A screw pipe unit intended to cooperate with and at least partly enclose a screw in a plug screw feeder for cellulose chips/pulp is provided. The screw pipe unit comprises a plurality of openings for filtrate dewatered from the chips/pulp in the screw pipe when the plug screw feeder is in operation. The screw pipe is, on at least a portion of its inner surface, provided with bars inclined with respect to the longitudinal axis of the screw pipe. The angle of inclination of the bars decreases in the longitudinal direction, from a first portion of the screw pipe unit with maximum inclination to a second portion of the screw pipe unit with bars substantially parallel to the axis, the second portion being further away from the inlet end of the screw pipe unit than the first portion.