Usually the riser ducts are design for th higher velocity than the ductwork system in conditioned space to minimize the shaft size. The reasonable air velocity in risers depend on the number of the story in building, for example for up to 10 or 12 story the air velocity can be 2,000 fpm and for higher buildings up to 3,000 fpm. The air velocity in a horizontal duct off the riser should not exceed 1,400 fpm to minimize the air flow sound level. The best velocity for the air distribution systm in the floor (conditioned space) is 1,100 fpm.
