Question

4. (25pts) A clothes dryer is installed in a basement. Air leaving the clothes dryer passes through an exhaust duct. The exhaust duct is made from a 4in diameter corrugated metal pipe which is 8ft long. Assume that relative roughness =0.015. The exhaust duct also contains two elbows, each with a K value of K=0.75. The outlet has a K value of K=0.50. Head loss in the elbows and the outlet is given by hq=K". Assume that the air flow is Q=200ft3/min and that the air at elevated temperature has kinematic viscosity v=0.00026ft2/s and mass density p=0.0019slug/ft3. 11 a. What the air velocity? Assume incompressible flow. Give answer in ft. b. What is the Reynolds number? Is the flow laminar or turbulent? C. What is the friction factor f? Show how you determined this by calculation or moody diagram as applicable. d. What is the total head loss in the duct? Give answer in ft.

Answer 1

Given: 4 D=4 in.= – ft 12 L = 8 ft = 0.015 D (Two elbows) Kelbow = 0.75 Koutlet = 0.5 y? h = K Zg Q= 200 ft / min v = 0.00026 ft /s p=0.0019 slug/ft? (a)V = ? (b)RE = ? (c) f = ? (d) hot = ?

(a) The velocity is: Oil 10 T 4 4 | 12 + V = 2291.83 ft/min x( 2010 V = 38.2 ft's 60 s (b) The Reynolds number is: Re-VD 38.2x{17) Re-0.00026 Re=48970.75 Since, the Reynolds number is greater than 2300, the flow is turbulent.

From Moody's chart the friction factor corresponding to Re = 48970.75 and = = 0.015 is as follows: If = 0.04463 (d) The Total head loss is: hot = h, the fly21 - hot hortLD2 +2 + B outlet 2g 2gD. elbow on 2g h 0.04463x8 x 38.2² +2/0.75x_38.2 0.5(38.2)" * 2(32.2))* 2(32.2) 32 how = 69.6 ft