A heat exchanger of the tube bundle type has a cylindrical body unit with an intake and outlet for the primary fluid. End sections of the body have closing flanges across which the tubes pass. Each flange is made of a material of rubber type and has a circular plate which bears on it periphery a set of circular lips. The flange rests with one of the sets of lips on a radial surface of the corresponding end section of the body unit. A mount element with a radial front surface rests on the other set of lips. A screw fastened to the outer wall of each end section locks the mouth element against the flange between the mount element and the body.