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More perforated tiles equal better data center cooling

A computational fluid dynamics company released a paper at last month's ASHRAE conference showing that raised floors in data centers often leak cold air into areas they shouldn't.

DALLAS -- When you pump cold air into a raised floor, you expect it will all arrive at its destination -- the server racks. But like a disobedient child, some of the cold air is probably sneaking off where it shouldn't, and it's costing you money.

A new study from Innovative Research Inc., a Plymouth, Minn.-based computational fluid dynamics (CFD) company, said a lot of that has to do with your perforated tiles; more specifically, how many of them you have.

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"Distributed air leakage can represent a significant portion of the total airflow into the plenum," said Kailash Karki, vice president of research and technology, Innovative, at a recent conference in Dallas for the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

Karki estimated that air leakage could account for as much as 15% of all airflow exiting into the data center room. Not only does that waste cooling, but it also requires more power to properly cool equipment.

When installing a raised-floor environment in a data center, so-called perforated tiles (like floor vents) are dispersed among solid floor panels so that cold air flowing from computer room air-conditioners (CRACs) can rise through the floor and into server racks, cooling off hot equipment. But if the raised floor isn't constructed correctly, air can push through other unintentional openings in the floor where it isn't useful to the equipment.

There are a number of places the leaked air could enter the data center: loose cable cutouts under cabinets, spaces between floor tiles and the walls, or between poorly aligned floor tiles, missing tiles or superfluous perforated tiles.

Innovative's study describes the distributed air leakage as the total area of gaps between floor panels on a raised-floor environment. Using CFD technology, Innovative was able to determine variables, such as the flow rate under a raised floor and the flow rate leaving the floor tile, which allows them to calculate the distributed air leakage. According to Karki, many of the gaps allowing cold air leakage are tiny, some less than one millimeter.

Karki and his co-authors, Amir Radmehr and Suhas Patankar, discovered that air leakage was more prominent when the ratio of perforated tiles to total floor space was smaller. That is, fewer perforated tiles equals more air leakage.

To research the problem, the company set up a 364-square-foot minidata center with a 12-inch raised floor and no obstructions, such as water pipes, underneath. They then installed two rows of perforated tiles sitting near a CRAC unit blowing at 10,000 cubic feet per minute (cfm). The study looked at air leakage with 4, 8, 12, 16 and 20 perforated tiles.

It found that there was more than twice as much air leakage with four tiles as there was with 20.

"What we were looking for was airflow through the gaps," Karki said. "Not all of the airflow is exiting through the perforated tiles."

The solution? Try increasing perforated tiles as long as they direct cold air to the equipment, and make sure that any opportunities for air leakage are shut down.

Let us know what you think about the story; e-mail: Mark Fontecchio, News Writer, and check out the Blog.

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