... to standard : CE ----approvalsaaUL , VDE 60hz : Axial and centrifugal fans with AC motor sizes 094/110/138 Asynchronous external rotor ... diameter from 500mm to 910mm , for backward curved centrifugal fans with impeller diameter 450mm to 630mm , and forward curved ... of special , fibre-reinforced plastic . For backward curved centrifugal fans up to 630mm in diameter , an impeller with three-dimensional ...