There is a general perception that direct air systems are more cost efficient and hence the default option. However, providing the design incorporates good air segregation, recommended ASHRAE equipment environmental conditions and adiabatic cooling of the outdoor air, for most cities warmer than London, the indirect air systems are considerably more efficient than the direct air systems. This is because many more hours of free cooling can be achieved with adiabatic cooling without affecting the indoor conditions. Furthermore, zero-refrigeration in most of the world is possible with this solution. For cooler climates, direct systems are only marginally more efficient.
Often when data centre free cooling is discussed, people assume this means direct fresh air cooling. However, in climates warmer than London, indirect air systems are more efficient than direct air systems and can allow refrigeration to be eliminated and considerably reduce the electrical plant sizing requirements. Use of adiabatic evaporative cooling on the outdoor airstream allows free cooling to be achieved for many more hours in the year when there are hot, dry conditions. Further detail on the application for free cooling in data centres is available in our technical papers.