This International Standard specifies the dimensions of the circular and rectangular flanges of general purpose fans, as well as the fan size designations. It is not applicable to cross flow fans or to fan appliances used for individual household or similar applications. For circular flanges, it provides for three different flange series: one for light duty casing thicknesses, another for medium duty fans and the third for heavy duty fans as used on sea-going vessels or in heavy industry. In order not to restrict fan design unduly, only the pitch diameter, hole numbers and hole diameters are specified. Flange thickness, as well as internal and external flange diameters, may be chosen freely within the limits of good engineering practice.