All rectangular ducts are fitted with flange frames made of sheet angles
and braced by cross-wise ribbing. Larger ducts are additionally braced
with galvanized tubes (baffles). The ducts are manufactured in the following
standard sections to standardize the fabrication, shipping and installation procedure:
If a or b ≤ 500, then L = 1250 mm
If a or b > 500, then L = 1500 mm
