December 26 2008

Figure Out The Right Height For Raised Flooring
A Floor That Is Too High Or Too Low Can Compromise Airflow Functions

To uninformed observers, a data center may appear to be one of the more humdrum elements of data center operations. However, most savvy data center administrators know that the raised flooring can be an important piece of the cooling and airflow puzzle. Figuring out the correct height for a raised floor is essential to the success of cooling and airflow functions necessary for keeping servers humming and enterprises running.

Equipment housed in a data center with properly calibrated cooling and airflow will generally suffer fewer inexplicable failures and malfunctions, while equipment housed where temperature and airflow are hard to control could cause more problems.

The Floor Function

A raised floor is an integral part of the airflow and cooling function that are so vital to maintaining a well-run data center. Lior Blik, CEO and president of NIT Connect (www.nitconnect.net), says cooling is generated from under the floor to the front of the rack. Cold air comes up through the raised floor, cools the front of the servers, and replaces warm air from the back of the servers.

Even though the primary benefit of a raised floor at the proper height is improved airflow and hardware cooling, there are other considerations. For example, Scott Coll, president of Commercial Flooring Systems (commercialflooringsystems.com), says a correct floor height can mean the difference between a quiet or noisy data center.

"If you are funneling airflow under the raised flooring and the duct is too small," Coll says, "there will be noise problems as air pushes through the smaller space." One way to alleviate this problem, he adds, is to use perforated panels that give the high-velocity air another escape route.

Randall Hayter, director of technical operations for Lee Technologies (www.leetechnologies.com), says floor height is determined primarily by the volume of air needed to cool the data center at design density along with the under-floor static pressure needed to deliver that air to all areas requiring cooling.

"Compromises must sometimes be made when dealing with an existing structure," he adds, "but, in general, the floor should be built to the designed height."

Before You Start

The first question administrators should ask before determining the correct height for a raised floor is whether a raised access floor is necessary, says Hayter. In fact, he adds, for many data center administrators considering a new facility, the answer has been no. Alternatives such as concrete slabs with in-row cooling or overhead ducted supply and return, he says, are attractive alternatives to raised access flooring.

"This question should be considered early in the requirements phase of the project as it directly impacts many basic design decisions," says Hayter. If a raised access floor is required, administrators must keep in mind the fact that its primary function is to provide a flexible system for delivering cool air to data center equipment, wherever you place the equipment.

Coll says administrators exploring the use of a raised access floor should "understand the dynamics of a data center." He adds that determining the weight load of equipment is also a crucial consideration in the overall budgeting of the project.

NIT Connect's Blik says the industry standard is to have data center floors raised 18 inches from the ground. But, he says the height determination on the number of servers and the amount of space you need to cool.

For example, to generate cooling for 50 or more racks, Blik's company would recommend a floor height of 24 inches or more. In fact, he says some data centers will construct their floors at 30 to 40 inches to allow for more room under the floor, letting personnel manage the cooling from there.

The Raised Floor Toolkit

Because a raised access floor is such a critical component of a data center's cooling and airflow function, tools that enable administrators to model airflows under different conditions are essential to success. Hayter says CFD (computational fluid dynamics) is a useful computer modeling tool that lets an engineer predict airflow delivery at design time.

"Using a CFD model," he says, "the effect of using different air handling unit configurations, tile perforations and placements, floor height, placement of air handlers, under-floor blockages, redundant unit operation for failover, and similar variables can be predicted and controlled."

If a CFD is not available, says Hayter, engineers and administrators can determine a floor height based solely on flow calculations. But, he warns, if you are using flow calculations alone, prepare to experiment with perforated floor tile flow rates to get the airflow right.

Coll says administrators should focus their efforts on creating a raised floor master plan. In fact, he says the room's ceiling height, door clearance, and ramp access should all be considered before construction begins.

Potential Pitfalls

So, what potential pitfalls should be avoided when considering the proper height for a raised floor? For starters, Hayter says if the floor height is too low, air speed under the floor will need to be high in order to transport enough air to the equipment. This fast-moving air can cause a pressure drop on the underside of the perforated tiles, which on the extreme end can cause air to flow down rather than up.

Conversely, he adds, if the floor height is too high, the air-handling equipment will have to work harder to compensate for low air velocity to build the static pressure needed for proper air delivery.

Blik says decreased or minimized raised floor height can cause cold air pockets to build under the floor, essentially creating an air blockage for air trying to move up.

Jill Eckhaus, CEO of AFCOM (www.afcom.com), says one key item to consider is the seismic zone that the data center is located in, because this will impact the stability of your data center.

The Bottom Line

The correct height for a raised floor is a vital component of a properly cooled data center. This seemingly mundane decision impacts a data center environmental concern: keeping hardware cool and air moving. As data centers evolve and increasing amounts of hardware gets crammed into place, this consideration will only grow in importance.

As Hayter points out, choosing raised access flooring height is a decision that can't be made in a vacuum. It is an integral part of the overall engineering process, so the calculations to determine proper floor height must be part of the overall project engineering scope.

by Sixto Ortiz Jr.

TOP TIPS

Mitigate the rat's nest-The space under a raised floor often houses the seemingly endless feet of cable needed for any good-sized data center. But, a large amount of unorganized cabling can cause airflow blockages.

Room to grow-Jill Eckhaus, AFCOM CEO, says administrators should consider accessibility for expansion when designing a raised floor to ensure there is enough real estate for cooling, cabling, and so on.

Consider cabling runs-Even the manner in which cabling is arranged under the floor can be a critical factor. For example, says NIT Connect's CEO and President, Lior Blik, administrators should make sure cables are run closer together, which lets the cool air rise.