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Figure 19
Gutter views
1st Module
.658
2.15
L is the fabricated gutter length. L = Lp + 1.316 inches.
DUCTWORK RECOMMENDATIONS
Sound Attenuation
Noise is produced by unit supply fans, and exhaust
fans. Supply fan noise is substantially attenuated by
the ductwork, provided it is properly constructed.
SMACNA recommendations for metal gauge
thickness and installation should be followed
carefully. Lightly constructed ductwork can produce
"oil canning"--a rapid in-and-out pulsating motion of
the duct walls--resulting in sound problems.
NOTE: IF ROOF DECKING CANNOT BE PLACED
UNDER THE UNIT, AN ACOUSTICAL BARRIER CAN BE
INSTALLED.
Corrugated steel decking is acoustically ideal for the
installation of the roof curb and connecting ductwork.
Closely toleranced holes must be cut in the roof deck
for the supply and return ductwork. When the duct is
installed, caulking must be used to seal the decking
to the duct. Even a small air leak between the duct
and the deck will destroy most of the attenuation
available from the steel decking.
Installation
L
2nd Module
In addition, special consideration may be required to
ensure that the weight of the unit does not crush the
roof deck at those points where the deck is between
the curb and support joist. Poured insulation will
usually add the required strength, while the use of
rigid insulation may require placement of three-inch
support channels on the decking.
As a general rule, exhaust fan noise is not a problem
and can be adequately attenuated by the return
ductwork. For critical applications where return
ductwork is not required, however, installation of an
acoustically-lined "return T" is recommended; see
Figure 20
.
Supply and Return Air Ductwork
Ductwork for all units must be fabricated and
installed by the installing contractor; to ensure proper
duct construction and installation, SMACNA
recommendations should be closely followed.
Below are several typically used guidelines for
ductwork construction:
- Connections to the unit should be made with three-
inch canvas connectors to minimize noise and
vibration transmission;
.625
.926
2.000
3.318
1.500
Last Module
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2.15
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7
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