Fans Have Few Fans, But Perhaps They Should? | Avnet Abacus

Fans have few fans, but perhaps they should?

By Mathias Goebel | May 28, 2015

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Fans are often looked at like insurance policies – they’re something you need, rather than something you want. A fundamental weakness of fans is that they have moving parts. However well they’re made, there will be some friction, some life-limiting wear, so they will often be the part of an electronic system with the shortest theoretical life expectancy. Added to this, fans create noise, use energy, require holes to be cut in electronic enclosures, and their accompanying filters may need regular servicing to keep them clean. So why would you ever want to use a fan?

The answer’s simple. Forced air-cooling with a fan means that you can make systems much smaller for a given power rating. And you can direct airflow to keep the components with greatest power dissipation below the critical temperature at which they might fail. Consider a typical AC-DC power supply as an example. The RPS-300 AC-DC (down) open frame unit from Mean Well is rated at 200W with convection cooling but this goes up to 300W with 20.5 CFM of forced air cooling. Using a fan can avoid having to use a significantly larger and more expensive power supply.

Careful design can mitigate the other potential downsides. Noise can be minimised by judicious placement of the fan and by choosing a ‘low noise’ model. In particular, it’s important not to position a fan too close to other components because this can restrict the intake and exhaust currents, causing air flow to diminish and noise levels to rise. As a rule-of-thumb, the distance between the fan and adjacent components should be at least equal to the depth of the fan.

Here’s a short overview of the types of small fan available today. Noise and energy consumption can be minimised with electronic controls that ensure the fan runs only when it’s needed, and then only at the speed required to achieve the desired airflow – just like automatic climate control in a car. In many applications, air filters won’t be needed, so the maintenance issue isn’t relevant, and the operating life of a high quality, small DC fan (at 40 degrees C ambient) is likely to be in excess of 100,000 hours, and possibly as high as 170,000 hours. That’s nearly 20 years, which could well be greater than the design-life of the end system.


	The fan curve plots pressure against airflow under a standardised set of conditions that are chosen to represent ‘typical’ applications

Calculating the required airflow and choosing the most appropriate fan for the job can be challenging, particularly as it’s something you may not have to do very often. DIN 24 163 is used to create fan curves, like the one shown, that determines the optimum operating range in which fans work best with respect to efficiency and noise levels.

The problem with relying upon fan curves is that few applications are ‘typical’. Every enclosure is different and even a relatively small physical obstruction, perhaps something as simple as a heat sink, can significantly disrupt the flow of air.

The most common approach to this dilemma is to over-specify the fan to ensure sufficient airflow under worst-case conditions. There can be a lot of trial-and-error involved. This extends product design time, and the end result is often to over-specify the fan in terms of the airflow capacity in order to account for worst-case scenarios. Equipment failures through over-heating can be too catastrophic to contemplate!

This is where thermal modelling tools can be invaluable. The ability to model airflow in complex 3D enclosures, and identify potential hotspots, means that effective thermal management can be achieved through simulation, rather than trial and error.

The market-leading product in thermal modelling software is FloTHERM from Mentor Graphics. First launched 25 years ago, it’s a computational fluid dynamics (CFD) tool that integrates with mechanical CAD and EDA software. However, it’s a rather specialist tool for which significant training is usually needed. Future Facilities with the 6SigmaET CFD tool (left) for thermal simulation is a relatively new entrant to the market. The company claims that the tool is so powerful that it can halve the time needed for thermal design. Their training course is only 2 days and most of the company’s customers don’t take it up, which seems to indicate that it’s a relatively straightforward tool to learn.

If you’d like to know more about fan types, how they work and how to calculate what you need, this guide from Papst is a great starting point, however if you would like to get in touch with one of our regional Product Specialists directly, click the Ask an Expert button below.