Abstract:
A water heater including a water container; a combustion chamber adjacent the container, the combustion chamber having at least one air inlet to admit air and extraneous fumes into the combustion chamber and confine ignition and combustion of the extraneous fumes within the combustion chamber; a burner associated with the combustion chamber and arranged to combust fuel to heat water in the container; and an air diverter positioned between the air inlet and the burner and adapted to channel at least a portion of combustion air passing through at least a portion of the air inlet to a position for mixture with the fuel prior to entering the burner to slow combustion and thereby reduce combustion temperatures and NO x  emissions.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to water heaters, particularly to improvements to gas fired water heaters adapted to render them safer for use and to reduce NO x  emissions.  
         BACKGROUND OF INVENTION  
         [0002]    The most commonly used gas-fired water heater is the storage type, generally comprising an assembly of a water tank, a main burner to provide heat to the tank, a pilot burner to initiate the main burner on demand, an air inlet adjacent the burner near the base of the jacket, an exhaust flue and a jacket to cover these components. Another type of gas-fired water heater is the instantaneous type which has a water flow path through a heat exchanger heated, again, by a main burner initiated from a pilot burner flame.  
           [0003]    For convenience, the following description is in terms of storage type water heaters but the invention is not limited to this type. Thus, reference to “water container,” “water containment and flow means,” “means for storing or containing water” and similar such terms includes water tanks, reservoirs, bladders, bags and the like in gas-fired water heaters of the storage type and water flow paths such as pipes, tubes, conduits, heat exchangers and the like in gas-fired water heaters of the instantaneous type.  
           [0004]    A particular difficulty with many locations for water heaters is that the locations are also used for storage of other equipment such as lawn mowers, trimmers, snow blowers and the like. It is a common procedure for such machinery to be refueled in such locations.  
           [0005]    There have been a number of reported instances of spilled gasoline and associated extraneous fumes being accidently ignited. There are many available ignition sources, such as refrigerators, running engines, electric motors, electric and gas dryers, electric light switches and the like. However, gas water heaters have sometimes been suspected because they often have a pilot flame.  
           [0006]    Vapors from spilled or escaping flammable liquid or gaseous substances in a space  5  in which an ignition source is present provides for ignition potential. “Extraneous fumes,” “fumes” or “extraneous gases” are sometimes hereinafter used to encompass gases, vapors or fumes generated by a wide variety of liquid volatile or semi-volatile substances such as gasoline, kerosene, turpentine, alcohols, insect repellent, weed killer, solvents and the like as well as non-liquid substances such as propane, methane, butane and the like.  
           [0007]    Many inter-related factors influence whether a particular fuel spillage leads to ignition. These factors include, among other things, the quantity, nature and physical properties of the particular type of spilled fuel. Also influential is whether air currents in the room, either natural or artificially created, are sufficient to accelerate the spread of fumes, both laterally and in height, from the spillage point to an ignition point yet not so strong as to ventilate such fumes harmlessly, that is, such that air to fuel ratio ranges are capable of enabling ignition are not reached given all the surrounding circumstances.  
           [0008]    One surrounding circumstance is the relative density of the fumes. When a spilled liquid fuel spreads on a floor, normal evaporation occurs and fumes from the liquid form a mixture with the surrounding air that may, at some time and at some locations, be within the range that will ignite. For example, the range for common gasoline vapor is between about 2% and 8% gasoline with air, for butane between 1% and 10%. Such mixtures form and spread by a combination of processes including natural diffusion, forced convection due to air current drafts and by gravitationally affected upward displacement of molecules of one less dense gas or vapor by those of another more dense. Most common fuels stored in households are, as used, either gases with densities relatively close to that of air (e.g. propane and butane) or liquids which form fumes having a density close to that of air, (e.g. gasoline, which may contain butane and pentane among other components is very typical of such a liquid fuel).  
           [0009]    In reconstructions of accidental ignition situations, and when gas water heaters are sometimes suspected and which involved spilled fuels typically used around households, it is reported that the spillage is sometimes at floor level and, it is reasoned, that it spreads outwardly from the spill at first close to floor level. Without appreciable forced mixing, the air/fuel mixture would tend to be at its most flammable levels close to floor level for a longer period before it would slowly diffuse towards the ceiling of the room space. The principal reason for this observation is that the density of fumes typically involved is not greatly dissimilar to that of air. Combined with the tendency of ignitable concentrations of the fumes being at or near floor level is the fact that many gas appliances often have their source of ignition at or near that level.  
           [0010]    Earlier efforts, such as those disclosed in U.S. Pat. No. 5,797,355, substantially raised the probability of successful confinement of ignition of spilled flammable substances from typical spillage situations to the inside of the combustion chamber. Other following structures, such as those disclosed in U.S. Pat. No. 5,950,573; 6,003,477; 6,082,310; 6,085,699; and 6,085,700, for example, have built on the break through success of &#39;355.  
           [0011]    Although the water heaters described in the above-identified patents have been well received and highly successful with respect to increasing the resistance to ambient flammable vapors, certain portions of the U.S., especially California, have stringent low NO x  emissions regulations and requirements. We have discovered an ongoing challenge associated with meeting these limits with such structures for the following reasons.  
           [0012]    An important element of such flammable vapor resistant water heaters is a flame arrestor or flame trap placed strategically in the air intake opening to the combustion chamber of the water heater. This is accomplished by creating a controlled opening for combustion air and otherwise sealing off the combustion chamber. It was discovered that placement of the flame arrestor relative to the pilot is important in consistently igniting the flammable vapors in different spill scenarios. Both the pilot and main burners must be able to light the flammable vapors soon after they enter the combustion chamber.  
           [0013]    There are also preexisting conditions that dictate the relative positioning of the pilot to the burner that it is to ignite. Thus, the design wherein the same pilot that would light the burner would also ignite the flammable vapors, if present, had the effect of setting a relationship of the inlet air relative to the burner. The result was that the air inset for the flame trap is off center and not symmetrical.  
           [0014]    Most natural gas burners used in storage water heaters are “pan” burners. They are made of sheet metal making them a very economical choice and their flame pattern is well suited for heating a typical storage tank bottom. Such pan burners are designed to operate with air evenly distributed around the inlet to the mixing chamber of the burner. They have the opening to the mixing chamber placed at a specific distance and concentricity from a gas orifice. This allows the gas to flow from the orifice and mix with the combustion air consistently and unencumbered.  
           [0015]    However, in that design, the relationship of the flame arrestor to pilot to burner creates imbalances in the availability and locations of primary and secondary air through out the combustion chamber. This imbalance can cause several side effects in the combustion process. It leads to higher levels of NO x  production as the flame temperature runs hotter on the non-arrestor side due to lack of excess air to help cool the process. The air at the mixing chamber opening is also distributed unevenly. This also contributes to higher NO x  production by disturbing the ratio of primary and secondary air. This can deprive the burner of enough air to mix with the amount of fuel. The fuel and air may also not mix as intended. With the air availability skewed from side to side, the mixture may not be as homogenous as a mixing chamber in a symmetrical environment and, therefore, produce pockets of rich mixture and pockets that are too lean. This has been found to increase NO x  production due to having isolated hot spots. It is known that one of the primary factors of NO x  production is flame temperature exceeding 2800 degrees F. Formation of thermal NO x  increases exponentially with combustion temperature, and increases by a square root relationship with the presence of oxygen in the combustion zone. This excessively rich primary mixture is also more likely to flashback on extinction when using alternate higher BTU input fuels that are required for ANSI testing. (Butane-Air). Accordingly, it has been a primary objective to produce a water heater that simultaneously addresses the issue of resistance to flammable vapors and can meet ever increasingly stringent low NOx emissions regulations and requirements by the various regulatory bodies.  
         SUMMARY OF THE INVENTION  
         [0016]    The invention relates to a water heater including a water container and a combustion chamber adjacent the container. The combustion chamber has at least one flame trap to admit air and extraneous fumes into the combustion chamber. The flame trap (sometimes also referred to as an “air inlet” or “air inlet plate”) has a plurality of ports. The ports are sized and shaped to cause air and extraneous fumes to pass through the ports, yet confine ignition and combustion of the extraneous fume species within the combustion chamber. The water heater includes a burner associated with the combustion chamber and arranged to combust fuel to heat water in the container. A low NO x  draft hood is positioned relative to the burner and the flame trap to enhance combustion dynamics to reduce production of NO x  emissions.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic partial cross-sectional view of a gas-fueled water heater having an air inlet and low NO x  hood according to the invention.  
         [0018]    [0018]FIG. 2 is a schematic perspective view of the combustion chamber of a water heater of the type shown in FIG. 1.  
         [0019]    [0019]FIG. 3 is a top plan view of the combustion chamber shown in FIG. 2.  
         [0020]    [0020]FIG. 4 is a cross-sectional view of the combustion chamber taken through the line A-A in FIG. 3.  
         [0021]    [0021]FIG. 5 is a schematic perspective view of a burner and low NO x  hood according to the invention.  
         [0022]    [0022]FIG. 6 is an inverted plan view of the burner and low NO x  hood of FIG. 5.  
         [0023]    [0023]FIG. 7 is a cross-sectional view of the burner and low NO x  hood taken through the line VII-VII of FIG. 6.  
         [0024]    [0024]FIG. 8 is an elevation view of a prior art combustion chamber generally similar to that shown in FIG. 4, partially cut-away to show a conventional burner. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    It will be appreciated that the following description is intended to refer to the specific embodiments of the invention selected for illustration in the drawings and is not intended to limit or define the invention, other than in the appended claims.  
         [0026]    Turning now to the drawings in general and FIGS.  1 - 4  in particular, there is illustrated a storage type gas water heater  62  including jacket  64  which surrounds a water tank  66  and a main burner  74  in an enclosed chamber  75  that addresses and solves the longstanding problems described above. Water tank  66  is preferably capable of holding heated water at mains pressure and is insulated preferably by foam insulation  68 . Alternative insulation may include fiberglass or other types of fibrous insulation and the like. Fiberglass insulation surrounds chamber  75  at the lowermost portion of water tank  66 . It is possible that heat resistant foam insulation can be used if desired. A foam dam separates foam insulation  68  and the fiberglass insulation.  
         [0027]    Located underneath water tank  66  is a pilot burner  73  and main burner  74  which preferably use natural gas as fuel or other gases such as LPG, for example. Other suitable fuels may be substituted. Main burner  74  receives combustion air through air inlet plate  90  and low NO x  draft diverter  91  and then combusts gas admixed with air and the hot products of combustion rise up through flue  70 , possibly with heated air. Water tank  66  is lined with a glass coating to provide corrosion resistance against the hot water content. It is also coated on part of the exterior surface with a glass coating to provide corrosion resistance to that portion particularly that forms the upper wall of the combustion chamber  75  and contacts the hot and corrosive products of combustion. The external glass coating is applied to about half the thickness of that of the internal lining since this results in a protective coating more resistant to cracking under the influence of sudden changes in temperature. Such cracking if it happens causes the coating to resemble a fish scale appearance and such scales could detach eventually and possibly partly block the air inlet plate  90 . However, without this external protective coating, the steel surface of the tank  66  and lower portion of the flue  70  may shed flake-like rust products, which could risk blocking the air inlet plate  90 .  
         [0028]    Fuel gas is supplied to both burners  73 ,  74  through a gas valve  69  and a fuel line  71 . Flue  70  in this instance, contains a series of baffles  72  to better transfer heat generated by main burner  74  to water within tank  66 . Near pilot burner  73  is a flame detecting thermocouple (not shown) which is a known safety measure to ensure that in the absence of a flame at pilot burner  73  the gas control valve  69  shuts off the gas supply. The water temperature sensor  63 , preferably located inside the tank  66 , co-operates also with the gas control valve  69  to supply gas to the main burner  74  on demand.  
         [0029]    The products of combustion pass upwardly and out the top of jacket  64  via flue outlet  76  after heat has been transferred from the products of combustion. Flue outlet  76  discharges conventionally into a draught diverter  77  which in turn connects to an exhaust duct leading outdoors.  
         [0030]    Water heater  62  is mounted preferably on legs  84  to raise the base  86  of the combustion chamber  75  off the floor. In base  86  is an aperture  28  which is closed gas tightly by an air inlet plate  90  which admits all air for combustion of the fuel gas combusted through the main burner  74  and pilot burner  73 , regardless of the relative proportions of primary and secondary combustion air used by each burner. Air inlet plate  90  is preferably made from a thin metallic perforated sheet of stainless steel.  
         [0031]    Where base  86  meets the vertical combustion chamber walls  79 , adjoining surfaces are thoroughly sealed by a circumferential seam  81  to prevent ingress of air or flammable extraneous fumes. Instead, the base  86  and walls  79  may be one piece. The underside of the base  86  is recessed inwardly from the lower end of the seam to create an air space  82  between it and a flat-bottomed support pan  56  illustrated in FIG. 8. This support pan  56  is used with the present invention also but is omitted from FIGS.  1 - 4  for clarity of illustration. The support pan  56  is provided with at least one hole  83  vertically below the air inlet plate in the base  86  to admit air into the air space  82 .  
         [0032]    Pilot flame establishment can be achieved by a piezoelectric igniter. A pilot flame observation window  93  can be provided which is sealed. Cold water is introduced at a low level of the tank  66  and withdrawn from a high level in any manner as already well known.  
         [0033]    Referring to FIGS.  4 - 7 , specifics of a burner  74  and air diverter  91 \ are shown. Burner  74  is constructed of an upper sheet metal plate  100  and a lower sheet metal plate  102  that are fixed together in a known manner such as by spot welding or the like at a multiplicity of nodes  104 . The diameter of upper sheet  100  is smaller than the diameter of lower sheet  102 . The nodes are elongated in a radially-extending direction and form a series of elongated radially-extending channels  106  through which a mixture of fuel and combustion air pass prior to combustion of the fuel. An opening  108  is located at the outer terminus of substantially all of channels  106 .  
         [0034]    Upper sheet  100  is substantially “bowl-shaped”, as is lower sheet  102 , except that lower sheet  102  has an opening  110  centrally located which is formed by a downwardly-extending lip  112 .  
         [0035]    Diverter  91  includes a pair of substantially vertically oriented side walls  114  and an end wall  116  connected therebetween. The side walls  114  and end wall  116  are also connected together by a cover  118  as well as a lower floor  120 . The side walls  114  taper towards each other in the direction which terminates in end wall  116 . Floor  120  is sized such that it leaves a substantial opening  122  in a lowermost portion of air diverter  91 . There is also an opening  124  formed by a lip  126  that is connected directly to lip  112  of burner  74 . Lower floor  120  also has an opening  128  that is sized to receive fuel line  71 . Fuel line  71  is terminated at opening  128  with a fuel injecting nozzle  129 . Cover  118  has a curved portion  130  that is especially designed to cause an even flow of air into diverter  91  as air passes through opening  122  and towards opening  124 . Curved portion  130  also causes the air flow to be such to maximize mixing with fuel prior to passage through opening  110  of burner  74  and through channels  106  of burner  74  prior to combustion.  
         [0036]    During normal operation, water heater  62  operates in substantially the same fashion as conventional water heaters except that all air for combustion enters through air inlet plate  90 . A portion of the air entering through air inlet plate  90  passes into air diverter  91  through opening  122 , assisted by aspiration from a pressure drop in fuel flow through nozzle  129  as the fuel&#39;s velocity is increased by passing through the small opening in it. The fuel and air mix prior to passing through opening  110  and channels  106 .  
         [0037]    The invention, including the use of diverter  91 , corrects the imbalance described above. We found that the diverter  91  increases the amount of primary air available by about 25%. This lowers the NO x  levels below the industry-accepted value of 40 Ng/J or 55 ppm corrected to 3% oxygen. This structure also completely neutralizes tendencies of the burner to flashback to the orifice on flame extinction. Although we do not wish to be bound by any particular theory, we believe that air diverter  91  increases the velocity of the air mixing may contribute to this factor being so effectively answered. Because of the robustness of this design, concerning this flashback solution, we can also lower costs by using inexpensive burners with relatively low port loading that would normally flashback in this environment. Low port loads in natural gas burners are known to produce less NO x .  
         [0038]    This mode of combustion is brought about so that NO x  emissions are reduced due to the proportioning and premixing of the air and fuel in proper ratios and so that combustion takes place in a slower and substantially even manner. Moreover, the multiplicity of holes  108  supply further even quantities of air calculated to lower flame temperatures, thereby reducing NO x  emissions still further. This configuration inhibits soot formation or “candling” at nozzle  129 . The remainder of the air in the combustion chamber can flow freely around the outer edge of the lower sheet  102  to contribute further secondary air to the main burner.  
         [0039]    It is important that the lower edge of diverter  91 , where opening  122  is located, does not physically contact air inlet plate  90 . This could reduce the ability of air inlet plate  90  to confine potential ignition within the combustion chamber. It is accordingly preferred that a space or gap of at least about ¼-inch is maintained. Also, it is important that the curved portion  130  of cover  118  not extend across both elongated side edges of air inlet plate  90  inasmuch as this will potentially disturb the even flow of air currents around and through air inlet plate  90 .  
         [0040]    Also, it is preferable that opening  122  be aligned over the center of the perforated area of the air inlet plate  90  because the velocity profile of the air entering the combustion chamber is greatest above the center of plate  90 . By taking air into the opening  122  of cover  118  at the highest available upward velocity a higher aeration is achieved in the primary stage of combustion, believed to amount to about 2 to 20% additional primary aeration. The higher upwardly then horizontally directed flow of incoming fresh air across the vertically emerging stream of gas or fuel from nozzle  129  is believed also to be a reason for the inhibiting of candling as mentioned above.  
         [0041]    As best illustrated in FIG. 3, both air diverter  91  and pilot burner  73  are located side by side generally as close to the center of the perforated area of the air inlet plate  90  where the velocity of incoming air is highest. When flammable extraneous fumes enter the combustion chamber  75  the alignment of the pilot burner  73  generally over the center of the plate  90  (or near the center) has been found to minimize instances of high velocity or explosive initial ignition of fumes in the chamber.  
         [0042]    If spilled fuel or other flammable fluid is in the vicinity of water heater  62  then some extraneous fumes from the spilled substance may be drawn through plate  90  by virtue of the natural draft characteristic of such water heaters. Air inlet  90  allows the combustible extraneous fumes and air to enter but confines potential ignition and combustion inside the combustion chamber  75 .  
         [0043]    The spilled substance taken into water heater  62  is burned within combustion chamber  75  and exhausted through flue  70  via outlet  76  and duct  78 . Because flame is confined by the air inlet plate  90  within the combustion chamber, flammable substance external to water heater  62  will not be ignited.  
         [0044]    With reference to FIG. 1, the size of air inlet plate  90  is dependent upon the air consumption requirement for proper combustion to meet mandated specifications to ensure low pollution burning of the gas fuel. Merely by way of general indication, the air inlet plate should be conveniently about 3700 mm 2  perforated area when fitted to a water heater having between 35,000 and 50,000 Btu/hr (approximate) energy consumption rating to meet ANSI requirements for overload combustion. Air diverter  91  is sized accordingly and preferably made from the same or similar material as air inlet plate  90 .  
         [0045]    With reference to FIG. 8, the lower portion is quite similar to FIGS. 2 and 4 and corresponding reference numerals are used. The burner  74  lacks the extended lower sheet  102  of FIG. 5 and the holes  108  therein. The diverter  91  is absent. The support pan  56  is included although this is also used in the invention but not illustrated, for clarity reasons only. The support pan  56  and base  86  define an air space  82  below the combustion chamber. Combustion air enters the air space  86  upstream of the combustion chamber  75  via the entry hole  83 .  
         [0046]    It is to be understood that the invention disclosed and defined herein extends to all alternative combinations of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention. The foregoing describes embodiments of the present invention and modifications, obvious to those skilled in the art can be made to them, without departing from the scope of the present invention.