Abstract: A fluidized bed combustion, unitized transportable fire tube boiler package supported by its framework and housed within an insulated housing that ducts incoming air to preheat it while serving as a noise abutment shield. The water and steam retaining pressure vessel has an upright lower portion tapered divergently upwardly and enclosing a like-configurated, fluidized bed combustion housing spaced therefrom to form a water jacket therebetween. The pressure vessel also has a horizontally elongated upper portion enclosing a plurality of sets of combustion-gas tubes interconnected by flow reversing end members and surrounded by water.