back side of the control disc 18 to propel such fibers radially outwardly and toward a collection chamber or channel 100 that is annularly disposed near the outer portion of the casing 12 and which is in fluid communication with the steam discharge conduit 66 this chamber collects steam as well as bypassed fibers the extent of pulp or fiber content in the steam bypass flow 64 will depend in large part on the kind of refining control that is implemented by adjustment of the relationship of pressures p1 p2 p3 and p4 this fine control is achieved with the present invention as an overlay to the two stage refining in which the first stage inner refining zone 40 operates with low energy at high intensity due to the counter rotation and resulting high relative speeds between the first and second plates 42 44 and the second stage in the outer refining zone 50 where high energy low intensity refining occurs due to the rotation of only the third plate 52 relative to the stationery fourth plate 54 as in conventional double disc refiners the control disc 18 is axially adjustable 102 relative to the feed end disc 14 and in accordance with the present invention the stationary plate holder 56 and therefore third plate 56 are axially adjustable 104 relative to the third plate 52 carried by the feed end disc 14 as a further control option in accordance with the present invention the relative speeds of the counter rotating discs can be adjusted preferably the feed end disc 14 is rotated at a conventional speed such as 1500 rpm whereas the control end disc 18 operates at a high speed for.