EQUAL FRICTION METHOD EXAMPLEDuct system for general office
;
Total air quantity5400 cfm18 air terminals300 cfm eachOperating
pressure for all terminals-0.15 in.wgRadius elbowsR/D = 1.25
Find ;Initial duct velocity, area, size and friction rate in the
ductTotal equivalent length of duct run, with highest resistance
Total static pressure required at Fan discharge
DUCT LAYOUT
SELECT AN INITIAL VELOCITYRECOMMENDED MAX DUCT VELOCITIES FOR
LOW VELOCITY SYSTEMS
Velocity dipilih 1700 fpm
Duct area : 5400 cfm/1700 fpm = 3.18 sq.ft
Duct size (TABLE 6)
Duct size : 22 in x 22 in
Circular equiv. diameter
Initial friction rate
Friction rate ; 0.145 in wg per 100 ft of eauivalent length
Percent section area in branches
Percent of cfm = air quantity in duct section/ total air
quantity
It appears that the duct run from the Fan to terminal 18 has the
highest resistance
Friction of rectangular Elbow
The total friction loss in the ductwork frfom the Fan to
terminal 18 :
Loss= total equiv length x friction rate
= 229 ft x (0.145/100 ft) = 0.332 in. wg
Total pressure at Fan DischargeThe sum of the terminal operating
pressure and the loss in the ductwork
