No one would deny that it makes sense to use outside air for cooling a data center in a place like Oregon, but can this practice work in a hot-and-humid climate such as North Carolina?
For Facebook, the answer is a qualified "yes."
As part of the ongoing disclosure of its data center efficiency measures, the social networking giant has published an Open Compute Project blog about its cooling methods for its facility in Forest City, N.C.
When the company designed this facility, it was forced to make a number of adjustment to the evaporative cooling and humidification system design it uses in Prineville, Ore., in order to achieve the same energy efficiency.
For a start, the company set the upper end of the server inlet temperature higher for the North Carolina data center - at 85 degrees Fahrenheit instead of 80 degrees Fahrenheit. It was also forced to expand the humidity range maximum to 90 percent relative humidity (RH) from 65 percent.
Facebook also installed a direct expansion cool system in the facility in case the free-cooling process didn't work.
But even though the summer of 2012 was the second hottest on record for the state, it never had to turn on that backup system - that's because even on those record hot days, the relative humidity was still low and the misting systems were able to cool things down. (See the chart below.)
And on the especially humid days, the company also got lucky - because the temperatures weren't especially high on those days.
"High RH is potentially problematic in an evaporative cooling system, as our ability to cool air by adding water decreases as the RH of the incoming air increases," writes Facebook in its blog post. "But as illustrated by the chart for June 25, the dry bulb temperature tended to be low on the days RH was high this summer - meaning that we didn't need to cool it before sending it to the data hall. (In fact, we actually added hot return air from the data hall to the supply air, to bring the humidity down within 90 percent RH cap.)"
What will happen if BOTH the temperature and humidity soar? So far, that remains to be seen, but the data Facebook is publishing should be of interest to data center engineers seeking to improve their own energy efficiency metrics. You should check out its blog for more of the relevant charts.