How Cool Is That? Ensuring Effective Thermal Design

Proper thermal design of any electronics-based system is key to its long-term reliability. NASA engineering expertise in this area is renowned. The International Space Station (ISS) operates in a temperature environment from 250 degrees F (121 °C), down to a minus 250 degrees F (-157 °C), while maintaining a survivable internal temperature. Yet, in the commercial electronics industry many systems engineers have limited knowledge about thermal design. Furthermore, military and industrial customers with wide temperature range applications, want to save money by using commercially available off-the-shelf (COTS) equipment.

The Uptime Institute, a leading data center authority, states that for every 18 degrees Fahrenheit increase in temperature above 70 degrees Fahrenheit, the long–term reliability of a data center is reduced by 50%. This is true for most COTS equipment, as it has been designed for use in a protected home, office, laboratory or computer room environment. A manufacturer’s specification sheet will indicate its operational and storage temperature ranges.

Equipment Cooling Methods

Before the electronics located inside the device’s enclosure can be kept within its operational temperature range, the heat generated by the electronics must be removed from the enclosure. For commercial equipment this is typically accomplished in two ways, convection or forced air (cooling fan).

Convection cooling is accomplished by properly placing cooling vents to the outside of the enclosure, often directly above the heat source. Digital-based equipment can produce a lot of heat and if convection cooled, like most home entertainment equipment, the equipment’s location and placement require special attention. It is convenient to stack other equipment on top, but this approach blocks or restricts the cooling convection air flow, often resulting in erratic operation or failure. For example, DIRECTV states in their converter box installation instructions that nothing should be installed above the cooling vents located on top of the converter box. Due to the large size of the converter box, customers often place a DVD player or other audio-video components on top, restricting the converter cooling vent airflow. According to DIRECTV, this is one of the major causes of the converter box malfunctioning or failing prematurely.

Forced air cooling is much more effective than the convection method, especially when coupled with good internal heatsink and plenum style enclosure designs. The heated air is removed from the equipment enclosure by a fan or blower, which also draws cool air into the enclosure through vents. Depending on the cooling requirements, low to high volumes of air can be moved through the enclosure. Typically the heated air is exhausted out the rear or side of the enclosure. Again, care must be exercised when installing the equipment. Installation in a confined space can cause the heated exhaust air to be directed back to the cool air inlet, causing the air to be recirculated, increasing the internal electronics temperature. Should the equipment be installed inside of an equipment rack with other heat generating equipment, the air flow direction should have cool air entering the equipment’s front panel and heated air out the rear panel. An air flow in the reverse direction can cause air superheated by other equipment inside the rack to overheat any equipment having the reverse air flow.