What is the operating velocity of a 20" round galvanized duct that is 100' long and delivers 2000 cfm of air?

What is the operating velocity of a 20" round galvanized duct that is 100' long and delivers 2000 cfm of air?

• A. 800 fpm
• B. 900 fpm
• C. 1000 fpm
• D. 1100 fpm

Answer: (C)

To determine the operating velocity of a duct, you can use the formula that relates air flow (in cubic feet per minute, or cfm), duct diameter (in inches), and velocity (in feet per minute, or fpm). The area of the duct can be calculated first, which is crucial for substituting in the appropriate values. For a round duct, the area (A) in square feet can be calculated using the formula A = π × (d/2)², where d is the diameter of the duct in feet. For a 20-inch duct, that would convert to 20 inches / 12 = 1.67 feet. Calculating the area: - d/2 = 1.67 / 2 = 0.835 feet - A = π × (0.835)² ≈ π × 0.697 = 2.19 square feet (approximately) Now, to find the velocity, we can use the equation: Velocity (fpm) = Air Flow (cfm) / Duct Area (sq ft) Substituting in the known values: Velocity = 2000 cfm / 2.19 sq ft ≈ 910 fpm

To determine the operating velocity of a duct, you can use the formula that relates air flow (in cubic feet per minute, or cfm), duct diameter (in inches), and velocity (in feet per minute, or fpm). The area of the duct can be calculated first, which is crucial for substituting in the appropriate values.

For a round duct, the area (A) in square feet can be calculated using the formula A = π × (d/2)², where d is the diameter of the duct in feet. For a 20-inch duct, that would convert to 20 inches / 12 = 1.67 feet.

Calculating the area:

• d/2 = 1.67 / 2 = 0.835 feet

• A = π × (0.835)² ≈ π × 0.697 = 2.19 square feet (approximately)

Now, to find the velocity, we can use the equation:

Velocity (fpm) = Air Flow (cfm) / Duct Area (sq ft)

Substituting in the known values:

Velocity = 2000 cfm / 2.19 sq ft ≈ 910 fpm