What is the formula for calculating duct size based on airflow?

The formula for calculating duct size based on airflow is derived from the relationship between the airflow in cubic feet per minute (CFM) and the diameter of the duct required to effectively manage that airflow while minimizing friction loss. The correct formula, which indicates that duct size is proportional to the square root of the airflow times a specific constant (0.257), reflects the dynamics of airflow in round ducts.

Using this formula, if you need to determine the duct size for a given airflow, you would take the square root of the airflow multiplied by 0.257. This effectively adjusts for the physical characteristics of air moving through a circular duct, considering factors like air density and velocity. The inclusion of the square root is crucial because duct size affects the velocity of airflow; as you increase the size of the duct, the velocity decreases, which affects the pressure drop and overall efficiency of the HVAC system.

In contrast, other formulas that multiply or divide by airflow without the square root do not accurately reflect this relationship and could lead to improper sizing, resulting in inefficiencies, increased energy use, or inadequate system performance. Therefore, recognizing the importance of this relationship is essential for HVAC design and ensuring optimal system operation.