Reduced size fire tube boiler system and method of operating same

A boiler system comprising:

• a housing having a generally cylindrical shape and extending a length between first and second walls of the housing to provide a generally cylindrical space,
• a head portion that includes a front housing and includes an air inlet and an air passage that provides air for an air-fuel mixture, wherein the air is drawn by a combustion air fan through the air inlet and is directed through the air passage;
• a fire tube positioned substantially within the generally cylindrical space, the fire tube having a shell structure extending longitudinally from a fire tube front wall at the first wall of the housing a portion of the length between the first and second walls of the housing to a fire tube end wall, wherein the fire tube has a length that is less than half of the length of the housing of the boiler system, the fire tube providing a combustion chamber bounded by the shell structure, which is the only combustion chamber, wherein the combustion chamber comprises an interior space extending over a length that is at least substantially the entire extent of the length of the fire tube such that complete combustion of an air-fuel mixture is accomplished within the combustion chamber;
• a burner, wherein the burner is mounted to the fire tube front wall, the burner having a burner head that extends from the fire tube front wall into the combustion chamber so as to discharge the air-fuel mixture into the combustion chamber, wherein there is space between the burner head and the combustion chamber in the radial direction of the burner head, wherein the fuel is a gas, and wherein the air-fuel mixture is ignited to produce a flame in the combustion chamber;
• a first set of tubes located within the housing, wherein the firetube end wall extends and opens to the first set of tubes, the first set of tubes extending longitudinally from and parallel with the fire tube and extending longitudinally from the fire tube end wall a remaining portion of the length between the first and second walls of the housing to and through the second wall of the housing;
• a second set of tubes located above and in parallel with a portion of the fire tube and a portion of the first set of tubes, the second set of tubes generally spanning the length extending between the first and the second walls of the housing;
• and
• a chamber providing a space between and connecting the first and second sets of tubes;
• wherein a flow of heated combustion gases is sequential such that the heated combustion gases flow from the fire tube directly to and through the first set of tubes, and then directly to and through the chamber space, and then directly to and through the second set of tubes, before the heated combustion gases are discharged from the boiler;
• and
• wherein the heated combustion gases that are discharged from the boiler comprise emissions that are characterized as low NOx emissions having 9 ppmv or less NOx formation.