Patent Application
20090008069
This patent application claims the benefit of Chinese Patent Application #200710043537 filed Jul. 6, 2007, the title of which has been translated as A HEAT TRANSFER TUBE FOR CONDENSING APPLICATION.
A. Field of the Invention
This invention relates to finned tubes, especially as such tubes are used for heat transfer.
B. 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 or depressions formed in the crest of the fin with peaks defined between the depressions. 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. There are many designs of finned tubes in existence, but any change which improves heat transfer is always welcome.
The current invention includes a tube with a helical fin extending from the tube's outer surface. The fin and tube are formed of one part so the fin is monolithic with the tube. The fin has a fin crest with peaks and depressions between the peaks, and there is a channel defined between adjacent fins. There is an indent defined in the fin peaks along a side surface, and the indent includes a flank surface intersecting the top of the fin peak. An indent base surface extends into the channel defined between two fins, such that the indent forms a step on the side of the fin peak. The tube fins and deformations enhance the rate of heat transfer across the tube.
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 the surface area of the tube, and by improving the tube'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.
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, or 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 to transfer heat to a cooling liquid inside the tube. As a vapor condenses, a specific amount of heat is given off, and this quantity of heat is referred to as the heat of condensation. 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 liquid 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 provides 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, such as a rough surface or helical grooves which promote swirling. 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 10 are used in shell and tube heat exchangers 8, and the ends of the tubes are affixed in tube sheets 6 of the heat exchanger, as shown in
Finned tubes 10 have design considerations specifically related to the collection of condensate on the tube outer surface. Often, a heat exchanger 8 will use several layers of horizontal tubes 10, with the condensate from the top layers dripping onto the bottom layers. The tubes 10 in the lower layers tend to become flooded, or covered in liquid condensate, which lowers the heat transfer efficiency. Some tubes 10 are better at shedding condensate than others. If condensate is shed more rapidly, the layer of condensate on the tube 10 is thinner and there is less resistance to heat flow. The condensate shedding ability is especially important for tubes 10 in the lower parts of a horizontal heat exchanger 8. Therefore, a tube 10 that more rapidly sheds condensate tends to be preferred because it provides a more rapid heat flow throughout the heat exchanger 8.
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. Concave areas tend to concentrate condensate into drops which can then more rapidly move around and fall from the tube, so the tube sheds condensate more quickly. Condensate tends to avoid convex surfaces because surface tension effects tend to pull or draw the condensate away from convex areas. Therefore, convex areas tend to remain relatively free of condensate and have less resistance to heat flow. 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 to 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.
One embodiment of the finned tube 10 of the current invention is shown in different perspectives in
The tube 10 has at least one fin 20 formed on its outer surface 14. The fin 20 generally protrudes or extends radially 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 crest 24 is opposite the fin base 22 and is the highest point of the fin 20 relative to a tube axis 26. A fin height 27 is the distance between the fin base 22 and the fin crest 24. A fin side wall 28 includes a left side wall 30 and a right side wall 32 opposite the left side wall 28. A channel 34 is defined between two adjacent fins 20 such that the channel 34 is between a left side wall 30 of one fin 20 and a right side wall 32 of an adjacent fin 20. 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 perpendicular from the tube axis 26. It is also possible for the fin 20 to be positioned at other angles to the tube body outer surface 14.
The fin crest 24 can have a plurality of depressions 36, as best seen in
As the depressions 36 are cut into the fin crest 24, the material that was in the depression 36 can be pushed to the side, such that the peak 42 is twisted at an angle to the fin 20. This results in a portion of the fin peak 42 extending into the channel 34 between adjacent fins 20, so the fin peak 42 can have what looks like a beak 45. This beak 45 is at least partially above the channel 34, so that a line perpendicular to the tube axis 26 would pass through the channel 34 and part of the beak 45, but not pass through any other part of the fin 20. Any further shape changes to the beak 45 area would also be at least partially above the channel 34, as opposed to being above the fin 20.
The beak 45 is part of what is referred to in this discussion as a platform 44. The platform 44 is a portion of the fin crest 24 which is formed to extend over an adjacent channel 34, instead of over the fin 20. The beak 45 is the tip of the platform 44, when the platform 44 is formed from the fin peak 42. It is also possible for a platform 44 to be formed from the depression 36. This results when the material from the depression 36 is pushed downward during the depression formation. It is even possible for there to be a platform 44 extending from both the peak 42 and the depression 36 at the same time. The plurality of platforms 44 provides additional curvature, angles, and surface area in the fin 20, and also provides material which can be formed and utilized to produce even more curvature and angles.
The fin peaks 42 can be viewed as having side surfaces 46, including a left peak side surface 48, a right peak side surface 50, a front peak surface 52, and a back peak surface 54. The left peak side surface 48 merges into and forms a part of the left fin side wall 30, and the right peak side surface 50 merges into and forms a part of the right fin side wall 32. For certain depression skew angles 38, the left and right peak side surfaces 48, 50 can partially merge or face the depression 36. Also, the left and right peak side surfaces 48, 50 can be pushed and formed to be over the channel 34 next to the fin 20. However, the left and right peak side surfaces 48, 50 are still considered a part of the left and right fin side walls 30, 32.
The peaks 42 also have a front surface 52 and a back surface 54, where the front and back surfaces 52, 54 generally face up or down the fin crest 24, or toward adjacent depressions 36. As with the left and right peak side surfaces 48, 50, the front and back peak surfaces 52, 54 can face somewhat into the channel 34 depending on the depression skew angle 38, as shown. The left and right peak side surfaces 48, 50 can also be generally vertical, but they face into the channels 34 on either side of the fin 20. When reference is made to the peak left, right, front and back surfaces 48, 50, 52, 54 being generally vertical, it means the surfaces tend to run more up and down relative to the tube outer surface 14, instead of more parallel with the tube outer surface 14.
The four peak surfaces 48, 50, 52, 54 do not necessarily form smooth, straight, flat surfaces, but may form bulging, rough, curves surfaces. There are not necessarily clear, angular demarcations between adjacent peak surfaces, so the left side surface 48 could have rounded corners and gradually meld into the front and back surfaces 52, 54. In fact, the peak 42 could be round when viewed from the top, but the four peak surfaces 48, 50, 52, 54 still provide a useful reference for discussing characteristics of the peak 42. There is also a fin peak top 56, which intersects the four peak surfaces 48, 50, 52, and 54. As with the peak surfaces 48, 50, 52, and 54, the top 56 is not necessarily a clearly defined, flat, surface, and the intersection between the top 56 and the peak surfaces 48, 50, 52, and 54 may be more of a blending than a sharp corner.
An indent 58 is defined on the peak side surface 46, and this indent 58 can be in the platform 44 extending from the peak 42. The indent 58 includes a flank 60, which is a surface which is more or less vertical. The flank 60 is not necessarily exactly vertical, which would be perpendicular to the tube axis 26, but the flank 60 tends to run more up and down than side to side relative to the tube outer surface 14. The indent 58 also includes a base surface 62, which is more or less horizontal. The base 62 is not necessarily perfectly horizontal relative to the tube outer surface 14, but does tend to run more parallel or in line with the tube outer surface 14. The indent 58 also includes a junction 64 where the flank 60 and base surface 62 meet. Preferably, the flank 60 and base surface 62 are defined well enough for there to be a distinct, definable intersection between the two surfaces, and this intersection defines the junction 64. The junction 64 is parallel with the fin base 22 when the indents 58 are formed parallel with and along the fin 20.
The flank 60 intersects the fin peak top 56, and this intersection is referred to as the seam 66. Preferably, the flank 60 is well enough defined for the seam 66 to be detectible, so there is a distinct, noticeable angle at the seam 66 between the flank 60 and the peak top 56. The base surface 62 intersects the fin side wall 28 at the hem 68. As with the flank 60, it is preferred the base surface 62 is well enough defined for the hem 68 to form a detectable angle. The indent 58 is positioned on the side of the peak 42, so the indent 58 generally faces into the channel 34 between two adjacent fins 20. Therefore, the base surface 62 intersects only one fin side wall 28, and the base surface extends into only one channel 34. Similarly, for one indent 58 the flank 60 faces only one channel 34, and the flank 60 faces the same channel 34 the base surface extends into.
The flank 60 and base surface 62 of the indent 58 forms a step-like structure or shape 70 on the side of the fin peak 42, so the fin peak 42 has a stepped 70 side. The stepped shape 70 includes a relatively horizontal surface intersecting a relatively vertical surface. The relatively horizontal surface is the base surface 62, and the relatively vertical surface is the flank 60. The stepped 70 side faces into a channel 34.
If there are a plurality of steps 70, where the indent includes one flank 60 above another flank 60, only the uppermost flank 60 intersects the peak top 56 at the seam 66. Similarly, multiple steps 70 provide for one base surface 62 above another base surface 62, and only one base surface 62 intersects the fin side wall 28 at the hem 68. There can be indents 58 on both the left and right fin peak side surfaces 48, 50, which provide for a stepped side surface 46 on opposite sides of the fin peak 42. Furthermore, each indent 58 can have more than one step 70. If both peak side surfaces 48, 50 had indents 58 with multiple steps 70, the peak 42 would include at least 4 steps 70. The hem 68, seam 66, and junction 64 of the indent 58 all provide for edges and sharp angles to enhance the performance of the finned tube 10.
Because the platform 44 can be part of a peak side surface 46, the indent 58 can be formed into the platform 44. An indent in the platform 44 results in at least part of the base surface 62 being received over the channel 34 between two adjacent fins 20. If there is a platform 44 on both sides of the peak 42, there can be two indents 58 on opposite sides of the peak 42 with the base surface 62 of each indent 58 at least partially over the channel 34, instead of over the fin 20.
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, as seen in
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
The arbor 80 generally includes several fin forming discs 86 which successively deform or shape 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, shape, and define aspects of the final tube 10. These remaining discs can be included or excluded, as desired. After the finning discs 86, a knurling tool 90 is used to knurl depressions 36 in the fin crest 24. If the knurling tool 90 is blunter, the material displaced to form the depressions 36 tends to accumulate at the base of the depression 36, and there can be a platform 44 projecting into the channel 34 from the base of the depression 36. A sharper knurling tool 90 cuts the depression 36, instead of mashing the depression 36, and pushes the material from the depression 36 to the side, so there is a platform 44 on the fin peak side surface 46. The knurling tool 90 can be set to form the depressions 36 at varying skew angles 38, which can have an affect on where and how the platform 44 is formed.
An indent tool 92 is then used to form the indents 58 in the peaks 42, although it is possible for the indent tool 92 to be used before the knurling tool 90. The shape of the indent tool 92 determines the shape of the indent 58. Possible shapes for the indent tool 92 include teeth or a smooth circumferential shape. The indent tool 92 can be shaped to produce a single step 70, or at least double steps 70 in the peak, as desired. The indent tool 92 is used to press off-center indents 58 into the fin peaks 42, so the fin peaks 42 become stepped. The indent tool 92 can shape or form either one side of the peaks 42, or both sides of the peaks 42, and if both opposite sides are deformed, it can be done either simultaneously or sequentially. Other tools for further shaping the tube body outer surface 14 can also be included on the arbor 80, as desired.
The inner support 84 uses recesses 88 to form the ridges 74 on the tube inner surface 16. The inner support 84 rotates relative to the tube 10, or vice versa, and the inner support 84 can rotate at a different speed relative to the tube 10 than the arbors 80. A wide variety of tools or designs can be included with the inner support 84, and a wide variety of different designs, shapes or textures can be formed on the tube inner surface 16. Improvements in the heat transfer for the tube outer surface 14 are most effective if the inner surface 16 rate of heat transfer is either comparable or faster than the outer surface 14 rate of heat transfer. A wide variety of considerations and factors influence the heat transfer rates, such as the material contacting a surface, flow rates, phase changes, and many others.
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 10 interior. This type of use is 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 flow downward and fall off the tube 10 more readily, so the tube 10 sheds condensate more quickly. Also, the channels 34 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 fin crest 24, the seam 66 and the hem 68, because of surface tension effects. These relatively condensate-free areas provide less resistance to heat flow, which further promotes condensation rates.
The fins 20, depressions 36, platforms 44, and indents 58 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 reduces 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.
Actual testing of the current invention against existing technology using R134A as the gas condensing outside the tubes has demonstrated the benefits of the stepped fin peak design. The tube outer surface heat exchange rate has been measured with rates increased by 6.5% to 15%. These improved rates were achieved by utilizing the stepped fin peak design.
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 which is referred to as the fin thickness, and this thickness can be between 0.05 and 0.3 millimeters.
The fin 20 has a height 27 measured from the fin base 22 to the fin crest 24, and the fin height 27 can be between 0.7 and 1.5 millimeters.
The depressions 36 formed in the fin crest 24 has a depth 40 which can vary between 0.1 and 0.5 millimeters, and the depression 36 has a width which can vary between 0.1 and 1 millimeter.
The ridge 74 formed on the tube body inner surface 16 has a depth 76, and this depth 76 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.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having the 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| ZL200710043537.6 | Jul 2007 | CN | national |
