1 meter above measured from the upper edge of the lower digester screening arrangement 1b to the lower edge of the newly fitted digester screening arrangement 2a wash liquor is supplied to the lower part of the digester through an inflow conduit arrangement 4 attached in the vicinity of the bottom 1a of the digester and cooking liquid with alkali addition through the central pipes 5a 5b the cooked pulp is taken out from the bottom of the digester via a conduit 1e valves 8 and 9 respectively control introduction of white digesting liquor through pipes 1f and 1g into the circuits for pipes 5b and sa upstream of the respective heat exchangers 6a and 6b to assist in maintaining the necessary control of the heat content of the liquors introduced as described one of these central pipes 5a which belongs to the original system of the digester penetrates down to the lower screening arrangement 1b of the digester after which a portion of the liquid is drawn off through screen 1b and passed to the heat exchanger 6a after heating via the first heat exchanger 6a the liquid is passed back through pipe 5a on a level with the digester screening arrangement 1b to maintain the desired isothermal temperature condition at this zone of the digester.