refiner or defibrator 24 in the embodiment shown the throat 22 comprises a conical bore that narrows in the direction of material flow and encloses a rotating conveyor screw 26 of the same conical contour connected to the outlet end of the pipe there may be a counter pressure device 27 e g in the form of a pipe fitting 84 in which flaps 28 are mounted which are powered by piston servomotors 30 in such a manner that they can be swung into the bore 32 of the pipe fitting which bore preferably forms a cylindrical continuation of the end of the screw compressor thus reducing the cross sectional flow area of this bore this achieves a high degree of compression of the starting material e g chips this material usually contains water which during the compression phase is forced out through perforations 34 in the compressor pipe and is carried away via a funnel 35 the refiner or defibrator 24 comprises refining discs enclosed in a housing 36 which in the embodiment shown comprises one stationary disc 38 that is rigidly united with the casing and one refining rotating disc 40 that is carried by a shaft 44 powered by a motor not shown a servomotor not shown arranged in a known manner between the motor and the rotating refining disc 40 transmits the pressure of a hydraulic medium e g in the manner disclosed in swedish pat no 179 337 which corresponds.