As already discussed in the chapter "Interaction of Fan and System", fan characteristics depend to a large extent on the installation situation and thus on the inlet and outlet flow conditions. In the case of fans with a diffuser housing, the outlet cross-section A_2=A\cdot H_z is smaller than the connection cross-section A_3=A\cdot H. In this case, special attention must be paid to the definition of the discharge area on the pressure side with regard to the calculated dynamic pressure component p_{d,2} in order to interpret the characteristic curve correctly.
Static pressure recovery can be described as follows:
\Delta p_{fs}=p_{d,2}-p_{d,3}
The dynamic pressures are calculated according to:
p_{d,2/3}=\frac{\rho}{2}\cdot v^2_{2/3}
For the calculation of v_{2/3} in all cases the last cross-section A_{2/3} flowed through must be used. The correct determination of v_2 would be the integral solution (summation of the velocities per surface element). In practice, the average flow velocity is determined as follows
v_{2/3}=\frac{q_V}{A_{2/3}}
The dynamic pressure calculated with the simplified discharge velocity and thus the total differential pressure is too small compared to the actual differential pressure present (See VDI 2044).