I think, though I'm not certain, that the big advantage is speed. The cylinder can be spun faster than the screw because the screw is much less stiff and will have a tendency to 'whirl' ; to bend as it rotates. This would lead to rubbing on the cylinder, with consequent wear and heat. The tube is stiffer and can be supported with as many bearings as needed.<br>Anyone got thoughts?,(reply>), No.. It can be spun faster because all the pressure is generated from below. Spinning a screw faster will cause splashes as the screw applies pressure everywhere not just at the bottom, but which mean the top of the screw applies pressure to the tiny part of material at the top and thus can throw splashes of stuff everywhere.,(reply>), It's also got less air transfer, so fewer particles of stuff you don't want. Big advantage to a lot of fields.