Attenuation. Witt&Sohn AG Oct-14

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Shon Maxwell
5 years ago
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1 Noise Generation & Attenuation

2 CONTENT Noise Generation & Attenuation 1. General 2. Jet fans 3. Axial Fans 2

3 General Sound Power L W depends on Vibrations Number of blades Turbulences Rotational speed Power consumption Sound power as a function of fan capacity and pressure at η max. 3

4 General Sound Pressure L P,r : depends on distance r from source L W Think of a soap bubble (constant amount of soap) The larger the bubble becomes (r)... the thinner the thickness of the sphere becomes The thickness of the sphere is proportional with L P,r L W r L P,r 4

5 General Sound Pressure L P,r : L P,r = L W L A Sphere A Sphere L P,r = L W 10 log ( A ) Sphere L P,r A Sphere = 4 π r 2 L W r A Semi-sphere = 2 π r 2 5

6 General Sound Measurement: Envelope Surface Method (ISO 13347) L P,4 L P,1 L P,2 L P,3 L W r L W = n i= 1 log n ( L ) P, i + 10 log ( A ) cube, r 6

7 General Sound Measurement: Free inlet/ outlet (DIN ) free field, 3m, L P,3m L W L P,3m 7

8 General Sound Measurement: Free inlet/ outlet (DIN ) free field, 3m, 45 free field, 10m, 45 L P,3m L W L P,3m L P,10m L P,10m 8

9 General Sound Measurement: Free inlet/ outlet (DIN ) free field, 3m, 45 Casing break-out Noise free field, 1m, 90 r L W L P,r 9

10 CONTENT Noise Generation & Attenuation 1. General 2. Jet fans 3. Axial Fans 10

11 Jet Fans Corrections of Noise Level: Batteries of Parallel Jet Fans No. of Parallel Fans DL P, install [db]

12 Jet Fans Corrections of Noise Level: from free field (3m, 45 ) to tunnel Tunnel Wall Type DL P, tunnel [db] Tiled Walls Approx.. +6 Concrete Walls Approx Blasted Rock ±0 12

13 Jet Fans Corrections of Noise Level: P,tunnel = L P,free field, 3m, 45 + DL P, install + DL L P, tunnel P,free field, 3m, 45 = L P,tunnel - DL P, install - DL P, tunnel Sample: - installed batteries of 2 jet fans in parallel: +3dB - concrete walls: +2.5dB - L P,tunnel = 85dB(A) - L P,free field, 3m, 45 = 85dB(A) 6dB 2.5dB L P,free field, 3m, 45 ~ 76.5 db(a) used value for jet fan selection 13

14 Jet Fans Corrections of Noise Level: Protection Grills = Bird Screens - approx. + 2dB(A) (tip: place grill in silencer at 50% of length) - approx. 2 % losses for each grill on top of shaft power 14

15 Jet Fans Attenuation of Noise Level: Length of Silencers DL P, silencer [db] 0.5D approx D approx D approx D approx D approx longer silencers not really reasonable 15

16 Jet Fans Silencers Types: WITT&SOHN (without pod) Competitors use (with pod) Advantages WITT&SOHN: Disadvantages with pod: - No additional losses - Fan efficiency decreased - Low Sound pressure level due to - Costs increased due to sophisticated design additional pod - High efficiency fan 16

17 CONTENT Noise Generation & Attenuation 1. General 2. Jet fans 3. Axial Fans 17

18 Axial Fans Attenuation of Noise Level: by round silencers (see jet fans above) by split silencers: - p loss ~ width width silencer opening module! ~ Pa - max. air speed inside silencer: c 15 air m s 18

19 Axial Fans Attenuation of Noise Level: by split silencers: - min. distances before impeller: s 12 width + 100mm safety silencer module 1300mm 2500mm 600mm OPTIMIZATION: 200mm 50mm otherwise stall operation likely! 19

20 Axial Fans Attenuation of Noise Level: by split silencers: - min. distances before impeller: s 12 width + 100mm safety silencer module 2500mm 200mm 20

21 Axial Fans Attenuation of Noise Level: by split silencers: - min. distances before impeller: s 12 width + 100mm safety silencer module OPTIM MIZATION 600mm 1300mm 50mm otherwise stall operation likely! 21

22 Summary The sound power level of a fan is mainly determined by it s volume flow rate / pressure characteristic Sound pressure can be measured, sound power is calculated Influence of number of jet fans installed in tunnel on sound pressure level Influence of tunnel wall types on sound pressure level How much attenuation you can achieve with increasing length of silencers for jet fans Disadvantages of silencers with pods for jet fans Silencers for axial fans 22

FAN TERMINOLOGY. t = 1.2 kg/m 3 (Standard air density at 20 C and 1013mb. V = duct air velocity, m/s.

FAN TERMINOLOGY. t = 1.2 kg/m 3 (Standard air density at 20 C and 1013mb. V = duct air velocity, m/s. +7 (0)1 55 1077 +7 (0)1 55 797 FAN TERMINOLOGY These notes are designed to explain some of the terms that are used in describing the characteristics of fans and the relationship between the fan performance