Abstract: The structural integrity and coolant flow characteristics of a heat exchanger which employs double tubes is improved by providing a structural element, which defines an annular chamber through which coolant may flow, about the internal structure which supports the opposite ends of the double tubes. The outer wall of the structure which defines each of the annular chambers in alignment with and welded to, a casing which in part defines a gas entry or exit chamber axially disposed at a first side of the internal tube support structure and to an external support ring located axially at the other side of the internal tube support structure.