IBC AHU Series - Duct Work; Sizing the Ducts

2.3 Duct work
Ducting design and installation should adhere to
SMACNA and/or ASHRAE guidelines. Install the
ducting system to NFPA 90B (latest edition).
Adhere to the following:
Size the supply air plenum duct fitting to
the size of the appliance's supply air
opening, and extend for 3' above the
appliance.
Size the supply and return air ducting
according to ASHRAE standards or
equivalent.
Seal all joints and connections to prevent
air leakage from the ducting system.
Choose accessories that will not
excessively restrict the air flow and create
excessive pressure drop on the system.
2.3.1 Sizing the ducts
If using flex / insulated flex ducting, ensure that you allow for a higher pressure drop.
Branch Duct - CFM at .25"wc external static pressure (galvanized duct)
Round CFM range
3" 20-30
4" 30-35
5" 55-65
6" 90-100
7" 135-160
Table 5 Sizing round galvanized duct for CFM range
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Section: Installation