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Technical Bulletin TBN021.2/2001

PREDICTION OF FAN SOUND POWER Fan noise is a function of the fan design (Kw), volume flow rate (Q), total pressure (Pt) and efficiency (c). The sound power generation of a given fan performing a given duty is best obtained from the fan manufacturer’s actual test data taken under approved test conditions. However, if such data are not readily available, the octave band sound power levels for various fans can be estimated by the following procedure. Fan noise can be rated in terms of the specific sound power level, which is defined as the sound power level generated by a fan operating at a capacity of 1m3/s (or 1 cfm) and a pressure of 1 Pa (or 1 in. of water). By reducing all fan noise data to this common denominator, the specific sound power level serves as a basis for direct comparison of the octave band levels of various fans and as a basis for a conventional method of calculating the noise levels of fans at actual operating conditions. Blade Passage Frequency ( Bf )

Recent study shows that on a specific sound power level basis, small fans are somewhat noisier than large fans. While any such size division is necessarily arbitrary, the size divisions indicated are practical for estimating fan noise. Fans generate a tone at the blade passage

frequency, and the strength of this tone depends, in part, on the type of fan. To account for this blade passage frequency, an increase should be made in the octave band into which the blade frequency falls. The number of decibels to be added to this band is called the blade frequency increment (BFI). Blade frequency (Bf ) is :

Bf = (rpm x no. of blades)/60 The number of blades and the fan rpm can be obtained from the fan selection catalogue. If this catalogue is unavailable, Table 1 may be used for estimation. Fan Type

Octave Band in which BFI occurs

Centrifugal Airfoil, backward curved backward inclined Forward curved Radial blade, pressure

blower Vaneaxial Tubeaxial Propeller

250 Hz 500 Hz 125 Hz 125 Hz 63 Hz 63 Hz

Table 1 Octave Band in which Blade Frequency Increment (BFI) Occurs*

Specific sound power levels and blade frequency increments are listed in Table 2. For a more complete desciption of fan types, construction, and applications, see Technical Bulletin TBN007.0 – Fan Types.

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Technical Bulletin - TBN021.2/2001

Octave Band Center Frequency, Hz Fan Type Wheel Size 63 125 250 500 1000 2000 4000 8000 BFI Centrifugal Airfoil, backward

curved, backward inclined

Forward curved Radial blade Pressure blower

> 0.9m < 0.9m All > 1m 1m to 0.5m < 0.5m

32 36

47 45 55 63

32 38

43 39 48 57

31 36

39 42 48 58

29 34

33 39 45 50

28 33

28 37 45 44

23 28

25 32 40 39

15 20

23 30 38 38

13 15

20 27 37 37

3

2

8

Vaneaxial

> 1m < 1m

39 37

36 39

38 43

39 43

37 43

34 41

32 38

26 35

6

Tubeaxial

> 1m < 1m

41 40

39 41

43 47

41 46

39 44

37 43

34 37

32 35

5

Propeller

All 48 51 58 56 55 52 46 42 5

Table 2 – Specific Sound Power Levels (dB re 1pW) and Blade Frequency Increments (BFI) for Various Type Fans

Point of Operation

The specific sound power levels given in Table 2 are for fans operating at or near the peak efficiency point of the fan performance curve. This conforms with the recommended practice of selecting fan size and speed so that operation falls at or near this point; it is advantageous for energy conservation and corresponds to the lowest noise levels for that fan. If, for any reasons, a fan is not or cannot be selected optimally, the noise level produced will increase and a correction factor C as shown in Table 3 shall accounts for this. Static Efficiency % of Peak

Correction Factor

90 to 100 85 to 89 75 to 84 65 to 74 55 to 64 50 to 54

0 3 6 9 12 15

Table 3 Octave Band in which Blade Frequency Increment (BFI) Occurs*

This correction factor should be applied to all octave bands.

Prediction of Fan Sound Power (Lw)

Sound power levels at actual operating conditions may be estimated by using the actual fan-volume flow rate and fan pressure, as:

cPP

log20QQ

log10KL11

ww +⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

where :

Lw = estimated sound power level of fan (dB re 1pw) Kw = specific sound power level (see table 2) Q = flow rate, m3/s (cfm) Q1 = 0.000472 when flow is in m3/s

(1 when cfm) P = pressure drop in pascals (in.H2O) P1 = 249 when pressure in pascals (1 when in.H2O) c = correction factor in dB, for point of fan operation.

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Values of the estimated sound power level are calculated for all eight bands , and the BFI is added to the octave band in which the blade passage frequency falls. Example : A forward curved fan FDA500C is selected to supply 4.15 m3/s at 750Pa. It has 41 blades and operates at 904rpm with static efficiency of 56%. What is the estimated sound power level ? Step 1 : Obtain the specific sound

power level (Kw) from Table 2 for forward curved.

Step 2 : Calculate the additional sound

power levels due to the volume flow rate and pressure.

4957.944.39

249750

log20000472.0

15.4log10

PP

log20QQ

log1011

=+=

⎟⎠⎞

⎜⎝⎛+⎟

⎠⎞

⎜⎝⎛=

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛

Technical Bulletin - TBN021.2/2001

Step 3 : Calculate the Bf to determine

the BFI falls at which octave bands.

Bf = (rpm x no. of blades)/60

= (904x41)/60 = 617 Hz The BFI falls on 500Hz octave

bands. (i.e between lower f, 355Hz to upper f, 710 Hz)

Step 4 : Determine correction factor c

for off-peak efficiency. From catalogue performance

data, this fan shows a peak efficiency of 62%.

% of peak static efficiency = (56/62)x100 = 90.3.

From Table 3, c = 0. Combine all 4 steps as shown in the Table 4. Lw(Linear) = 98.2 dB LwA = 85.8 dB(A)

Table 4 – Sample Calculation

Octave Band Center Frequency, Hz Sound Reference 63 125 200 500 1000 2000 4000 8000 Power

Step 1 47 43 39 33 28 25 23 20 Step 2 49 49 49 49 49 49 49 49 Step 3 - - - 2 - - - - Step 4 0 0 0 0 0 0 0 0 Lw(dB)(Linear) = 96 92 88 84 77 74 72 69 98.2 A-weighted factor LwA (dB(A))=

-25.5 70.5

-15.5 76.5

-8.5 79.5

-3.0 81

0 77

+1 75

+1 73

-1 68

85.8