1/29/2013 - confex · 2013. 1. 30. · 12 blades, 1094 rpm, 219 hz bpf 9 blades, 1175 rpm, 178 hz...
TRANSCRIPT
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Basics of HVAC Noise Control Sponsored by TC 2.6 – Noise & Vibration
Mark Schaffer, P.E.
Schaffer Acoustics Inc.
310-459-6463
Learning Objectives
1. Explain the impact of noise on human comfort.
2. Describe the operating conditions and fan design parameters that provide the most acoustic benefit.
3. Describe methods for reducing HVAC noise.
4. Explain the effectiveness of noise mitigation measures with respect to frequency.
5. Distinguish between the behavior of sound with respect to transmission and absorption.
6. Explain the decibel scale and the differences in numeric sound pressure levels.
ASHRAE is a Registered Provider with The American Institute of Architects Continuing Education
Systems. Credit earned on completion of this program will be reported to ASHRAE Records for AIA members. Certificates of Completion for non-AIA members are available on request.
This program is registered with the AIA/ASHRAE for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or
endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to
specific materials, methods, and services will be addressed at the conclusion of this presentation.
Sound Levels
LW or PWL - Sound Power Level
The amount of sound generated by a noise source
LP or SPL – Sound Pressure Level
The level of sound at a specific location
Both values are expressed in decibels (dB)
Frequency Bands
Octave Band
A range of frequencies for which the highest frequency is twice the lowest frequency
(centered at 16, 31, 63, 125, . . . , 8000 Hertz)
1/3-Octave Band
A range of frequencies for which the highest frequency is 26% higher than the lowest frequency
(centered at 12.5, 16, 20, . . . , 10,000 Hertz)
Loudness Descriptors
dBA - A-weighted decibels
Logarithmic scale value - 20 to 130 dBA
Sones
Linear scale value – ¼ to 512 Sones
Fan Noise
Broadband noise due to air turbulence
Light breeze – hurricane
Tonal noise due to blade pass rotation
BPT Blade pass Tone
BPF Blade pass Frequency
BPI Blade pass Increment
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Tonal Noise
BPF (Hertz) =
BPF = = 200 Hertz
BPI – up to 15 dB
Fan RPM x # of Blades
60
1000 RPM x 12 Blades
60
Fan Selection Goals
• Minimize overall noise
• Minimize BPI – more distracting than broadband noise
• Select for BPF >200 Hertz, if possible
• Avoid Stall and Surge – causes throbbing noise that is heard/felt
Fan Curves 33” Plenum Fan 20,000 CFM @ 5” w.g.
Do not select
to the left of
this curve
BHP
VAV
System
Curve
36” Plenum Fan 20,000 CFM @ 5” w.g.
Do not select
to the left of
this curve
BHP
VAV
System
Curve
Plenum Fan Comparison 20,000 CFM @ 5” w.g.
36” fan
33” fan
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Fan Types (88)
Centrifugal – airflow turns 90 degrees
•Forward-curved (FC)
•Backward Inclined (BI)
•Airfoil (AF)
•Radial
Inline – airflow doesn’t change direction
•Propeller
•Vane-axial
•Airfoil
Centrifugal Scroll Fan
Centrifugal
Plenum
Fan
Inline Fans
Vane-axial
Tube-axial
Centrifugal
Mixed Flow
Inline Tube-Axial Fan Inline Tube-Axial Fan
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Inline
Centrifugal
Fan
Inline Centrifugal Fan
Mixed Flow Fan Mixed Flow Fan
Inline Fan Noise Spectra 20,000 CFM @ 0.75” w.g., 5 - 6 BHP
Centrifugal
Tube-Axial
Mixed Flow
Plenum Air Handling Units
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Plenum
Fan
Inlet
Plenum
Fan
9 or 12
blades
BPF usually
higher with
12 blades
Plenum Fan Noise Spectra 25,000 CFM @ 6” w.g., 39” Fans
80
85
90
95
100
105
110
63 125 250 500 1000 2000 4000
Sound P
ow
er
Level in
dB
Octave Band Center Frequency
12 blades,
1094 RPM,
219 Hz BPF
9 blades,
1175 RPM,
178 Hz BPF
33 BHP
Plenum Fan Noise Spectra 25,000 CFM @ 6” w.g., 36” Fans
80
85
90
95
100
105
110
63 125 250 500 1000 2000 4000
Sound P
ow
er
Level in
dB
Octave Band Center Frequency
12 blades,
1320 RPM,
264 Hz BPF
9 blades,
1426 RPM,
214 Hz BPF
34 BHP
Fan Array
15” to 24” Diameter
Direct-Drive
Plenum Fans
Large Fan Array
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Low Noise Propeller Fans
• High solidity
• Wide chord, shaped blades
• Standard & custom diameters
• Lower rpm
• Lower noise
Blade Shape
Standard Blades Sickle-shaped Blades
Fixed Blade Pitch
36” Diameter Adjustable Blade Pitch
40-foot Fan Diameter
Assembled Super-Quiet Fan Propeller Fan Noise Spectra 25,000 CFM @ ½” w.g., 3.5 – 4 BHP
50
55
60
65
70
75
80
85
90
95
100
63 125 250 500 1000 2000 4000
Sound P
ow
er
Level
in d
B
Octave Band Center Frequency
48”
Low Noise
48” Standard
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Custom Propeller Fans
• Blade Material
• Number of Blades
• Blade Shape
• Blade Length
• Blade Angle
Customization Options
What are Sones ?
Calculated from either octave or 1/3-octave
band LP values
Sones Approx. dBA 1 40 dBA 2 50 dBA 4 60 dBA 8 70 dBA 16 80 dBA 32 90 dBA 64 100 dBA 128 110 dBA 256 120 dBA 512 130 dBA
Don’t Confuse AMCA & HVI Sones
AMCA HVI
LW values measured in a reverberation chamber
LP values calculated at 5’ from noise source
Mounted on wall or ceiling Suspended in mid-air
8 Octave Bands 24 1/3-octave bands
Don’t Confuse AMCA & HVI Sones
SonesAMCA = 0.70 * Smax + 0.30 * (S1+S2+S3 . . . +S8)
SonesHVI = 0.85 * Smax + 0.15 * (S1+S2+S3 . . . +S24)
Example – small FC fan
SonesAMCA = 4.8 SonesHVI = 3.5
HVI rating appears to be 27% “quieter” than
AMCA rating
How Not to Select a Fan
for Low Noise ASHRAE Estimation Method (1960s)
LW = Base LW+10 Log CFM+20 Log S.P.+C+BFI
Base LW values come from a handbook table
C = efficiency adjustment
BFI = Blade Pass Frequency Increment adjustment
This method has been obsolete for 20+ years.
Use only measured LW values
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36.5” AF Fan 15,000 CFM @ 3” S.P.
65
70
75
80
85
90
95
100
105
63 125 250 500 1000 2000 4000
L W i
n d
B r
e 1
pic
oW
att
Octave Band Frequency in Hertz
Measured Discharge
Measured Inlet
ASHRAE Estimate
Propeller Fan 10,000 CFM @ 0.6” S.P.
50
55
60
65
70
75
80
85
90
95
100
63 125 250 500 1000 2000 4000
L W i
n d
B r
e 1
pic
oW
att
Octave Band Frequency in Hertz
48" Low Cost
36" Low Noise
ASHRAE Estimate
Summary of
Fan Selection Guidelines
• Consider several fan types and blade shapes
• Select near peak static efficiency
• System curve should intersect down-sloping
part of fan curve
• Blade Pass Frequency >200 Hertz
• Do not compare SonesAMCA and SonesHVI
• Do not estimate LW values with the obsolete
ASHRAE Estimation Method
Thank You
Questions ?
Mark Schaffer, P.E.