Abstract: The invention concerns a heat exchanger which is intended for exchanging heat between a first medium passing through a tube system and a second medium which flows around the tube system whereby the tube system comprises a plurality of tubes which are connected in parallel between an inlet and an outlet for the first medium. The tube system is enclosed in a casing which is composed of two halves, which casing is so designed that a channel is formed for the second medium from an inlet to an outlet for this second medium, said channel being formed by mutually substantially parallel parts connected with each other by means of substantially semi-circular parts. The channel comprises the tubes which form the tube system, said tubes having corresponding straight parts and semi-circular parts. The tubes, which may be provided with cross grooves, can be arranged in a direction which is perpendicular to the mean plane of the casing and can be flattened in this direction.