Smart Data Center Design for More Effective Cooling

As Director of the Liebert Precision Cooling Application Engineering group, Dave Kelley is responsible for all of the technical support for precision cooling products. Dave has been with Emerson Network Power for more than 33 years in a variety of technical and management capacities.

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The data center is one of the most dynamic and critical operations in any business. Complexity and criticality have only increased in recent years as data centers experienced steady growth in capacity and density, straining resources and increasing the consequences of poor performance. This Industry Perspective looks at a pair of design practices data center managers can carry out on the cooling side to increase efficiency, availability and capacity.

Maximize Cooling Units Return Temperature to Improve Capacity and Efficiency

This best practice is based on the hot-aisle/ cold-aisle rack arrangement (Figure 1), which improves cooling unit performance by reducing mixing of hot and cold air, thus enabling higher return air temperatures. A 10 degree F increase in return air temperature typically results in a 30 to 38 percent increase in cooling unit capacity and a 15-20 percent increase in system efficiency.

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In the hot-aisle/cold-aisle arrangement, racks are placed in rows face-to-face, with a recommended 48-inch aisle between them. Cold air is distributed in the aisle and used by racks on both sides. Hot air is expelled at the rear of each rack into the “hot aisle.”

To mitigate air mixing as it returns to the cooling unit, perimeter cooling units should be placed at the end of the hot aisle. If the cooling units cannot be positioned at the end, a drop ceiling can be used as a plenum to prevent hot air from mixing with cold air as it returns to the cooling unit. Cooling units can also be placed in a gallery or mechanical room.

Containing the Cold Aisle  Is Easier, Less Risky

Cold aisle containment is favored over hot aisle containment because it is simpler to deploy and reduces risk during the event of a breach of the containment system. With hot aisle containment, open doors or missing blanking panels allow hot air to enter the cold aisle, jeopardizing the performance of IT equipment. In a similar scenario with the cold aisle contained, cold air leaking into the hot aisle decreases the temperature of the return air, slightly compromising efficiency, but not threatening IT reliability.

Row-based cooling units can operate within the contained environment to supplement or replace perimeter cooling. This brings temperature and humidity control closer to the source of heat, allowing more precise control and reducing the energy required to move air across the room. By placing the return air intakes of the precision cooling units directly in the hot aisle, air is captured at its highest temperature and cooling efficiency is maximized. The possible downside of this approach is that more floor space is consumed in the aisle. Row-based cooling can be used in conjunction with traditional perimeter-based cooling in higher density “zones” throughout the data center.

Match Cooling Capacity and Airflow with IT Loads

The most efficient cooling system is one that matches needs to requirements. This has proven to be a challenge in the data center because cooling units are sized for peak demand, which rarely occurs in most applications. This challenge is addressed through the use of intelligent cooling controls capable of understanding, predicting and adjusting cooling capacity and airflow based on conditions within the data center, paired with variable capacity fans and compressors. Intelligent controls enable a shift from cooling control based on return air temperature, to control based on conditions at the servers, which is essential to optimizing efficiency.

This often allows temperatures in the cold aisle to be raised closer to the safe operating threshold now recommended by ASHRAE (max 80.5 degrees F). According to an Emerson Network Power study, a 10 degree increase in cold aisle temperature can generate a 20 percent reduction in cooling system energy usage.

The control system also contributes to efficiency by allowing multiple cooling units to work together as a single system utilizing teamwork. The control system can shift workload to units operating at peak efficiency while preventing units in different locations from working at cross-purposes. Without this type of system, a unit in one area of the data center may add humidity to the room at the same time another unit is extracting humidity from the room.

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