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The duct should be supported by supports at a distance not exceeding 8’. 24-G duct should be supported
by ½” MS rod with 1 ¼” x 3/16” angle on which the ducts will be rested. ¼” felt / foam should be provided
between the duct bottom and support face of the angle. All support rods and angles should be painted
with red oxide primer.
For all branches, guide vanes / splitters / dampers should be provided. Provide straightening vanes for
collars and smooth bends. Interconnection of ducts between two units should have individual dampers
of opposed blades / sliding type.
Provide acoustic lining for the first 5 feet of ducting after fan outlet with 12 mm fiberglass rigid board with
RP tissue paper / 26 G perforated aluminum sheet.
In case plenums are used, the velocity in plenum should not exceed 300 FPM and the plenum should
be lined for acoustic purpose. If two branches are to be taken from the same plenum, the same should
be provided with dampers. These should be taken on the same side but opposite to the incoming duct
connection.
Provide for suitable fresh air arrangement.
Provide a straight duct length of at least 2D after the fan outlet for streamlining airflow; D is the equivalent
round duct diameter. This is VERY IMPORTANT.
III. Return and supply air grille sizing
Return air at the inlet of the indoor unit should not exceed 300 FPM. Return air path should be
designed for a maximum velocity of 1000 FPM.
Supply air grilles / diffusers should be sized as per manufacturer’s specification.
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