What is the small opening diameter of a round Y branch that has a large diameter of 18" and a smaller diameter of 12"?

To determine the small opening diameter of a round Y branch with a large diameter of 18 inches and a smaller diameter of 12 inches, it’s important to understand the typical proportions involved in the design of such a branch.

In ductwork applications, especially with Y branches, the smaller diameter is generally designed to facilitate effective transitions between the main duct and the branch. A Y branch typically maintains a proportion of the larger diameter for the small opening to ensure optimal airflow and efficiency.

Given that the larger diameter is 18 inches and the smaller diameter is provided as 12 inches, one can infer that common practice often involves the smaller diameter being less than half of the larger diameter for proper airflow management. When examining the choices, 13 inches aligns with this understanding as it's a reasonable and practical diameter that balances the need for airflow while maintaining structural integrity.

The choice of 13 inches also reflects common values you might encounter in standard engineering practices. The other options do not fit well within the typical ranges seen for round Y branches where good airflow and fitment must be taken into account.

By revisiting the design principles and understanding common diameter ratios helps solidify why 13 inches is the most applicable choice in this scenario.