Abstract:
A gas-fired air conditioning furnace has a cavity burner configured to combust an air-fuel mixture at least partially within an interior space of the cavity burner. A method of operating a gas-fired furnace by flowing an air-fuel mixture into a cavity burner through a perforated wall of the cavity burner, combusting at least a portion of the air-fuel mixture within an interior space of the cavity burner, and flowing at least partially combusted air-fuel mixture into a heat exchanger. A gas-fired air conditioning device has a cavity burner that has a cylindrically shaped body and a cap on a first end of the body, each of the body and the cap being perforated. The device has a cylindrically shaped heat exchanger inlet tube and the cavity burner is at least partially concentrically received within the heat exchanger inlet tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     Gas-fired furnaces are widely used in commercial and residential environments for heating, including space heating for air conditioning interior spaces. However, gas-fired furnaces are known to generate and emit oxides of nitrogen (NO X ). NO X  is a term used herein to describe the various oxides of nitrogen, in particular NO, N 2 O and NO 2 . NO X  emissions from gas-fired furnaces are typically attributable to less than optimal air-fuel mixtures and combustion temperatures. 
     SUMMARY 
     In an embodiment, among others, a gas-fired air conditioning furnace is provided that comprises a cavity burner configured to combust an air-fuel mixture at least partially within an interior space of the cavity burner. 
     In another embodiment, among others, a method of operating a gas-fired furnace is provided. The method comprises flowing an air-fuel mixture into a cavity burner through a perforated wall of the cavity burner, combusting at least a portion of the air-fuel mixture within an interior space of the cavity burner, and flowing at least partially combusted air-fuel mixture into a heat exchanger. 
     In yet another embodiment, among others, a gas-fired air conditioning device is provided that comprises a cavity burner comprising a cylindrically shaped body and a cap on a first end of the body. Each of the body and the cap are perforated. The device further comprises a cylindrically shaped heat exchanger inlet tube and the cavity burner is at least partially concentrically received within the heat exchanger inlet tube. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is an oblique exploded view of a gas-fired furnace comprising cavity burners according to an embodiment of the disclosure; 
         FIG. 2  is an orthogonal simplified view of a gas-fired furnace with cavity burners according to an embodiment of the disclosure; 
         FIG. 3  is a block diagram of a method of air conditioning according to an embodiment of the disclosure; 
         FIG. 4  is a simplified oblique view of a cavity burner received within an inlet tube; and 
         FIG. 5  is a simplified schematic view of a gas-fired furnace comprising a cavity burner and an associated heat exchanger. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     Lowering NO X  emissions attributable to a gas-fired furnace may be accomplished by lowering the burn temperature of an air/fuel mixture in the burners of the gas-fired furnace. It may be desirable to lower the NO X  production to below 14 nano-grams per joule (ng/J) of energy used. Accordingly, a gas-fired furnace with cavity burners for lowering the burn temperature of an air/fuel mixture is provided. The furnace may comprise one or more cylindrical premix cavity burners similar to the cylindrical metal premix burners sold by Worgas of Formigine, Italy, although other cavity burners may be used. The cavity burners may each be inserted into a heat exchanger inlet tube. The burner tubes may be housed in a heat exchanger inlet tube assembly such that a mixture of air and fuel is provided to a first side of the cavity burners. A second side of the burner tube assembly may be connected to a heat exchanger for venting hot flue gasses, such that the air flow through the furnace passes through the burners. 
     Referring to  FIG. 1 , an oblique exploded view of a gas-fired furnace  100  is illustrated. The furnace  100  comprises an air/fuel mixing box  105 , an air/fuel mixing baffle  110 , a partition panel  115 , a plurality of heat exchanger inlet tubes  120 , a plurality of cavity burners  125 , a burner box  130 , a post combustion chamber  135 , a plurality of heat exchangers  140 , and a heat exchanger exhaust chamber  145 . 
     The air/fuel mixing baffle  110  may be connected to a portion of the partition panel  115  above an opening for the heat exchanger inlet tubes  120 . The air/fuel mixing box  105  may be mounted to the partition panel  115  such that a cavity is created around the air/fuel mixing baffle  110  and the openings for the heat exchanger inlet tubes  120 . Fuel and air may be introduced to the air/fuel mixing box  105  to allow mixing before combustion. The air/fuel mixing baffle  110  aids in the mixing of air and fuel in the air/fuel mixing box  105  by altering the direction of air and fuel flow through the air/fuel mixing box  105 . The mixing of the air and fuel may also be aided by a mixing device to encourage homogeneous mixing of the fuel and combustion air in the air/fuel mixing box  105 . Fuel may be introduced to the air/fuel mixing box  105  by a gas supply valve. The gas supply valve may be adjusted either electrically of pneumatically to obtain the correct air to fuel ratio for increased efficiency and lower NO X  emissions. The gas supply valve may be configured for either staged operation, or modulation type operation. For example, staged operation may have two flame settings, where modulation type operation may be incrementally adjustable over a large range of outputs, for example from 40% to 100% output capacity. 
     The air/fuel mixture may travel from the air/fuel mixing box  105  into the heat exchanger inlet tubes  120 . The heat exchanger inlet tubes  120  may be constructed of a cylindrical piece of metal having a slightly larger inner diameter than the outer diameter of cavity burners  125 . The cavity burners  125  may be perforated to allow the air/fuel mixture through the walls of the cavity burners  125 . For example, the cavity burners  125  may comprise a great number of small perforations over a substantial portion of the cylindrical walls and end walls of the cavity burners  125 . 
     The cavity burners  125  may be substantially coaxially received within the heat exchanger inlet tubes  120 . By positioning the cavity burners  125  within the heat exchanger inlet tubes, the cavity burners  125  are within a combustion airflow path, therefore substantially all of the combustion air passes through the cavity burners  125 . The cavity burners  125  may be substantially cylindrical in shape, open on one end, and closed on the opposite end. The open end of the cavity burners  125  may be positioned at input openings of the heat exchangers  140 . Each cavity burner  125  may have an associated heat exchanger  140  for venting hot flue gasses such that the heat exchanger  140  is in the combustion airflow path of the associated cavity burner  125 . While four cavity burners  125  are depicted, the total number of cavity burners  125  may vary depending upon the desired capacity of the furnace. 
     An igniter mounted to the post combustion chamber  135  may be positioned at the opening of one of the cavity burners  125  to ignite the air/fuel mixture in one of the cavity burners  125 . The remaining cavity burners  125  may be ignited by a flame carry over path. The flame carry over path may connect the cavity burners  125 . The flame in the cavity burners  125  may be counter-flow to the direction of combustion gas flow in the system, resulting in substantially all of the air/fuel mixture passing through the perforations in the cavity burners  125  to the flame. The combustion of the air/fuel mixture substantially occurs inside the cavity burners  125  along the inner perforated surfaces of the cavity burners  125 . Combustion inside the cavity burners  125  may allow substantially all of the heat of combustion to be focused at the opening of the cavity burners  125 . Combustion air may be introduced either in induced draft mode, by pulling air through the system, or in forced draft mode by pushing air through the system. Induced draft mode may be accomplished by attaching a blower or fan at the exhaust of the heat exchanger exhaust chamber  145  and pulling air out of the system by creating a relatively lower pressure at the exhaust of the heat exchanger exhaust chamber. Forced draft mode may be accomplished by placing a blower or fan at the air/fuel mixing box and forcing air into the system through the air/fuel mixing box. A control system may control the fan or blower to an appropriate speed to achieve adequate air flow for a desired firing rate through the cavity burners  125 . Increasing the fan speed of the combustion blower will introduce more air to the air/fuel mixture, thereby changing the characteristics of the combustion in the cavity burners  125 . 
     Substantially enclosing the cavity burners  125  within the heat exchanger inlet tubes  120  and substantially containing combustion within the cavity burners  125  may reduce the amount of thermal radiation emitted to parts of the furnace  100  other than the heat exchangers  140 . The open ends of the cavity burners  125  are attached to the post combustion chamber  135 . However, in alternative embodiments, the cavity burners  125  may be positioned differently and/or the flow of the air/fuel mixture may be passed through the cavity burners  125  in a different manner. The post combustion chamber  135  is attached directly to an opening on the heat exchangers  140  to ensure that substantially all of the heat generated by the cavity burners  125  may be transferred directly into the heat exchangers  140  by directing hot flue gasses into the heat exchangers  140 . The post combustion chamber  135  seals the system from secondary dilution air as well as positions the cavity burners  125  for transfer of the hot flue gasses to the heat exchangers  140 . The heat exchangers  140  may be, for example, be clamshell, tubular, drum or shell and tube type heat exchangers. 
     Turning now to  FIG. 2 , another gas-fired furnace  100  with cavity burners is depicted. In this embodiment, the furnace  100  further comprises a draft inducer  210 , an air/fuel mixer  220 , an igniter  230 , and a flame sensor  235 . The draft inducer  210  may be a fan attached to the heat exchanger exhaust chamber  145  for pulling hot flue gasses through the heat exchangers  140 . The draft inducer may be controlled by a control system to ensure appropriate air flow through the system. The igniter  230  may, for example, comprise a pilot light, a piezoelectric device, or a hot surface igniter. The igniter  230  may be controlled by a control system or may be manually ignited. The igniter  230  may also comprise a flame sensor such as a thermocouple or another safety device. The flame sensor  235  may comprise a thermocouple, a flame rectification device, or any other suitable safety device. 
     Referring now to  FIG. 3 , a block diagram depicting a method  300  of conditioning air is depicted. The method begins at block  310  by mixing a fuel and air together. The fuel may be natural gas available from a gas valve attached to an air/fuel mixing box. The air may be introduced to the air/fuel mixing box by a forced draft or an induced draft. The mixing process may be aided by an air/fuel mixing baffle installed within the air/fuel mixing box. The air fuel mixing baffle may be placed in front of the outlet of the air/fuel mixing box, altering the flow of the air and fuel within the air/fuel mixing box and thereby causing an improved mixing of the air and the fuel. An air/fuel mixer may also be part of the air/fuel mixing box to actively mix the air and fuel within the air/fuel mixing box. 
     The method continues at step  320  where the air/fuel mixture may be moved through a cavity burner. The cavity burner may have a cylindrical body with an open end and a closed end. The closed end and the cylindrical body may be perforated to allow the air/fuel mixture to pass through into the cavity created by the walls of the cavity burner. The cavity burner may be contained within a heat exchanger inlet tube such that the air/fuel mixture leaving the air/fuel mixing box passes through the perforations of the cavity burner. 
     The method continues at step  330 , where the air/fuel mixture may be ignited. The open end of the cavity burner may face a post combustion chamber. An igniter may be mounted in the post combustion chamber near the opening of the cavity burner. The igniter may be a pilot light, a piezoelectric spark, or a hot surface igniter. As the cavity within the cavity burner fills with the air/gas mixture, the igniter may ignite and cause combustion to begin within the cavity burner. 
     The method continues at step  340  by venting hot flue gasses through a heat exchanger. Combustion may occur at least partially within an interior space of the cavity burner so that heat is generated and forced out of the open end of the cavity burner and into the post combustion chamber. In this embodiment, the combustion may occur generally within a space bound by the cylindrical wall of the cavity burners  125 . Of course, in other embodiments, combustion may occur both within the interior space and outside the interior space, such as in a space generally associated with the open end of the cavity burners  125 . Other embodiments may even have the cavity burners  125  with the opening adjacent to the mixing box  105 , and the flame situated on the exterior surface of the cavity burner  125 . The post combustion chamber may have a heat exchanger attached. The heat exchanger may be tubular in design with a first end connected to the post combustion chamber and a second end connected to a heat exchanger exhaust chamber. The hot flue gasses may be a result of the combustion of the air/fuel mixture and may contain NO X . The level of NO X  in the hot flue gasses may be lowered by varying the combustion temperature of the air/fuel mixture. Combustion within a cavity burner may occur at lower temperatures and have a much smaller flame front area thereby reducing the level of NO X  generated and thereafter present in the flue gasses. 
     The method continues at step  350  by conditioning air outside of the heat exchanger. As the hot flue gasses travel through the heat exchanger to the heat exchanger exhaust chamber, the heat exchanger may be heated. Air that is exterior to the heat exchanger may be moved across the heat exchanger. As the air moves across the heat exchanger heat may be transferred from the heat exchanger to the air. 
     The method concludes at block  360  by venting the conditioned air into an air conditioned space, for example, an office space or living area of a home. The heated air may be used to warm the space in order to increase comfort levels for occupants or to maintain the contents of the space at a pre-determined temperature. 
     Referring now to  FIG. 4  in the drawings, a cutaway view of a cavity burner  125  located within an inlet tube  120  and connected to burner box  130  and post-combustion chamber  135  is shown. In  FIG. 4 , a portion of the inlet tube  120  is cut away to show that cavity burner  125  resides therein and to show that cavity burner  125  is connected to burner box  130  which is connected to post-combustion chamber  135 . 
     Referring now to  FIG. 5 , a gas-fired furnace  500  is shown. Gas-fired furnace  500  comprises a circulation air blower  502  that receives incoming airflow  504  and passes incoming airflow  504  into contact with heat exchangers  140  to transfer heat from the heat exchangers  140  to the air. Exiting airflow  506  is distributed to an area that is to be conditioned with the heated air. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted or not implemented. 
     Also, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.