Calculate the required air changes per hour for a confined space with lead work under Cal/OSHA regulations. The space volume is 1,200 cubic feet and the exhaust fan moves 400 CFM. How many air changes per hour are being provided?
Correct Answer
B) 20 air changes per hour
Air changes per hour = (CFM × 60) ÷ Volume in cubic feet = (400 × 60) ÷ 1,200 = 24,000 ÷ 1,200 = 20 air changes per hour.
Why This Is the Correct Answer
Option B is correct because the air changes per hour calculation follows the standard formula: (CFM × 60 minutes) ÷ Volume. With 400 CFM and 1,200 cubic feet: (400 × 60) ÷ 1,200 = 24,000 ÷ 1,200 = 20 air changes per hour. This calculation determines how many times the entire volume of air in the confined space is replaced each hour, which is critical for lead work safety under Cal/OSHA regulations.
Why the Other Options Are Wrong
Option A: 30 air changes per hour
30 air changes per hour is incorrect. This would require 600 CFM (30 × 1,200 ÷ 60 = 600). The calculation error likely comes from incorrectly multiplying or dividing by factors in the formula, resulting in an overestimate of the actual air change rate.
Option C: 25 air changes per hour
25 air changes per hour is incorrect. This would require 500 CFM (25 × 1,200 ÷ 60 = 500). This error typically results from miscalculating the numerator in the formula or rounding incorrectly during intermediate steps of the calculation.
Option D: 15 air changes per hour
15 air changes per hour is incorrect. This would only require 300 CFM (15 × 1,200 ÷ 60 = 300). This underestimate likely results from calculation errors in the formula or confusion about the relationship between CFM and air changes per hour.
Memory Technique
Remember 'CFM times Sixty, Divided by Volume' - CSDV. Think 'Clean Space, Divide Volume' to recall the air changes formula: (CFM × 60) ÷ Volume.
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