a first plate of a plurality of first plates and a second plate of a plurality of second plates each comprising an inner surface and an outer surface; wherein

in each of the heat exchanger elements, the first plate has a pair of side edges and the first and second plates of each heat exchanger element are sealingly joined together, wherein an external manifold portion comprises a flattened tubular structure enclosing both an inlet manifold channel and an outlet manifold channel, the external manifold portion extending across all the heat exchanger elements along an axis which is perpendicular to side edges of the first and second plates, wherein at least one stiffening element comprises a plurality of bends in the external manifold portion, the plurality of bends creating a plurality of first portions and a plurality of second portions of the external manifold portion which are located in different planes and which are separated by inclined shoulders, and wherein the plurality of first portions are located in a first plane and the plurality of second portions are located in a second plane, the first and second planes being parallel and spaced apart by a thickness of the plurality of heat exchanger elements.