Abstract: A heat exchanger includes a core structure formed by a plurality of plate members in stacked arrangement. Each plate member is square or rectangular and includes along two opposed sides a portion bent at right angles to the basic plate and then bent inwardly to wrap around the next adjacent plate member. The other two sides of the plate member are bent in the opposite direction and then turn inwardly to act as spacers to hold each plate member from the next by a predetermined dimension thus defining cells between each plate member and the next adjacent plate member. This core construction is then assembled in a plurality of separate core members into a frame structure defined by a pair of side frames defining rails along which the core members can slide. The core members are compressed inwardly by a pair of end doors leaving open top and bottom open sides of the frame structure for connection to suitable duct work.