Can coolers be turned off in favor of outdoor air, even when temperatures top 100 degrees? According to Facebook, the answer is yes.
In a Nov. 14 blog posting, Facebook engineer Daniel Lee described how the company has taken the design for the outside-air system that cools its Prineville, Ore. facility and used it for its data center in Forest City, North Carolina. The primary concern: whether or not Facebook could successfully eliminate the need for mechanical chillers with a combination of blown outside air (at ambient temperatures) and a misting system.
The experiment was a success—a rousing success, actually, although the company hedged its bets by designing in a direct-expansion coil anyway, just to be sure. Facebook estimated the power usage effectiveness (PUE) for Forest City at 1.07 this summer, versus a score of 1.09 for Prineville over roughly the same period.
Facebook summarizes the Prineville system as “a built-up penthouse that utilizes 100 percent outside air economization with a direct evaporative cooling and humidification (ECH) misting system.” As the cool air is pulled from outside, a direct ECH misting system lowers the temperature still further via evaporative cooling. (The water used for cooling is filtered and purified in order to avoid clogging the array of microscopic nozzles with just 150-micron orifices.)
That works fine for central Oregon’s climate, a desert plain with hot, dry summers and cool evenings and winters, Lee wrote. But how would it fare in North Carolina?
In order to accommodate the higher heat and humidity, Facebook set the upper tolerances of the sever inlet temperature range from 80 degrees Fahrenheit to 85 degrees F. Facebook also adjusted the relative humidity tolerances from 65 percent to a whopping 90 percent. Since July was the second hottest month on record for North Carolina, it proved a perfect test for the design. At one point the dry-bulb temperature topped 100°F outside the data center, Lee noted.
“We also saw several days of high relative humidity over the course of the summer,” he wrote. “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. 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 into the data hall. (In fact, in some cases we actually added hot return air from the data hall to the supply air, to bring the relative humidity down within 90% RH cap.)”
Did Facebook get lucky? According to the company’s charts, the temperature hit 102 degrees F on July 1, one of the hottest days of the year; the relative humidity that day was 26 percent. That allowed Facebook’s misters to operate efficiently. Likewise, the day where the humidity was highest, at 97 percent, was accompanied by only a 67-degree high.
However, a list of historical weather data for the area suggests that the average humidity never tops 90 when the temperature soars above 85 degrees—although both come quite close (The assumption is also that humidity is highest in the early morning, when temperatures are cool.)
Still, the data collected by both Green Grid and Facebook questions the concept of mechanical cooling solutions as always necessary for the data center.