When the free area of a metal louver is not known, which free area assumption is correct?

When determining the free area of a metal louver when the specific dimensions are not provided, the correct assumption typically falls within the range of 60%-75%. This range is based on standard practices in HVAC design and airflow calculations, where the effective opening of the louver contributes significantly to air movement but does not equate to the full size of the louver itself.

The justification for choosing the 60%-75% range is rooted in real-world testing and the common understanding of how louvers are designed. While they may look large, several factors affect the actual airflow through them, including the arrangement of the blades and any obstructions present. This range serves as a conservative estimate that balances adequate airflow with realistic expectations based on varying installations.

Higher percentage estimates, such as those in the other choices, would imply a greater effective airflow than is typically achievable due to physical limitations and the design characteristics of most metal louvers, making it less practical in several scenarios.