When a fan serves a whole duct network, the duty is a system operating point rather than a single airflow: the tool builds the network resistance curve from segment lengths, cross-sections, roughness and fitting zeta values, then solves it against the fan curves.
Segment losses and pressure profile：Every segment reports inlet pressure, its own loss and outlet pressure (gauge); the terminal may be a device resistance or an outlet static pressure that can be negative
Velocity, friction rate, plate thickness：The conveying duty sets the minimum transport velocity (below it ducts silt up), vertical runs may be 2 m/s lower, and standard diameters with steel plate thickness are recommended
Parallel branches and booster fans：The trunk can split into parallel branches (flow splits by resistance, with imbalance warnings), and long runs can take series booster fans
FAQ
Why is the operating airflow below the requirement?
The network resistance exceeds the head the fan can supply; the page then suggests a larger duct, a lower velocity or more units in series.
Guangzhou XIN FENG Co., Ltd. · Industrial Fan Selection System