Using the discussed air-management metrics, detailed calculations were made for a mid-range data center with bypass and re-circulation factors of 0.5. Eliminating airflow bypass can reduce CRAC fan energy use by 60kW. By eliminating re-circulation, compressor energy savings of 74kW are possible, due to the evaporator operating at higher temperatures. For a 10,000 square foot data center, this can represent a saving of $90,000 annually. These estimates do not include free cooling.
How can facility managers improve the air management of their data centers? From the practical applications the following is a list of measures that reduce the issues of poor air management. They are listed in descending order of impact and feasibility:
Bypass Air:
Recirculation Air:
The effectiveness of those measures can be seen from work done at a 5,000-square-foot data center. During an energy assessment of the facility, measurements were taken from more than 20 percent of the servers present and all of the CRAC units. The general data center floor layout is back-to-front, but some server discharges and intakes are on the same side. The load density was 27 watts per square foot. Recirculation, bypass, and balance were calculated to be 0.61, 0.21, and 0.50, respectively. Recall that the ideal for recirculation and bypass is zero, and for balance it is 1. Clearly there was room for improvement.
The balance ratio of 0.5 indicates that the servers are not getting enough air. That was attributed to the fairly low height (18 inches) of the raised floor with many obstructions. The obstructions resulted in low air velocities from the floor grills, so that cool air didn't reach the servers in the top of the cabinets. Another problem is that many server intakes and discharges are located in the same aisle.
To reduce recirculation and increase balance, the assessment team recommended better management of underfloor obstructions; more efficient layout such as cold/hot aisle; retrofitting the data center with a return air plenum; blanking-off all the unused server locations; fitting blanking panels between cabinets; and fitting brush seals at all cable cutouts. Based on measurements taken from the data center at the same locations after implementing most of the recommendations, new air management metrics were 0.34 for recirculation, 0.18 for bypass and 0.82 for balance, a substantial improvement. However, the facility manager is looking at installing a return air plenum to reduce the recirculation flow ratio even more.
The following lessons can serve as guidelines for such analyses.
Many data center operators take a relaxed attitude toward bypass air, which is produced by the CRACs and returns directly without cooling servers. Bypass air causes a gap between the temperature of air produced by the CRAC units and the temperature of the air at the inlet to the server, although in theory those two should be the same. CRAC units are designed to produce lower temperatures for the sole reason of offsetting the re-circulation of air from the outlet of the server to its inlet. Air bypass also starves servers of air, which increases recirculation.
Better air management within data centers — mainly minimizing bypass and recirculation air flow — has the potential to improve temperature control for more reliable server cooling. It can also save energy. What's more, improved air management is essential to maximizing data center free cooling opportunities. 
Munther Salim, PhD, is principal, energy efficiency and sustainability in the Chicago office of HP Critical Facilities Services - EYP MCF. Robert Tozer, PhD, is technical director - HP EMEA.
Download the technical treatment of this article.
Understanding the Metrics for Chilled Air in Data Centers
Improving Data Center Air Management
