HomeMy WebLinkAbout32 t
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+ 101og1oQ + 20log1oP+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 x RPM/ 60 (Hz)
CN = efficiency correction (because fans that are operated off their optimum flow
conditions get noisier)
CN = 10+ 10 loglo (1-i1)/ 11 typical values:
1 Cn
90% 0
75% 5.2
40% 12.2
11= Hydraulic efficiency of the fan = QxP/ (6350xHP)
HP= nominal horsepower of the fan drive motor
216 FAN ENGINEERING—BUFFALO FORGE COMPANY
mpa-'loo
95 I I
¢w gOUNE FOYER live-,WT 50
9p
WO
0`85 - FCl 80Z-40 POWER LEYEI'PI-g,
pn.60
UERE Q2 20 siceR 10OO%
16
so
8
6 BO(0 4
w E��'C{ -� 70 U
I.2 F�o"�
z2 iipp%A."1"11111-441':-4114
"1"- so di cul-
t...2
aZO 1.050WS .8 / �4
w 6 etc.`c`O \ 40 1 2 gW
4__ 30 : S
.o BPEc1 JC_014METER 2p a I =
MO
10 52
WO B t
0 2 4 6 8 10 12 14 16 O w 0
CAPACITY IN 1000'S OF CFM
Figure 95—Typical Constant Speed Performance Curves
for Ventilating Fans
Fan noise 7/12/2000 11.1
e 120 30
S O.u•P.
EV
J 110 �,
120 9
J 100 2
A 110 8 `--
O
90
N NI ' 7.411111.1.919W100 W.
I. R L L V W
90 th
W
IAT, -F W
g TO y
0 60t 12
N
sou to
F
m ` 20 6.11
P
gI 20 G14 ♦_
10 W 2
0
0 2 4 6 • 10 I2 N 16 II 20 0 0
CAPACITY IN 1000'S OF CFM
Figure 102—Typical Constant Speed Performance Curves for Vaneaxial Fans
Fan Application
3.10 1
The choice of a fan depends on the desired 2'6
ventilation requirements (volume,pressure, 2 4 X31111: a mrdts»84884 �
density,and speed)and other considerations 2 2 a - tArtsNi.
FC "Forometliv Cwild goat q
including noise,initial cost,operating costs, 12 g'armaive nr -0881881 r,p 80811*
1 8c 14X,- kssol Ilton
environment,etc. Aerodynamic selection of a.6 Eft I
type and size can be done with the aid of s.4
1111
charts such as Figure 95 and 102(ref. Fan ' 2 � IIS
� v
Engineering,Buffalo Forge, 1970). Figure 17 0# 1111111111141"1114111 11....i.,
_
(ref. Handbook for Mechanical Engineers, OA
Baumeister and Marks) compares data for o,
various commercial fan types. Specific
diameter (Ds)and efficiency vs.specific speed loo 11�®® X11111
01111111 1111
1111
am
(Ns)are shown. p=pressure drop (inchesI to I lira
water),D=fan wheel diameter (ft),d= density '- 1111
111
of air (.075 lb/ ft3 at standard temperature and f 20 viii
pressure),Q =flow volume (cfm),RPM =fan 'alital' � ��
rotational m speed r '° ° W,.
P (rpm). a tow 04,020:w t1C0:1
q *Smolt speed
Ns /�/ld DS = (Pl d)�D ,t7. Symeific *owed.
lP ) cue diameter a ad A-
Id (teary for co mereial sate le islet fags type*
Generally,efficiencyincreases and fan size
BC—backwardly cu blade®
BCA—backwardly curved*Weil blades
decreases as specific speed increases. This w..�,forrardly blade*
WV—mind-tip blades
figure can be used to determine the most AV—axial fan
efficient size and type of fan for a particular D.'"` `X 12
application.
Fan noise 7/12/2000 11.2
II
Noise Comparison
For lowest noise output,fans should always be operated near their peak efficiency
point. A common mistake is to use a fan that is too small (or too large)for the
application,so that it cannot be run at its most efficient point. Variable airflow
applications can also cause noise problems. The cheapest way to achieve variable
volume (and the noisiest) is with VAV (Variable air volume) units,which basically
throttle the flow with louvers. A better way from a noise standpoint (but more
expensive),is a variable speed motor drive. The typical noise characteristics of various
fans are compared in Table 1 below.
Table 1. Comparison of noise from various fan types
Fan Type Noise(broad band) Blade passing tone Flow
Centrifugal
Airfoil blades Lowest Moderate Very efficient
Backward Inclined Blades Lower Moderate
Forward Inclined Blades Moderate Lowest Low pressure drop
Radial Blades High applications
Axial g High
Vane Higher than Can be high,depends Very efficient
centrifugal on flow obstructions
Tube More than vane °°
Propeller Highest
Fan noise 7/12/2000 11.3
SPECIFIC
SOUND-POWER
GO
TYPE DESIGN LEVEL,Kw BFI APPLICATIONS CD
tJ
CENTER FREQUENCY-Hz Highest efficiency of all centrifugal fan design contains
]0 to 16 blades of airfoil shape.
--IP. o Q o Used for general heating,ventilating,and air-condition-
,\ �+ v� $ 25 S 8 in systems, usually applied to central station units
AIRFOIL 0 3 where the horsepower saving will be significant. Can be
• dB re 10-"z watt and used onto low,medium,and high-pressure systems and will
1 cfm at 1 inch ftp operate satisfactorily in parallel.
Is also used in large sizes,for clean-air industrial appli- T
35 35 34 32 31 26 18 !0 cations where power savings will be significant. Can be D
used on industrial exhaust systems,where the air-clean- 2
ing system is of high efficiency.
>
co Z
---0. Efficient slightly only slihtly less than that of the airfoil v
BACKWARD fn. Contains 10 to 16 blades. T
W
INCLINED Uased for the same eneral ar
BACKWARD 35 35 34 32 31 26 18 10 3 g ppilations as the airfoil
U CURVED fan. Can be used in industrial applications where the gas 0
is essentially clean,but does not meet the standards re-
rai
qured for airfoil fan selection. N
N
ITI
m
—a. Simplest of all centrifugal fans-relatively low effi-
• ciency,usually has 6 to 10 blades;includes both radial z
RADIAL 48 45 43 43 38 33 30 29 5-8 blades(R),and modified radial blades(M). 0
�r# Used primarily for industrial exhaust,including dirty
gas fans and recirculating gas fans. This design also used to
for high-pressure industrial applications, m
Efficiency less than the airfoil and backwardly curved
—41. fans,this fan Is usually fabricated of lightweight and
0 low-costlCantypes conandstruction.operates Itat maythe havelowest fromspeed.24 to 64
FORWARD 40 38 38 34 28 24 21 15 2 gablades. This design will be the smallest of the centeifu-
CURVED
Used primarily in low-pressure heating,ventilating, S
and au-conditioning applications, such as: domestic •
furnaces,small central station units,and packaged air- tD
s conditioning equipment.
High-efficiency axial flow fan with airfoil blades and
high pressure capability. Blades may be fixed or adjust-
"1` able and the hub diameter is usually greater than 50 per
cent of the fan tip diameter. There may be from 3 to -
I`I ---9.• 42 39 41 42 40 37 35 25 6-8 16 blades. This fan design has guide vanes downstream
VANEAXIAL from the wheel which permits good air flow pattern on co
the discharge side of the fan.
1' Used for general hes ting,ventilating,and air-condition-
ing applications in low,medium,and high-pressure sys-
tems. May also be used in industrial applications such
as:drying ovens,paint spray booths,and fume exhaust
systems.
This fan is more efficient than the propeller fan design m
and can develop a more useful pressure capability. The D
number of blades may vary from 4 to 8 and the hub is Z
TUBEAXIAL —R 44 42 46 44 42 40 37 30 6-8
usually about 50 per cent of the fan tip diameter. The
blades may be of airfoil or single thickness cross-section. D
The fan is built without downstream guide vanes. z
Used in low- and medium-pressure ducted heating,
ventilating,and air-conditioning applications where the
w - poor air flow pattern downstream from the fan is not '}I
..l detrimental. This fan is also used in some industrial ap- f
plications such as:drying ovens,paint spray booths and 0
fume exhaust systems.
CO)
Low efficiency wheels are usually of inexpensive con- y
struction and are limited to very-low-pressure applies- ,.
tions. Usually contains 2 to 8 blades of single thickness m
PROPELLER i--0••• 51 48 49 47 45 45 43 31 5-7 construction attached to a relatively small hub. The g
housing is a simple circular ring or orifice plate.
This fan is used for low pressure, high-volume air- 2
moving applications such as air circulation within a 0
space or as exhaust fans in a wall or roof.
N
m
' ` This fan usually has a wheel similar to the airfoil or
,-fes- backwardly inclined wheel,described above,which is
built into an axial flow type housing. This results in
TUBULAR tower efficiencies than the centrifugal fans of similar
CENTRIFUGAL —i 46 43 43 38 37 32 28 25 4-6 wheel design. The air is discharged radially from the
wheel and must change direction by 90 degrees to flow
TT---- through the guide vane section. Used primarily for
low-pressure return-air systems in heating,ventilating, G.)
and air-conditioning applications. CD
W
Fig.9-3. Acoustic properties of various fan types.
Fan noise 7/12/2000 11.4
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).
FAN NOISE 41.19
Qu = Qb(du/db)3(nalnb) (41.3)
f'ta =Pab(daldb)2(nalnb)2 (41.4)
Pa =Pb(daldb)5(nalnb)3 (41.5)
LWa = Lwb + 70 log10(da/db) + 50 log10 (na/nb) (41.6)
where Q = volume flow rate, m3/s
p� = total pressure, kPa
P = fan power, kilowatts
LK, = sound power level, decibels re 1 picowatt
d = rotor diameter, meters
n = rotor speed, number of revolutions per minute
Note the additional subscripts:
a = data at required performance conditions
b = data at base curve performance conditions
Although Eq. (41.6) is less accurate than the equations which predict other
performance characteristics, it is sufficiently accurate for estimating purposes.
The fan laws express mathematically that when two fans are both members of
a similar series, their performance curves are similar, and at the equivalent point
of rating on each performance curve, the efficiencies are equal. The fan laws can
be applied only to one point of rating on the fan curve for each calculation; this
point can be used to calculate only the equivalent point on any new speed curve.
To define the new speed curve accurately, it is necessary to use enough individ-
ual base curve data points to calculate the new curve so that the new curve will
be defined with a minimum of interpolation error between the chosen points.
To apply the fan laws, it is necessary to have actual test data for one fan in the
same series. The use of the fan laws is restricted to cases where the linear di-
mensions of the larger or smaller fans are all proportional to the fan for which
there are test data.
A misunderstood parameter in fan noise generation is the speed of the fan.For
a given type of fan, volume flow rate, and pressure, there is one particular size
that is more efficient than all other sizes. Neither a larger size nor a smaller size
can be more efficient. It is a common misconception that if a larger fan is used
which runs at a slower speed, the noise will be reduced. This is in error; the
larger fan does run at a slower rotational speed, but it does not operate at peak
efficiency. Therefore, the noise is greater. While the larger fan is running at a
slower speed,the outer diameter of the larger rotor must run at the same velocity
as the outer diameter of the smaller rotor in order to develop the required pres-
sure. The specified pressure cannot be developed at lower velocities. Therefore,
the noise-generating mechanism is the same, and the noise produced by the larger
fan at lower speed is not less than that produced by the smaller fan. The change
in efficiency will result in higher noise levels for the larger fan. Once the optimum
size of fan for a particular application is determined, it is not possible to reduce
the noise levels by using a larger and slower-speed fan.
An exception to the above statement may occur if the speed change is suffi-
cient to lower the blade frequency significantly; since the human ear responds
more poorly at lower frequencies (see Fig. 17.1), there is a reduction in loudness
Fan noise 7/12/2000 11.5
,
. _.... -.,'
I
•. -:,, ., i
/ •
. . . ..
..''''''.:„.''.. , •,. ...,,,, ,' ' ':::
. .
. ,.
''. --- -; IIP . . . _ I .•,. ,
'P1114 111
... , , ...,
.. 4 . .
, • ,,,'„ ''
. .
.,. „...'
. ,
' '' 14 111110
1
-T.
...
-.,- -
-';--,
„._.
;•' '':-,".4 -,-,:,„, i , , . . ,,,:.
•-,,,,....--..„-K-...-:!-,..:..,--77T4i-,,,--4,-, - A .
,
,... '..!' • ;.':, :'It. ,„I
. f -,•-.---y' -1,
- -• -;-:-.--
,-..
--,!..,..,, - . .......
” .
.._,
-_.-
1 .
;•--*- ',-,-;„,-:•-;:: ' '' ' i
• -
I
t
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+ 101og1oQ + 20log1oP+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 x RPM/ 60 (Hz)
CN = efficiency correction (because fans that are operated off their optimum flow
conditions get noisier)
CN = 10+ 10 loglo (1-i1)/ 11 typical values:
1 Cn
90% 0
75% 5.2
40% 12.2
11= Hydraulic efficiency of the fan = QxP/ (6350xHP)
HP= nominal horsepower of the fan drive motor
216 FAN ENGINEERING—BUFFALO FORGE COMPANY
mpa-'loo
95 I I
¢w gOUNE FOYER live-,WT 50
9p
WO
0`85 - FCl 80Z-40 POWER LEYEI'PI-g,
pn.60
UERE Q2 20 siceR 10OO%
16
so
8
6 BO(0 4
w E��'C{ -� 70 U
I.2 F�o"�
z2 iipp%A."1"11111-441':-4114
"1"- so di cul-
t...2
aZO 1.050WS .8 / �4
w 6 etc.`c`O \ 40 1 2 gW
4__ 30 : S
.o BPEc1 JC_014METER 2p a I =
MO
10 52
WO B t
0 2 4 6 8 10 12 14 16 O w 0
CAPACITY IN 1000'S OF CFM
Figure 95—Typical Constant Speed Performance Curves
for Ventilating Fans
Fan noise 7/12/2000 11.1
e 120 30
S O.u•P.
EV
J 110 �,
120 9
J 100 2
A 110 8 `--
O
90
N NI ' 7.411111.1.919W100 W.
I. R L L V W
90 th
W
IAT, -F W
g TO y
0 60t 12
N
sou to
F
m ` 20 6.11
P
gI 20 G14 ♦_
10 W 2
0
0 2 4 6 • 10 I2 N 16 II 20 0 0
CAPACITY IN 1000'S OF CFM
Figure 102—Typical Constant Speed Performance Curves for Vaneaxial Fans
Fan Application
3.10 1
The choice of a fan depends on the desired 2'6
ventilation requirements (volume,pressure, 2 4 X31111: a mrdts»84884 �
density,and speed)and other considerations 2 2 a - tArtsNi.
FC "Forometliv Cwild goat q
including noise,initial cost,operating costs, 12 g'armaive nr -0881881 r,p 80811*
1 8c 14X,- kssol Ilton
environment,etc. Aerodynamic selection of a.6 Eft I
type and size can be done with the aid of s.4
1111
charts such as Figure 95 and 102(ref. Fan ' 2 � IIS
� v
Engineering,Buffalo Forge, 1970). Figure 17 0# 1111111111141"1114111 11....i.,
_
(ref. Handbook for Mechanical Engineers, OA
Baumeister and Marks) compares data for o,
various commercial fan types. Specific
diameter (Ds)and efficiency vs.specific speed loo 11�®® X11111
01111111 1111
1111
am
(Ns)are shown. p=pressure drop (inchesI to I lira
water),D=fan wheel diameter (ft),d= density '- 1111
111
of air (.075 lb/ ft3 at standard temperature and f 20 viii
pressure),Q =flow volume (cfm),RPM =fan 'alital' � ��
rotational m speed r '° ° W,.
P (rpm). a tow 04,020:w t1C0:1
q *Smolt speed
Ns /�/ld DS = (Pl d)�D ,t7. Symeific *owed.
lP ) cue diameter a ad A-
Id (teary for co mereial sate le islet fags type*
Generally,efficiencyincreases and fan size
BC—backwardly cu blade®
BCA—backwardly curved*Weil blades
decreases as specific speed increases. This w..�,forrardly blade*
WV—mind-tip blades
figure can be used to determine the most AV—axial fan
efficient size and type of fan for a particular D.'"` `X 12
application.
Fan noise 7/12/2000 11.2
II
Noise Comparison
For lowest noise output,fans should always be operated near their peak efficiency
point. A common mistake is to use a fan that is too small (or too large)for the
application,so that it cannot be run at its most efficient point. Variable airflow
applications can also cause noise problems. The cheapest way to achieve variable
volume (and the noisiest) is with VAV (Variable air volume) units,which basically
throttle the flow with louvers. A better way from a noise standpoint (but more
expensive),is a variable speed motor drive. The typical noise characteristics of various
fans are compared in Table 1 below.
Table 1. Comparison of noise from various fan types
Fan Type Noise(broad band) Blade passing tone Flow
Centrifugal
Airfoil blades Lowest Moderate Very efficient
Backward Inclined Blades Lower Moderate
Forward Inclined Blades Moderate Lowest Low pressure drop
Radial Blades High applications
Axial g High
Vane Higher than Can be high,depends Very efficient
centrifugal on flow obstructions
Tube More than vane °°
Propeller Highest
Fan noise 7/12/2000 11.3
SPECIFIC
SOUND-POWER
GO
TYPE DESIGN LEVEL,Kw BFI APPLICATIONS CD
tJ
CENTER FREQUENCY-Hz Highest efficiency of all centrifugal fan design contains
]0 to 16 blades of airfoil shape.
--IP. o Q o Used for general heating,ventilating,and air-condition-
,\ �+ v� $ 25 S 8 in systems, usually applied to central station units
AIRFOIL 0 3 where the horsepower saving will be significant. Can be
• dB re 10-"z watt and used onto low,medium,and high-pressure systems and will
1 cfm at 1 inch ftp operate satisfactorily in parallel.
Is also used in large sizes,for clean-air industrial appli- T
35 35 34 32 31 26 18 !0 cations where power savings will be significant. Can be D
used on industrial exhaust systems,where the air-clean- 2
ing system is of high efficiency.
>
co Z
---0. Efficient slightly only slihtly less than that of the airfoil v
BACKWARD fn. Contains 10 to 16 blades. T
W
INCLINED Uased for the same eneral ar
BACKWARD 35 35 34 32 31 26 18 10 3 g ppilations as the airfoil
U CURVED fan. Can be used in industrial applications where the gas 0
is essentially clean,but does not meet the standards re-
rai
qured for airfoil fan selection. N
N
ITI
m
—a. Simplest of all centrifugal fans-relatively low effi-
• ciency,usually has 6 to 10 blades;includes both radial z
RADIAL 48 45 43 43 38 33 30 29 5-8 blades(R),and modified radial blades(M). 0
�r# Used primarily for industrial exhaust,including dirty
gas fans and recirculating gas fans. This design also used to
for high-pressure industrial applications, m
Efficiency less than the airfoil and backwardly curved
—41. fans,this fan Is usually fabricated of lightweight and
0 low-costlCantypes conandstruction.operates Itat maythe havelowest fromspeed.24 to 64
FORWARD 40 38 38 34 28 24 21 15 2 gablades. This design will be the smallest of the centeifu-
CURVED
Used primarily in low-pressure heating,ventilating, S
and au-conditioning applications, such as: domestic •
furnaces,small central station units,and packaged air- tD
s conditioning equipment.
High-efficiency axial flow fan with airfoil blades and
high pressure capability. Blades may be fixed or adjust-
"1` able and the hub diameter is usually greater than 50 per
cent of the fan tip diameter. There may be from 3 to -
I`I ---9.• 42 39 41 42 40 37 35 25 6-8 16 blades. This fan design has guide vanes downstream
VANEAXIAL from the wheel which permits good air flow pattern on co
the discharge side of the fan.
1' Used for general hes ting,ventilating,and air-condition-
ing applications in low,medium,and high-pressure sys-
tems. May also be used in industrial applications such
as:drying ovens,paint spray booths,and fume exhaust
systems.
This fan is more efficient than the propeller fan design m
and can develop a more useful pressure capability. The D
number of blades may vary from 4 to 8 and the hub is Z
TUBEAXIAL —R 44 42 46 44 42 40 37 30 6-8
usually about 50 per cent of the fan tip diameter. The
blades may be of airfoil or single thickness cross-section. D
The fan is built without downstream guide vanes. z
Used in low- and medium-pressure ducted heating,
ventilating,and air-conditioning applications where the
w - poor air flow pattern downstream from the fan is not '}I
..l detrimental. This fan is also used in some industrial ap- f
plications such as:drying ovens,paint spray booths and 0
fume exhaust systems.
CO)
Low efficiency wheels are usually of inexpensive con- y
struction and are limited to very-low-pressure applies- ,.
tions. Usually contains 2 to 8 blades of single thickness m
PROPELLER i--0••• 51 48 49 47 45 45 43 31 5-7 construction attached to a relatively small hub. The g
housing is a simple circular ring or orifice plate.
This fan is used for low pressure, high-volume air- 2
moving applications such as air circulation within a 0
space or as exhaust fans in a wall or roof.
N
m
' ` This fan usually has a wheel similar to the airfoil or
,-fes- backwardly inclined wheel,described above,which is
built into an axial flow type housing. This results in
TUBULAR tower efficiencies than the centrifugal fans of similar
CENTRIFUGAL —i 46 43 43 38 37 32 28 25 4-6 wheel design. The air is discharged radially from the
wheel and must change direction by 90 degrees to flow
TT---- through the guide vane section. Used primarily for
low-pressure return-air systems in heating,ventilating, G.)
and air-conditioning applications. CD
W
Fig.9-3. Acoustic properties of various fan types.
Fan noise 7/12/2000 11.4
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).
FAN NOISE 41.19
Qu = Qb(du/db)3(nalnb) (41.3)
f'ta =Pab(daldb)2(nalnb)2 (41.4)
Pa =Pb(daldb)5(nalnb)3 (41.5)
LWa = Lwb + 70 log10(da/db) + 50 log10 (na/nb) (41.6)
where Q = volume flow rate, m3/s
p� = total pressure, kPa
P = fan power, kilowatts
LK, = sound power level, decibels re 1 picowatt
d = rotor diameter, meters
n = rotor speed, number of revolutions per minute
Note the additional subscripts:
a = data at required performance conditions
b = data at base curve performance conditions
Although Eq. (41.6) is less accurate than the equations which predict other
performance characteristics, it is sufficiently accurate for estimating purposes.
The fan laws express mathematically that when two fans are both members of
a similar series, their performance curves are similar, and at the equivalent point
of rating on each performance curve, the efficiencies are equal. The fan laws can
be applied only to one point of rating on the fan curve for each calculation; this
point can be used to calculate only the equivalent point on any new speed curve.
To define the new speed curve accurately, it is necessary to use enough individ-
ual base curve data points to calculate the new curve so that the new curve will
be defined with a minimum of interpolation error between the chosen points.
To apply the fan laws, it is necessary to have actual test data for one fan in the
same series. The use of the fan laws is restricted to cases where the linear di-
mensions of the larger or smaller fans are all proportional to the fan for which
there are test data.
A misunderstood parameter in fan noise generation is the speed of the fan.For
a given type of fan, volume flow rate, and pressure, there is one particular size
that is more efficient than all other sizes. Neither a larger size nor a smaller size
can be more efficient. It is a common misconception that if a larger fan is used
which runs at a slower speed, the noise will be reduced. This is in error; the
larger fan does run at a slower rotational speed, but it does not operate at peak
efficiency. Therefore, the noise is greater. While the larger fan is running at a
slower speed,the outer diameter of the larger rotor must run at the same velocity
as the outer diameter of the smaller rotor in order to develop the required pres-
sure. The specified pressure cannot be developed at lower velocities. Therefore,
the noise-generating mechanism is the same, and the noise produced by the larger
fan at lower speed is not less than that produced by the smaller fan. The change
in efficiency will result in higher noise levels for the larger fan. Once the optimum
size of fan for a particular application is determined, it is not possible to reduce
the noise levels by using a larger and slower-speed fan.
An exception to the above statement may occur if the speed change is suffi-
cient to lower the blade frequency significantly; since the human ear responds
more poorly at lower frequencies (see Fig. 17.1), there is a reduction in loudness
Fan noise 7/12/2000 11.5
,
. _.... -.,'
I
•. -:,, ., i
/ •
. . . ..
..''''''.:„.''.. , •,. ...,,,, ,' ' ':::
. .
. ,.
''. --- -; IIP . . . _ I .•,. ,
'P1114 111
... , , ...,
.. 4 . .
, • ,,,'„ ''
. .
.,. „...'
. ,
' '' 14 111110
1
-T.
...
-.,- -
-';--,
„._.
;•' '':-,".4 -,-,:,„, i , , . . ,,,:.
•-,,,,....--..„-K-...-:!-,..:..,--77T4i-,,,--4,-, - A .
,
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