Abstract: A vertical heat exchanger including a vertical tube bundle connected between a lower inlet manifold and upper outlet manifold. The tube bundle comprises numerous pairs of concentric tubes with the inner tube being spaced from the outer tube to form an annular flow passage positioned around an inner flow passage. The inner tubes are welded between outer tube sheets that form a part of the upper and lower manifolds while the outer tubes are welded between inner tube sheets forming part of these manifolds. Each of the manifolds has a distributor ring which has radially directed openings throughout its periphery so as to provide for a radial flow in the manifold. Fluid enters the lower manifold through suitable inlet nozzles and exits the upper manifold through suitable outlet nozzles. A high temperature heating gas is directed up through the inner tubes of each pair of tubes by a conical inlet distributor and exits the upper end of the tube bundle through a similar conical distributor.