Abstract: A heat exchange assembly is provided which employs concentric flat plate heaters, the heat exchange assembly having a common fixed tube sheet and floating tube sheet; the floating tube sheet comprising two concentric portions having frustoconical interface thereby, the frustoconical interface when projected has its apex in a plane containing the adjacent face of the fixed tube sheet in the axis of generation of the flat plate heater. Thermal stress on the floating tube sheet is thereby relieved. The heat exchanger is useful as a reactor particularly where the temperature profile of the material flowing therethrough is controlled or varied.