assembly generally designated 44 the windbox assembly 44 includes a heat exchanger inlet plenum 45 including top walls 46 bottom wall 48 and a side wall 50 a circulating fluid inlet 52 connects into the wall 50 of the inlet plenum 45 to provide for flow of a fluid which will provide for cooling of heat exchanger tubes generally designated 54 which are also referred to as fluidized bed regulating or cooling tubes the circulating fluid is admitted in the direction of arrow 56 shown in fig3 and it flows around tubes 58 which extend through the windbox and which provide for passage of combustion air therethrough from an air chamber or lower section chamber 60 fed by a fluidizing or combustion air inlet pipe 62 the air passes through the tubes 58 into the air chamber or windbox 42 the combustion air flows in the direction of the arrow 64 into the air chamber 60 and from the air chamber in the direction of the arrow 66 through the tubes 58 and into the chamber 42 the coal which has been reduced to a small size is delivered through a coal gun 75 into the chamber 26 and the air is delivered through the inlet 40a of the tuyeres 40 for discharge into the fluidized bed above the distributor plate 28 in accordance with a feature of the invention the heat exchanger tubes 54 comprise an outer tube 54a which extends through the distributor plate 28 and is.