5.2 FAN NOISE

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There are several types of fans used in industrial and residential applications.

The fans may be classified according to the nature of flow through the

fan and by the blade geometry (Avallone and Baumeister, 1987). Generally,

the noise signature is different for each type of fan. Various fan types and

typical applications are as follows:

(a) Centrifugal fan, airfoil blades. The airfoil blades have backwardcurved

chord lines, with the leading edge of the airfoil pointing

forward and the trailing edge pointing backward with respect to

the direction of rotation of the fan. All centrifugal fans have a

164 Chapter 5

Copyright © 2003 Marcel Dekker, Inc.

volute or scroll-type housing. This fan is used in large heating,

ventilating and air conditioning systems in which relatively clean

air is handled.

(b) Centrifugal fan, backward curved blades (BCB). The blades are

flat plates with uniform thickness. The leading edge points in a

direction opposite to the rotation of the fan. BCB fans are used

for general ventilating and air conditioning applications. The fan

efficiency is somewhat higher than that of the other types of

centrifugal fans. The fan speed must be higher for a given flow

rate than that of other centrifugal fans.

(c) Centrifugal fan, radial blades. The blades are flat plates of uniform

thickness, oriented along the radial direction of the fan

cage. This fan is often used in material handling systems in industrial

applications in which sand, wood chips, or other small

particles are present in the air.

(d) Centrifugal fan, forward curved blades (FCB). The blades are

shallow and curved, such that both the leading and trailing

edges point in the direction of rotation. The fan efficiency is

somewhat lower than that of the other centrifugal fans. As a

result, this fan is used for low pressure rise, low speed applications,

including domestic furnaces and packaged home air

conditioning units.

(e) Tubular centrifugal fan. These fans use a tubular casing so that

both the entering and leaving flow is in the axial direction. The

blades may be either backward curved or airfoil type. The fan is

often used for low-pressure return-air systems in heating and

ventilating applications.

(f) Vaneaxial fan. These fans usually have blades of airfoil design,

which allows the fan to be used in the medium to high pressure

rise range at relatively high efficiency. Guide vanes are located in

the annular space between the casing and the inner cylinder.

Noise generation for vaneaxial fans is generally higher than

that of the centrifugal fans. Typical applications for the vaneaxial

fan include fume exhaust, paint spray booths, and drying ovens.

(g) Tubeaxial fan. This fan is similar to the vaneaxial fan, except that

straightening vanes are not used. The fan is generally used in low

to medium pressure rise applications.

(h) Propeller fan. The fan blades are usually wider than those of the

vaneaxial fan. The propeller fan is mounted in a ring with no

attached ductwork. The pressure rise is relatively small, but the

fan can handle a large volume flow. Applications for this type of

Noise Sources 165

Copyright © 2003 Marcel Dekker, Inc.

fan include roof exhaust systems and induced-draft cooling

towers.

There are several paths through which noise may be radiated from a

fan, including (a) sound power radiated directly from the fan outlet and/or

inlet, if there is no attached ductwork to the inlet and/or outlet; (b) sound

radiated through the fan housing; and (c) sound induced by vibrations

transmitted from the fan through the fan supports to the adjoining structure.

These paths are illustrated in Fig. 5-1. With proper vibration isolation

by the support, as discussed in Chapter 9, it is possible to reduce the noise

from vibrations to a negligible level, compared with the first two contributions

given above.

If we denote the sound power level generated internal to the fan by

LW, then the sound power level for sound radiated out of the inlet and/or

outlet openings or radiated down the attached ductwork may be estimated

from the following expressions (note that if ductwork is attached to an inlet

or outlet, there is no sound radiated into the surroundings from the inlet or

outlet source):

LWрoutletЮ ј LW _3dB (5-8)

LWрinletЮ ј LW _3dB (5-9)

The sound power level for sound transmitted through the fan housing is

related to the transmission loss TL of the fan housing:

LWрhousingЮ ј LW _TL (5-10)

166 Chapter 5

FIGURE 5-1 Fan noise paths.

Copyright © 2003 Marcel Dekker, Inc.

The noise generated internally by each of the fans mentioned previously

is composed of two components: broadband noise generated by

vortex shedding from the fan blades and a discrete tone (blade tone) produced

as the blade passes by the inlet or outlet opening of the fan.

The sound power level of noise generated by the fan for any octave

band may be estimated from the following correlation (Graham, 1972):

LW ј LWрBЮ ю 10 log10рQ=QoЮ ю 20 log10рP=PoЮ ю BT (5-11)

The term LWрBЮ is the basic sound power level, and is given in Table 5-1

for each of the fan types discussed previously; Q is the volumetric flow

rate through the fan, and Qo is a reference volumetric flow rate,

0.47195dm3=s ј 1 ft3=min; P is the pressure rise through the fan, and Po

is a reference pressure rise, 248.8 Pa ј 1 in H2O; and BT is the blade tone

component, which is zero except for the octave band in which the blade pass

frequency lies. For this one octave band, the value of the blade tone component

is given in Table 5-1. The blade pass frequency fB is the number of

times a blade passes one of the fan openings and is given by the following

expression:

fB ј nrNb (5-12)

The quantity nr is the rotational speed of the fan, rev/sec, and Nb is the

number of blades on the fan.

The sound power level calculated from Eq. (5-11) is the sound power

level generated internally in a particular octave band for the fan only. The

Noise Sources 167

TABLE 5-1 Basic Sound Power Level Spectrum LWрBЮ for Fans

Fan type

Blade tone,

BT, dB

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

Centrifugal fans:

Airfoil blade 3 35 35 34 32 31 26 18 10

BCB 3 35 35 34 32 31 26 18 10

Radial blade 5–8 48 45 45 43 38 33 30 29

FCB 2 40 38 38 34 28 24 21 15

Tubular 4–6 46 43 43 38 37 32 28 25

Vaneaxial 6–8 42 39 41 42 40 37 35 25

Tubeaxial 6–8 44 42 46 44 42 40 37 30

Propellor 5–7 51 48 49 47 45 45 43 31

Source: Graham (1972). By permission of Sound and Vibration, Acoustical

Publications, Inc.

Copyright © 2003 Marcel Dekker, Inc.

additional noise due to the fan motor and drive system is not included in

Eq. (5-11).

Example 5-1. A forward curved blade (FCB) centrifugal fan operates at a

speed of 552 rpm against a pressure of 190 Pa (0.7626in H2O) to deliver

1.80m3/s (3814 cfm) of air. The fan is located outdoors (air at 300K or

808F), and the fan has both inlet and outlet ducts, so that noise is radiated

only through the housing of the fan. The transmission loss for the fan is

given in Table 5-2. The directivity index for the fan may be taken as DI ј 3dB for all frequencies. Determine the overall sound pressure level produced

by the fan at a distance of 3m (9.8 ft) from the fan.

The blade pass frequency is found from Eq. (5-12):

fB ј р552=60Юр64 bladesЮ ј 589 Hz

This frequency lies in the 500Hz octave band (354–707 Hz), and the blade

tone component for the 500Hz octave band is found in Table 5-1 for an

FCB centrifugal fan:

BT ј 2 dB in the 500 Hz octave band

BT ј 0 dB for all other octave bands

The internal noise generation sound power level can be calculated

from Eq. (5-11). For the 500 Hz octave band, we obtain the following value:

LW ј 34 ю 10 log10р1800=0:47195Ю ю 20 log10р190=248:8Ю ю 2

LW ј 34 ю 35:8 ю р_2:3Ю ю 2 ј 34 ю 33:5 ю 2 ј 69:5dB

For the 250 Hz octave band, the sound power level is as follows:

LW ј 38 ю 33:5 ю 0 ј 71:5dB

The corresponding values for the other octave bands are given in Table 5-2.

168 Chapter 5

TABLE 5-2 Solution for Example 5-1

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

LW, dB 73.5 71.5 71.5 69.5 61.5 57.5 54.5 48.5

TL, dB 15 21 27 33 39 40 40 40

LW(housing), given 58.5 50.5 44.5 36.5 22.5 17.5 14.5 8.5

Lp(octave band), dB 41.1 33.1 27.1 19.1 5.1 0.1 _2.9 _8.9

Copyright © 2003 Marcel Dekker, Inc.

The sound power radiated from the fan through the housing is found

for each octave band from Eq. (5-10). For the 500Hz octave band, we find

the following value:

LWрhousingЮ ј LW _TL ј 69:5_33 ј 36:5dB

The sound pressure level in the 500Hz octave band is found from Eq. (5-4).

The atmospheric attenuation is negligible, because the distance рr ј 3m) is

small:

Lp ј LWрhousingЮюDI_20log10рrЮ_10:9

Lp ј 36:5ю3_20log10р3:00Ю_10:9 ј 36:5ю3_9:5_10:9

ј 36:5_17:4

Lp ј 19:1dB

The other octave band sound pressure levels are shown in Table 5-2.

The overall sound pressure level is found by combining the octave

band values of acoustic energy, as discussed in Sec. 2-8.

LpрoverallЮ ј 10 log10Ѕ_10LрOBЮ=10_

LpрoverallЮ ј 10 log10р104:11 ю 103:31 ю 102:71 ю_ _ _Ю ј 41:9dB

Suppose the inlet duct were removed, so that sound could be radiated

out the inlet opening of the fan. For the 500 Hz octave band, the sound

power radiated out the inlet is given by Eq. (5-9):

LWрinletЮ ј 69:5 _ 3 ј 66:5dB

The total power radiated from the fan in the 500 Hz octave band would have

the following value:

LW ј 10 log10р103:65 ю 106:65Ю ј 66:5dB

In this case, the effect of sound radiated through the housing is negligible.

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