The choice between direct or indirect in a data center depends on several factors including their respective benefits, capital and operating costs, geographic locations, and availability risks.
Paul Lin, Senior Research Analyst at Schneider Electric’s Data Center Science Center, notes that both modes have pros and cons, but they must be evaluated with the above factors in mind.
Direct air economization is exactly what it sounds like, explained Paul in a previous post — using fans and louvers to draw in cold air, which is then filtered directly into the data center. Indirect economizer mode keeps the outside air separate from the inside air by using an air-to-air heat exchanger. This mitigates the effect of impure or damp outside air.
Not So Fresh
According to White Paper 215, “Choosing Between Direct and Indirect Air Economization for Data Centers,” of which Paul is a co-author, the main barriers to using direct fresh air are also the biggest differences between the two modes: air pollution and high humidity.
Research has shown that good levels of filtration can be achieved in the U.S. when using direct fresh air systems, but in other regions like China and India, and in certain other environmental types (e.g. deserts), the amount of filtration needed is much more substantial, as is the need for filter maintenance. Having to replace clogged filters every few months can be a significant operating expense and resource drain.
If adequate filtering capabilities are not in place or filters are not kept clean, “particulate and gaseous contamination such as dust, chemicals from spills, smoke/ash, etc. may have a chance to enter the data center.”
The presence of these toxins alone can cause corrosion, plus humidity acts as a catalyst for exceling deterioration. To prevent equipment fail, corrosion monitoring systems would need to be implemented along with specific strategies to control cleanliness.
The paper also states: “Other data center availability risks associated with these two economizer mode include expanded IT inlet temperature range, DX backup configuration, loss of evaporative water, impact on the fire suppression system and human error.”
Expanded IT inlet temperature range may decrease energy consumption in one area while raising it overall. Increasing DX backup means more capital cost. Loss of water can happen for any number of reasons and must be addressed through back up equipment or right-sizing the mechanical cooling system. In case of fire, direct economizer mode would call for closing off the outside fresh air supply entirely.
Plus, people are responsible for the majority of data center outages and direct air systems are highly customized and are therefore, arguably more susceptible to human error. So the direct air economizer can have a lower initial capex cost, but the added cost to mitigate all of its risk factors greatly diminishes its appeal.
Ample details on all the key factors for making the decision between direct and indirect air economization appear in whitepaper 215, which clearly illustrates the comparison.
Given the availability risks outlined here, can you guess which mode Paul concludes is smarter?