Which formula is used to calculate airflow in CFM for a duct?

The correct answer involves using the area of the duct, calculated in square feet, multiplied by the velocity of the air, measured in feet per minute. This approach provides the airflow in cubic feet per minute (CFM), which is the standard measurement for airflow in HVAC systems.

To elaborate, airflow through a duct can be understood through the basic principle that the volume of air moving through a specified cross-sectional area over a given period of time equals the flow rate; hence, the calculation involves both the area and the velocity of the air moving through that area.

When employing the area (in square feet) and the air velocity (in feet per minute), the product yields the total volume of air moving through the duct per minute, classified as CFM. This method encapsulates both the physical dimensions of the duct and the dynamics of airflow, which is crucial for effective heating, ventilation, and air conditioning design.

The other options do not provide the correct methodology for finding CFM. For instance, diameter times velocity might suggest a relationship to flow, but it lacks the necessary conversion to area for accurate calculation. Volume divided by time gives an average flow rate but doesn't directly relate to the duct size and air movement efficiently. Likewise, width times height provides