Patent Publication Number: US-6663381-B2

Title: Burner arrangement for low NOX emissions

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to gas burners for residential furnaces and for commercial heating and cooling packaged products and, more particularly, to apparatus for reducing NOX emissions from such devices. 
     Continued concern about atmospheric pollution has created renewed interest in lowering the emissions of various combustion devices. Of particular concern are nitric oxide (NO) and nitrogen dioxide (NO2) emissions because of their roles in forming ground level smog and acid rain, and in depleting stratospheric ozone. For simplicity, NO and NO2 are often grouped together as NOx. Many jurisdictions have stringent NOx emissions regulations. For example, California limits NOx emissions from gas furnaces to a maximum of 40 ng/J. It is expected that over the coming years, the regulations will become more stringent. 
     The common mechanism for the formation of NOx in a gas fired furnace is referred to as thermal NOx. In this mechanism, high flame temperatures, generated by the combustion process, result in the formation of NOx. The primary strategy to control NOx formation is to lower the flame temperature. One method is shown in U.S. Pat. No. 4,904,179, wherein a radiant member is inserted in the flame. The member heats up, radiates energy away from the flame and thereby cools the flame down. The disadvantage of this method is that the NOx benefit gained will not be enough to meet possible future regulations. Another method to lower flame temperature is to recirculate flue gas into the flame. The presence of combustion products suppresses flame temperature. One method is achieving this, is described in a preferred embodiment of U.S. Pat. No. 6,071,115. High momentum secondary air jets are injected into the primary fuel-air mixture from the outside, thereby promoting mixing and causing a recirculation of the combustion products into the flame. However, it is difficult to apply this concept to a furnace in a simple, and cost-effective way. 
     It is therefore an object of the present invention to provide an improved apparatus and method for reducing NOx in a gas burner. 
     SUMMARY OF THE INVENTION 
     Briefly, in accordance with one aspect of the invention, a tube is inserted into the downstream area of a burner and air is supplied to one end of the tube and emerges at the other end thereof so as to thereby reduce the resulting generation of NOX. 
     By another aspect of the invention, the tube extends radially inwardly near the end of the burner and then turns to extend substantially along an extension of the burner axis, with the air being discharged at the end of the tube, the position of which is optimized to obtain a desired degree of NOX reduction while maintaining a low noise level. 
     In the drawing as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of a furnace with the present invention incorporated therein. 
     FIG. 2 is a schematic illustration of a gas burner and flame in accordance with the prior art. 
     FIG. 3 is a schematic illustration of a gas burner and flame in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, the present invention is shown generally at  10  as applied to a typical hot air furnace  11  having a sheet metal covering  12  which encases a series of heat exchangers  13 , an air circulation blower  14 , a plurality of burners  16  and a pressure regulator  17 . The burners  16  are so arranged that they receive gas from the pressure regulator  17  to be injected by the burners  16  into the open ends of the heat exchangers  13  for ignition. Secondary air is drawn through the heat exchangers  13  by way of a common header  18 , which is fluidly connected to an inducer  19  driven by a motor  21 . The exhaust gases are then discharged through a vent  22 . The structure is all common to most induced draft furnaces. 
     In accordance with the present invention, additional apparatus is provided to enhance the combustion process by reducing NOX emissions. An air pump  23  is mounted adjacent the inducer drive motor  21  and is drivingly connected thereto by way of a driveshaft  24 . The air pump  23  takes its suction from the ambient air adjacent the furnace and discharges to an air manifold  26  that passes along the burners  16  as shown. From the air manifold  26 , the air is then routed to the fuel/air mixture of the individual burners  16  by way of air inlet conduits or tubes  27 . This air is then applied to the fuel/air mixture of the burners  16  as a source of primary air for augmenting the combustion process for the purpose of reducing NOX in a manner to be described below. 
     Before examining the effect of the present invention, it may be well to review the combustion process in a conventional burner arrangement as shown in FIG.  2 . As fuel from a fuel injection spud  28  is introduced into the inlet  29  of a burner  16 , primary air is drawn into the inlet  29  as indicated by the arrows. This primary air/fuel mixture passes through the burner  16  and into the area downstream thereof where secondary air is introduced as indicated by the arrows. At the boundary  30  between the primary air/fuel mixture and the secondary air, combustion occurs and a primary flame  31  results. 
     Referring now to the present invention as shown in FIG. 3, a burner  16  is shown along with its air inlet tube  27  which projects radially inwardly into the primary air/fuel mixture or the flame and then turns to extend along an extension of the centerline of the burner, to remain within the flame throughout its length. From its open end  29 , primary air from the air pump  23  is discharged into the fuel/air mixture to thereby enhance the burning process and reduce the production of NOX gases. The length of the tubes  27  can be varied in order to meet specific performance requirements. In this regard, it is recognized that shorter tubes tend to provide for greater NOX reductions, but will produce greater noise levels. Thus, these parameters may be optimized by experimenting with various lengths of tubes. 
     The theory of NOX reduction by the introduction of primary air into the internal portion of the flame as described above can be explained by reference to FIGS. 2 and 3. In the conventional burner arrangement of FIG. 2, the combustion which occurs produces a relatively high temperature primary flame  31  and relatively high NOX levels. In the case of the present invention as shown in FIG. 3, where primary air is introduced into the heart, or core, of the primary flame  31  as shown on the right side of the figure, there is again a boundary  32  wherein the fuel/air and air (and in this case, it is primary air) interact to produce combustion and a secondary flame  33 . However, in this case the secondary flame  33  is internal to the primary flame  31 , thereby producing combustion byproducts within the primary flame  31 . These combustion byproducts are dispersed within the primary flame, thereby reducing of the temperature thereof and causing a reduction in NOX gases. As a side effect, since the secondary, or internal, flame causes a faster burning of the available fuel, the primary flame is shortened in length from what it would otherwise be. 
     While the present invention has been described in terms of a preferred embodiment, it will be apparent to those skilled in the art that various other embodiments and forms thereof can be employed without departing from the basic principles of the invention. For example, even though the tube is shown to have only one opening at its end, it can also have a number of openings along its length so as to thereby provide primary air at a number of locations within the fuel/air mixture. Also, while the air inlet tube is shown and described as extending along the extended axis of the burner, it need not be and could simply pass through the heat exchangers and extend radially inwardly into the flame area.