Flow velocity in air ducts should be kept within certain limits to avoid noise and unacceptable friction loss and energy consumption. Low velocity design is very important for the energy efficiency of the air distribution system. Doubling the duct diameter reduces the friction loss by factor 32.
It is common to keep main duct velocity above 20 m/s (3940 ft/min).
Engineering ToolBox, (2005). Air Duct Velocities. [online] Available at: https://www.engineeringtoolbox.com/duct-velocity-d_928.html [Accessed Day Mo. Year].