What is the assumed percentage of free area for metal louvers and grilles when the design and free area are not known?

The assumption of 75 percent free area for metal louvers and grilles is based on industry standards for airflow calculations in HVAC systems. This percentage represents the portion of the total area of the louver or grille that can effectively contribute to airflow, accounting for obstructions and the geometry of the louvers or grilles themselves.

In practical applications, this value is used to provide a reasonable estimate when specific performance data is not available, allowing designers and engineers to make informed decisions regarding ventilation and air distribution systems. Specifically, a 75 percent free area is often considered a good balance, allowing for sufficient airflow while still recognizing that some area is not entirely effective due to design features.