| Tabbed transfer fins for air-cooled heat exchanger -> Monitor Keywords |
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Tabbed transfer fins for air-cooled heat exchangerTabbed transfer fins for air-cooled heat exchanger description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060169019, Tabbed transfer fins for air-cooled heat exchanger. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application No. 60/486,071, filed Jul. 10, 2003, which is incorporated by reference herein in its entirety. BACKGROUND OF THE INVENTION [0003] 1. Field of the Invention [0004] The present invention relates generally to heat exchangers that utilize fins or plates on or in contact with tubes, pipes, or plates to transfer heat away from the working fluid in the tubes, pipes, or plates, and more particularly, to heat transfer fins, and heat exchangers or condensers that include such fins, that include a plurality of tabs extending from the fins to provide enhanced heat transfer on the air side of the heat exchanger with low and acceptable increases in pressure drop. [0005] 2. Relevant Background [0006] Heat exchangers are used extensively in industrial and consumer applications, and typically employ two moving fluids, one fluid being hotter than the other, to transfer heat to the colder fluid. Many heat exchangers currently in use, such as in air conditioners, automotive radiators, process industry air-cooled condensers, and boilers, transfer heat between a gas and a single or multi-phase liquid. Typically, such heat exchangers include a number of liquid conduits, e.g., circular, oval, or flat tubes, pipes, or plates, that are positioned within a shell or housing that defines a gas flow passage or chamber. The heat exchanger uses a fan or blower to force a gas, e.g., air, to flow within the gas flow chamber in a perpendicular (i.e., cross-flow) or parallel (i.e., counter-flow) direction relative to the liquid conduits. The resulting heat transfer between the liquid and the gas is directly proportional to the heat transfer surface area between the liquid and the gas, to the temperature difference between the liquid and the gas, and to the overall heat transfer coefficient of the heat exchanger. The overall heat transfer coefficient is defined in terms of the total thermal resistance to heat transfer between the gas and the liquid, and it is dependent on a number of characteristics of the heat exchanger design, such as the thermal conductivity of the material used to fabricate the conduit and the local film coefficients along the conduit, i.e., measurements of how readily heat can be exchanged between the gas and the exterior surfaces of the conduit. [0007] Although gas-liquid heat exchangers are widely used, the heat transfer effectiveness of these heat exchangers is low. The low heat transfer effectiveness leads to relatively high operating and capital costs for gas-liquid heat exchangers because a greater number of units and/or larger capacity units that require more power must be used to obtain a desired heat transfer. For example, air-cooled geothermal power plants operate at low temperature differences between the gas and the liquid and, in these power plants, more than 25 percent of the cost of producing electricity is the expense of purchasing and operating gas-liquid heat exchangers or condensers. As a result of these high costs, continuing efforts are being made to improve heat transfer effectiveness of gas-liquid heat exchangers while at the same time controlling the manufacturing and operating cost to increase the likelihood that new heat exchanger designs will be adopted by industry and consumers. [0008] Geothermal plants provide one example of a situation in which there is often not a sufficient supply of water or other cooling liquid for evaporative cooling, and heat must be rejected to atmospheric air. This heat rejection is accomplished through the use of large air-cooled condenser units in which air is forced through several rows of long individually finned tubes by large fans, i.e., a gas-liquid heat exchanger or condenser is employed. Each of the tubes carrying the hot working fluid has fins on their outer surfaces in order to provide a large heat transfer surface area. Finned-tube heat exchangers have been used for many years to improve the gas-side heat transfer rate by increasing the heat transfer surface area available for contacting the gas as it flows through the heat exchanger. In general, finned-tube heat exchangers are cross-flow heat exchangers that include a number of tubes, i.e., conduits, for carrying the liquid fabricated from aluminum, copper, steel, or other high thermal conductivity materials. [0009] The tubes pass through and contact a series of parallel, high thermal conductivity material sheets or plates, i.e., fins, which provide an extended heat transfer area for the tubes. The overall heat transfer area is based on the number and size of the included fins. The fins are separated a fixed distance, i.e., a fin separation distance, and define relatively parallel channels that direct the gas flow across and among the tubes. Heat transfer occurs as the gas flows through the channel and contacts the surface of the fins and as the gas contacts the outer surfaces of the tubes. The highest heat transfer rate on a flat surface like a flat fin occurs at the leading edge of the surface and decreases with distance from the leading edge as a boundary layer develops and thickens causing the local heat transfer coefficient to decrease. [0010] However, although finned-tube heat exchangers are widely used because they are relatively inexpensive to produce and do not create a large pressure drop, there are several operational drawbacks to finned-tube heat exchangers. For example, finned-tube heat exchangers have low heat transfer coefficients on large portions of the fins due to the development of thick boundary layers. Additionally, these heat exchangers have poor heat transfer in the wake or shadowed regions behind tubes as a majority of the gas flowing over a tube does not contact the back side of the tube or contact the portion of the fin surface that is shadowed by the tube. [0011] In an attempt to increase the effectiveness of finned-tube heat exchangers, efforts have been made to vary the surface and overall geometry of the parallel fins to interrupt gas boundary layers or to make it more difficult for thick boundary layers to form on the fins. For example, finned-tube heat exchangers have utilized triangular or s-shaped wavy fins to enhance the heat transfer coefficient by disrupting boundary layer development and, also, by increasing the available heat transfer area. Alternatively, the surface geometry of flat, parallel fins can be enhanced, as is often done in refrigerant condensers, by slitting the fin three or four times in the areas of the fin between the tubes, thereby interfering with boundary layer development by creating offset surfaces on the fin that cause repeated growth and wake destruction of boundary layers. A number of heat exchangers have been developed that include structures on the fin surfaces that are designed to create turbulence in the channel between the fins to break up the boundary layer and increase heat transfer. Generally, these structures have been configured with a major portion of their surface area, such as winglets, vortex generators, and the like, facing the flowing gas or directed toward or into the gas flow path, e.g., to have a large profile relative to the gas flow path within the fin channel. [0012] However, the larger the profile or "form" placed in the flow path of the gas, including the liquid tubes, the larger the pressure drop in the cooling gas as form drag is increased, which is generally an undesirable and often unacceptable result. [0013] While some of the above changes in the fin surface and fin shape may provide somewhat higher heat transfer coefficients in finned-tube heat exchangers, the design changes also result in unacceptably large increases in pressure drop on the gas side of the heat exchanger that require increased expenditures on fan power. Additionally, many of these design changes have not been adopted due to unacceptably high manufacturing costs in producing the fins or due to increased maintenance costs as some of the fin surface structures snag or collect debris often found in unfiltered air often used in air-cooled heat exchangers. [0014] Hence, there remains a need for a more effective finned-tube, gas-liquid heat exchanger that provides improved heat transfer capabilities on the gas side of the exchanger while creating an acceptable increase in the pressure drop for the gas passing through the tubes and fins and while controlling manufacturing and maintenance costs. DISCLOSURE OF THE INVENTION [0015] The present invention addresses the above problems by providing an improved design for heat transfer fins that enhances the heat transfer rate on the gas or air side of heat exchangers with relatively low increase in pressure drop. Briefly, the fins include numerous tabs or secondary fins that are bent upward and downward from the body of the fin at a selected bend angle (such as between about 70 and 110 degrees and more typically, about 90 degrees). In this manner, the material of the fin body is retained for use in heat transfer with the air or gas flowing over the fins. Preferably, all or a majority of the tabs are aligned with the flow path(s) of the cooling gas to minimize the creation of turbulence and pressure drop (i.e., by minimizing creation of flow drag by only "showing" the tab's leading edge to the flowing gas). [0016] For example, the tabs may be substantially planar and aligned with their surfaces parallel to the main flow path or simple flow path or line (or in some cases, the local flow paths) of the cooling gas relative to the fin. In a first embodiment, the tabs are positioned with their planar surfaces perpendicular to a leading edge of the fin to align the tabs substantially parallel with the main flow path of gas across the fin. In a second embodiment, some or all of the tabs are positioned to be more aligned with local flow paths or with streamlines to guide air flowing in the channel between fins to reduce the size of wakes behind tubes and to reduce pressure drop relative to the first embodiment by producing less turbulent flow. In the second embodiment, the tabs may be positioned substantially parallel to or angled less than about 5 degrees relative to the streamlines. [0017] This is achieved by positioning the tabs at various, differing offset angles, e.g., 0 degrees (or substantially parallel to the simple flow path), 10 degrees as measured from either side of the simple flow path, and the like. The offset angles are typically less than about 20 degrees and more preferably less than about 10 degrees as measured from either side of the simple flow path with the offset angle, at least in some embodiments, being selected to be substantially parallel (such as within 5 to 10 degrees or less to being parallel) to the local stream line or flow path. In this manner, heat transfer is significantly enhanced by reducing the thickness of the thermal boundary layer on each tab and by placing heat transfer surface area in contact with cooler portions of the flowing gas (for a cooling application), e.g., the surface area of the tabs extends outward into cooler portions of the flow channel between adjacent fins. [0018] The tabs of the fin serve four main functions. First, the tabs are preferably arranged so as to serve as a plurality of sites for starting new boundary layers. This is achieved generally by offsetting the tabs (or adjacent rows of the tabs) such that downstream tabs are not shadowed by upstream tabs. Second, the tabs are preferably positioned relative to the flowing gas to enhance heat transfer. More particularly, the tabs typically have a tab height as measured from the surface of the fin body that allows the tab to extend out into the region of high air flow rate and cool air (in the case of cooling applications), i.e., forming on both sides of the fin body. In one embodiment, the fin height is selected to between about 40 and 50 percent (e.g., about one half) of the size of the channel between adjacent fins, i.e., a fin separation distance and tabs are extended outward from both sides of the fin body. In other embodiments, the fin height is greater than 50 percent with one specific embodiment using a tab height of about two thirds or about 67 percent of the fin separation distance. In this manner, the tabs place fin material into the coolest portion of the gas flowing on both sides of the tab. Third, the openings in the main fin surface disrupt the boundary layer on that surface thus enhancing heat transfer. Fourth, due to their angles, their flow resistance, and the channels they create, the tabs direct air flow so that the fin surface is more uniformly covered and relatively stagnant wake regions behind tubes are reduced. To achieve these functions, the tabs are formed by punching holes in the fin body but retaining a connection to the fin body on at least one edge. The material is then bent upward and/or downward relative to the fin body to extend at a bend angle from one or both of the surfaces of the fin body, i.e., to allow the tabs to extend into the boundary layers that form on one or both sides of a fin. [0019] According to one aspect of the invention, a method is provided for fabricating heat transfer fins for heat exchangers. The method comprises providing a plain fin, such as an aluminum fin typically utilized in finned-tube heat exchangers. A tab pattern is selected or provided for the particular fin to define the quantity, size, and location of heat transfer tabs on the fin. The tab pattern selection may comprise performing a variety of flow and heat transfer tests on the fin implementing a number of potential tab patterns to obtain a useful pattern to enhance heat transfer while not unacceptably increasing pressure drop. With a tab pattern selected, a punch mechanism or tool can be fabricated or provided based on the pattern. The punch mechanism can be adapted for punching the tabs in one operation with tabs extending from one or both sides of the fin body. The method continues with forming, such as with the punch mechanism, the heat transfer tabs defined by the tab pattern by creating openings or holes in the fin by removing material from the fin body while retaining a connecting edge between the fin body and the removed material or tab body. The forming comprises bending the removed fin body material along the connector edge to a bend angle, such as 90 degrees, relative to one of the two sides of the fin body. The tab pattern is configured such that all or a majority of the tab bodies are aligned parallel (or within about 10 to 20 degrees) to a simple flow path (i.e., a directional line drawn perpendicular to the leading edge of the fin body) or are aligned parallel (or within about 5 to 10 degrees) of local flow paths. In one embodiment, the tab pattern is configured such that the surface area of the removed material or tab bodies is such that the tabbed fin has porosity of less than about 50 percent and more typically between about 15 and 30 percent. In some embodiments, about half of the tabs extend from one side of the fin body while the remaining tabs extend from the second side of the fin body. Preferably, the tabs on each side of the fin body are arranged in the tab pattern such that adjacent upstream and downstream tabs (or proximal and distal tabs relative to a fin body leading edge) are offset to avoid shadowing of downstream tabs. The tabs are also arranged in such a way that they do not adversely interfere with the tabs on adjacent fins. Further, their pattern encourages uniform flow over the main fin and maximized heat conduction within the fin. BRIEF DESCRIPTION OF THE DRAWINGS [0020] FIG. 1 is a simplified heat exchanger according to the present invention illustrating one configuration in which tabbed fins or plates (such as those shown in later figures) can be employed to enhance heat transfer on the air or gas side; [0021] FIG. 2 illustrates two fins according to the invention, one that is partially punched or has less tab density and one that is fully fabricated or has higher tab density, and a template that can be used for producing a tool to fabricate the fins shown; Continue reading about Tabbed transfer fins for air-cooled heat exchanger... Full patent description for Tabbed transfer fins for air-cooled heat exchanger Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Tabbed transfer fins for air-cooled heat exchanger patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. 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