Frictional pressure losses are inversely proportional to duct diameter raised to the fifth power. Thus, simply choosing a duct diameter that is one standard size larger than initially calculated can produce a large reduction in energy use. Ducts should always be at least 150 mm (6") in the smallest dimension to allow for the use of tools during installation and repair. Ducts, except for runs to outlets, should be sized in 50-mm (2") increments, e.g., 250, 300, 350 mm (10," 12," and 14") diameters. Some ducts may appear slightly oversized using this approach, but avoiding extra transition fittings between different sizes of duct will reduced installation costs and performance will be nearly the same. Recommendations for round duct diameters for laboratories are:
Branch ducts should be at least 50-mm (2") smaller than the trunk ducts to allow use of standard 25-mm (1") flanges for connections to branch ducts. The final duct connection to an outlet in a suspended ceiling grid should be reinforced flexible duct in lengths of 2 m (6 feet) or shorter and installed without any kinks. This length allows easy offsets to make final connections even if the ceiling grid turns out to be quite different than anticipated.
Using larger ducts often requires thicker or heavier gauge material. Murphy (1976) presents a case where two parallel 1,500 x 900 mm (60"x36") ducts with total cross-section area of 2.79 m2 (30 ft2) have lower installation cost than one duct of 3,000 x 750 mm (120"x30") with cross-section area of 2.32 m2 (25 ft2) because the metal gauge for the single duct is 18 and the gauge for the two smaller ducts is 20.
