A rectangular supply duct is 24 in. by 12 in. inside and carries air at 800 fpm. What airflow is carried by that duct?
Correct Answer
C) 1,600 cfm
Duct area is 24 in. x 12 in. = 2 ft x 1 ft = 2 sf. Airflow is area x velocity = 2 x 800 = 1,600 cfm. Other choices use one duct dimension, multiply by inches, or double the area.
Why This Is the Correct Answer
Duct area is 24 in. x 12 in. = 2 ft x 1 ft = 2 sf. Airflow is area x velocity = 2 x 800 = 1,600 cfm. Other choices use one duct dimension, multiply by inches, or double the area.
Why the Other Options Are Wrong
Option A: 800 cfm
airflow in duct, choice a: 800 cfm uses a different airflow factor.
Option B: 2,400 cfm
airflow in duct, choice c: 2,400 cfm uses a different airflow factor.
Option D: 3,200 cfm
airflow in duct, choice d: 3,200 cfm uses a different airflow factor.
Memory Technique
Keep a scope box beside airflow in duct: if one number is skipped, the option drifts away from 1,600 cfm.
Reference Hint
Open Mechanical and Electrical Systems for Construction Managers, 4th Ed., 2024; use the index entry or section for "Duct airflow calculation" to confirm that Duct area is 24 in.
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