A mechanical room is 40 ft by 30 ft by 12 ft high. If temporary exhaust is set for 6 air changes per hour, what exhaust rate is required?
Correct Answer
D) 1,440 cfm
Room volume is 40 x 30 x 12 = 14,400 cf. Six air changes per hour require 14,400 x 6 / 60 = 1,440 cfm. Other choices use the wrong room height, omit the hour-to-minute conversion, or use 10 ACH.
Why This Is the Correct Answer
Room volume is 40 x 30 x 12 = 14,400 cf. Six air changes per hour require 14,400 x 6 / 60 = 1,440 cfm. Other choices use the wrong room height, omit the hour-to-minute conversion, or use 10 ACH.
Why the Other Options Are Wrong
Option A: 1,200 cfm
air changes, choice a: 1,200 cfm uses a different airflow factor.
Option B: 1,800 cfm
air changes, choice c: 1,800 cfm is wrong because 1,800 cfm uses a different airflow factor.
Option C: 2,400 cfm
air changes, choice d: 2,400 cfm uses a different airflow factor.
Memory Technique
Keep a stormwater map beside air changes: if one number is skipped, the option drifts away from 1,440 cfm.
Reference Hint
Open Mechanical and Electrical Systems for Construction Managers, 4th Ed., 2024; use the index entry or section for "Air-change ventilation calculation" to confirm that Room volume is 40 x 30 x 12 = 14,400 cf.
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