Tabbed transfer fins for air-cooled heat exchanger
Heat transfer fins (110), and heat exchangers (100) incorporating such fins (110), enhancing heat transfer with acceptable pressure drop increase. The fins (110) include tabs or secondary fins bent upward and downward from the fin body at a selected bend angle. All or a majority of the tabs are aligned with a simple flow path or with local flow paths for cooling air adjacent the fins (110) to minimize turbulence and pressure drop. The tabs are planar and generally aligned parallel to the simple flow path or local flow paths and are arranged so as to serve as a plurality of sites for starting new boundary layers by offsetting the tabs such that downstream tabs are not shadowed by upstream tabs. The tabs have a height sufficiently large to extend the tabs out into boundary layers on the fin (110). The tabs provide more uniform flow over fins (110) and shrink wake size behind tubes (120).
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.
CONTRACTURAL ORIGIN OF THE INVENTIONThe United States Government has rights in this invention under Contract No. DE-AC36-99GO-10337 between the United States Department of Energy and the National Renewable Energy Laboratory, a Division of the Midwest Research institute.
BACKGROUND OF THE INVENTION1. Field of the Invention
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.
2. Relevant Background
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.
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.
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.
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.
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.
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.
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.
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.
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 INVENTIONThe 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).
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.
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.
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.
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
The present invention is directed to heat exchanger fins employing tabs to increase heat transfer effectiveness, heat exchangers or condensers incorporating such tabbed fins or plates such as air-cooled, finned-tube heat exchangers, and methods of making tabbed fins. Generally, each fin of the invention includes a multiplicity of small tabs or secondary fins that are formed by punching material (i.e., metal) out of the main fin body (i.e., creating a hole or opening) and the punched material is bent outward away from the main fin body in one or both directions from the surface of the fin. To minimize or control the creation of a vortex or increased pressure drop, the tabs are generally planar and aligned with the direction of the fluid, e.g., air, flowing over the fins in the channel between adjacent fins. In other words, a leading edge of the tab first contacts the flowing gas and the substantially planar body of the tab is aligned substantially parallel to the gas flow path or direction over the fin.
In some embodiments, it is assumed that there is one flow direction through the fins, such as perpendicular to the leading edge of the fins, and all of the fins are aligned parallel to this flow direction. These embodiments can be described as tabbed fins with tabs positioned at a zero offset angle relative to the simple flow path (i.e., a line drawn perpendicular to the leading edge of the fin body) such that the planar tab bodies are substantially parallel to the simple flow path. In other embodiments, two or more flow directions within the fin channel are identified, and fin tabs in different locations of the fin are aligned with these different flow paths to better limit creation of pressure drop.
The different flow paths can be termed “local flow paths” or “local streamlines” and in these embodiments, a portion or all of the tabs are positioned at offset angles greater than zero relative to the simple flow path (such as less than 20 degrees and more preferably less than about 10 degrees offset). In some cases, a portion or subset of the tabs are aligned somewhat, such as less than about 5 percent or about 5 degrees, off of the local flow direction or local flow path in order to redirect flow into areas of low flow such as the area behind or shadowed by a tube, whereby heat transfer is enhanced without creating a vortex at the tab.
The tabs can take many shapes, such as square, rectangular, triangular, semi-circular, and a combination of these shapes, and are generally bent outward at about a right angle relative to the body of the fin but a smaller bend angle can be utilized to practice the invention. The tabs can also be curved, e.g., into an L-shape or U-shape, to place more tab area into the most advantageous flow regions. Tabs in adjacent rows are preferably offset from each other so as to avoid shadowing of subsequent tabs and to promote the creation of multiple boundary layers within the channels. The tabs have a height defined by the amount of material removed from the fin body, and this tab height is generally (but not necessarily, such as when a bend angle of less than 90 degrees is utilized) less than the separation distance between fins, i.e., fin separation distance, and in many embodiments, the tab height is selected to be about one half and three fourths of the fin separation distance, e.g., two thirds or 67 percent, to place a large portion of surface area of the tab bodies within the coolest air flowing between the fins, i.e., at about the top of the boundary layer formed by the fin body. As will become clear from the following description, the use of tabbed fins according to the invention can significantly enhance the air-side heat transfer coefficient in finned-tube heat exchangers, with some tests indicating an increase of up to approximately 100 percent with a corresponding increase in pressure drop of approximately 60 percent relative to smooth or plain fins. Additional test results are provided and explained with reference to
In the following description, the use of tabbed fins according to the invention is explained most fully with finned-tube arrangements in which the fins or plates are arranged in a parallel fashion. However, it should be understood that the invention covers the use of tabs on many other arrangements of fins than just the ones shown. For example, it is anticipated that tabbed fins would be useful with helically wound fins.
Additional applications of tabs to fins, whether or not the fins are applied to tubes, will be understood by those skilled in the art and are considered within the breadth of the invention and following description.
Generally, the heat exchanger 100 would further include a housing enclosing fins and tubes and, at least in part, defining air flow channels, i.e., causing the air or other gas to flow between the fins and to define a gas inlet and a gas outlet. The heat exchanger 100 further would include one or more fans to draw (or push) air across the tubes and between the fins. These components are well known and hence, it is not believed necessary to illustrate or describe these components further to allow one skilled in the arts to understand and practice the invention. The heat exchanger 100 transfers heat energy from one fluid, i.e., the fluid in, FIN, to another fluid or gas, i.e., the air in, AIN, which results in a cooler fluid being output, i.e., the fluid out, FOUT, and a hotter fluid or gas being output, i.e., the air out, AOUT, from the heat exchanger 100. Of course, the fluids being cooled may be a gas or liquid or any mixture thereof, and the invention applies to heating as well as cooling.
Referring again to
The incoming air, AIN, is passed through the channels between the fins 110 and strikes the leading edges of the plurality of secondary fins or tabs (which are shown in a rows that are diagonally offset in
Referring now to
As shown, a relatively large portion of the fin body surface area has been removed to form the tabs. For example, the surface area of the fin body removed or used to form the tabs may be selected from the range of 0 to 50 percent, and more preferably between 10 and 40 percent, and in one preferred embodiment, the surface area removed is about 20 to 25 percent of the fin body surface area, i.e., 20 to 25 percent of the original fin body surface area or material is used to form the tabs. While initially it may appear that the area used to form the tabs should be maximized, there are limitations to how much material can or should be removed from the fin body. More particularly, the removal of fin body material reduces the volume of or mass of the fin body that is available to conduct the heat from the tube collar and tube contacting the collar to the tabs. Hence, testing may likely be required to identify for a particular fin and tube arrangement the amount of fin body surface area that should be used in forming the tabs. The amount of material removed defines the surface porosity of the fins 210, 220, and this porosity may be varied to practice the invention and may vary with fin materials and the makeup of the fluids passed through the heat exchanger. In one embodiment, the fin porosity is selected to be between 20 and 30 percent with tested embodiments utilizing about 25 percent porosity. The number and size of the tabs may be varied while maintaining a desired porosity with larger tabs resulting in fewer tabs and smaller tabs resulting in fins with more tabs. Also, the amounts of heat transfer improvement and pressure drop increase can be controlled by varying the tab dimensions (height and length) and the number of tabs or porosity. The tabs shown are generally square (as can be seen clearly from the material removed to form the tab openings or holes) but numerous other shapes can be utilized, such as rectangular (such as shown in
In other words, downstream tabs are generally not positioned immediately behind an upstream tab or in the same air flow path to minimize shadowing and to encourage development of new boundary layers by each tab. Item 230 is a template useful for designing a punch tool for creating the tabs in a plain fin with darkened or colored holes indicating tabs to be punched to extend from a first surface of the fin body and the other holes indicating tabs punched the other direction. Again, the template 230 is useful for showing that downstream tabs are offset from upstream tabs. Arrangement of the tabs is also done with due consideration for providing heat conduction paths in the base fin from the tube to outer regions of the fin.
The tab 404 is shown to include a leading edge 406 and a trailing edge 408. The tab 404 is bent or formed in a manner that positions the leading edge 406 to contact the incoming air, AIN, and such that the planar area of the tab 404 is substantially parallel to the flow path of the air, AIN. In some embodiments, the tabs 404, 410 are substantially parallel to the simple flow path while in other embodiments, some, a majority, or all of the tabs 404, 410 are arranged substantially parallel (such as within about 5 to 10 degrees) to the local flow paths or streamlines. Tab 410 is shown to be generally square in shape but to include rounded shoulders 414 such that its leading edge is less likely to snag or catch debris in the incoming air, AIN, that might clog the air flow channel between the fins and reduce heat transfer and/or increase pressure drop. Some of the tabs can be more semicircular in shape indicating that the shape of the tabs 404, 410 can vary on differing fins or within a single fin to practice the invention. The tabs can also bend at angles less than 50 degrees. In some situations, it may be advantageous to have tabs bend downward in the direction of gravity to facilitate water drainage from the fin surface that could result from rain, dew formation (as occurs in the case of an evaporator), and spray cooling enhancement.
For example, a first tab 614 is bent downward relative to the surface 610 and the material removed from the body or surface 610 forms a tab opening or hole 616 adjacent the tab 614. A second tab 620 extends upward relative to the surface 610 at a substantially right angle with the removed material (i.e., the material retained in tab 620) creating a tab hole or opening 622 adjacent the tab 620. As shown, the tab holes 616, 622 (and corresponding tabs 614, 620) are substantially square in shape but other embodiments of fins of the invention may utilize other shapes.
As shown, the tabs are arranged generally in rows that extend substantially parallel to the leading and trailing edges 611, 612 of the fin 600. Note, that this particular configuration is not required but is useful for ease of tab pattern selection (such as relative to amount of surface area to be utilized), for ease of manufacturing, and for assuring that tabs are positioned to achieve a desired sequentially offset arrangement.
The offset feature of the invention can be seen by looking at the tabs in Rows 1-4 and particularly the four tabs of Rows 1-4 shown with the dashed line 630 that can be said to be corresponding or adjacent tabs in adjacent ones of the Rows 1-4. Row 1 can be thought of as the first row or most upstream row of tabs with Row 2 being the second row and immediately downstream row relative to Row 1 (or adjacent to Row 1). The tabs shown by line 630 in Rows 1 and 2 can be seen to be offset from each other.
Likewise, the tab in element 630 in Row 3 is offset from the tab in element 630 in Row 2 (immediately upstream or in the adjacent row), and the tab in element 630 in Row 4 is offset from the tab in Row 3.
The amount of offset can vary to practice the invention, with the offset shown being one useful embodiment, e.g., the opening and tab in the downstream row is positioned substantially in the space between adjacent openings/tabs in the upstream row. Note, also, that the tabs in the element 630 are offset on a “diagonal” and this pattern is continued in several additional rows of tabs. However, other offset patterns may be utilized as long as corresponding tabs in adjacent rows are offset from each other. Preferably, the offset pattern is selected so as to provide a spacing between similarly positioned tabs, such as by skipping a number of rows before placing a tab in a similar position within a row (e.g., as shown in
In some embodiments, a subset (such as a small percentage such as 10 percent or less) of the tabs are purposely not aligned with the main air flow through the fin (i.e., not perpendicular to a leading edge of the fin) but are instead skewed or angled relative to the main or simple flow path or flow direction of the cooling gas so as to act as directional vanes. Typically, these directional vane tabs are positioned near (e.g., beside and/or slightly behind) the collars (and inserted tubes) of the fins to direct the air or gas flow into areas that otherwise would be starved for flow such as in the wake region behind the collar/tube. In other cases, the direction vane tabs may be further upstream to begin diversion of flow to the wake area prior to the tube collar and tube so that the flow redirection can be more gentle, i.e., less dramatic or turbulent. In one embodiment, at least some of the tabs near the collar 636 are angled to direct some air flow from the main flow path into the wake region 640. Preferably, the angle for the directional vane tabs is selected so as to avoid or minimize the creation of vortices behind these tabs so as to control increases in pressure drop, e.g., the angle may be less than about 5 degrees relative to the local flow paths or streamlines and the like. Unlike a delta winglet pair, the tabs in this embodiment gently direct the flow into the wake regions without causing turbulence. The reduction in wake size reduces form drag and overall pressure drop while at the same time providing better heat transfer coverage in the wake region behind the tubes/collar.
The introduction of the fins, although parallel to the simple flow path or local flow paths or streamlines, does alter the flow of the cooling gas relative to the fin, such as by increasing friction and by creating multiple thermal boundary layers within the gas flow channel or passage. In some embodiments, patterns of the tabs are selected with the express purpose of gently redirecting flow of the cooling gas. For example, the pattern of the tabs are selected purposely to fold or direct more of the cooling gas into the wake areas and areas of low pressure or flow between two fins. In one case, the tabs are offset in diagonal patterns to gently orient (e.g., with minimal turbulence) toward the wake regions behind collars and tubes.
In still other embodiments of the invention (not shown), fins are fabricated according to the invention so as to generate at least some vortices in the cooling air or gas. In one vortex generating application, the tabs illustrated and described in the invention are utilized in combination with delta winglet pairs, such as near the tube collars or other areas of low flow. In this manner, the beneficial effects of the tabs of the present invention and of winglet pairs are combined to enhance heat transfer. The amount of pressure drop can be controlled by limiting the number of winglet pairs utilized, and/or this embodiment may be employed when a higher pressure drop is acceptable. In another vortex generating application, tabs that are sharply angled (such as over 5 degrees up to 90 degrees) relative to the main or local flow paths of the cooling gas are included on a fin. Typically, in these embodiments of the invention, the majority of tabs would remain aligned parallel with the main or local flow paths with a minority or small number of unaligned tabs being added in strategic locations, such as locations at which winglet pairs are often employed or other locations at which it is desirable to create turbulence.
While machining costs and pressure drops may be increased, some embodiments of the invention can be fabricated with the tabs aligned more particularly with the flow path of gas corresponding to the location of the tab, e.g., alignment along the local flow path lines. In other words, the tabs may be arranged with many differing alignments to suit the flow in that particular region of the fin. It is expected, though, that the placement of the tabs in the fin would change the flow relative to the fin and it may take numerous iterations to “match” such a tab alignment to the flow. Additionally, flow patterns vary with other parameters such as gas or air velocity and the like, although laboratory tests have demonstrated that one tab pattern can work well over a wide range of air flow rates.
An exemplary tabbed-fin was fabricated according to the invention (similar to that shown in
Measurements at air flow of 3 m/s in early fin samples showed that the tabbed fins provided 68% more air-side heat transfer and had a 33% higher pressure drop than similar untabbed fins. For comparison purposes, small cores made up of advanced fin materials were also tested, e.g., wavy fins and louvered fins. These cores were of a different fin density, but it was believed useful to compare the performance of the tabbed fins to the plain fins for these other arrangements designed for enhanced heat transfer. Table 1 shows the test results for these cores and compares the results to the results obtained for the tabbed fins of the invention. Note that the wavy fins and louvered fins were in a test core containing only one row of tubes. Also, the tabbed fin design had fins spaced at only 5 fins per inch (0.20 inch spacing) because that was the stock available. The tab-forming tool used produced tabs that are 0.050 inch on a side, so each tab extended only one-quarter way across the gap whereas it is believed that tabs that extend halfway or more across the gap perform better.
As discussed above, some embodiments of the invention comprise tabbed fins in which some or all of the tabs are arranged to be generally aligned with or to be substantially parallel (such as within 5 to 10 degrees or less) to the local flow path or local streamlines. Additionally, some embodiments may comprise tabs each with a body that is rectangular in shape so as to provide a more desirable aspect ratio to enhance heat transfer rates relative to the same overall porosity of a tabbed fin. With these design ideas in mind, a tabbed fin (or fin portion) 1000 is shown in
Further, the tabs can be oriented at slight angles to the usual local path lines to help direct the flow into the stagnant wake regions behind the tubes.
As shown, the fin 1000 comprises a fin body 1010 with a plurality of tabs 1030 extending at bend angles of about 90 degrees from a first fin surface 1014 and a plurality of tabs 1040 extending in an opposite direction at bend angles of about 90 degrees from a second fin surface 1018. Generally, the tabs 1030, 1040 are alternated in each tab row, such as rows 1050, 1054, 1058 so as to create an overall tab pattern in which corresponding tabs or upstream/downstream tab pairs are offset or purposely not aligned along a line drawn perpendicular to the leading edge 1020 of the fin body 1010 so as to avoid shadowing downstream tabs with upstream tabs. In other words, heat transfer achieved in downstream tabs is enhanced by not placing the downstream tabs directly in wakes or vortices created by upstream tabs. Wherever possible, consistent with the above considerations, the hinged edge of a tab is located closest to the tube to minimize the heat conduction path length.
The fin 1000 further comprises tube collars 1026 for mating with heat transfer or fluid tubes (not shown). The tube collars 1026 define fin separation distances when the fin 1000 is mated with another fin in a heat exchanger (such as exchanger 100 in
A simple flow line, LSimple Flow, is illustrated as showing generally air flow across the fin 1000 during its use in a heat exchanger. As can be seen, the tabs 1030, 1040 are positioned at offset angles, θ1, θ2, relative to the simple flow path or line, LSimple Flow, that enables the tabs 1030, 1040 to align with the usual local flow direction and, further, to direct the flow more uniformly over the fin surface and into the wake regions. As shown, the offset angles, θ1, θ2, relative to the simple flow line, LSimple Flow, are about 10 degrees but the offset angles, θ1, θ2, may be larger (such as up to about 20 to 30 degrees) or smaller. The measurement of the offset angles, θ1, θ2, is provided as an absolute value or variance from the simple flow line, LSimple Flow, but could be also provided, as shown, as 10 degrees, 170 degrees, 190 degrees, and 350 degrees for the various tabs 1030, 1040 on the fin 1000. A streamline or line representing a local flow path, LStreamline, is also shown in
To more fully describe the invention, it may be useful to more fully describe one fabricated embodiment of the fin 1000 of
To model and understand flow of air through channels defined by two adjacent, tabbed fins, a bubbler device was utilized by the inventors. In
Testing of a set of fins 1000 shown in
Turning to
As can be seen, there is a significant improvement in the heat transfer coefficient for the tabbed fin compared with the plain fin. Specifically, in the same face velocity range of 2 to 3 m/s, the heat transfer coefficient ranges from about 65 to about 85 W/m2K This represents about a 70 percent increase in the heat transfer coefficient for the tabbed fin relative to the plain fin of similar size, material, and thickness. The “cost” of this added efficiency or effectiveness of the tabbed fin is an increase in pressure drop. However, as shown, the pressure drop for the tabbed fin in the 2 to 3 m/s face velocity range is only about 31 to 62 Pa which represents a relatively low increase in pressure drop of about 50 percent that would likely be an acceptable tradeoff for the significant increase in the fins heat transfer rate and in effectiveness of heat exchangers incorporating such tabbed fins.
Line 1820 represents pressure drop values for the varying face velocities, and in the range of 2 to 3 m/s face velocity ranges from about 28 to 55 Pa
As can be seen, the tabs may be selected to have a height as measured from the fin body that varies significantly to practice the invention, but that will typically be selected to be about the fin separation distance or less to avoid problems in fabricating the heat exchanger. More typically, the tab height is selected to remove adequate material from the fin body to have the tab body extend out into the cooler flowing air.
Generally, testing has shown that it is desirable for the edge of the tab distal to the fin body to extend beyond the top of the boundary layer so as to place a significant portion of the tab body surface area in the very coolest air. With this in mind, most embodiments of the invention utilize a tab height in the range of about 25 to 75 percent of the gap or fin separation distance. For an 8 FPI exchanger, the gap is about 0.115 inches and the tab height is selected from the range of about 0.029 inches to about 0.087 inches. More typically, the tab height is selected from the range of about 40 to 67 percent of the gap. In these cases, the tab height is in the range of about 0.0460 inches to about 0.077 inches for the 8 FPI exchanger. In a 10 FPI heat exchanger, the gap is smaller at about 0.09 inches and hence, the tab heights would be selected from the overall range of about 0.022 inches to about 0.0675 inches while the narrower range is about 0.036 inches to about 0.06 inches.
While generally the tabs or a majority of the tabs are aligned so as to control pressure drop, it may be useful in some embodiments of the invention to have a subset of the tabs purposely arranged or configured to generate turbulence. These turbulence generating tabs may, for example, be arranged with offset angles greater than 10 degrees, e.g., at or near 90 degrees but often at lower offsets, and may be located on the fin in select areas to create higher heat transfer in otherwise low heat transfer areas (such as behind the collars and/or tubes) or may be interspersed among the other tabs. The number of tabs in the subset relative to the other tabs may vary widely to practice the invention and will typically be driven by allowable pressure drop for a heat exchanger application.
In other embodiments (not shown), a transition to turbulence can be promoted by the configuration of the tabs and base fin by manipulating surface roughness or texture.
Generally, the tabs and base fin have low surface roughness, e.g., are smooth metal. In some embodiments, though, surface roughness of the base metal is increased to a desired amount to cause adjacent flow to begin to transition to turbulent flow. The surface roughness can be thought of as a surface treatment and may include (or be replaced by) dimples or other surface treatments that alter the surface texture from smooth to a level of roughness that promotes turbulent flow. In one embodiment, the surface treatment is applied only to tabs (or portions of each tab or a subset of the tabs) while in others the treatment is applied only to the fin and/or tubes. In other cases, the surface treatment may be applied to all of these components or any combination.
The above disclosure sets forth a number of embodiments of the present invention Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims. Particularly, the use of tabs aligned and configured as discussed in the above description is readily applicable with fin arrangements other than parallel plate arrangements. For example, air-cooled condensers are often configured with tubes upon which fin material (such as aluminum foil) is helically wound. Such condensers may readily incorporate tabbed fins to enhance heat transfer, such as by punching the fin material prior to winding the material onto the tube. The tabs illustrated in the figures are generally planar with a rectangular cross-section when viewed from the leading edge. Other tab cross sections can readily be envisioned and are considered within the breadth of the above disclosure. For example, tabs with an upside down “L” cross-section can be substituted for the illustrated tabs and may be useful for placing greater tab surface area in the cooler air flow while not unacceptably increasing drag and/or manufacturing costs. Other tab cross sections include a stepped cross section, a wavy or serpentine cross section, an L-bend, a U-bend, and the like. Fin bodies can be fabricated with tabs extending from both sides by forming a composite fin from two fins with tabs extending from one side and their planar surfaces abutting.
Claims
1. A method for fabricating a heat transfer fin for a heat exchanger, comprising:
- providing a fin for use in a heat exchanger with a fin body having first and second sides;
- selecting a tab pattern for the fin, wherein the tab pattern defines a quantity of and location of heat transfer tabs; and
- forming the heat transfer tabs defined in the tab pattern by creating openings in the fin by removing material from the fin body while retaining a connecting edge between the fin body and the removed material, whereby a tab body is formed from the removed material extending outward from the fin body;
- wherein the forming step comprises bending the removed material to a bend angle relative to one of the first and second sides and wherein the tab bodies are substantially planar with a majority of the tab bodies aligned parallel to a predetermined directional line.
2. The method of claim 1, wherein the predetermined directional line is an anticipated simple flow path for a cooling gas across the fin body.
3. The method of claim 1, wherein the predetermined directional line is transverse to an anticipated simple flow path to channel flow across the fin in a predetermined direction.
4. The method of claim 1, wherein a minority of the tabs are vortex generators and are aligned at an angle greater than 5 degrees relative to the simple flow path of the cooling gas.
5. The method of claim 4, wherein the minority of the tabs are positioned proximal to a wake region for the plain fin.
6. The method of claim 1, wherein the tab pattern is selected such that at least a portion of the tabs extending from the first or the second side are arranged to direct flow to areas of low flow for the plain fin.
7. The method of claim 1, wherein the fin body includes a leading edge and the majority of the tabs are aligned substantially perpendicular to the leading edge.
8. The method of claim 1, wherein a minority of the tabs are direction vanes at an angle of less than about 10 degrees from the simple flow path to direct flow of a gas flowing over the minority of the tabs into anticipated wake regions.
9. The method of claim 8, wherein the minority of the tabs are aligned such that the tab bodies of the minority tabs are substantially parallel to a local flow path.
10. The method of claim 1, wherein the tab pattern is selected such that during the tab forming step a first portion of the tabs are bent to extend from the first side and a second portion of the tabs are bent to extend from the second side at the bend angle.
11. The method of claim 10, wherein the bend angle is between about 30 and 90 degrees.
12. The method of claim 1, wherein the tabs extend a tab height measured from fin body, the tab height being less than about seventy-five percent of a fin separation distance defining a gap between adjacent ones of the fin in a heat exchanger.
13. The method of claim 12, wherein the tab pattern is selected such that the tab bodies have a combined surface area that is less than about 50 percent of a surface area of the first side of the fin body.
14. The method of claim 13, wherein the combined surface area of the tab bodies is between about 10 and 30 percent of the area of the first side.
15. The method of claim 1, wherein the creating of the openings in the fin comprises applying a punch mechanism to the fin body, the punch mechanism configured according to the tab pattern and adapted to concurrently form the tabs extending from the first and second sides of the fin body.
16. The method of claim 1, wherein the tab pattern is selected such that the heat transfer tabs only extend from the first or the second side.
17. The method of claim 1, wherein the tabs extend a tab height measured from the side of the fin body from which the tabs extend, the tab height being less than about a fin separation distance.
18. A fin for use with tubes in a finned-tube, air-cooled heat exchanger, comprising:
- a metallic fin body with first and second heat transfer surfaces and a leading edge;
- tube collars formed in the fin body for receiving and contacting the tubes of the heat exchanger; and
- a plurality of tabs extending at a bend angle from the first and second heat transfer surfaces, wherein each of the tabs comprises a substantially planar body and wherein the tab bodies are positioned at offset angles, the offset angles being less than about 20 degrees as measured from a simple flow path extending across the fin body substantially perpendicular to the leading edge of the fin body.
19. The fin of claim 18, wherein the bend angle is between about 70 and 110 degrees as measured from the first or the second heat transfer surface.
20. The fin of claim 18, wherein about 50 percent of the tabs extend from the first heat transfer surface.
21. The fin of claim 18, wherein the tabs have a height as measured from the first or second heat transfer surface that is less than about two thirds of a predetermined fin separation distance for the heat exchanger.
22. The fin of claim 18, wherein the tab bodies are generally square or generally rectangular in shape and include at least a partially curved shoulder at a leading edge.
23. The fin of claim 18, wherein the tabs are positioned on the fin body such that the tabs are less densely distributed in a wake region near the tube collars and distal to the leading edge of the fin body.
24. The fin of claim 18, wherein the tabs are arranged in rows relative to the leading edge, and wherein in each of the rows a first portion of the tabs extend from the first heat transfer surface and a second portion of the tabs extend from the second heat transfer surface.
25. The fin of claim 24, wherein each of the tabs extending from a same one of the heat transfer surfaces in each of the rows is offset an offset distance relative to corresponding ones of the tabs in adjacent ones of the rows.
26. The fin of claim 24, wherein adjacent ones of the rows are offset relative to each other such that the tabs in the adjacent rows are not coplanar.
27. The fin of claim 18, wherein the offset angles are less than about 10 degrees.
28. The fin of claim 27, wherein the offset angles differ for at least some of the tabs and the offset angles are selected to position the tab bodies substantially parallel with a plurality of predetermined local flow paths for a fluid flowing adjacent to the heat transfer surfaces.
29. The fin of claim 28, wherein the tab bodies are positioned at angles of less than about 10 degrees as measured from the local flow paths.
30. The fin of claim 18, further including a delta winglet pair associated with each of the tube collars on the first heat transfer surface of the fin body.
31. The fin of claim 30, wherein the delta winglet pairs are positioned proximal to the tube collars.
32. The fin of claim 18, wherein a minority of the tabs are aligned at an angle relative to the majority of the tabs, the minority of tabs being positioned proximal to the tube collars and the angle being selected to direct a gas flowing over the fin body around the tube collar.
33. The fin of claim 18, wherein the tabs are positioned adjacent the tube collars to disrupt heat conduction pathways in the fin body that extend substantially parallel to the leading edge away from the tube collars.
34. The fin of claim 18, wherein the fin body comprises a first body half comprising the first heat transfer surface and a planar mating surface and the fin body further comprises a second body half comprising the second heat transfer surface and a planar mating surface, the mating surfaces of the first and second body halves being adjacent.
35. The fin of claim 18, wherein a subset of the tabs are positioned at offset angles greater than 20 degrees to generate turbulence in air flowing across the fin body.
36. The fin of claim 18, wherein at least a subset of the tabs have a surface roughness greater than the heat transfer surfaces of the fin body to promote a transition to turbulence adjacent the portion of the tabs.
37. The fin of claim 18, wherein at least a portion of the first heat transfer surface or the second heat transfer surface of the fin body has a surface treatment selected to promote turbulence adjacent the surface treated portion.
38. An air-cooled heat exchanger, comprising:
- a plurality of conduits for passing a hot fluid through the heat exchanger; and
- a plurality of fins contacting the conduits, the fins being spaced apart a fin separation distance and defining an air flow passage between adjacent pairs of the fins;
- wherein the fins comprise:
- a metallic fin body with first and second sides and a leading edge; and
- a plurality of tabs extending at a bend angle from the first and second sides, wherein the tabs are arranged with a leading edge proximal to a leading edge of the fin body and within about 5 degrees of local flow paths in the air flow passage.
39. The heat exchanger of claim 38, wherein the tabs are arranged in rows relative to the leading edge in which a first portion of the tabs extend from the first heat transfer surface and a second portion of the tabs extend from the second heat transfer surface and wherein adjacent ones of the rows are offset relative to each other.
40. The heat exchanger of claim 38, wherein adjacent pairs of the fins are connected and the fins comprise metallic foil, and wherein the fins are attached to the conduit by winding in a helical pattern about the outer surface of the conduit.
41. The heat exchanger of claim 38, wherein the tabs are substantially rectangular in shape and the bend angle is about 90 degrees.
42. The heat exchanger of claim 38, wherein the tabs have a tab height as measured from the fin body to an edge distal to the fin body in the range of about 25 to about 75 percent of the fin separation distance.
43. The heat exchanger of claim 42, wherein the tab height is in the range of about 40 to about 67 percent of the fin separation distance.
44. An air-cooled heat exchanger, comprising:
- a plurality of conduits for passing a hot fluid through the heat exchanger; and
- a plurality of fins contacting the conduits, the fins being spaced apart a fin separation distance and defining an air flow passage between adjacent pairs of the fins;
- wherein the fins comprise:
- a metallic fin body with first and second sides and a leading edge; and
- a plurality of tabs extending at a bend angle from the first side, wherein the tabs are arranged with a leading edge proximal to a leading edge of the fin body and within about 10 degrees of local flow paths in the air flow passage.
45. The heat exchanger of claim 42, wherein the tabs comprise a body having a shape comprising a square, a rectangle, a trapezoid, a triangle, or a semi-circle.
46. The heat exchanger of claim 45, wherein the tab body is non-planar with a larger percentage of the tab body surface area proximal to the fin body.
47. The heat exchanger of claim 46, wherein the tab body has an L-shaped or U-shaped cross section when viewed from the leading edge of the fin body.
48. The heat exchanger of claim 44, wherein at least a portion of the tabs extend across the fin separation distance to abut an adjacent one of the fin bodies, whereby the portion of tabs act as spacers between the fins.
49. The heat exchanger of claim 44, wherein the first side is proximal to a lower portion of the heat exchanger such that the tabs extend substantially parallel to the direction of gravity when the heat exchanger is mounted for use.
Type: Application
Filed: Jul 1, 2004
Publication Date: Aug 3, 2006
Inventors: Charles Kutscher (Golden, CO), Eric Kozubal (Boulder, CO)
Application Number: 10/549,962
International Classification: B21D 53/04 (20060101);