By Walter R. Schwarz, HVAC&R Industry Manager, Fluent Inc.

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The thermal cooling of high-density computer data centers is a challenging yet mission-critical task in many companies today. The complex nature of airflow and the frequency at which equipment is added, changed, or relocated only add to the challenge. Fluent Inc. is not immune to these types of problems since its continued growth in business has been accompanied by a continued growth in computing resources. Some Fluent office locations around the world have data centers that are cooled with underfloor air supply, and have equipment arranged in hot-aisle/cold-aisle configurations. At the corporate headquarters in Lebanon, NH, however, the central data center is cooled using overhead air supply. As is true for many such facilities, it was probably designed for occupant comfort rather than for the cooling needs of a data processing environment. Until recently, not much attention was given to the thermal cooling of the room, since most of the computing resources were deployed in individual offices. With the changing trend toward clustering computer resources in a central location, however, cooling has become an important concern.

During the past year, the Director of Computer Services and Building Manager were experiencing difficulties in maintaining optimal cooling conditions in the data center. According to ASHRAE’s Thermal Guidelines for Data Processing Environments (2004), the recommended equipment environment temperatures are 20°C to 25°C (68°F to 77°F). Compounding the situation in Lebanon were plans to bring more computer horsepower and its associated thermal load into the existing data center. Initial thoughts were to spend considerable money to bolster the cooling capacity with more HVAC capital equipment. Before proceeding with such a large investment, both managers approached the HVAC industry team to see if Fluent software could be used to analyze the problem.

“ It was remarkable to identify local cold spots predicted by the simulation and to then go to the data center and verify the results with your hands. The simulation was able to precisely pinpoint the temperature variations in the room.”
- Alister Cate
Building Manager, Fluent Inc.

“ I knew I had a problem. The HVAC system in my data center was not keeping up with the existing thermal load and I was about to add a lot more computing power. I was almost convinced by several contractors to spend $40,000 on more AC, when I turned to Fluent’s own HVAC team for recommendations. Following their advice, we changed and relocated the diffusers, and rearranged the existing equipment. Afterwards, I was able to add all the planned new hardware, without any additional AC. And the temperatures in the room have never been cooler or more uniform. I am very impressed!”
- Earl Zmijewski
Director of Computer Services, Fluent Inc.

Airpak airflow modeling software was used to quickly create a virtual model of the data center, which is approximately 67 m² (725 ft²) in area with a power density of about 394 W/m² (37 W/ft²). The actual power of the equipment was directly measured and used in the simulation, instead of the equipment nameplate ratings, which can lead to over design of the HVAC system. The first analysis was of the data center with the existing equipment and air distribution system that made use of four-way ceiling diffusers. The simulation results reflected much of what the managers already knew – that there was a wide range of rack inlet temperatures, and several locations where the equipment was subjected to higher than desired temperatures. The managers were impressed by the close agreement between the equipment temperature readouts and the temperatures predicted by Airpak. Moreover, the simulation revealed some short-circuiting of supply air into the returns before it had a chance to cool the equipment.

Learning from the initial analysis, the equipment was repositioned in the room to develop a hot aisle/cold aisle arrangement, and the four-way diffusers were replaced with one-way diffusers that discharged cool supply air vertically down. Simulation of this improved design showed far better cooling of the equipment. Other analyses were performed to assess the vulnerability of critical servers in the room to hypothetical failure scenarios, where a refrigerant compressor might fail in one of the HVAC systems. Future simulations will be done to help design the cooling system to accommodate an expected doubling of computer resources in the room. The goal is to make sure that the data center is adequately protected and capital investments in HVAC system upgrades are wisely planned.

Pathlines colored by temperature in the data center at Fluent Inc. illustrate the airflow distribution from the new vertical diffusers

Plan view of temperature contours two feet above the floor

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