Design considerations for air cooling electronic systems in high altitude conditions

Covers the impact of altitude on electronics cooling. MIL-STD 210 is reviewed to demonstrate the relationship of altitude to density as well as to outside air temperature. Once these relationships are understood the paper discusses the impact of altitude on the performance of air moving devices using the widely accepted fan laws. A discussion of both constant speed and altitude compensating fans addresses performance issues such as power dissipation, pressure drop, and free delivery of fans under these conditions. Once the fan performance has been characterized, the system impedance (or pressure drop) must be determined for the appropriate altitude. The paper covers how system impedance changes under laminar and turbulent flow regimes as altitude varies. This is most important since poor thermal designs have resulted when the flow regime was not known. Since every altitude has a unique system impedance, a modified system impedance curve, called the Sigma-Delta P curve, is presented. This collapses all of the altitude curves to one curve and is a powerful way to characterize a system for all altitudes. The impact of altitude on heat transfer is also discussed along with the effects of operating in the turbulent and laminar regimes. In order to demonstrate the concepts, an example problem is worked.