An apparatus and method serves, e.g., to deaerate, clean or screen a fibrous suspension and is provided with a closed housing and a rotor arranged centrally therein. The rotor puts at least a part of the fibrous suspension guided into the housing into a rotational flow. The fibrous suspension flowing into the housing through the inlet is put into rotation by a guide element acting hydraulically. This increase the efficiency of the apparatus and prevents disturbing turbulences in the apparatus.