In part 1 of our 2 part series this week, “Selecting a Fan for Your Thermal Management System”, we talked about how the type of fan needs to be chosen based on chassis design and allowable space. We discussed a bit about fan types and how to estimate the amount of airflow a system needs. Today in part 2, we’ll talk about fan impendance curves, pressure drop, and the effect of multiple fans.
For high heat loads, with concentrated heat sources you must design to the worst case component. Spot cooling may be accomplished with internal fans, heatsinks, ducting etc.
Next the total system impedance curve is needed, or the very least, the system pressure drop at the desired flow rate. System resistance is defined as:
Where:
For practical purposes the value of can only be found experimentally or using computational fluid dynamics. Due to the complex nature of modern electronics enclosures an accurate value of cannot be derived analytically. In a modern electronics enclosure the airflow is turbulent and the value of can be conservatively chosen as 2.
Typical Fan and Impedance Curves (3)
The calculated flowrate at a specific static pressure can then be compared to a specific fan curve to determine if the fan will be adequate. An example fan curve is shown in figure 5. Point A is known as the “no flow” point of the fan curve, where the fan is producing the highest pressure possible. Next is the stall region of the fan, B, which is an instable operating region and should be avoided. The area from point C to D is the low pressure region of the fan curve; this is a stable region of fan operation and should be the design goal. It is best to select a fan that operates to the higher flow area of this region to improve fan efficiency and compensate for filter clogging.
In many systems a single fan cannot deliver the entire volumetric flow rate needed. In these situations multiple fans can be used, either in parallel or series configurations. In order to determine which configuration is more appropriate the system impedance curve is once again needed (figure 6). For a high impedance system two fans in series will produce a higher flow rate than in parallel. The opposite behavior can be expected in a low impedance system, where parallel fans are preferred.
Figure 6. Effect of multiple fans on system pressure and flow rate [Comair Rotron]
An important consideration that needs to be addressed once the fan has been selected is to configure them in a blow through or pull through configuration. The airflow into a fan can roughly be modeled as laminar, whereas the exit airflow is highly turbulent. This phenomenon can be useful in thermal management, for instance in a typical telecom sub rack. Due to the varied resistance that the PCBs impose and the close proximity of the fans (fan tray) to the cards, the laminar flow will assist in better velocity distribution in the sub-rack, which also functions as a plenum.
In the blow through configuration the turbulent air has a positive effect on the heat transfer coefficient which can be useful when dealing with concentrated heat sources. The blow through design allows the fan to push cooler air, which improves its pressure capability, and extends the life of the fan. In this configuration the enclosure is slightly pressurized which prevents unfiltered air from being drawn through the joints and gaps in the chassis.
References:
1. Fan Cooled Enclosure Analysis Using a First Order Method, Ellison, Gordon, N., Electronics Cooling, Vol. 1, No. 2, October 1995, pp. 16-19.
2. Practical Guide to Fan Engineering, Daly, Woods, Woods of Colchester, Ltd, 1992
3. All you need to know about fans, Mike Turner, Electronics Cooling, Vol.1 May 1996
4. Comair Rotron, Establishing Cooling Requirements: Air Flow vs. Pressure, www.comairrotron.com/airflow_noe.shtml, March 12, 2007
