High turn down low nox burner

Abstract

An air-fuel burner includes a heat-transfer tube, an air-fuel mixing chamber, and an air-fuel nozzle. The air-fuel nozzle is coupled to the air-fuel chamber to communicate a combustible air-fuel mixture into a combustion chamber defined between the air-fuel nozzle and the heat-transfer tube. The combustible air-fuel mixture, when ignited, establishes a flame in the combustion chamber to produce heat which is transferred through heat-transfer tube to an adjacent medium external to the heat-transfer tube.

1 . An air-fuel burner comprising a heat-transfer tube formed to include an interior region and adapted to discharge heat to an adjacent medium located outside the heat-transfer tube when exposed to heat from a flame generated in the interior region, an air-fuel mixing chamber adapted to mix air from an air supply and fuel from a fuel supply to establish a combustible air-fuel mixture therein, and an air-fuel nozzle coupled to the air-fuel mixing chamber and arranged to extend into the interior region of the heat-transfer tube, the air-fuel nozzle being configured to provide means for forming three nozzle exits communicating with a combustion chamber defined in the interior region and located between the air-fuel nozzle and the heat-transfer tube to cause the combustible air-fuel mixture to exit from the air-fuel nozzle into the combustion chamber through a first nozzle exit formed in the air-fuel nozzle to establish, when a portion of the combustible air-fuel mixture flowing through the first nozzle exit is ignited, a detached first flame extending in radially outward directions in the combustion chamber from the air-fuel nozzle toward the heat-transfer tube, and the detached first flame includes a root positioned to lie between the air-fuel nozzle and the heat-transfer tube and a tip arranged to stabilize on an interior surface of the heat-transfer tube, a second nozzle exit formed in the air-fuel nozzle and arranged to lie in spaced-apart relation to the first nozzle exit in a downstream direction away from the air-fuel mixing chamber to establish, when a portion of the combustible air-fuel mixture flowing through the second nozzle exit is ignited, a second flame extending in radially outward directions in the combustion chamber from the air-fuel nozzle toward the interior surface of the heat-transfer tube, and the second flame includes a root positioned to lie between the air-fuel nozzle and the heat-transfer tube and a tip arranged to stabilize on the interior surface of the heat-transfer tube when the firing rate of the combustible air fuel mixture is high, wherein the second flame is attached to the air-fuel nozzle when the firing rate of the combustible air fuel mixture is low and detaches from the air fuel nozzle when the firing rate of the combustible air fuel mixture is increased to exceed a threshold firing rate, and a third nozzle exit formed in the air-fuel nozzle and arranged to lie in spaced-apart relation to the second nozzle exit in the downstream direction to locate the second nozzle exit between the first and third nozzle exits and to establish, when a portion of the combustible air-fuel mixture flowing through the third nozzle exit is ignited, an attached third flame extending in the downstream direction away from the air-fuel nozzle and the detached first and second flames, and the attached third flame includes a root stabilized on the air-fuel nozzle and a tip extending in the downstream direction. 
     
     
         2 . The air-fuel burner of  claim 1 , wherein the air-fuel nozzle includes an air-fuel transfer conduit and an air-fuel discharge plate, the air-fuel transfer conduit has an upstream end and a downstream end arranged to lie in spaced-apart relation opposite the upstream end and the air-fuel transfer conduit is coupled to the air-fuel mixing chamber at the upstream end and to the air-fuel discharge plate at the downstream end. 
     
     
         3 . The air-fuel burner of  claim 2 , wherein the first nozzle exit is defined by a series of air-fuel discharge slots formed in the air-fuel transfer conduit and arranged to lie in circumferentially spaced-apart relation to one another around a circumference of the air-fuel transfer conduit. 
     
     
         4 . The air-fuel burner of  claim 3 , wherein the second nozzle exit is defined by a band of perforations formed in the air-fuel transfer conduit and arranged to lie circumferentially around the circumference of the air-fuel transfer conduit. 
     
     
         5 . The air-fuel burner of  claim 4 , wherein the third nozzle exit is defined by a series of staged air-fuel discharge apertures formed in the air-fuel discharge plate and arranged to extend in a pattern between a center of the air-fuel discharge plate and a perimeter edge of the air-fuel discharge plate to cause the attached third flame, when ignited, to extend between the center and the perimeter edge to maintain ignition of the detached second flame. 
     
     
         6 . The air-fuel burner of  claim 1 , wherein the first nozzle exit is configured to provide means for communicating about 10% to about 20% of the combustible air-fuel mixture, the second nozzle exit is configured to provide means for communicating about 40% to about 80% of the combustible air-fuel mixture, and the third nozzle exit is configured to provide means for communicating about 10% to about 20% of the combustible air-fuel mixture by volume through the air-fuel nozzle. 
     
     
         7 . The air-fuel burner of  claim 1 , wherein a distance d 1  between the first nozzle exit and the second nozzle exit is between about 1.8 and about 4 times a diameter d 2  of the air-fuel nozzle. 
     
     
         8 . The air-fuel burner of  claim 1 , wherein the root of the detached first flame is positioned to lie in spaced-apart relation to the air-fuel nozzle a first distance D 1  and the root of the detached second flame is positioned to lie in spaced-apart relation to the air-fuel nozzle a relatively smaller second distance D 2 . 
     
     
         9 . The air-fuel burner of  claim 1 , wherein the air-fuel nozzle includes an air-fuel transfer conduit and an air-fuel discharge plate, the air-fuel transfer conduit has an upstream end and a downstream end arranged to lie in spaced-apart relation opposite to the upstream end and the air-fuel transfer conduit is coupled to the air-fuel mixing chamber at the upstream end and to the air-fuel discharge plate at the downstream end, and wherein the first nozzle exit is defined by a series of air-fuel discharge slots formed in the air-fuel transfer conduit and arranged to lie in circumferentially spaced-apart relation to one another around a circumference of the air-fuel transfer conduit, the second nozzle exit is defined by a band of perforations positioned circumferentially around the downstream end of the air-fuel transfer conduit. 
     
     
         10 . The air-fuel burner of  claim 9 , wherein the series of air-fuel discharge slots is defined by a first discharge slot, a second discharge slot, a third discharge slot, a fourth discharge slot, a fifth discharge slot, and a sixth discharge slot and each discharge slot is positioned to lie in spaced-apart relation equally to one another around the circumference of the air-fuel transfer conduit from one another. 
     
     
         11 . The air-fuel burner of  claim 4 , wherein the second nozzle exit provides a means to improve operating range of the burner between a low firing rate and a high firing rate, wherein at low firing rate, the second flame exiting from the second nozzle exit stabilizes and attaches to a surface of the air-fuel nozzle into which the second nozzle exit is formed. 
     
     
         12 . An air-fuel burner comprising a heat-transfer tube formed to include an interior region, an air-fuel mixing chamber configured to establish a combustible air-fuel mixture therein, and an air-fuel nozzle coupled to the air-fuel mixing chamber and arranged to extend into the interior region of the heat-transfer tube, the air-fuel nozzle formed to include three nozzle exits communicating with a combustion chamber defined in the interior region between the air-fuel nozzle and the heat-transfer tube to move the combustible air-fuel mixture from the air-fuel nozzle into the combustion chamber through a first nozzle exit formed in the air-fuel nozzle to establish, when a portion of the combustible air-fuel mixture flowing through the first nozzle exit is ignited, a detached first flame extending in radially outward directions in the combustion chamber from the air-fuel nozzle toward the heat-transfer tube, and the detached first flame includes a root positioned to lie between the air-fuel nozzle and the heat-transfer tube and a tip arranged to stabilize on an interior surface of the heat-transfer tube, a second nozzle exit formed in the air-fuel nozzle and arranged to lie in spaced-apart relation to the first nozzle exit in a downstream direction away from the air-fuel mixing chamber to establish, when a portion of the combustible air-fuel mixture flowing the through the second nozzle exit is ignited, a second flame attached to and extending in radially outward directions from the air fuel nozzle into the combustion chamber toward the interior surface of the heat-transfer tube, wherein the attached second flame is stabilized on the air fuel nozzle within the heat-transfer tube, and a third nozzle exit formed in the air-fuel nozzle and arranged to lie in spaced-apart relation to the second nozzle exit in the downstream direction to locate the second nozzle exit between the first and third nozzle exits and to establish, when a portion of the combustible air-fuel mixture flowing through the third nozzle exit is ignited, a attached third flame extending in the downstream direction away from the air-fuel nozzle and the detached first and second flames, and the attached third flame includes a root stabilized on the air-fuel nozzle and a tip extending in the downstream direction. 
     
     
         13 . The air-fuel burner of  claim 12 , wherein the second flame is attached to the air-fuel nozzle when the firing rate of the combustible air fuel mixture is low. 
     
     
         14 . The air-fuel burner of  claim 13 , wherein the second flame detaches from the air fuel nozzle when the firing rate of the combustible air fuel mixture is increased to exceed a threshold firing rate. 
     
     
         15 . The air-fuel burner of  claim 14 , wherein the air-fuel nozzle includes an air-fuel transfer conduit and an air-fuel discharge plate, the air-fuel transfer conduit has an upstream end and a downstream end arranged to lie in spaced-apart relation opposite the upstream end and the air-fuel transfer conduit is coupled to the air-fuel mixing chamber at the upstream end and to the air-fuel discharge plate at the downstream end. 
     
     
         16 . An air-fuel burner comprising an elongated air-fuel nozzle adapted to receive a combustible air-fuel mixture and configured to provide means for forming three nozzle exits to cause three separate flames to be established in the combustion chamber when the combustible air-fuel mixture is ignited and wherein the three nozzle exits are defined by a first nozzle exit formed in the elongated air-fuel nozzle and positioned to lie in spaced-apart relation to an inner end of the elongated air-fuel nozzle, a relatively larger second nozzle exit formed in the elongated air-fuel nozzle and positioned to lie in spaced-apart relation to first nozzle exit near an opposite outer end of the elongated air-fuel nozzle, and a relatively smaller third nozzle exit formed in the elongated air-fuel nozzle and positioned to lie at the opposite outer end of the elongated air-fuel nozzle to locate the relatively larger second nozzle exit between first nozzle exit and the relatively smaller third nozzle exit and the first, second, and third nozzle exits are arranged in the elongated air-fuel nozzle to cooperate to provide means for minimizing NO x  formation associated with the three flames during combustion while maximizing operating efficiency of the air-fuel burner and to provide a means for improved burner turn-down, whereby the operating range of the burner between a low firing rate and a high firing rate is improved through the configuration of the second nozzle exit that facilitates attachment to and stabilization there upon of a second flame exiting from the second nozzle exit to a surface of the air-fuel nozzle into which the second nozzle exit is formed. 
     
     
         17 . The air-fuel burner of  claim 16 , wherein the elongated air-fuel nozzle includes an air-fuel transfer conduit and an air-fuel discharge plate, the air-fuel transfer conduit has an upstream end and a downstream end arranged to lie in spaced-apart relation opposite the upstream end and the air-fuel transfer conduit is coupled to an air-fuel mixing chamber at the upstream end and to the air-fuel discharge plate at the downstream end. 
     
     
         18 . The air-fuel burner of  claim 17 , wherein the first nozzle exit is defined by a series of air-fuel discharge slots formed in the air-fuel transfer conduit and arranged to lie in circumferentially spaced-apart relation to one another around a circumference of the air-fuel transfer conduit. 
     
     
         19 . The air-fuel burner of  claim 17 , wherein the second nozzle exit is defined by a band of perforations formed in the air-fuel transfer conduit and arranged to lie circumferentially around the circumference of the air-fuel transfer conduit. 
     
     
         20 . The air-fuel burner of  claim 17 , wherein the third nozzle exit is defined by a series of staged air-fuel discharge apertures formed in the air-fuel discharge plate and arranged to extend in a pattern between a center of the air-fuel discharge plate and a perimeter edge of the air-fuel discharge plate to cause the attached third flame, when ignited, to extend between the center and the perimeter edge to maintain ignition of the detached second flame.