Variation in sound power with flowrate

AXIAL FLOW FAN

подпись: axial flow fan Variation in sound power with flowrateAt 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—

подпись: 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

подпись: figure 14.25 variation in fan noise levels with speed according to motor type and control methodA 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

Variation in sound power with flowrate

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


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