ASHRAE Journal Supplement - AMCA InMotion - November 2023 - 28

Ventilation fans. Typically, in the case of an
exhaust fan mounted atop a commercial or industrial
building, the fan intake is ducted with the fan discharging
to atmosphere. Environmental noise is a function of the
noise propagating from the fan discharge and radiating
from the fan casing and motor drive train. Knowing
how fans work and selecting the correct noise-control
products can help keep noise within acceptable levels.
Noisier fans tend to be oversized and operated under
the design speed or undersized and operated above the
design speed. Sometimes, re-adjusting a fan to run closer
to its design speed and pressure will help to resolve a
noise issue, lessening reliance on noise-control products
and saving money.
Often, a fan-discharge silencer (Photo A) is all that is
needed to solve a noise problem. Sometimes, however, a
combination of fan silencer and acoustic barrier wall or
enclosure (Photo B) is needed. With an acoustic barrier
wall, accessibility and clearance between the wall and
fan unit are important to meeting local electrical codes.
If a full enclosure is applied, it is important to determine
the amount of ventilation required to keep interiortemperature
rise above ambient-air temperature within
a set limit (e.g., 20˚F [10˚C]). Once the required ventilation
is determined, enclosure ventilation silencers can
be selected. These devices allow required ventilation
airflow at minimum pressure loss while controlling
noise propagated through enclosure ventilation openings.
Sometimes, forced ventilation is needed, and, sometimes,
convection (stack effect) ventilation will work. Taking all
of these factors into consideration will keep a fan unit
from overheating and shutting down.
Air-cooled chillers. The noise produced by
an air-cooled chiller is a combination of mid-to-highfrequency
sound generated by the screw compressor
(bottom of the unit) and low-frequency sound generated
by the upblast fan (top of the unit). It is important
to understand an air-cooled chiller is a noise-generating
system, and all components that generate noise must be
considered. In many cases, the fi rst attempt to reduce
noise involves wrapping a compressor with a composite
quilt and mass-loaded vinyl jacket. Although this will
control some mid-to-high-frequency noise, rarely does
it solve a system-noise problem, as it considers only one
aspect of the system noise.
Silencer baffl es, barrier walls, and fi xed-blade acoustic
louvers can be used to control air-cooled-chiller noise;
the combination depends on the application, equipment
footprint, location of noise-sensitive areas, and any local
noise ordinance:
■
If a barrier wall of suffi cient height can be placed at
the distance specifi ed by the chiller manufacturer,
one double-leaf acoustic access door is all that will
be needed to achieve up to a 23-dBA noise reduction
perpendicular to the outside of the wall.
■ If the distance between a barrier wall and a chiller is
PHOTO A. Fan intake and discharge silencers.
Photo courtesy of Kinetics Noise Control Inc.
PHOTO B. Dust-collector fan with acoustic barrier
wall and silencer. Photo courtesy of Kinetics Noise Control Inc.
28
2023 AMCA inmot ion
www.amca.org
http://www.amca.org
ASHRAE Journal Supplement - AMCA InMotion - November 2023

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