A conveyor comprises perforation bands arranged in the floor of a storage space. Some of the bands emit an air stream that in quantity or pressure differs from the air stream in the remaining bands and have apertures with a larger flow area than the apertures in the remaining bands. The larger apertures are provided: along with a first perforation band placed in the middle of a sector of the floor and extending across the entire sector; along one further band arranged on one side of the first band with a shorter extension than in the first band; and in portions of bands which portions are closest to the walls of the storage space.