Airflow Management’s Role in Data Center Cooling Capacity

Airflow management (AFM) is changing the way data centers cool the IT thermal load. In thesimplest terms AFM is the science of separating the cooling supply from the hot return airflow.  

AFM’s impact on cooling capacity is huge. This is because the traditional cooling scenario without the full separation of supply and return airflowrequires as much as four times the cooling capacity to satisfy the same thermal load. This creates the unnecessary need for cooling units due to airflow inefficiency.  

Data center managers can easily determine the percentage of inefficiency by counting the tons of available cooling capacity and measuring it against the IT thermal load measured in kW.

For example, 40 tons of cooling capacity will cool 140.67 kW. Thus the average data center without AFM might have as much as 160 tons of cooling to mediate 140 kW.  

It’s easy to conclude that if AFM can reduce the excessive cooling capacity, that operating cost could likewise be reduced. There are three typical ways to reduce the energy use of cooling units:

  1. Turning off cooling units
  2. Reducing fan speeds
  3. Increasing temperature and decreasing relative humidity set points.

In order to understand this difference the engineers at Subzero Engineering coined the term UNIFLOW in 2005. ‘Uniflow’ describes air that moves in only one direction.  

Data center airflow should flow from the cooling unit directly to the IT thermal load and back to the cooling unit intake. This unidirectional airflow should correspond to the volume of air that is requiredto carry the thermal load of the IT equipment. Anything in excess of this airflow requires additional cooling energy to create the volume or CFM. Thus anytime you plug leaks in the UNIFLOW a reduction in fan speed could be made.  

As you can imagine, the energy saved due to excessive volume has more to do with the amount of airflow leaks. It is here that some AFM companies have over-estimated potential energy saved. It is common to hear that if you plug your cable cutouts that you will save 30% of energy. That is only true if you have leakage that amounts to 30% of excessive volume. And, that this volume can be adjusted.Note: In some cases additional energy canbe wasted when cold supply air bypasses the thermal load and returns to the cooling unit. This is when the cooling unit is required to lower the RH of the return air.  

The other part of the cooling efficiency equation is adjusting of the cooling units’ set points.This can only be accomplished when the intake temperature and RH across the face of the IT equipment (intake) is within the acceptable manufacturer’s range.This can be likened to the ‘weakest link’...

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