1. A ventilating apparatus adapted for placement in a cylindrical air duct ( 260 ), comprising: an axial fan ( 120 ) having an axial fan wheel ( 122 ) dimensioned to fit inside the cylindrical air duct; an electronically commutated electric motor ( 110 ) serving to drive the axial fan so that, during operation, the fan transports air axially through the duct; an air-conducting tube ( 248 ) mounted in the duct ( 260 ) adjacent the axial fan ( 120 ); a thermal anemometer ( 140 ) in said air-conducting tube for sensing a volumetric air flow generated by the fan ( 120 ) and for generating a measured volumetric air flow value (Vmess); a volumetric flow rate control arrangement ( 160 ) for automatically controlling the volumetric air flow rate ( 125 ) generated by the fan ( 120 ) to a predetermined target volumetric air flow value (V_s), said volumetric air flow rate control arrangement ( 160 ) being adapted to generate a target rotation speed value (N_s) for the electric motor ( 110 ); and a rotation speed controller ( 170 ) which automatically controls the rotation speed of the electric motor ( 110 ) to the target rotation speed value (N_s ) generated by the volumetric air flow rate control arrangement ( 160 ).