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Hot/Cold Aisle Containment Pros and Cons

Hot and cold aisle containment compared in CFD models, including fire code limits and an approach that improves airflow without physical barriers.

This webinar compares hot aisle and cold aisle containment using computational fluid dynamics (CFD) models of an older raised floor data hall. It is for operators and engineers weighing containment against its cost, fire code exposure and effect on flexibility. Three presenters take part: the president of Applied Math Modeling, which develops CoolSim, and two guests from companies that sell airflow products and services.

What the webinar covers

The talk opens with the pressure on data centers: rising electricity costs, the role of cooling in Power Usage Effectiveness (PUE), and common ways to cut cooling energy. These include a higher supply temperature, variable speed fans and a larger temperature rise across the servers.

The presenter then models a naturally convective hall with an 18 inch raised floor and underfloor cable trays. Path lines show high underfloor velocities, and air leaving the computer room air conditioner (CRAC) units does not turn up into tiles near them. The same room is modeled with hot aisle containment using passive chimneys, and then with cold aisle containment. In both cases the containment leaks a little, and the models show how leakage under racks, through rack rails and past missing blanking panels hurts rack inlet temperatures.

The guests then describe a different approach. They compare standard tiles with a tile that has an underfloor structure that turns the air upward and creates turbulence. Back in the hall model, the guests’ tile was staggered at about two thirds of the original tile count with no physical containment, and the model showed rack inlets close to the contained cases. The guests present two field case studies, then take questions on fans, in-row cooling, cable trays, tile open area and rack density.

Key points

  • Containment is not a perfect seal. The models assume a small amount of leakage, and gaps under racks, in rack rails or from missing racks can undo much of the benefit.
  • Cold aisle containment made the room the hot side, which some people find hard to work in. Hot aisle containment kept the room cool, but needs a ceiling plenum or chimneys.
  • Containment can conflict with fire protection rules. The presenters cite National Fire Protection Association (NFPA) standards 75 and 76, and say that requirements differ by jurisdiction.
  • Containment is hard to maintain where equipment changes often, such as in colocation halls, because every moved or removed rack can open a gap.
  • Higher tile open area is not automatically better. A tile with large openings can send air up in a fast jet that bypasses the servers, cools the return air and makes the fans work harder.
  • The guests say their tile approach delivers containment-like results without barriers. Their savings and payback figures come from their own case studies and are not independently verified here.
  • Underfloor velocity is present in every raised floor the presenters have seen. Obstructions can raise pressure by slowing the air, but they can also starve an area, so the effect depends on the room.