Variation in sound power with flowrate
AXIAL FLOW FAN |
At a constant fan speed, the sound power generated will be dependent on the system resistance against which the fan has to operate. It is, therefore, of importance to ensure that this has been correctly calculated.
The change in noise at constant fan speed for some typical designs was shown in Figure 14.17. Differences up to 15 dB are common and will occur quite sharply if the characteristic contains a marked stall point.
LEGEND A — MAX STARTING TORQUE B — ‘HIGH’ STARTING TORQUE C — MIN STARTING TORQUE D-GENERATED SINUSOIDAL SUPPLY "T — r— |
In recent years, variable air volume systems have become of great importance and are recognised as an energy efficient solution to the whole question of air conditioning. It is rare for a building to continuously require the design flowrate determined by temperature, occupancy, solar heat gain, relative humidity or other criteria. Some percentage of the maximum flowrate must, therefore, be delivered by the fan.
Figure 14.25 Variation in fan noise levels with speed according to motor type and control method |
A distribution curve (Figure 14.24) can be constructed and this indicates the percentage running time against percentage flow.
10
UJ
CL
0
_J
1
0
40 60 80
% DESIGN FLOWRATE Figure 14.24 Typical fan operating load profile
100
How this affects the noise produced, depends on the method used.
A) Simple damper control: in this case the fan simply works along its characteristic. Noise will generally increase according to fan design as previously stated.
B) Speed control: noise may be expected to reduce with decreasing fan speed according to the relationship
, log n2
PWL2 — PWL1 = 10a
Log N,
Where:
A = exponent between 3.5 and 7.5 with an average value of say 5.5.
Note: Because of resonances and phenomena still the subject of analysis, the variation may not be continuous. It is also likely to vary at different points along the fan characteristic.
There can be “peaks” on the graph (see Figure 14.13). It should also be remembered that this curve does not take account of motor noise. Where the motor is contained within the fan duct, as with a typical direct driven axial flow fan, the reduction in noise may be less. With certain types of inverter control the electrical waveform may be sufficiently distorted to increase the motor noise at reduced speed. Figure 14.25 shows the overall effect.
Caution should still be used when controlling by inverter on lightweight fans, or where fans are mounted on flimsy structures and in any installation where the fan is run in an open environment. In these situations the torque characteristics suited to fans should always be utilised, (V oc f2). The fan type will also affect the amount of noise radiated into the system, and if possible indirect drive should be considered for critical applications.
It is likely that with careful application of damping materials and the design of fan hardware to suit the problems of general inverter drives, a reduction in the resultant noise level could be
NOISE LEVELS ARE IN-DUCT OUTLET LPA IN 710mm o DUCTWORK re: 2×1 O’5 N/m2
CENTRIFUGAL FAN |
JT |
Expected. However, this can result in considerable increase in initial fan cost, and may make the option of inverter control less attractive.
Some inverters are available that have a fundamental switching frequency in the ultrasonic range, and these noise problems can then be eliminated.
C) Inlet vane control: this type of control may be used with mixed flow fans, with a noise penalty of up to 10dB at small opening angles (see Table 14.6). It should not be used with axial fans where the noise penalties are severe (Table 14.7) With centrifugal fans, the effect on noise down to about 50% design flow is small, but below this figure instability can be a problem with the wider high flow designs, such that noise will increase (Table 14.8).
Vane Angle |
Flowrate M3/s |
Fan Pressure Pa |
In duct PWLdB re10’12W |
||||||||
Tot |
63 |
125 |
250 |
500 |
1k |
2k |
4k |
8k |
|||
Full Open |
2.4 |
410 |
91 |
84 |
79 |
83 |
86 |
83 |
80 |
75 |
67 |
O CO |
2.37 |
405 |
92 |
85 |
80 |
84 |
87 |
84 |
80 |
75 |
67 |
70° |
2.3 |
382 |
94 |
88 |
82 |
86 |
88 |
85 |
80 |
75 |
67 |
60° |
2.17 |
356 |
96 |
91 |
85 |
87 |
89 |
86 |
81 |
76 |
67_ |
I O |
2.05 |
320 |
97 |
94 |
87 |
89 |
90 |
87 |
81 |
76 |
67 |
40° |
1.89 |
277 |
99 |
96 |
89 |
90 |
90 |
87 |
81 |
75 |
67 |
30° |
1.67 |
221 |
100 |
98 |
91 |
90 |
90 |
86 |
80 |
75 |
67 |
To O |
1.39 |
154 |
98 |
96 |
90 |
89 |
88 |
84 |
79 |
74 |
67 |
10° |
0.76 |
56 |
100 |
98 |
92 |
89 |
87 |
85 |
80 |
76 |
69 |
Closed |
0 |
6 |
101 |
98 |
96 |
92 |
91 |
88 |
84 |
81 |
74 |
Table 14.6 Typical noise levels of mixed flow fan with inlet vane control |
Vane Angle |
Flow Rate M3/s |
Fan Pres Sure Pa |
In duct PWL dB re 10 12 W |
||||||||
Tot |
63 |
125 |
250 |
500 |
1k |
2k |
4k |
8k |
|||
Fan Only |
24.1 |
; Free inlet and delivery |
105 |
93 |
90 |
96 |
94 |
94 |
92 |
98 |
95 |
Full Open |
23.5 |
122 |
98 |
100 |
122 |
110 |
108 |
101 |
97 |
96 |
|
79° |
22.8 |
122 |
98 |
100 |
122 |
115 |
111 |
103 |
96 |
94 |
|
67° |
21.8 |
123 |
97 |
101 |
119 |
117 |
111 |
101 |
96 |
92 |
|
56° |
20.6 |
122 |
98 |
102 |
118 |
116 |
109 |
100 |
95 |
90 |
|
I to |
19.2 |
120 |
100 |
103 |
118 |
112 |
106 |
100 |
94 |
87 |
|
34° |
17.4 |
118 |
101 |
104 |
118 |
109 |
106 |
100 |
93 |
86 |
|
23" |
14.9 |
117 |
102 |
104 |
116 |
107 |
105 |
99 |
93 |
85 |
|
11° |
13.1 |
117 |
102 |
102 |
116 |
107 |
105 |
98 |
91 |
84 |
|
Closed I |
0 |
116 |
101 |
101 . |
115 |
106 |
104 |
97 |
90 |
83 —— 1 |
Table 14.7 Typical noise levels of axial flow fan with inlet vane control |
I Vane Angle |
Flow Rate M3/s |
Fan Pres Sure Pa |
In duct PWL dB re 10 12 W |
||||||||
Tot |
63 |
125 |
250 |
500 |
1k |
2k |
4k |
8k |
|||
Fan Only |
4.85 |
790 |
107 |
104 |
100 |
96 |
92 |
93 |
87 |
82 |
74 |
Full Open |
4.73 |
751 |
107 |
105 |
101 |
97 |
93 |
94 |
88 |
83 |
75 |
IT |
4.37 |
641 |
106 |
103 |
100 |
97 |
93 |
94 |
88 |
83 |
75 |
O> O |
4.12 |
570 |
106 |
102 |
100 |
97 |
93 |
94 |
88 |
83 |
75 |
54° |
3.90 |
520 |
106 |
102 |
100 |
97 |
94 |
95 |
89 |
84 |
76 |
24° |
2.30 |
178 |
107 |
103 |
101 |
98 |
95 |
96 |
90 |
86 |
78 |
Closed |
0.34 |
4 |
108 |
104 |
102 |
99 |
96 |
97 |
91 |
88 |
80 |
Table 14.8 Typical noise levels of centrifugal fan with inlet vane control 234 FANS & VENTILATION |
BE B |
Flow |
Fan |
In duct PWL dB re 10’12 W |
||||||||
Rate M3/s |
Pressure Pa |
Tot |
63 |
125 |
250 |
500 |
1k |
2k |
4k |
8k |
|
Fan Only |
4.85 |
790 |
107 |
104 |
100 |
96 |
92 |
93 |
87 |
82 |
74 |
Full Open |
4.85 |
790 |
107 |
104 |
100 |
96 |
92 |
93 |
87 |
82 |
74 |
80% |
4.51 |
683 |
105 |
103 |
99 |
95 |
92 |
93 |
87 |
82 |
74 |
60% |
4.2 |
592 |
103 |
100 |
96 |
92 |
90 |
91 |
85 |
80 |
72 |
40% |
3.90 |
520 |
100 |
95 |
95 |
92 |
88 |
89 |
83 |
78 |
70 |
20% |
2.30 |
178 |
100 |
91 |
97 |
92 |
86 |
88 |
82 |
77 |
69 |
Closed |
0 |
0 |
99 |
91 |
97 |
91 |
87 |
88 |
82 |
77 |
69 |
Table 14.9 Typical noise levels of centrifugal fan with disc throttle control |
D) Disc throttle control: this patented control for centrifugal fans (UK number 2,119,440B) varies the flow by narrowing the effective blade width and a monotonic reduction in noise with decreasing flowrate is achieved (see Table
14.9) . The reductions are especially noteworthy at low frequencies where other controls are ineffective.
E) Variable pitch in motion axial fans: noise reduces with decreasing flow throughout the whole range of performance and no discontinuities or distortions are apparent (Figure 14.26). This graph also shows the differences in
FAN CODE: 100 JG 40A-4-9 REV./MIN: 1470 Hz:50
CORRECTION TO D TYPE OUTLET TOTAL SOUND POWER LEVEL dB
F |
TOTAL |
63 |
125 |
250 |
500 |
1k |
2k |
4k |
8k |
^0 |
-6 |
-10 |
-17 |
-9 |
-18 |
-18 |
-20 |
-22 |
-25 |
Bo |
-4 |
-8 |
-16 |
-9 |
-17 |
-19 |
-17 |
-24 |
-31 |
^0 |
-10 |
-17 |
-23 |
-16 |
-19 |
-18 |
-16 |
-27 |
-33 |
Do |
0 |
-7 |
-12 |
-2 |
-15 |
-20 |
-20 |
-30 |
-35 |
Al |
-8 |
-12 |
-19 |
-10 |
-20 |
-20 |
-18 |
-23 |
-29 |
B, |
-10 |
-28 |
-20 |
-15 |
-22 |
-20 |
-15 |
-21 |
-24 |
°l |
-11 |
-21 |
-26 |
-15 |
-26 |
-20 |
-17 |
-26 |
-30 |
-5 |
-10 |
-19 |
-7 |
-12 |
-13 |
-19 |
-27 |
-33 |
|
FtVmin. 9 |
, , |
, |
20000 . ‘ j |
, |
40000 |
, |
Mm |
0 50000 P, = 1.2kg/m3 Pm = 1 -202kg/m3 TYPE 8.D |
Spectra for both inlet and outlet noise according to ducting configuration.
Posted in Fans Ventilation A Practical Guide