Hot air oven

Hot air oven provided with a cross-flow tube heat exchanger comprising at least two rows of a plurality of heat exchanger tubes, the heat exchanger is arranged inside a heating chamber in which cooking air which is to be heated flows past the hot heat exchanger, hot exhaust gases for warming the cooking air are generated by a burner, the exhaust gases pass through a gas-tight duct system, including said heat exchanger tubes, which leads from the burner to a chimney, and that the heat exchanger tubes in each row are arranged in an essentially horizontal plane, and that heat exchanger tubes in a first row are in a predetermined relationship in relation to the heat exchanger tubes in a second vertically adjacent row, wherein said predetermined relationship involves that the heat exchanger tubes in the first row are horizontally and vertically offset from the heat exchanger tubes in the second vertically adjacent row, each heat exchanger tube comprising an elongated tube with a tube wall, the interior of the elongated tube is provided with at least one longitudinally extending interior wall which extends from one side of the interior surface of the elongated tube towards another side of the interior surface, wherein, during use, the interior walls are heated by the exhaust gas and the interior walls transfer this heat by conduction to the heat exchange tube wall, wherein each tube has a cross-section with two straight and parallel long sides, separated by a distance D, and connected by a pair of convex curved ends of diameter of curvature D, and wherein the at least one interior wall extends from one long side to the other long side.