Data Center

Common Questions Raised When Considering Direct and Indirect Air Economization

A survey of data center economizer use completed by The Green Grid and discussed in Green Grid WP #41, “Survey Results: Data Center Economizer Use”, showed that use of economization will result in an average cooling energy savings of 20% when compared to data center cooling systems without economizer. Direct and indirect air economizer modes are two approaches of air-side economizer mode which uses outdoor air to cool the data center.

I think most people understand the idea of direct air economization which is often referred to as “fresh air” economizer. It uses fans and louvers to draw a certain amount of cold fresh outside air through filters and then directly into the data center, when the outside air conditions are within specified set points.

What’s indirect air economization? As the name implies, this kind of economizer mode uses outside air to indirectly cool data center air when the outside air conditions are within specified set points. An air-to-air heat exchanger is used to isolate the impact of outside humidity and prevent outdoor air pollutants from entering the IT space.

Let’s look at some of the common questions raised when considering these two air economizer modes?

  • Which locations are favorable for each kind of economizer mode?
  • What’s the capital investment?
  • What’s the operating cost?
  • Do I need a full mechanical DX system for backup or just a mechanical cooling assist?
  • What’s the impact of increased temperature, elevated humidity, and air pollutants on the reliability of the IT equipment housed in my data center for a direct air system?
  • Can my IT equipment withstand external conditions for a direct air system?
  • Can my data center be retrofitted with this kind of economizer mode?
  • What are the design considerations to install an indirect air economizer system?
  • What are the main differences between direct and indirect air economization?
  • What’s the energy efficiency penalty due to the air-to-air heat exchanger in the indirect air economizer mode?


Knowing the answers to the above questions can help data center designers and users select a cost effective and high efficiency cooling system. In my next post, I’ll tell you where you can find answers to the above questions, compare direct and indirect air economizer modes, and provide practical guidance for selection.

2 Responses
  1. Sam Roborh

    Hi there,
    What about taking the air from a basement area where it is likely to be cooler, rather than the outside air when its temperature is likely to low only at certain times of the day?


    • Paul Lin

      Hi Sam,

      I agree with you that the air from the basement is likely to be cooler. But, there are also some issues for using the air from the basement. First, not every data center has a basement, and it would be a very big project and high investment to taking the air from the basement to pass through the direct and indirect air economizer systems. Second, I don’t think there is enough cold air in the basement to cool the data center. After several heat transfer cycles, the temperature of the basement will increase faster. So, you will need an extremely large basement.




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