All of the aerodynamic aspects of a fan are exhibited in a fan curve such as is shown in Figure 3. The fan performance curve is one of the few curves that are read from right to left, because you start with healthy aerodynamic flow and follow it through to aerodynamic stall.
Figure 3: Fan/system interaction
However, in contrast to an airplane wing, there is life after stalling in a fan. A stalled fan continues to deliver air, but at an increased static pressure and a decreased volumetric flow rate, and also at the cost of an increase in noise. If noise is not a consideration, the fan can be utilized in this condition.
An energy viewpoint is helpful in understanding the fan performance curve. For example, at the shut-off point, the fan is in the condition of the maximum potential energy. At free delivery, the fan is in the condition of the maximum kinetic energy. Although neither of these extreme conditions are likely to occur in practice, they can be useful parameters in comparing fans.
The governing principle in fan selection is that any given fan can only deliver one flow at one pressure in a particular system. This "operating point" is determined by the intersection of the fan static pressure curve and the system pressure curve. Figure 3 illustrates the operating points of both high and low resistance systems. It is best to select a fan that will give an operating point being toward the high flow, low pressure end of the performance curve to maintain propeller efficiency and to avoid propeller stall. Each particular electronic packaging system should be analyzed for possible reduction in the overall resistance to airflow. Other considerations, such as available space and power, noise, reliability, and operating environment should also be brought to bear on fan choice.
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