Building Up: Nine-Foot High Server Racks

Some data center users are building up, rather than out. By using oversize racks that are 9 feet tall instead of the standard 7 feet, they are able to pack more servers into each square foot of data center space.

How do you get more mileage out of your data center space? Some companies are building up, rather than out. By using taller racks and cabinets, these users are able to pack more servers into each square foot of data center space. This practice is being reported in leased data center facilities designed to support higher power densities.

"Our customers are more and more sophisticated, and now they understand that power density is just a matter of how high the racks can go," said Hossein Fateh, President and CEO of DuPont Fabros Technology. "You can build vertically as well as horizontally, and when you build your rack higher they don’t have to pay extra for it. To be more efficient, they will build a nine foot rack. We just went through the exercise with one customer who hadn’t done it before."

A standard server rack is seven feet high and can accommodate 42 units (42U) of rackmount server equipment in a 19-inch wide slot. There are many options in rack size, with manufacturers offering enclosures that are up to 23 inches wide (a size often used for telecom equipment) and height options ranging from 44U and a nine-foot tall 58U rack.

Taller racks allow you to pack more gear into the same space, but can also create cooling problems, particularly if the racks are completely filled to take advantage of the additional height. One of the challenges in data center air conditioning is addressing temperature fluctuations between the top and bottom of racks.

Watch Out for Hot Spots
A historic problem in raised-floor data centers – where cold air from a plenum area under the floor is pumped into the equipment area – is the tendency for “hot spots” near the top of racks. If the air flow isn’t strong enough, the top of the racks don’t get enough cool air. If the airflow is too strong, air is driven past the top of the rack and can cause recirculation that mixes hot and cold air in the upper section of the cold aisle.

Using containment for the hot aisle or cold aisle can prevent recirculation of waste heat from the server outlets. Taller racks can also work well in designs that feature top-down cooling, either in a slab-floor design or by using rack-top cooling units to supplement a raised floor.

HP uses taller racks to increase the server density in its POD data center container. The POD (short for Performance Optimized Datacenter) features 50U floor-to-ceiling racks, which allow users to pack in more servers, but also separate the hot and cold aisles. See our video tour of the HP POD for a closer look at its design.

Vern from SwiftWater Telecom notes that some telco central offices have used racks as tall as 11 feet. "The biggest concern here, aside from supporting them safely, is to make darn sure you don’t exceed your floor load capacity," he writes.

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