Screw thickener

In a screw thickener for the dewatering of fiber suspensions having a starting concentration in the range of 1 to 10% and comprising a shaft rotatable in a screen drum provided with at least one flight mounted in helicoid form, the screen drum and the envelop of said at least one flight tapering towards the discharge end and the diameter of the shaft increasing conically toward the discharge end, said screw thickener also including means for feeding at one end of said screen drum and plug forming and discharge means at the other end, the improvement comprising means maintaining an internal hydrostatic overpressure of at least 10 kPa as measured at the highest interior point of the screen drum and means for pulling said plug forming means comprising a plurality of pulling means evenly distributed around the periphery of said plug forming means and externally of said screen drum, and said shaft at said discharge end passing through said plug forming means.