Today I’m going to explain an important concept in one of the most popular ways of doing duct design. I’ve been writing a series on duct design over at my blog and began with a look at the basic physics of air moving through ducts. The short version is that friction and turbulence in ducts results in pressure drops. Then in part 2 I covered available static pressure. The blower gives us a pressure rise.
from Building Science http://www.greenbuildingadvisor.com/articles/dept/building-science/using-total-effective-length-duct-design