Data Center Design: Raised Floor Versus Slab Floor?

on March 2 | in Data Centers | by | with No Comments

By Dave Pickut

sv5 slab floor 001 Data Center Design: Raised Floor Versus Slab Floor?

SV5, Equinix's newest data center in Silicon Valley, was designed using slab floors.

In the data center industry, few topics raise more heated discussion than the subject of raised floor versus slab floor construction.

Twenty years ago, the topic didn’t even come up in conversation – nearly all data centers used raised floor construction.

However, in the last five to ten years, the debate has generated a wide variety of whitepapers and analyses, including computational fluid dynamics (CFD) modeling of cooling systems performance and new terminology to define cooling effectiveness.

To further fuel the flames, various manufacturers of cooling systems and equipment have attempted to end the debate once and for all, with one manufacturer going so far as to claim raised floor data centers are obsolete.

As an operator of more than 90 IBXs globally, Equinix has a lot of experience with the design, construction and operation of both types of data center. We raised industry eyebrows when we built our first IBXs starting in 1998 – with slab floor and overhead cooling rather than raised floor.

In the ensuing years, those IBXs have performed exceptionally well, particularly in handling high power density (HPD) loads such as the first blade servers and high-performance optimized compute platforms such as the Sun Fire UltraSPARC boxes. While classic data centers were struggling to cool these loads, our IBXs cooled them with ease.

Today, Equinix constructs both types of data centers. Why? The decision is driven by business needs and market demands.

The business and operational needs for any data center or telecommunications facility should drive the design. Factors other than cooling need to be considered, particularly in a multi-tenant facility such as an Equinix IBX.  While a data center built for a specific purpose by a single owner or tenant can be optimized in a variety of ways, a multi-tenant colocation facility has to meet a broad spectrum of needs.

Our analyses have indicated that when it comes to cooling efficiency for base design power density, neither design has a significant advantage. A pressurized raised floor system delivering cooling air via perforated floor tiles consumes about the same energy as pushing cooling air through overhead ducts and diffusers. From a layout flexibility perspective, raised floor has an advantage because perforated tiles can be easily rearranged. Overhead cooling, however, has fewer issues with air leakage or air bypass. System air balancing is a little more difficult with raised floor.

Construction cost comparisons also indicate that neither approach has a compelling advantage.  The various component costs associated with each approach yield a similar cost per square foot of data center area.

Consequently, business needs become the deciding factor for one system versus the other. For future flexibility in a multi-tenant facility, the raised floor approach has an advantage. 

The accessible raised floor plenum facilitates future changes such as added cooling or electrical infrastructure – it is easier to run new conduits or piping under a raised floor than it is to run overhead in a “live” data center. (Note: in our IBXs with raised floor, we use the underfloor plenum primarily for air delivery and routing of chilled water piping, if any. Power and data cabling are run overhead. This result in an underfloor area with few obstructions).  The flexibility to run future services to HPD cooling equipment such as door coolers, in-row cooling units, or heat transfer media direct to servers is important for some facilities.

For equipment installation and movement, and overall maintenance cost, the advantage goes to the slab floor. The concerns with point loading or moving loads associated with raised floor systems are not an issue with a slab floor. Maintenance costs for a slab floor are lower than those for a raised floor.

Seismic (earthquake) performance is also less costly for a slab floor. Equipment anchoring is easier, and the added lateral bracing and reinforcement associated with a raised floor system are not issues with a slab.  Market demand is a factor for a global operator like Equinix. In various regions of the world, the market expects data centers to have raised floor. For that reason, some of our designs incorporate raised floor – but still keep power, data and some cooling functions (such as return air plenums for close-coupled cooling) overhead.

In summary, the best design for a data center is that which best fits the operational and business objectives that have been identified. Let the debate rage on.

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