Underfloor Air Distribution System

Most large data centers use an underfloor air distribution (UFAD) system to cool IT equipment. Why are UFADs commonly used in data centers and not in typical building environments? Building cooling systems typically use duct distributed ventilation, versus under floor systems. Understanding the benefits and drawbacks of a UFAD and why they are commonly used in data centers can shed light on how to make them more efficient. The following are some key features, along with some pros and cons of using a UFAD system.

  • Open Space Layout
    • Pros     Because a data center is one large open space, individual ducts aren’t needed to deliver air to specific zones or rooms (like you’d find in a building). Air delivered via a UFAD can provide cooling to the entire space. Also, tiles allowing ventilation can be self-configured to provide cooling only where needed (such as cold aisles).

    • Cons     Ducts are simple; they originate at the AHU, flow in only one direction, and end in the zone without breaks or holes in between.  Airflow characteristics such as pressure and flow can be measured and analyzed.  A UFAD system is like driving through a rotary in Rome, it is crazy and unpredictable.  Subfloor airflows and pressure gradients are complex and unstable.  Turning on and off cooling units will have effects on the entire systems that are impossible to predict.
  • Uniform Loads
    • Pros     Data centers typically have a uniform cooling load across the floor.  This is quite different than buildings which typically have varying cooling and heating loads that change on daily and seasonal basis.  UFADs can take advantage of this relatively constant stable load distribution in a data center.
    • Cons     If loads aren’t uniform (a few racks or a rows may have higher density IT equipment), UFAD systems will be driven by the larger loads and overcool the rest of the space – in turn wasting energy.   A solution for high density IT equipment can be supplemental cooling, provided directly at rack level.
  • Redundancy
    • Pros     Zone level cooling and heating in a building depend on a single air handling unit (AHU).  If an AHU fails, the zones served by that unit will not receive ventilation.  That is unacceptable for a data center.  Providing airflow through a common plenum in a UFAD provides redundancy if a CRAC/CRAH unit should fail.
    • Cons     The drawback for a common plenum system, as mentioned above, is that airflows are unpredictable and complex.  If I dropped a feather in a ducted system, I know it will come out the other end.  If I dropped a feather in a UFAD system, it’s anyone’s guess where that feather will go.

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