Abstract:
A heat transfer element includes a plurality of adjacent heat transfer plates. Each of the heat transfer plates has oppositely disposed first and second heat transfer surfaces and a plurality of laterally spaced notches. A first heat transfer plate of each pair of adjacent heat transfer plates has at least one tall notch and the second heat transfer plate of each pair of plates has at least one short notch, where the notch height of the tall notch is greater than the notch height of the short notch. Each of the tall notches of a heat transfer plate are received in a short notch of an adjacent heat transfer plate to thereby define flow channels therebetween.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to heat transfer element assemblies. More particularly, the present invention relates to heat transfer element assembly adapted for use in a rotary regenerative air preheater.  
           [0002]    Rotary regenerative air preheaters are commonly used to transfer heat from the flue gases exiting a furnace to the incoming combustion air. A typical rotary regenerative heater has a cylindrical rotor divided into compartments in which are disposed and supported spaced heat transfer plates which, as the rotor turns, are alternately exposed to a stream of heating gas and then upon rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. As the heat transfer plates are exposed to the heating gas, they absorb heat therefrom and then when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heating gas by the heat transfer plates is transferred to the cooler gas. Most heat exchangers of this type have their heat transfer plates closely stacked in spaced relationship to provide a plurality of passageways between adjacent plates for flowing the heat exchange fluid therebetween.  
           [0003]    Heat transfer elements for regenerative air preheaters have several requirements. Most importantly, the heat transfer element must provide the required quantity of heat transfer or energy recovery for a given depth of the heat transfer element. Conventional heat transfer elements for preheaters use combinations of flat or ribbed form-pressed or rolled-pressed steel sheets or plates. When in combination, the plates form flow passages for the movement of the flue gas stream and air stream through the rotor of the preheater. The surface design and arrangement of the heat transfer plates provides contact between adjacent plates to define and maintain the flow passages through the heat transfer element  
           [0004]    Due to the close proximity of the heat transfer sheets, any relative movement between the sheets will result in wear. Since the individual sheets generally have a relatively small thickness, such wear can result in the development of holes in the sheet material. Further, the wear allows for greater relative movement between the heat transfer sheets, thereby accelerating the process. In applications where the flue gasses are expected to contain highly corrosive elements, the surfaces of the heat transfer sheets are coated with a porcelain enamel material to provide greater corrosion resistance. Movement induced wear will result in localized failure of the enamel material and a loss of corrosion resistance. Conventional heat transfer elements include a perimeter structure, such as a basket, which applies an external pressure to the heat transfer surfaces to lock the heat transfer sheets together and thereby prevent relative movement therebetween. However, it is common for heat transfer elements installed in horizontal shaft air preheaters to lose the packing pressure provided by the basket structure, allowing relative movement between the heat transfer sheets during rotation of the rotor.  
           [0005]    Heat transfer elements are subject to fouling from particulates and condensed contaminants, commonly referred to as soot, in the flue gas stream. Therefore, another important performance consideration is low susceptibility of the heat transfer elements to significant fouling, and furthermore easy cleaning of the heat transfer element when fouled. Fouling of the heat transfer elements is conventionally removed by soot blowing equipment emitting pressurized dry steam or air to remove by impact the particulates, scale and contaminants from the heat transfer elements. The heat transfer elements must therefore also survive the wear and fatigue associated with soot blowing.  
         SUMMARY OF THE INVENTION  
         [0006]    Briefly stated, the invention in a preferred form is a heat transfer element which comprises a plurality of adjacent heat transfer plates. Each of the heat transfer plates has oppositely disposed first and second heat transfer surfaces and a plurality of laterally spaced notches. Each of the notches includes adjacent, mutually parallel, first and second lobes, with each first lobe extending transversely from the first heat transfer surface to a crest and each second lobe extending transversely from the second heat transfer surface to a crest. The crests of the first and second lobes define a notch height. A first heat transfer plate of each pair of adjacent heat transfer plates has at least one tall notch and the second heat transfer plate of each pair of plates has at least one short notch, where the notch height of the tall notch is greater than the notch height of the short notch. Each of the tall notches of a heat transfer plate are received in a short notch of an adjacent heat transfer plate to thereby define flow channels therebetween.  
           [0007]    In a first embodiment of the invention, the notches of each heat transfer plate comprise at least one tall notch and at least one short notch. In a second embodiment of the invention, the notches of each first heat transfer plate of each pair of plates comprises a plurality of tall notches and the notches of each second heat transfer plate of each pair of plates comprises a plurality of short notches.  
           [0008]    The notches of each heat transfer plate are substantially equidistantly laterally spaced apart. Each tall notch of each heat transfer plate has substantially the same notch height and each short notch of each heat transfer plate has substantially the same notch height. Each lobe has a lateral width, with the widths of the first and second lobes of each short notch being greater than the widths of the first and second lobes of each tall notch. The intermediate surface formed between adjacent notches of a heat transfer plate may include one or more protuberances extending transversely from at least one of the first and second heat transfer surfaces.  
           [0009]    An object of the invention is to provide a heat transfer element having improved heat transfer capacity.  
           [0010]    Another object of the invention is to provide a heat transfer element having constituent heat transfer plates which do not move relative to each other.  
           [0011]    A still another object of the invention is to provide a heat transfer element that is easy to clean.  
           [0012]    These and other objects of the invention will be apparent from review of the specification and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which:  
         [0014]    [0014]FIG. 1 is a perspective view of a conventional rotary regenerative air preheater which contains heat transfer element assemblies made up of heat transfer plates.  
         [0015]    [0015]FIG. 2 is a perspective view of a conventional heat transfer element assembly showing the heat transfer plates stacked in the assembly.  
         [0016]    [0016]FIG. 3 is a fragmentary end-on-view of a first embodiment of a heat transfer assembly in accordance with the invention.  
         [0017]    [0017]FIG. 4 is an exploded view of the heat transfer element of FIG. 3.  
         [0018]    [0018]FIG. 5 is a fragmentary end-on-view of a second embodiment of a heat transfer assembly in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    With reference to FIG. 1, a conventional rotary regenerative preheater is generally designated by the numerical identifier  10 . The air preheater  10  has a rotor  12  rotatably mounted in a housing  14 . The rotor  12  is formed of diaphragms or partitions  16  extending radially from a rotor post  18  to the outer periphery of the rotor  12 . The partitions  16  define compartments  20  therebetween for containing heat exchange element assemblies  22 .  
         [0020]    The housing  14  defines a flue gas inlet duct  24  and a flue gas outlet duct  26  for the flow of heated flue gases through the air preheater  10 . The housing  14  further defines an air inlet duct  28  and an air outlet duct  30  for the flow of combustion air through the preheater  10 . Sector plates  32  extend across the housing  14  to divide the air preheater  10  into an air sector and a flue gas sector. The arrows of FIG. 1 indicate the direction of a flue gas stream  34  and an air stream  36  through the rotor  12 . The hot flue gas stream  34  entering through the flue gas inlet duct  24  transfers heat to the heat transfer element assemblies  22  mounted in the compartments  20 . The heated heat transfer element assemblies  22  are then rotated to the air sector of the air preheater  10 . The stored heat of the heat transfer element assemblies  22  is then transferred to the combustion air stream  36  entering through the air inlet duct  28 . The cold flue gas stream  34  exits the preheater  10  through the flue gas outlet duct  26 , and the heated air stream  36  exits the preheater  10  through the air outlet duct  30 . FIG. 2 illustrates a typical heat transfer element assembly or basket  22  showing a general representation of heat transfer plates  38  stacked in the assembly  22 .  
         [0021]    [0021]FIG. 3 depicts a first embodiment of the invention showing portions of three stacked heat transfer plates  38 . All of the heat transfer plates  38  are basically identical, composed of thin sheet metal capable of being rolled or stamped to the desired configuration. This is advantageous in that only one type of plate  38  needs to be manufactured. Each plate  38  has a series of notches  40 ,  42  at spaced intervals which extend longitudinally and parallel to the direction of the flow of the airstream  36  and the gas stream  34  through the rotor  12  of the air preheater  10 . These notches  40 ,  42  maintain adjacent plates  38 ,  38 ′ a predetermined distance D apart and form the flow passages or channels  44  between the adjacent plates  38 ,  38 ′. Each notch  40 ,  42  comprises one lobe  46  projecting outwardly from a first surface  48  of the plate  38 ,  38 ′ on one side and another lobe  50  projecting outwardly from the second surface  52  of the plate  38 ,  38 ′ on the other side. Each lobe  46 ,  50  is essentially in the form of a U-shaped groove with the apexes of the grooves directed outwardly from the plate  38 ,  38 ′ in opposite directions.  
         [0022]    [0022]FIG. 4 is an exploded view of the assembly  22  of FIG. 3. As is more readily apparent in this view, each heat transfer plate  38 ,  38 ′ has alternating short and tall notches  40 ,  42 , where each tall notch  42  has substantially the same height Ht and each short notch  40  has substantially the same height Hs and where Ht&gt;Hs. The pitch  58  of the notches, i.e., the distance between adjacent tall and short notches  42 ,  40 , is substantially equal for all short/tall notch pairs such that the lobes  46 ,  50  of the tall notches  42  nest within the lobes  46 ,  50  of the short notches  40  of an adjacent plate  38 ,  38 ′ with the outer surface of the crest  60  of each tall notch  42  engaging the inner surface of the crest  60  of the short notch  40 . The width Wt of the tall notches  42  is substantially equal to the width Ws of the short notches  40 , with the greater slope of the tall notches  42  facilitating insertion of tall notches  42  into the short notches  40 . In a preferred embodiment Ht=0.690 inches, Hs=0.322 inches, Wt=0.350 inches, and Ws=0.350 inches.  
         [0023]    The heat transfer surface  62  intermediate the notches  40 ,  42  of a plate  38  is held at a distance D from the heat transfer surface  62  of the adjacent plate  38 ′ which is substantially equal to difference in height of the notches  40 ,  42 , that is D=Ht−Hs. The ratio of Ht to Hs can be adjusted so the frontal area of each air flow opening is equalized for uniform air flow through the element. The heat transfer surface  62  between the notches may have protuberances to cause air turbulence in the spaces between the heat transfer sheets.  
         [0024]    [0024]FIG. 5 depicts a second embodiment of the invention showing portions of three stacked heat transfer plates  64 ,  66 . In this embodiment, the assembly  22 ′ is comprised of two types of heat transfer plates  64 ,  66 , with all of the plates of the first type  64  being substantially identical and all of the plates of the second type  66  being substantially identical. All of the heat transfer plates  64 ,  66  are composed of thin sheet metal capable of being rolled or stamped to the desired configuration. Each plate  64 ,  66  has a series of notches  68 ,  70 , respectively, at spaced intervals which extend longitudinally and parallel to the direction of the flow of the heat exchange fluid through the rotor  12  of the air preheater  10 . Each notch  68 ,  70  comprises one lobe  72  projecting outwardly from a first surface  74  of the plate  64 ,  66  on one side and another lobe  76  projecting outwardly from the second surface  78  of the plate  64 ,  66  on the other side. Each lobe  72 ,  76  is essentially in the form of a U-shaped groove with the apexes of the grooves directed outwardly from the plate  64 ,  66  in opposite directions.  
         [0025]    Each plate of the first type  64  has alternating tall notches  68  and each plate of the second type  66  has alternating short notches  70 , where each tall notch  68  has substantially the same height Ht′ and each short notch  70  has substantially the same height Hs′ and where Ht′&gt;Hs′. The pitch of the notches  68 ,  70 , i.e., the distance between adjacent notches  68 ,  70 , is substantially equal and is substantially the same for each type of plate  64 ,  66 . As is apparent from FIG. 5, the lobes  72 ,  76  of the tall notches  68  nest within the lobes  72 ,  76  of the short notches  70  of an adjacent plate with the outer surface of the crest of each tall notch  68  engaging the inner surface of the crest of the short notch  70 . The width Wt′ of the tall notches  68  is substantially equal to the width Ws′ of the short notches  70 , with the greater slope of the tall notches  68  facilitating insertion of tall notches  68  into the short notches  70 . In a preferred embodiment Ht′=0.690 inches, Hs′=0.322 inches, Wt′=0.350 inches, and Ws′=0.350 inches.  
         [0026]    The heat transfer surface  80  intermediate the tall notches  68  is held at a distance D′ from the heat transfer surface  82  intermediate the short notches  70  which is substantially equal to difference in height of the notches, that is D′=Ht′−Hs′. The ratio of Ht′ to Hs′ can be adjusted so the frontal area of each air flow opening is equalized for uniform air flow through the element. The heat transfer surface  80 ,  82  between the notches  68 ,  70  may have protuberances to cause air turbulence in the spaces between the heat transfer plates  64 ,  66 .  
         [0027]    The nesting of the tall notches  42 ,  68  in the short notches  40 ,  70  form closed channels  44 ,  84  that facilitate cleaning of the heat transfer surface  62 ,  80 ,  82 . The closed channel  44 ,  84  contains the energy of the sootblower or water washing jet, allowing maximum cleaning action from the energy and preventing the dissipation of energy allowed by conventional heat transfer assemblies.  
         [0028]    It should be appreciated that the interlocking heat transfer surface design provides for longer life by preventing the relative motion between the two plates of heat transfer surface thus preventing the wearing and ultimate loosening of the heat transfer surfaces. The interlocking design also facilitates cleaning. This assembly  22 ,  22 ′ is especially suited, but not exclusively, to horizontal shaft air preheaters  10  where the heat transfer plates  38 ,  64 ,  66  of the assembly  22 ,  22 ′ are coated with enamel to prevent corrosion. The interlocking notches form a rigid block of heat transfer material that will not shift during the rotation of the rotor. The heat transfer plates  38  are easily trimmed to assure proper nesting and to form the proper overall shape for the basket.  
         [0029]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.