More on power and cooling
The cooling of servers used to be taken care of exclusively by large computer room AC (CRAC) units stationed at the side of a building. These units pumped cold air under the floor and then fed it up to servers via perforated tiles. But server density increased to the point where more cooling was required. Vendors introduced a variety of products: Localized spot coolers that could be stationed near to hot spots as well as liquid-based cooling solutions harnessing water or a refrigerant that could be taken right to the server.
The problem with most of these approaches, though, has been the price tag. That's why Facebook and other organizations are looking to slash costs by combining outside air with evaporative cooling. Facebook is building a 147,000 square foot facility in Prineville, Ore. that meets the rigorous U.S. Green Building Council requirements for Leadership in Energy and Environmental Design (LEED) Gold certification. It will also have a Power Usage Effectiveness (PUE) rating of 1.15. A PUE of 2.5 is regarded as decent for a data center. To give some reality on what that means, a 1.15 PUE basically indicates that for every 115 watts going to the data center, 100 watts goes to the computing equipment, and only 15 watts goes to the infrastructure.
"The facility will be cooled by simply bringing in colder air from the outside," said Facebook vice president of technical operations, Jonathan Heiliger. "This feature will operate for between 60 percent and 70 percent of the year. The remainder of the year requires the use of the evaporative cooling system to meet temperature and humidity requirements."
The evaporative cooling system in place at Facebook is based on fogging technology. Used for decades in building cooling and humidification, it is being supplied by Mee Industries.
"This system evaporates water to cool the incoming air, as opposed to traditional chiller systems that require more energy-intensive equipment," said Heiliger. "This process is highly energy efficient and minimizes water consumption by using outside air."
When the air outside is cool enough, it is pumped into the facility to cool it. During the summer months, when temperatures climb, fogging is used to cool the air.
"The capital cost, operating expense and parasitic load of evaporative cooling are a tiny fraction of those of traditional chiller units," said Thomas Mee III, CEO of Mee Industries. "Fogging systems can be part of a new data center design or can be installed in existing data centers, often without requiring a shutdown of the air handling equipment."
Fogging uses the natural cooling power of evaporation to achieve better control of temperature and humidity. Side benefits include the elimination of static electricity and suspending of airborne dust. For those afraid of water getting into contact with the computer equipment, the good news is that it is kept well away. Water is piped into the ducting system and released into the incoming air via an array of nozzles that create a fine mist. The water droplets produced measure only a few microns in diameter enabling them to be suspended in the air and thus bring the temperature down significantly.
According to Mee, a high-pressure fogging system contains elements such as a high-pressure pump, a demineralized water system, piping, filters and fog nozzles. Demineralized water at 1,000 to 2,000 pounds per square inch (psi) is fed to a series of fog nozzles installed inside the air duct. These nozzles have an orifice diameter of around six-thousandths of an inch. Water jetting out of them is broken into as many as 5 billion micron-sized droplets per second.
Operating costs are relatively low. A typical fog system uses about one horsepower for every 700 pounds of water per hour. The evaporation of one pound of water at room temperature removes around 1100 BTUs, and each horsepower of pumping removes about 770,000 BTUs/Hour, which equates to more than 64 tons of cooling. Thus, a fogging system with a 7.5 HP motor could potentially replace a 500 ton chiller.
With companies such as Facebook leading the way to lower cost cooling, expect to see more of these fogging systems showing up in data centers in the near future.
Drew Robb is a freelance writer specializing in technology and engineering. Currently living in California, he is originally from Scotland, where he received a degree in geology and geography from the University of Strathclyde. He is the author of Server Disk Management in a Windows Environment (CRC Press).