Abstract:
A rotary regenerative heat exchanger [ 1 ] employs heat transfer elements [ 100 ] shaped to include notches [ 150] , which provide spacing between adjacent elements [ 100 ], and undulations (corrugations) [ 165,185 ] in the sections between the notches  150.  The elements [ 100 ] described herein include undulations [ 165,185 ] that differ in height and/or width. These impart turbulence in the air or flue gas flowing between the elements [ 100 ] to improve heat transfer.

Description:
BACKGROUND 
       [0001]    The present invention relates to heat transfer elements of the type found in rotary regenerative heat exchangers. 
         [0002]    Rotary regenerative heat exchangers are commonly used to transfer heat from flue gases exiting a furnace to the incoming combustion air. Conventional rotary regenerative heat exchangers, such as that shown as  1  in  FIG. 1 , have a rotor  12  mounted in a housing  14 . The housing  14  defines a flue gas inlet duct  20  and a flue gas outlet duct  22  for the flow of heated flue gases  36  through the heat exchanger  1 . The housing  14  further defines an air inlet duct  24  and an air outlet duct  26  for the flow of combustion air  38  through the heat exchanger  1 . The rotor  12  has radial partitions  16  or diaphragms defining compartments  17  therebetween for supporting baskets (frames)  40  of heat transfer elements. The rotary regenerative heat exchanger  1  is divided into an air sector and a flue gas sector by sector plates  28 , which extend across the housing  14  adjacent the upper and lower faces of the rotor  12 . 
         [0003]      FIG. 2  depicts an end elevation view of an example of an element basket  40  including a few elements  10  stacked therein. While only a few elements  10  are shown, it will be appreciated that the basket  40  will typically be filled with elements  10 . As can be seen in  FIG. 2 , the elements  10  are closely stacked in spaced relationship within the element basket  40  to form passageways  70  between the elements  10  for the flow of air or flue gas. 
         [0004]    Referring to  FIGS. 1 and 2 , the hot flue gas stream  36  is directed through the gas sector of the heat exchanger  1  and transfers heat to the elements  10  on the continuously rotating rotor  12 . The elements  10  are then rotated about axis  18  to the air sector of the heat exchanger  1 , where the combustion air stream  38  is directed over the elements  10  and is thereby heated. In other forms of rotary regenerative heat exchangers, the elements  10  are stationary and the air and gas inlet and outlet portions of the housing  14  rotate. 
         [0005]      FIG. 3  depicts portions of conventional elements  10  in stacked relationship, and  FIG. 4  depicts a cross-section of one of the conventional elements  10 . Typically, elements  10  are steel sheets that have been shaped to include one or more various notches  50  and undulations  65 . 
         [0006]    Notches  50 , which extend outwardly from the element  10  at generally equally spaced intervals, maintain spacing between adjacent elements  10  when the elements  10  are stacked as shown in  FIG. 3 , and thus form sides of the passageways  70  for the air or flue gas between the elements  10 . Typically, the notches  50  extend at a predetermined angle (e.g. 90 degrees) relative to the fluid flow through the rotor ( 12  of  FIG. 1 ). 
         [0007]    In addition to the notches  50 , the element  10  is typically corrugated to provide a series of undulations (corrugations)  65  extending between adjacent notches  50  at an acute angle Au to the flow of heat exchange fluid, indicated by the arrow marked “A” in  FIG. 3 . The undulations  65  have a height of Hu and act to increase turbulence in the air or flue gas flowing through the passageways  70  and thereby disrupt the thermal boundary layer that would otherwise exist in that part of the fluid medium (either air or flue gas) adjacent to the surface of the element  10 . The existence of an undisrupted fluid boundary layer tends to impede heat transfer between the fluid and the element  10 . The undulations  65  on adjacent elements  10  extend obliquely to the line of flow. In this manner, the undulations  65  improve heat transfer between the element  10  and the fluid medium. Furthermore, the elements  10  may include flat portions (not shown), which are parallel to and in full contact with the notches  50  of adjacent elements  10 . For examples of other heat transfer elements  10 , reference is made to U.S. Pat. Nos. 2,596,642; 2,940,736; 4,396,058; 4,744,410; 4,553,458; and 5,836,379. 
         [0008]    Although such elements exhibit favorable heat transfer rates, the results can vary rather widely depending upon the specific design and the dimensional relationship between the notches and the undulations. For example, while the undulations provide an enhanced degree of heat transfer, they also increase the pressure drop across the heat exchanger ( 1  of  FIG. 1 ). Ideally, the undulations on the elements will induce a relatively high degree of turbulent flow in that part of the fluid medium adjacent to the elements, while the notches will be sized so that the fluid medium that is not adjacent to the elements (i.e., the fluid near the center of the passageways) will experience a lesser degree of turbulence, and therefore much less resistance to flow. However, attaining the optimum level of turbulence from the undulations can be difficult to achieve since both the heat transfer and the pressure loss tend to be proportional to the degree of turbulence that is produced by the undulations. An undulation design that raises the heat transfer tends to also raise the pressure loss and, conversely, a shape that lowers the pressure loss tends to lower the heat transfer as well. 
         [0009]    Design of the elements must also present a surface configuration that is readily cleanable. To clean the elements, it has been customary to provide soot blowers that deliver a blast of high-pressure air or steam through the passages between the stacked elements to dislodge any particulate deposits from the surface thereof and carry them away leaving a relatively clean surface. To accommodate soot blowing, it is advantageous for the elements to be shaped such that when stacked in a basket the passageways are sufficiently open to provide a line of sight between the elements, which allows the soot blower jet to penetrate between the sheets for cleaning. Some elements do not provide for such an open channel, and although they have good heat transfer and pressure drop characteristics, they are not very well cleaned by conventional soot blowers. Such open channels also allow for the operation of a sensor for measuring the quantity of infrared radiation leaving the element. Infrared radiation sensors can be used to detect the presence of a “hot spot”, which is generally recognized as a precursor to a fire in the basket ( 40  of  FIG. 2 ). Such sensors, commonly known as “hot spot” detectors, are useful in preventing the onset and growth of fires. Elements that do not have an open channel prevent infrared radiation from leaving the element and from being detected by the hot spot detector. 
         [0010]    Thus, there is a need for a rotary regenerative heat exchanger heat transfer element that provides decreased pressure loss for a given amount of heat transfer and that is readily cleanable by a soot blower and compatible with a hot spot detector. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention may be embodied as a heat transfer element [ 100 ] for a rotary regenerative heat exchanger [ 1 ] including: 
         [0012]    notches [ 150 ] extending parallel to each other and configured to form passageways [ 170 ] between adjacent heat transfer elements [ 100 ], each of the notches [ 150 ] including lobes [ 151 ] projecting outwardly from opposite sides of the heat transfer element [ 100 ] and having a peak-to-peak height Hn; 
         [0013]    first undulations [ 165 ] extending parallel to each other between the notches [ 150 ], each of the first undulations [ 165 ] including lobes [ 161 ] projecting outwardly from the opposite sides of the heat transfer element [ 100 ] having a peak-to-peak height Hu 1 ; and 
         [0014]    second undulations [ 185 ] extending parallel to each other between the notches [ 150 ], each of the second undulations [ 185 ] including lobes [ 181 ] projecting outwardly from the opposite sides of the heat transfer element [ 100 ] having a peak-to-peak height Hu 2 , wherein Hu 2  is less than Hu 1 . 
         [0015]    It may also be embodied as a heat transfer element [ 100 ] for a rotary regenerative heat exchanger [ 1 ] including: 
         [0016]    notches [ 150 ] extending parallel to each other and configured to form passageways [ 170 ] between adjacent heat transfer elements [ 100 ], each of the notches [ 150 ] including lobes [ 151 ] projecting outwardly from opposite sides of the heat transfer element [ 100 ]; 
         [0017]    first undulations [ 165 ] disposed between the notches [ 150 ], the first undulations [ 165 ] extending parallel to each other and having a width Wu 1 ; 
         [0018]    second undulations [ 185 ] disposed between the notches [ 150 ], the second undulations [ 185 ] extending parallel to each other and having a width Wu 2 , wherein Wu 1  is not equal to Wu 2 . 
         [0019]    The present invention may also be embodied as a basket [ 40 ] for a rotary regenerative heat exchanger [ 1 ] including: 
         [0020]    a plurality of heat transfer elements [ 100 ] stacked in spaced relationship thereby providing a plurality of passageways [ 170 ] between adjacent heat transfer elements [ 100 ] for flowing a heat exchange fluid therebetween, each of the heat transfer elements [ 100 ] including: 
         [0021]    notches [ 150 ] extending parallel to each other and configured to form passageways [ 170 ] between adjacent heat transfer elements [ 100 ], each of the notches [ 150 ] including lobes [ 151 ] projecting outwardly from opposite sides of the heat transfer element [ 100 ] and having a peak-to-peak height Hn; 
         [0022]    first undulations [ 165 ] extending parallel to each other between the notches [ 150 ], each of the first undulations [ 165 ] including lobes [ 161 ] projecting outwardly from the opposite sides of the heat transfer element [ 100 ] having a peak-to-peak height Hu 1 ; and 
         [0023]    second undulations [ 185 ] extending parallel to each other between the notches [ 150 ], each of the second undulations [ 185 ] including lobes [ 181 ] projecting outwardly from the opposite sides of the heat transfer element [ 100 ] having a peak-to-peak height Hu 2 , wherein Hu 2  is less than Hu 1 , and Hu 1  is less than Hn. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0025]      FIG. 1  is a partially broken away perspective view of a prior art rotary regenerative heat exchanger; 
           [0026]      FIG. 2  is a top plan view of a prior art element basket including a few heat transfer elements; 
           [0027]      FIG. 3  is a perspective view of a portion of three prior art heat transfer elements in stacked configuration; 
           [0028]      FIG. 4  is a cross-sectional elevation view of a prior art heat transfer element; 
           [0029]      FIG. 5  is a cross-sectional elevation view of a heat transfer element in accordance with an embodiment of the present invention; and 
           [0030]      FIG. 6  is a perspective view of a portion of a heat transfer element in accordance with the embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]      FIGS. 5 and 6  depict a portion of a heat transfer element  100  in accordance with an embodiment of the present invention. The element  100  may be used in place of conventional elements  10  in a rotary regenerative heat exchanger ( 1  of  FIG. 1 ). For example, elements  100  may be stacked as shown in  FIG. 3  and inserted in a basket  40  as depicted in  FIG. 2  for use in the rotary regenerative heat exchanger  1  of the type depicted in  FIG. 1 . 
         [0032]    The invention will be described in connection with reference to both  FIGS. 5 and 6 . The element  100  is formed from thin sheet metal capable of being rolled or stamped to the desired configuration. Element  100  has a series of notches  150  at spaced intervals which extend longitudinally and approximately parallel to the direction of flow of the heat exchange fluid past element  100  as indicated by the arrow labeled “A”. These notches  150  maintain adjacent elements  100  a predetermined distance apart and form the flow passages  170  between the adjacent elements  100  when the elements  100  are stacked. Each notch  150  comprises one lobe  151  projecting outwardly from the surface of the element  100  on one side and another lobe  151  projecting outwardly from the surface of the element  100  on the opposite side. Each lobe  151  may be in the form of a U-shaped groove with the peaks  153  of the notches  150  directed outwardly from the element  100  in opposite directions. The peaks  153  of the notches  150  contact the adjacent elements  100  to maintain the element  100  spacing. As also noted, the elements  100  may be arranged such that the notches  150  on one element  100  are located about mid-way between the notches  150  on the adjacent elements  100  for maximum support. Although not shown, it is contemplated that the element  100  may include a flat region that extends parallel to the notches  150 , upon which the notch  150  of an adjacent element  100  rests. The peak-to-peak height between the lobes  151  for each notch  150 , is designated Hn. 
         [0033]    Disposed on the element  100  between the notches  150  are undulation (corrugation)  165 ,  185  having two different heights. Each of these comprises a plurality of undulations  165 ,  185 , respectively. While only a portion of the element  100  is shown, it will be appreciated that an element  100  may include several notches  150  with undulations  165  and  185  disposed between each pair of notches  150 . 
         [0034]    Each undulation  165  extends parallel to the other undulations  165  between the notches  150 . Each undulation  165  includes one lobe  161  projecting outwardly from the surface of the element  100  on one side and another lobe  161  projecting outwardly from the surface of the element  100  on the opposite side. Each lobe  161  may be in the form of a U-shaped channel with the peaks  163  of the channels directed outwardly from the element  100  in opposite directions. Each of the undulations  165  has a peak-to-peak height Hu 1  between the peaks  163 . 
         [0035]    Each undulation  185  extends parallel to the other undulations  185  between the notches  150 . Each undulation  185  includes one lobe  181  projecting outwardly from the surface of the element  100  on one side and another lobe  181  projecting outwardly from the surface of the element  100  on the opposite side. Each lobe  181  may be in the form of a U-shaped channel having peaks  183  of the channels directed outwardly from the element  100  in opposite directions. Each of the undulations  185  has a peak-to-peak height Hu 2  between the peaks  183 . 
         [0036]    In one aspect of the present invention, Hu 1  and Hu 2  are of different heights. The ratio of Hu 1 /Hn is a critical parameter because it defines the height of the open area between adjacent elements  100  forming passageways  170  for the fluid to flow through. 
         [0037]    In the embodiment shown, Hu 2  is less than Hu 1 , and both Hu 1  and Hu 2  are less than Hn. Preferably, the ratio of Hu 2 /Hu 1  is greater than about 0.20 and less than about 0.80; and more preferably the ratio of Hu 2 /Hu 1  is greater than about 0.35 and less than about 0.65. The ratio of Hu 2 /Hn is preferably greater than about 0.06 and less than about 0.72, and the ratio of Hu 1 /Hn is preferably greater than about 0.30 and less than about 0.90. When the Hu 2 /Hu 1  ratio drops below 0.20, the smaller undulations have less effect on creating turbulence, and are less effective. 
         [0038]    When the Hu 2 /Hu 1  ratio is above 0.80, the two undulation heights are nearly equal and there is minimal improvement over prior art. 
         [0039]    Once the Hu 1 /Hn ratio and the Hu 2 /Hu 1  ratios have been chosen, the Hu 2 /Hn ratio is fixed. 
         [0040]    In another aspect of the present invention, the individual width of each of the undulations  165  may be different than the individual width of each of the undulations  185 , as indicated by Wu 1  and Wu 2 . Preferably, the ratio Wu 2 /Wu 1  is greater than 0.20 and less than 1.20; and more preferably, Wu 2 /Wu 1  is greater than 0.50 and less than 1.10. The selection of the Wu 1  and Wu 2  are, to a great degree, dependent on the values used for Hu 1  and Hu 2 . One of the overall objectives of the preferred embodiment of the present invention is to create an optimal amount of turbulence near the surface of the elements. This means that the shapes, as viewed in cross-section, of both types of undulations need to be designed in accordance with that goal, and the shape of each undulation is determined largely by the ratio of its height to its width. In addition, the choice of the undulation widths can also affect the quantity of surface area provided by the elements, and surface area also has an impact on the amount of heat transfer between the fluid and the elements. 
         [0041]    In contrast, as shown in  FIG. 4 , the undulations  65  in conventional elements  10  are all of the same height, Hu, and are all of the same width, Wu. Wind tunnel tests have surprisingly shown that replacing the conventional, uniform undulations  65  with the undulations  165  and  185  of the present invention can reduce the pressure loss significantly (about 14%) while maintaining the same rate of heat transfer and fluid flow. This translates to a cost savings to the operator because reducing the pressure loss of the air and the flue gas as they flow through the rotary regenerative heat exchanger will reduce the electrical power consumed by the fans that are used to force the air and the flue gas to flow through the heat exchanger. 
         [0042]    While not wanting to be bound by theory, it is believed that the difference in height and/or width between undulations  165  and  185  encountered by the heat transfer medium as it flows between the elements  100  creates more turbulence in the fluid boundary layer adjacent to the surface of the elements  100 , and less turbulence in the open section of the passageways  170  that are farther away from the surface of the elements  100 . The added turbulence in the boundary layer increases the rate of heat transfer between the fluid and the elements  100 . The reduced turbulence away from the surface of the elements  100 , serves to reduce the pressure loss as the fluid flows through the passageways  170 . By adjusting the two undulation heights, Hu 1  and Hu 2 , it is possible to reduce the fluid pressure loss for the same amount of total heat transferred. 
         [0043]    The superior heat transfer and pressure drop performance of the element  100  of the present invention also has the advantage that the angle between the undulations  165  and the primary flow direction of the heat transfer fluid can be reduced somewhat, while still maintaining an equal amount of heat transfer when compared to elements  10  having conventional, uniform undulations  65 . This is also true of the angle between the undulations  185  and the primary flow direction of the heat transfer fluid. 
         [0044]    This allows for better cleaning by a soot blower jet since the undulations  165  and  185  are better aligned with the jet. Furthermore, because a decreased undulation angle provides a better line-of sight between the elements  100 , the present invention is compatible with an infrared radiation (hot spot) detector. 
         [0045]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated by those skilled in the art to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.