Abstract: A heat exchanger having an outer shell with an elongated, semi-circular fluid duct plate mounted in the shell. A tube bundle slidable between the duct plate and the shell is fixed at one end relative to the shell. The fluid inlet and outlet configuration, and transverse fluid flow pattern, are typical of those described in U.S. Pat. No. 3,532,160. The exterior of the tube bundle is partially covered with a shrouding to contain the fluid flow within the tube bundle by allowing the fluid to exit only through the outlet side. Sealing mechanisms running longitudinally between the shrouding and the upper duct plate, and between the shrouding and the shell, provide a seal which restricts the fluid flow to a transverse flow through the tube bundle. A seal between the supported end of the tube bundle and the shrouding confines the fluid flow in the bundle and allows for relative expansion of the shrouding and the bundle.