Data Center Economizer Contamination and Humidity Study

Data centers require continuous air conditioning to address high internal heat loads (heat release from equipment) and maintain indoor temperatures within recommended operating levels for computers. Air economizer cycles, which bring in large amounts of outside air to cool internal loads when weather conditions are favorable, could save cooling energy. There is reluctance from many data center owners to use this common cooling technique, however, due to fear of introducing pollutants and potential loss of humidity control. Concerns about equipment failure from airborne pollutants lead to specifying as little outside air as permissible for human occupants.

To investigate contamination levels, particle monitoring was conducted at eight data centers in Northern California. Particle counters were placed at three to four different locations within and outside of each data center evaluated in this study. Humidity was also monitored at many of the sites to determine how economizers affect humidity control. The results from this study indicate that economizers do increase the outdoor concentration in data centers, but this concentration, when averaged annually, is still below current particle concentration limits. Study results are summarized below:

1. The average particle concentrations measured at each location, both outside and at the servers, are shown in Table 1. Measurements show low particle concentrations at all data centers without economizers, regardless of outdoor particle concentrations. Particle concentrations were typically an order of magnitude below both outside particle concentrations and recently published ASHRAE standards.
2. Economizer use caused sharp increases in particle concentrations when the economizer vents were open. The particle concentration in the data centers, however, quickly dropped back to pre-economizer levels when the vents closed. Since economizers only allow outside air part of the time, the annual average concentrations still met the ASHRAE standards. However, concentration were still above the levels measured in data centers that do not use economizers.
3. Current filtration in data centers is minimal (ASHRAE 40%) since most air is typically recycled. When using economizers, modest improvements in filtration (ASHRAE 85%) can reduce particle concentrations to nearly match the level found in data centers that do not use economizers. The extra cost associated with improve filters was not determined in this study.
4. Humidity was consistent and within the ASHRAE recommended levels for all data centers without economizers. Results show that, while slightly less steady, humidity in data centers with economizers can also be controlled within the ASHRAE recommended levels. However, this control of humidity reduces energy savings by limiting the hours the economizer vents are open.
5. The potential energy savings from economizer use has been measured in one data center. When economizers were active, mechanical cooling power dropped by approximately 30%. Annual savings at this center is estimated within the range of 60-80 MWh/year, representing approximately a 5% savings off the mechanical energy load of the data center. Incoming temperatures and humidity at this data center were conservative relative to the ASHRAE acceptable temperature and humidity ranges. Greater savings may be available if higher temperature humidity levels in the data center area were permitted.

The average particle concentrations measured at each of the eight data center locations are shown in Table 1. The data centers ranged in size from approximately 5,000 ft² to 20,000 ft² . The indoor concentrations and humidity in Table 1 represents measurements taken at the server rack. Temperature measurements at the server rack consistently fell between 65-70°F. The Findings section contains a discussion of the individual findings from each center.