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Q4 (a) A contractor wants to install a duct handling 3000 CFM in a hung ceiling that has 12 in. of vertical clear space available for the duct. By considering 1 in. of insulation on the duct, the maximum duct depth can be is 10 in. The velocity in the duct is not to exceed 1600 FPM to avoid excessive noise. Referring to Figure Q4 (a-i) on page 15 and Figure Q4 (a-ii) on page 16, what size duct should the contractor install by considering the good duct aspect ratio? (8 marks)
100,000 80,000 80,000 40,000 30,000 20,000 10,000 8000 8000 4000 3000 2000 1000 800 500 400 300 100 80 8 30 20 10 -30 28 01 26 24 200-12 22 20 18 18 14- 10 9 8 Fpm velocity In duct diam. 12 02 03.04 06 08.1 3 B -% % 2 34 6 81.0 Friction Loss, in. w. per 100 It Figure Q4 (a-i): Friction loss for air flow in galvanized steel round ducts. 18 PB 400 12 1800 2 3 4 6810
Side of rectangular duct, in. hadema 2015 5 6 7 8 9 10 Side of rectangular duct. in. Figure Q4 (a-ii): Equivalent round duct sizes AVA Table Q4 (c): Loss Coefficients, transition (Diverging Flow) for Rectangular, Pyramidal Duct. 2 For equat friction per ti of length 4 6 entround duct 210 Note A 15 20 25 30 8 16 20 30 45 60 90 120 180 0.18 0.22 0.25 0.29 0.31 0.32 033 030 0.36 043 050 0.56 0.61 0.63 0.63 0.63 0.42 0.47 0.58 0.68 0.72 0.76 0.76 0.75 042 049 0.59 0.70 0.80 0.87 0.85 0.86 area (Entering airstream), A1-Area Leaving airstream