the portion of the flow that is radially inward becomes essentially free of fibers about 60 per cent of the flow denoted by the symbol f 1 then enters the two air return ducts 58 and 59 and is returned to the rotor chamber 21 the remaining portion of the air flow denoted by the symbol f 2 which contains a more concentrated volume of the cellulosic fibers exits through the outlet duct 13 and proceeds on for further processing as explained earlier maintaining a proper clearance between the raker bars 99 and the screen 61 is essential additionally rotor speed air velocity through screen 61 and mesh size for the screen 61 must be properly selected it is theorized that the bulk of the fiberization process is attributable to the high velocity flow of air through the screen 61 and that the raker bars serve primarily to inhibit screen blinding and to agitate continuously the material adjacent to the screen 61 by reason of the radial chambers 90 narrowing radially outwardly a velocity increase of the air flow correspondingly occurs in the outer peripheral portions of the rotor 70 also a higher pressure zone occurs adjacent to the leading surface of each vane 86 providing for maximum pressure differential over the screen 61 in the regions immediately adjacent to the raker bars 99 the air flow at the raker bars 99 passes not only through the screen 61 fiberizing the material but also between teeth 102 aiding in the material agitation process typically air flow through the screen 61 ranges between four 4 and fifteen 15 cubic feet per minute per square inch of screen residence time of the material within rotor 70 should be.