Since the total loss

Since the total loss of .11 in. in the longest run is less than the .2 in. expected to be available from the air handler, all the sections of the longest run will be sized using the .05 in. /100ft loss factor.  Table 2 illustrates the procedure. The size of duct section A has been found.  For the other sections, the flow quantities for a section are found by deducting all branch quantities head of it.  The flow in duct section C is 2137– 198 = 1939.  Lengths of each section plus applicable entrance and elbow equivalent lengths are listed.  Then for section C, enter an air friction chart at 1939cfm and .05 in. /100 ft friction factor.  Read the duct velocity, 880 ft/min, and 20 inch duct diameter directly from the chart.  The remaining sections of this run are sized similarly.  Losses in each section are found from the lengths and friction factor.  The round duct sizes can be converted to equivalent rectangular sections from available charts.
© Gary D. Beckfeld  Page 14 of 21
The remaining branches, K, G, E, L, M, N, and B, are now sized to balance each branch with the same total pressure loss.  For equal pressure loss in branch K, this branch must have the same loss as branch J.  From Table 2, the loss must be .026 in. of water.  Since the K Branch is 19 feet long, the pressure loss is .026(100/19) = .1368 in. /100 ft.  Entering an air friction chart at .1368 and 604cfm gives the duct velocity as 950 fpm and the duct diameter as 11 inche