Patent Publication Number: US-2013228318-A1

Title: Heat exchanger

Description:
This disclosure relates to a heat exchanger, more particularly a condensing type heat exchanger. The disclosure also relates to a method of assembling a heat exchanger, more particularly a condensing type heat exchanger. 
     A known fuel-fired forced air heat exchanger is set forth in U.S. Pat. No. 4,960,102. The heat exchanger includes a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger. Advantageously, the secondary heat exchanger is arranged to condense the combustion gases discharged from the primary heat exchanger, and so capture latent heat from the available combustion gas. Such heat exchangers are commonly referred to as “condensing” heat exchangers. 
     The increasing cost of fuel and materials means that there is a need to improve the heating efficiency and assembly of such heat exchangers. 
     According to one aspect of the invention, there is provided a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, wherein the primary heat exchanger comprises a drum having a longitudinal axis and the secondary heat exchanger comprises a plurality of tubes extending parallel with the longitudinal axis of the drum. 
     In exemplary embodiments, the tubes are provided in an array adjacent, e.g. to the side of, the drum. 
     In exemplary embodiments, a spiral formation is formed in the outer surface of each tube, for promoting flow of combustion gases through the tube. 
     In exemplary embodiments, the spiral formation begins at a predetermined distance from one end of the tube and stops at a predetermined distance from the opposite end of the tube, so that the ends of the tube are plain. 
     In exemplary embodiments, the tubes have a small diameter relative to the diameter of the drum. 
     In exemplary embodiments, the tubes have a diameter in the range 4 mm to 8 mm. In another embodiment the tubes have a diameter in the range 5 mm to 7 mm. In a further embodiment the tubes have a diameter of 6 mm. 
     In exemplary embodiments, the tubes are arranged in staggered rows. 
     In exemplary embodiments, the tubes are provided between inlet and outlet plates, as a subassembly. 
     In exemplary embodiments, the inlet and/or outlet plates are preformed with tube apertures for receiving the ends of the tubes in a pre-defined array. 
     In exemplary embodiments, the tube apertures have a peripheral flange projecting from the associated plate to provide a sleeve for a section of the tubes. 
     In exemplary embodiments, the primary heat exchanger consists of a subassembly including a cylindrical drum sealingly attached to an outlet plate; wherein the secondary heat exchanger consists of a subassembly including said plurality of tubes sealing attached to an inlet plate; and wherein the two subassemblies are united with one another by attachment between the outlet plate and inlet plate. 
     In exemplary embodiments, a closure is attached to the united subassemblies and defines a passageway for combustion gases from the primary heat exchanger to the secondary heat exchanger. 
     In exemplary embodiments, an array of concentric circles pressed into a surface of the closure to reduce noise during flexure of the closure under thermal expansion. 
     In exemplary embodiments, the closure is of box-type configuration having four side walls and a base wall, wherein the base wall is pressed outwards, to extend beyond the side walls, and defines four generally triangular sloping surfaces. 
     According to another aspect of the invention, there is provided a method of assembling a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, and in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, comprising the steps of:
         providing the primary heat exchanger in the form of a drum having a longitudinal axis, and providing the secondary heat exchanger in the form of a plurality of tubes extending parallel with the longitudinal axis of the drum.       

     In exemplary embodiments, the method includes the step of providing an inlet plate and an outlet plate for the tubes, the inlet and outlet plates being pre-formed with tube apertures for receiving the ends of the tubes in a pre-defined array, arranging the plates in a spaced array with said tubes arranged between the plates with the tubes in alignment with the tube apertures, and driving the plates together in the direction of one another to force the ends of the tubes into the respective tube apertures. 
     In exemplary embodiments, each tube aperture is provided with a peripheral flange projecting from the associated plate to provide a sleeve for a section of a respective tube. 
     In exemplary embodiments, the peripheral flange is swaged to fixedly couple the flange to the tubes. 
     In exemplary embodiments, a spiral thread is formed in the outer surface of each tube prior to incorporation in the assembly. 
     In exemplary embodiments, each tube is drawn and then the spiral thread is formed on the drawn tube, prior to a second drawing operation, to remove any significant deformations generated when the thread is formed. 
     In exemplary embodiments, the primary heat exchanger is formed as a subassembly including a cylindrical drum sealingly attached to an outlet plate; the secondary heat exchanger is formed as a separate subassembly including said plurality of tubes sealing attached to an inlet plate; and the two subassemblies are united with one another by attachment between the outlet plate and inlet plate. 
     In exemplary embodiments, a closure is attached to the united subassemblies to define a passageway for combustion gases from the primary heat exchanger to the secondary heat exchanger. 
     In exemplary embodiments, an array of concentric circles pressed into a surface of the closure to reduce noise during flexure of the closure under thermal expansion. 
     In exemplary embodiments, the closure is of pre-formed box-type configuration having four side walls and a base wall, wherein the base wall is pressed outwards, to extend beyond the side walls, and defines four generally triangular sloping surfaces. 
     In a further aspect of the invention, there is provided a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, wherein the primary heat exchanger comprises a drum having a longitudinal axis and the secondary heat exchanger comprises a plurality of tubes extending parallel with the longitudinal axis of the drum, wherein a spiral formation is formed in the outer surface of each tube, for promoting flow of combustion gases through the tube. 
    
    
     
       Other aspects and features of the invention will be apparent from the attached claims and the following description of preferred embodiments, made by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view of an outlet plate for a secondary heat exchanger; 
         FIG. 2  is an enlarged view of encircled region A from  FIG. 1 ; 
         FIG. 3  is a schematic perspective view of an inlet plate for a secondary heat exchanger; 
         FIG. 4  is an enlarged view of encircled region B from  FIG. 3 ; 
         FIG. 5  is a schematic perspective view of a secondary heat exchanger subassembly incorporating the inlet and outlet plates of  FIGS. 1 and 3 ; 
         FIG. 6A  is schematic side view of a heat exchanger tube for use in the subassembly of  FIG. 5 ; 
         FIG. 6B  is an enlarged view of encircled region C from  FIG. 6A ; 
         FIG. 6C  is schematic cross-section through the heat exchanger tube of  FIG. 6A ; 
         FIG. 7  is a schematic perspective view of a primary heat exchanger subassembly; 
         FIG. 8  is a schematic perspective view showing the pre-assembly formed when the subassembly of  FIG. 5  is united with the subassembly of  FIG. 7 ; 
         FIG. 9  is a schematic perspective view of the pre-assembly of  FIG. 8  prior to attachment of a cover element to provide a passageway between the primary heat exchanger and the secondary heat exchanger; 
         FIG. 10A  is schematic side view of the closure from  FIG. 9 ; and 
         FIG. 10B  is cross-sectional view of the closure from  FIG. 10A  taken along line D-D. 
     
    
    
     Referring firstly to  FIG. 1 , there is shown a plate  10  of rectangular form. The plate includes an array of primary fixing apertures  12  arranged about the periphery of the plate  10 , e.g. one at each corner  14  of the plate  10  and one positioned mid way along each of the two longest sides  16  of the plate  10 . The plate  10  also includes a plurality of secondary fixing apertures  18  arranged in two rows, each row being spaced inwardly of the primary fixing apertures  12  and extending in a direction parallel with the respective longest sides  16  of the plate  10 . 
     The plate  10  further includes an array of tube apertures  20  arranged centrally on the plate  10  in a plurality of rows extending in a direction parallel with line the longest sides  16  of the plate  10 . Adjacent rows in the array of tube apertures  20  are staggered relative to one another, e.g. so that the apertures do not align in a transverse direction relative to the longest sides of the plate. The tube apertures  20  are formed by a punching operation and so define a peripheral flange  22  which projects upwardly from the upper surface  24  of the plate  10 , as viewed in  FIG. 1  (and as seen most clearly in  FIG. 2 ). 
     Plate  10  is preformed to the configuration shown in  FIG. 1 , ready for use as an outlet plate  10  for a secondary heat exchanger assembly according to an exemplary embodiment of the invention. As can be seen, the edges  26  of the plate  10  are turned to project downwardly as viewed in  FIG. 1 , to form an open box type structure (as would be apparent if viewed from below in  FIG. 1 ). 
     Referring now to  FIG. 3  there is a shown a plate  30  of rectangular form, and including three fixing apertures  32  arranged at predetermined locations along one of the longer sides  34  of the plate  30 . 
     Plate  30  also includes an array of tube apertures  36  arranged centrally on the plate  30  in a plurality of rows extending parallel with the longest sides  34  of the plate  10 . Again, adjacent rows in the array of tube apertures  36  are staggered relative to one another. The tube apertures  36  are formed by a punching operation and so define a peripheral flange  38  which projects downwardly from the lower surface  40  of the plate  30 , as viewed in  FIG. 3  (and as seen most clearly in  FIG. 4 ). 
     Plate  30  is pre-formed to the configuration shown in  FIG. 3 , ready for use as an inlet plate  30  for a secondary heat exchanger according to an exemplary embodiment of the invention. 
     Unlike the plate  10  in  FIG. 1 , plate  30  does not include turned edges. However, the position of the tube apertures  36  on the plate  30  matches the position of the tube apertures  20  on plate  10 . 
     According to an exemplary method of assembly, the two plates  10  and  30  are arranged in a spaced array with a plurality of stainless steel heat exchanger tubes  50  arranged therebetween, e.g. as shown in  FIG. 5 . The plates  10  and  30  and tubes  50  are arranged with the tubes  50  in alignment with the tube apertures  20 ,  36 , e.g. using a special purpose assembly machine. The plates  10  and  30  are then moved in the direction of one another, to force the ends of the tubes  50  into the respective tube apertures  20 ,  36 . The diameter of the tube apertures  20 ,  36  is selected to create an interference fit between the associated flanges  22 ,  38  and the respective ends of the tubes  50 . In an exemplary method of assembly, a swaging tool (not shown) may be used to swage the ends of the tubes  50  in the flanges  22 ,  38  to securely couple the tubes  50  to the inlet/outlet plates  12 ,  30 . The two ends may be swaged simultaneously, to reduce process time. 
     An example of a heat exchanger tube  50  for use in the secondary heat exchanger assembly and method of assembly described above is shown in  FIG. 6 . The tube  50  is of seamless drawn construction, having a predefined outer diameter (for providing on an interference fit with the flanges  22 ,  38  on the plates  10 ,  30 ) and a predefined internal diameter. In exemplary embodiments, the tubes  50  are of small diameter. 
     Test results have shown a 6 mm diameter bore to provide beneficial performance characteristics. In exemplary embodiments, the diameter of the tubes is within the range 4 mm to 8 mm. 
     A spiral thread  52  is provided as a recessed formation in the outer surface of the tube  50 . The configuration of the thread  52  (in terms of pitch and depth relative to the length of the tube) is configured to promote the flow of combustion gases though the tube  50 , in use. 
     Test results show that a pitch of the spiral formation in the region of 6-12 mm (e.g.12 mm), at a depth of 0.69-0.7 mm, with a wall thickness of 0.5±0.03 mm provides optimum strength and heat transfer characteristics for exemplary embodiments. 
     In exemplary embodiments, the tube  50  is drawn and then the spiral thread  52  is formed on the drawn tube  50 . A second drawing operation is then carried out to remove any significant deformations generated when the thread is formed, so as to maintain the accuracy of the tube diameter axial alignment, to promote optimum performance and avoid condensate being trapped within the tube. 
     The spiral thread  52  does not extend to the ends  54  of the tube  50 ; the thread  52  begins at a predetermined distance from one end  54  of the tube  50  and stops at a predetermined distance before the opposite end  54  of the tube  50 , so that the ends  54  of the tube are plain, to ensure a tight fit with the tube apertures  20 ,  36  on the plates  10 ,  30 . 
     A primary heat exchanger assembly  60  according to an exemplary embodiment of the invention will now be described with reference to  FIG. 7 . 
     The primary heat exchanger assembly  60  has a cylindrical drum  62  of pre-selected diameter. An outlet plate  64  having a central outlet aperture  66  is fitted to the lower end of the drum  62 , as viewed in  FIG. 7 , and affixed thereto (e.g. by seam welding to a peripheral flange  68 ) to form an airtight seal with the drum  62 . 
     An inlet assembly  70  is fitted to the upper end of the drum  62 , with a thermal insulation plate  72  fixed in place beneath the inlet assembly  70 . The inlet assembly  70  affixed to the drum (e.g. by seam welding to a peripheral flange  68 ) to form an airtight seal with the drum  62 . 
     X-type strengthening formations  74  are formed adjacent each of the corners  76  of the outlet plate  64 . These serve as strengthening braces and also allow the material to move during the expansion and contraction cycles. Allowing the material to move eliminates noise issues which would otherwise result, such as ‘bonging’ and ‘ticking’, due to the different expansion rates of the mating materials. 
     Referring now to  FIG. 8 , it can be seen that the primary and secondary heat exchanger assemblies  60 ,  40  form separate sub assemblies of a condensing heat exchanger according to an exemplary embodiment of the invention. The two sub assemblies  60 ,  40  are then brought together (e.g. using a mechanical jig) and attached to one another. In the illustrated embodiment, the two assemblies  60 ,  40  are affixed to one another by a seam weld  78  formed between respective edges regions of the outlet plate  64  of the primary heat exchanger assembly  60  and the inlet plate  30  of the secondary heat exchanger assembly  40 . 
     As can be seen from  FIG. 9 , a box closure  80  is then be applied over the lower end of the united sub assemblies  40 ,  60 . The box closure  80  includes a peripheral flange  82  and a seam weld is used to affix united plates  30 ,  64  to the peripheral flange  82 , and create an airtight seal between the closure  80  and the united sub assemblies  40 ,  60 . 
       FIGS. 10A and 10B  show a box closure  80  of exemplary configuration, having side walls  84  and a base wall  86 . An array of concentric circles  88  is pressed into the base wall  86  (on the outer side of the box  80 ). The base wall  80  is then pressed outwards, to extend beyond the side walls  84  (e.g. as viewed in  FIG. 10B ), and bent so as to define four generally triangular sloping surfaces  90 . This configuration has been found to provide reduced noise from flexure of the material (e.g. bongs and ticks) and reduces the tendency for splitting, in use. 
     Once assembled, the finished assembly is ready for incorporation in a condensing heat exchanger, e.g. with a fuel burner (not shown) in communication with the inlet assembly  70  of the primary heat exchanger  60 , so that combustion gasses pass through the drum  62 , into the box closure  80  and out through the tubes  50  of the secondary heat exchanger  40 . 
     The provision of an array of small diameter tubes as the secondary heat exchanger has been found to be particularly effective, especially when incorporating a spiral thread and/or when arranged in staggered rows. The arrangement of the tubes between the pre-formed inlet and outlet plates provides a convenient and efficient sub-assembly, which can be readily incorporated with the primary heat exchanger sub-assembly described herein.