In recent years, the aerodynamic noise generated by centrifugal fans is receiving increasing attention because of strict environmental noise level restrictions and customer demands. The noise generated by fans is due to aerodynamic sources and to other several sources, such as, for instance, by the fan drive, by bearings and gearing, and, when present, by the inverter. Additional noise sources can be also due to structural resonance effects induced by periodic forces associated with the blade passing frequency or vortex shedding. Usually, these additional noise sources are dominated by aerodynamic noise generated by the fan, in particular when the intake and outlet of the fan are free. On the other side, if fan intake and outlet are ducted, the additional sources can relevantly contribute to overall sound generation. In this paper, an experimental characterization of the noise generated by industrial centrifugal fans when both inlet and outlet are ducted is presented. To do this, an experimental facility has been design and set up, and the sound power measured by means of the procedures outlined in the ISO 3746 international standard. A number of different type of centrifugal fan (straight-, forward- and backward blade) in different working conditions were tested, resulting in 133 different runs. These amount of data were then processed and a general formula for fan noise estimation obtained as a function of the geometrical and fluid dynamic parameters is derived. Moreover, specific coefficients with respect to blade geometry for the determination of the A-weighted frequency spectrum are presented. Finally, auxiliary devices or other features, such as inverter, thickness of the casing, acoustic insulation, electric motor shaft, are analyzed and some general rules to estimate their influence on sound power level quantified.

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