3.1. Fan Pressure Curve

Fans have the task of conveying a volume flow and supplying it with as much energy as is necessary to overcome the resistance. The flow through the respective system creates a pressure gradient between the beginning and the end of the flow. The design basis for the pressure increase of a fan is the total pressure difference  p_f between fan inlet and outlet. This is referred to as the fan pressure.

Figure 3.1. shows schematically how the static, dynamic pressures and the total pressure increase in a suction and discharge line differ. They are plotted as negative pressure (-) or positive pressure (+) with respect to the atmosphere and are accordingly given negative or positive signs. Dynamic pressures are always given positive signs, since they have no direct relationship to atmospheric pressure. For our body, the atmosphere is apparently pressureless, since an equal counterpressure acts in it. In air and valve gate technology, the approach is similar and all pressures are related to atmospheric pressure as the "zero level".

Figure 3.1: Fan Pressure Curve

Fan pressure:  p_f=\Delta p_{tot}=p_{tot,2}-p_{tot,1}=p_2+p_{d,2}-\left(-p_1+p_{d,1}_ )

Static fan pressure:  p_{fs}=p_2-p_{tot,1}=p_2-\left(-p_1+p_{d,1}_ )

In practice, it is often common to specify  \Delta p to the fan manufacturer as the design operating point. However, from the following installation situations with a schematic pressure curve, it can be seen that the pressure specification at the operating point is not unambiguous and depends on the installation case. The guideline VDI 2044 "Acceptance and performance tests on fans" therefore states:

"The specification of a pressure increase  \Delta p only makes sense if the location and size of the free flow cross section of the measuring cross sections at (1) and (2) of the fan are precisely defined at the same time."