Using outdoor air in the cooling process allows a data centre to run in economiser mode, in which the load on chillers and condensers can be greatly reduced or even removed. The choice between using direct or indirect economization depends on a number of factors particular to each data centre, including the ambient climactic conditions, the degree of risk associated with availability and costs.
Co-written by Paul Lin, John Niemann and Leo Long of Schneider Electric, White Paper #215 “Choosing Between Direct and Indirect Air Economization for Data Centers” describes in detail the differences between each approach and recommends scenarios in which each is the preferable option.
Direct air economization uses fans and louvers to draw cold external air through filters and then directly into the data centre when outside air conditions are within specified set points. Indirect air economization also draws in cold external air when outside conditions are within specified set points but in this case air-to-air heat exchangers, heat wheels or heat pumps are used to isolate the impact of outside humidity and filter pollutants before the air enters the IT space.
White Paper 215 covers in depth the pros and cons of each mode. Direct air economization has lower capital costs for prefabricated systems because it does not use an air-air heat exchange stage to isolate external air from the air in the IT equipment space and can be a cost-effective option for geographical regions with cool temperatures, low humidity and air quality with low levels of pollutants. However, such systems are highly susceptible to changes in outdoor air quality and costs can increase if extra filters are needed or if input air has to be treated for humidity.
Indirect air economization has higher capital costs but typically these are only a small percentage of the overall cost of a data centre. Operating and maintenance costs are lower as the outdoor air is completely isolated from the data centre air by the heat exchangers. The heat-exchange phase also lowers the efficiency of indirect air economization compared with direct options but availability risks are also lower.
The white paper can be downloaded HERE