from the digester in this respect if a given screen or screens is noted to be plugging the operator can close down outflow from such screen or screens thereby allowing the downflowing chip stream to sweep the surface of the screen interiorly of the digester and remove all or part of any material which is attempting to plug the screen or screens if this technique is unsuccessful the operator further has the option of back washing the screen or screens individually employing the first second and third manually operable valve which are associated with the stub pipe of each screen in accordance with one aspect of the present invention hot liquor withdrawn from the digester through the screens is preferably reintroduced to the interior of the digester through the downcomer that is aligned with the vertical centerline 74 in the present invention contrary to the prior art the discharge ports in the bottom end of the downcomer are disposed both centrally of the interior of the digester and radially aligned with the screens that surround the downcomer in this manner the present inventors provide for the injection into the chip mass of a substantially circular sheet of fresh hot liquor that flows from the downcomer ports radially toward the screens this flowing sheet of hot liquor has been found to eliminate or substantially discourage the development of upflows or downflows within the chip mass at substantially all points radially between the downcomer and the screens in the digester wall this effect has been particularly noted in the regions of the perpendicular cross section of the digester at the level of the screens.