hvac noise control
TRANSCRIPT
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Commercial & Industrial Air Conditioning
Architectural Acoustics
HVAC Noise Control
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Resources
• Manufacturers Trane Industrial Acoustics Many others
• ASHRAE (American Society for Heating, Refrigerating, and Air-Conditioning Engineers)
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Basic HVAC Functionality
• Room air is blown over a heat exchanger through which heated liquid (hot water) or cooled liquid (cold water or other refrigerant) liquid is circulated.
• Unwanted thermal energy is released outdoors• This requires…
Ren
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aerMain HVAC Noise Sources
• Fans (to move the air) Axial Centrifugal Propeller
• Compressors (to convert gas to liquid) Piston Rotary Scroll Centrifugal Screw
• Pumps (to circulate liquids)
• Diffusers and Ductwork (to distribute air) Turbulent aerodynamic
noise “Break-out” noise
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Other MEP Noise Sources
• Waste and Rain Leader Piping• Transformers• Dimmer Racks• Lights & Ballasts• Elevator Equipment
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Noise Control Approaches
• Location of equipment• Sealing penetrations • Resilient mounting of equipment & connected
services• Flexible connections to equipment• Lower fluid velocities• Internal duct lining and duct attenuators• Routing of ductwork and piping• Enclosing ductwork and piping
From Kirkegaard Associates
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Fan Coil Units• Opportunity for
significant noise issues: Fan and coil in
close proximity: high turbulence
Applications: typically close to “listeners” (hotel rooms, etc.)
Water flow noiseFrom Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Packaged Air Handler
• Includes fan or fans• Heating coil• Cooling coil• Air filters• Humidifier• Air dampers and
controls
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Packaged Air Handler
From Kirkegaard Associates
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aerTypical Air-Handler Design
MJR Figure 9.3, p. 192
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Equipment Location: Rooftop
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Equipment Location: Mechanical Equipment Room
• Noise inside the MER• Noise outside the
MER• Duct Breakout• Active Noise Control
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Isolator TypesElastomeric Pads
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Isolator TypesElastomeric Pads
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Isolator TypesNeoprene-In-Shear Floor Mount
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Isolator TypesNeoprene-In-Shear Floor Mount
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Isolator TypesNeoprene-In-Shear Floor Mount
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Isolator Types
Open Spring Floor Mount
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Isolator Types
Open Spring Floor Mount
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Isolator Types
Restrained Open Spring Floor Mount
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Isolator Types
Restrained Open Spring Floor Mount
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Reciprocating and Centrifugal Chillers Noise
• Reciprocating chillers tend to be quieter than centrifugals for the same load
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Fan Noise Components
• 1 duct length• 3 duct length• 5 duct length
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
• Aerodynamic noise• Blade-passage noise
fB = (RPM/60) ·N N = number of blades
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Fan Noise
Fan noise depends on the fan operation point on the fan curve
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Fan Noise
Fan noise depends on the fan operation point on the fan curve
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Estimating Fan Noise
• LW = fan sound power level
• KW = fan specific value
• Q = volume flow rate (cfm)• P = static pressure (in H20)
• BFI = blade frequency increment• C = efficiency correction
CBFIPQKL WW 1010 log20log10
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
1
log1010 10C
η = Hydraulic efficiency of the fan = Q·P/(6350 · HP)
HP = nominal horsepower of the fan drive motor
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Estimating Fan Noise
US Army TM 5-805-4 Technical Manual, “Noise and Vibration Control”, Table C-13
CBFIPQKL WW 1010 log20log10
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Diffuser Noise
• Flow sets the noise level at a given static pressure level forcing the flow
• Good aerodynamics are important to low noise from air terminals
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (Long Fig. 13.23, p. 474)
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Indoor Diffusers• Linear or Slot Diffusers • Round or Rectangular Diffusers • Grilles• Registers
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Specifications for Diffuser Noise
• Ideal: sound power data in octave bands versus static pressure & CFM
• Reality: most manufacturers only provide the NC “rating” at a fixed “room effect” (typically 10 dB)
• Sound power from NC:
• Sadly, this only provides a noise estimate based on a perfect NC curve (diffusers are typically high-frequency elements, therefore this tends to over-estimate low frequency power)
dB10)( NCLL PW
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
• 400 sabins• 12 feet
Ren
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aerEstimating Diffuser Noise
• LW = sound power level (dB re. 10-12 Watts)
• SD = cross-sectional face area of diffuser (ft2)
• UD = flow velocity prior to the diffuser (ft/s)
• ξ = normalized pressure-drop coefficient
3.31log60log30log10 101010 DDW USL
20
9.334DU
P
ΔP = pressure drop across the diffuser (in. H20)
ρ0 = density of air (0.075 lb/ft3)
Long, p. 475
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aerEstimating Diffuser Noise
Long, Fig. 13.24, p.476
DWW CLL Oct,
• Octave-band power levels can be calculated from the overall level LW
213.115.082.5 AACD 213.115.082.11 AACD
Generalized Diffuser Spectrum
for round diffusers
for rectangular diffusers
GP Uf 8.48
fNfNA BPB peak frequency
NB(x) = octave-band number of frequency x (32 Hz = 0, 63 Hz = 1, 125 Hz = 2, …)
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Recommended Velocity Limits
• Plant Rooms 5m/s• Aud. Shafts 4m/s• Within Aud. 2.5m/s
Branch RunoutsRC-35 2.75 m/sRC-25 2 m/sRC-15 1.25 m/s
Terminal velocities are critical because there is nothing after the diffuser to provide additional attenuation!
From Kirkegaard Associates
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Unlined Ducts
• Not much attenuation in unlined ducts Little absorption from surfaces (although some
energy is lost to break-out noise) Plane-wave propagation → no spreading loss
• Plane-wave propagation when duct dimensions (not length) are less than half a wavelength
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Attenuation in Unlined Ducts
MJR Figure 9.6, p. 193
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Duct Liner
MJR Figure 9.5 and 9.7, pp. 193 and 194
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Duct Liner• Attenuation in lined rectangular ducts can be
approximated with this equation
P = duct perimeter (ft) S = duct cross-sectional area (ft2) t = thickness of lining (in)
DC
duct tS
PBL
63 125 250 500 1000 2000 4000 8000
B 0.0133 0.0574 0.2710 1.0147 1.7700 1.3920 1.5180 1.5810
C 1.959 1.410 0.824 0.500 0.695 0.802 0.451 0.219
D 0.917 0.941 10.79 10.87 0.000 0.000 0.000 0.000
Octave-Band Center Frequency (Hz)
Long, Eq. 14.12, p. 487
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Duct Liner
MJR Figure 9.5 and 9.7, pp. 193 and 194
x xx
x
xx
x
x
x
x
x x
x
Data from Long’s equation
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Duct Liner Data
http://www.owenscorning.com/comminsul/documents/FiberglasDuctBoardLiner.pdf
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Internal Fiberglass Duct Lining
From Kirkegaard Associates
Duct Liner
Ren
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aerAirflow: Turbulent Noise in
Ductwork
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Fig. 9.12, p. 198)
Ren
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aerAirflow: Turbulent Noise in
Ductwork
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Table 9.1, p. 197)
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aerHow Ductwork Radiates Noise
(Break Out)
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Duct Shape and Noise Control
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
• Stiffness of round ductwork reduces break-out noise since motion of the duct walls is restricted
• However, this means that more noise energy stays within the duct and may produce higher noise levels at the outlet
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Long, p. 486
• The ratio of perimeter to cross-sectional area is also important, and can be used to approximate duct attenuation.
P = perimeter (ft) S = cross-sectional area (ft) l = duct length (ft) f = octave-band center frequency between 63 and 250 Hz
3 ,0.17 85.025.0
S
Plf
S
PLduct
3 ,64.1 58.073.0
S
Plf
S
P
Duct Shape and Noise Control
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• For octave bands above 250 Hz
P = perimeter (ft) S = cross-sectional area (ft) l = duct length (ft)
lS
PLduct
8.0
2.0
Long, p. 486
Duct Shape and Noise Control
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Long, p. 486
Frequency (Hz)
63 125 250 500 1000 2000 4000
Loss (dB/ft)Circular
0.03 0.03 0.03 0.05 0.07 0.07 0.07
Loss (dB/ft)Square
0.36 0.20 0.11 0.06 0.06 0.06 0.06
• Data for circular duct from Long, Table 14.1• Data for square duct from previous equations with P/S = 4
Duct Shape and Noise Control
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Discharge Noise
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
High noise levels near the discharge of the AHU
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Discharge Noise Control
• Stiffen the initial 25-50 ft of the discharge duct
• Often done by wrapping the duct with gypsum board or loaded vinyl
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Duct Lagging
Make the ducts stiff using lagging, typically fire-rated drywall.
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Duct Lagging
MJR Figure 9.14, p. 200
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Duct Lagging
From Kirkegaard Associates
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From Kirkegaard Associates
Duct Lagging
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From Kirkegaard Associates
Duct Penetrations
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From Kirkegaard Associates
Ductwork Crossing an Isolation Joint
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Resilient Duct Hangers
Elastomeric Hanger
From Kirkegaard Associates
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Resilient Duct Hangers
Spring-and-Neoprene-in Series Isolator (Hanger)
From Kirkegaard Associates
Precompressed Spring-and-Neoprene-in-Series
Isolator (Hanger)
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Resilient Duct Hangers
Spring-and-Neoprene-in-Series Isolator (Hanger)
From Kirkegaard Associates
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Resilient Hangers
From Kirkegaard Associates
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Resilient Hangers
From Kirkegaard Associates
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aerFlexible Duct Connections
From Kirkegaard Associates
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Improvements in Design for Noise Performance
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
Poor Design
Better Design
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End Effects
• Change in cross-sectional area when a duct terminates in a room
88.1
010 1log10
fd
cLend
88.1
010
8.01log10
fd
cLend
Termination in free space:
Termination flush with wall:
c0 = speed of sound f = frequency d = duct diameter ( for a rectangular duct)
S
d4
Long, p. 490
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Air Plenums, Passive and Active Silencers
• Plenum used near equipment outlet; promotes laminar airflow and provides acoustical insertion loss (< 12 dB)
• Passive silencers used when large insertion loss is required; must account for pressure drop
• Active silencer has no pressure drop, but is typically impractical
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Application of Duct Liner in Underfloor Plenum
From Kirkegaard Associates
Lined Plenum(For under-floor air supply)
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Silencer Location
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Duct Sound Attenuators
From Kirkegaard Associates
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Active Noise Control in Ducts
MJR Figure 9.19, p. 205
Using data from the input microphone, the controller generates a signal to be played by the loudspeaker which is out of phase (180º) with the duct-borne noise at the loudspeaker position. Feedback from the error microphone (which ideally senses no noise) helps fine tune the process.
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A Sample Interior Noise Prediction
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (MJR Table 9.2, p. 204)
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1/3 vs. 1/1 Octave Band Data
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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aerFan and Compressor Noise
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
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Thanks!