A fan’s performance cannot easily be described by a single figure. Thus it differs from a motor car, which for many years was specified by its horsepower, under a known set of conditions e. g., RAC or DIN etc.
There are two quantities which are of interest to the user — the volumetric flowrate and the pressure rise. Both quantities vary over a wide range, but they do have a fixed relationship with each other. The best way of defining this relationship is to plot a characteristic curve on graph paper. Ideally it will be plotted at a fixed rotational speed, although for some direct driven fans an “inherent-speed” curve may be desirable.
Almost invariably the volumetric flowrate is plotted along the baseline (the x axis) whilst the fan pressure is plotted as the ordinate or y axis. This is the minimum amount of information which would be given. Other performance characteristics such as absorbed power, efficiency and noise level can also be added as further ordinates. Examples of these are shown against specific blade forms in Section 1.6 and onwards.
The peak efficiency of the fan can always be found at a specific point or duty on the curve. Where efficiencies are also added as curve information, this is easily identified as the “best efficiency point” (b. e.p.). As operation here gives the lowest power consumption of a particular design, it is desirable from an energy efficiency viewpoint. It usually achieves the added benefit of the lowest possible noise level for that particular design.
Fans can however be operated at other points on their characteristic curves, where, for example a smaller fan at higher speed can be selected, albeit at lower efficiency and higher noise level. These duties will be to the right of the b. e.p. In like manner a fan, which is oversized, will to the left of b. e.p. when the fan could be “stalled” with increased noise and vibration and unsteady flow. In the case of axial fan sit could even result in inadequate cooling of the electric motor and/or motor overloading.
Posted in Fans Ventilation A Practical Guide