Abstract:
A water heater has a tank having an upper and a lower domes which are penetrated by tubular flues in a 1-1-5 arrangement of a first flue connected to a second flue which in turn is connected to tertiary condensing flues. A gas-fired burner on the upper dome fires into the first flue. Both the first and second flue have heat exchange capacity enhanced by a multiplicity of rectangular metal fins welded in a helical arrangement. Some or all of the fins in the first flue may be stainless steel, while the remaining fins may be a different material, such as mild steel. The first and second flues are arranged to remove approximately 82-89 percent of the heat generated by the burner with minimal or no formation of condensate. Approximately 5.5-9 percent of the heat generated by combustion in the burner is removed in third flues where condensation takes place.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to water heaters in general, and more particularly to water heaters with multi-flues, at least one of which is condensing. 
     Heat exchange between a liquid and a gas is a process which has many industrial and domestic applications. Perhaps one of the most widely used applications of heat exchange between a gas and a liquid is in heating water. Typically a water heater has a tank which holds the water to be heated, and a burner producing combustion gases. The water is heated by the combustion of fuel with air in the burner to produce combustion gases which heat the water in the tank by passing through one or more flues or tubes which extend through the water tank. Two considerations which are paramount in the design of a water heater are durability and efficiency. Ever since the early 1970s there has been a heightened awareness of the importance of efficiency for cost, environmental, and geopolitical reasons. Efficiency is a measure of how effectively the heat energy present in the fuel is transferred to the water contained within the water heater tank. 
     The combustion gases pass through the flues, exchanging heat with the walls of the flues and thus the water contained within the water tank. It has long been known that internal baffles within a flue can increase heat transfer between the flue gases and the water within the water tank. The baffles create turbulent flow which mixes the combustion gases within the flue, bringing more of the flue gases into contact with the flue wall which transfers heat to the water. Further, if the baffles are welded to the wall of the flue, heat is conducted from the baffles to the wall of the flue. 
     As efforts are made to increase efficiency, i.e. the percentage of the combustion energy which is transferred to the hot water, at some point increased efficiency requires utilizing heat released by condensing water vapor which is produced by combustion of the hydrogen contained in the fuel. Because the latent heat of water vapor is relatively high, approximately a thousand BTUs per pound, a relatively large amount of the energy of combustion is contained in the latent heat of evaporation of the water vapor (i.e., in the steam), formed as a combustion byproduct. A pound of natural gas when combusted with dry air will produce about 2¼ pounds of water. A pound of heating oil will produce approximately 1.4 pounds of water. When the relative heating values of the fuels are taken into account approximately 7 percent of the heat of combustion of number 2 oil is contained in the latent heat of the water produced during combustion, and approximately 10 percent of the heat of combustion of natural gas is contained in the latent heat of the water produced during combustion. Therefore, a number of gas water heaters have been developed which employ heat exchangers which condense at least some of the water contained in the flue gases. Such systems have been described as having efficiencies of 90 to 96 percent. Condensing heat exchangers must be arranged to drain downwardly, and must be designed to overcome the corrosion potential of liquid water, which is often contaminated by corrosive constituents in the intake air or corrosive products of combustion. 
     What is needed is a water heater which utilizes the heat transfer capabilities of a finned flue, but achieves greater efficiencies by also utilizing a condensing flue. 
     SUMMARY OF THE INVENTION 
     The water heater of this invention has a cylindrical tank which extends between a circular upper dome and a circular lower dome. The tank is penetrated by seven tubular flues which extend between the upper and lower domes and have a 1-1-5 arrangement of a first flue connected to a second flue which in turn is connected to five tertiary condensing flues. A gas-fired burner of approximately 100,000 to 500,000 BTUs per hour is mounted on the upper dome to fire into the first flue. The heat exchange capacity of the first flue is enhanced by a multiplicity of rectangular metal fins which are welded in a helical arrangement on the inside of the vertical flue. The first flue is connected to a second flue by a junction box mounted on the lower dome so the combustion gases are transferred from the first flue to flow upwardly through the second flue which is also lined with a multiplicity of rectangular metal fins which are welded in a helical arrangement on the inside of the second flue. A second junction box is mounted on the upper dome so that combustion gases from the second flue are transferred through a plenum to five tertiary glass lined condensing flues. The first and second flues are arranged to remove approximately 82-89 percent of the heat generated by combustion in the burner with minimal or no formation of condensate. Approximately a further 5.5-9 percent of the heat generated by combustion in the burner is removed in third or tertiary flues where condensation takes place. The condensate formed in the third flues drains downwardly along the tertiary flue walls, into a third junction box which connects the third flue to a condensate drain in the vent tubes. The heat exchange in the first and second flues is tailored by the arrangement of fins to accomplish a removal without condensation. The five tertiary flues accomplish condensing heat exchange utilizing cold drawn mild steel tubes with a stainless steel baffle plate suspended along the length of the tertiary tubes. The fins welded to the first flue are at least in the upper part of the flue exposed directly to the combustion gas from the burner and so are manufactured of heat resistant alloy such as 309S stainless steel which has good strength and oxidation resistance in continuous service temperatures up to 2000° F. (1093° C.). 
     It is an object of the present invention to provide a water heater which combines the advantages of low flow resistance flues with a condensing heat exchanger. 
     It is another object of the present invention to provide a water heater wherein the heat transfer in the vertical flues can readily be adjusted by changing the number and placement of the fins in the flues. 
     It is yet another object of the present invention to provide commercial water heating in the 150,000 to 500,000 BTU/hr class. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of the condensing water heater of this invention. 
         FIG. 2  is a top view of the condensing water heater of  FIG. 1  showing five tertiary condensing flues. 
         FIG. 3  is a top plan view of an alternative embodiment of the condensing water heater of  FIG. 1  showing ten tertiary condensing flues. 
         FIG. 4  is an exploded isometric view of the water heater of  FIG. 1 . 
         FIG. 5  is a illustrative diagrammatic view of the operation of the water heater of  FIG. 1   
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more particularly to  FIGS. 1-5  wherein like numbers refer to similar parts, a water heater  20  is shown exploded in  FIG. 2 , and schematically in  FIG. 1 . The water heater has a tank  22  formed of a mild steel cylindrical outer shell  24  to which are welded two circular domes, comprising an upper dome  26 , and a lower dome  28 . Mounted below the water tank  22  as a downward continuation of the outer shell is a cylindrical stand  32 . Seven cylindrical flues or heat exchanging pipes  34 ,  40 , and  44  are arranged between the upper dome  26  and the lower dome  28  such that the axes of the flues are parallel to an axis defined by the outer shell  24  of the tank  22 . A power burner  36  is mounted to the first flue and is fired downwardly into first flue  34  from the upper dome  26 . The burner  36  supplies approximately 20%-30% excess air and natural gas, which are burnt to form combustion gases  37 , indicated in  FIG. 1 , which flow downwardly through the first flue  34 . The combustion gases are transferred via a junction box  38  mounted on the lower dome  28  to a second flue  40 . The combustion gases travel in the second flue  40  upwardly through the water tank  22  to the upper dome  26 , where they are transferred by a second junction box  42  mounted to the upper dome  26  to five tertiary and final flues  44  in which the combustion gases can travel downwardly to a third junction box  46  which connects the third flue to a condensate drain  48  and an exhaust vent  50 . 
     As the combustion gases travel from the power burner  36  through a volume of water  52  contained within the tank  22 , the combustion gases exchange heat with the walls of the three different flue types  34 ,  40 ,  44 . The first flue  34  has fins  56  welded to the inner surface  57  of the flue and is typically greater in diameter than the second flue  40 , which also has fins  62  welded to its inner surface  59 , and which in turn is typically greater in diameter than the third flues  44 . For example, in a 130 gallon water heater which is fired at a rate of up to 500,000 BTUs per hour, the first flue  34  may be 8 inches in diameter, the second flue in the same water heater may be 6 inches in diameter, and the third flue may be 5 inches in diameter. In the arrangement of the first, second and third flues,  34 ,  40 ,  44  it is desirable that the third and final flues  44  be arranged so that the combustion gases flow downwardly, so that condensation which forms on the walls  54  and within the flow of combustion gases moves downwardly to the condensate drain  48 . 
     As hot combustion gases flow down through the first flue  34 , the flow of hot combustion gases is mixed and impeded by a multiplicity of discrete high temperature resistant metals fins  56  which form the fins within at least the upper part  58  of the first flue  34  and so make up at least the upper half of fins  56  in the first flue. A suitable material is type 309S stainless steel which can be used continuously at temperatures up to about 2000° F. Each fin  56  has a generally rectangular plan, i.e., two long sides and two short sides, and has a thickness of, for example, ⅛ or ¼ inches, and extends radially inwardly substantially toward the axis of the first cylindrical flue  34 . The location of the fins along the first flue  34  is shown in  FIG. 1 . The design of the finned flues and the placement of the fins forming the heat exchanger is described more completely in U.S. Pat. No. 6,957,629 which issued on Oct. 25, 2005, and in U.S. Pat. No. 4,761,532 which issued on Aug. 2, 1988, the disclosures of both of which are incorporated herein by reference. It should be noted that to weld stainless steel fins to the mild steel tubes of the flue  34 , removal of the oxide layer, clamping pressure, current, and voltage must be adjusted to account for the changes in material properties. 
     Fins  62  arranged in the lower portion  60  of the first flue  34  and within the second flue  40  may be formed of mild steel to the extent the temperature of the gases has fallen sufficiently so as not to degrade the mild steel fins over time i.e., below about 1000° F. (540° C.) where mild steel is suitable for continuous service. The junction box  38  may be welded to the lower dome  28  and is lined with vacuum formed ceramic fiber material  39  to minimize the heat loss from the junction box. Examples of a suitable material for lining the junction box  38  are materials with continuous use temperatures of over 2000° F. and composed of over 90% Al 2 O 3  and SiO 2  with R values of 1-2 in the range of 2000° F. to 600° F. 
     The second flue  40  extends upwardly through the tank  22  and is of a smaller diameter than the first flue, because of the greatly reduced volume of combustion gases as the result of the falling temperature. When absolute temperature falls by more than half, the cross-sectional area of the second flue can also decrease to about half that of the first flue  34 . The heat exchange capability of the second flue  40  is selected by modifying the arrangement of the fins  62  and the percentage of the axial length of the second flue which is covered by fins. The heat exchange capacity of the second flue  40  is matched to the heat exchange capacity of the first flue  34  such that the flue gases exiting the second flue have a temperature somewhat above the dew point i.e. the point at which water begins to condense out of the combustion gases. In this way the presence of liquid water in the second flue  40  with the attendant problem of corrosion is substantially controlled or eliminated. 
     The five tertiary flues  44  are designed to achieve heat recovery by condensing water vapor in the combustion gases  37 . As shown in  FIG. 1 , the combustion gases  37  from the second flue  40  empty into the shallow junction box  42  welded to the upper dome  28  of the tank  22  as shown in  FIG. 2 . The five tertiary flues  44  have their combustion gas receiving ends  66  also extending into the junction box  42 . The tertiary flues  44  are arranged equidistant from the secondary flue  40  so that the plenum formed by the junction box  42  evenly distributes the combustion gases  37  to each of the five tertiary flues. The ends or openings  66  of the tertiary cylindrical flues  44  are equally spaced along an arc of a circle centered on the upper end  67  of the second cylindrical flue. The tertiary flues  44  are designed to have identical or nearly identical flow characteristics so that the flow of combustion gases  37  approximates an even split between all the tertiary flues. Because liquid water is present in the condensing tertiary flues  44 , the flues are lined with glass i.e. a porcelain enamel coating, and the tertiary flues are preferably formed of cold drawn mild steel. Glass lining better adheres to cold drawn mild steel. In the tertiary flues  44  stainless steel baffles  68  are hung from the gas receiving ends  66  of the tertiary flues as shown in  FIG. 1  to increase turbulence within the tertiary flues. The baffles extend axially within the tertiary flues and have regular radially extending projections which deflect the flow casing turbulence. 
     The tertiary flues  44  empty into a third junction box  46  which may be constructed of enamel coated steel or uncoated stainless steel welded to the lower dome  28  of the tank  22 , or can be constructed of plastic or other moderate temperature material which is compatible with liquid water. The third junction box  46  slopes downwardly and outwardly of the tank  22  such that condensation water drains to the drain  48 . The drain  48  typically is connected to a floor drain through a water lock (not shown) which allows only water and not combustion gases to pass. The gases proceed through an exhaust vent  50  then pass up a stack (not shown) and exit the building to prevent the buildup of oxygen depleted air, carbon dioxide, and humidity inside the building housing the water heater  20 . Because the power burner  36  supplies the combustion gases under pressure it is not necessary to use an exhaust fan for the combustion gases. 
     As previously discussed, for a natural gas fired water heater, approximately 10 percent of the total heat produced by combustion is contained in the latent heat of the water vapor produced during combustion. 
     The operational arrangement of the principal components of the water heater  20  are shown arranged schematically in  FIG. 5  to illustrate the operational design parameters for the water heater. Beginning on the left side of  FIG. 5  the power burner  36  is shown connected to a drive motor  70  which is connected to a Proportional Integral Derivative controller or PID controller  72  which is in turn connected to a temperature sensor  74  which senses the temperature of the water in the tank  22 . The temperature sensor  74  together with the PID controller  72  is used to control the motor  70  which in turn controls the fan speed of the burner  36 . The burner  36  is of the type that draws a vacuum which automatically draws in the gaseous fuel in proportion to the burner air provided by the burner fan (not shown). Thus the control of a single variable, namely fan speed, which is controlled by motor speed, controls the amount of air and the amount of gas passing through the burner and so controls the total BTU output of the burner  36 . The motor  70  is for example a DC brushless motor and is controlled by the PID controller  72 . HD controllers form a generic control loop with feedback and are widely used in industrial control systems. The PID controller calculates an error value as the difference between measured water temperature and a desired setpoint. The controller attempts to minimize the error by adjusting the motor speed. The MD controller algorithm involves three separate parameters: the proportional, the integral, and the derivative values. The three parameters are tuned to minimize overshoot of the setpoint and system oscillation. It is also possible to provide a factory setting to limit the total BTU output for a given use of the water heater. It may also be desirable to employ an algorithm which minimizes the BTU output consistent with meeting the requirements for hot water output and to thereby maximize the efficiency of the water heater  20  which is more efficient at lower firing rates. 
     In  FIG. 5  the heat exchanging flues  34 ,  40 ,  44  are shown schematically and for illustrative purposes are shown arranged sequentially. The theoretical flame temperature for natural gas at 20 to 30% excess air fuel may be taken as about 3000° F. which exceeds the use temperature for most materials so the stainless steel fins  56  are spaced some distance from the burner  36  along the first flue  34  such that in practice the blades do not exceed the continuous use temperature for 309S stainless of about 2000° F. As the gases progress through the heat exchanger, the temperature continues to drop as the gases exchange heat with the water  52  in the tank  22  until the temperature drops to a level where the fins reach a temperature of only 1000° F. and mild steel fins  62  may be used. The maximum temperature experienced by the fins  56  or  62  is dependent on the maximum firing rate (BTUs/hr) of the water heater and so the arrangement of the fins must be selected with the maximum firing rate in mind. Correspondingly, as the firing rate is decreased, the combustion gases  37  cool sooner. Since the temperature of the first junction box  38  is well above the dew point of the combustion gases, the ceramic insulation  39  is located on the inside of the junction box, whereas the second junction box  42  which may see some condensation is insulated externally. 
     Along the bottom of  FIG. 5  are shown the approximate temperatures associated with combustion gas movement through the first, second and third flues. Because the first flue  34  and second flue  40  are constructed of mild steel and are not glass lined it is desirable that combustion gas temperatures exceed the dewpoint of the combustion gases  37  as they travel through the first and second flues. So in addition to the design constraint due to the use maximum temperature associated with the use of 309S stainless, or mild steel fins, there is also a minimum temperature of the gases exiting the second flue. The minimum temperature will control the height to which the fins  62  extend in the second flu  40 , which is dependent on the minimum firing rate. Finally, the condensing flues  44  are affected by the firing rate inasmuch as they extract more of the available latent heat value of the steam or water vapor contained in the combustion gases at lower firing rates principally because of the longer time the smaller volume of combustion gases spend within the condensing flues  44 . In an exemplary case the dewpoint of the combustion gases may be, for example, 130° F. and the combustion gas temperature entering the second junction box  46  may be 150° F. in order to prevent or limit condensation in the second flue  40 . As shown in  FIG. 4 , a water inlet  76  is arranged at the bottom of the tank  22  and the water outlet  78  is arranged at the top of the tank which is consistent with the hot water naturally rising to the top of the tank. The exit temperature 100° F.-130° F. of the combustion gases  37  is controlled by the temperature of the water  52  within the tank  22 , particularly at the bottom of the tank where the cold water inlet  76  is located. When hot water is withdrawn, cold water enters the tank  22  and eventually the burner  36  is turned on in response to the temperature drop sensed by the temperature sensor  74 . So when the burner is the cold water is being introduced into the tank bottom to facilitate the functional condensing flues  44 . The normal setpoint of the water heater  20  may be 130° F. and yet the exit gases may be cooled below 100° F. by the incoming water which in the mid-latitudes is in the neighborhood of 50-60° F. 
     How the flues  34 ,  40 ,  44  contribute to the overall efficiency is also illustrated in  FIG. 5 . At a low rate of fire of 183,000 BTUs/hr more of the total heat transfer occurs in the first flue  34 , and less in the second flue  40  and third flues  44 , compared to a higher rate of fire. Overall a higher efficiency is achieved with lower rates of fire. At high rates of fire such as 500,000 BTUs/hr comparatively more heat transfer takes place in the second flue  40  and third flues  44 , but the overall efficiency is lower. In all cases most of the heat transfer occurs in the first flue  34  because the large temperature differential between the combustion gases and the water  52  effects very large heat transfer rates of about 84 to 71 percent of the total available heat corresponding to firing rates of 183,000 BTUs/hr to 500,000 BTUs/hr. The secondary flue  40  transfers between about 4.5 to 11 percent of the total available heat, and the final five parallel flues  44  transfer between about 6.5 and 9 percent of the total available heat. The overall efficiency for the water heater  20  for a 130 gallon tank as illustrated is about 94 percent at a firing rate of 183,000 BTUs/hr and about 91 percent at 500,000 BTUs/hr. The graph at the top of  FIG. 5  shows schematically the cumulative heat transfer, and the changing temperature of the combustion gases as the combustion gases pass through each of the flues  34 ,  40 ,  44 . 
     Another alternative embodiment water tank  122  is shown in  FIG. 3  where, instead of five tertiary flues  54 , ten tertiary flues  154  are employed. The tertiary flues  154  are 2 inches in diameter and correspondingly have stainless steel baffles  156  and the tertiary flues are ideally equally distantly spaced from the secondary flue  40 , but this ideal need only be approximated as shown in  FIG. 3 , especially because of the higher flow resistance inherent in the smaller flues  154 . The tertiary flues  154  extend to the third junction box  46  in a configuration similar to that shown in  FIG. 1 . A series of 0.75 NPD Spuds  80  are shown in  FIGS. 2 and 3  and are used for insertion of protective anodes, or a pressure release valve. 
     It should be understood that the 309S stainless steel fins could be constructed of other high temperature metals or alloys. It should also be understood that the stainless steel baffles  68  in the tertiary flues  44  may be of any various designs typically used in the prior art, such as a singular rectangular plate which extends the length of the flue and is twisted into a spiral, or folded into a zigzag, or an arrangement of two rectangular plates which extend the length of the flue and are bent in a triangular wave pattern, wherein the peaks of the triangles are welded together to form a series of open parallelograms. Generally any baffle will work which is arranged to substantially occupy the cross-section of the tertiary flue and cause turbulent mixing of the combustion gases. 
     The enamel coating used within the condensing flues and also for all surfaces exposed to water within the tank may be for example of the type described in US publication No. US 2003/0082306, published May 1, 2003. This type of porcelain enamel coating is prepared as a water suspension of borosilicate glass, milled silica, and zirconia compounds. 
     It should be understood that the description of the upper dome  26 , and lower dome  28  is intended to include the flat plates illustrated, and other functionally equivalent shapes such as conical, spherical or elliptical or a combination of such shapes whether convex or concave relative to the water tank  22 . 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.