What is the name of the fitting that modifies duct size without altering airflow direction?

The fitting that modifies duct size without altering airflow direction is known as a transition. A transition fitting facilitates a change in duct size, either increasing or decreasing it, while maintaining a straight airflow path. This is crucial in HVAC systems to ensure that the air moves efficiently from one duct size to another without creating turbulence or altering the direction of airflow.

In practical applications, a transition fitting would typically be used when connecting ducts of different diameters, allowing for an effective and seamless flow of air. This is particularly important in maintaining the system's overall performance and efficiency.

The other options are fittings used for specific purposes. A reducer decreases duct size but typically may not be considered a transition as it might involve a change in airflow conditions. An elbow changes the direction of airflow, which is the opposite of what a transition fitting does. An offset, while also altering direction, involves adjustments in placement and might include bends, which again does not align with the function of a transition fitting.