Five Approaches to Cooling Military Electronics
Seventy-one degrees C is the temperature of a steak done medium-well. It is also the temperature of an oven used to test thermal characteristics of military electronics. Electronic components in the COTS industry often have temperature limits of 85°C, leaving 14°C of thermal potential to move the heat generated by the components away. Among various cooling approaches, the correct solution for your application depends on meeting your requirements at the lowest possible cost and complexity.
Approaches to Thermal Management
The enclosure type identifies the cooling approach used. A vented approach encloses the CCA with a sheet metal box with holes provided in the top and bottom to allow air to directly cool the CCA. An example of a vented enclosure is a television. Fan-driven venting adds forced air to the enclosure, as used in laptop computers. Sealed with exposed fins means that the CCA is separated from external environments by the enclosure. Fins are added to the exterior of the box. Computers in armored fighting vehicles often use this approach. Sealed with fan-cooled fins adds forced air to the exterior of the enclosure. This approach shares popularity with fandriven open venting in aircraft electronics. Cold-plate presumes that cooling happens away from the enclosure, with heat moving through the material of the enclosure to a cooler temperature. The possible sources of the cool temperature are not included here.
Other factors to be considered in the evaluation of appropriate technologies for military electronics include thermal performance, enclosure weight, weight-specific performance, environmental susceptibility, and design and manufacture. The higher the thermal performance rating, the better. Thermal performance is the amount of heat in watts dissipated for each degree C that the CCA increases in temperature. Enclosure weight, measured in kilograms, depends on the substances used to create the enclosure. Simple sheet metal is the lightest component. A sealed box with large external fins can become impractically heavy. Weight-specific performance divides the performance number by the weight added to the CCA simulation by the cooling enclosure. Environmental susceptibility includes factors such as mechanical shock, electro-magnetic disturbances, and airborne contaminants. Design and manufacture takes into consideration the complexity of the design including a parts count and ease of procurement/ assembly.
Simulation
The vented design shown in Figure 2 is as simple to design and build as possible. The air in direct contact with the CCA heats up, creating air movement. After factoring in weight, a free-air vented system provides superior performance per pound of enclosure when compared to the sealed free-air system. However, at an ambient temperature of 71°C, most 3U CCAís dissipate more heat than this design can handle.
A sealed enclosure with exposed fins is shown in Figure 3. It was constructed from the previous enclosure by sealing the vents and adding external finning. Minimal environmental susceptibility is expected from this design, as the CCA is protected from exposure to environmental contamination and the lack of a cooling fan eliminates moving parts. The trade-offs are weight, parts count, and parts complexity. Although the highest power CCA can be kept cool with this design, the amount of cooling surface needed becomes impractically large.
The fan-cooled version of the sealed enclosure is shown in Figure 4. It shares all aspects of the previous design regarding parts complexity and adds a fan to the parts count and to environmental susceptibility. The payoff is in thermal performance. This finning approach weighs one tenth of the previous version, has only one sixth of the cooling area, but still performs twice as well. It performs 4 times better than the previous version if weight is factored in.
The trade-off is dependency. Implied in this approach is a separate heat sink, whether it be a vehicle, the hull of a ship, or a refrigeration system.
No matter what the application in military electronics, available thermal technologies can accommodate the heat generated and dissipated. If the next breakthrough in electronic hardware technology is significant, every enclosure would ideally be a vented box. Until the creation of such revolutionary technology, thermal engineers need to evaluate their options in selecting a cooling method on cost, complexity and weight to keep todayís electronics from damaging applications.
This article was written by Mark Dimick, Mechanical Engineer, Curtiss-Wright Electronic Systems (Santa Clarita, CA). For more information, visit http://info.hotims. com/34459-502.
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