Hvac noise control

Rensselaer
Commercial & Industrial
Air Conditioning
Architectural Acoustics
HVAC Noise Control

Rensselaer
Resources
• Manufacturers
 Trane
 Industrial Acoustics
 Many others
• ASHRAE (American Society for
Heating, Refrigerating, and Air-
Conditioning Engineers)

Rensselaer
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…

Rensselaer
Main 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

Rensselaer
Other MEP Noise Sources
• Waste and Rain Leader Piping
• Transformers
• Dimmer Racks
• Lights & Ballasts
• Elevator Equipment

Rensselaer
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

Rensselaer
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 noise

Rensselaer
Packaged Air Handler
• Includes fan or fans
• Heating coil
• Cooling coil
• Air filters
• Humidifier
• Air dampers and
controls

Rensselaer
Packaged Air Handler

Rensselaer
Typical Air-Handler Design
MJR Figure 9.3, p. 192

Rensselaer
Equipment Location: Rooftop

Rensselaer
Equipment Location:
Mechanical Equipment Room
• Noise inside the MER
• Noise outside the
MER
• Duct Breakout
• Active Noise Control

Rensselaer
Isolator Types
Elastomeric Pads

Rensselaer
Isolator Types
Neoprene-In-Shear Floor Mount

Rensselaer
Isolator Types
Open Spring Floor Mount

Rensselaer
Isolator Types
Restrained Open Spring Floor Mount

Rensselaer
Reciprocating and Centrifugal
Chillers Noise
• Reciprocating chillers
tend to be quieter than
centrifugals for the
same load

Rensselaer
Fan Noise Components
• 1 duct length
• 3 duct length
• 5 duct length
• Aerodynamic noise
• Blade-passage noise
 fB = (RPM/60) ·N
 N = number of blades

Rensselaer
Fan Noise
Fan noise depends on the fan
operation point on the fan curve

Rensselaer
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





 


1
log1010 10C
 η = Hydraulic efficiency of
the fan = Q·P/(6350 · HP)
 HP = nominal horsepower
of the fan drive motor

Rensselaer
Estimating Fan Noise
US Army TM 5-805-4 Technical Manual, “Noise and Vibration Control”, Table C-13
    CBFIPQKL WW  1010 log20log10

Rensselaer
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)

Rensselaer
Indoor Diffusers
• Linear or Slot Diffusers
• Round or Rectangular Diffusers
• Grilles
• Registers

Rensselaer
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
• 400 sabins
• 12 feet

Rensselaer
Estimating 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 
2
0
9.334
DU
P




 ΔP = pressure drop across the diffuser (in. H20)
 ρ0 = density of air (0.075 lb/ft3)
Long, p. 475

Rensselaer
Estimating Diffuser Noise
Long, Fig. 13.24, p.476
DWW CLL Oct,
• Octave-band power levels can be calculated from the overall
level LW
2
13.115.082.5 AACD 
2
13.115.082.11 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, …)

Rensselaer
Recommended Velocity Limits
• Plant Rooms 5m/s
• Aud. Shafts 4m/s
• Within Aud. 2.5m/s
Branch Runouts
RC-35 2.75 m/s
RC-25 2 m/s
RC-15 1.25 m/s
Terminal velocities are critical because there is
nothing after the diffuser to provide additional
attenuation!

Rensselaer
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

Rensselaer
Attenuation in Unlined Ducts

Rensselaer
Duct Liner
MJR Figure 9.5 and 9.7, pp. 193 and 194

Rensselaer
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)
D
C
duct t
S
P
BL 






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

Rensselaer
Duct Liner
MJR Figure 9.5 and 9.7, pp. 193 and 194
x x
x
x
x
x
x
x
x
x
x x
x
Data from Long’s equation

Rensselaer
Duct Liner Data
http://www.owenscorning.com/comminsul/documents/FiberglasDuctBoardLiner.pdf

Rensselaer Internal Fiberglass Duct Lining
Duct Liner

Rensselaer
Airflow: Turbulent Noise in
Ductwork
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Fig. 9.12, p. 198)

Rensselaer
Airflow: Turbulent Noise in
Ductwork
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Table 9.1, p. 197)

Rensselaer
How Ductwork Radiates Noise
(Break Out)

Rensselaer
Duct Shape and Noise Control
• 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

Rensselaer
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 85.0
25.0






 

S
P
lf
S
P
Lduct
3,64.1 58.0
73.0






 
S
P
lf
S
P

Rensselaer
• For octave bands above 250 Hz
 P = perimeter (ft)
 S = cross-sectional area (ft)
 l = duct length (ft)
l
S
P
Lduct 






8.0
2.0
Long, p. 486

Rensselaer
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

Rensselaer
Discharge Noise
High noise levels near the
discharge of the AHU

Rensselaer
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

Rensselaer
Duct Lagging
Make the ducts stiff using lagging,
typically fire-rated drywall.

Rensselaer
Duct Lagging

Rensselaer
Duct Penetrations

Rensselaer
Ductwork Crossing an
Isolation Joint

Rensselaer
Resilient Duct Hangers
Elastomeric Hanger

Rensselaer
Spring-and-Neoprene-in
Series Isolator (Hanger)
Precompressed Spring-
and-Neoprene-in-Series
Isolator (Hanger)

Rensselaer
Spring-and-Neoprene-in-Series Isolator (Hanger)

Rensselaer
Resilient Hangers

Rensselaer
Flexible Duct Connections

Rensselaer Improvements in Design for Noise
Performance
Poor Design
Better Design

Rensselaer
End Effects
• Change in cross-sectional area when a duct
terminates in a room















88.1
0
10 1log10
fd
c
Lend














 

88.1
0
10
8.0
1log10
fd
c
Lend

Termination in free space:
Termination flush with wall:
 c0 = speed of sound
 f = frequency
 d = duct diameter ( for a rectangular duct)

S
d
4

Long, p. 490

Rensselaer 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

Rensselaer Application of Duct Liner in Underfloor Plenum
Lined Plenum
(For under-floor air supply)

Rensselaer
Silencer Location

Rensselaer
Duct Sound Attenuators

Rensselaer
Active Noise Control in Ducts
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.

Rensselaer
A Sample Interior Noise Prediction
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (MJR Table 9.2, p. 204)

Rensselaer
1/3 vs. 1/1 Octave Band Data

Rensselaer
Fan and Compressor Noise

Hvac noise control

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (20)

Destaque

Destaque (20)

Semelhante a Hvac noise control

Semelhante a Hvac noise control (20)

Último

Último (20)

Hvac noise control