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Managing Data Center Air Flow With Hot Aisles, Cold Aisles Can Help Drive Energy Efficiency

By Loren Snyder - January 2014 - Data Centers

hvac, hvac upgrades, energy efficiency, commissioning


Configuring data centers for hot-aisle/cold-aisle setup is nothing new — the idea has been around for the better part of a decade. But increasingly, data centers are now enclosing the hot and cold aisles to allow less air mixing and more efficiency for the cooling system. Initially, partitions were made relatively simply, perhaps with a curtain that helped separate thermal gradients within the data center. Increasingly, says Cassiliano, those partitions are becoming more permanent than just a curtain. “These days, it’s more like a room within a room,” he says.

Griffin also notes that the operating inlet aisle has a greater permissible temperature range than data centers have historically have had. “Essentially, you want to match the quantity of the air flow of the system to that required by the servers,” he notes.

Historically, mechanical cooling was designed to meet the needs of an entire facility, and it was made extra-cold for the data center. Increasingly, however, data centers are tailoring their air-flow management and consequent cooling needs.

“Increasingly, the industry is pushing air-flow management,” Griffin says. “DCIM — data center infrastructure management — allows users to update controls programs with temperature sensors and match the quantity of air with what’s needed for the space.”

What needs to be avoided, he says, is hot-air recirculation or cold air short-cycling tendencies within a data center.

According to ASHRAE, recirculated hot air raises the temperature of all components within the system and will cause a boost in air mover speeds to compensate, increasing power consumption.

Proper air-flow management often represents the low-hanging fruit for data center facility operators, largely because it can be managed via low-cost methods. Blanking panels should be installed in racks where there aren’t servers to prevent hot exhaust from recirculating to server intakes; end-of-aisle doors or some containment curtains can be used to further separate hot and cold aisle arrangements; and using temperature sensors – even spot-checking with an infrared camera several times a day — at known hotspots (top of the racks and ends of the aisles) can help facility managers optimize air-flow management.

According to the latest ASHRAE standards, however, what is fundamentally required, and is generally available, are inlet ambient temperature values as documented in Energy Star server requirements.

Waste Heat Reuse

In some applications, particularly for data centers that are exceedingly energy conscious, waste heat from servers can be captured and redirected for use elsewhere — a kind of low-level co-generation scheme.

Salim notes that waste heat could be recirculated to warm battery space for a data center, but cautions that value of this energy may or may not affect PUE, depending on the location of the reuse.

“If the waste energy is reused within the data center, the value of the reused energy will already be accounted for in the PUE calculation, but if the waste energy is reused outside the data center, the PUE of the data center is not affected and waste heat reuse cannot be accounted for in the calculation of the data center PUE,” he says.

“In an era of constrained budgets, limited resources, increased corporate social responsibility, and threat of upcoming potential regulations, the data center’s high energy bills are uncomfortably on the agenda,” notes Salim.

To make it easier to tackle high data center energy costs, Cassiliano recommends giving the CIO responsibility for the data center energy bill. That shift, he says, can help get data center operators the C-suite support they need.

Loren Snyder, a contributing editor for Building Operating Management, is a writer who specializes in facility issues. He was formerly managing editor of Building Operating Management.





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