Introduction:

Every now and then we receive the (somewhat surpised) question or statement that our system uses fans for cooling. This is quite understandable as there is a major tendency in industrial computing to go fanless.

However, at White Bream, we have some very good reasons not to follow that same route, and to stick with good ol´ fan-based active cooling of the system. Read on to learn why, and to learn why fans don´t have to be so bad at all!

Cooling requirements:

The size of these computer systems is among the smallest in the industry. Needless to say that this is one of our unique features. But when considering fanless, one needs to start with a rather big case, especially if the demand on operating temperature range is high.

To cool a 35W CoreDuo processor at an ambient of 60°C, the cooling solution needs to have a thermal resistance less than ( Tcpu(max) - Tamb ) / Pcpu = (100°C - 60°C / 35W) = 1.1°C/W. A quick browse in an electronics components catalog learns that this requires a passive heatsink no less than 150x150x27mm (6x6x1.1"). That´s already ~75% of the total volume of our system, and that is just the heat coming from the CPU...

There is one more reason not to rely on passive cooling, and that is the power distribution throughout the system. Next table lists some system configurations and the estimated total (internal) power consumption, and the power consumption that is attributed to the (passive) cooled components, typically just the Intel chipset and processor:

System:Options:System power:CPU Power:
UNIGOT2050, 1GB, GPRS, HD, QUSBxx.x Wxx.x W = xx %
UNIGO/MINIZ510, 512MB, GPRS, SD, DVI15.9 W3.5 W = only 22%

With the UNIGO/MINI as worst case example, a passive cooling solution which provides a thermal coupling between CPU, chipset and the system case will dissipate only 22% of the heat generated in the system. The other 78% is cooled only by internal passive convection, which is usally far from enough. Only a rather big case can provice the necessary material surface to accomodate this heatflow. Pity that having exceptional small cases is one of our unique selling points...

This overview makes it pretty clear that a cooling solution that only targets the ´classic´ hot-spots, CPU and chipset, is not sufficient anymore with modern low-power microprocessors.

Reliability & quality:

It is a very common misconception that fans in the system have to reduce reliability of the system. Of course it does, but so does every other component that is added to the system. It is also true that fans are very oten the least reliable parts of a computer. But the truth is that this really depends on how much you are willing to spend on these fans...

Sunon Maglev The fans that are used in the White Bream mobile computer systems are Sunon Maglev units, one of the most reliable fan architectures available. In addition, we have chosen to configure these fans as ´inlet compressors´, which means they can work at ambient temperatures, instead of the much hotter temperatures found at the outlet air vents. If we assume that the fans operate at a temperature of 30 °C, then they have a typical lifetime of no less than 224752 hours. Yes; that is equal to a lifetime of approximately 26 years! Hence much longer than the lifetime of the average vehicle the system is build into, and certainly way much longer than the practical lifespan of the computer system itself (particulary in terms of obsolecence due to new software, new communication technologies, etc).

Just like S.M.A.R.T.:

Despite the long life expectancy of the fans, they still can fail prematurely. An undetected fan failure will jeopardize the systems reliability due to elevated temperatures, which ultimo results in premature complete system failure. Therefore the system controller monitors the fan speed setpoint and effective RPM speed. When these two deviate too much, an alarm is signalled to alert the user or operator of a possible fan failure. (Which in most cases will be a loose wire or something stuck in the air intake, and blocking the fan.)

Pros & cons:

Of course there can be very good reasons not to use fans. The best reason is probably ingress protection (as in IP44, IP65, IP67, etc). Our fans transport the ambient air through the system, and this air cointains some dust as well. Over time, this dust can accumulate and cause problems inside the system, especially when combined with high humidity.

A fanless cooled system (with sufficient IP rating) does not have this problem since all the dust remains outside. However, this is not a warranty against problems. After all, the dust will settle on the outside of the case equally well. After a while this dust will form a thermally isolating cover over the case, which will cause less cooling capacity, increasing temperature, and thus early failure.

The thermal block of the fanless system is easily solved by cleaning, but this is getting less easy if that system is hidden away in a difficult spot. And then the difference between just cleaning and a preventive overhaul gets rather small...

References:

Please check the download button above for links to Sunon reliability data, and to a report on fan reliability from another Sunon customer.