Abstract: An improved lightweight heat exchanger particularly suited for use in systems having low volume flow, high longitudinal gradient and high effectiveness requirements. The heat exchanger is characterized by a shell of an annular configuration, an endless plate of minimal thickness and of a substantially uniformly convoluted configuration disposed within the annular shell for defining therewithin a plurality of endless, juxtaposed passages, each having a low Reynold's number and being of an annular configuration. A pair of manifolds disposed 180.degree. apart is mounted on the shell in communication with the passages through which counterflowing fluids having different temperatures are simultaneously introduced and extracted from the passageways for thus achieving a continuous transfer of heat through the convoluted plate.