We're a Big Fan of Forced Convection
We're a Big Fan of Forced Convection
In the thermal management industry, we are highly concerned with the convection type. Is it natural or is it forced convection? The answer to that question makes a big difference when it comes to designing a cooling solution. With natural convection, we use buoyancy to do the lifting. Forced convection solutions get outside help to move fluid through a system. Many applications need to make the switch 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.
Up to Their Own Devices
For air cooling, fan, blowers, and other air movers 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. All of 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.
Forced Convection for Air: Fans and Blowers
What's the Difference Between a Fan and a Blower?
Both a fan and a blower have a motor with engineered blades that produce a pressure differential when spun. The pressure differential is what drives the air flow through the device. The difference between the fan or blower is the direction the air is expelled from the blade. Axial fans pull air from one end of its axis of rotation, then force the air out along the other direction along that same axis.
Blowers, also known as centrifugal fans, pull air from the axis of rotation. It can either be one side or both sides of the axis. So instead of pushing out along the axis, the air is directed outward, generally guided by a housing or case that points the air flow perpendicular to the axis of rotation.
Things to Consider When Using a Fan or Blower
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 only so much oomph 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. Unfortunately, fans and blowers don't magically power themselves, so we 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. You don't want any burnt out boards and unhappy customers since you weren't good to your fans.
Ingress Protection (IP) Rating of Fans and Blowers
Since most fans used in thermal management applications are electric fans, environmental factors are an important consideration when picking out a fan. A common way to determine what sort of environment a fan can handle is by the Ingress Protection Rating, or IP Rating. The first number of the IP rating refers to the solid particle ingress and what size particle the device is protected against. The second number refers to the amount of liquid protection the device has from just little droplets to dunking the device in liquid.
Users are going to put your product through all sorts of torture. Unless your product will live out its days 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. The point is that your devices, and therefore your fans and blowers, must be designed and fabricated to handle whatever your users will throw at it. Take some time to consider how abusive your users will be to your product before you start picking out a fan or blower.
Fan and Blower Reliability
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 might get some short responses from unhappy customers in return. 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.
Other Air Movers for Forced Convection Solutions
There are other types of air movers that generate a forced convection cooling. They're a little different, but sometimes they're the right technology for a specific application.
You already know about oscillating diaphragms. When your own diaphragm pulls downwards, it quickly creates a larger volume within your lungs and generates a pressure differential. Air rushes in through your nose and mouth to correct the difference in pressure. That extra air is expelled when your diaphragm relaxes.
What you may not know is that that same process can be used to cool hot devices. By using this inhale-exhale process on a small and rapid scale, we can generate devices that blow out puffs of air quickly. This is how Aavid's Synjets™ and RazorJets™ can generate turbulent air flow. Since the air is so turbulent, it can actually be more effective than traditional fans and blowers.
The LED lighting industry has benefited from these oscillating diaphragm air movers. These air movers don't use any frictional parts like fans and blowers do. That makes then quieter and more reliable than fans and blowers since those have bearings that will wear out over time.
Fans and Blowers in Your Application
We're a bunch of fanboys and fangirls of forced convection, so we would love to help you upgrade your natural convection solution. Contact us through email, our social media pages or try comparing a natural convection and forced convection heat sink in Aavid Genie.