Hot-aisle/cold-aisle containment stokes fire-code issues

Containing a data center's hot and cold aisles in may create energy-efficiency savings, but the design strategy can leave a facility vulnerable to fire detection and suppression issues.

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Hot-aisle/cold-aisle containment has gained popularity as a data center design feature, especially in power-dense environments. But with this energy-efficient data center strategy, fire prevention can be tricky and requires special attention.

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What is hot/cold aisle?

Hot-aisle/cold-aisle containment – a variation on the traditional data center best practice of hot-aisle/cold-aisle design – is a way for data centers to isolate hot and cold air streams so they don't mix with one another and cause energy inefficiencies. But when isolating those air streams, data centers can leave themselves vulnerable to violating fire codes that require detection and prevention devices throughout the room.

"You have to be able to show that you have sprinkler coverage everywhere in the room," said Rob Menuet, a mechanical engineer and fellow at the Uptime Institute, a data center industry group. "The other issue is detection. If you have compartments, you have to make sure you have the right detection. It's a combination of sprinkler coverage and detection."

Some data center design experts recommend against it altogether.

"In short, there is no good fire suppression methodology for inside rack containment systems," said Pete Sacco, the president of PTS Data Center Solutions in Franklin Lakes, N.J. "For that purpose, coupled with temperature variance [in a hot aisle], we choose to do it in different ways."

The evolution of hot-aisle/cold-aisle design
Hot-aisle /cold-aisle design is a long-standing data center best practice. It requires lining up server racks in alternating rows with cold-air intakes all facing one aisle and hot-air exhausts facing the other. Over the past several years, it became the de facto standard. But as server density has increased, efficiency gains have consequently eroded.

In dense environments, the air from the hot aisle goes over the top of the racks and mixes with the cold air entering the IT equipment. As a result, the cold air must be made cooler – or the fans blowing that air need to blow harder – so the IT equipment can get the right temperature air entering it.

Enter the idea of isolating the hot and cold aisles from one another. Hot-aisle/cold-aisle containment systems use a physical barrier that separates the hot- or cold-aisle airflow through makeshift design solutions such as vinyl plastic sheeting used in meat lockers, ducted plenum systems and other commercial offerings.

We will probably have to put sprinkler drops in each of the hot aisle. Absolutely, it's a big concern.
Bob Seese,
chief data center architectAdvanced Data Centers

The separation of hot and cold air provides better uniformity of air temperature from the top to the bottom of the rack. That uniformity enables data center pros to raise the set point temperature more safely. William Tschudi, a project manager in the Energy Technologies Division at Lawrence Berkeley National Laboratory, has studied the effectiveness of hot-aisle/cold-aisle containment. Tschudi said that the combination of aisle containment and variable frequency drives can reduce fan energy use by 75%

One data center aware of fire code issues
Hot/cold aisle containment is one of the features that data center designer and operator Advanced Data Centers (ADC) will use to boost its facility efficiency. ADC will attach a ducted plenum to the back of the hot aisle to exhaust the hot air and thus it can raise the set-point temperature in the room and reduce fan power to reduce cooling costs.

Bob Seese, the company's chief data center architect, knows that this kind of design introduces fire prevention and suppression issues.

"We will probably have to put sprinkler drops in each of the hot aisles," he said. "Absolutely it is a big concern. We will have to put a sprinkler in each contained hot aisle, that's our belief right now. We're working with manufacturers to get this designed as a single piece of equipment."

Watch out for temperature variance
One of the phrases often tossed about in discussions of hot-aisle/cold-aisle design is "delta T," often seen as ΔT. It's the difference in temperature between the air coming into the IT equipment and the air exiting it. When you contain the hot aisle, even though the delta T doesn't change, that hot air that comes out the back stays hot because it doesn't have a chance to dissipate in the room.

"The hot aisle is just too hot," Sacco said. "If you're going to exceed 110 degrees, you could actually exceed the [National Electric Code] standards."

Forgetting about the fire code issues, Menuet added that you have to think of your employees.

"It will be over 100 degrees in the hot aisle," he said. "It certainly isn't comfortable for technicians that have to get in there and work on the equipment."

Using vinyl curtains for separation
Using vinyl sheeting, much like what you'd find in a meat locker, is one method that data centers have used to contain their hot aisles. Storage vendor NetApp uses them and says the curtains alone save it 1 million kWh of energy per year. Yahoo uses them in one of its data centers as well.

But there are fire suppression issues with this method as well. If you have plastic sheets over your racks and don't have sprinklers in the contained area, how can a potential fire be squelched? Most companies that manufacture this sheeting say it melts at 130 degrees, thus eliminating the aisle containment and allowing the sprinklers to work from there. But what if for some reason the curtains malfunction?

"The curtain attaches to the ceiling with heat-sensitive fusable links," Menuet says. "If there's a fire, the heat melts the links and the curtain falls. But if I was the fire marshal, I'd be concerned about a curtain hanging from some fusable links. Most places would have trouble with that. If I were proceeding with good hot-aisle/cold-aisle containment, I would design a fire system around it and put enough heads in the hot aisle and enough in the cold aisle."

Never mind the aesthetic ramifications. Menuet said he was working on a project in Minnesota, where discussion included using freezer curtains to contain the hot aisle, "but it didn't show well." In other words, company executives might not like the fact that their data center looks like the back of a butcher shop.

Inform the local fire inspector
Sacco added that one of the most important things is being in line with local fire authorities, because if they conduct an inspection, see something they weren't notified about and don't like it, they could shut you down.

"Many people deploy these systems in ignorance of the law," he said. "Many people do it and manufacturers are manufacturing the pieces. But the local inspector is the final authority. If the local inspector doesn't realize what's going on, the whole job might not be compliant."

Let us know what you think about the story; email Mark Fontecchio, News Writer. You can also check out our Data Center Facilities Pro blog.

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