In area-based duct sizing, what is determined from cross-sectional area?

In area-based duct sizing, what is determined from cross-sectional area?

• A. The required flow and velocity
• B. The color of duct
• C. The length of duct
• D. The weight of duct

Answer: (A)

In area-based sizing, air flow and speed are linked through the cross-sectional area: Q = V × A. The cross-sectional area sets the velocity you’ll achieve for a given flow, or the flow you’ll get for a given area. That’s why the cross-section determines the required flow and velocity—the design is choosing duct size to meet the desired air volume while keeping the velocity within acceptable limits. For example, if you need 1000 CFM and want about 900 ft/min of air in the main duct, you’d pick an area A ≈ Q/V = 1000/900 ≈ 1.11 ft². From that area, you’d determine the duct dimensions to fit that size. Other options—color, length, or weight—don’t follow from cross-sectional area in the same way, since they depend on materials, routing, and construction rather than the airflow performance alone.

In area-based sizing, air flow and speed are linked through the cross-sectional area: Q = V × A. The cross-sectional area sets the velocity you’ll achieve for a given flow, or the flow you’ll get for a given area. That’s why the cross-section determines the required flow and velocity—the design is choosing duct size to meet the desired air volume while keeping the velocity within acceptable limits.

For example, if you need 1000 CFM and want about 900 ft/min of air in the main duct, you’d pick an area A ≈ Q/V = 1000/900 ≈ 1.11 ft². From that area, you’d determine the duct dimensions to fit that size. Other options—color, length, or weight—don’t follow from cross-sectional area in the same way, since they depend on materials, routing, and construction rather than the airflow performance alone.