What is the name of the rectangular duct fitting that elevates a duct without changing its airflow direction or size?

The fitting that specifically elevates a rectangular duct without altering its airflow direction or size is called an ogee offset. This fitting is designed to provide a vertical rise while maintaining the duct's cross-sectional area and airflow characteristics. The design of an ogee offset allows for a smooth vertical transition, making it essential in applications where ducts need to navigate around obstacles or rise to meet other duct runs without causing turbulence or restricting airflow.

In contrast, other options, such as a taper, transition, and transitional elbow, are used for different purposes. A taper reduces or increases the duct size, which directly affects the airflow direction and capacity. A transition fitting serves to change the duct shape from one size to another, and a transitional elbow changes the direction of airflow, which does not apply in situations where an elevation is needed without altering direction or size.