Patent Publication Number: US-9423127-B2

Title: Radial burner air inlet with linear volumetric air control

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 13/553,485, filed Jul. 19, 2012, now allowed and which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a burner apparatus and methods of burning fuel gas in the presence of air. More specifically, the present invention relates to apparatus and methods for air introduction and air damping in such a gas burner apparatus. 
     2. Description of the Related Art 
     The majority of process burners get the air required for combustion through “natural draft.” In such natural draft burners, air flows through the burner into a process heater because of the light negative pressure inside the heater firebox. As a result, the air side pressure drop over the burner is low, typically less than about 1 in H 2 O (about 249 Pa) and often from about 0.25 to about 0.6 in H 2 O (about 62 Pa to about 149 Pa). This low pressure means there is a lack of resistance necessary to correct flow maldistributions. Burners are sensitive to flow maldistributions, which create less than optimal flame conditions, and hence, can lead to higher emissions of carbon monoxide and NO X , uneven and longer flames, detrimental impact on flue gas recirculation patterns inside the firebox, and poor excess air control during turndown of burner duty. 
     Generally, such natural draft burners have one of three types of air inlets or registers. An early design uses two concentric metal cylinders. Each cylinder has slots. One cylinder is stationary while the other can be rotated. By rotating the cylinder, all or a portion of the slots on one cylinder can be aligned with the slots on the other cylinder, thus allowing more or less air to flow through the slots. A subsequent design uses only a single stationary cylinder with slots. In this design, each slot is fitted with a damper blade on a shaft. By adjusting the angle of the damper blade, more or less air is allowed through the slots. A third type of air register utilizes an air inlet box located on the side of the burner plenum. The air inlet box has a single or multi-blade damper. The angle of the damper blades determines the amount of air flow into the plenum. 
     These conventional registers have difficulty with flow maldistributions. The conventional cylinder designs are subject to unwanted air turbulence and spiral air flow. The air inlet box design relies on side entry that causes uneven distribution of air across the plenum and, hence, the burner. Additionally, damper blades as air flow regulators do not regulate air in a linear fashion and, thus, are difficult to adjust to achieve the desired air flow. Accordingly, because burners are sensitive to flow maldistributions, as discussed above, there are often problems with such burner systems such as higher than optimal carbon monoxide and NO x  emissions. Further, it is difficult to achieve tight shutoff with damper blades and concentric cylinders even when they are fully closed. It is desirable to have a register that overcomes these difficulties. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention there is provided a burner system comprising a plenum, a burner tile, a primary fuel nozzle, a front plate, and a damper plate. The plenum has a perimeter wall with a first end having an air inlet disposed therein and a second end having an air outlet disposed therein. The first end opposes the second end. The burner tile is attached to the second end at the air outlet. The burner tile has an opening formed therein for allowing air to flow therethrough. The primary fuel nozzle is connectable to a fuel source and positioned in relation to the burner tile such that air and fuel are burned within and adjacent to the top of the burner tile. The front plate is mounted a fixed distance below the air inlet of the plenum. The damper plate is mounted between the first end and the front plate to thus form a peripheral flow path for the introduction of air into the inlet of the first end, wherein the front plate, damper plate, air inlet, air outlet and the opening of the burner tile are axially aligned along a longitudinal axis and the damper plate is moveable along the longitudinal axis such that the size of the flow path is changed. 
     In a further embodiment there is provided an air damper for controlling air flow to a burner plenum. The burner plenum has a housing defining a first opening and a second opening opposing the first opening wherein air is introduced into the burner plenum through the first opening and the second opening supplies air to a burner. The air damper comprises a damper plate mounted adjacent the first opening so as to define a flow path between the damper plate and the housing so that air enters the first opening through the flow path and is introduced into the burner plenum. The burner, first opening, second opening and damper plate are axially aligned along a longitudinal axis and the damper plate is moveable relative to the first opening along the longitudinal axis so as to change the size of the flow path. 
     In another embodiment of the invention there is provided a damper system comprising a top ring, a front plate, a damper plate and an actuator. The top ring defines an aperture therein and has a flat bottom surface. The front plate is mounted a fixed distance from below the flat bottom surface of the top ring. The damper plate comprises a flat disk having an upper surface. The damper plate is mounted between the top ring and the front plate so as to define a flow path between the damper plate and the top ring so that air enters the aperture of the top ring peripherally through the flow path. The damper plate is moveable relative to the top ring so as to change the size of the flow path and so as to have an uppermost position where the upper surface is in contact with the flat bottom surface of the top ring. The actuator is operationally connected to the damper plate to provide linear volumetric air flow control. 
     In yet another embodiment there is provided a burner system comprising a primary plenum having an air inlet. Located within the primary plenum are at least two secondary plenums having a perimeter wall having a first end with an air inlet disposed therein and a second end with an air outlet disposed therein. The first end opposes the second end. Associated with each secondary plenum is a burner tile attached to the second end at the air outlet. The burner tile has an opening formed therein for allowing air to flow therethrough. Also associated with each plenum is a primary fuel nozzle connectable to a fuel source and positioned in relation to the burner tile such that air and fuel are burned within and adjacent to the top of the burner tile. At the first end of the secondary plenum is a front plate mounted outside the secondary plenum and a fixed distance from the air inlet of the plenum. The front plate is located inside the primary plenum. A damper plate is mounted between the first end of the secondary plenum and the front plate to thus form a peripheral flow path for the introduction of air into the inlet of the first end. The front plate, damper plate, air inlet, air outlet and the opening of the burner tile are axially aligned along a longitudinal axis and the damper plate is moveable along the longitudinal axis such that the size of the flow path is changed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a damper system in accordance with one embodiment of the invention, shown without a cam. 
         FIG. 2  is a side elevation view of the damper of  FIG. 1 , shown with a cam. 
         FIG. 3  is a front elevation view of the damper of  FIG. 1 , shown with a cam. 
         FIG. 4  is a section view of a burner utilizing a damper system in accordance with one embodiment of the invention. 
         FIG. 5  is a graphical illustration of burner duty versus damper setting for a traditional opposed blade damper system and for a linear volumetric air control damper system. 
         FIG. 6  is a schematic illustration of a forced air multi-burner system utilizing an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIGS. 1-4 , an air damper system according to the invention is illustrated and generally designated by the numeral  10 . In  FIG. 1 , a perspective of the damper system  10  is shown. The view of  FIG. 1  is not shown with a cam. Side and front elevation views of the damper system  10 , with a cam, are illustrated in  FIGS. 2 and 3 .  FIG. 4  illustrates the damper plate attached to a burner system  100 . 
     Air damper system  10  comprises a damper plate  12  and fixed plate  14 . Fixed plate  14  is sometimes called a front plate and for vertical burner arrangements it can be located below damper plate  12 . Fixed plate  14  is mounted a fixed distance from the bottom of a burner system by mounts  16 , see  FIG. 4 . Mounts  16  can be a spacer-type bolt or any similar mounting system that will not significantly impede air flow. Fixed plate  14  can be a flat disk as shown and can optionally have a ring  13  mounted thereon (as shown in  FIGS. 1 and 3 ). Fixed plate  14  is designed to attach to the bottom of a plenum and to support the damper plate  12  and actuator  22 . Fixed plate  14  can have a shape and thickness suitable for those functions. 
     Damper plate  12  is mounted between fixed plate  14  and the bottom of a plenum. Generally, it will be mounted between fixed plate  14  and a top ring or fixed ring (shown as flange  119  in  FIG. 4 ), which is generally a ring having a flat bottom surface  102  and defining an aperture therein but can be designed as a plate having an aperture therein. Damper plate  12  can be a flat disk but may have other shapes if desired. Generally, damper plate  12  has a flat upper surface  11  that can contact fixed ring  119  so as to substantially stop air flow through flow path  44 . Damper plate  12  is operationally connected to fixed plate  14  so that it is moveable relative to fixed plate  14  and the open end  116  of burner system  100 . Generally the burner system will have a longitudinal axis  104 , as illustrated in  FIG. 4 , and it is desirable that damper plate  12  move along longitudinal axis  104 . Thus, for a vertical burner system, damper plate  12  will move generally upwards or downwards relative to fixed plate  14  and bottom  102  or open end  116 . 
     Mounted on fixed plate  14  are guide tubes  18  and  20 . Guide tubes  18  and  20  aid in guiding damper plate  12  when it is moving and help to keep it level with respect to bottom  102 . Additionally, guide tubes  18  and  20  can serve as access ports into the burner system  100 . For example, guide tube  18  can serve as an entrance port for the pilot assembly  106  and guide tube  20  can serve as a sight access port. 
     Mounted to the bottom surface  15  of fixed plate  14  is actuator  22 . Actuator  22  is operationally connected to damper plate  12  to move it along longitudinal axis  104 . Actuator  22  can be any suitable such actuating device. As shown, actuator  22  comprises a slide plate  24 , slide shaft  28 , cam  34  and ratcheted handle  38 . Slide plate  24  is connected to fixed plate  14  and extends generally outward from the bottom surface  15  of fixed plate  14 . Slide plate  24  has a channel  26  in which lies slide shaft  28 . Slide shaft  28  can move longitudinally in channel  26  and is held in place by bolt  30 , which extends through longitudinal aperture  32  located in channel  26 . Bolt  30  is in sliding relation with aperture  32  so that bolt  30  can slide longitudinally within aperture  32 . Additionally, bolt  30  connects to first end  29  of slide shaft  28 . Slide shaft  28  extends slideably through fixed plate  14 , and second end  27  of slide shaft  28  is connected to damper plate  12  so that longitudinal movement of slide shaft  28  results in longitudinal movement of damper plate  12 . Bolt  30  is also slidingly connected to cam  34  and rests in slot  35  of cam  34  such that bolt  30  follows the lower edge  36  of slot  35 . Bolt  30  may have a means to facilitate movement in aperture  32  and/or slot  35 , such as a grooved wheel. Cam  34  comprises a radial arm  33  and an arced cam arm  31  having slot  35  and lower edge or cam edge  36 . Radial arm  33  of cam  34  is pivotally connected at axis  37  of ratcheted handle  38 . Ratcheted handle  38  comprises a ratcheted wheel  40  and grip handle  42 . Ratcheted wheel  40  has teeth  41  that engage grip handle  42  such that grip handle  42  can be locked in discrete positions about ratcheted wheel  40 . Additionally, ratcheted handle  38  and radial arm  33  of cam  34  are operatively connected such that turning ratcheted handle  38  pivots cam  34  about axis  37  and, when grip handle  42  is locked in a position, cam  34  is locked in place. Cam  34  can be any suitable cam but, as illustrated, is a non-linear cam, or a rotational cam, and is operatively connected to bolt  30  and slide shaft  28  so that rotational movement of cam  34  results in linear movement or straight-line movement of slide shaft  28  and, hence, damper plate  12 . 
     As described above, “linear movement” or “straight-line movement” refers to the general direction of the movement of the damper plate and not the proportional movement of the cam and damper plate. While in one embodiment the damper plate and the cam can have a linearly proportional movement, in a preferred embodiment the damper plate and cam movements are not linearly proportional. Rather, the cam is designed to give linear volumetric air flow control of air passing through flow path  44 . By “linear volumetric air flow control” it is meant that movement of the cam is sequenced with the movement of the damper plate to result in a linear change of the volume of air per unit time flowing through flow path  44 . Accordingly, by way of nonlimiting example, moving the cam to a 50% of maximum air flow position will result in moving the damper plate to allow 50% of the maximum volume of air per unit time that can flow through flow path  44  even though the resulting movement of the damper plate may not be 50% of the longitudinal size of the fully open position, i.e. it may be 25% of the longitudinal size of the fully open position. As a further nonlimiting example, in natural draft burners, the pressure loss across flow path  44  will be directly proportional to the square of the burner duty; thus, when the burner duty drops by 50%, the pressure loss across flow path  44  will reduce by a factor of four. In such situations, the burner throat pressure loss will also be reduced as a result of drops in burner duty. To compensate for this, the longitudinal damper opening will need to be reduced to increase the pressure loss. The pressure loss across flow path  44  is approximately proportional to the inverse square of the longitudinal damper opening. Accordingly, the cam can be designed to give linear volumetric air flow control such that movement of the cam takes into account this inverse proportionality and pressure loss reduction based on burner duty; i.e., when there is a 50% drop in burner duty in a burner having a six inch maximum flow path (based on longitudinal size), the cam is designed such that, when the cam is moved to a fifty percent indicator the damper plate will be moved to reduce the flow path to one inch to increase the pressure loss by a factor that will maintain a stable pressure loss for the flow path and burner throat. To avoid confusion between “linear volumetric air control” and the movement of the cam and/or damper plate, hereinafter the word “linear” will be reserved to describe volumetric air control and movement of the cam, slide shaft, damper plate and such will be referred to as “rotational movement” or “straight-line movement” and similar. 
     Focusing now on  FIG. 4 , a burner system  100  using the above described air damper  10  can be seen. Burner system  100  is a natural draft burner system, thus air flows through the burner because of the light negative pressure created inside the burner system by the heat generated therein. Burner system  100  is sealingly attached to the bottom wall  108  of a furnace space over an opening therein. While gas burner apparatus are commonly mounted vertically and fired upwardly as shown in  FIG. 4 , it is to be understood that the burner apparatus can also be mounted horizontally and fired horizontally or vertically and fired downwardly and that terms used herein, such as up, upper, down or lower, are used for convenience in indicating direction. 
     The burner system  100  is comprised of a housing or plenum  110  having perimeter wall  112 . Plenum  110  has an open end  114  and an open end  116  defining an air outlet and air inlet respectively. The plenum  110  is attached to the furnace wall  108  by means of a flange  118  and a plurality of bolts  120 , which extend through complementary openings in the flange  118  and the wall  108 . The furnace wall  108  includes an internal layer of insulating material  122  attached thereto, and the open end  114  of the plenum  110  includes a burner tile  124  formed of flame and heat resistant refractory material attached thereto. As illustrated in  FIG. 4 , the interior surface of the insulating material  122  attached to the furnace wall  108  and the top of the base portion  126  of the burner tile  124  define a furnace space within which the fuel gas and air discharged by the burner system  100  are burned. The burner tile  124  has a central opening  128  formed in the base portion  126  thereof through which air introduced into the plenum  110  by way of the air damper  10  is discharged. The burner tile  124  also includes a wall portion  130  having a recessed interior surface  132  which surrounds the opening  128 , forms a circular ledge  134  and extends into the furnace space. The burner tile  124 , the interior surface  132  of the wall portion  130  and the central opening  128  in the base portion  126  of the burner tile  124  as well as the plenum  110  can take various shapes, e.g., circular, rectangular, square, triangular, polygonal or other shape. However, the burner system  100  preferably includes a circular burner tile  124  having a circular opening  128  therein and a circular wall portion  130 . Also, the plenum  110  preferably includes circular openings  114  and  116  therein and the plenum is preferably cylindrical. However, the plenum can also include square openings  114  and  116  therein and can have square or rectangular perimeter wall  112 . In a preferred embodiment as shown in  FIG. 4 , the circular opening  128  in the circular burner tile  124  is smaller than the interior surface  132  of the wall  130  thereof so that the circular ledge  134  is provided within the burner tile  124  which functions as a flame stabilizing surface. 
     A central primary fuel gas nozzle  136  can be positioned within the opening  128  near the bottom of the burner tile  124 . Nozzle  136  is connected by a conduit  178  to a fuel gas manifold  140 . A conduit  142  connects manifold  140  to a source of pressurized fuel gas. Also, the burner system  100  can optionally include a plurality of nozzles  136  in lieu of the single nozzle  136 . Nozzle  136  is lit by pilot tip  107  of pilot assembly  106 . Pilot assembly  106  extends longitudinally through plenum  110 , although other arrangements will be readily apparent to those skilled in the art. Additionally, a plurality of secondary fuel gas discharge nozzles  144  can be positioned in spaced relationship around the burner tile  124 . The nozzles  144  are connected to fuel gas conduits which are connected to the fuel gas manifold  140 . The burner tile  124 , primary fuel gas nozzle  136  and/or secondary fuel gas discharge nozzles  144  make up the burner. While a specific burner arrangement has been described, it will be understood that the invention is not limited to the specific burner arrangement but can be utilized with other burner tiles and nozzle designs. 
     Air flow rate regulating register or damper system  10  is connected to the plenum  110  at its open end  116  for regulating the flow rate of combustion air entering the plenum  110 . The damper system  10  is attached to the open end  116  by means of a fixed ring or flange  119  and a plurality of mounts  16  which extend through complementary openings in the flange  119 , damper plate  12  and the fixed plate  14 . Mounts  16  are in fixed engagement with flange  119  and fixed plate  14  and are in sliding engagement with damper plate  12  to allow movement of damper plate  12  along longitudinal axis  104 . Optionally, damper plate  12  can have a smaller radius such that its outer edge is within the radius of the mounts. Flow path  44  is defined by damper plate  12  and the bottom  102 , which in the illustrated embodiment is also the bottom of flange  119 . Flow path  44  is a peripheral flow path. Accordingly, flow path  44  allows air to radially move through it into the plenum  110  from around the periphery of the bottom  102  of the plenum  110 . Preferably the air will enter the flow path from substantially the entire perimeter around the bottom of the plenum; that is, from 360° around the perimeter. As indicated above, the plenum  110  can be any of various shapes. In one embodiment, damper plate  12  can be a matching shape to the cross-sectional shape of the plenum  110 . Thus, in the case of a cylindrical plenum, the damper plate  12  can be a circular disk and the flow path  44  will be a circumferential flow path. In one embodiment, the shapes of the plenum and damper plate are matching so as to ensure that there is even flow of air into the flow path  44  from the entire perimeter and to ensure, when the damper plate is in its uppermost position, it is in contact with and is closed against flange  119  such that the air flow is substantially stopped. In this regard, as described above, flange  119  can have a flat bottom surface  102  and damper plate  12  can have a flat upper surface. 
     Optionally, damper plate  12  can have a perforated can or screen  46  mounted around its perimeter to shield the flow path from debris and birds and to serve as a noise muffler and to protect personnel. The screen  46  can be mounted on the damper plate, the front plate  14  or the flange  119 . In the case of mounting on the front plate or flange, the damper plate should fit slideably within the screen. As illustrated in  FIG. 4 , screen  46  extends upward from the top surface of damper plate  12  into plenum  110 . Screen  46  is slideably mounted in plenum  110  so as to not impede damper plate  12  from longitudinal movement. The perforations of screen  46  should be sufficient to allow adequate air flow into plenum  110  and to block debris from entering the plenum. However, screen  46  can also serve to change the amount of air flowing into air path  44 . Thus, by varying the porosity of screen  46  for a given burner system, the linear volumetric air flow can be controlled. Additionally, by varying the porosity of the screen between different burner systems designs, different burner capacities can be achieved without changing the overall dimensions of the burner. 
     In operation of the burner system  100 , fuel gas is introduced into the furnace space to which the burner system  100  is attached and burned therein at a flow rate which results in the desired heat release. Air is also introduced into the plenum  110  through damper system  10  and a column of the air flows into the furnace space. The flow rate of air introduced into the furnace space is in the range of from about 0% to about 100% in excess of the flow rate of air required to form a stoichiometric mixture of air and fuel gas. Preferably, the flow rate of air is in excess of the stoichiometric flow rate of air by about 15%. Stated another way, the mixture of fuel gas and air discharged into the furnace space contains from about 0% to about 100% of excess air. 
     As shown in  FIG. 4 , a peripheral flow path  44  is provided for the introduction of air into the open end  116  of the plenum  110 . Air flows into flow path  44  from around the periphery of the damper system  10  and the bottom  102  of the plenum  110 . The flow path allows for air introduction substantially 360° around the perimeter of the open end  116 . The size of the flow path  44  can be adjusted by moving the damper plate  12  along the longitudinal axis  104 . The damper plate  12  can be adjusted by moving a rotating cam  34  and translating the rotational movement of the cam into straight-line movement of the damper plate  12 . Upward movement of damper plate  12 , or movement towards bottom  102 , decreases the flow path size and downward movement of damper plate  12 , or movement away from bottom  102 , increases the flow path size. At its uppermost position, damper plate  12  is in contact with bottom  102  so that the flow path is closed and substantially no air passes into the plenum. Additionally, the actuator and its cam  34  are operationally connected to the damper plate to provide linear volumetric air flow control of the air flowing through flow path  44 . 
     Air traveling through the flow path  44  into the open end  116  of the plenum  110  forms a unified column of air. By “unified column of air” it is meant that there is substantially even distribution laterally across the plenum of longitudinally moving air, or as illustrated in  FIG. 4 , upward moving air. The unified column of air flows through the plenum  110  and through the opening  128  in the burner tile  124  into the mixing zone formed within the interior and above the wall  130 . While within the mixing zone, the air mixes with the fuel gas and flue gases. The resulting primary fuel gas-flue gases-air mixture containing a large excess of air is burned within and adjacent to the top of the burner tile  124  and the flue gases formed therefrom have very low NO X  content due to the dilution of the fuel gas by the excess air and flue gases. In one embodiment, the burner (burner tile  124 , primary fuel gas nozzle  136  and/or secondary fuel gas distribution nozzles  144 ), plenum  110 , and damper plate  12  are all axially aligned along longitudinal axis  104 . This embodiment, as illustrated in  FIG. 4 , ensures that air flowing in through air flow path  44  creates a unified column of air in plenum  110  and that the unified column of air reaches the burner (more specifically central opening  128  and primary fuel gas nozzle  136 ) with a minimum of maldistribution in the air flow. Further in this embodiment, flow path  44  can be the only air introduction path for the plenum, can be the only air introduction path for the burner and can be the only air introduction path for the burner system. Alternatively, flow path  44  can provide the primary air introduction and, hence, provide the major portion of air to the burner. A minor portion of the air introduced into the burner system can be introduced around the fuel gas nozzles. Accordingly, the invention can also be useful in staged air burners with the primary air being introduced through flow path  44  and the secondary air being introduced by nozzles around the fuel gas nozzles or in the furnace above the burner tile. 
     Accordingly, the inventive damper system allows for a unified column of air to enter the plenum and for linear volumetric control of air flowing through the flow path. In regard to linear volumetric control of the air flow,  FIG. 5  is a graphical representation of burner duty versus damper setting for an opposed blade damper system and for a linear volumetric air control damper system. As can be seen by reference to  FIG. 5 , bladed damper systems, such as opposed blade dampers, do not provide for a linear air flow adjustment. Because of this, it is difficult to control the air flow into the plenum and difficult to adjust the air flow for changing conditions or changing burner operating parameters in a bladed damper system. The present air damper provides for more even air introduction into the plenum and more proportional control of the air flow in relation to the burner duty and, hence, more consistent air control. 
     While the invention is advantageous in natural draft burners because of its even air introduction and, hence, low susceptibility to flow maldistributions, the invention can also be used advantageously in forced draft burners. The invention can be particularly useful in an embodiment where forced draft systems utilize multiple burners sharing a common plenum.  FIG. 6  schematically illustrates two burners having primary nozzles  150 A and  150 B, and burner tiles  152 A and  152 B. Primary housing or primary plenum  156  surrounds the lower portion of the burner system including secondary plenums  158 A and  158 B and air damper systems  160 A and  160 B. Air damper systems  160 A and  160 B have front plates  162 A and  162 B, damper plates  164 A and  164 B, flanges  166 A and  166 B, and screens  170 A and  170 B. Also, the damper systems  160 A and  160 B can have an actuator (not shown) to adjust the height of the damper plates  164 A and  164 B. The actuator can be in accordance with the description above for  FIGS. 1-4 . Additionally, each damper system can have a separate control of the actuator by a ratcheted handle or automated control or can have a common control such as utilizing a jack shaft. 
     In operation, forced air or pressurized air is introduced into plenum  156  at air inlet  172 . The forced air flows throughout plenum  156  and enters the burner systems through damper systems  160 A and  160 B by the flow paths created at screens  170 A and  170 B. The general flow of the forced air is shown by the arrows in  FIG. 6 . It will be realized by one skilled in the art by examination of  FIG. 6 , that the forced air will be subject to flow maldistribution across plenum  156  and can be susceptible to greater volumes of air being available to those burners closer to the air inlet  172 . Damper plates  166 A and  166 B can be adjusted to different heights so that each burner system receives sufficient air to eliminate flow maldistribution resulting in efficient burning of fuel with low NO X . Accordingly, damper systems  160 A and  160 B correct flow maldistributions caused by side entry of the forced air. 
     Other embodiments of the current invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Thus, the foregoing specification is considered merely exemplary of the current invention with the true scope thereof being defined by the following claims.