The fan draws in freely, thus the static negative pressure and the dynamic pressure component at the fan inlet cancel each other out. The fan has to overcome only pressure-side resistances (see figure 3.1.2.a).
If the complete dynamic pressure has not been converted into static pressure in a diffuser (see figure 3.1.2.b), the static and the remaining dynamic proportion at the end of the diffuser section can be used as the fan pressure difference. If atmospheric pressure is present at the end of the diffuser section / exhaust section, only the dynamic component can be used as fan pressure.
