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THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

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14.4 Fan or Blower Noise 361Figure 14.3. Three types of centrifugal fans. (Harris, 1991, p. 41.3.).fan and an axial-flow fan, respectively, along with listing of commonly usednomenclature.Centrifugal fans come in with a variety of blades. Three types are shown inFigure 14.3. Axial-flow fans divide into three main categories (vaneaxial, tubeaxial,and propeller), illustrated in Figure 14.4. In addition to flow and pressure requirements,fans are selected to meet environmental conditions, withstand corrosion,allow ease of maintenance, budget limits, and so on. The noise characteristics ofvarious types of fans are fairly predictable, as they are not significantly altered byminor changes in the fan geometry.Fan Noise CharacteristicsTable 14.2 lists the broadband noise characteristics of typical fan designs. Theaverage specific sound power levels in eight octave bands are given for welldesignedfans installed in well-designed systems. This data can be utilized toestimate fan noise at the design stage. In the selection process during the designstage, actual noise data should be obtained from the manufacturer. Octave-bandnoise levels should be used in calculations. Single-number ratings for fan noiseshould be avoided.When a blade passes over a given point, the air receives an impulse. The repetitionrate of this impulse, termed the blade frequency, determines the fundamentaltone that is produced by the blade. It can be predicted fromf B = nN(14.2)60wheref B = blade frequency, Hzn = fan speed, number of revolutions per minute (rpm)N = number of blades in the fan rotorAccording to Equation (11.23), the sound pressure level L p from a specific sourcein a room depends on room conditions as well as on the sound power level L Wof that source. In order to predict the contribution of the fan noise to the room

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