pipes 34 36 and 38 can be discharged through the pipes into the return conduit 54 for reinjection into the bed 22 through the feeder pipe 56 this of course prevents the build up of material in the bed 22 during regular operation since the bed portion extending above the chamber 26d is fluidized at all times the pipe 40 functions as a normal bed drain or additional source of fluidizing air depending on the position of the valves 48 and 46 respectively a pair of horizontal headers 60 are connected in fluid communication with the tubes 18 forming the front wall 12 and the rear wall 14 respectively and another pair of horizontal headers 62 are connected in fluid communication with the tubes 18 forming the sidewalls 16 it is understood that headers similar to the headers 60 and 62 are provided in communication with the upper ends of the walls 12 14 and 16 as a result a fluid to be heated can be sequentially or simultaneously passed through the walls 12 14 and 16 to pick up the heat from the fluidized bed in a conventional manner before it is passed to external apparatus for further processing it is further understood that variations in the above arrangements can be made without departing from the scope of the invention for example the number and specific locations of the pipes 40 as well as the zonal fluidization techniques can be varied as long as the above objectives and results are achieved further a bank or series of heat exchange tubes can be provided in the enclosure 10 for circulating water in a heat exchange relationship with the bed 22 in a conventional manner a latitude of modification change and substitution is intended in.