How Data Center Cooling Is Evolving to Meet the Needs of Slab Floor Data CentersTop images show how standard perimeter CRAH units create high-velocity airflows that translate into negative pressure at the front of racks at the beginning of the row. Bottom images show how chilled water units for non-raised floor applications help lower airflow velocities to balance pressure across the row.

Andrea Moscheni
Product Manager,
Vertiv
Slab, or non-raised floor, data centers have helped cloud and colocation providers meet growing capacity demand by accelerating speed-to-market and reducing capital costs. Those benefits have, however, come with new data center cooling challenges. Cooling solutions not tailored to the needs of slab floor facilities can jeopardize equipment reliability and reduce cooling system efficiency. But with new challenges come new opportunities, and recent developments in control strategies and cooling technologies are enabling high performing cooling in non-raised floor environments.

Data Center Cooling Challenges in Slab Floor Facilities

When slab floor data centers were first gaining traction, the airflow control strategy that had proven effective in raised floor environments was applied to these non-raised floor data centers. But this strategy — which manages airflow and fan speed based on pressure differential, or Delta P — hasn’t been as effective in slab floor data centers as it is in raised floor environments.

Without the duct provided by the space beneath the floor, pressure is more difficult to measure and manage in slab floor data centers. Data center designers also lose the ability to control airflow to racks using properly sized and positioned floor tiles. Instead of cold air being distributed directly to the front of racks through the tiles, air must travel the length of the row. To compensate, many operators drive fan speeds too high, wasting fan energy and resulting in lower return air temperatures that prevent cooling units from operating at their design efficiency.

The need for air to travel down the row also creates airflow patterns that can limit the ability to cool racks closest to the cooling units when standard data center cooling units are used. The velocity of the air at the beginning of the row has to be high enough to ensure adequate airflow at the end of the row. With standard cooling units, that requires velocities so high they create negative pressures in front of racks at the beginning of the row. This increases the potential for temperature-related failures in these racks.

As a result, operators of slab data centers have had to compromise both cooling system efficiency and equipment reliability. But that is no longer necessary, as new strategies and technologies designed specifically for slab floor data centers are now available.

A More Effective Control Strategy for Slab Floor Data Center Cooling

With control based on Delta P proving inefficient in slab floor data centers, Vertiv developed a control strategy based on the temperature differential (Delta T) between the supply air leaving the cooling units and the return air to the cooling units.

Temperature is much easier to measure than pressure, and by setting a temperature control point for return air above the supply air temperature, operators can ensure enough airflow is reaching each rack.

This strategy takes into consideration numerous failure conditions, such as blocked cold aisles, and provides monitoring to ensure air temperatures at the rack are precisely controlled and consistently meet temperature service level agreements (SLAs) — something that isn’t possible with a Delta P control strategy. The need to run fans at higher-than-necessary speeds to compensate for pressure variations across the row is eliminated, and return air temperatures are maintained at the setpoint to optimize cooling unit efficiency. For more on this control strategy, see the Vertiv white paper, Overcoming the Challenges in Cooling Non-Raised Floor Data Centers.

More Effective and Efficient Products for Slab Floor Data Center Cooling

Chilled water cooling systems offer a number of benefits to cloud and colocation provides developing or operating slab floor data centers. One of the most significant
is the ability of chilled water systems to reduce direct and indirect greenhouse
gas emissions compared to other cooling technologies. Reductions in direct emissions are enabled by a chiller’s ability to use low global warming potential (GWP) hydrofluoroolefin (HFO) refrigerants. Indirect emissions are reduced through the overall efficiency of these systems, which can achieve very low power usage effectiveness (PUE) values through the use of intelligent control systems. To learn more, read the Vertiv white paper, How Chilled Water Systems Meet Data Center Availability and Sustainability Goals.

To address the challenge of airflow distribution discussed previously, new chilled water cooling units have been engineered to meet the airflow requirements of slab floor data centers, including perimeter and thermal wall cooling units.

Perimeter units, for example, have been redesigned to relocate the fan at the top of the unit and create a larger surface area for air distribution. This allows these units to distribute more air at lower speeds, improving the ability to move air down the length of the row and reducing the risk of negative pressure at the beginning of the row.

New thermal wall units adapt the air handling unit (AHU) concept to the needs of slab floor data centers. Installed in the service corridor, they blow air horizontally to the server room, providing high volumes of air that move at low speeds. These systems are particularly well suited when high-density cooling units are required.

Both products can be integrated to the chilled water system manager, which optimizes the entire system by coordinating operation of external and internal units.

Optimizing Cooling in Slab Floor Data Centers

Developers and operators of slab floor data centers no longer have to accept compromises in cooling system performance to realize the cost and speed benefits enabled by eliminating the raised floor. By using control strategies and cooling technologies engineered specifically for slab floor data centers, they can leverage the environmental and operating benefits of chilled water cooling while effectively managing airflow and temperature across the facility. For more information on selecting the right cooling system for your data center see the white paper, Chilled Water Data Center Cooling for Non-Raised Floor Applications.