a heat exchanger apparatus includes a plurality of tanks assembled together . each of the tanks is a separate structural member that provides opposed parallel surfaces . the surfaces carry a plurality of parallel longitudinally extending u - shaped grooves that are correspondingly placed . when the grooves are aligned , they form a first plurality of channels for holding a first fluid system . a load - carrying portion of each plate - like tank extends between the opposed surfaces . a second plurality of fluid conveying channels carries a second fluid system through the load carrying portion of the tank . fluid inlets and outlets are provided for each fluid system . the design of the present invention provides an improved plate - like tank structure that allows the separate fluid systems to be maintained at substantial pressure differentials such as , for example , between about 1 , 000 - 3 , 200 p . s . i .