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June 5, 2009 • Vol.31 Issue 16
Page(s) 27 in print issue

Smart Air Management & Circulation
Best Practices To Keep Increased Thermal Loads From Stressing Computer Room Air Systems

Key Points

• A hot aisle/cold aisle configuration with strict air separation can double a data center’s server cooling capacity.

• Maximize return air temperature by supplying air directly to loads with appropriate air diffusers and proper room design and cabinet layout.

• Minimize cable clutter with short, custom-made power and patch cables and cable trays both under raised floors and overhead.

Global warming isn’t just a weather phenomenon—many IT managers are facing their own versions of climate change inside increasingly packed data centers. Computer rooms originally designed for bulky servers with a few systems per rack are struggling to handle an influx of new equipment that’s sucking power and generating heat.

The Lawrence Berkeley National Laboratory, or LBNL (www.lbl.gov), has been studying energy usage by high-tech industries since the mid-1990s with the goal of benchmarking performance and developing technologies, tools, and strategies for improving overall efficiency. Culling information from 22 data centers, it established a set of best practices that many other organizations, such as Pacific Gas & Electric and the Climate Savers Computing Initiative, have used as the basis for their own design guidelines.

According to PG&E, data center air management encompasses "all the design and configuration details that go into minimizing or eliminating mixing between the cooling air supplied to equipment and the hot air rejected from the equipment." According to Michael Petrino, vice president at PTS Data Center Solutions (www.ptsdcs.com), airflow problems have a number of causes, including poor cabinet layout, bypass airflow from cold to hot aisles due to inadequate separation, improper placement of vented floor tiles, and cable clutter constricting air paths.

Use Hot Aisle/Cold Aisle Configurations

LBNL outlines two key strategies for achieving efficient airflow management: eliminating the mixing and recirculation of hot equipment exhaust and maximizing the return (exhaust) air temperature. The most basic and universally recommended tactic for efficient air circulation and reduced mixing is a hot aisle/cold aisle configuration. This entails laying out equipment racks in rows that alternate between air intake (cold aisle) and exhaust (hot aisle).

In order to reduce efficiency-sapping air mixing, it’s imperative that most of the chilled air flows through active equipment; thus, gaps in racks and empty spaces above them and at the end of rows should be physically blocked. A number of vendors have developed easily installed products to assist with air isolation. Roger Jette, CEO of Snake Tray (www.snaketray.com), says baffles and adjustable panels can fill unused positions in equipment racks and “keep conditioned air in the right spot.”

Aftermarket products are especially important when incorporating older equipment with nonstandard exhaust patterns into a hot aisle/cold aisle configuration. However, if building custom ductwork is impractical, LBNL advises “configuring racks to ensure that equipment with side-to-side, top-discharge, or other airflow configurations reject heat away from other equipment air intakes.” Racks themselves can also affect airflow, so it’s important to select those with minimal internal obstruction. PG&E notes that specialized rack products with integral plenums that duct cool air directly from floor vents and exhaust to the return air plenum can enhance circulation and reduce mixing. Zach Wilson, executive vice president at NetStructures (www.netstructures.biz), says it’s also important to use the shortest possible custom cable power and patch cords within racks and to minimize cabling under raised floors.

Increased Return Air Temperature

The second key objective to enhancing airflow efficiency is maximizing the return air temperature. According to PG&E, “A higher difference between the return air and supply air temperatures increases the maximum load density possible in the space and can help reduce the size of the cooling equipment required, particularly when lower-cost mass-produced package air handling units are used.”

Peter Sacco, president of PTS Data Center Solutions, believes that the best way to understand and optimize a room’s air distribution and temperature profile is by using CFD (computational fluid dynamics) simulations along with centralized, automated environmental monitoring.

Air diffusers on cold air vents can make a significant difference. LBNL’s best practices note that “standard office-style diffusers, designed to create a fully mixed environment and avoid creating drafts, are inappropriate for data centers.” It adds, “Diffusers should be selected that deliver air directly to the IT equipment, without regard for drafts or throw concerns that dominate the design of most office-based diffusers.”

Overall System Design Guidelines

These supplemental tactics for improving airflow management and circulation may still not be sufficient for legacy data centers trying to handle today’s loads. “You still need to have a fundamentally sound HVAC design,” according to Sacco. This means having adequate cooling capacity, sufficient fan volume for each rack, and a well-considered floor plan. As Petrino notes, “There are many products that block air, change floor pressure, use fans to direct airflow, and add additional venting capability; however, used improperly, these products can create problems in other areas of the data center.”

According to PG&E, using best practices for air management can double the cooling capacity of a data center and, when coupled with air-side economizers, can reduce cooling costs by more than 60%. Many of these techniques, such as tidy cabling, proper air diffusion, hot/cold-aisle design, and increased air isolation between zones, need not be huge investments, and given today’s electric rates, LBNL estimates investments payback in only three months.

by Kurt Marko


Biggest Improvement: Preventing Airflow Spillover

Assuming a well-designed hot aisle/cold aisle layout with adequate airflow, proper diffusers, and well-managed cables, the most significant improvement to air management is preventing air mixing and spillover from cold to hot areas. Flexible strip curtains, which can be nothing more sophisticated than those plastic refrigeration covers seen in most supermarkets, are an effective means of restricting airflow around racks. Peter Sacco, president of PTS Data Center Solutions (www.ptsdcs.com), does advise caution before installing these above racks, however, because they may block fire suppression equipment such as sprinklers. He believes that most of the benefits of hot/cold-aisle separation can be achieved by blocking air from spilling around row ends. PTS has found this is easily achieved by using overlapping thermal strip curtains between rows at the end of each aisle, a technique Sacco says is particularly effective in conjunction with in-row cooling.


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