Data Center

Air Economizers – Which Architecture is Best: Direct or Indirect?

Besides your data center’s IT equipment, the cooling system is the biggest energy consumer in the data center. Cooling systems that operate primarily in economizer mode (aka free cooling mode), allow you to significantly reduce that cooling energy, making it easier to hit your power usage effectiveness (PUE) or cost reduction targets. We’ve all heard this before, but our Cooling Economizer PUE Calculator Tradeoff Tool quantifies it for you.

Your Home’s “Economizer Mode”

Think about what you do when your home gets too hot in the summer. You may choose to turn on your air conditioner, or you may simply open a window and run some fans, depending on the outdoor conditions. In making that decision, you probably consider the temperature, humidity, air quality, and so on. Think of the window and fans as your home’s economizer mode. In the latest Meet the Expert video, I use this analogy to illustrate the importance of economizer modes and the tradeoffs in choosing one cooling architecture over another.

The question we often hear is: I know I want an air economizer architecture, but should I go with direct fresh air or indirect with a heat exchanger? The home analogy would be “direct” since the outside air makes its way into the space being cooled.

Quick Definitions

Direct air economization – Fans and louvers are used to draw a certain amount of cold, outside air through filters and then directly into the data center when the outside air conditions are within specified set points.

Indirect air economization – Outside air is used indirectly (through air to air heat exchanger, heat wheel, or heat pipe) to cool data center air when the outside air conditions are within specified set points, isolating the impact of outside humidity and preventing outside pollutants from entering the IT space.

The Data Center Science Center wrote White Paper 215 that goes into greater detail on the differences between these two cooling architectures.

A Calculator to Compare Cooling Efficiency

In a nutshell, direct air economization may be more efficient than indirect in some climates, but it comes with some added risks. Specifically, the risk of air contamination, moisture intrusion, and increased maintenance. Our Tradeoff Tool compares the expected annual PUE, energy cost, and carbon emissions for these two approaches (as well as five other cooling architectures). The Tool shows how variables like location (climate), and power and cooling configuration inputs (IT inlet temperature, % load, etc.) impact your results. Knowing how each architecture performs relative to one another gives you concrete answers to help you make the right decision.

Data Center Resources, from the Experts

The Cooling Economizer PUE Calculator is one of the many tools the Data Center Science Center has developed to help you justify the tradeoffs you may be faced with for your data center. Is there a decision you’re faced with that our tools don’t address? Contact us directly, the Data Center Science Center continues to invest in developing new tools that cater directly to data center professionals. For more insights on data center knowledge and expertise, watch the Meet the Expert video series on our Data Center Science Center playlist on YouTube.


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