Thermal management systems feel the heat

The last 10 years in particular have seen a huge shift towards smaller, higher performance fans with a wider range of voltages, as Roy Sexton, business manager, fans, at Aerco, has observed. He describes the progress as 'remarkable', both technologically and in terms of demand. "Fans now come in many different thicknesses, in a wider range of speeds, and with performance levels that would not have been possible just a few years ago," he said. "All fans, in particular dc fans, are now better than ever." Sexton believes this is due to improved motor technology, better bearings and new housings. "These improvements, as well as the introduction of Venturi and impeller designs, have led to fans that spin at 16,000rev/min, compared with a maximum speed of 3600rev/min just a few years ago. They also now work with five times the airflow and at eight times the pressure." One of the most significant developments in recent years, notes Sexton, has been the move to double bladed and dual fans. "Companies like Delta, for example, are racing to produce extra deep frame sizes to give maximum performance in applications that require high airflow in high pressure environments." Delta's recently launched GFB range consists of fans that are 50.8mm deep and are available in frame sizes of 60, 80, 92 and 120mm2. Both frames and impellers are made from UL94V-O plastic and the twin ball bearing motor construction is designed to provide a mean time between failure of 70,000hr. New developments in ac fans are also helping engineers to achieve effective cooling solutions. Fulltech's UF60D series of small ac equipment fans is built in a 60 x 60 x 30mm frame size; the smallest ac axial fan frame available. This new series lends itself to applications that require small, densely packed electronic enclosures or housings that will not accommodate the previously smallest fan frame size of 80 x 80 x 25mm. The units feature a low noise emission of 28dBA at the maximum airflow of 4.8litre/s, enabling them to be used in noise sensitive applications such as desktop or medical instrumentation. "It's been a long time since the days when a simple heat sink, applied with thermal tape and coupled with a fan, were adequate enough to cool electronic systems," said Sexton. "Complex air flows now have to be detected. We also have to consider hot spots and identify and account for gradients. The demand for higher operating speeds and equipment with higher density has left us with significant thermal management challenges." Battling the laws of physics One such challenge, as Cyntech's president Dave Mellor points out, is the constant battle with the laws of physics. "The miniaturisation trend has significantly increased the density and amount of equipment and components that can be introduced into an enclosure," he asserted. "Thermal management is critical to performance and life expectancy and it's often an uphill battle trying to create more powerful devices at such small sizes." Mellor has witnessed considerable decreases in fan size over the last few years. "In 2009, the average fan size was 48mm. Then, before we knew it, it was 40, then 30, and now companies are working on 15mm designs. We've really had to keep pace." As well as the demand for smaller, more efficient devices, Mellor says there is increasing call for fans with lower power consumption, as more pressure is put on companies to 'go green'. One company trying to address this issue is GardTec. The fan accessory specialist recently announced a thermostatically controlled fan cord that allows ac fans to turn on/off as system ambient temperature calls for additional cooling. The patent pending Green Fan Cord turns on an ac cooling fan when the temperature reaches 30°C ±3°. The device will then turn off the cooling fan when the temperature reaches approximately 25°C. Its life expectancy is 100,000 cycles. According to company president David Peterson, the driving force behind the product was a need to create an easy to use, low cost alternative to large, expensive devices. "The Green Fan Cord's thermostat is built into the actual ac fan cord assembly," he explained. "The device requires no additional wiring, pc boards or mounting of controls as everything is built into a single fan cord assembly. The device costs just a few dollars and significant cost savings can be made." In terms of future development, Sexton warns that greater priority should be given to thermal management solutions during the design process. "Although designers are now more aware of thermal issues, requirements are so high these days that we would encourage people to come forward earlier to avoid unnecessary redesigns. Engineers no longer have to risk heat induced failure, but this is provided that thermal management issues have been tackled early in the product design life cycle." Mellor concurred: "The need for forced air cooling should be determined at as early a stage as possible in system electronics enclosure design. It is important that the systems designer has electrical, mechanical, magnetic, thermal, acoustic and chemical knowledge to design good airflow to heat generating components. It is also important to allow adequate space and power for the physical cooling solution." Despite other advances in thermal management, both Sexton and Mellor are confident that fans are still the best option for cooling and will continue to be so for at least the next 10 to 15 years. "Whilst other technologies have emerged over the years from various companies and educational institutions, none of them have actually come to fruition," concluded Mellor. "Engineers now have the choice of ac or dc fans, axial fans for high air flow, centrifugal blowers and much more. The wide choice now available only reaffirms that fans are the cheapest and most effective option."