Abstract: A heat exchanger apparatus (which can be a boiler) 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.