A duct is handling 6000 CFM of air in a central air conditio

A duct is handling 6,000 CFM of air in a central air conditioning system. Length of duct is 600 ft. Air velocity is 1, 200 ft/min. The duct has to fit in a 40 \"x 40\" space. What would be the optimum duct size that would give the best aspect ratio? Total friction loss in inches of water.

Solution

(a) flow rate = 6000 ft^3 / min   = 100 ft^3/s

      Area * velocity = flow rate

      Area = 6000 / 1200 = 5 ft^2

      Assume square cross section for duct

      Side = sqrt(5) = 2.236 ft   or 26.833 in

(b) equivalent dia of cross section = 1.3 * (2.236 * 2.236 )^0.625 / (2 * 2.236)^0.25 = 2.44 ft

      from the friction loss in ducts chart

      we get for 6000 cfm, 600 ft length and 2.44 ft dia

      friction loss = 0.42 inches of water