The ongoing trend in the electronics industry is for increasingly high-powered components to meet the ever-growing demands of consumers. Coupled with greater component-density in smaller packages, thermal management is more and more of a priority to ensure performance and reliability over the life of an electronics system.
As thermal needs have grown, engineers have sought out different cooling methods to supplement convection cooling. While options such as liquid cooling have grown in popularity in recent years, still one of the most common techniques is to add fans to a system.
Through the years, fan designs have improved. Fan blades have been streamlined to produce great flow rate with less noise and fans have become more power-efficient to meet the desires of customers trying to use less resources and save costs.
While much has changed in the presentation of fans, there are many basic concepts that engineers must consider when deciding how to implement fans in a project.
This is part one of a two-part series on how to select the best fan for a project. Part one will cover the types of fans that can be used. Part two, which can be found at https://www.qats.com/cms/2017/03/10/analysis-of-fan-curves-and-fan-laws-in-thermal-management-electronics, will cover fan laws and analyzing fan curves.
COMMON TYPES OF FANS AND BLOWERS
As described by Mike Turner of Comair Rotron in an article for Electronics Cooling Magazine, “All You Need to Know About Fans,” fans are essentially low pressure air pumps that take power from a motor to “output a volumetric flow of air at a given pressure.” He continued, “A propeller converts torque from the motor to increase static pressure across the fan rotor and to increase the kinetic energy of the air particles.”
In a white paper from Advanced Thermal Solutions, Inc. (ATS) entitled, “Performance Difference Between Fans and Blowers and Their Implementation,” it was added that fans are at their core, dynamic pumps. The article added, that in dynamic pumps “the fluid increases momentum while moving through open passages and then converts its high velocity to a pressure increase by exiting into a diffuser section.”
The biggest difference between a fan and a blower is the direction in which the air is delivered. Fans push air in a direction that is parallel to the fan blade axis, while blowers move air perpendicular to the blower axis. Turner noted that fans “can be designed to deliver a high flow rate, but tend to work against low pressure” and blowers move air at a “relatively low flow rate, but against high pressure.”
The three types of fans are centrifugal, propeller, tube axial, and vane axial:
• In centrifugal fans, the air flows into the housing and turns 90 degrees while accelerating due to centrifugal forces before being flowing out of the fan blades and exiting the housing.
• Propeller fans are the simplest form of a fan with only a motor and propellers and no housing.
• Tube axial fans, according to Turner, are similar to a propeller fan but “also has a venture around the propeller to reduce the vortices.”
• Vane axial fans have vanes trailing behind the propeller to straighten the swirling air as it is accelerated.
The most common fans used in electronics cooling are tube axial fans and there are a number of manufacturers creating options for engineers. A quick search of Digi-Key Electronics, offered options such as Sunon, Orion Fans, Sanyo Denki, NMB Technologies, Delta Electronics, Jameco Electronics, and several more.
FACTORS TO CONSIDER WHEN PICKING A FAN
When selecting a fan, engineers must consider the specific requirements of the system in which they are working, including factors such as the necessary airflow and the size restrictions of the board or the chassis. These basic factors will allow engineers to search through the many available options to find a fan that fits his or her needs.
In addition, engineers may look towards combining multiple fans in parallel or in a series to increase the flow rate across the components without increasing the size of the package or the diameter of the fan.
Parallel operation means having two or more fans side-by-side. When two fans are working in parallel, then the volume flow rate will be increased, even doubled when the fans are operating at maximum. Turner added. “The best results for parallel fans are achieved in systems with low resistance.”
In a series, the fans are stacked on top of each other and results in increased static pressure. Unlike parallel operations, fans in a series work best in a system with high resistance.
The ATS white paper noted, “In real situations, the fans may interfere with each other. The end results is a lower than expected performance.” Turner warns that in either parallel or series configurations there is a point in the combined performance curve that should be avoided because it creates unstable and unpredictable performance, but analyzing fan performance and fan curves will be covered in more detail in part two of the blog.
Efficiency is a major factor when selecting a fan. As noted in an article from Qpedia Thermal eMagazine, “A large data center contains about 400,000 servers and consumes 250 MW of power. It has been estimated that about 20% of the total power supplied to a high end server is consumed by fans.”
Clearly, finding a fan that can work efficiently with lower power will save a considerable about of resources. The article details several methods for creating efficiency in designing a system that includes fans:
“Fan power consumption is traditionally reduced by controlling the motor speed to produce only the airflow required for adequate cooling, rather than operating continuously at full speed. Significant energy savings can be achieved beyond this technique through fan efficiency increase. Optimizing the motor and electronic driver, increasing fan aerodynamic efficiency through careful redesign, and optimizing fan-system integration are three ways of achieving this.”
Read more about the techniques for achieving efficiency at https://www.qats.com/cms/wp-content/uploads/2015/03/Designing_Efficient_Fans_for_Electronics_Cooling