Is the screw entirely necessary in the rotating tube or is it invalidated if the rotation of the spiral is reversed? I can understand why the tube works but I'm confused as to how the screw isn't a hindrance. I suppose it doesn't really matter. I can see a problem with this though if the screw isn't turning then a portion of substance is never moved but resolves into part of the central core.<br><br>I suppose the Whole pipe must turn and not just a lower section or bit for the reason of compaction?