cross sectional flow downstream from the pivot axis 3 1 the grating 2 arranged directly behind the shutoff valve 3 equalizes the flow i e results in uniform velocities of flow over the flow cross section of the air channel 1 downstream from the grating 2 in particular in the cross section of the end opening 1 2 due to the great obstruction of the reduced flow area 1 3 i e the great flow resistance there which is assigned to the area near the wall and thus to the wake of the open slot 3 2 and also due to the low velocity of flow of the flow through area 1 4 which is assigned to the pivot axis 3 1 and or the wake of the shutoff valve body the direct structural assignment of the reduced flow area 1 3 to the zone of a high absolute pressure and the direct assignment of the flow through area 1 4 to the zone of a low absolute pressure make it possible to achieve a uniform flow in a very small installation space and with a low pressure drop fig2 shows a sectional diagram across the longitudinal extent of the air channel 1 along the sectional line.