Transitions are usually placed in a trunk or branch duct following a tap-in or branch because the airflow rate is reduced. An expanding transition at the fan discharge is usually used to reduce the main trunk velocity and its associated pressure drop. The least expensive transition maintains three sides straight while changing the fourth side. The changing of two or more sides presents both a layout and an installation challenge. Automated fitting fabricating machines have reduced the magnitude of this problem but it still exists. Ideally, a change in one dimension on one side in a one-section transition should create no more than a 5° angle (C=0.02).
