Abstract:
A gas-fired combustion apparatus includes a burner disposed in a substantially sealed combustion chamber and a permeable flexible member arranged to suppress pressure fluctuations within the combustion chamber while concurrently providing for intake of combustion air and restriction of particulate which may interfere with burner operation and increase the risk of ignition of flammable vapor remote of the combustion chamber. The permeable flexible member forms a portion of the combustion chamber wall and is displaceable to provide volume changes. Water heater applications of the burner apparatus are illustrated.

Description:
FIELD OF INVENTION 
     The present invention relates to gas-fired apparatus, and more particularly, to gas-fired heaters, in which combustion occurs within a sealed or substantially sealed combustion chamber. A gas flow device and method provide regulation of gaseous and/or particulate flows in respect to the combustion chamber. In an illustrated embodiment, the gas flow device provides in-take of ambient air for use as primary or secondary air during combustion with suppression of low frequency start-up vibrations and effective filtration of airborne particulate. 
     BACKGROUND OF INVENTION AND RELATED ART 
     The present invention is particularly advantageous in connection with gas-fired heaters having a sealed combustion chamber and a burner that is supplied with a combustible fuel such as natural gas and combustion air from the surrounding environment. The invention is illustrated and described hereafter with respect to residential hot water heaters. 
     In such water heaters, the burner may comprise a porous surface burner such as an infrared burner or a blue flame burner. In the former, the primary air flow may exceed the stoichiometric ratio and no secondary air is required to complete combustion of the fuel. In the case of a blue flame burner, secondary air is generally required to complete the combustion process. In both cases, the primary or secondary air may be delivered to the burner after passing through an entrance opening in the sealed combustion chamber. 
     One problem encountered in connection with the operation of the foregoing heaters is the occurrence of low frequency resonant vibrations in the range of 1-100 hertz during combustion start-up. Such low frequency vibrations or noise may be suppressed by relieving pressure buildup in the combustion chamber. Such pressure relief may be provided by combustion chamber volume change or pressure relief as taught in U.S. Pat. No. 5,435,716 owned by the assignee of the present application. 
     As discussed in greater detail in U.S. Pat. No. 5,435,716, it is theorized that when a gas-fired heater is first placed into operation and the burner is lit there is an initial rapid expansion of the air/gas mixture in the combustion chamber. The expansion occurs so rapidly that the inertia of the column of air in the flue pipe or exhaust pipe open to the atmosphere is unable to accelerate fast enough to remove all of the expanding gases. As a result the pressure in the combustion chamber increases during a high pressure start-up cycle and restricts the flow of combustion air or an air/fuel mixture to the burner so as to inhibit combustion as well as the formation of combustion products and commence a low pressure start-up cycle. The increase in pressure in the combustion chamber may in fact cause the combustion air or air/fuel mixture to flow backwards. This causes the burner flame to greatly decrease in intensity and may even force the flame to extinguish. Once the hot combustion gases in the combustion chamber have had sufficient time to overcome the inertia of the gases in the flue pipe, they will have moved up the flue pipe, causing the low pressure cycle to exist in the combustion chamber. The low pressure cycle or vacuum created will rapidly draw in the combustion air or air/fuel mixture causing rapid expansion of the flame and the hot gases will result in a pressure increase, thereby starting the high pressure cycle over again. 
     The pressure relief device allows for the increase of pressure in the combustion chamber caused by the inertia of the air in the flue pipe or exhaust pipe to be relieved and not restrict the flow of the combustion air or air/fuel mixture, thereby greatly reducing or completely eliminating the noise that the hot water heater makes when engaged, and inhibiting non-uniform flow. Thereafter, there exists little fluctuation in the pressure in the combustion chamber and generally uniform combustion follows. 
     As described above the fluctuation in air pressure causes a low resonance frequency to be observed. The low resonance frequency is best characterized as a rumbling. Although over an extensive period of time the low resonance frequency will dissipate, most gas-fired heaters are not in continuous operation, and the resonance frequency occurs each time the burner is placed into operation. 
     Another problem often encountered in connection with the operation of the foregoing heaters is burner intake of flammable or ignitable vapor contained in the ambient air supply. The intake of such flammable vapor may result in an explosion and/or ignition of the vapor portions remote of the heater. Such explosion and/or remote ignition may be associated with a pilot light used in connection with the heater or the intermittent operation of the heater burner itself. 
     The vapor may result from any number of volatile liquid or gaseous sources such as gasoline, solvents, insecticides, propane and other such sources typically encountered at the sites of household heater applications. The obvious resolution of this problem is to pass the intake ambient air together with any flammable vapor contained therein through a flame trap or arrester. Such flame traps are well-known in the art and tend to contain combustion of the flammable vapor within the sealed combustion chamber. 
     However, the use of a flame trap is not entirety satisfactory since the ambient air also tends to contain particulate which may collect and block the passage of flammable vapor through the flame trap. Illustrative particulate includes lint from fabrics or the like, dust and other conventional solid contaminants found in household or residential ambient air. If particulate collection is sufficient to block gas flow through the flame trap, the burner combustion may be interrupted. On the other hand, if the particulate is combustible, it may serve as a wick or flame path to ignite flammable vapors outside the combustion chamber. 
     A related problem described in U.S. Pat. No. 5,317,992, which is also owned by the assignee herein, arises when it is necessary to achieve high burner loading in a relatively small space. In such cases, maintaining low NO x  emissions becomes even more difficult as increased loading tends to increase the combustion temperature and carbon monoxide and NO x  concentrations in the products of combustion. Substantially sealing the combustion chamber overcomes this tendency by causing a subatmospheric pressure condition in the combustion chamber sufficient to pull excess primary air through the burner to cool the flame and reduce the emissions of carbon monoxide and NO x  to low levels. However, substantially sealing the combustion chamber also exacerbates the tendency of burners operating in combustion chambers to produce a resonance or combustion noise upon ignition of combustion. This resonance can persist for long times and can be unacceptably loud. The tighter the seal, the louder the noise, and the more difficult it becomes to control it. 
     It is the object of the present invention to overcome or minimize the disadvantages of the existing technology as described above. Such disadvantages specifically include the low resonance frequency noise observed when the gas-fired heater is initially placed into operation, the risk of explosion and/or ignition of flammable vapor that may be contained in ambient air and the avoidance of interference of burner operation by airborne particulate. 
     SUMMARY OF THE INVENTION 
     As indicated, the present invention contemplates a device and method for accommodating or suppressing pressure fluctuations within a sealed or substantially sealed enclosure or combustion chamber while concurrently providing for intake of combustion air. Further, the device and method may be arranged to provide regulation of the risks of particulate interference with burner operation and flammable vapor ignition. 
     Specifically, a gas-fired combustion apparatus or heater having a sealed or substantially sealed enclosure or combustion chamber includes a pressure relief device that operates to relieve or suppress pressure changes which may otherwise occur in the combustion chamber with commencement of the burner operation. To that end, ambient air enters the combustion chamber through a gas flow device arranged to pass ambient air for use in the combustion process and restrict particulate flow which may interfere with the combustion process and/or serve as a flame path to ignite flammable vapor remote of the combustion chamber. The particulate is concomitantly cleared from the gas flow device by the latter&#39;s operation to relieve or suppress pressure fluctuations in the combustion chamber. 
     In an illustrated embodiment, the gas flow device comprises a flexible member extending across the entrance to the combustion chamber. The flexible member is gas permeable to allow passage of ambient air into the combustion chamber while restricting the entrance of particulate such as lint or dust. The flexible member is fixed about its periphery to the combustion chamber. The flexible member is sufficiently elastic to deform and oscillate or reciprocally move in the manner of a drum skin in response to pressure fluctuations. In this manner, the flexible member provides both volume changes and venting of gas from the interior of the combustion chamber in order to reduce the low frequency vibrations. 
     The pulsing oscillations or reciprocal movement of the flexible member tends to dislodge particulate collected on the surface of the flexible member remote of the combustion chamber. That is, the particulate falls away from the surface of the flexible member and back into the environment. 
     The flexible member may be formed of any suitable extensible material. The extensible material may be formed of polymers, woven or nonwoven fibrous webs and combinations thereof. As used herein, extensible contemplates a degree of elastic deformation sufficient to accommodate the oscillations of the flexible member. 
     If the material does not have sufficient permeability for the desired gas flow, openings may be provided in the member. Illustrated materials include synthetic and natural rubbers, silicone polymers, urethane polymers and fluoride polymers as are well-known in the art. 
     It should be appreciated that the overall size, thickness and degree of permeability or opening sizes of the flexible member together with the modulus properties of the material forming the member cooperatively provide sufficient movement of the member in response to pressure fluctuations to achieve the desired pressure relief. In addition to pressure relief, sufficient movement of the flexible member is provided to shake free particulate collected on the surface of the member remote of the combustion chamber. 
     The foregoing regulation of particulate permits the safe use of a flame trap located downstream of the flexible member to receive the incoming ambient air. The incoming ambient air flows through the flame trap in order for the latter to quench flame propagation to the exterior of the combustion chamber. Suitable flame traps or flame arresters are well-known in the art and typically include a heat resistant permeable material. 
     Flame traps typically include one or more woven or mesh screens formed of metal. Suitable metals include woven stainless-steel, inconel  601  wire mesh or the like. The flame trap may also be formed of a porous ceramic element such as a SCHWANK type ceramic tile. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic longitudinal sectional view of a water heater having a burner and a gas flow device according to the invention; 
     FIG. 2 is a schematic fragmentary sectional view on an enlarged scale of the gas flow device and a flame trap taken along dashed line A in FIG. 1; 
     FIG. 3 is a schematic plan view of the gas flow device shown in FIG. 1; 
     FIG. 4 is a fragmentary sectional view similar to FIG. 1 showing the gas flow device and flame trap used in a water heater having a blue flame burner; and 
     FIG. 5 is a fragmentary sectional view of the water heater of FIG. 1 modified in accordance with a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, heater  1  includes a gas-fired burner or combustion unit  2  having a combustion surface  5  located in a water heater  1  and positioned below a water tank or container  10 . More particularly, the burner  2  is located within a substantially sealed combustion chamber  8  in the base of the water heater  1 . 
     The combustion unit  2  includes a plenum chamber  4  positioned below the burner combustion surface  5  and an air/fuel mixing and delivery device comprising an air duct  7 . The air duct  7  may be in the form of a venturi as shown or in the form of a cylindrical tube or pipe. The cross-sectional area of the air duct should be sufficiently large to minimize pressure flow losses relative to the subatmospheric pressure driving force above the combustion surface  5 . 
     Gaseous fuel, such as natural gas, is provided to the burner  2  via fuel line  6 . The fuel enters the air duct  7  from a nozzle  6 a and aspirates or induces environmental ambient air to enter the plenum chamber  4  with the fuel. In this manner, the air duct  7  operates in response to the flow of fuel to aspirate and combine environmental air with the fuel to form a combustible air/fuel mixture which is delivered to the plenum chamber  4  at a plenum pressure. The air duct  7  is arranged to allow an excess of primary air to mix with the fuel in the plenum  4 . The majority of the primary combustion air is provided by the driving force of the subatmospheric pressure maintained above the combustion surface  5 . 
     Combustion chamber  8  is in fluid communication, via pores in the burner combustion surface  5 , with the plenum chamber  4 , which in turn is in fluid communication with the air duct  7 . Air duct  7  provides at least partial mixing of the air and fuel, which is completed within the plenum chamber  4 . 
     The heater  1  may be provided with a pilot light (not shown) or other conventional means to provide ignition at burner start-up. Further, a thermocouple (not shown) may be provided for regulation of a gas supply valve (not shown) in a known manner. 
     The burner combustion surface  5  is preferably made of wire mesh, and is more preferably made of inconel 601 wire mesh. Burner combustion surface  5  may also be made of other heat resistant porous materials, such as ceramics. 
     As indicated above, the burner combustion surface  5  is disposed within the substantially sealed combustion chamber  8 . Combustion chamber  8  may enclose the burner element  2 , or the burner element  2  may be attached to the bottom wall  12  of the combustion chamber, whereby burner combustion surface  5  constitutes a portion of the inner wall of chamber  8 . Chamber  8  is sufficiently sealed to prevent entry into the combustion chamber  8  of secondary air in quantities which could adversely affect burner operation. 
     Flue stack  9  constitutes an opening to the environmental air. Thus, as used here in relation to combustion chambers, “sealed” or “closed” refers to minimizing entry of ambient air into the combustion chamber relative to the amount of primary or secondary air. “Sealed” or “substantially sealed” also refers to the condition that exists when the combustion gases within the combustion chamber  8  are not in fluid communication with the outside environment to a degree sufficient to adversely affect the combustion products and limitation of pollutants. 
     Combustion chamber  8  has an opening  13  preferably located in the lower wall  12  which is covered by a gas flow device such as flexible member or diaphragm  14  that is permeable to gas flow but restricts particulate flow into the combustion chamber  8 . The gas flow device also provides pressure relief to suppress the low frequency resonance. The diaphragm  14  covering the opening  13  may be formed of a resilient deformable material such as a polymeric material, a woven structure or a combined woven and polymeric structure. Woven structures may include metallic, graphite, carbon or polymeric fibers such as KEVLAR. Suitable polymeric materials include butyl rubber, natural rubber, silicone rubber, vinyl polymers, urethane polymers, polyethylene polymers or fluoride polymers such as to polytetrafluoroethylene. 
     The resilient deformable material must meet the high temperature requirements of the combustion chamber  8 , and can be evaluated using the following criteria: flexibility, low cost, high temperature rating, ease of manufacture, and durability. The criteria is not absolute and if the material that the diaphragm  14  is made from meets one or more of the criteria it may be suitable. Silicone rubber is used in the preferred embodiment because of its flexibility, relatively low cost, ease of manufacture (injection molding), and because it satisfies the high operating temperatures of the combustion chamber. The flexibility of the diaphragm of course is effected by the diameter of the opening  13  and the thickness of the diaphragm  14 . A suitable thickness range is 0.1 to 0.3 mm. 
     The material used to form the diaphragm  14  and/or its construction may provide sufficient permeability per se to allow the desired intake of ambient air for purposes of combustion. On the other hand, sufficient permeability and gas flow may be provided by openings or apertures  15  (FIG. 2) extending through the diaphragm. As shown, the openings  15  extend through the thickness of the diaphragm and open to the opposed faces  14   a  and  14   b  of the diaphragm  14 . The openings  15  should be no greater than about 1.6 mm in size. In order to filter or restrict particulate, the openings  15  may range in size from 1 mm to 3 mm. 
     The diaphragm  14  is adapted to vent gas through openings  15  and to move in response to variations in the pressure of the chamber. Accordingly, the diaphragm  14  may be considered to be both a volume change device and a venting device. The diaphragm  14  of the present invention adjusts the volume of the combustion chamber  8  in response to pressure fluctuations in the combustion chamber  8 . The inward or outward contraction or expansion of the diaphragm causes the hot water tank to make less noise by suppressing or dampening pressure fluctuations that exist in the combustion chamber  8  upon burner start-up. The diaphragm  14  is shown in FIG. 2 in dotted line in an outwardly extended position and displaced away from the combustion chamber  8  as occurs during a high-pressure cycle. For a diaphragm having a diameter in the range of 20-30 cm, the axial travel of the center point of the diaphragm may be 1 to 5 cm in each direction. 
     The openings  15  are sized to restrict the passage of particulate “P” through the diaphragm  14 . Accordingly, particulate is collected along the face  14   b  of the diaphragm  14  during intake of ambient air. The particulate P is periodically dislodged from the surface  14   b  upon movement of the diaphragm  14 . The dislodgment of the particulate may occur due to the motion of the diaphragm  14  and/or the flow gas from the combustion chamber  8  through the openings  15 . In addition, the remote surface  14   b  of the diaphragm  14  and/or the diaphragm itself should not tend to assume static charges that may attract or secure particulate. The particulate is separated from the diaphragm  14  and may fall to the room floor or be otherwise conveyed away by ambient air movement to a different location. In any event, the particulate is prevented from entering the combustion chamber  8  or collecting in excessive amounts that may interfere with proper combustion. 
     The effective exclusion of particulate from the combustion chamber  8  enables a flame trap  20  to be mounted safely over the opening  13  and downstream of the combustion air flow through the diaphragm  14 . The flame trap  20  may comprise one of more layers of wire mesh arranged to define interstices sized to pass the gas or intake combustion air, but to prevent flame propagation through its thickness. The exclusion of the particulate by the diaphragm  14  prevents particulate from accumulating on the flame trap  20  or within the interstices thereof so as to effectively block intake air flow and possibly provide a flame conducting medium which could ignite flammable vapors outside of the combustion chamber  8 . 
     The pressure relief device such as the membrane or diaphragm  14  can be retroactively fitted with a snap in design that would lead to the benefits of the present invention. The membrane or diaphragm may be provided with any convenient configuration, such as circular, square, rectangular or combination thereof. 
     Turning to the operation of the heater  1 , as best shown in FIG. 1, flue stack  9  extends upwardly from an upper dome-shaped wall surface  11  of the combustion chamber  8  through the center of the water tank  10 . The flue stack may extend above the water heater to increase the natural draft and further decrease the subatmospheric pressure in the combustion chamber  8 . The dome-shaped upper wall  11  functions to guide the combustion products into the flue stack  9 . Further, the domed-shaped upper wall  11  operates as a heat exchange surface since it is part of the water tank. 
     The domed-shaped upper wall  11  of the combustion chamber  8  is in direct heat exchange relationship with the water within the water heater and its concave, domed shape accommodates the upward flow of the combustion products from the combustion chamber  8  into an upwardly extending flue stack  9 . The combustion chamber  8  and flue stack  9  are structured so that the natural draft results in the flow of the buoyant combustion products up through the flue stack  9  and produces a subatmospheric pressure (e.g. 0.015 inches water column) within the combustion chamber  8 . This facilitates the flow of fuel and primary air through the burner and the combustion surface so that a given size burner operates at a higher loading than the same burner would operate if the combustion chamber  8  were maintained at atmospheric pressure. This permits the manufacture of a water heater of a given rating with a smaller size burner than would be possible if the combustion chamber were at atmospheric pressure. 
     Flue stack  9  may also contain baffle device (not shown) to improve efficiency of heat transfer from combustion gases to the water. The baffle should be designed to reduce frictional flow losses in the flue stack  9 . 
     The burner may be operated at conditions which result in primarily convective heat transfer, e.g. 70% to 80% or more, and reduced emissions of pollutants. To that end, the combustion or flame temperature is maintained in the range of 600 to 900° C. by the use of excess primary combustion air. Generally, the excess air is in the range of from about 110% to about 200% in order to maintain the desired combustion temperatures. The combustion loading of the burner surface may range from about 500 to about 2,000 MJoules/m 2  hr. These operating conditions reduce the NO 2  emissions to less than about 5 ng/Joule and provide a CO/CO 2  ratio of less than about 0.003 discussed in assignee&#39;s U.S. Pat. No. 5,340,305. Accordingly, the outdoor ventilation of the flue products is not required. Such combustion temperatures also favor convective heating over radiant heating so that the burners provide primarily convective heat transfer. Heretofore, burners having combustion surfaces were operated at significantly higher temperatures to promote radiant heat transfer. The lower temperature operating conditions also increase the selection of suitable materials for the flexible member or membrane because this material must be able to withstand the temperatures that exist in various locations within the combustion chamber. 
     The second embodiment of the present invention is shown in FIG.  4 . This embodiment is similar to that described above and for convenience, similar elements are identified with the same number and elements that have been modified are indicated by a prime symbol (′). The same or similar elements operate in the manner discussed above. 
     The heater  30  has a combustion unit  31  comprising a blue flame burner positioned below the upper wall  11  of the combustion chamber  8 . Once again, a pilot burner (not shown) and thermocouple (not shown) may be provided in a known manner. Gaseous fuel is delivered to the burner  31  through fuel line  6  with primary air and secondary air being drawn from the combustion chamber  8 . The burner  31  combusts the fuel and air mixture to yield hot combustion products that rise through the flue stack  9  passing through the center of the water tank  10 . 
     Ambient air is drawn into the combustion chamber  8  through the opening  13  in the lower wall  12 . The flexible member or diaphragm  14  covers the opening  13  and, as described above, again allows passage of gas or ambient air to provide combustion air and filters particulate to prevent it from entering the combustion chamber. Accordingly, the flame trap  20  may be used effectively and safely to prevent flame propagation and ignition of flammable vapors remote of the combustion chamber. 
     Referring to FIG. 5, a further embodiment of the present invention is shown. This embodiment is similar to those described above and for convenience similar elements are identified with the same number and operate in the same manner as described above. The burner  2  is mounted in the combustion chamber  8 . The diaphragm  14  extends across the opening  13  in the lower wall of the combustion chamber  8  and the flame trap  20  is arranged to receive the ambient air entering the combustion chamber through the diaphragm  14 . 
     In this embodiment, the diaphragm  14  is structurally supported by perforated protective covers  16  and  17  located at each side of the diaphragm. The perforated protective covers  16  and  17  have perforations  18  which allow the diaphragm to be in fluid communication with the combustion chamber  8  and the outside environment. Perforated covers  16  and  17  prevent unwanted displacement of the diaphragm  14 . 
     In may be desirable in certain applications to position on the lower wall  12  insulation  19  (shown in dotted line) which may comprise, for example, a glass or mineral fiber batt. The insulation  19  provides multiple functions. The insulation  19  reduces the low frequency resonance by allowing passage of gas during pressure fluctuations and, if provided with an appropriate thickness, it operates as a flame trap. Of course, the insulation  19  also reduces heat loss through the wall  12 . The placement of the insulation over the diaphragm  14 , protective covers  16  and flame trap  20  also shields one or more of the latter from excessive heat. Such protection is particularly desirable in respect to the diaphragm  14 , and enables the diaphragm to be formed of desirable materials which may not meet the above temperature criteria. 
     Furthermore, the positioning of the flexible membrane or diaphragm  14  on the lower wall  12  is very advantageous. First, combined with the insulation  19  the flexible diaphragm  14  reduces the noise to a point that it is barely, if at all, audible. Also, positioning of the flexible membrane or diaphragm  14  at the bottom of the tank on the lower wall  12  allows the membrane  14  to exist in the coolest place in the combustion chamber  8 . The temperature directly below the plenum chamber  4  is not as hot as the rest of the chamber because of the buoyancy of the hot air/gas products and the positioning of the pressure relief device in the lower wall  12  prevents the diaphragm  14  from being damaged and provides for the longest life of the diaphragm  14 . 
     Although preferred embodiments of this invention have been shown and described, it should be understood that various modifications and rearrangements of the parts may be resorted to without departing from the scope of the invention as disclosed and claimed herein.