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Forced convection thermal management
In thermal management, we are concerned with the convection type. Is it natural or is it forced convection? The answer has a significant impact on the cooling system design. With natural convection, buoyancy drives fluid flow. Forced convection solutions bring in external energy to move fluid through the system. Many applications transition from a natural convection solution to a forced convection one when more heat transfer is required and adding surface area isn’t an option. Adding more surface area can add more weight to a cooling solution or the extra surface area might choke flow produced from natural convection.
For air cooling, fan and blowers mechanically force air through a cooling solution like a heat sink, thus increasing the amount of heat transfer with a given amount of surface area. These devices come in different sizes, performances and other options. In this post, we’ll focus on air forced convection and the devices that create those air flows.
PQ curves, also known as pressure-flow curves, characterize the performance of a fan or blower. The motor that drives the fan has a certain amount of speed and torque it can handle. A motor has a limited amount of energy to push air through the impeller blades. This relationship on how much air it can move at particular speed is depicted in the PQ curve for a fan or blower. If you try to exceed this curve, the motor will stall out and then you won’t have any flow for your application. That leads to overheating and potential product failure.
Not only do you need to consider the mechanical flow performance of your fans and blowers, you also need to check how to power your fan. You need to design a board or power supply to give the voltage and current a fan needs to push air. While the voltage a fan utilizes can be consistent, the current it draws can vary greatly from manufacturer to manufacturer. Make sure you supply your fan with the right voltage and current while you still maintain a significant safety factor on the current draw.
Users are going to put your product through tough environmental conditions and use scenarios. Unless your product will stay in a clean room, your device and your fan or blower will experience dust. Consumer electronics experience all sorts of dust sources, from humans and pets shedding to stray fibers from clothing and furniture. Plus those same consumers are prone to spilling their beverages, both hot and cold, on your devices. Products on manufacturing floors will not only see those particles, but they might also see grease and grime from machines. Your fans and blowers must be designed and fabricated to meet expected application conditions. Take some time to consider how your product will be used before you select a fan or blower.
How long is your product’s life span? If your product requires active cooling, it’s going to require robust fans or blowers over the entire course of the product’s life. Specifying a fan that will wear out before the expected lifetime of your product will cut the product’s usability window short and you may receive warranty claims or negative customer feedback. Otherwise you may need to plan for mid-life maintenance where you need to replace the fans to extend the lifetime of your product. In some cases, that is difficult and impractical for your end users.
Fan and blower reliability and lifetime is heavily dependent on what type of bearings the fan or blower uses. Less expensive fans typically use sleeve bearings to support the fan impeller within the fan frame. Fans using ball bearings will wear more slowly over time, giving those fans a longer lifetime. Other fans have been known to use rifle bearings, fluid bearings or even maglev bearings, all of which have the aim to increase the overall lifespan of the fan. Be careful in selecting what sort of bearing your fan or blower uses and how that affects your overall product lifespan.
Need assistance in updating your natural convection application into a forced convection one? Need help picking the right fan or blower? Contact our experienced Eaton engineers today.
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