DIN EN ISO 5801 specifies the test conditions, the evaluations and the fan characteristic curve display forms of the measurement results. Thus, it generally applies only to separate fans offered for general installation conditions in catalogs, but not to fans that are part of various inflow variations with elbows, edges, guard grilles and similar individual resistances. Such inlet flow variations, which cause inhomogeneous velocity distribution (e.g. swirl), have a direct influence on fan performance. They cause so-called "system losses" in the fan itself. These losses cannot be determined theoretically in all details and are additionally dependent on the entire fan design. In the case of correspondingly unfavorable inlet and outlet flow conditions, the fan standard characteristics determined on a standard test rig in accordance with ISO 5801 cannot be assumed. A distinction must be made here between fan, unit and system characteristic curves.
The operating point of the fan is always the point of intersection between the fan characteristic curve and the system characteristic curve (resistance parabola). This results in the actual volume flow conveyed and the actual pressure difference to be overcome by the fan. Figure 3.5. shows two fan curves at different speeds and one system curve. It can be seen that the operating point always shifts to the system characteristic curve as the speed is increased.
Figure 3.5: Interaction Fan & System