suction opening 11 the duct 12 leads into a further duct 19 this further duct 19 is bounded by the horizontal separating wall 18 of the casing the side wall 15 and a further side wall 25 and a top cover 26 as well as a cover side wall between edges 60 61 and leads to the underside of the main pressing surface element 2 for guiding suction air or blowing air on its side facing away from the fan the duct 19 opens through a cut out in the horizontal separating wall 18 of the casing into a duct 27 the duct 27 is bounded by the partition wall 16 a front wall 28 the side wall 25 a bottom 29 which extends obliquely upwards from the fan discharge opening 32 towards the side wall 25 and a separating wall 31 and a wall 29a both shown in fig3 on the rear of the blower assembly it is open to atmosphere in the lower part of the partition wall 16 the duct 27 is connected to the discharge opening 32 of the fan 10 a shaft 20 on the separating wall 18 extends along that edge which delimits the opening of the duct 19 into the duct 12 up to and including the opening of the duct 19 into the duct 27 this shaft 20 is rotatably mounted in bearings not shown in the side walls 15 25 and in the separating wall 16 a sheet metal vane 21 is secured to the shaft 20 by screwing or welding and is of such a size that in the manner shown in fig2 it can sub divide the duct 12 into two parts and can completely close off the opening where the duct.