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Benefits and Risks of Evaporative Cooling in Data Centers

While evaporative cooling (EC) has been used in homes for years as an alternative to traditional air conditioning, EC is finding new applications in data centers. Unlike HVAC units that use vapor compression or absorption refrigeration and consumer large amounts of electrical energy in the process, EC uses water evaporation as the coolant. Heat is absorbed by water, which turns to cooling vapor.

EC’s simplicity translates into low installation costs, often half that of more conventional HVAC alternatives. The water pump in EC systems uses little power and overall cost of operation is estimated to be about 25 percent of HVAC units, which leads some to refer to EC as "free cooling."

For optimum benefits, EC needs a reliable water source and a warm climate (averaging around 75 degrees Fahrenheit) with naturally low humidity. And, it helps if servers are capable of operating at high temperatures, an advantage new servers today afford.

The American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) Technical Committee 9.9 recently revised allowable heat ranges for data processing environments. Allowable temperatures in new class 1 data centers is now 89.6 degrees Fahrenheit; for class 2 and class 3 the upper range is 95 degrees; and class 4 is 104 degrees. Because EC data center hot aisles are operating at temperatures too hot for human tolerances, elements that need maintenance need to be set up in the cold aisle.

Rob Bernard, chief environmental specialist at Microsoft, reported in an executive strategy brief on modular data centers that the company had experimented with operating servers "at higher temperatures than traditional ASHRAE standards."

The ASHRAE standards are conservative numbers, often more restrictive than the operating conditions warrantied by server manufacturers. "Elevating operating temperatures enables the broader use of free air cooling instead of extensive air conditioning," maintains Bernard in the strategy brief.

Other companies, including Google and Facebook also are using evaporative cooling along with higher server temperatures, reported to be nearer 120 degrees, in major data centers, such as Microsoft's Redmond Ridge 1.

Google installed evaporative cooling for its large Douglas County, Ga., computing center in 2007. Initially, the center used potable water from the local utility. However, earlier this year, Google revealed its system would use 100 percent recycled water to cool the facility. Google is working with the Douglasville-Douglas County Water and Sewer Authority on the project. After cleaning at a Google pretreatment plant, the gray water flows to Google's EC system. Heated air from the servers is fanned into the much cooler water, which evaporates most of it into the Georgia air. Backup chillers are on hand for heat shaving during certain hot hours.

One significant risk of EC is that the process itself does not afford dehumidification, as conventional HVAC systems do. If high humidity occurs inside the data center, it could cause condensation, leading to corrosion and other issues with electronic equipment. As the dew point rises, less cooling is available, which is why EC is more attractive in areas with low humidity and high temperatures.

Facebook's Prineville, Ore., data center is using water misting evaporative cooling at its LEED Gold data center. Reportedly, the data center did encounter a build up of condensation that was fixed by modifying the center’s control system that adjusts for rapid temperature swings.

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