3.1.1. Suction Side Operation

The fan discharges freely and has to overcome resistances on the suction side (see figure 3.1.1.a). The static pressure at the outlet corresponds to the ambient pressure. The dynamic pressure at the outlet cannot be used to overcome pressure resistances.

Figure 3.1.1.a: Suction-side operation

By means of an outlet diffuser following the fan, the dynamic pressure component can be reduced and converted into static pressure (see Figure 3.1.1.b).

Figure 3.1.1.b: Suction-side operation with discharge diffuser

As described above, the dynamic pressure at the outlet is to be considered as a loss. Therefore, the usable pressure increase of the free discharge fan is defined as  p_{fs}. In the case of suction-side fan test benches according to ISO 5801, the suction-side dynamic pressure component must also be neglected due to the cross-sectional size of the measuring chamber. Figure 3.1.1.c shows an example of this with a fan without scroll housing.

Figure 3.1.1.c: Suction-side test bench