This tip is the first in a series on choosing a raised floor for a data center. Read part two about choosing raised floors and panels.
Choosing an appropriate raised floor
Evaluating the need for raised floors
Raised access floors have been with us since the early days of computers, and the computer room is often referred to as the “raised floor space.” For many IT managers, a data center is simply not a data center without one. It’s included automatically in most projects and rarely questioned. But data centers have different requirements today. Much of what has historically been under the raised floor--primarily power and data cabling–is now often run overhead, and cooling is also done with overhead systems and/or in-row air conditioners. So the question is, are raised access floors still useful in today’s data centers and, if so, for what?
The key word is “access.” Raised floors were originally put in computer rooms because of the large cables and connectors that attached peripherals to the mainframes. For those too young to remember, those cables were often an inch or more in diameter, and the connectors could be one inch by four inches or more. They were factory-made in fixed lengths, so there was always excess cable to be coiled up. Equipment was changed fairly often as computing needs mushroomed, so data center managers had to run new cables quickly to minimize expensive downtime. Power was also on flexible “whips” so that it could be moved around. The raised floors, usually only six to eight inches high, provided the means to handle these requirements while keeping the equipment space clean and safe.
It was only later that the concept of air delivery under the floor came about. In the early days of data centers, cooling was accomplished by virtually freezing the rooms. Working in a data center without a jacket was rare. As systems grew and peripherals spread-out, it became impractical to simply over-cool large spaces, so equipment was designed to be “spot cooled” from below via underfloor air. It was still unnecessary to have particularly high floors; 12 inches of space was usually sufficient. And uniform underfloor pressures were not really important--if you put enough air under the floor, the hardware would receive adequate cooling. That has changed dramatically.
The challenges of raised floors
Massive underfloor air delivery is often problematic and accounts for many of today’s data center cooling problems. Higher floor plenums are required for the quantity of air needed to cool high-density computing hardware, but putting so much air through the floor cavity requires careful placement of air conditioners. Air turbulence, which is like a small tornado, causes uneven air pressure and spotty air delivery. Floor heights of 18 inches, 24 inches and even 30 inches or more are needed, and few buildings have the slab-to-slab clearance for that.
Ramps also take up a lot of space. The Americans with Disabilities Act (ADA) requires at least a 1:12 slope, which means a ramp must have one foot of length for every inch of floor height. In new buildings, a depressed slab will keep the raised floor even with the surrounding corridors, but that takes a special structure. In either case, if height is not available and an attempt is made to deliver large air quantities through a lower than necessary floor plenum, the results are likely to be disappointing, with a lot of money spent for inadequate cooling.
There are also code considerations with underfloor air delivery. A data center that uses the raised floor space for cooling may be required by Article 645 of the National Electrical Code to also have an emergency power off button next to exit doors. Data center owners can avoid this requirement in a number of ways, including not using a raised floor at all. In that case, major piping, such as chilled water lines, can often be run in the floor under the data center or on the wall behind it.
So does this mean raised floors are obsolete? Definitely not! Putting everything overhead can take as much height as a raised floor and can get a good deal more complicated. If you’re using in-row coolers, rear door coolers, self-cooled cabinets or servers that need chilled water connections, you may not want to run water piping overhead. Equipment may not have convenient water ports closer to the ceiling, and there are always concerns about potential leaks from overhead piping (although that really shouldn’t be a concern if it’s done right).
Another thing to consider is slab levelness. Most building floor slabs are uneven, and they’re designed to flex as weight is added. Raised floors use adjustable pedestals that result in a very level floor surface without needing to level the slab. This makes it easier to align rows of cabinets, as well as to roll equipment into place. So whether you choose to use a raised floor is really a matter of how you see the pros and cons.
More resources on choosing a raised floor
About the author: Robert McFarlane is a principal in charge of data center design for the international consulting firm Shen Milsom &Wilke LLC. McFarlane has spent more than 35 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professional program, is a data center power and cooling expert, is widely published, speaks at many industry seminars and is a corresponding member of ASHRAE TC9.9 which publishes a wide range of industry guidelines.
This was first published in August 2011