A pair of round corrugated heat transfer plates provide a cassette with the corrugations of one heat exchanger plate angled relative to the other so as to form channels for fluid flow of a primary fluid and a secondary fluid. A plurality of the cassettes are contained within a housing and have a pair of port holes. The housing has a cylindrical shell, a bottom cover and a top cover. The shell has an inlet nozzle and an outlet nozzle for the secondary fluid. The top cover has an inlet nozzle and an outlet nozzle for a primary fluid. The nozzles are aligned with the port holes. A spring device compensates for any mechanical or thermal expansion of the cassettes. A prevents short-circuiting of the fluid.