A commercial building requires 1800 L/s of supply air. The maximum allowable velocity in the main supply duct is 6 m/s. Assuming a rectangular duct with an aspect ratio of 2:1 (width:height), what are the approximate minimum dimensions of the main supply duct?

Question: A commercial building requires 1800 L/s of supply air. The maximum allowable velocity in the main supply duct is 6 m/s. Assuming a rectangular duct with an aspect ratio of 2:1 (width:height), what are the approximate minimum dimensions of the main supply duct?

Answer options:

• 600 mm x 300 mm ✅ 900 mm x 450 mm
• 400 mm x 200 mm
• 1200 mm x 600 mm

Correct answer: 900 mm x 450 mm

Explanation: First, calculate the required duct area: Area = Volume Flow Rate / Velocity = 1.8 m³/s / 6 m/s = 0.3 m². For a 2:1 aspect ratio, if height is H, width is 2H, so Area = 2H². Solving for H: 0.3 = 2H², H² = 0.15, H ≈ 0.387 m. Width = 2 × 0.387 ≈ 0.774 m. Approximating to standard sizes gives 900 mm × 450 mm.