Patent Publication Number: US-9844807-B2

Title: Tube with fins having wings

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The current invention describes finned tubes used for heat transfer, such as the tubes used in shell and tube heat exchangers. 
     Description of the Related Art 
     Finned tubes have been used for heat transfer for many years. Heat flows from hot to cold, so heat transfer is accomplished by conducting heat from a warmer material to a cooler material. There is also heat given off when a material condenses from a vapor to a liquid, and heat is absorbed when a liquid vaporizes or evaporates from a liquid to a vapor. When finned tubes are used for heat transfer, the warmer material is on either the inside or the outside of the tube and the cooler material is on the other side. Usually the tube allows for the transfer of heat without mixing the warmer and cooler materials. 
     For cooling purposes, a cooling medium can be a liquid such as cooling water flowing through a shell and tube heat exchanger, or it can be a gas such as air blown over a finned tube. Similarly, a heating medium can be either a liquid or a gas. Finned tubes are sometimes used instead of relatively smooth tubes because finned tubes tend to increase the rate of heat transfer. Therefore, a smaller heat exchanger with finned tubes may be able to transfer as much heat in a given application as a larger heat exchanger with relatively smooth tubes. The design of finned tubes affects the rate of heat transfer and sometimes the tubes are designed differently for specific heat transfer applications. For example, finned tubes used for condensation tend to have different designs than finned tubes used for evaporation. 
     Examples of the prior art include finned tubes with helical ridges formed on an inner surface of the tube and fins formed on an outer surface of the tube. A channel is defined by adjacent fins on the tube outer surface, and this channel can have a curved, “U” shaped bottom or the channel can have a flat bottom. When used as condensing tubes with the condensing vapor on the outside of the tube and coolant inside the tube, the channels tend to become filled with liquid condensate. The condensate serves to insulate the tube and restrict the cooling needed for further condensation. The flat bottom is preferred because condensate tends to spread out along the bottom of the flat channel instead of creeping up the sides of the fins. This leaves more surface area on the fins free of condensate which enhances heat transfer. 
     Finned tubes also have had breaks formed in the fins so condensate flowing within a channel between two fins could flow through a break and enter a different channel. Other finned tubes have had the outer portion of the fin bent over so that a bend is formed part of the way between a base of the fin and a top of the fin. This creates additional angles in the fin which tends to cause the tube to shed liquid condensate more rapidly. When liquid condensate is shed from a tube more rapidly, it tends to enhance heat transfer. Other fins have had notches formed in the fin tip with peaks defined between the notches. In some cases the peaks are bent over to form a curl shape. This again increases curvature and angles in the fin and thereby tends to cause the tube to shed liquid condensate more rapidly. 
     Some finned tubes are produced by attaching fin material to a relatively smooth tube so the fins are not formed from the material of the tube body. This increases the area available for heat transfer, which does improve heat transfer rates, but the interface between the fin and the tube does cause some resistance to heat flow. The fins attached to the tube can extend radially from a tube axis so they stand straight up from the tube, but they can also be curved or bent in various ways to improve heat transfer. There are many designs of finned tubes in existence, but any change which improves heat transfer is always welcome. 
     BRIEF SUMMARY OF THE INVENTION 
     A tube used for heat transfer has fins extending from an outer surface of the tube. The fins are formed from the material of the tube outer surface, so the fins are monolithic with the tube body. Wings extend from a side surface of the fin between a fin base and a fin top. The wings can extend to approximately the center of a channel defined by two adjacent fins such that the wings split the channel into an upper channel and a lower channel. The tube can include helical ridges formed on an inner surface of the tube, and the tube can include depressions formed in the fin tops. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view of a section of the finned tube. 
         FIG. 2  is a side sectional view of the finned tube. 
         FIG. 3  is a top view of the outer surface of the tube. 
         FIG. 4  is a perspective view of a section of the finned tube with depressions in the fin top. 
         FIG. 5  is a side sectional view of an embodiment of the finned tube with opposite wings at different wing heights. 
         FIG. 6  is a side section view of an embodiment of the finned tube with a wing on only one fin side wall. 
         FIG. 7  is a side view of an arbor and inner support with a sectional view of a tube side wall between the arbor and inner support. 
     
    
    
     DETAILED DESCRIPTION 
     The finned tube of the current invention is used for heat transfer, and primarily for condensation of a liquid onto the tube outer surface. In a typical example, a cooling liquid flowing through the tube interior absorbs the heat of condensation as a vapor condenses. The design of the fins on the tube outer surface increase heat transfer by increasing surface area of the tube, and by improving the tube&#39;s condensate shedding ability. Other aspects of the tube design also improve heat transfer rates. The tube is most often used in the construction of shell and tube heat exchangers, but it is also possible to use the finned tube in other heat transfer applications. 
     Condensation Principles 
     When heat is transferred from a condensing vapor on the outside of a tube to a cooling liquid on the inside of a tube, the heat transfer is considered in several distinct steps. The same basic steps apply when heat is transferred through a barrier, such as a tube wall, between any two mediums with different temperatures. This description is directed towards a condensing vapor on the outside of the tube and a cooling liquid on the inside of the tube, but different applications are possible. 
     The vapor outside the tube has transfer heat to cooling liquid inside the tube. As a vapor condenses, a specific amount of heat referred to as the heat of condensation is given off. There is generally a layer of liquid condensate on the tube outer surface, so the first step is the transfer of heat from the vapor to the condensate on the tube. The heat then flows through the condensate, and condensate often resists heat flow because it acts as an insulator. After heat flows through the condensate, it is transferred from the condensate to the tube outer surface. There is an interface between the condensate and the tube outer surface, and any interface provides some resistance to heat flow. 
     Once heat is transferred to the outer surface of the tube, it has to flow from the outer to the inner surface of the tube. To facilitate this heat flow, heat transfer tubes are usually made out of a material which readily conducts heat, or a heat conductor. Generally there is a thin layer of liquid contacting the inner surface of the tube wall which is essentially stagnant. After the heat flows through the tube wall, it must be transferred through the interface between the inner surface of the tube wall to the adjacent layer of cooling liquid inside the tube. Heat then has to flow through this thin layer of liquid. 
     The more turbulent or rapid the flow of cooling liquid within the tube, the thinner the layer of stagnant liquid sitting next to the tube wall. Therefore, tube designs which cause mixing or agitation of the liquid within the tube provide a benefit. Turbulent flow causes mixing of the cooling liquid, as compared to laminar flow, and higher cooling water flow rates can increase the turbulence of the cooling water. Features of the tube inner surface can also increase the turbulence and mixing of the cooling liquid inside the tube. Heat transferred to the flowing cooling liquid in the tube is then carried away as the liquid exits the tube. 
     An interface between the fins and the tube exists if the fins are constructed separately from the tube, and then attached. This is true if the fin and tube are constructed of the same material, such as copper, or from different materials. Any interface causes some resistance to heat flow. If the fins are formed from the tube wall, there is no interface and heat flow is improved. In this discussion, fins formed from the tube wall are referred to as being monolithic with the tube, and it is preferred that fins be monolithic with a tube to minimize resistance to heat flow. 
     The tube should be made from a malleable substance so the fins can be formed from the tube without cracks or breaks forming in the tube wall. Cracks or breaks limit the structural integrity and strength of a tube. Generally these tubes are used in shell and tube heat exchangers, and the ends of the tubes are affixed in tube sheets of the heat exchanger. A malleable tube can be easier to install in a heat exchanger tube sheet. The tube should also be constructed from a material which readily conducts heat. Copper is often used in tube construction because of its malleability and heat conducting properties. 
     Finned tubes have design considerations specifically related to the collection of condensate on the tube outer surface. Some tubes are better at shedding the condensate than others. If condensate is shed more rapidly, the layer of condensate on the tube is thinner and there is less resistance to heat flow. Therefore, a tube that more rapidly sheds condensate tends to be preferred because it provides a more rapid heat flow. 
     One aspect that causes a tube to shed condensate more quickly is the ability of the outer surface to concentrate the condensate into drops. This is frequently done by having sharp points or curves on the outer surface. If a sharp point or curve is concave in nature, it tends to act as an accumulation site for condensate drops because surface tension tends to cause the condensate to collect in concave surface features. Convex surfaces tend to avoid condensate because surface tension effects tend to cause the condensate to avoid these areas. Therefore, convex areas tend to remain relatively free of condensate and have less resistance to heat flow. Concave areas tend to concentrate condensate into drops which can then more rapidly fall from the tube, so the tube sheds condensate more quickly. Curves or sharp points generally produce both convex and concave surfaces at different locations, which promotes more rapid condensate shedding, as well as areas on the tube with very little or no condensate which more rapidly transfer heat. 
     It is also true that the more surface area on a condensing tube, the more rapid the flow of heat. When fins are formed on a tube it increases the surface area of the tube, which serves to increase the rate of heat transfer across the tube. Other deformations in the tube outer surface which increase surface area will also tend to increase the rate of heat transfer. 
     Finned Tube Main Body 
     One embodiment of the finned tube  10  of the current invention is shown in different perspectives in  FIGS. 1, 2 and 3 . The tube  10  includes a main body  12  which has an outer surface  14  and an inner surface  16 . The main body  12  is the base for any shapes or structures on the outer or inner surface  14 ,  16 . This main body should be made of a material which conducts heat readily. Metals are generally good conductors and are frequently used for the construction of tubes of the current invention. The material should also be malleable such that the various structures on the inner and outer surface  14 ,  16  can be formed without damaging the integrity of the tube body  12 . This allows for the structures formed from the tube body  12  to be monolithic with the tube body  12 . 
     Tube Fins 
     The tube  10  has at least one fin  20  formed on its outer surface  14 . The fin  20  generally protrudes or extends circumferentially from the tube body outer surface  14 , and is usually helical. It is possible that one single fin  20  is helically wound around the entire length of the tube  10 . It is also possible that there will be a plurality of fins  20  which are all received helically around the tube  10 . In either case, when looking at a section of the tube body outer surface  14 , it will appear as though there are several adjacent circumferential fins  20  protruding from the tube body outer surface  14 . When viewed along the axial direction of the tube  10 , fin  20  sections next to each other are referred to as adjacent fins  20  despite the fact that they might be the same fin  20  helically wrapping around the tube body outer surface  14 . The fin  20  is formed from the material of the tube body  12 , so the fin  20  is monolithic with the tube body  12 . 
     Each fin  20  has several parts including a fin base  22  at the point where the fin  20  connects to the tube body outer surface  14 . The fin top  24  is opposite the fin base  22  and is the highest point of the fin  20  relative to an axis of the tube  10 . A fin side wall  26  includes a first side wall  28  and a second side wall  30  opposite the first side wall  28 . A channel  32  is defined between two adjacent fins  20 , and the channel  32  has a channel center  34 . The channel center  34  is equidistant from the two adjacent fins  20  which form the channel  32 . The fin  20  can be approximately perpendicular to the tube body  12  such that the fin  20  extends essentially straight out from the tube body outer surface  14 . In such a case, the fin  20  would extend radially from the tube  10 . It is also possible for the fin  20  to be positioned at other angles to the tube body outer surface  14 . 
     The fin top  24  can have a plurality of depressions  36 , as best seen in  FIG. 4 . The depressions  36  have a skew angle  38  which is defined by the angle of the depression  36  relative to the fin top  24 . The skew angle  38  can range between 0 to 90° such that the depression  36  can be perpendicular to the fin  20  or the depression  36  can be set at a different angle to the fin  20 . The depression has a depth  40  which generally ranges between 0.01 to 0.5 millimeters. A plurality of peaks  42  are defined between adjacent depressions  36 . When depressions  36  are formed in the fin top  24 , a platform  44  can be formed extending from the fin top  24 . The platform  44  extends from the fin top  24  at the depressions  36  because the fin top  24  undulates up and down with the depressions  36  and peaks  42 . The plurality of platforms  44  provides additional curvature, angles, and surface area in the fin  20 . 
     Wings 
     Referring again to  FIGS. 1, 2 and 3 , the fin  20  includes a wing  50  extending or protruding from the fin side wall  26  between the fin top  24  and the fin base  22 . The wing  50  can be positioned near the middle of the side wall  26 , closer to the fin top  24 , or closer to the fin base  22 , but not at the fin top  24  or the fin base  22 . Preferably, there are a plurality of wings  50 , and the wings  50  can be approximately perpendicular to the fin side wall  26  or they can be set at other angles to the fin side wall  26 . When more than one wing  50  is on one fin side wall  26 , a gap  58  is defined between adjacent wings  50 . The wings have a height  52  defined as the distance from the fin base  22  to a wing upper surface  54 . If the wing  50  is set at an angle other than 90° to the fin side wall  26 , the wing height  52  is defined as the distance from the fin base  22  to the highest point on the wing upper surface  54 . 
     The wing  50  has a wing base  56  at the point where the wing  50  connects to the fin side wall  26 . Generally, the wing base  56  is approximately parallel to the fin base  22 , but it is possible for the wing base  56  to be at an angle which is not parallel with the fin base  22 . The wing  50  can extend from the side wall  26  to approximately the channel center  34 , but the wing  50  can extend to a point short of the channel center  34  or even a point beyond the channel center  34 . Wings  50  can extend from both the fin first side wall  28  and the second side wall  30  such that wings  50  from adjacent fins  20  each reach into the channel  32  defined between the adjacent fins  20 . The wings  50  extending from adjacent fins  20  into the channel  32  can be aligned, as shown, but it is also possible that wings  50  are staggered such that a wing  50  extending into the channel  32  would be positioned across from the gap  58  between two wings  50  on the adjacent fins  20 . 
     The surface area of the wings  50  is maximized by extending the wings  50  to approximately the channel center  34 . When reference is made to extending the wings  50  to approximately the channel center  34 , it is intended to mean that wings  50  opposite each other extending together form an effective barrier such that liquids will not easily pass between the wings  50 . This does not mean the opposite wings  50  have to actually touch at the channel center  34 , but the wings  50  should be close to each other, and it is acceptable if the wings  50  do actually touch. This effective meeting of opposite wings  50  at the channel center  34  can aid in condensation, because the wings  50  can interact with each other to affect the surface tension of the liquid to aid in the overall condensation efficiency of the tube  10 . 
     The wing  50  splits the channel  32  into an upper channel  60  and a lower channel  62 . Condensate can flow through both the upper and lower channels  60 ,  62  and more inter-channel flow can be accommodated by various positions for the wing  50 . One example of this is shown in  FIG. 5 , with different wing heights  52  on the first and second fin side walls  28 ,  30 . In this manner, the wings  50  extending into the channel  32  have a larger space  63  between the wings  50 , which does not restrict the flow of condensate as much as if the facing wings  50  were at the same wing height  52 . A second option for improving inter-channel flow involves positioning a wing  50  on only one fin side wall  26 , as seen in  FIG. 6 . In this case, one of the first and second side walls  28 ,  30  has a wing  50  extending from it, and the other of the first and second side wall  28 ,  30  does not. 
     Referring now to  FIGS. 1, 2, and 3 , the wing  50  has a side surface  64  extending from the wing base  56  to a wing terminus  66 . The side surface  64  faces the gap  58  defined between adjacent wings  50  on a single fin side wall  26 . A plurality of shelf walls  68  can be included, wherein the shelf walls  68  extend between the wing side surfaces  64  and the fin side wall  26 . The shelf wall  68  extends upwards from the wing  50 , so the shelf wall  68  extends towards the fin top  24 . An angle or sharp point is produced at the wing side surface  64 , where the wing  50  and the shelf wall  68  intersect. This angle generally varies between about 90 degrees and about 170 degrees. The shelf wall  68  can point essentially directly towards the fin top  24 , but a more gradual angle at the wing side surface  64  results in the shelf wall  68  pointing at an angle towards the fin top  24 . 
     The shelf wall  68  aids in condensation, because it provides several sharp points and angles. The shelf wall  68  has sharp points  69  (as shown in  FIG. 1 a   ) and angles at the intersection and connection point with the fin side wall  26 , at the intersection and connection point with the wing side surface  64 , and at a shelf wall outer edge where the end of the shelf wall  68  projects into the channel  32 . There are angles and sharp points on both the top and bottom surfaces where the shelf wall  68  intersects other structures. Besides connecting the wing side surface  64  and the fin side wall  26  and creating sharp points and angles, the shelf wall  68  also increases surface area, which aids in condensation. 
     The wing  50  generally provides a relatively flat wing upper surface  54  with clearly defined boundaries. The wing base  56  is generally a straight line, as well as the two wing side surfaces  64  and the wing terminus  66 . These four generally straight boundaries provide a wing upper surface  54  with a quadrilateral shape. Each boundary of the wing  50  provides a sharp point or an angle to improve condensation. 
     It is possible to provide too many wings  50  such that condensate can become trapped in the lower channel  62 . This could hinder the ability of the tube  10  to shed condensate. Therefore, the gap  58  between adjacent wings  50  on a single fin side will  26  and the space  63  between wings  50  on facing fin side walls  26  has to be considered in the design of the current invention. Related considerations include the wing heights  52  of opposing wings  50  extending into a single channel  32 , and the distance between the wing terminus  66  and the channel center  34 . 
     Channel Mark 
     Channel marks  70  can be formed on the tube body outer surface  14  within the fin channel  32 . Channel marks  70  are basically a recess defined in the tube body outer surface  14 . The channel mark  70  can be continuous or intermittent, wherein a continuous channel mark  70  would be similar to a groove formed circumferentially around the tube  10  within the fin channel  32 , and intermittent channel marks  70  would be a plurality of discreet depressions defined in the fin channel  32 . The channel marks  70  shown are intermittent. The channel marks  70  can be formed basically a line, so that the channel marks  70  define a channel line  72 . The channel line  72  can be approximately parallel with the fin channel  32  or the fin base  22 . The channel line  72  is defined by the row of channel marks  70 . 
     There can be one channel line  72  or a plurality of channel lines  72  within one fin channel  32 . The channel lines  72  can be at or near the channel center  34 , they can be offset from the channel center  34  near the fine base  22 , or they can be anywhere in between. If there are two or more channel lines  72  and the channel marks  70  are intermittent, the channel marks  70  can be simultaneous or alternating. If the channel marks  70  are simultaneous, they will be aligned directly across from each other, as shown. If the channel marks  70  are alternating, they will be aligned such that the channel marks  70  in one channel line  72  are not directly across from channel marks  70  in another channel line  72  within the same fin channel  32 . 
     The channel marks  70  can have a multitude of shapes. They can be square, rectangular, trapezoidal, polygonal, triangular or almost any other shape. The channel marks  70  tend to serve as nucleation sites for condensation. They also serve as sharp corners or angles which tend to aid in drop formation because they provide an accumulation site for the condensate. The channel marks  70  also increase surface area, which helps with heat transfer. The channel marks  70  can extend into the tube body  12  and therefore they can reduce the strength of the tube  10 . Therefore, the channel marks  70  and channel line  72  can be positioned near the fin base  22 , where the thickness of the tube body  12  can be larger. 
     Inner Surface Ridges 
     Heat transfer across the tube  10  can be improved by providing better transfer of heat from the tube body inner surface  16  to the cooling liquid within the tube  10 . Ridges  74  can be defined on the tube body inner surface  16  to help facilitate more rapid heat transfer. The ridges  74  on the inner surface  16  are generally helical and have a depth  76  and a frequency. The frequency is the number of ridges  74  within a set distance. The ridges  74  are also set at different cut angles relative to the tube axis. The depth  76  and the frequency of the ridges  74  can vary, and the cut angle can be set to cause the cooling liquid to swirl within the tube  10 . A swirling liquid tends to increase heat transfer by increasing the amount of agitation within the cooling liquid. 
     Tube Forming Process 
     Finned tubes  10  are generally formed from relatively smooth tubes  10  with a tube finning machine, which is well known in the industry. The tube finning machine includes an arbor  80  as seen in  FIG. 7 , with continuing reference to  FIGS. 1, 2, and 3 . Frequently, a tube finning machine will include three or more arbors  80  positioned around the tube  10 , so the tube  10  is held in place by the arbors  80 . The arbors  80  are positioned and angled such that each complements the others. A tube is provided and fed through the finning machine such that a tube wall  82  is positioned between the arbor  80  and an inner support  84 . The arbor  80  deforms the tube outer surface  14 , and the inner support  84  can deform the tube inner surface  16 . The tube wall  82  is generally rotated relative to the arbor  80  and moves axially with the inner support  84  as it rotates. 
     The arbor  80  generally includes several fin forming discs  86  which successively deform the tube wall  82  to form one or more helical fins on the tube outer surface  14 . Successive finning discs  86  tend to project deeper into the tube wall  82  such that fins  20  are formed and pushed upwards by the finning discs  86 . The inner support  84  can include recesses  88  such that helical ridges  74  are formed on the tube inner surface  16  as fins  20  are formed on the tube outer surface  14 . 
     After the finning discs  86  have formed the fins  20 , various other discs can be included on the arbor  80  to further deform and define aspects of the final tube  10 . These remaining discs can be included or excluded, as desired. After the finning discs  86 , the channel mark disc  90  can be used to form channel marks  70  in the channel  32  defined by adjacent fins  20 . After the channel mark disc  90 , one or more wing forming discs  92  can be used to form wings  50  on the fin side surfaces  28  between the fin base  22  and the fin top  24 . The wing forming disc  92  also forms the shelf wall  68 , and the shape of the teeth on the wing forming disc  92  determine the shape and angle of the shelf wall  68 . After the wing forming disc  92 , a depression forming disc  94  can be mounted on the arbor  80 . The depression forming disc  94  creates depressions in the fin top  24 . In this manner, the various deformations of the original relatively smooth tube  10  are produced. There are other possible orders and designs of discs which can be used to achieve similar results. 
     Example Dimensions 
     The dimensions of the current invention can vary, but example dimensions are provided below which will give the reader an idea as to at least one embodiment of the current invention. 
     The inter-fin distance is the distance between a center point of two adjacent fins  20  and this distance can be between 0.3 and 0.7 millimeters. 
     The fin  20  has a thickness above the wing  50  which is referred to as the fin thickness, and this thickness can be between 0.05 and 0.2 millimeters. 
     The fin  50  has a height measured from the fin base  22  to the fin top  24 , and the fin height would be measured from the fin base  22  to the fin top  24  at a peak  42  if the fin had depressions  36 , and the fin height can be between 0.7 and 1.5 millimeters. 
     The wing  50  has a height  52  measured from the tube body outer surface  14  to the wing upper surface  54 , and this wing height  52  can be between 0.15 and 0.6 millimeters. 
     The wing  50  has a thickness from the wing upper surface  54  to a bottom portion of the wing  50  which can be between 0.1 and 1 millimeter. 
     The fin side wall  26  has a depth below the wing  50  which can be between 0.2 and 0.6 millimeters. 
     The channel marks  70  have several dimensions. They have a length which is measured along the circumference of the tube  10 , and this length can be between 0.1 and 1 millimeter. The channel mark  70  has a width which is measured along the axis of the tube  10 , and this width can be between 0.1 and 0.5 millimeters. The channel mark  70  also has a depth which can be between 0.01 and 0.2 millimeters. 
     The depression  36  formed in the fin top  24  has a depth  40  which can vary between 0.01 and 0.5 millimeters, and the depression  36  has a width which can vary between 0.01 and 1 millimeter. 
     The ridge  74  formed on the tube body inner surface  16  has a height, and this height can be between 0.1 and 0.5 millimeters. The internal ridge angle with the axis can be set at 46°, and the ridge starts can vary between 8 and 50. 
     The outside diameter of the tube  10  can be 19 millimeters. The tube wall  82  has a thickness which can be 1.04 millimeters. 
     The wing spread, which includes the gap between adjacent wings  50  and one wing  50 , can be between 0.6 and 6 millimeters. The wing spread would be measured from the start of one wing  50  to the start of the next adjacent wing  50 . The wing width as measured along the wing base  56  can be between 0.1 and 0.5 millimeters. 
     Tube Benefits 
     The tube  10  as described is very effective when used for condensing a vapor on the outside surface  14  with a cooling liquid passed through the tube interior. This type of use in one example of how the tube  10  can be used. Condensation is facilitated because the outer surface  14  has lots of angles and sharp corners, and these angles and sharp corners provide areas where surface tension tends to cause the condensate to form into drops. When these drops are formed, they fall off the tube  10  more readily, so the tube  10  sheds condensate more quickly. Also, the channels  32  between the fins  20  facilitate flow of the condensate, which improves the rate at which drops escape or fall from the tube  10 . This also improves the condensate shedding ability of the current invention. Condensate tends to avoid areas with convex curves, such as the edges of fins  20 , wings  50 , shelf walls  68 , and platforms  44 , because of surface tension effects. These relatively condensate-free areas provide less resistance to heat flow, which further promotes condensation rates. 
     The fins  20 , wings  50 , shelf walls  68 , depressions  36 , platforms  44 , and channel marks  70  all add surface area to the tube outer surface  14 . Heat flows across a surface, so more surface area tends to increase the rate of heat flow. Therefore, any formations on the tube outer surface  14  which increase surface area tends in increase the rate of heat flow. 
     The tube inner surface  16  also promotes heat transfer because the ridges  74  can cause turbulence and swirling of the cooling liquid. This turbulence and swirling cause a mixing which minimizes laminar flow, and also tends to minimize the depth of the liquid layer directly adjacent to the tube inner surface  16 . The ridges  74  also increase the surface area of the inner surface  16 , which facilitates heat transfer. A higher ridge frequency and/or a larger ridge depth  76  tends to increase heat transfer rates, but higher ridge frequencies and/or deeper ridges  74  also tend to increase resistance to flow of the cooling liquid through the tube  10 . A lower flow rate of cooling liquid can slow heat transfer. Therefore, a balance must be struck for the best heat transfer conditions. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed here. Accordingly, the scope of the invention should be limited only by the attached claims.