5.7 COOLING TOWER NOISE

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There are several different cooling tower designs, and each has a somewhat

different noise spectrum associated with it. The cooling towers may be

classified as either mechanical-draft types or natural-draft types, depending

on the mechanism producing motion of the air through the tower.

The mechanical-draft towers may be classified according to the type of

fan used in moving the air. Induced-draft towers generally use a propeller fan

located on the top of the tower. Air is drawn in through the intake louvers to

cool the water flowing from the top of the tower over the tower packing.

Forced-draft towers utilize a centrifugal fan located near the base of the

tower. Air is exhausted from the fan into the cooling tower near the lower

portion of the tower.

178 Chapter 5

TABLE 5-7 Conversion Factor CF4 to Convert from

the Overall Sound Power Level to Octave Band

Sound Power Levels for Transformers

Octave band center frequency, Hz

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

CF4, dB 7 3 9 13 13 19 24 30

Copyright © 2003 Marcel Dekker, Inc.

The noise from a mechanical-draft cooling tower is produced by two

primary mechanisms: (a) the fan on the tower and (b) the splashing water

within the tower. The fan noise is predominant in the octave bands from

63Hz to 1000 Hz. The splashing water contributes to noise mainly in the

2000Hz to 8000Hz octave bands. The fan noise is usually 15–20 dB higher

than the water noise (Thumann and Miller, 1986).

The following correlations may be used to estimate the overall sound

power level for mechanical-draft cooling towers. For an induced-draft tower

using a propeller fan, the following expression applies:

LW ј 96ю10log10рhp=hpoЮ (5-29)

For a forced-draft tower using a centrifugal fan, the following expression

may be used:

LW ј 87ю10log10рhp=hpoЮ (5-30)

The quantity hp is the power input to the tower fan, and hpo ј 1:00hp ј 745:7W. The overall sound power level for induced-draft or

forced-draft cooling towers may be converted to octave band values by

using the following expression:

LWрoctave bandЮ ј LW _CF5 (5-31)

The values for the conversion factor CF5 are given in Table 5-8.

The only source of noise for natural-draft towers, as shown in Fig. 5-3,

is the water-generated noise. If the cooling tower packing extends below the

air inlet opening of the tower, noise due to water splashing over the packing

material is radiated directly from the tower. In addition, the water falling

from the packing material produces noise as it strikes the surface of the

water in the pond at the bottom of the tower.

Noise Sources 179

TABLE 5-8 Conversion Factor CF5 to Convert from the Overall

Sound Power Level to Octave Band Sound Power Levels for

Induced-Draft and Forced-Draft Cooling Towers

Cooling tower type

Octave band center frequency, Hz

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

Propeller fan, induced 6 5 7 9 16 21 29 35

Centrifugal fan, forced 4 5 9 10 14 16 22 31

Copyright © 2003 Marcel Dekker, Inc.

The overall sound power level for noise from a natural-draft tower

may be determined from the following correlation (Ellis, 1971):

LW ј 10 log10рmghЮ ю 10 log10Ѕ0:95рhp=hЮ2 ю 1:80рho=hЮ2_ ю 60:0

(5-32)

The quantities in Eq. (5-32) and the required units are defined as follows:

m ј mass flow rate of cooling water, kg/s

g ј local acceleration due to gravity ј 9:806m=s2

h ј total distance that the water falls in the tower, m

hp ј depth of the packing material below the tower ring beam, m

ho ј distance between the bottom of the packing and the pond

surface, m

The A-weighted sound power level may be found from the overall sound

power level:

LWрAЮ ј LW ю 0:1 dB (5-33)

The sound pressure level at any distance r from the edge of the pond

may be determined from the following relations, depending on whether the

receiver is near the tower or farther from the tower. The region near the

tower is defined by the following relationship:

r < r_ ј 1

2DtfЅ1 ю 2рhp ю hoЮ=Dt_1=2 _ 1g (5-34)

180 Chapter 5

FIGURE 5-3 Natural-draft cooling tower.

Copyright © 2003 Marcel Dekker, Inc.

The quantity Dt is the diameter of the tower. If the tower is rectangular with

plan dimensions a _ b, use Dt ј р4ab=_Ю1=2. The relationship between the

overall sound pressure level and the overall sound power level for the region

near the tower is as follows:

Lp ј LW _ 10 log10f_Dtрhp ю hoЮЅ1 ю р2r=DtЮ_g ю 10 log10р_ocWref=p2

ref Ю

(5-35)

For atmospheric air around 300K (808F), the numerical value of the last

term in Eq. (5-35) is 0.1 dB.

For the region farther from the tower, r             r_, the following expression

may be used to determine the overall sound pressure level:

Lp ј LW ю 10 log10рQЮ _ 20 log10рrЮ _ 10 log10р4_p2

ref=_ocWref Ю

(5-36)

For atmospheric air around 300K (808F), the numerical value of the last

term in Eq. (5-36) is 10.9 dB. The quantity Q is given by the following

expression:

Q ј

4 tan_1fЅ1 ю рDt=rЮ_1=2g

_Ѕ1 ю рDt=rЮ_

(5-37)

The argument of the inverse tangent function in Eq. (5-37) must be

expressed in radians when making numerical calculations.

The octave band values of the sound power level may be obtained

from the overall sound power level by using the following conversion:

LWрoctave bandЮ ј LW _ CF6 (5-38)

Values of the conversion factor CF6 are given in Table 5-9.

Example 5-4. A natural-draft cooling tower has a mass flow rate of water

through the tower of 120 kg/s (196,000 lbm=hr). The tower diameter is 7.50m

(24.6 ft). The packing extends 3.00m (9.8 ft) below the tower ring, and the

Noise Sources 181

TABLE 5-9 Conversion Factor CF6 to Convert from the

Overall Sound Power Level to Octave Band Sound

Power Levels for Natural-Draft Cooling Towers

Octave band center frequency, Hz

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

CF6, dB 17.7 19.4 19.8 13.0 7.8 6.3 5.3 7.2

Copyright © 2003 Marcel Dekker, Inc.

open height of the tower is 6.50m (21.3 ft). The water falls a total distance of

20m (65.6 ft) in the tower. Determine the overall sound pressure level at a

distance of 25m (82.0 ft) from the edge of the tower pond.

The overall sound power level may be determined from Eq. (5-32):

LW ј 10 log10Ѕр120Юр9:806Юр20Ю_ ю 10 log10Ѕр0:95Юр3=20Ю2

ю р1:8Юр6:5=20Ю2_ ю 60:0

LW ј 10 log10р23,534Ю ю 10 log10р0:0214 ю 0:1901Ю ю 60:0

LW ј 43:7 ю р_6:7Ю ю 60:0 ј 97:0dB

The characteristic distance for the cooling tower may be evaluated

from Eq. (5-34):

r_ ј 1

2

             

р7:5ЮfЅ1 ю р2Юр3:0 ю 6:5Ю=р7:5Ю_1=2 _ 1g ј 1

2

             

р7:5Юр0:8797Ю

r_ ј 3:30m р10:8 ftЮ

For this problem, the location r ј 25m > r_ ј 3:30 m; therefore, the sound

field corresponds to far-field conditions. We must use Eq. (5-36) to evaluate

the sound pressure level.

The directivity factor may be calculated from Eq. (5-37):

Q ј р4Ю tan_1fЅ1 ю р7:50=25Ю_1=2g

р_ЮЅ1 ю р7:50=25Ю_ ј р0:9794Ю tan_1р1:1402Ю ј 0:8333

The overall sound pressure level may be evaluated:

Lp ј 97:0 ю 10 log10р0:8333Ю _ 20 log10р25Ю _ 10:9

Lp ј 97:0 ю р_0:8Ю _ 28:0 _ 10:9 ј 57:3dB

Since all factors are independent of frequency, the A-weighted sound level

may be found from Eq. (5-33) in terms of the sound pressure level.

LA ј Lp ю 0:1 ј 57:3 ю 0:1 ј 57:4 dBA

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