the drying air flow coming from the air distribution chamber is accelerated because of flow contraction caused by openings 88 between the air distribution chamber 18 and the air distribution channels 36 the said acceleration in the supply air flow immediately after the inlet openings of the nozzle boxes may in some cases cause an insufficient velocity in the nozzle impingement profile in the first part of the nozzle box by dividing air from the supply air distribution chamber 18 into distribution channels 36 in an appropriate way the said reduction in velocity may be compensated fig1 thus schematically shows a section along the line ee taken from the wall between the supply air distribution chamber 18 and the suction chamber 20 shown in fig3 the opening 88 is in an exemplary way provided with a grate 90 for reducing the contraction of the entry flow into the air distribution channel 36 in fig1 the grate 90 is shown enlarged and seen diagonally from the side the width of the grate is chosen taking into consideration the velocity of the incoming flow the width of the nozzles and the length of the nozzles the width of the grate apertures is typically about 30 mm and the length about 80 mm but the measures may vary according to need the lower plates 92 of the grate may be shaped to direct.