The radial dampers “inlet-vanes”, also known as vortex damper, are used in all kinds of facilities for regulating the flow and pressure in the centrifugal fans.
They are designed to generate an inlet vortex in the same direction as the fan rotor (pre-rotation), causing a reduction in energy consumption as the air flow is reduced, thus reducing the work done. That is, the relative velocity between the fan and the inlet flow is reduced.