Abstract: Disclosed is a heat exchanger assembled of a plurality of oval heat exchanging pipes whose end portions are mounted in a leakage proof manner in oval openings of top and bottom plates. The ratio of maximum and minimum diameters of intermediate portions of the pipes if 2.5:1 through 8:1 whereas the ratio of the maximum and minimum diameters of the oval openings in the end plates is 1.2:1 through 3:1. A method of mounting the pipe end portions in the oval openings is based on a preliminary shaping of the end portions by the application of pressure from the outside toward the center axis of the end portions in the direction of their maximum diameter. The preshaped end portions are inserted into sealing collars surrounding the edges of respective plate openings. Thereafter the end portions are expanded by means of mandrels to sealingly close the openings.

Abstract: A heat conducting fin for a heat exchanger is provided with an array of deep drawn collars defining passages with flattened, oval cross-sections for receiving heat exchanging pipes of a corresponding cross-section. In order to prevent the formation of cracks in the collars particularly when a large ratio of the maximum diameter to the minimum diameter of the oval collar involves very small radii of curvatures, the height of the collar in the region of its small radius of curvature is less than in the region of its large radius of curvature. The invention also relates to a heat exchanger including a stack of such fins and a series of heat exchanging pipes passing through the passages in the collars and being attached to the inner walls of the collars by expanding corresponding pipe portions.