These charts relate

These charts relate duct friction, duct size, air flow in cubic foot per minute and flow velocity.  Reference 2 indicates that duct velocities should be about 900 ft/min or less.   Entering an air friction chart at 2137 ft3 /min flow quantity and a velocity of 900 ft/ min at the air handler outlet, gives a friction loss of .05 inches of water per 100 feet and an outlet duct of 21 inches diameter.  To get the maximum pressure loss in the system, determine the longest run of duct. Inspection of Figure 5 shows that the longest run consists of duct sections A+C+D+F+H+J.  Including the entrance and elbow equivalent lengths, this run length is  L = 10ft entrance +6+6+35ft elbow +10+20+2+35ft elbow +17 = 141 ft The total pressure loss is
© Gary D. Beckfeld  Page 13 of 21
P = .05 (141/100) + .04 in. diffuser loss = .1105 in. of water