flap 4 is of rectangular design and has a height h and a width b the height h corresponds at least to the depth t cf fig1 of the tubes 3 so that the tube cross sections are covered when the flap is closed the width b in fig1 a is selected in such a manner that the tubes 3 2 to 3 n u2212 1 are covered when the flap 4 is closed and only the two outer tubes 3 1 and 3 n remain open and have charge air passing through them in exemplary embodiments which are not shown only one tube or a plurality of tubes in particular more than two tubes remains or remain open these tubes are arranged on one or both edges of a respective tube row however tubes in the center of a tube row may also remain open for this purpose a plurality of flaps or preferably one flap with cutouts are used with the cutouts being assigned to the tubes which are to remain open fig2 shows a further exemplary embodiment of the invention with an air box 6 on the outlet side which is connected in a plane 6 a to a tube plate not illustrated or to a heat exchanger unit with tubes opening into the tube plate a charge air outlet 6 b is arranged on that side of the air box 6 which faces away from the plane 6 a arranged within the charge air box 6 is an angled partition 7 which is composed of three regions 7 a 7 b 7 c and divides the air box.