be oriented at an incline with respect to the tube surface, so that they will frictionally engage the tube surface and cause the tube to be longitudinally displaced. 
       FIG. 12  illustrates the fin connecting station  88 . After tube  72  exits indentation forming station  74 , it is fed to fin connecting station  87 . While tube  72  continues to spin, the aluminum ribbon is fed to station  87  after exiting the base forming station. Fin connecting station  88  comprises a synchronized external friction wheel  92  driven by a piston mechanism  94  for applying force onto the base so that the latter will be secured into a corresponding indentation throughout the circumference of the tube. 
     While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without exceeding the scope of the claims.