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Fan Noise Production - Noise Control - Handout, Exercises of Noise Control

National Institute of Industrial EngineeringNoise Control

Some of topics included in this course are: Fundamentals of Acoustics, Levels and Decibels, Divergence and Directivity, Hearing, Human Response to Noise, Frequency Analysis, Sound Sources and Fields, Room Acoustics, Sound Power, Noise Barriers, Outdoor Sound Propagation, Helmholtz Resonator and Vibration Control. Key points of this lecture are: Fan Noise Production, Sound Power Level, Specific Sound Power Level, Volume Flow Rate, Total Pressure, Blade Frequency Increment, Blade Passing Frequency

Typology: Exercises

2012/2013

Uploaded on 10/02/2013

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Fan noise 7/12/2000 11.1
11. FAN NOISE PREDICTION
The sound power produced by centrifugal and axial fans can be approximated by a
simple equation (ref. ASHRAE Handbook)
LW = KW + 10 log10 Q + 20 log10 P +BFI +CN
where:
LW= sound power level (dB)
KW = specific sound power level depending on the type of fan (see Fig 9-3), from
empirical data provided by fan manufacturer
Q = volume flow rate (cfm)
P = total pressure (inches of H20)
BFI = Blade Frequency Increment = correction for pure tone produced by the blade
passing frequency (bpf) from Fig 9-3, add this correction only to the octave band
whose center frequency is closest to the blade passing frequency.
bpf= blade passing frequency = #of blades × RPM/60 (Hz)
CN = efficiency correction (because fans that are operated off their optimum flow
conditions get noisier)
C
N = 10 + 10 log10 (1-η)/η typical values:
η Cn
90% 0
75% 5.2
40% 12.2
η= Hydraulic efficiency of the fan = Q×P/(6350×HP)
HP = nominal horsepower of the fan drive motor
pf3
pf4
pf5

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11. FAN NOISE PREDICTION

The sound power produced by centrifugal and axial fans can be approximated by a simple equation (ref. ASHRAE Handbook)

LW = K (^) W + 10 log 10 Q + 20 log 10 P +BFI +C (^) N where: LW= sound power level (dB) K (^) W = specific sound power level depending on the type of fan (see Fig 9-3), from empirical data provided by fan manufacturer Q = volume flow rate (cfm) P = total pressure (inches of H 2 0) BFI = Blade Frequency Increment = correction for pure tone produced by the blade passing frequency (bpf) from Fig 9-3, add this correction only to the octave band whose center frequency is closest to the blade passing frequency. bpf= blade passing frequency = #of blades × RPM/60 (Hz) C (^) N = efficiency correction (because fans that are operated off their optimum flow conditions get noisier) C (^) N = 10 + 10 log 10 (1-η)/η typical values: η Cn 90% 0 75% 5. 40% 12. η= Hydraulic efficiency of the fan = Q×P/(6350×HP) HP = nominal horsepower of the fan drive motor

Fan Application

The choice of a fan depends on the desired ventilation requirements (volume, pressure, density, and speed) and other considerations including noise, initial cost, operating costs, environment, etc. Aerodynamic selection of type and size can be done with the aid of charts such as Figure 95 and 102 (ref. Fan Engineering, Buffalo Forge, 1970). Figure 17 (ref. Handbook for Mechanical Engineers, Baumeister and Marks) compares data for various commercial fan types. Specific diameter (DS) and efficiency vs. specific speed (NS) are shown. p=pressure drop (inches water), D=fan wheel diameter (ft), d= density of air (.075 lb/ft 3 at standard temperature and pressure), Q = flow volume (cfm), RPM = fan rotational speed (rpm).

Q

p d D D p d

RPM Q

N S S

(^14) (^34)

Generally, efficiency increases and fan size decreases as specific speed increases. This figure can be used to determine the most efficient size and type of fan for a particular application.

Fan Laws: Size and Speed

Fan performance can be predicted over a wide range of sizes and speeds using basic scaling relations (ref. Handbook of Acoustical Measurements and Noise Control, by C. Harris, 1991).