When fans discharge freely, which is often the case in practice, the entire dynamic pressure component, related to the discharge cross-section, is lost. It is the flow energy that is lost when the air passes out of the system into the atmosphere. Lost means that the dynamic portion must be considered as a loss, since the power requirement of a fan must be considered proportional to the total pressure increase. Diffusers significantly reduce the discharge velocity and thus reduce the dynamic pressure loss. With the same static pressure difference, the total pressure difference to be generated can therefore be reduced and thus also the torque or motor power.
Finally, it should be mentioned that the standardization basically allows power measurement with and without compensation distance. This alone results in the following theoretical and practical fundamentally different power curves. The three underlying test specimen arrangements shown in Figures 3.6.1, 3.6.2 and 3.6.3 have been compared by measurement on a suction-side chamber test bench. The influences of the different discharge geometries are shown in the characteristic curves
