Patent ID: 7300550

Claim:
In a primary wood chip refiner having flat, relatively rotating discs, each having a working plate thereon, the working plates being arranged in confronting coaxial relation thereby defining a refiner gap which extends substantially radially outward from the inner diameter of the discs to the outer diameter of the discs and defines a flow area for the chip material to be refined, the improvement comprising: each plate having a radially inner fiberizing band and a radially outer fibrillating band, each band having an inner feeding region and an outer working region, wherein the working region of the inner band is defined by a first pattern of alternating bars and grooves, and the feeding region of the outer band is defined by a second pattern of alternating bars and grooves, and said flow area increases immediately after the inner working region into the outer feeding region; the refiner gap includes an outer gap defined between the confronting outer bands and an inner gap defined between the confronting inner bands; the refiner is a thermomechanical refiner that generates steam in the outer gap between the working regions of the outer bands; at least one of the confronting plates includes a steam bypass channel for directing some of said steam from the outer gap to the inner gap at the inner feeding region while bypassing the inner gap at the inner working region; and the steam bypass channel is formed as a groove on the surface of the working region of the inner band, oriented diagonally through the bars and grooves of said pattern of bars and grooves.