TROX swirl diffusers of the FD type consist of a diffuser face with fixed, radially arranged air control elements. They are available in square or round design. Connection to the air duct system is effected via the plenum box, in either vertical or horizontal configuration. Swirling, horizontal discharge of the supply air at a high induction rate guarantees swift temperature equalisation and fast reduction of the flow velocity. In order to stabilize the supply air flow, all sizes must be installed flush with the ceiling.