Aisle containment an option for thermal management

April 1, 2009
For some, the question becomes whether to contain the cold aisle or the hot aisle.

For some, the question becomes whether to contain the cold aisle or the hot aisle.

One of the data center's longest-standing truths has met up with one of its most significant recent trends. The trend—virtualization—is having a legitimate impact on the truth, that thermal management is a complex and never-ending task. Kevin Dunlap, general manager of cooling solutions with APC by Schneider Electric (, explains: “As managers virtualize their data centers, heat loads do not run constant and remain in one spot in the data center. The loads can move from one rack to another, and from one row to another.”

This thermal dynamic created at least in part by virtualization is one example of why row-based cooling, either in place of or in addition to room-based cooling, is an advantageous solution for some users. Dunlap points out that APC does not have a strictly targeted approach: “We do not advocate row-cooling only or room-cooling only. Many users are likely to end up with a hybrid approach.” But he adds that many users, including data centers using virtualization where energy consumption and heat generation are not static, can benefit from rack-level or row-level cooling.

“Room-level cooling has its place, especially in low-density data centers,” he explains. “It has a lower cost of entry, although a higher total cost of ownership. Room-level cooling allows flexibility in that you can redistribute air by moving a perforated tile.” Many data centers have non-racked information-technology (IT) equipment, which is a factor in the approach taken to cooling. “For rack-based IT loads, row-based is the better way to cool it,” Dunlap says. “But you still have to cool the entire space, including the non-racked equipment. So, you will need some room cooling to accomplish that.”

Data center managers gain the most efficiency from row-based cooling by the approach's close-coupling characteristics. By moving the cooling equipment closer to the heat load, they do not have to distribute air over as great a distance as they would using room-cooling alone. Additionally, because the cooling equipment is close to the hot air, that air does not have the opportunity to mix with cooler air in the room.

And that's the essence of data center cooling, according to Dunlap. Cooling really comes down to the removal of heat. Some ways may be deemed more efficient than others, but he says APC's approach to thermal management is to focus on removing heat.

These cooling methods are predicated on the hot-aisle/cold-aisle room layout. Lennart Stahl, senior marketing manager for Liebert cooling products with Emerson Network Power (, explains some fundamentals of the hot-aisle/cold-aisle setup: “Most equipment manufactured today is designed to draw air in through the front and exhaust it out the rear. This allows equipment racks to be arranged to create hot aisles and cold aisles. This approach positions racks so that rows of racks face each other, with the front of each opposing row of racks drawing cold air from the same aisle—the cold aisle. Hot air from two rows is exhausted into a hot aisle, raising the temperature of the air returning to the computer-room air-conditioning [CRAC] unit and allowing it to operate more efficiently.”

The hot-aisle cold-aisle concept was developed in 1993 by Robert F. Sullivan, PhD, now a staff scientist with The Uptime Institute (, and has become the globally accepted means of laying out a data center. One-and-a-half decades later, vendors including APC and Emerson Network Power, have begun to advocate the physical separation of hot and cold aisles from one another. Going beyond row-based cooling, the aisle-containment method calls for the placement of barriers within the data center to keep the hot and cold air separated.

The quote earlier attributed to Emerson's Stahl comes from his recent white paper entitled “Focused cooling using cold aisle containment.” In that paper he summarizes by stating, “Aisle containment to prevent the mixing of hot and cold air stands out as a method that can dramatically reduce costs, minimize hot spots, and improve the carbon footprint of data centers.”

Organizations that study the possibility of aisle containment face a research project of potential dos and don'ts. One of the “don'ts” is violating fire codes by placing the physical barriers. Stahl addressed the issue in his white paper and in an interview with Cabling Installation & Maintenance: “Very early in the process, discuss your plans with local fire authorities. You may find you have to move some sprinkler nozzles.” But Stahl says he has not heard of an instance in which a user absolutely could not implement aisle containment because of a code issue. While the project may necessarily include adaptation of alarm and sensor systems because it amounts to building a room within a room, “If you deal with code issues early and contact authorities, typically it works,” he explains.

Another decision data center managers must make before moving forward is whether to contain the cold aisle or the hot aisle. In other words, do you “block off” the cold aisle and allow the rest of the data center to be hot, or “block off” the hot aisles and use energy cooling for the rest of the data center?

The title of Emerson's paper suggests, and Stahl confirms, his organization favors cold-aisle containment.

On that point, Emerson and APC disagree. Says APC's Dunlap, “Both approaches achieve certain goals—eliminating mixing and increasing energy efficiency. But there are advantages to containing the hot aisle.” One advantage in particular, he says, comes back to the cases in which a data center has freestanding equipment. When only the cold aisles are contained, the freestanding equipment resides in the (hot) remainder of the data center. By contrast, hot-aisle containment prevents the grouping of contained racks from affecting the rest of the environment. “It comes back to the basic philosophy of removing heat,” Dunlap says. Hot-aisle containment captures heat and locks it down so it does not mix with the rest of the room.

Stahl sees it differently: “Cold-aisle containment has a focused cooling approach that can be used with or without conventional raised-floor cooling. It is easily retrofitted into existing raised-floor data centers, can be done with air flow control of the cooling units based on the server needs, and will create highly efficient cooling solutions when used in tandem with raised-floor and/or extreme density cooling systems.”

Emerson Network Power and APC have published white papers on the topic, available at their respective Web sites.

Patrick McLaughlin is chief editor of Cabling Installation & Maintenance.

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