Abstract:
A water heater including a water container; a combustion chamber adjacent the container, the combustion chamber having at least one flame arrestor 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; an air diverter positioned between the flame arrestor and the burner and adapted to channel at least a portion of combustion air passing through at least a portion of the flame arrestor to a position for mixture with the fuel prior to entering the burner to ensure uniform combustion; and a flange positioned above the air diverter to which the flames attach and thereby reduce combustion temperatures; the combined effect of which is to optimize combustion performance and minimize NO x  and CO emissions.

Description:
FIELD OF THE INVENTION 
     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 
     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. 
     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. 
     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. 
     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. 
     Vapors from spilled or escaping flammable liquid or gaseous substances in a space 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. 
     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 capable of enabling ignition are or are not reached given all the surrounding circumstances. 
     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). 
     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. 
     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. Nos. 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. 
     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. 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 NO x  emissions regulations and requirements by the various regulatory bodies. 
     One attempt to limit NO x  emissions is U.S. Pat. No. 5,645,413 to Benedek et al., which discloses a water heater designed to operate with unlimited burner primary air, and a key feature is to recirculate secondary air to the primary combustion flame region. In &#39;413, the flame guide and burner are an integral system such that the burner does not function separately from the flame guide. 
     SUMMARY OF THE INVENTION 
     This invention relates to a water heater including a water container and a combustion chamber adjacent the container. The combustion chamber has a side wall and at least one flame arrestor 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 having a multiplicity of burner ports is associated with the combustion chamber and arranged to combust fuel to heat water in the container. 
     An air diverter including a substantially flat plate having a central opening larger than the diameter of the burner is positioned in the combustion chamber and below the burner ports in the burner. The plate is sized to create a gap between its outer edge and the side wall and is adapted to channel combustion air passing through at least a portion of the flame arrestor through the gap. A flange is positioned above the air diverter and has a central opening of a size and alignment substantially the same as that of the air diverter and the flange is angled upwardly and away from the air diverter in the radially outwardly direction and is positioned relative to the burner ports such that burner flames tend to attach thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic partial cross-sectional view of a gas-fueled water heater having an air inlet and low NO x  air distributor according to the invention. 
     FIG. 2 is a front elevational view of the combustion chamber of the water heater of FIG. 1 with the access door and fuel supply means removed for ease of viewing. 
     FIG. 3 is a schematic perspective view of a burner and low NO x  air distributor according to the invention broken apart for ease of understanding. 
     FIG. 4 is a plan view taken through the line IVA—IVA of FIG. 2, with portions taken through line IVB—IVB. 
     FIG. 5 is a cross-sectional view taken through the line V—V of FIG.  4 . 
     FIG. 6 is a cross-sectional view taken through the line VI—VI of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Turning now to the drawings in general and FIG. 1 in particular, there is illustrated a storage type gas water heater  2  including jacket  4  which surrounds a water tank  6  and a main burner  74  in an enclosed chamber  15 . Water tank  6  is preferably capable of holding heated water at mains pressure and is preferably insulated by foam insulation  8 . Alternative insulation may include fiberglass or other types of fibrous insulation and the like. Fiberglass insulation  9  surrounds chamber  15  at the lowermost portion of water tank  6 . It is possible that heat resistant foam insulation can be used if desired. A foam dam  7  separates foam insulation  8  and fiberglass insulation  9 . 
     Located underneath water tank  6  is a pilot burner (not shown) 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 flame arrestor  30 , which is located at opening  28 , and then combusts gas admixed with air and the hot products of combustion rise up through flue  10 , possibly with heated air. Water tank  6  is lined with a glass coating (not shown) for corrosion resistance. The thickness of the coating on the exterior surface of water tank  6  is about one half of the thickness of the interior facing surface to prevent “fish scaling”. Also, the lower portion of flue  10  is coated (not shown) to prevent scaling that could fall into chamber  15  and possibly partially block off flame arrestor  30 . 
     The fuel is supplied to both burner  74  through a gas valve  48  and fuel line  84 . Flue  10  in this instance, contains a series of baffles  12  to better transfer heat generated by main burner  74  to water within tank  6 . Near the pilot burner is a flame detecting thermocouple (not shown) which is a safety measure to ensure that, in the absence of a flame at the pilot burner, the gas control valve  48  shuts off the gas supply. The water temperature sensor  67 , preferably located inside the tank  6 , co-operates also with the gas control valve  48  to supply gas to the main burner  74  on demand. 
     The products of combustion pass upwardly and out the top of jacket  4  via flue outlet  16  after heat has been transferred from the products of combustion. Flue outlet  16  discharges conventionally into a draught diverter  17  which in turn connects to an exhaust duct  19  leading outdoors. 
     Water heater  2  is mounted preferably on legs  24  to raise the base  26  of the combustion chamber  15  off the floor. As noted above, an aperture  28  is closed gas tightly by flame arrestor  30  which admits air for combustion of the fuel gas combusted through main burner  74  and the pilot burner, regardless of the relative proportions of primary and secondary combustion air used by each burner. Flame arrestor  30  is preferably made from a thin metallic perforated sheet of stainless steel, such as described in U.S. Pat. No. 6,085,699, for example. 
     Where base  26  meets the vertical combustion chamber wall or skirt  79 , adjoining surfaces can be either one piece or alternatively sealed thoroughly to prevent ingress of air or flammable extraneous fumes. Gas, water, electrical, control or other connections, fittings or plumbing, wherever they pass through combustion chamber wall  79 , such as at opening  80 , are sealed with a closure plate (not shown). The combustion chamber  15  is air/gas tight except for means to supply combustion air through flame arrestor  30  and to exhaust combustion products through flue  10 . 
     Pilot flame establishment can be achieved by a piezoelectric igniter. A pilot flame observation window (not shown) can be provided which is sealed. Cold water is introduced at a low level of the tank  6  and withdrawn from a high level in any manner as already well known. 
     Referring now to FIGS. 1-6, the invention also includes an air distribution, metering, and combustion staging apparatus  99  for combustion chamber  15  of water heater  2  equipped with flame arrestor  30 . We found that flame arrestor  30  imposes a large flow restriction of the combustion air entering combustion chamber  15  as well as asymmetry in air distribution to burner  74 . We also found that this flow imbalance produces nonuniform stoichiometry around the periphery of the typically axisymmetric burner, with resulting performance penalties in NO x  production in the regions where stoichiometry is not optimal. These nonuniformities arise in both the primary combustion zone, where there may be incomplete mixing of the gas and primary combustion air, and the secondary region where combustion is completed by additional air available at the exit of the burner ports. 
     The reactions by which NO x  is formed are strongly dependent on temperature, with higher flame temperatures producing substantially more NO x  than the amounts created at lower temperatures. Since these high flame temperatures occur in mixtures closest to stoichiometric air/fuel ratios, it is desirable to avoid operating in such a regime. Generally, this is accomplished by ensuring that the overall combustion air is sufficient to increase the stoichiometric ratio well above a value of 1.0, typically to a value of 1.2 or above. Local stoichiometries, however, can vary significantly from the bulk value if air is provided in a nonuniform distribution to the burner and is incompletely mixed prior to combustion at the burner ports. Thus, some regions of the burner can be operating in a manner which produces high levels of NO x  while other regions do not, resulting in an elevated average NO x  concentration in the total flow of combustion products in the flue. Similar mechanisms can produce undesirable levels of CO emissions in nonuniform or poorly mixed gas/air mixtures if localized stoichiometries are such that the oxidation of CO to CO 2  cannot be completed before flame temperatures drop below a critical level. 
     Additional control of NO x  emissions may be achieved through the implementation of staged combustion, in which combustion is initiated under fuel-rich conditions and allowed to proceed for a certain time without the addition of secondary air. During this time, heat is drawn from the flame to minimize NO x  formation when secondary air is eventually added. This heat removal may be accomplished by radiation or conduction away from the flame, and one way to do this is by attaching the flame to a metal surface such as a plate. The secondary air is then added in sufficient quantity to produce fuel-lean bulk conditions, and is mixed in rapidly to minimize the time that any localized region spends near stoichiometric conditions. 
     Apparatus  99  of the invention improves the performance of the combustion system by providing a means to more evenly distribute the air entering chamber  15  via the flame arrestor  30  and thus produce a more uniform stoichiometry around the burner periphery. Referring particularly to FIGS. 5 and 6, a circular flat plate  100  is installed in chamber  15  substantially concentric with burner  74  with the surface from about 0.05 to 0.5 inches, preferably ¼″, lower than the lower edge of burner ports  102  as shown by distance D. Plate  100  is positioned to allow burner  74  to be inserted and removed while plate  100  remains fixed in combustion chamber  15 . Burner  74  is formed from two metallic sheets fixed together. The upper sheet thereof has a smaller diameter than the lower sheet. The metallic sheets are shaped to form a multiplicity of elongated and radially extending channels  103  through which premixed gas and air flow prior to combustion and the lower sheet has an opening positioned at a distal end portion of substantially all of the channels  103 . Channels  103  terminate at burner ports  102 . 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 diameter of plate  100  is sized to create a gap  104  of about 0.125 to about 0.75 inches between its outer edge  106  and skirt  79  of combustion chamber  15  that is small relative to the overall diameter of combustion chamber  15 . The impingement and subsequent redistribution of air on the underside of plate  100  results in a more even flow to and around burner  74 . Additionally, the pressure drop of the secondary air around outer edge  106  of plate  100  can be adjusted by the width of gap  104  between plate  100  and skirt  79  and/or gap between the top of the outer edge  115  (see below) and lower surface  108  (see FIG. 1) of water storage tank  6 , thus allowing more or less secondary air to be admitted. 
     Since the overall airflow into the chamber is restricted by flame arrestor  30 , control of the secondary air accordingly provides a means to control the amount of primary air entering burner  74  and thus the overall primary fuel/air ratio. Additionally attached to the upper surface of plate  100  by five spot welds  101  is a staging flange  110 , comprising a ring  112  with an inner edge  111  having the same inner diameter of plate  100  and an outer edge  115  having diameter that is preferentially smaller than outer edge  106  of plate  100 . The top surface  114  of the flange  110  serves as a means for the flame from the burner ports to attach to the flange  110 , thereby reducing the flame temperature via heat loss due to radiation from the surface  114 . The flange  110  angles upwardly from inner edge  111 , preferably at an angle between about 5° and about 10°, more preferably at about 7°, such that the flange  110  only contacts plate  100  at its inner edge  113 . 
     The flange  110  is positioned vertically so that the inner portion of top surface  114  is substantially flush with the bottom surface of the burner ports  102 . This position and geometry is important for two reasons. First, the upward slope/angle causes the flames to attach substantially continuously to top surface  114  of flange  110 , thereby transferring heat from the flames to flange  110  which reduces peak temperatures and minimizes NO x  production. Second, since outer edge  115  of flange  110  is raised above plate  100 , the flames will attach only to flange  110  and not to any exposed surface of plate  100 . This allows the function of each component to remain separate and can be easily and independently adjusted. Plate  100  controls the relative amounts of primary and secondary air and provides a means for more even distribution of both flows. Flange  110  controls both the radiative heat loss and the time the combustion gases spend in the primary combustion zone prior to secondary air being introduced by adjustment of its width. These in turn control various combustion processes such as formation of NO x  and the burnout of carbon monoxide. 
     Apparatus  99  is supported by three brackets  120 ,  122  and  124 . Brackets  122  and  124  are the same and simply act as support legs. Bracket  120  is substantially in a square “U” shape and, not only acts as a support leg, but has a positioning function by virtue of the length of support  126 , holder  128  and angles α and β, which cause plate  100  to be substantially horizontally and vertically fixed into a desired position. This is especially important to maintain gap  104  substantially even between skirt  79  and outer edge  106 . 
     Thus, the invention serves to control the combustion processes by distributing total combustion air more uniformly and metering the relative proportions of the primary and secondary air, as well as by controlling the heat release and staging. Each function can be individually tailored so that the net effect is the optimization of the overall burner system including, but not limited to, improvements in the emissions of NO x  and carbon monoxide, the efficiency of heat transfer to the water storage tank, and the peak metal temperatures of the combustion apparatus. 
     Installation of apparatus  99  is accomplished as follows: 
     The air distribution plate  100  with staging flange  110  is installed permanently in combustion chamber  15  prior to attaching tank  6  to skirt  79 . Later, burner  74  and the manifold assembly are installed into combustion chamber  15  through opening  80  such that burner  74  is raised up through the center of plate  100  and a seal is formed between edge  106  of plate  100  and the extended lower lip of burner  74 . Burner  74  is then supported by the tip  82  of fuel line  84  at support bracket  86  and by the front cover. The joint between burner  74  and plate  100  is recessed to capture condensate and keep scale away from burner ports  102 . 
     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.