conduit 9 opening in the end wall 3 within the drum coaxial to the rotational axis of the drum 1 is arranged a plurality of cylindrical walls in the example shown five walls 10 14 every second such wall is in its one end tightly connected to the end wall 3 and every second to the end wall 4 the walls 10 and 11 are tightly connected to the end wall 3 while there is an axial space between the opposite end wall 4 and the cylindrical walls 10 and 11 correspondingly the walls 12 13 and 14 are tightly connected to the end wall 4 and there is a space between the opposite end wall 3 and the cylindrical walls the end wall 4 ends at its connection to the cylinder wall 14 so that the channel formed by the cylinder wall 14 and the drum mantle 2 is open in an axial direction radially outside the end wall 4 there forming an annular outlet 15 adjacent cylinder walls 12 and 10 10 and 13 13 and 11 11 and 14 and cylinder wall 14 and drum mantle 2 form together with the spaces just mentioned a flow channel leading to and fro between the end walls 3 and 4 of the drum the channel starting at the centre of the drum and terminating at the outlet 15 for stiffening of the drum a radially inner cylindrical wall 16 extends as an extension of the inlet conduit 9 between the end walls 3 and 4 and is connected thereto further the wall 16 is provided with a plurality of perforations 17 in the example shown three.