Patent ID: 6971336

Claim:
A fluid heating apparatus comprising: an elongated fluid containment vessel having opposed first and second end walls, said first end wall forming a first opening adapted to receive a burner and said second end wall forming a second opening, said second opening closed off by a fluid-cooled door connected to said fluid containment vessel; a first stage burner secured to said first end wall, said first stage burner having a first stage fuel inlet, a first stage oxidant inlet, a first stage fuel outlet opening and a first stage oxidant outlet opening, and a first stage combustion chamber wall enclosing a first stage combustion chamber and forming at least one first stage combustion products outlet opening, said first stage combustion chamber disposed within said fluid containment vessel; a second stage burner disposed within said fluid containment vessel, said second stage burner having a second stage fuel inlet, a second stage oxidant inlet, and a second stage combustion chamber wall enclosing a second stage combustion chamber and forming at least one second stage combustion products outlet opening; at least one first tubular heat transfer element connecting said at least one first stage combustion products outlet opening to said second stage fuel inlet and providing fluid communication therebetween; at least one flow reversal wall forming a first flow reversal plenum within said fluid containment vessel, said first flow reversal plenum in fluid communication with said second stage combustion products outlet opening; at least one combustion products exhaust wall forming a combustion products exhaust plenum disposed within said fluid containment vessel downstream of said first flow reversal plenum, said combustion products exhaust plenum in fluid communication with said first flow reversal plenum; and at least one second tubular heat transfer element providing said fluid communication between said first flow reversal plenum and said combustion products exhaust plenum.