ASHRAE Journal - December 2010 - 41

technology award case studies
unacceptably high. Beaty and Davidson point out the common problem of cold air becoming hot by the time it reaches the inlet of electronic equipment that is located near the top of the racks as a result of diffusion of hot air.2 The tile diffusers must be readjusted periodically for air balance to adapt to changes in rack loads. Increasing airflow in cold aisles helps to reduce diffusion of hot air in the cold aisle, but also increases energy use. It is not uncommon to observe more than 100% oversupply of cold air in data centers and, we observed the same in our existing data center space. The extra cold supply air bypasses electronic equipment and mixes directly with the hot air from the racks and significantly lowers the return air temperature to the CRAC units. Since most of the CRAC units are controlled by the return air temperature, this requires a lower thermostat setpoint and, therefore, the cooling efficiency suffers. And as more air is supplied to limit hot air diffusion, the return air temperature setpoints must be set even lower. For example, in our simulation study, the return air temperature to CRAC, initially set at 78°F (26°C) with just 40% oversupply, was reduced to 73°F (23°C) with a 100% oversupply of the cold air.3 This may explain why the CRAC unit thermostats are commonly set to lower temperature setpoints in the alternating cold and hot aisles configuration.
New System Design

airside economizers were not practical. The waterside economizers were not practical due to the limited number of hours of free cooling available at temperatures low enough to supply the lower temperature air required by the existing data center CRAC units. Additional chillers with variable frequency drives (VFDs) were found to be energy efficient even at part-load conditions. The cooling towers were oversized to provide better condensing temperatures for all new and existing chillers. The outdoor makeup air is supplied through separate direct outside air preconditioning units. These units were operated to remove moisture at the source during humid conditions so that the CRAC units did not have to dehumidify the air on the data center floor. This strategy improved the energy efficiency of the CRAC units. In addition, nearly a million gallons (3.8 million L) of the condensate from the preconditioning units and air-handling units for the electrical rooms is recycled to the cooling towers, which reduces water use and chemical treatment.
Airflow Management

Building at a Glance
Name: Oracle Austin Data Center Location: Austin, Texas Owner: Oracle Principal Use: Data Center Includes: Office to Support
Operations

Employees/Occupants: 97 Gross Square Footage: 130,500 Conditioned Space: 103,000 Substantial Completion/ Occupancy: 2004 Occupancy: 100%

The raised floor data center was increased by 32,000 ft2 (2973 m2) to a total of 82,000 ft2 (7618 m2). The office space supporting the data center is 21,000 ft2 (1951 m2). The building’s total area, including equipment room, is 130,500 ft2 (12 124 m2). The design team had some constraints such as the cooling system architecture (e.g., underfloor air distribution with CRAC units on the data center floor) that needed to be compatible with the existing system. Some options such as
December 2010

The main challenge for energy-efficient operation lies in air distribution. The reported high temperatures at the inlet of the IT equipment near the top of the rack, in the industry and our own observations, led us to conclude that we needed to find a solution to prevent mixing of the hot air into the cold air at the inlet of the IT equipment. In the state-of-the-art alternating hot and cold aisles layout, the diffusion of hot air in the cold aisles caused increasing hot spots as the IT equipment power densities increased. It required cooler and more air to overcome these hot spots leading to inefficient operation, and yet there was no assurance that a certain area would not have hot spots due to complexity in airflow on the data center floor and cold aisles.

The team determined that the definitive way to prevent mixing was to create a physical barrier between hot and cold air. The team reviewed the three options: 1) enclosing the cold air aisle; 2) enclosing the hot air aisle; and 3) enclosing individual racks. Each of these options offered its own cost and energy advantages and limitations. The team selected individual rack containment with hot air discharged directly to an overhead return air plenum. It offered higher efficiency from warmer supply air since no personnel were exposed to the hot discharge air. It also provided deployment flexibility. Once the decision was made to install a physical barrier between hot and cold air, it became clear that the supply airflow must be able to vary to meet the airflow through the IT equipment. The airflow across the IT equipment could vary widely, from about 100 to 300 cfm (47 to 142 L/s) per kW of power use, based on the individual equipment design. In addition, the airflow may also vary with workload on some IT equipment. The use of variable speed
ASHRAE Journal 41

ASHRAE Journal - December 2010

Table of Contents for the Digital Edition of ASHRAE Journal - December 2010