How to avoid costly flaws with data centre flooring

By Gordon Johnson, Senior CFD Engineer at Subzero Engineering
Gordon Johnson, Senior CFD Engineer at Subzero Engineering, shares his insights into how data centre flooring can improve sustainability standards

With the growing cost of power in Europe, energy efficiency should be a top concern for today’s data centre owners and operators, and in the future, design teams must carefully evaluate their infrastructure options. 

Average monthly wholesale electricity prices, per megawatt-hour, across Europe have risen from well under 100 euros in January 2020, breaking the 100 euros barrier in July/August 2021, to today’s levels of typically mid-300 euros, with Italy already over the 400 euros level. And, at the end of August, Bloomberg reported that German power for 2023 was being traded at 1,050 euros per megawatt-hour.

Thanks to the drive towards Net Zero, data centre owners and operators were already looking at improving their energy efficiency levels significantly, by adopting both innovative design and technology developments. The runaway energy price levels have served to speed up this process.

The idea of moving away from the traditional raised floors towards a design which incorporates slab flooring, can offer many benefits to data centre operators. However, in the past, most legacy environments were equipped with raised flooring systems, used for underfloor air distribution to deliver cold supply air to the IT equipment. Additionally, some raised floor areas were used to run data cabling and wiring. 

Slab floors have been around for decades, but haven’t enjoyed the same popularity as raised flooring, and until recently, slab floor designs had meant that the cold supply air was delivered to the IT equipment via overhead ducts and diffusers. One issue with this design has been the lack of flexibility with moving or adding new racks to the data centre, and thereafter, how to reconfigure the ducts and diffusers to match such changes?

Another challenge has been the ability to deliver hot exhaust air back to the cooling units, which was also problematic since the returns are located on the front/bottom of the CRAC/CRAHs and hot exhaust air naturally rises towards the ceiling. In this instance, cold aisle containment (CAC) can and should be utilised but can be difficult to install due to the location and height of the existing ducts.

Airflow optimisation and cooling efficiency also became a problem, and some experts may say that from a design and flexibility perspective, a raised floor is better, primarily because it’s easier to add or rearrange perforated tiles as the infrastructure scales or evolves. Others, however, would argue that overhead cooling tends to produce fewer issues with air leakage or wasted air.    

So, considering these challenges, when should a raised floor be used, and what are some advantages to its design?  

Traditional raised floors

One situation where a raised floor might still be the better choice is with data centres that serve as multi-tenant or colocation facilities. The reason a raised floor is advantageous is that it can offer flexibility for adding cooling and electricity when needed, such as when new customers move in or need to scale their footprint. Often, it’s easier to run new conduit and piping under the raised floor than overhead. 

Another scenario where raised floors might be preferred is when a data centre houses low density racks utilising a lower power draw. This might mean a lot of rack movement and changes, especially in caged hosting spaces, so a raised floor is often the better choice. 

In addition, distributing the cold supply air efficiently to where it is needed via overhead ducts, and in a room with a slab floor, can be difficult. With a raised floor, rearranging the perforated tiles is all that may be needed to correctly change airflow distribution to the IT equipment.  

Regardless of these and other advantages, there are many reasons why more data centre designs, both large and small, are moving away from the traditional raised flooring systems and using a slab floor design instead. What are the advantages to this approach? 

The benefits of slab flooring

When it comes to equipment installation and movement, as well as maintenance costs, a slab floor is clearly a better choice. There are no concerns about placing and moving newer, heavier equipment on areas of the floor that aren’t designed for the weight. For regions prone to earthquakes, seismic performance is also important, again making the slab floor the preferred choice. Furthermore, not only does it cost less than a raised floor, but equipment anchoring is easier, and the added lateral bracing and reinforcement associated with a raised floor system is no longer an issue.    

Many large hyperscale data centres, with their unprecedented demand for storage, are choosing the slab floor over the raised floor. Even smaller data centres, with space at a premium, are moving in this direction since they save vertical space by not installing the raised floor and associated ramps that usually are part of this design. 

Moreover, colocation data centre providers seeking to meet Open Compute Project or OCP-Ready™ criteria, like Kao Data in the UK, have also embraced slab flooring, which helps them to accommodate the GPU-accelerated workloads required by businesses utilising high performance computing (HPC) and artificial intelligence (AI). 

Whilst we are not recommending this approach for every new design, the benefits are many, and include greater efficiency and sustainability, as well as lower installation and maintenance costs.

Simplify the cooling technology

With innovative developments in cooling technologies, the need to provide underfloor supply air has been reduced, as has providing it via overhead ducts and diffusers. Instead, many new data centre designs are opting to use slab floors with the simple approach of flooding the room with cold supply air, while exhausting hot air through containment into the plenum ceiling. This design is being used in both smaller sites and is becoming the standard in large hyperscale deployments due to its advantages.

The cooling used to flood the room can be via CRAC/CRAH units located on perimeter walls or outside the data centre, via indirect evaporative cooling (IDEC) units, and both airside and waterside economisers just to name a few. Additional cooling technologies including fan walls, consisting of an outside wall with cooling units and a fan wall inside the data centre are also excellent candidates for this design.  

Another benefit is that hot aisle containment (HAC) is used to remove the hot exhaust air from the data centre. From a thermodynamic standpoint, both hot and cold aisle containment have similar results since they’re both preventing cold supply air and hot exhaust air from mixing. Since HAC encloses the hot aisles and turns the rest of the data centre into one large cold aisle, data centre operators and facility managers tend to run higher supply air temperatures with HAC compared with CAC.

The result is the IT equipment receives the cold supply air needed to meet demand airflow, and the process is simplified since the data centre can be viewed as one large cold aisle system. 

 

Today more hyperscale, colocation and smaller, on-premises data centre operators are beginning to utilise slab floors and reaping the benefits. However, it's critical to remember that one size does not fit all, and as the data centre industry continues to evolve, so should our designs as we pursue objectives for greater energy efficiency and sustainability.

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