Typical sound ratings

From the Section above, it will be seen that it is virtually impos­sible to determine the sound power of a fan for a specific duty, without knowing the characteristics of the particular design to be used.

Nevertheless, it is appreciated that a demand will still exist for some predictive measurement. In an attempt to meet this de­mand, Figure 14.27 has therefore been produced. Again, it is assumed that the fan has been selected to operate at its best efficiency point and is handling air of standard density.

PWL is the level of sound power transmitted along a duct at­tached to the fan inlet or outlet (this in itself may be ±3 dB).

LP is derived from the fan total pressure and Lp from the volu­metric flowrate.

PWL =Lp + Lq dBW re 10“12 W Equ 14.29

The air duty has been used rather than the size, speed or me­chanical power input so that fans of differing type or efficiency may be compared. On the diagram in Figure 14.27, straight lines have been drawn through 84 dBW, 250 Pa at slopes corre­sponding to PWL oc N35 to N7 5 where N is the fan speed.

The area bounded by the dashed lines covers the range within which LP may be expected to lie. Axial and forward curved cen­trifugal fans will be located around the middle of the band, whilst backward curved and mixed flow designs will be in the lower half. The lowest values will be found from aerofoil bladed cen­trifugal fans. At very high pressures radial tipped blades often have to be used for strength considerations. These are not so quiet and hence the power limit line has been curved upwards.

It will be noted that this graph shows a much wider spread be­tween the best and worse fans than previously thought.

Posted in Fans Ventilation A Practical Guide