This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
With the blower type of fan the size of the main ducts may be based on a velocity of 1200 to 1500 feet per minute, the branches on a velocity of 1000 to 1200 feet per minute, and as low as 600 to 800 feet when the pipes are small. Flue velocities of 500 to 700 feet per minute may be used although the lower velocity is preferable. The size of the inlet register should be such that the velocity of the entering air will not exceed about 300 feet per minute. The velocity between the inlet windows and the fan or heater should not exceed about 800 feet.
Regulating dampers should be placed in the branches leading to each flue for increasing or reducing the air supply to the different rooms. Adjustable deflectors are often placed at the fork of a pipe for the same purpose. One of these is shown in Fig. 19.