Abstract:
A high performance louvered fin for a heat exchanger is disclosed, wherein adjacent entrance louvers have increased widths and adjacent exit louvers have decreased widths in order to optimize thermal efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/403,311 filed on Apr. 13, 2006. The entire disclosure of the above application is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to heat exchangers and more particularly to high performance louvered fins for heat exchangers. 
       BACKGROUND OF THE INVENTION 
       [0003]    An air-cooled fin-type heat exchanger is very well known. Heat exchangers are used for changing the temperature of various working fluids, such as an engine coolant, an engine lubricating oil, an air conditioning refrigerant, and an automatic transmission fluid, for example. The heat exchanger typically includes a plurality of spaced apart fluid conduits or tubes connected between an inlet tank and an outlet tank, and a plurality of heat exchanging fins disposed between adjacent conduits. Air is directed across the fins of the heat exchanger by a cooling fan or a motion of a vehicle, for example. As the air flows across the fins, heat in a fluid flowing through the tubes is conducted through the walls of the tubes, into the fins, and transferred into the air. 
         [0004]    One of the primary goals in heat exchanger design is to achieve the highest possible thermal efficiency. Thermal efficiency is measured by dividing the amount of heat that is transferred by the heat exchanger under a given set of conditions (amount of airflow, temperature difference between the air and fluid, and the like) by the theoretical maximum possible heat transfer under those conditions. Thus, an increase in the rate of heat transfer results in a higher thermal efficiency. 
         [0005]    Typically, to improve thermal efficiency the airflow must be improved and/or a pressure drop through the heat exchanger must be reduced. Improved heat exchanger performance can be accomplished by forming the fins and/or louvers on the fins at a predetermined angle in a manner also well known in the art. Pressure drop is associated with the change in airflow direction caused by the louvered fins. A higher air pressure drop can result in a lower heat transfer rate. Various types of fin and louver designs have been disclosed in the prior art with the object of increasing the heat exchanger efficiency by making improvements in the fins, louvers, and airflow pattern. 
         [0006]    Examples of these prior art fin and louver designs include an addition of fin rows in order to increase the amount of air encountered by the heat exchanger. Other designs include louvers formed at an angle to the fin wall, rather than square to the fin wall. Further, the prior art discloses heat exchangers with multiple changes of airflow direction. Air flows through the louvers until a middle transition piece or turnaround rib is reached. The air then changes direction and flows through exit louvers to exit the heat exchanger. Fin design continues to play an important role in increasing heat exchanger efficiency. 
         [0007]    It would be desirable to produce a fin for a heat exchanger whereby a pressure drop associated therewith is minimized and an airflow through the heat exchanger is maximized. 
       SUMMARY OF THE INVENTION 
       [0008]    In concordance with the instant disclosure, a fin for a heat exchanger whereby a pressure drop associated therewith is minimized and an airflow through the heat exchanger is maximized, has been discovered. 
         [0009]    In one embodiment, a flat tube heat exchanger comprises at least one header; a plurality of spaced apart tubes in fluid communication with the header; and a plurality of fins disposed between the tubes, the fins further comprising: a base wall having a longitudinal axis, a first end, a second end, and a middle portion; at least one turnaround rib disposed in the middle portion of the base wall; a plurality of spaced apart entrance louvers disposed between the first end of the base wall and the turnaround rib, the entrance louvers having a first edge and a second edge, a width of each of the entrance louvers defined as a distance between the first edge of each of the entrance louvers and the second edge of each of the entrance louvers, wherein the width of at least one of the entrance louvers is greater than the width of a remainder of the entrance louvers; and a plurality of spaced apart exit louvers disposed between the turnaround rib and the second end of the base wall, the exit louvers having a first edge and a second edge, a width of each of the exit louvers defined as a distance between the first edge of each of the exit louvers and the second edge of each of the exit louvers, wherein the width of at least one of the exit louvers is greater that the width of a remainder of the exit louvers, is disclosed. 
         [0010]    In another embodiment, a high performance heat exchanger fin comprises a base wall having a first end, a second end, and a middle portion; at least one turnaround rib disposed in the middle portion of the base wall; a plurality of spaced apart entrance louvers having a longitudinal axis, a first edge, and a second edge, the entrance louvers disposed between the first end of the base wall and the turnaround rib, a width of each of the entrance louvers defined as a distance between the first edge and the second edge, wherein the width of the entrance louvers increases moving in a direction from the first end to the second end of the base wall, each of the entrance louvers disposed at a predetermined angle in respect of the longitudinal axis of the entrance louver, the predetermined angle decreasing for at least one of the entrance louvers moving in a direction from the first end to the second end of the base wall; and a plurality of spaced apart exit louvers having a longitudinal axis, a first edge, and a second edge, the exit louvers disposed between the turnaround rib and the second end of the base wall, a width of each of the exit louvers defined as a distance between the first edge and the second edge, wherein the width of the exit louvers decreases moving in a direction from the first end to the second end of the base wall, each of the exit louvers disposed at a predetermined angle in respect of the longitudinal axis of the base exit louvers, the predetermined angle decreasing for at least one of the exit louvers moving in a direction from the first end to the second end of the base wall, is disclosed. 
         [0011]    In another embodiment, a high performance heat exchanger fin comprises a base wall having a first end, a second end, and a middle portion, the base wall having a first longitudinal axis extending from the first end to the middle portion and a second longitudinal axis extending from the middle portion to the second end, whereby the first longitudinal axis and the second longitudinal axis are non-linear; at least one turnaround rib disposed in the middle portion of the base wall; a plurality of spaced apart entrance louvers having a first edge and a second edge, the entrance louvers disposed between the first end of the base wall and the turnaround rib, a width of each of the entrance louvers defined as a distance between the first edge and the second edge, wherein the width of the entrance louvers increases moving in a direction from the first end to the second end of the base wall; and a plurality of spaced apart exit louvers having a first edge and a second edge, the exit louvers disposed between the turnaround rib and the second end of the base wall, a width of each of the exit louvers defined as a distance between the first edge and the second edge, wherein the width of the exit louvers decreases moving in a direction from the first end to the second end of the base wall, is disclosed. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]    The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
           [0013]      FIG. 1  is a perspective view of a flat tube heat exchanger including a high performance heat exchanger fin in accordance with an embodiment of the invention; 
           [0014]      FIG. 2  is a perspective view of the high performance heat exchanger fin illustrated in  FIG. 1 ; 
           [0015]      FIG. 3  is a top sectional view of a plurality of louvers of the high performance heat exchanger fin of  FIG. 2  taken along line  3 - 3 ; 
           [0016]      FIG. 4  is a top sectional view of a plurality of louvers in accordance with another embodiment of the invention; 
           [0017]      FIG. 5  is a top sectional view of a plurality of louvers in accordance with another embodiment of the invention; 
           [0018]      FIG. 6  is a top sectional view of a plurality of louvers in accordance with another embodiment of the invention; 
           [0019]      FIG. 7  is a top sectional view of a plurality of louvers in accordance with another embodiment of the invention; and 
           [0020]      FIG. 8  is a top sectional view of a plurality of louvers in accordance with another embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. 
         [0022]      FIG. 1  shows a flat tube heat exchanger  1  in accordance with an embodiment of the current invention. The heat exchanger  1  includes a tank or header  2  having a fluid inlet  4  and a fluid outlet  6 . A plurality of flat tubes  8  are in fluid communication with the tank  2 . A plurality of high performance heat exchanger fins  10  is disposed between each of the flat tubes  8 . It is understood that more or fewer flat tubes  8  and fins  10  can be used as desired without departing from the spirit or scope of the invention. 
         [0023]    The high performance heat exchanger fins  10  are more clearly shown in  FIG. 2 . The heat exchanger fins  10  include a plurality of base walls  12 . It is understood that more or fewer base walls  12  can be used without departing from the spirit or scope of the invention. The base walls  12  include a first end  14 , a spaced apart second end  16 , and a middle portion  18  disposed therebetween. 
         [0024]    The base walls  12  include a leading edge louver  17 , a trailing edge louver  19 , a plurality of entrance louvers  20 , a plurality of exit louvers  22 , and a turnaround rib  24 . The leading edge louver  17  and the entrance louvers  20  are connected to the base wall  12  at a first end  26  and a spaced apart second end  28 . The entrance louvers  20  are pivoted about a bend axis  37  to dispose each of the louvers  20  at a predetermined angle α from the base wall  12 . The trailing edge louver  19  and the exit louvers  22  are connected to the base wall  12  at a first end  30  and a spaced apart second end  32 . The exit louvers  22  are pivoted about a bend axis  39  to dispose each of the louvers  22  at a predetermined angle β from the base wall  12 . The turnaround rib  24  is connected to the base wall  12  at a first end  34  and at a spaced apart second end  36 . 
         [0025]    As more clearly shown in  FIG. 3 , each of the entrance louvers  20  includes a first edge  38  and a spaced apart second edge  40 . A gap  41  is formed between adjacent entrance louvers  20 . A first distance  43  is measured in the gap  41  between the first edges  38  of adjacent entrance louvers  20 , and a second distance  45  is measured between the second edges  40  of adjacent entrance louvers  20 . 
         [0026]    A width W of each of the entrance louvers  20  is defined as the distance between the first edge  38  and the second edge  40  thereof. In the embodiment shown, the width W of adjacent entrance louvers  20  varies. Each adjacent entrance louver  20  has a slightly greater width W from the entrance louver  20  adjacent the first end  14  of the base wall  12  to the entrance louver  20  adjacent the turnaround rib  24 . Thus, the width W of the entrance louver  20  adjacent the first end  14  of the base wall  12  is smaller than the width W of each of the remaining entrance louvers  20  leading to the turnaround rib  24 . The first edge  38  and the second edge  40  of each entrance louver  20  extend laterally outwardly from a longitudinal axis of the entrance louvers  20  further than the first edge  38  and the second edge  40  of each entrance louver  20  moving from the first end  14  of the base wall  12  to the turnaround rib  24 . This change in lateral extension is a result of the difference in width W of adjacent entrance louvers  20 . In this embodiment, the predetermined angle α from the base wall  12  remains substantially constant for each of the entrance louvers  20 . 
         [0027]    Each of the exit louvers  22  includes a first edge  42  and a spaced apart second edge  44 . A gap  47  is formed between adjacent exit louvers  22 . A first distance  49  is measured in the gap  47  between the first edges  42  of adjacent exit louvers  22 , and a second distance  51  is measured between the second edges  44  of adjacent exit louvers  22 . 
         [0028]    A width W of each of the exit louvers  22  is defined as the distance between the first edge  42  and the second edge  44  thereof. In the embodiment shown, the width W of adjacent exit louvers  22  varies. Each adjacent exit louver  22  has a slightly smaller width W when moving from the turnaround rib  24  to the second end  16  of the base wall  12 . To account for a difference in the width W of adjacent exit louvers  22 , the first edge  42  and the second edge  44  of each exit louver  22  does not extend laterally outwardly as far as the first edge  42  and the second edge  44  of an adjacent exit louver  22  moving from the exit louver  22  adjacent the turnaround rib  24  to the exit louver  22  adjacent the second end  16  of the base wall  12 . In this embodiment, the predetermined angle β from the base wall  12  remains substantially constant for each of the exit louvers  22 . 
         [0029]    As in known in the art, air is caused to flow through the gaps  41  between the entrance louvers  20 . Heat removed from the fluid located in the flat flow tubes  8  is transferred through the heat exchanger fin  10  and the entrance louvers  20  to the air. The air is then turned at the turnaround rib  24 . The air flows through the gaps  47  between the exit louvers  22  where additional heat is transferred from the exit louvers  22  to the air. 
         [0030]    A pressure drop through the louvers  20 ,  22  is minimized. The increase in the width W of adjacent entrance louvers  20  and the decrease in the width W of adjacent exit louvers  22  helps accomplish these benefits by minimizing frictional losses and maximizing an exposed surface of the louvers  20 ,  22 . For the embodiment shown in  FIGS. 1 and 2 , at least a 15% reduction in pressure drop has been measured. 
         [0031]      FIG. 4  shows a leading edge louver  117 , a trailing edge louver  119 , a plurality of entrance louvers  120 , a plurality of exit louvers  122 , and a turnaround rib  124  in accordance with another embodiment of the invention. The leading edge louver  117  is connected to a base wall (not shown) as discussed above for  FIG. 2 . The entrance louvers  120  include a first edge  138  and a spaced apart second edge  140 , and are connected to a base wall as discussed for the  FIG. 2 . The entrance louvers  120  are pivoted about a bend axis  137  to dispose each of the louvers  120  at a predetermined angle α from the base wall. A gap  141  is formed between adjacent entrance louvers  120 . A first distance  143  is measured between the first edges  138  of adjacent entrance louvers  120 . A second distance  145  is measured in the gap  141  between the second edges  140  of adjacent entrance louvers  120 . 
         [0032]    Each of the entrance louvers  120  is disposed at the predetermined angle α from the base wall. In this embodiment, to account for a difference in the width W of adjacent entrance louvers  120 , the predetermined angle α of each entrance louver  120  moving from the first end of the base wall to the turnaround rib  124  is decreased. The angle α is decreased by an amount in order to maintain all of the first edges  138  of the entrance louvers  120  in substantially the same plane, and all of the second edges  140  of the entrance louvers  120  in substantially the same plane. 
         [0033]    The trailing edge louver  119  is connected to the base wall as discussed above for  FIG. 2 . The exit louvers  122  include a first edge  142  and a spaced apart second edge  144 , and are connected to a base wall as discussed for the  FIG. 2 . The exit louvers  122  are pivoted about a bend axis  139  to dispose each of the louvers  122  at a predetermined angle β from the base wall. A gap  147  is formed between adjacent exit louvers  122 . A first distance  149  is measured in the gap  147  between the first edges  142  of adjacent exit louvers  122  and a second distance  151  is measured in the gap  147  between the second edges  144  of adjacent exit louvers  122 . 
         [0034]    Each of the exit louvers  122  is disposed at the predetermined angle β from the base wall. The predetermined angle β of each exit louver  122  moving from the turnaround rib  124  to the second end of the base wall is decreased. The angle β is decreased by an amount to maintain the first edges  142  of the exit louvers  122  in substantially the same plane. Likewise, the decreasing angle β maintains the second edges  144  of the exit louvers  122  in substantially the same plane. Air flow through the louvers  117 ,  119 ,  120 ,  122  is the same as described above for  FIG. 3 . 
         [0035]      FIG. 5  shows a leading edge louver  217 , a trailing edge louver  219 , a plurality of entrance louvers  220 , a plurality of exit louvers  222 , and a turnaround rib  224  in accordance with another embodiment of the invention. The leading edge louver  217  is connected to a base wall (not shown) as discussed above for  FIG. 2 . Each of the entrance louvers  220  includes a first edge  238  and a spaced apart second edge  240 , and is connected to a base wall as discussed above for  FIG. 2 . The entrance louvers  220  are pivoted about a bend axis  237  to dispose each of the louvers  220  at a predetermined angle α from the base wall. Adjacent entrance louvers  220  include a gap  241  formed therebetween. A first distance  243  is measured in the gap  241  between the first edges  238  of adjacent entrance louvers  220 , and a second distance  245  is measured in the gap  241  between the second edges  240  of adjacent entrance louvers  220 . 
         [0036]    The trailing edge louver  219  is connected to the base wall as discussed above for  FIG. 2 . Each of the exit louvers  222  includes a first edge  242  and a spaced apart second edge  244 , and is connected to a base wall as discussed above for  FIG. 2 . The exit louvers  222  are pivoted about a bend axis  239  to dispose each of the louvers  222  at a predetermined angle β from the base wall. A gap  247  is formed between adjacent exit louvers  222 . A first distance  249  is measured in the gap  247  between the first edges  242  of adjacent exit louvers  222  and a second distance  251  is measured in the gap  247  between the second edges  244  of adjacent exit louvers  222 . 
         [0037]    A first convex curved surface  253  and a second convex curved surface  255  extend between the first edge  238  and the second edge  240  of the entrance louvers  220 , and the first edges  242  and the second edges  244  of the exit louvers  222  over an entire length thereof. The first convex curved surface  253  and the second convex curved surface  255  cooperate to generally form an oval or football shape in cross section. 
         [0038]    Adjacent entrance louvers  220  and exit louvers  222  include the same width pattern as discussed above for  FIG. 4 . The entrance louvers  220  have a width W that increases from the entrance louver  220  adjacent the first end of the base wall to the entrance louver  220  adjacent the turnaround rib  224 . The exit louvers  222  have a width W that decreases from the exit louver  222  adjacent the turnaround rib  224  to the exit louver  222  adjacent the second end of the base wall. 
         [0039]    Each of the entrance louvers  220  is disposed at the predetermined angle α from the base wall. In this embodiment, the predetermined angle α is decreased by an amount necessary to maintain the first edges  238  of the entrance louvers  220  in substantially the same plane and the second edges  240  of the entrance louvers  220  in substantially the same plane. 
         [0040]    Each of the exit louvers  222  is disposed at the predetermined angle β from the base wall. Similar to the description above for the entrance louvers  220 , the predetermined angle β is decreased. The angle β is decreased by an amount necessary to maintain the first edges  242  of the exit louvers  222  in substantially the same plane. Similarly, the second edges  244  of the exit louvers  222  are maintained in substantially the same plane. It is understood that the louvers  220 ,  222  can include the same width W pattern as those described above for  FIG. 3 , wherein the angles α, β between adjacent louvers  220 ,  222  remain substantially constant. Air flow through the louvers  217 ,  219 ,  220 ,  222  is the same as described above for  FIG. 3 . 
         [0041]      FIG. 6  shows a leading edge louver  317 , a trailing edge louver  319 , a plurality of entrance louvers  320 , a plurality of exit louvers  322 , and a turnaround rib  324  in accordance with another embodiment of the invention. The leading edge louver  317  is connected to the base wall (not shown) as discussed above for  FIG. 2 . Each of the entrance louvers  320  includes a first edge  338  and a spaced apart second edge  340 . Each of the louvers  320 ,  322  is connected to a base wall as previously described for  FIG. 2 . The entrance louvers  320  are pivoted about a bend axis  337  to dispose each of the louvers  320  at a predetermined angle α from the base wall. A gap  341  is formed between adjacent entrance louvers  320 . A first distance  343  is measured between the first edges  338  of adjacent entrance louvers  320 . A second distance  345  is measured in the gap  341  between the second edges  340  of adjacent entrance louvers  320 . 
         [0042]    A first bend  346  and a second bend  348  are formed between the first edge  338  and the second edge  340  of the entrance louvers  320 . In the embodiment shown, the first bend  346  is formed in a direction opposite the second bend  348 , resulting in a generally S-shaped structure in cross section. 
         [0043]    The trailing edge louver  319  is connected to the base wall as discussed above for  FIG. 2 . The exit louvers  322  include a first edge  342  and a spaced apart second edge  344 , and are connected to a base wall as discussed for the previous embodiments. The exit louvers  322  are pivoted about a bend axis  339  to dispose each of the louvers  322  at a predetermined angle β from the base wall. A gap  347  is formed between adjacent exit louvers  322 . A first distance  349  is measured in the gap  347  between the first edges  342  of adjacent exit louvers  322  and a second distance  351  is measured in the gap  347  between the second edges  344  of adjacent exit louvers  322 . 
         [0044]    A first bend  350  and a second bend  352  are formed in the exit louvers  322  between the first edge  342  and the second edge  344  thereof. Thus, a cross sectional shape of the exit louvers  322  is generally a reverse S. 
         [0045]    Adjacent entrance louvers  320  and adjacent exit louvers  322  include the same width pattern as discussed above for  FIG. 4 . A width W of the entrance louvers  320  increases from the entrance louver  320  adjacent the first end of the base wall to the entrance louver  320  adjacent the turnaround rib  324 . The increase in the width W can result from a change in the distance between the first edge  338  and the first bend  346 , the first bend  346  and the second bend  348 , the second bend  348  and the second edge  340 , or any other combination thereof. 
         [0046]    The exit louvers  322  have a width W that decreases from the exit louver  322  adjacent the turnaround rib  324  to the exit louver  322  adjacent the second end of the base wall. The decrease in the width W can result from a change in the distance between the first edge  342  and the first bend  350 , the first bend  350  and the second bend  352 , the second bend  352  and the second edge  344 , or any other combination thereof. 
         [0047]    The first edges  338  of the entrance louvers  320  and the second edges  340  of the entrance louvers  320  are disposed at the predetermined angle α from the base wall. In this embodiment, to account for a difference in the width W between adjacent entrance louvers  320 , the predetermined angle α of each entrance louver  320  is decreased. The angle α is decreased by an amount necessary to maintain all of the first edges  338  of the entrance louvers  320  in substantially the same plane and all of the second edges  340  of the entrance louvers  320  in substantially the same plane. 
         [0048]    The first edges  342  of the exit louvers  322  and the second edges  344  of the exit louvers  322  are disposed at the predetermined angle β from the base wall. The predetermined angle β of each exit louver  322  moving from the middle portion to the second end is decreased. The angle β is decreased by an amount to maintain the first edges  342  of the exit louvers  322  in substantially the same plane. Likewise, the decreasing angle β maintains the second edges  344  of the exit louvers  322  in substantially the same plane. It is understood that the louvers  320 ,  322  may have the same width W pattern as those described for  FIG. 3  above, wherein the angles α, β between adjacent louvers  320 ,  322  remain substantially constant. Air flow through the louvers  317 ,  319 ,  320 ,  322  is the same as described above for  FIG. 3 . 
         [0049]      FIG. 7  shows a leading edge louver  417 , a trailing edge louver  419 , a plurality of entrance louvers  420 , a plurality of exit louvers  422 , and a turnaround rib  424  in accordance with another embodiment of the invention. The leading edge louver  417  is connected to a base wall (not shown) as discussed above for  FIG. 2 . Each of the entrance louvers  422  includes a first edge  438  and a spaced apart second edge  440 , and is connected to a base wall as discussed above for  FIG. 2 . The entrance louvers  420  are pivoted about a bend axis  437  to dispose each of the louvers  420  at a predetermined angle α from the base wall. A gap  441  is formed between adjacent entrance louvers  420 . A first distance  443  is measured in the gap  441  between the first edges  438  of adjacent entrance louvers  420 , and a second distance  445  is measured between the second edges  440  of adjacent entrance louvers  420 . 
         [0050]    A width W of the entrance louvers  420  is defined as the distance between the first edge  438  and the second edge  440 . The width W of adjacent entrance louvers  420  varies. Each adjacent entrance louver  420  has a slightly greater width W from the entrance louver  420  adjacent the first end of the base wall to the entrance louver  420  adjacent the turnaround rib  424 . Thus, the width W of the entrance louver  420  adjacent the first end of the base wall is smaller than the width W of each of the remaining entrance louvers  420  leading to the turnaround rib  424 . In this embodiment, the predetermined angle α from the base wall remains substantially constant for each of the entrance louvers  420 . 
         [0051]    In this embodiment, to account for a difference in the width W of adjacent entrance louvers  420 , a decrease in the predetermined angle α between louvers as described in  FIG. 4  is combined with the increase of the extension of the edges of the adjacent louvers as described in  FIG. 3 . A gap  441  is formed between adjacent entrance louvers  420 . A first distance  443  is measured in the gap  441  between first edges  438  of adjacent entrance louvers  420 , and a second distance  445  is measured in the gap  441  between second edges  440  of adjacent entrance louvers  420 . 
         [0052]    The trailing edge louver  419  is connected to the base wall as discussed above for  FIG. 2 . The exit louvers  422  include a first edge  442  and a spaced apart second edge  444 , and are connected to a base wall as discussed for the  FIG. 2 . The exit louvers  422  are pivoted about a bend axis  439  to dispose each of the louvers  422  at a predetermined angle β from the base wall. A gap  447  is formed between adjacent exit louvers  422 . A first distance  449  is measured in the gap  447  between the first edges  442  of adjacent exit louvers  422 , and a second distance  451  is measured between the second edges  444  of adjacent exit louvers  422 . 
         [0053]    A width W of the exit louvers  422  is defined as the distance between the first edge  442  and the second edge  444 . The width W of adjacent exit louvers  422  varies. Each adjacent exit louver  422  has a slightly smaller width W when moving from the turnaround rib  424  to the second end of the base wall. In this embodiment, the predetermined angle β from the base wall remains substantially constant for each of the exit louvers  422 . 
         [0054]    In this embodiment, to account for a difference in the width W of adjacent exit louvers  422 , the predetermined angle β for each of the exit louvers  422  is decreased for each of the exit louvers  422  moving from a turnaround rib  424  to the second end of the base wall (not shown). Additionally, a decrease in the extension of the first edges  442  and the second edges  444  of the adjacent exit louvers  422  as described in  FIG. 3  is provided. 
         [0055]    Air flow through the louvers  417 ,  419   420 ,  422  is the same as described above for  FIG. 3 . It is understood that football shaped louvers as discussed in  FIG. 5 , and S-shaped louvers and reversed S-shaped louvers as discussed in  FIG. 6  can be replaced for the louvers shown in this embodiment. 
         [0056]    In another embodiment shown in  FIG. 8 , the fin (not shown) is bent along the length of the middle portion of a base wall (not shown) to form a first portion of the base wall and a second portion of the base wall. The bend along the middle portion forms the entrance louvers  520  and the exit louvers  522  in a staggered pattern. 
         [0057]    There is shown a leading edge louver  517 , a trailing edge louver  519 , a plurality of entrance louvers  520 , a plurality of exit louvers  522 , and a turnaround rib  524 . The leading edge louver  517  is connected to the base wall as discussed above for  FIG. 2 . Each of the entrance louvers  520  includes a first edge  538  and a spaced apart second edge  540 , and is connected to the base wall as discussed above for  FIG. 2 . The entrance louvers  520  are pivoted about a bend axis  537  to dispose each of the entrance louvers  520  at a predetermined angle α from the base wall. Adjacent entrance louvers  520  include a gap  541  formed therebetween. A first distance  543  is measured in the gap  541  between the first edges  538  of adjacent entrance louvers  520 , and a second distance  545  is measured in the gap  541  between the second edges  540  of adjacent entrance louvers  520 . 
         [0058]    The trailing edge louver  519  is connected to the base wall as discussed above for  FIG. 2 . Each of the exit louvers  522  includes a first edge  542  and a spaced apart second edge  544 , and is connected to a base wall as discussed above for  FIG. 2 . The exit louvers  522  are pivoted about a bend axis  539  to dispose each of the louvers  522  at a predetermined angle β from the base wall. A gap  547  is formed between adjacent exit louvers  522 . A first distance  549  is measured in the gap  547  between the first edges  542  of adjacent exit louvers  522  and a second distance  551  is measured in the gap  547  between the second edges  544  of adjacent exit louvers  522 . 
         [0059]    Adjacent entrance louvers  520  and exit louvers  522  include the same width pattern as discussed above for  FIG. 3 . The entrance louvers  520  have a width W that increases from the entrance louver  520  adjacent the first end of the base wall to the entrance louver  520  adjacent the turnaround rib  524 . The exit louvers  522  have a width W that decreases from the exit louver  522  adjacent the turnaround rib  524  to the exit louver  522  adjacent the second end of the base wall. In the embodiment shown, the predetermined angles α, β from the base wall remain substantially constant for each of the louvers  520 ,  522 . However, it is understood that these angles could vary between adjacent louvers as described for  FIGS. 4-7  above. 
         [0060]    Air flow through the louvers  517 ,  519   520 ,  522  is the same as described above for  FIG. 3 . It is understood that football shaped louvers as discussed in  FIG. 5 , and S-shaped louvers and reversed S-shaped louvers as discussed in  FIG. 6  can be replaced for the louvers shown in this embodiment. 
         [0061]    From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.