Digester for cooking pulp having staggered screens

A digester for continuous cooking under raised pressure and temperature of fiber material in said digester, said digester including a vessel having an interior surrounded by a wall where input of fiber material and cooking liquid takes place at the top of the digester, withdrawal of spent cooking liquor is carried out from at least one digester screening means located between the top and the bottom of the digester, and fiber material is fed out from the bottom of the digester, and at least one screening system is provided in the lower half of the digester, said at least one screening system comprising a plurality of separate screens each having a noncircular screen face of which the total area is less than 1 m.sup.2 and which is attached to the digester wall with seal means to form a sealed volume from which liquid only can be supplied and withdrawn through each said screen face from said digester interior, said screening system including conduit means for providing an inlet and outlet for liquid, said conduit means penetrating the digester wall and communicating with said sealed volume of each said screen of said system, said screens of said system being distributed about the inner surface of said digester vessel with each said screen including a said seal means forming a separate sealed volume and separate conduit means for each screen, said digester being vertically disposed in use and including a lower widened portion and said plurality of screens of said screen system being located in said lower widened portion, said plurality of screens comprising two rows each extending about said inner surface of said lower widened portion of said digester vessel, said screens in one of said rows being positioned in staggered relation to said screens of the other of said rows.