Abstract: This invention relates to fluid flow apparatus such as a heat exchanger. The heat exchanger includes inlet means, outlet means and a core element. The core element has a plurality of flow passages for a process fluid and a multiplicity of flow passages for a working fluid wherein each respective process fluid passage is located adjacent to a corresponding working fluid passage. There is also provided means for selectively controlling the direction of process fluid and/or working fluid on passage through the core element. It is also within the scope of the invention that the heat exchanger may be utilized to treat a plurality of different fluids or working fluids. Suitably the fluid flow apparatus includes an inlet component, an outlet component and the core element and it is the specific juxta position of these members that provides the means for selectively controlling direction of process fluid and/or working fluid through the core element.