With Thermal Engineer day approaching (7/24) we here at ATS would like to thank all of the PR firms and digital news magazines who covered our new clipKIT campaign.
As a token of our appreciation, we have provided a link to our customers and viewers to download our clipKIT data sheet for all your attachment needs. HERE.
Posted in ATS News, Uncategorized
Tagged Advanced Thermal Solutions, electronics, electronics cooling, engineering, engineering education, Fan, Fan Characterization, heat sink, heat sink attach, publication, QPedia, temperature measurement, thermal analysis, thermal conductivity, thermal design, thermal engineering, thermal management, webinar, Wind Tunnel
In today’s electronics industry, there is a constant and well documented push to higher powered components, tighter grouping of devices, and overall increased system thermal dissipation. The higher dissipation must be managed effectively to ensure long term reliability of the system.
With forced convection being the dominant mode of electronics cooling, more efficient heat sinks are often used for cooling these increased thermal loads. But, they are only half the solution. Due to volumetric constraints, it may not be possible to design an adequate heat sink for a given component. A large amount of air preheating may occur if multiple components lie in the flow path. The increased ambient temperature resulting from this preheated air often brings the need for a larger heat sink, but the space may not be available.
The solution to higher power levels and decreasing heat sink space is to increase the system’s air flow rate. A boost in flow rate has a twofold benefit: first, it lowers the thermal resistance of the heat sink, which reduces the temperature difference from junction to ambient. Secondly, it reduces the overall temperature rise in the chassis. The reduced temperature rise allows downstream components to suffer less preheating and operate at lower ambient temperatures.
This direct relationship between air velocity and component temperature indicates the importance of understanding how fans behave in electronics cooling.
System Curve
Prior to selecting any fan it is important to characterize the overall system with respect to air flow and pressure drop. For example, a tightly packed 1U chassis will require a much different fan configuration than a larger desktop one, even if both systems use the same CPU. In the 1U chassis, components are spaced very tightly and exhibit a large resistance to flow. This requires a fan with a high pressure head. A benefit of the 1U chassis design is less bypass flow, reducing the need for larger volumetric flow rate. In an ATX style desktop chassis the requirements are very much the opposite. There is typically much more open space in the ATX chassis, which lowers the chassis pressure drop. The widely spaced components create a less efficient flow path, and thus a larger volumetric flow is needed to ensure adequate cooling of all components.
Figure 1. Typical Overlay of a System Curve and Fan Curve
Fan Curve
A fan curve example is shown in Figure 1. Point A is the no flow point of the fan curve, where the fan is producing the highest pressure possible. Next on the curve is the stall region of the fan, Point B, which is an unstable operating region and should be avoided. From point C to point D is the low pressure region of the fan curve. This is a stable area of fan operation and should be the design goal. It is best to select a fan that operates to the higher flow point of this region to improve fan efficiency and compensate for filter clogging.
The system pressure curve can then be compared to a specific fan curve to determine if the fan is adequate. To compare fan curves from different manufacturers, it is important to follow a testing standard. For electronics applications, the relevant standard is the AMCA 210-99/ ASHRAE 51-1999 test guidelines.
Figure 2a. AMCA Fan Testing Chamber
The AMCA fan testing chamber, shown in Figure 2a, consists of a supply fan, a variable blast gate, two test chambers, flow nozzles and an opening to place the test fan. A commercialized testing module from Advanced Thermal Solutions, Inc. is shown in 2b.
During a typical fan test, a dozen or more operating points are plotted for pressure and flow rate, and from this data a fan curve is constructed. To obtain the highest pressure rating of the fan, the blast gate shown in Figure 2a, is closed to ensure zero flow while the fan is running. The chamber pressure is then read from the static pressure manometer to obtain the maximum pressure rating of the fan. The blast gate is then slightly opened in successive steps to obtain additional operating points. Finally, the maximum flow capability of the fan is found by opening the blast gate completely and running the supply fan. The supply fan ensures the secondary chamber is operating at atmospheric pressure, which removes the flow losses in the system.
Figure 2b. FCM-100 Fan Characterization Module from Advanced Thermal Solutions, Inc.
The operating pressure of the fan curve is found by taking measurements from a static manometer. The volumetric flow rate, Q, is found by measuring the pressure drop across an AMCA nozzle (Figure 3) using a differential manometer. The flow rate through an AMCA nozzle is a function of its size and differential pressure as shown in the following equation.
In contrast, the FCM-100 is void of any nozzles and works based on volumetric flow rate measurement using the ATVS technology flow sensing system. It is compact, portable and capable of characterizing single fans or fan trays.
Figure 3. Various AMCA Nozzles (CTS, Inc.)
Air Flow
Fan Laws
Fan laws are a set of equations applied to geometrically identical fans for scaling and performance calculations.
Published fan laws apply to applications where a fan’s air flow rate and pressure are independent of the Reynolds number. In some applications, however, fan performance is not independent and thus the change in Reynolds number should be incorporated into the equation. To determine if the Reynolds number needs to be considered, it must first be calculated.
According to AMCA specifications, an axial fan’s minimum Reynolds number is 2.0×106 When the calculated Reynolds number is above this value, its effects can be ignored.
Fan Law Application
During a product’s life cycle a redesign may be carried out which replaces older components with new, higher powered ones. Due to the resulting higher heat flux, increased cooling is often needed to maintain adequate junction temperatures and reduce temperature rise within the system.
Consider for example a telecom chassis using a single 120 mm fan for cooling. The maximum acceptable temperature rise in the box is 15°C. The chassis dissipates 800 W, but a board redesign will increase the power to 1200 W. The current 120 mm fan produces a 3³/min flow rate at 3000 RPM using 8 W of power. How do we calculate the requirements of a substitute fan for the higher powered system?
Next, calculate the change in RPM needed:
Thus, to meet this example’s cooling requirement for 1200 W, a fan is needed with a 4³/min flow rate, 4,000 RPM speed and 18.9 W of power. Note that the system power, flow rate and fan RPM all increased in a linear fashion from those in the original system. However, the fan power increased by nearly a factor of three.
Summary
Bulk testing of electronics chassis provides the relationship between air flow and pressure drop and determines the fan performance needed to cool a given power load. The fan rating is often a misunderstood issue and published ratings can be somewhat misleading. Knowledge of fan performance curves, and how they are obtained, allows for a more informed decision when selecting a fan. Continued and ever shortening product design cycles demand a “get it right the first time” approach. The upfront use of system curves, fan curves and fan laws can help meet this goal.
References:
1. Ellison, G., Fan Cooled Enclosure Analysis Using a First Order Method, Electronics Cooling, October 1995.
2. Daly, W., Practical Guide to Fan Engineering, Woods of Colchester, Ltd, 1992.
3. Turner, M., All You Need to Know About Fans, Electronics Cooling, May 1996.
4. Certified Ratings Program – Product Rating Manual for Fan Air Performance, AMCA 211-05 (Rev. 9/07).
This article was first published in Qpedia. To buy the complete Qpedia book set, please visit: http://www.qats.com/eShop/Qpedia
To learn more about Fan Characterization and the FCM-100, please visit: http://www.qats.com/Products/Specialty-Instruments/Fan-Characterization
Posted in Fan
Tagged education, engineering formulas, Fan, Fan Characterization, fan curve, fan law, fan test, heat transfer
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
Posted in Fans, Thermal Analysis, Thermal Design, thermal management
Tagged Fan, thermal design, thermal management
Tony Koryban’s mail bag archive today addresses a problem whereby the engineer only needs it to go full blast when the room temperature goes over 40ºC. Here’s the question:
I don’t need it to run at full capacity. I only need it to go full blast when the room temperature goes over 40ºC. Maybe I could put in a speed controller to slow the fan down when I don’t need all that air, and a thermal sensor to tell it to speed up again if the room gets hot. Slowing the fan down will definitely make it quieter. Before I do that, is there a way I can calculate the new operating point at reduced fan speed? And how much does the audible noise go down with reduced fan speed?
And Tony has his usually full, practical and humorous answer of course. Read Tony’s solution to this fan fun at Tony Koryban’s Mail Bag Archive.
During a product’s life cycle a redesign may be carried out which replaces older components with new, higher powered ones. Due to the resulting higher heat flux, increased thermal management is often needed to maintain adequate component junction temperatures and reduce temperature rise within the system. Fan Laws are useful mathematical tools to understand, compare and contrast different fan approaches. Using Fan Laws before choosing a fan can help engineers to make solid choices to reduce cost, experimentation and time to market. ATS’s five-page presentation on Fan Laws for thermal management which includes examples on how to use them, is a good introduction to this important engineering tool:
[slideshare id=4205722&doc=atsslidecastfanlaws-100521141058-phpapp02]
Posted in Fan, Fans, thermal management
Tagged air flow, Fan, Fan Characterization, thermal management