Air Flow Management

One of the most common complaints design engineers hear from data center owners and operators is that they need additional cooling capacity because the existing system doesn’t maintain an acceptable temperature at the data equipment inlets. But in most cases, the problem isn’t one of insufficient capacity, but of poor air flow management. The good news is that adopting a strategy to improve data center air flow results in two positive changes. First, by reducing the amount of air that needs to be supplied, less energy is used for data center cooling. Second, temperature distribution across cabinets is improved.

The biggest culprits leading to poor air flow conditions in data centers are recirculation and bypass air flow. Bypass air flow is cold supply air that does not lead to productive cooling at the IT load. Both reduce the overall available airflow for cooling, but do so by two different mechanisms.

  • Recirculation is the mixing of the hot exhaust with the cold intake of the IT equipment
  • Bypass circumvents the cooling path by going around the IT equipment

The primary method for minimizing the impact of both recirculation and bypass is to separate the supply and return air streams using air flow management or containment. With containment, the rack density is only limited by the capacity of the cooling equipment to provide the proper airflow at an acceptable differential temperature (I.e. server inlet temperature to server outlet temperature).

Why don’t all data center managers use containment? Some don’t like that containment restricts access to the cabinets, cable trays, or aisles. However, a less obvious problem is containment requires a carefully planned control strategy to prevent excessive pressure differences between hot and cold aisles. If the pressurization control strategy is wrong, the server fans could starve for air, which could cause them to increase speed in order to maintain acceptable processor temperatures. Thus, to respond to pressure issues, the servers will increase energy consumption and operations costs will increase.

Data Center Air Flow Management Best Practices

  • Air Flow Management Hot Cold Aisle ContainmentHot Aisle / Cold Aisle Containment. Hot aisle containment encloses and captures the hot IT exhaust and ducts the hot air directly back to the computer room air conditioners/handlers.

    While hot aisle containment captures the return, cold aisle containment, as the name implies, contains the cold air supply.

    Both methodologies have the benefit of isolation of the hot air return from the supply air, allowing for increased CRAC/H efficiencies gained from higher return cooling coil temperatures. Additionally, both will reduce the need for humidification and dehumidification as the air is delivered directly back to the CRAC/H without mixing. The two differ in three areas—scalability, thermal mass and operator comfort.

  • Install blanking panels in all openings within each cabinet. Don’t forget bypass and recirculation can occur inside cabinets. An air flow management system cannot effectively cool the equipment in a cabinet without eliminating internal paths of bypass and recirculation. Blanking panels reduce these air flows and are considered a must for proper air flow inside a cabinet.

  • Deploy perforated tiles in cold aisles. Placing perforated tiles in any location but cold aisles increases bypass. There is never a justification for placing perforated tiles in hot aisles unless it’s a maintenance tile. During maintenance, if necessary, a perforated tile can be used in a hot aisle to support an IT technician. However, make sure not to leave it behind.

  • Polargy-obstructed-cutoutsSeal gaps between raised floors, walls, doors, cable cutouts, etc. Sealing the spaces between the raised floors and room walls is a no-brainer. Those gaps are easily identified by a simple visual inspection. A more subtle form of bypass can be found when column walls are not finished above the ceiling and below the floor. Often, the sheet rock used to enclose a column forms a chase for direct bypass of air into the return air stream. These chases must be sealed to reduce bypass air flow.
  • Use right type of floor tile. Many data center managers install high capacity grates at perceived hot spots (i.e. racks with blade servers) but this can actually be counter-intuitive. Rather, it is critical to perform a computational fluid dynamic analysis (CFD) above and below the floor of the hot spot to fully understand air flow and pressure changes which will drive the type of floor tile (I.e. high-capacity flow) to deploy.

  • Manage the placement of perforated tiles. Calculate the load for the cold aisle and place an appropriate number of perforated tiles or grates (but not both) to cool the load in that aisle. Placing too few tiles in the cold aisle will cause recirculation. Placing too many will increase the amount of bypass.

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