the central screen element in a batch digestion vessel where in connection with the filling of the chips and filling of the impregnation liquor the chip column can heavily move upwards and break the constructions the connection of the central screen element to the liquid flows is preferably made at the upper part of the screen element for this reason when installing the screen element into an existing digester the existing pipe connections must be taken into consideration when designing and sizing the apparatus the central screen element can be made of any material suitable for that purpose like austenitic and ferrite austenitic steels the construction of the screen segment of the screen zone can be a round bar screen as described in patent wo01 31117 which is flow technically efficient and would significantly increase the hydraulic capacity of the screen on the other hand also any other possible constructions implementing the characteristic features in accordance with the invention are covered by the scope of the invention the screen element can be implemented for example with a perforated plate or a slotted plate the choice is often based on a plurality of criteria among others on the price the transversal cross section x x of fig1 illustrates one embodiment in accordance with the invention showing the channel construction of the central screen element also other channel constructions for example pipe constructions for conducting.