Abstract:
A water heater apparatus includes an elongated radial burner extending along a longitudinal central axis of the apparatus. First and second concentric rows of longitudinally extending fin tubes around the radial burner. The fin tubes may have multiple wiped circumferentially laterally portions allowing close packing of the fin tubes for improved efficiency and reduced footprint.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure relates generally to water heaters and boilers, and more particularly, but not by way of limitation, to an arrangement in construction of the fin tubes of a heat exchanger for a water heater. 
         [0003]    2. Description of the Prior Art 
         [0004]    One heat exchanger architecture which is found in the prior art includes an elongated radial burner concentrically received within a circular array of fin tubes. Such heat exchangers have previously been sold by the Assignee of the present invention under the Trademark POWER-FIN®. Examples of such heat exchangers are shown for example in U.S. Pat. No. 4,793,800 to Vallett et al., and U.S. Pat. No. 6,694,926 to Baese et al. 
         [0005]    Elongated burners used in such heat exchangers may be constructed in accordance with the disclosures of Baese et al. U.S. Pat. No. 6,694,926; Bodnar et al., U.S. Pat. No. 6,619,951; and/or Smelcer et al., U.S. Pat. No. 6,428,312. 
         [0006]    Additionally, it is known in the prior art to use an architecture similar to that described above but having two concentric rings of fin tubes surrounding the elongated burner. An example of a dual concentric ring fin tube architecture is seen in U.S. Pat. No. 9,074,792 to Ellingwood et al. 
         [0007]    There is a continuing need in the construction of water heater apparatus to improve the operating efficiency and reduce the foot print or space occupied by the water heater. 
       SUMMARY OF THE INVENTION 
       [0008]    A water heater apparatus is disclosed including an upper header having a water inlet and a water outlet, and including a lower header. An inner ring of inner fin tubes extends between the upper and lower headers and is communicated with the upper and lower headers to flow water through the inner fin tubes. Each inner fin tube includes a plurality of annual inner fin tube fins with circumferentially lateral portions of the inner fin tube fins bent to reduce a lateral cross-section dimension of the inner fin tube fins. An outer ring of outer fin tubes extends between the upper and lower headers and is located radially outward of the inner ring of inner fin tubes. The outer fin tubes are communicated with the upper and lower headers to flow water through the outer fin tubes. Each outer fin tube includes a plurality of annular outer fin tube fins with circumferentially lateral portions of the outer fin tube fins bent to reduce a lateral cross-section dimension of the outer fin tube fins. A burner tube is located radially inward of the inner ring of inner fin tubes and is configured to combust a fuel and air mixture and to radially project heated gases past the inner and outer rings of fin tubes to heat water flowing through the fin tubes. 
         [0009]    In another embodiment, a water heater apparatus is disclosed including an elongated radial burner extending along a longitudinal center axis of the apparatus. A plurality of longitudinally extending fin tubes are provided. Each fin tube includes a plurality of fins having multiple wiped circumferentially lateral portions. The plurality of fin tubes are arranged to form a first ring concentrically disposed about the burner and a second ring concentrically disposed about the first ring. Each fin tube of a respective ring is arranged with one of the lateral portions thereof facing a corresponding lateral portion of an adjacent fin tube of the respective ring. 
         [0010]    In any of the above embodiments the inner fin tubes may be close packed so that the bent circumferentially lateral portions of the inner fin tube fins of each inner fin tube contact the bent circumferentially lateral portions of the inner fin tube fins of each adjacent inner fin tube. 
         [0011]    In any of the above embodiments the outer fin tubes may be close packed so that the bent circumferentially lateral portions of the outer fin tube fins of each outer fin tube contact the bent circumferentially lateral portions of the outer fin tube fins of each adjacent outer fin tube. 
         [0012]    In any of the above embodiments at least some of the outer fin tubes made contact radially adjacent one of the inner fin tubes. 
         [0013]    In any of the above embodiments the inner tube fin fins may each have a radially outer bent portion, at least some of the radially outer bent portions of the inner fin tube fins of at least some of the inner fin tubes contacting the outer fin tubes. 
         [0014]    In any of the above embodiments the outer fin tube fins may each have two radially outer bent portions on either side of a radially outermost point of each outer fin tube fin, so that the outer fin tube fins of adjacent outer fin tubes define a V-shape space between their radially outermost points. 
         [0015]    In any of the above embodiments a V-shaped baffle may be located in each of the V-shaped spaces. 
         [0016]    In any of the above embodiments the bent circumferentially lateral portions of the fin tubes may be bent along lines extending substantially radially outwardly from a central axis of the apparatus. 
         [0017]    A method of manufacturing a heat exchanger for a water heater is also disclosed, which method may include the steps of: 
         [0018]    (a) providing a first plurality of fin tubes having laterally opposed wiped sides on each fin of the first plurality of fin tubes; 
         [0019]    (b) providing a second plurality of fin tubes having laterally opposed wiped sides on each fin of the second plurality of fin tubes, the fins of the second plurality of fin tubes being wiped differently than the fins of the first plurality of fin tubes; 
         [0020]    (c) assembling a first ring of the first plurality of fin tubes so that the laterally opposed wiped sides of adjacent fin tubes of the first plurality of fin tubes face each other; and 
         [0021]    (d) assembling a second ring of the second plurality of fin tubes, the second ring concentrically disposed about the first ring, so that the laterally opposed wiped sides of adjacent fin tubes of the second plurality of fin tubes face each other. 
         [0022]    The method may further include in step (b) providing two radially outer wiped portions on either side of a radially outermost point of each of the fins of the second plurality of fin tubes, so that the fins of adjacent fin tubes of the second ring define a V-shaped space between their radially outermost points. 
         [0023]    The method may further include a step of placing a V-shaped baffle in one of the V-shaped spaces. 
         [0024]    The method may further include in step (a) providing a radially outer wiped side on each fin of the first plurality of fin tubes, and in steps (c) and (d) assembling the first and second rings such that at least some of the fin tubes of the second plurality of fin tubes touch the radially outer wiped sides of the fins of the first plurality of fin tubes. 
         [0025]    The method may further include in step (c) the assembling of the first ring of the first plurality of fin tube so that the laterally opposed wiped sides of adjacent fin tubes of the first ring touch each other. 
         [0026]    The method may further include in step (d) assembling the second ring of the second plurality of fin tubes so that the laterally opposed wiped sides of adjacent fin tubes of the second ring touch each other. 
         [0027]    Numerous objects features and advantages of the present invention will be readily apparent of those skilled in the art upon a reading of the following disclosure when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic elevation view of a water heater apparatus. 
           [0029]      FIG. 2  is a perspective elevation view of the heat exchanger of the water heater apparatus of  FIG. 1 . 
           [0030]      FIG. 3  is an exploded perspective view of the heat exchanger of  FIG. 2 . 
           [0031]      FIG. 4  is a cross-section plan view through the upper header of the heat exchanger of  FIGS. 2 and 3 . 
           [0032]      FIG. 5  is a cross-section plan view through the lower header of the heat exchanger of  FIGS. 2 and 3 . 
           [0033]      FIG. 6  is a schematic plan view showing the shape and arrangement of the fin tubes of the inner and outer concentric rings of fin tubes for the heat exchanger of  FIG. 2 . 
           [0034]      FIG. 7  is an enlarged view of the circled area identified as  7  in  FIG. 6 . 
           [0035]      FIG. 8  is a schematic elevation view of a fin tube blank prior to bending of the fins to form one of the inner or outer fin tubes. 
           [0036]      FIG. 9  is a plan view of the fin tube blank of  FIG. 8  viewed along line  9 - 9  of  FIG. 8 . 
           [0037]      FIG. 10  is a schematic elevation view of one of the inner fin tubes forming the inner ring of fin tubes for the water heater apparatus of  FIG. 2 . 
           [0038]      FIG. 11  is a plan view of the inner fin tube of  FIG. 10  viewed along line  11 - 11  of  FIG. 10 . 
           [0039]      FIG. 11A  is a plan view of a die used to form the inner fin tube of  FIGS. 10 and 11 . 
           [0040]      FIG. 12  is a schematic elevation view of one of the outer fin tubes forming the outer ring of fin tubes for the water heater apparatus of  FIG. 2 . 
           [0041]      FIG. 13  is a plan view of the outer fin tube of  FIG. 12  viewed along line  13 - 13  of  FIG. 12 . 
           [0042]      FIG. 13A  is a plan view of a die used to form the outer fin tube of  FIGS. 12 and 13 . 
           [0043]      FIG. 13B  is a cross-section view taken along line  13 B- 13 B of  FIG. 13  showing a schematic cross-section of one of the fins, and showing the bent or wiped sides of the fin. 
           [0044]      FIG. 14  is a view similar to  FIG. 7  schematically illustrating with arrows the flow path of hot gases flowing radially outward past the inner and outer rings of fin tubes. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    Referring now to the drawings, and particularly to  FIGS. 1-3 , the water heater or boiler apparatus of the present invention is shown and generally designated by the numeral  10 . As used herein, the term water heater refers to an apparatus for heating water, including both steam boilers and water heaters that do not actually “boil” the water. This discussion may refer to the apparatus  10  as a boiler  10 , but it will be understood that this description is equally applicable to water heaters that do not boil the water. The water heater  10  includes a heat exchanger  12 . The heat exchanger  12  includes an upper header  14  including a water inlet  16  and a water outlet  18 . Heat exchanger  12  further includes a lower header  20 . 
         [0046]    An inner ring  22  of inner fin tubes  22 A,  22 B,  22 C, etc., extends between the upper and lower headers  14  and  20 . Inner fin tubes  22  are communicated with the upper and lower headers  14  and  20  to flow water through the inner fin tubes  22 . An outer ring  24  of outer fin tubes  24 A,  24 B,  24 C, etc., is located radially outward of the inner ring  22  and also extends between the upper and lower headers  14  and  20 . 
         [0047]    A burner tube or burner  26  is located radially inward of the inner ring  22  and is configured to combust a fuel and air mixture and radially project heated gasses as indicated by arrows  28  past the inner and outer rings  22  and  24  so as to heat water flowing through the fin tubes. The burner  26  may be constructed in any suitable manner including that disclosed in Baese et al. U.S. Pat. No. 6,694,926, or in U.S. Pat. No. 6,619,951 to Bodnar et al., or U.S. Pat. No. 6,428,312 to Smelcer et al., all of which are incorporated herein by reference. The burner  26  is of the type referred to as a pre-mix burner which burns a previously mixed mixture of combustion air and fuel gas. In the system shown in  FIG. 1 , a venturi  30  is provided for mixing combustion air and fuel gas. An air supply duct  32  provides combustion air to the venturi  30 . A gas supply line  34  provides fuel gas to the venturi  30 . The venturi  30 , may, for example, be a model VMU680 provided by Honeywell. A gas control valve  36  is disposed in supply line  34  for regulating the amount of gas entering the venturi  30 . The gas control valve  36  includes an integral shut-off valve. 
         [0048]    In order to provide the variable output operation of the burner  26 , a variable flow blower  38  delivers the pre-mixed combustion air and fuel gas to the burner  26  at a controlled burner flow rate within a burner flow rate range. The blower  38  may be driven by a variable frequency electric drive motor. 
         [0049]    The gas line  34  may be connected to a conventional fuel gas supply (not shown) such as a municipal gas line, with appropriate pressure regulators and the like being utilized to control the pressure of the gas supplied to the venturi  30 . 
         [0050]    The gas control valve  36  is preferably a ratio gas valve for providing fuel gas to the venturi  30  at a variable gas rate which is proportional to the air flow rate entering the venturi  30 , in order to maintain a predetermined air-to-fuel ratio over the flow rate range in which the blower  38  operates. 
         [0051]    Combustion gases from the burner  26  exit the water heater  10  through a combustion gas outlet  40  which is connected to an exhaust gas flue  42 . 
         [0052]    The heat exchanger  12  may be contained within an internal liner  44 , which may for example be made of stainless steel plate and be rectangular in cross-sectional plan. The exhaust gas flue  42  may be connected to the internal liner  44  and the combustion gas outlet  40  may be defined in the internal liner  44 . The entire heat exchanger  12  and internal liner  44  may be enclosed within an outer housing  46 . 
         [0053]      FIGS. 4 and 5  show cross-section plan views of the upper and lower headers  14  and  20 , respectively. It is noted that the upper header  14  includes an inner ring of upper header tube openings such as  48  and an outer ring of upper header tube openings such as  50 . Similarly, the lower header  20  includes an inner ring of lower header tube openings  52  and an outer ring of lower header tube openings  54 . 
         [0054]    As is further described below, each of the fin tubes such as inner fin tube  22  is connected to the upper header via one of the inner ring upper header tube openings  48  and to the lower header via one of the inner ring lower header tube openings  52 . Similarly, each of the outer tubes  24  is connected to the upper and lower headers via openings  50  and  54 . 
         [0055]    The upper header  14  includes first, second and third upper header baffles  56 ,  58  and  60 . Lower header  20  includes first and second lower header baffles  62  and  64 . 
         [0056]    The arrangement of header baffles provides that the heat exchanger  12  operates in 4 passes. Thus in plan view a first quadrant of the heat exchanger  12  is defined between first and second upper header baffles  56  and  58 , a second quadrant is defined between second upper header baffle  58  and second lower header baffle  64 , a third quadrant is defined between second lower header baffle  64  and third upper header baffle  60 , and the fourth quadrant is defined between third upper header baffle  60  and the first upper header baffle  56  and/or the first lower header baffle  62 . 
         [0057]    Thus water flowing into inlet  16  of the upper header  14  first flows downward through the tubes located in the first quadrant, then upward through the tubes located in the second quadrant, then downward through the tubes located in the third quadrant, then back upward through the tubes located in the fourth quadrant and out the water outlet  18  of upper header  14 . 
       Construction of the Fin Tubes 
       [0058]    Each of the inner fin tubes  22  and outer fin tubes  24  are manufactured from a fin tube blank such as the fin tube blank  66  shown if  FIGS. 8 and 9 . The fin tube blank  66  includes a central tube body  68  which is a length of cylindrical tubing of internal diameter  70  and length  72 . A plurality of fins  74  are attached to and extend generally radially outward from central tube body  68 . As seen in  FIG. 9 , the fins  74  initially are generally circular in shape and extend concentrically from the central tube body  68 . 
         [0059]    As best seen in  FIG. 8 , each of the fins  74  is initially slightly dished so that an upper surface  76  thereof slopes downward as seen in  FIG. 8 . 
         [0060]    The inner fin tubes  22  and outer fin tubes  24  are each made from blanks like the fin tubes blank  66  shown in  FIGS. 8 and 9 , by a process known as wiping. In the wiping process, the fin tube blank  66  is pulled through a die having a shape corresponding to the desired final plan shape of the fins of the inner and outer fin tubes  22  and  24 . The wiping will occur by pulling the fin tube blank  66  in a direction generally indicated by the arrow  78  through the die. As the fin tube blank  66  passes through the die, those portions of the fins engaging the edges of the die will be bent so that the fins  74  have a shape upon exiting the die generally corresponding to the internal shape of the die. Thus, referring for example to  FIGS. 10 and 11  illustrating one of the inner fin tubes  22 , a fin tube blank  66  as shown in  FIGS. 8 and 9  is pulled through a die  80  having a shape generally as shown in  FIG. 11A  thus resulting in fins on the inner fin tube  22  having a shape generally as shown in  FIG. 11 . 
         [0061]    Similarly, the outer fin tubes  24  may be formed by pulling a fin tube blank  66  through a die  82  having a shape generally as shown in  FIG. 13A . 
         [0062]    It is noted that  FIGS. 10 and 12  are both schematic, and do not attempt to show the individual fins like was done in  FIG. 8 . In  FIGS. 10 and 12  only the general outer profiles of the collection of wiped fins  23  and  25 , respectively are shown. Due to the folding of the fins, such as best shown in  FIG. 13B , it will be understood that when viewed as in  FIG. 10 or 12 , the fins overlap vertically at the bent edges. 
         [0063]    Regarding inner fin tubes  22 , as shown in  FIG. 11 , each of the fins  74  of the fin tube blank  66  has assumed a shape as indicated and as designated by the fins  23  of the inner fin tube  22 . Similarly regarding outer fin tubes  24 , as shown in  FIG. 13 , each of the fins  74  of the fin tube blank  66  has assumed the shape of the fins  25  of outer fin tubes  24 . 
         [0064]    As is best illustrated with reference to  FIGS. 6, 7 and 11 , each of the annular inner fin tube fins  23  of one of the inner fin tubes  22  has circumferentially lateral portions  84  and  86  bent to reduce a lateral cross-sectional dimension of the inner fin tube fins  23 . Thus, with reference to  FIG. 9 , the fins  74  of the fin tube blank  66  started with a lateral cross-sectional dimension equal to the diameter  88  of the fins  74  as seen in  FIG. 9 . That diameter  88  has been reduced for the inner fin tubes  22  to a range between a narrowest circumferential cross-section dimension  90  to a largest circumferential cross section dimension  92 , both of which are less than the initial diameter  88 . 
         [0065]    In the embodiment shown in  FIG. 11 , each of the inner fin tube fins  23  also has a radially outer bent portion  94  which has a reduced radius  96  from central axis  98  of tube body  68 , as contrasted to the radially inner most unbent portion of fins  23  which have a radius equal to that of the original unbent fins  74  of the fin tube blank  66 , namely one half of the original diameter  88 . 
         [0066]    Referring now to the die  80  which is used to form the bent fins  23  of inner fin tubes  22 , the die  80  has internal surfaces  94 ′ corresponding to bent surface  94 ,  84 ′ corresponding to bent surface  84 ,  86 ′ corresponding to bent surface  86 , and  88 ′ corresponding to unbent outer diameter. 
         [0067]    It will be understood that due to the resilience of the fins  74  of the fin tube blank  66 , the internal dimensions of surfaces  94 ′,  84 ′,  86 ′ and  88 ′ of the die  80  will be slightly smaller than the desired final dimensions of the corresponding surfaces of the fin  23  so that after the fins pass through the die and spring back slightly toward their initial shapes, the final dimensions of the fin  23  will be as needed for the final assembly of the heat exchanger  12 . 
         [0068]    Referring now to  FIGS. 6, 7, and 13 , each of the fins  25  of each of the outer fin tubes  24  may have circumferentially lateral portions  102  and  104  bent to reduce a lateral cross section as indicated at  106  of the outer fin tube fins  25 . 
         [0069]    Each of the outer fin tube fins  25  may also have two radially outer bent portions  108  and  110  on either side of a radially outermost point  112  of each outer fin tube fin  25 , so that the outer fin tube fins  25  of adjacent outer fin tubes  24  define a V-shaped space  114  there between as is best seen in  FIGS. 6 and 7 . 
         [0070]    As seen in  FIGS. 3 and 7 , a V-shaped baffle  116  may be located in each of the V-shaped spaces  114 . 
         [0071]    The V-shaped baffles  116  as shown in plan view in  FIG. 7  may comprise an angle  118  which may for example be in a range of from 80 degrees to 110 degrees, more preferably in a range from 90 degrees to 100 degrees, and most preferably about 95 degrees. 
         [0072]    As is best appreciated in viewing  FIGS. 6 and 7 , the inner fin tubes  22  may be close packed so that the bent circumferentially lateral portions  84  and  86  thereof of the inner fin tube fins  23  of each inner fin tube  22  contact the bent circumferentially lateral portions  84  and  86  of the inner fin tube fins  23  of each adjacent inner fin tube  22 . Similarly, the outer fin tubes  24  may be close packed so that the bent circumferentially lateral portions  102  and  104  of the outer fin tube fins  25  of each outer fin tube  24  contact the bent circumferentially lateral portions  102  and  104  of the outer fin tube fins  25  of each adjacent outer fin tube  24 . Also, as seen in  FIGS. 6 and 7 , at least some of the outer fin tubes  24  may contact radially adjacent ones of the inner fin tubes  22 . More particularly, the outer fin tubes fins  25  may contact the radially outer bent portions  94  of the fins  23  of the inner fin tubes  22 . 
         [0073]    As is also apparent from  FIGS. 6 and 7 , each of the bent circumferentially lateral portions such as  84  and  86  of the inner fin tubes  22 , and such as  102  and  104  of the outer fin tubes  24 , are bent generally along lines extending substantially radially outward from a central axis  99  of the heat exchanger  12 . 
       Methods of Manufacturing 
       [0074]    The methods of manufacturing and assembling the heat exchanger  12  can best be appreciated with reference to  FIGS. 3, 6 and 7 . 
         [0075]    As previously described, a plurality of the inner fin tubes  24  are constructed as described with regard to  FIGS. 10 and 11  using the die of  FIG. 11A , and a plurality of the outer fin tubes  24  are constructed as described with regard to  FIGS. 12 and 13  using a die as shown in  FIG. 13A . 
         [0076]    Then as seen in  FIGS. 3, 6 and 7 , an inner ring or first ring of the inner fin tubes  22  may be assembled so that the laterally opposed wiped sides  84  and  86  of adjacent inner fin tubes  22  face each other to allow for close packing of the first ring  22 . Preferably those wiped sides engage each other, although in some embodiments of the invention it is not required that the sides actually touch. 
         [0077]    Similarly, the outer or second ring of fin tubes  24  is assembled so that laterally outer wiped sides  102  and  104  face each other, and preferably touch each other. 
         [0078]    It will be appreciated that the positions of each of the inner fin tubes  22  and outer fin tubes  24  relative to each other is defined by the construction of the upper header  14  shown in  FIG. 4  and the lower header  20  shown in  FIG. 5 . Particularly the tube locations are defined by the locations of the tube receiving holes  48  and  50  of upper header  14  and  52  and  54  of lower header  20 . 
         [0079]    It will be appreciated that the central tube body  68  of each of the fin tubes  22  or  24  is received in the openings in the upper and lower headers. In the construction illustrated, the central tube bodies  68  are received in the openings  48 ,  50 ,  52 ,  54  as appropriate, and then are swaged or otherwise formed so as to hold the central tube bodies  68  firmly in place within the headers  14  and  16 . This is a form of construction sometimes referred to as a gasketless header. It is noted that with this construction the heat exchanger  12  does not include tube sheets to hold the array of tubes in place. 
         [0080]    As best seen in  FIG. 3 , after the inner and outer rows  22  and  24  of fin tubes are assembled with the upper and lower headers  14  and  16 , the V-shaped baffles  116  may be put in place, and then three circumferentially extending baffle retainers or bands  120  may be placed around the V-shaped baffles  116  to hold them in place. 
         [0081]    As seen in  FIG. 3 , the heat exchanger  12  may also include brass plugs  122  which close the outer openings in the headers corresponding to the position of the various fin tubes. Brass plugs  122  may be removed to allow cleaning of the bores of the central tube bodies  68 . Brass plugs  124  may be provided to plug various access ports such as  126  in the headers. A tube template  128  may be provided to aid in placement of the tubes during assembly. One or more bulbwells  130  may be provided to aid in connecting various temperature sensors or the like to the headers. 
         [0082]      FIG. 13B  is a schematic cross-section view taken along line  13 B- 13 B of  FIG. 13  to show how the fin  25  is folded or bent at bent portions  102  and  104  during the wiping process. It will be understood that  FIG. 13B  is schematic only and not to scale and shows only a single one of the fins  25  and exaggerates the bent portions thereof. The other wiped, folded or bent portions or edges such as  108  and  110  of fins  25  and such as  84 ,  86  and  94  of fins  23  are similarly bent or folded when seen in cross-section view. 
         [0083]    The wiping of tubes so that they may be arranged in a close packed manner as illustrated results in both a smaller footprint for the assembled water heater, and a higher efficiency of the water heater due to the improved flow pattern of gases through the fins. 
         [0084]      FIG. 14  is a view similar to  FIG. 7  to which has been added schematic representations of the various gas flow paths of the hot combustion gases as they flow radially outward past the fin tubes. Thus the gases represented by the arrows  28  in  FIG. 1  will typically follow paths such as indicated by the arrows  28  in  FIG. 14 . As will be appreciated, the gases will typically flow between each of the folded sides  84  and  86  and the associated central tube bodies  68  as they flow radially outward past the inner fin tubes  22 . Then the gases will flow between each of the folded sides  102  and  104  of the outer fin tubes  24  and their associated central tube bodies  68  and then through the small gaps between adjacent V-shaped baffles  116  to exit the outer ring of fin tubes  25 . 
         [0085]    Thus the wiping of the tube fins generally causes the gas flow to closely adhere to the central tube bodies  68  thus enhancing heat transfer from the hot combustion gases  28  to the water flowing through the central tube bodies  68 . 
       EXAMPLE 
       [0086]    With reference to  FIG. 6 , one example of an arrangement of fin tubes  22  and  24  to construct a heat exchanger  12  in accordance with the present disclosure includes a total of twenty-eight inner fin tubes  22  and thirty-four outer fin tubes  24 . Thus, a total of  62  fin tubes are provided. In this embodiment, each of the fin tubes may have a central tube body  68  having a nominal inside diameter of approximately 0.88 inch and having fins with a nominal outside diameter prior to wiping of 1.86 inch. The tubes may have an overall length of the finned area of between 39 and 40 inches. Both the central tube bodies  66  and the fins are preferably made of copper. 
         [0087]    The central axes of the central tube bodies of the inner fin tubes  22  may be placed upon a diameter  200  of approximately 14.25 inches, and the axes of the outer fin tubes  24  may be placed on a diameter  202  of approximately 17.5 inches about the central axis  99  of heat exchanger  12 . With this arrangement each of the inner fin tubes  22  subtend an angle of approximately 12.9 degrees about the central axis  99 , and each of the outer fin tubes  24  subtend an angle of approximately 10.6 degrees about the central axis  99 . 
         [0088]    Such an arrangement may provide a water heater apparatus  10  having a nominal output of 3.5 MBtu per hour. With this arrangement operating efficiencies as high as 87% may be achieved. Other arrangements may be used to provide nominal outputs ranging for example from 2.5 MBtu per hour to 5.0 MBtu per hour. 
         [0089]    Thus it is seen that the apparatus and methods of the present invention readily achieve the ends and advantages mentioned, as well as those inherent therein. While certain preferred embodiments of the invention have been described for purposes of the present disclosure, numerous changes in the arrangement and construction of parts and steps may be made by those skilled in the art, which changes are encompassed within the scope and spirit of the present invention as defined by the appended claims.