When Server Racks Undermine Data Center Cooling
February 11, 2009
By Mark Fontecchio
Appeared in SearchDataCenter.com
Server cabinets are an often overlooked component of data center design. But according to speakers and attendees at last week's American Society of Heating, Refrigerating, and Air-conditioning Engineers' conference, selecting the right server cabinet can have a marked effect on data center airflow.
How does a server cabinet's design have impact on data center airflow? According to ASHRAE's Technical Committee 9.9 members, it happens mainly in two ways:
- In a hot-aisle/cold-aisle configuration, server cabinets prevent the recycling of hot air to server intakes. When hot air from the server exhausts mixes with cold air from a data center's air conditioners, it forces air conditioners to work harder to get cool-enough air back to server intakes and wastes cooling.
- Cabinets can prevent bypass air, or cold air that returns to air conditioners without cooling anything. This happens when there are openings in the server cabinets that allow the cold air to pass through without cooling off servers. This prompts air conditioners to pump out more cold air than is necessary, again, wasting cooling.
Racks are an integral part of data centers, and data center facilities experts should oversee their selection, said Kishor Khankari, a TC 9.9 member and associate partner and computational fluid dynamics (CFD) specialist at the engineering consulting firm Syska Hennessy Group. "We cannot just let the IT guys decide what racks should be installed," he said during a presentation at the ASHRAE show.
IT staffers, for example, might not consider how a given server rack performs in the data center as a whole. Khankari said that servers perform differently when they are placed in racks than when the server manufacturers do bench tests. So the airflow dynamics also differ.
"The vendor tests the single-server airflow," he said. "But we stack them, then we put them in a cabinet with a front-perforated door and a back-perforated door. Then you add cables and other obstructions. This is not just a cabinet; it is more than a cabinet."Designing server cabinets for proper airflow
Generally speaking, a well-designed server cabinet should provide segregation between the cool supply air and the hot-return air, said Ian Seaton, a technology manager at equipment rack vendor Chatsworth Products Inc., said. To that end, data center managers have a few options to minimize recycled and bypass air traveling through a server cabinet.
Blanking panels are the most well-known way to prevent bypass air. The panels fit into empty rack space, preventing cool air from slipping through a cabinet without touching hot electronics. Blanking panels can also prevent hot air from servers installed above and below an empty space from pushing back into the cold aisle and getting into the server intakes.
At one time, installing blanking panels was a laborious job that required a drill and screws. But many of today's blanking panels can be installed without tools – they just snap on and off. (In fact, SearchDataCenter.com awarded its first-place prize for 2008 infrastructure product of the year in 2008 to Upsite Technologies' tool-less blanking panel.)
But as Seaton put it, "blanking panels by themselves are not sufficient." Often there is space on either side of servers inside the cabinet, leaving room for cool air to slip through or hot air to push its way back. Seaton suggested using "internal air dams" inside cabinets to better separate the front of cabinets from the back, and prevent recycled or bypass air.
The leveling feet or casters that server cabinets sit on can also introduce space through which bypass or recycled air can move through. Some data center managers recommend removing them or blocking off those spaces.
And then there is the space in between cabinets to contend with. Even if two server cabinets abut each other, air can still pass through a seam. Most manufacturers provide a way to bolt cabinets together, which data center managers should take advantage of.
Finally, there is the question of the server cabinet's doors. When it comes to doors, the more perforations, the easier it is for cold air to get to the server intakes and for hot air to get out away from the servers. This helps alleviate pressure buildup and heat in the back of the cabinet that can then reverberate back to servers.
"You need to take cholesterol measurements of the environment," Khankari said. "If you block that artery, the pressure buildup could have a significant effect on the servers."