... motor size, the axial fans have sickle-shaped plastic blades ( Hyblades ) or die-cast blade: in both variants , the blade ... , fibre-reinforced plastic . For backward curved centrifugal fans up to 630mm in diameter , an impeller with three-dimensional, backward curved ... provides the best possible results in terms of noise and efficiency . The forward curved centrifugal fans , with impeller diameters form ...