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Duct sizing

The following ducts sizes (inches) are given as rough equivalents to deliver the same CFM. Based on shape, which offers the least resistance to air movement and the least friction?
A. 5x12, B. 6x10, C. 7x8, D. 9 dia.




Answer = D. 9 in. diameter round duct. Although yielding slightly more cross-sectional area, this duct has the least perimeter and based on shape is the most efficient (least resistance and least friction).  ⭕

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