What is the net free air in square inches of a 12 by 12 metal louver opening. (formula = total area x 75%)

• A. 144
• B. 108
• C. 120
• D. 96

Answer: (B)

Net free area is the portion of the opening that actually allows air to pass, after accounting for the obstruction of the louvers. To find it, first calculate the gross opening area: 12 inches times 12 inches equals 144 square inches. Apply the given efficiency factor for the louver, 75%, by multiplying: 144 × 0.75 = 108 square inches. So the net free air is 108 square inches. The other numbers would come from using different efficiency factors or no obstruction, which aren’t what the problem specifies.