High turn down low nox burner

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.

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 NOx 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.