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.
Avoid Thermal Problems
Even when equipment has been installed in a room or area with a good air exchange and cooling rate, other challenges exist with thermal design. What happens when a military or industrial customer wants to install the COTS equipment inside of a sealed NEMA 4 enclosure? Further the NEMA 4 enclosure is to be installed outside in Phoenix, Arizona having an ambient temperature range of 0 °F to 131 °F. This is exactly what is often requested. One such case involved a 3kVA on-line UPS that generates about 450 watts of heat. Since the NEMA 4 enclosure was to be sealed, there was no way to get the heat out of the enclosure. Special NEMA 4 air conditioning could be used, but would require an excessively large NEMA 4 enclosure to accommodate the large heat exchanger necessary for proper cooling. Even if air conditioning could have been used, preventing the recirculation of heated UPS exhaust air inside the NEMA 4 enclosure would have been almost impossible to control. Even with the facts stacking up against a successful project implementation, the engineer still wanted to attempt the project.
Seeing a reasonable demand, the company solved the problem by designing a family of wide temperature range UPS products using existing COTS technology as a base design to reduce the design costs. The goal was to develop a family of 1kVA to 3kVA double conversion UPS models having an operational temperature range of -22 °F to +149 °F (-30 °C to +65 °C). To accomplish this task, our design engineers first made modest design changes to improve operating efficiencies. Next, all components were reviewed with respect to their thermal parameters and where needed, were replaced with higher temperature versions. Cooling was improved by replacing fans with ones having a higher flow rate. Since the original UPS model had an LCD display panel which would not function at the higher temperatures, it was replaced with a high temperature LED display panel. Plastics used to mold the UPS front panel had to be changed to a different type. Finally, the UPS battery type had to be changed to maximize its life when operated at the higher temperatures. The end result was the development of a family of wide temperature range UPS models with the desired operational temperature range.
Batteries Versus Temperature
Most high-power-density battery types inside COTS products have a couple of things in common. Because they are electrochemical devices, at temperatures below -10 °C to -30 °C their ability to deliver current becomes impaired. At high temperatures above 30 °C, the batteries’ service life will start to be reduced. For example, per the battery manufacturer, a 12-year-rated Valve Regulated Sealed Lead-Acid (VRLA) battery maintained at 25 °C has an expected service life of 12 years. The same battery used in a 30 °C environment should have a 10 year service life. At 60 °C its expected service life will only be one year. Once this is understood, battery replacement schedules can be adjusted accordingly.
Successful Real-World Implementation
Shortly after the development of the wide temperature range UPS models, we were approached by one of the world's leading manufacturers of wind turbine control systems. This company’s primary focus is on the lifetime performance of their control systems and their customers’ wind turbines. The engineering challenge is keeping thousands of wind turbines operating at more than 99% uptime, while the controllers are subjected to the most extreme temperature swings. ROI (return on investment) is the most critical factor when choosing a wind turbine control system.
They needed to integrate an online UPS having a wide temperature range to power the segment of their control system that assures continuous control of wind turbine blade pitch and braking. This control segment is essential to safely operate the wind turbine. Failing to control the blade pitch could destroy the turbine, its tower, and possibly cause loss of life. Since wind turbine systems are intended to be installed in worldwide locations having a wide range of temperatures, the company required a UPS that could operate at the widest temperature range possible. Since the UPS was to be installed in an enclosed area inside the turbine, along with other heat generating equipment, low temperature operation was not an issue. Due to the amount of energy required, air conditioning could not be provided, requiring a UPS with a high operational temperature rating.
In conclusion, most COTS electronic equipment has been designed to operate and provide the highest reliability when operated in temperatures between 32 °F to 70 °F (0 °C to 21 °C). Higher temperatures above this range will reduce the equipment’s service life. Temperatures above 40 °C can affect equipment performance and will drastically reduce the equipment’s service life and reliability. To prevent the costs associated with thermal issues, proper equipment installation and cooling are essential. Where high temperature operation is mandated, only use equipment that has proper thermal specifications.