Abstract:
A burner for use with an induced draft furnace and which satisfies reduced nitrous oxide (NO x ) emission standards is disclosed. The burner may employ a mechanical swirler that introduces a rotational vector to the emitted air and fuel mixed by the burner. By introducing the rotational vector, the resulting flame is more stable and sustainable even with the relatively low air flow afforded by an induced system. Such flame stability can be enhanced by positioning the burner directly within an inlet to a heat exchanger and manufacturing the inlet with reception surfaces that form a frusto-conically shaped flame expansion zone. In doing so, a secondary source of air is avoided and NO x  emissions are reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a non-provisional US patent application, which claims priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 61/421,974 filed on Dec. 10, 2010. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to gas burners and furnaces and, more particularly, relates to gas burners and furnaces which employ an induced draft. 
     BACKGROUND OF THE DISCLOSURE 
     Induced draft gas furnaces are commonly used to generate heat for residential and commercial use. Such furnaces vary in design, but at their core serve the basic function of igniting gas (typically natural gas or propane) and air, and directing the resulting combustion gases to a heat exchanger. The combustion gases are of an elevated temperature and by directing same through serpentine conduits provided as part of the heat exchanger, air to be heated can then be directed across the heat exchanger to extract heat from the heat exchanger. A blower motor provided as part of the furnace can be used to create the air flow across the outside surface of the heat exchanger. The heated air then exits the furnace and by way of ductwork is communicated to the rooms or space needing to be heated. 
     The heat exchangers of such furnaces typically employ a plurality of heat exchanger coils, each one having a burner associated with an inlet to the coil. The burner serves the function of mixing the gas and air and igniting same to generate a flame. The burner outlet with such prior art designs is positioned close to, but spaced from, the heat exchanger coil so as to direct at least a portion of the flame into the heat exchanger coil. The gas is typically introduced into the burner by way of a gas supply controlled by a processor of the furnace. The air needed for combustion is typically provided by way of another blower motor which pulls (induced draft) air through the burner and pulls the flame and combustion gases through the heat exchanger. 
     While effective and commercially successful, air quality regulations are becoming increasing stringent. For example, federal, state and local authorities regulate acceptable emissions standards of nitrous oxide (NO x ), among others. The SCAQMD (South Coast Air Quality Management District) of California is one example of a regulatory body dictating a maximum emission rate of NO x . Given the current climate and popular opinion regarding the environment, these standards are likely to only get more restrictive in the future. 
     As a result of such regulations, prior art burners have had to be redesigned. Certain prior art burners, known as “in-shot” burners, included two sources of air: a primary source providing air to the inlet of the burner for mixing with the gas, and a secondary source at the outlet of the burner and prior to introduction of the flame to the heat exchanger. However, in order to reduce NO x  emissions, that secondary source of air has to be eliminated. While reduction in NO x  emissions have been achieved in forced drafted system (blower at inlet) burners for use with induced draft furnaces which satisfy the emissions standards have not been introduced. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the disclosure, a furnace is disclosed which comprises a heat exchanger having an inlet and an outlet, the outlet being connected to a vent, an inducer motor operatively associated with the heat exchanger outlet to draw air through the heat exchanger, a burner tube adapted to direct a flame into the heat exchanger inlet, the burner tube having an inlet and an outlet, a swirler provided with the burner tube between the inlet and the outlet, a source of fuel connected to the burner tube inlet, a source of air operatively associated with the burner tube inlet, and a blower motor adapted to direct air flow across the heat exchanger to extract heat from the heat exchanger. 
     In accordance with another aspect of the disclosure, a heat exchanger assembly is disclosed which comprises a heat exchanger coil having an inlet and outlet, the inlet including reception surfaces forming a frusto-conically shaped flame expansion zone, an inducer motor operatively associated with the heat exchanger coil outlet, and a burner tube positioned within the heat exchanger coil inlet, the burner tube including an inlet and an outlet with a swirler between the inlet and outlet. 
     In accordance with yet another aspect of the disclosure, a method of operating an induced draft furnace is disclosed which comprises providing a heat exchanger having an inlet and an outlet, connecting a motorized fan to the heat exchanger outlet and thereby inducing an air flow through the heat exchanger, positioning a burner in the heat exchanger inlet, the burner including an inlet, an outlet, and a swirler between the inlet and the outlet, pre-mixing air and fuel in the inlet of the burner, inducing flow of the mixed air and fuel through the burner with the motorized fan, introducing a swirling flow pattern to the mixed air and fuel by passing the mixed air and fuel through the swirler, igniting the mixed air and fuel into a flame, and directing the flame into the heat exchanger inlet. 
     These and other aspects and features of the disclosure will be explained in further detail herein in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a furnace constructed in accordance with the teachings of the present disclosure; 
         FIG. 2  is a sectional view of a burner constructed in accordance with a prior art design utilizing an in-shot ignition system; 
         FIG. 3  is a sectional view of a burner and heat exchanger assembly constructed in accordance with the teachings of the present disclosure; 
         FIG. 4  is a sectional view of the burner of  FIG. 3 , taken along line  4 - 4  of  FIG. 3 ; and 
         FIG. 5  is a sectional view the burner and heat exchanger assembly constructed in accordance with the teachings of the present disclosure. 
     
    
    
     While the following detailed description will be given with respect to certain illustrative embodiments, it is to be understood that the teachings of the present disclosure can be used in conjunction with other embodiments not specifically disclosed but encompassed by the spirit and scope of the appended claims. 
     DETAILED DESCRIPTION 
     Referring now to the drawings, and with specific reference to  FIG. 1 , a furnace constructed in accordance with the teachings of the present disclosure is generally referred to by reference numeral  20 . While described herein primarily in conjunction with a furnace, it is to be understood the burner disclosed can be used in additional settings as well, including but not limited to, boilers and other heat generation equipment. 
     The furnace  20  may include a heat exchanger  22  having a plurality of individual heat exchanger coils  24 . The heat exchanger coils  24 , which may be metallic conduits, are provided in a serpentine fashion to provide a large surface area in a small overall volume of space, the importance of which will be discussed in further detail below. Each heat exchanger coil  24  includes an inlet  26  and an outlet  28 . A secondary or condensing heat exchanger  29  may be provided as well. A burner  30  is operatively associated with each inlet  26 , and a vent  32  is operatively associated with each outlet  28 . The plurality of burners  30  may collectively be provided in a burner box  31 . The burners  30  introduce a flame and combustion gases  34  (see  FIG. 3 ) into the heat exchanger coils  24 , while the vent  32  releases the combustion gases  34  to the atmosphere after the heat of the flame and combustion gases  34  is extracted by the heat exchanger  22 . 
     In order to extract that heat, a blower motor  36  may be provided to create significant air flow across the heat exchanger coils  24 . As the air circulates across the heat exchanger coils  24  it is heated and can then be directed to a space to be heated such as a home or commercial building by way of appropriate ductwork (not shown). The furnace  20  may also provide combustion air inlet  38 . 
     To generate the flame and hot combustion gases  34 , the burners  30  mix fuel and air and ignite same. Referring now to  FIG. 3 , the fuel is typically natural gas or propane and is provided to a spray nozzle or jet  42  positioned at an inlet  44  to the burner  30 . More specifically, the burner  30  may include a burner tube  46  having the inlet  44  and an outlet  47 . All of the air necessary for combustion is also introduced into the burner  30  at inlet  44 . Such air (represented by arrow  48  in  FIG. 3 ) is introduced by inducing an air flow using a motorized fan  49  downstream of the burner outlet  46 . More specifically, a motor  50  having the fan  49  coupled thereto is operatively associated with the outlet of  28  the heat exchanger coils  24  to induce a draft and pull the pre-mixture and flame  37  therethrough. When energized, the fan rotates and induces an air flow pulling air through the heat exchanger coils  24  and burners  30 . Control of the motor  50 , as well as the motor  36  may be controlled by a processor  52  such as an integrated furnace control (IFC). The motors  36  and  50  may be variable speed motors adapted to rotate at differing velocities as dictated by signals received from the IFC  52 . 
     Comparing  FIG. 3  to  FIG. 2 , the differences between the presently disclosed burner  30  of  FIG. 3  and the prior art burner of  FIG. 2  are shown in more detail. As indicated above, the burner  30  of the present disclosure may include the burner tube  46  having the inlet  44  and outlet  47 , with the outlet  47  integrated into the heat exchanger inlet. As all of the air needed for combustion is provided by inlet  44 , the inlet  44  also serves as and defines a mixing chamber  54  with the fuel. In order to reduce NO x  emissions, the fuel and air must be premixed prior to ignition. No source of secondary air can be provided. This is a significant departure from the prior art “in-shot” burner depicted in  FIG. 2 , wherein primary air  55  enters through inlet  44 ′ and secondary air  56  enters through gap  57  after initial ignition and thereby leads to the unacceptably high NO x  emissions levels associated with the prior art. 
     In order to provide a stable flame  34  in such an induced draft furnace  20 , the burner  30  may further includes a mechanical swirler  58 . As shown both in  FIG. 3  and  FIG. 4 , the swirler may include an annular plenum  60  surrounding a central passageway  62 . The annular plenum  60  may include a plurality of vanes  64  provide at an angle relative to the longitudinal axis  66  of the burner  30 . In so doing the premixed air and fuel flowing through the annular plenum  60  is deflected by the vanes  64 . A tangential or rotational vector is therefore introduced to the flow of the mixed air and fuel. In combination with the mixed air and fuel flowing through the central passageway  60  this creates an exiting plume  67  of fuel and air that can be controlled and results in a stable flame  34 . The central passageway  62  may be provided with a flow restrictor  68  to create a pressure drop from the inlet  44  to the outlet  46 . The amount of restriction lets the flow split between the central and annular flow paths. The flow restrictor  68  can be provided in the form of a wire mesh, screen or filter, or the aforementioned venturi, with the level of restriction being selected to result in the flame characteristics desired. Two examples of such low swirl burners are set forth in U.S. Pat. Nos. 5,879,148 and 5,735,681, both assigned to Lawrence Berkeley National Laboratories and both herein incorporated in their entireties by reference. 
     Upon exit from the swirler  58 , the plume  67  of mixed air and fuel encounters an igniter  69 . With ignition, the flame and combustion gases are created and directed into the heat exchanger coils  24  as indicated above. To supplement the stability of the flame  34 , the burner  30  may be provided directly within the inlet  26  of the heat exchanger coils  24  as shown best in  FIG. 5 . In so doing, the flame  34  is held within the heat exchanger  22  in its entirety. Moving the flame  34  into the heat exchanger  24  where air is present enables the heat to be more efficiently extracted, while at the same time making a more compact assembly and enabling the heat exchanger inlet and burner outlet to be integrated and sealed against the introduction of any secondary air. In addition, the inlets  26  of the heat exchanger coils  24  may be fabricated, as by stamping, so as to have reception surfaces  70  which form a frusto-conically shaped flame expansion zone  72 . Provision of the frusto-conically shaped flame expansion zone  72  encourages creation and maintenance of the flame  34 , while at the same time facilitating manufacturability. Moreover, as will be noted from  FIG. 5 , the diameter of the heat exchanger coil  24  is significantly greater than the diameter of the burner tube  30  (roughly double in one embodiment) to confine and yet maintain the proper flowfield for flame  34  stabilization. 
     In operation, it can therefore be seen that the present invention provides a furnace  20 , a burner and heat exchanger assembly  74 , and a method of operation same that works with an induced draft air flow and provides reduced NO x  emissions. The method of operation may include the steps of providing a furnace  20  or burner and heat exchanger assembly  74  as indicated above, inducing air flow through the burner  30  and heat exchanger  22  using a downstream motor  50 , introducing fuel flow through the fuel nozzle  42 , and energizing the igniter  69 . In so doing, a swirling, and conically expanding, flame  34  is created using a single air source and thus with reduced NO x  emissions. In addition, by providing the burner  30  directly within the heat exchanger inlet  26 , and providing the inlet  26  in the form of a frusto-conically shaped expansion zone  72 , the resulting flame  34  is both reduced in terms of NO x , and stable. 
     Industrial Applicability 
     From the foregoing, it can be seen that the technology disclosed herein has industrial applicability in a variety of settings such as, but not limited to, residential and commercial furnaces. Using an induced draft approach sufficient air needed for combustion can be pulled through the burner and heat exchanger without needing a secondary air source. Eliminating any secondary air source also reduces NO x  emissions. In addition, using a mechanical swirler, the flame produced by the burner, even though used in an induced draft system is stable and sustainable. This stability and sustainability are supplemented by positioning the burner within the heat exchanger inlet, and shaping the heat exchanger inlet to have a frusto-conical shape so as to support the stability of the flame. Such a burner or burner and heat exchanger assembly can also be used in other heating equipment such as boilers, among others. 
     It is to be understood that the teachings of the present disclosure can be practiced by the foregoing embodiments as well as other embodiments not specifically disclosed but encompassed by the literal and equivalent scope afforded by the appended claims.