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A Basic Fan/Blower Description
Types of Fans and Blowers
There are several types of fans, some of the most common being propeller, tube axial and vane axial styles. Propeller fans are the simplest type of fan, consisting of only a motor and propeller. One problem with propeller fans is that tip vortices are produced by the pressure differential across the airfoil section.
A tube axial fan (the most common type in electronic cooling systems) is similar to a propeller fan, but also has a venturi around the propeller to reduce the vortices. The vane axial fan has vanes that trail behind the propeller in the airflow to straighten the swirling flow created as the air is accelerated.
Centrifugal blowers may have a forward curved wheel, a backward curved wheel, or be of the squirrel cage variety.
The elevation angle is referred to as the angle of attack (AOA). The greatest airflow delivery from a fan occurs when the AOA is at a minimum, but the pressure differential across the fan is zero. As the AOA is increased, the airflow delivery decreases and the pressure differential increases. The airflow can decrease to nearly zero, but will also deliver the maximum pressure differential in this condition, which is called the shut-off point. When an AOA is reached where the air will no longer flow smoothly and begins to separate from the blades, an "aerodynamic stall" condition exists.
Since a fan is a constant volume machine, it will move the same volumetric flow rate of air irrespective of the air density. However, the mass flow rate does change as the density changes. This becomes important when equipment is expected to operate at altitudes significantly greater than sea level. Therefore the volumetric flow rate required at altitude (low density air) will be greater than that required to achieve the same cooling as at sea level.
The Fan Curve
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.
Steps to Fan Selection
Q· = m· Cp T
m· = G
This yields a rough estimate of the airflow needed to dissipate a given amount of heat at sea level. It should be noted that it is the mass flow rate of air, not its volumetric flow rate, that governs the amount of cooling..
Estimate the actual airflow
The experimental procedure can be used to measure the total airflow for specific fans or several pressure-airflow data pairs can be measured to develop a complete system resistance curve. The latter experimental method will then require the Engineer to superimpose the selected fan pressure vs. airflow curve and system resistance curve to obtain the operating airflow.
The airflow network procedure provides adequate results when the geometry is simple and the flow path within the cabinet is known or a rough estimate can be made. In many practical applications, however, the Designer deals with complex three dimensional flow paths that are not known from the very first instance. In these situations, CFD software can be used. The fan performance curve can be supplied as an input to the CFD software and the software system allowed to determine the operating point and system resistance. CFD works by numerically solving the governing equations of flow and heat transfer in three dimensions and takes into account the effects of turbulence and gravity. CFD can be used to study the performance of fans in series and parallel arrangements as well as optimize the location with respect to other objects inside the cabinet. Both of the computational procedures require a static pressure vs. airflow curve for the fan in question.
Irrespective of which method is chosen to estimate the system airflow, all packaging systems are characterized by a system resistance curve of the type shown in Figure 3. System resistance curves may usually be expressed as a non-linear expression of pressure vs. airflow:
P = KGN
Consider multiple fans
In series operation, the fans are stacked one upon the other, resulting in an increase of static pressure (doubling at shut-off, but less elsewhere). The best results for series fans are achieved in systems with high resistance. A fan curve simulating multiple, identical fans in series, may be constructed by scaling the fan curve pressure axis data in direct proportion to the number of fans.
In both series and parallel operation, particularly with multiple fans (5, 6, 7, etc.), certain areas of the combined performance curve will be unstable and should be avoided. This instability is unpredictable and is a function of the fan and motor construction and the operating point.
It is also important to consider fan placement in the enclosure. Pressurizing the enclosure is the preferred method, since incoming air can be readily filtered. In addition, a pressurized enclosure will prevent dust entering through cracks or crevices. The fan is also handling cooler, denser air, and it will therefore have a slightly higher pressure capability (this may be a very slight advantage for low heat dissipating systems). An important feature of a pressurized system is that the fan life and reliability are increased due to the fan ambient temperature being lower. The disadvantage of pressurization is that heat generated by the fan is dissipated into the enclosure.
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