The device has a gas flow levelling pipe (3), which defines a continuous curved development surface (Z), at comprising a collector (4) to which a nozzle (10) is fixed, a delivery manifold (1), in order to introduce pressurized gas into the pre-chamber (2) through the holes (12), a first holed partition (5) and a second holed partition (6) within the levelling pipe (3), arranged perpendicular to the curved development surface (Z) of the pipe (3).