The total fan ESP should overcome the system pressure drop, which should be the sum of pressure drop due to friction of air along the duct, and all losses through duct accessories and equipment such as dampers and grilles. Along the length of ducts with the greatest loss, only ONE terminal grille (exclude the branches) but ALL in-line dampers are to be considered.

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A general example of pressure drop of few types of duct accessories are shown below. The higher the "restriction" of the accessories to the airflow of the duct system, the higher the pressure drop of the system across the accessory.