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Noise Control in HVAC - CIBSE ASHRAE

1 Noise Control in HVACA ctive Passive or NoneDr Geoff LeventhallNoise and Vibration Sources in hvac - B57 Fan Noise Propagation8910 Control of Noise Transmission in DuctsAttenuation of Travelling Noise1112 All duct systems haveFrequency dependentattenuationFrequencydB13 Lined Ducts used widely in the Ducts used widely in the Noise attenuation and thermal insulation, but raises concerns for health. Not now recommended for sensitive locations except with special precautions. Older systems taken out, for example in schools and health uses more localised silencers than USA. UK ~100%. USA ~50%14 Lined DuctRectangular duct attenuation depends onPerimeter/area, (Pd/Ad), and Lining ThicknessAttenuation is not as well known for circular ducts15 Johns Manville Superduct RC Air Duct Board16[From CIBSE B5]17 Significance of Pd/AdDuct DimensionsPd/Ad3m x x x x x AttenuationParallel baffle (passive) silencer20 Parallel Baffle Silencer21 Duct Silencers Duct Silencers --parallel baffleparallel baffleConsiderations Length, width, weight Baffle thickness, Air gap Air velocity Pressure loss - varies as (velocity)2 Location wrt fan, bends etc22 Silencer attenuation 200mm/200mm Variation with length0102030405060 63 125 250

1 Noise Control in HVAC Active Passive or None Dr Geoff Leventhall Noise and Vibration Consultant geoff@activenoise.co.uk

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Transcription of Noise Control in HVAC - CIBSE ASHRAE

1 1 Noise Control in HVACA ctive Passive or NoneDr Geoff LeventhallNoise and Vibration Sources in hvac - B57 Fan Noise Propagation8910 Control of Noise Transmission in DuctsAttenuation of Travelling Noise1112 All duct systems haveFrequency dependentattenuationFrequencydB13 Lined Ducts used widely in the Ducts used widely in the Noise attenuation and thermal insulation, but raises concerns for health. Not now recommended for sensitive locations except with special precautions. Older systems taken out, for example in schools and health uses more localised silencers than USA. UK ~100%. USA ~50%14 Lined DuctRectangular duct attenuation depends onPerimeter/area, (Pd/Ad), and Lining ThicknessAttenuation is not as well known for circular ducts15 Johns Manville Superduct RC Air Duct Board16[From CIBSE B5]17 Significance of Pd/AdDuct DimensionsPd/Ad3m x x x x x AttenuationParallel baffle (passive) silencer20 Parallel Baffle Silencer21 Duct Silencers Duct Silencers --parallel baffleparallel baffleConsiderations Length, width, weight Baffle thickness, Air gap Air velocity Pressure loss - varies as (velocity)

2 2 Location wrt fan, bends etc22 Silencer attenuation 200mm/200mm Variation with length0102030405060 63 125 250 500 1000 2000 4000 8000 Octave band frequency HzAttenuation dB2400mm2100mm1800mm1500mm1200mm900mm23 Limitations of Passive SilencersLimitations of Passive Silencers Poor low frequency attenuation LF attenuation improved by narrow air gaps and longer length This leads to higher pressure loss24 Energy loss in silencersPower (W) used against pressure lossAir flow (m3/s) x pressure loss(Pa) fan-motor efficiency10m3/s into 100Pa > 1KW25 Energy use by fans in the UK(Lockwood - FETA Magazine December 2005)Fans in the range to 400kW per year of per year of electricityPotential saving of ( ) ( )by improved fan and system Electricity Use UK 2003 SectorUse TWh27 SignalmicLoudspeakerControllerControlmic Active silencer28 Active silencersActive silencers Good low frequency attenuation Normally combine active/passive for full range attenuation Lower pressure loss Smaller size and weight than passive for similar LF performance Higher initial cost?

3 29 Active Silencer attenuationActive Silencer attenuation30 Silencer m length; <50 Pa pressure loss; >25 dB attenuation at 125 HzNon-porous mediaNot possible by passive silencer31 Silencer Dimensions:1500 L x 1400 W x 1600 H321400 mm1600 mmView from reverb room into end of silencer from discharge side3335 Gas Platform AHU Active-Passive Silencer01020304050607080901006312525050 01000200040008000 Octave Band FrequencySound Pressure LevelPassiveActiveVibro-AcousticsWise Associates36 Passive silencing Active silencing Is no silencing possible?37Is no silencing possible? Very quiet fan operating optimally Good duct design Modify our Noise level requirements- less demanding criteria [One of the necessary lifestyle changes in the future?]38[ ASHRAE 2003]Fan is quietest at maximum efficiencyFan is quietest at maximum efficiency39 Fan outlet configuration - 1 Slope 1 in 7A > SMACNA Guide - hvac Sound and Vibration Manual 200540 Fan outlet configuration - 2 Full radius turning vanesMinimum150mm radiusFrom SMACNA Guide41 Reducing air flow generated noiseFrom SMACNA Guide42 From SMACNA Guide43 From SMACNA Guide44 From SMACNA Guide45 Most duct elements have attenuation which increases with frequency, although attenuation may drop at higher bends simple and effectiveHzdBLinedUnlinedResidual low frequency attenuation requirements obtained by active silencers46?

4 Do we over-silence?How much are criteria conditioned by culture and expectations?


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