6. A 5-ft- diameter air duct is used to carry ventilating air into a tunnel at a rate of 7000 ft3/min. Tests show that the pressure drop is 2.5 inch

of water per 1300 feet of duct. Calculate the value of the friction factor for this duct and the approximate size of the equivalent roughness of the surface of the duct. (10 points)

Fig: 1