Abstract: The heat exchanger which acts as a recuperator is constructed of a plurality of identical sub-assemblies which can be manufactured and tested in a plant and shipped as individual units to a construction site. Each sub-assembly includes a plurality of straight tubes which interconnect with spherical shells at each end and which serve to carry a flowable medium. Each sub-assembly also includes a guide tube about the straight tubes which is open at both ends to convey a flowable medium over the straight tubes but which has an outer flange which cooperates with similar flanges on the other sub-assemblies to block any flow of this medium over the outside surfaces of the guide tube from one end to the other.