Abstract: A method and apparatus of heating or cooling a charge of a solid material are disclosed. The method includes the steps of supplying a charge of the material to form a packed bed of solids in a process vessel having one or more passageways through which a heat transfer fluid can flow, passing a heat transfer fluid through the one or more passageways and supplying a working fluid to said vessel. The method is characterized by circulating the working fluid through the bed, the circulation step including removing at least part of the working fluid from one or more sections of the vessel and then returning the working fluid to other sections of the vessel.