ASHRAE Journal - August 2009 - 51

its way through this gap and enters into the bottom servers from the front. CFD analysis was performed with a 20% excess of cold supply air than what is required for a 20°F (11°C) rise through the servers. In spite of such an excess supply, the predicted inlet air temperatures are higher than the supply air temperature and exceeded ASHRAE recommended higher temperature limit of 80.6°F (27°C).1 This is due to the recirculation of air within the rack and entrainment of hot air under the rack. Rack Heat Load Matters As shown in Figures 5a and 5b, air recirculation becomes worse with increasing heat loads in the rack. With increase in the rack heat loads from 5 kW to 20 kW and proportionate increase in the airflow rates through the servers, the pressure difference between the back and front of the rack increases by 21%. As a result the extent of air recirculation into the server intakes increases from 20% to 45%. Recirculation within the rack is avoided by placing additional plates to block the flow of air from the back of the servers. However, Figure 5b shows such a blocking can further increase the pressure buildup that, in turn, can adversely affect the server fan performance and reduce the airflow rate through the server. The design of the back plenum is crucial to avoid such a pressure buildup. To achieve adequate cooling, the speed of a server fan increases with the increase in the temperature of electronic components and with a rise in the inlet air temperature. However, such a ramp up in the fan speeds can increase energy consumption (fan energy is proportional to the cube of the fan speed). The dc fan power of servers can increase 3.5 to 7 times when server inlet air temperature increases from around 80°F to 95°F (44°C to 53°C).4 In addition, increased airflow rates, such as at ramp up, can further promote pressure buildup behind the servers, which can affect fan performance. Summary Rack enclosures are a crucial link between data center airhandling units and servers. Design of rack enclosures can impact the fan performance, and therefore, cooling performance of servers. Careful consideration should be given before selecting the rack enclosures for a certain data center. Just following the best practices for the design and layout of data centers is not enough to obtain the optimum cooling performance. This article illustrates the importance of rack enclosure design and airflow management within the rack enclosures. References 1. ASHRAE. 2008. “2008 ASHRAE Environmental Guidelines for Datacom Equipment—Expanding the Recommended Environmental Envelope.” 2. Artman, P., D. Moss, G. Bennett. 2002. “Dell™ PowerEdge™1650: Rack Impacts on Cooling for High Density Servers.” Dell White Paper. Enterprise Systems Group (ESG). 3. Moss, D. 2009. Personal communications. 4. Jones, R., et al. 2008. “Seven Strategies to Improve Data Center Cooling Efficiency.” The Green Grid White Paper 11. August 2009 www.info.hotims.com/ 25207-17 ASHRAE Journal 51

ASHRAE Journal - August 2009

Table of Contents for the Digital Edition of ASHRAE Journal - August 2009