Air Circulation in a Data Center
The air-flow pattern in a data center decides the equipment and rack layout. Following are some common air-flow patterns:
■■ Airflow pattern with subfloor supply and overhead return vents — The air flow follows the basic convection principles: “Hot air rises; cold air sinks.” The pattern is common in data centers with raised floors and has the following stages: 1. The HVAC unit takes in air, which is cooled and dried. 2. The air is forced into the plenum (between the subfloor and the tiled floor) and then directed into the data center and racks via cutouts and perforated tiles. 3. The chilled air goes through the equipment and racks, where it mixes with warm air. As it cools the components, the air gets warmer. 4. The warm air from the racks rises toward the ceiling. 5. From the ceiling, the warm air is drawn back into the HVAC unit, where it is cooled and dried, and then forced back into the subfloor plenum.
■■ Airflow pattern with overhead supply and return vents — Data centers with- out raised floor pump cold air from overhead supply vents. The cold air mixes with warm air that is trying to find its way to the return vents, which are also located above the racks in the ceiling plenum or on the side walls. The contact warms the cold air before it can reach the equip- ment. The contact also creates minor turbulences above the racks.
7. Placement of Hardware Racks
Equipment from different manufacturers has different air-flow requirements. However, all equipment placed within a rack must have the same require- ment. If two pieces of equipment have conflicting requirements and were placed in the same rack, hot exhaust air from one would enter the other. Each stand-alone equipment or rack has a physical footprint, as well as a cooling footprint. The latter is the amount of area a rack or equipment needs to be cooled. Vertically cooled racks (top-to-bottom or bottom-to-top air flow) require less cooling footprint than horizontally cooled racks (front-to-back or side-to-side). The cooling footprint depends on how densely devices are packed within the rack and their heat-generating characteristics. Newer devices pack more electronic components within smaller boxes, thus increasing the heat generated within each device. It is straightforward to understand the air-flow pattern of cold and hot air if you think of the data center as a closed cocoon. The relative locations of racks within the closed space should be primarily based on the rack’s cooling require- ments. The secondary factors are the location of the available power connec- tions, as well as breaks in rack rows and structures (such as aisles, columns, entrances, and ramps).
8. Bottom-to-Top Cooled Racks
This is the most efficient cooling strategy. The cold air enters the rack from the bottom. In raised-floor data centers, the forced air is directed to the equipment via cutout or perforated tiles, placed below the racks. The cold air mixes with the warm air and cools the devices. It is then drawn out through the top of the racks into return vents in the overhead ceiling. Once the lower portions of the rack get the chilled air, the upper portions do not receive any chilled air from the perforated tiles in the floor. It then draws warmer air from other portions of the room such as the rear of the rack. This leads to a recirculation cell near the top of the rack. Air inlet temperatures near the rack top can be 50°F (10°C) to 68°F (20°C) higher than the chilled air near the perforated tiles. To reduce the rack top inlet temperature, it is necessary to use high-flow rate racks (which have a higher air-flow rate between the front and back of the racks) and tiles with higher larger perforated areas (up to 60 percent). When racks are added or moved around, it is simple to reposition solid and perforated tiles to direct cold air from the subfloor plenum to the new rack locations. Figure 8-3 shows an example of this design.