Basics of
Vibration Isolation for
HVAC Equipment & Systems
Mark E. Schaffer, P.E.
Schaffer Acoustics Inc.
Pacific Palisades, CA
mark@schaffer-acoustics.com
Introduction
• Why isolate vibration?
• Vibration sources & frequencies
• How to select isolators
• Isolator types
• Isolation system effects
Source-Path-Receiver
Vibration Sources
Fans, pumps & compressors
Transmission Paths
Slabs, floors, walls, etc.
Receivers
Bldg. occupants, sensitive instrumentation
Airborne & Structure-borne
Sound Transmission
SOUND TRANSMISS
S
Vibration Mechanisms
• Imbalances in rotating equipment
• Flow obstructions near blade tips
• Oscillating motor windings
• Airflow stall in fans
Vibration Frequencies
in Rotating Equipment
• Expressed in cps or Hertz (Hz)
• Equipment Shaft Rotation Frequency
SRF = RPM / 60
• Blade Passage Frequency (BPF)
BPF = # of blades x RPM / 60
• Harmonics & intermodulations
Vibration Frequencies
in Motors
• Motor Rotation Frequency
MRF = RPM / 60
• Motor Windings Frequency
MWF = 2 x 60 hertz = 120 Hz
• Harmonics & intermodulations
Frequency Ranges of Interest
• Equipment Shaft Rotation
400 to 3600 RPM, 6.7 to 60 Hz
• Blade Passage
1600 to 21,600 BPM, 26.7 to 360 Hz
• Motor Rotation:
870 to 3600 RPM, 14.6 to 60 Hz
• Motor windings - 120 Hz
• Harmonics & intermodulations
Pump Impeller & Cutwater
3500 RPM Pump
with 6 impeller blades
• Impeller BPF = 6 x 3500 / 60 = 350 Hz
• Motor RPM = 3500 RPM / 60 = 58.3 Hz
• Line frequency x 2 = 120 Hz
• Harmonics and intermodulations
Belt-driven 9-bladed fan,
1200 RPM,1770 RPM motor
• Fan BPF = 9 x 1200 / 60 = 180 Hz
• Fan Shaft Rotation = 1200 RPM / 60 = 20 Hz
• Motor RPM = 1770 RPM / 60 = 29.5 Hz
• Line frequency x 2 = 120 Hz
• Harmonics and intermodulations
Attenuation vs.
Isolator Efficiency
0
-5
Transmissibility
dB
Transmissibility
inindB
-10
-15
-20
-25
-30
-35
-40
-45
-50
0%
10%
20%
30%
40%
50%
60%
Isolator Efficiency
70%
80%
90%
100%
Transmissibility vs. Frequency
100 +40 dB
F = Forcing Frequency
F n = Natural Frequeny of isolated Equipment
Transmissibility
Ratio
Transmissibility
Ratio
10 +20 dB
1
0.1 -20 dB
0.01 -40 dB
0.001 -60 dB
0.1
1
10
F/Fn
100
Isolator Selection re % Efficiency
Selection Guide
for Vibration Isolation
Vibration Isolation Products
Neoprene or rubber
Steel or iron
Compressed fiberglass
Neoprene Pads
Fiberglass & Neoprene Mounts
Open Spring Mount, Inertia Base
& Seismic Restraint
Restrained Spring Mounts
Avoid this Restrained Spring Mount
Pneumatic Isolator (Airbag)
Pneumatic Isolator
& Leveling Valve
System Factors that Affect
Vibration Isolation
• Stiffness of supporting structures
• Piping, conduit & ductwork
• Isolation at equipment connections
• Isolation at building support points
Supporting Structure Stiffness
Kstructure > 10 * Kisolators
Flexible Pipe (Pump) Connectors
Pipe Hangers
Isolated Cooling Tower
Isolated Makeup Water Pipe
Refrigerant Piping
Isolated Rooftop Piping
Attachment of Suspended Piping
Flexible Conduit
Isolated Conduit Clamp
Flexible Duct
Connector
&
Thrust Restraint
Pipe Penetration
Effects of VFDs
Reduced
RPM
Reduced Vibration Strength
Reduced Vibration Frequency
Floor FN
as low as 4 Hz
Select isolators per
lowest practical equipment shaft RPM
Summary
• Select equipment isolators per ASHRAE
Applications Handbook (Chapter 47 in
2003
Handbook)
• Structure stiffness >10X isolator stiffness
• Isolate piping, conduit & ductwork
connections to equipment
• Isolate piping & conduit connections at
building attachment points & penetrations
• Consider effects of VFD control
For More Information
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