Raised and slab floors are the two main types of computer room environments. Both can be designed and managed to provide optimized cooling performance, and both, if not properly designed and managed, can be inefficient and limit power density. Raised-floor designs utilize a raised-floor plenum to distribute cooling, whereas in a slab-floor design, all equipment sits on the slab floor and cold air reaches the racks from either overhead ducts or room flooding. For many years raised floors have been the standard; however, new data centers are increasingly being built around a slab-floor design. Is it time for something new, or do the advantages of a raised floor still outweigh those of a slab floor?
Raised vs. Slab
A slab-floor design has racks of servers sitting directly on the structural floor, and cold air moves to the racks from above the floor. Some facilities use a series of overhead ducts without containment, some slab floors have flooded conditioned air supply and hot-aisle containment, and some slab floors have cold-aisle containment with in-row-cooling units.
Slab floors with overhead cooling can easily adjust to an increased IT load and support increased density in a room, but only if the IT load remains in the locations where ducts were originally placed. Power and IT wiring can usually be added without compromising cooling or airflow, but reconfiguring HVAC ducts to accommodate moved racks can be quite expensive. Raised-floor environments do not suffer this inflexibility; to move a cabinet in a raised floor environment, it is usually necessary simply to place a perforated tile in front of the cabinet.
Since the raised floor plenum is, in essence, one giant duct through which the perforated tiles connect to access cold air, there is much greater flexibility in changing equipment location. Since cooling is available no matter where racks are located in a room, then no matter the time, the rack, the size or the customer, perforated tiles can be quickly moved to where air needs to be delivered. However, complications such as power, adequate pressure and obstructions must be addressed to ensure that the raised floor can be used to its full potential.
Issues with the Raised Floor
Cooling in a raised-floor environment requires pressure below the floor to force cool air up through perforated tiles. Constant pressure distribution can be difficult to maintain due to under-floor airflow obstructions, airflow stratifications and AC placement relative to perforated tiles. While the raised-floor approach requires consideration of the maximum load that can be cooled in each cabinet, there are sites that successfully cool 30kW per cabinet on a raised floor.
The plenum creates a space for running required infrastructure, but under-floor obstructions can block airflow if not properly managed. Similarly, raised-floor openings must be managed to ensure that no air is leaked from unintentional openings like unsealed cable openings, openings under electrical equipment, and perforated tiles where no equipment is being used. In order to mitigate this problem, there are multiple best practices to address these airflow management (AFM) challenges in raised-floor data centers.
To meet both the efficiency goals and airflow demands of increasing power densities it is important to deliver as much conditioned air as possible through perforated tiles. Removing obstructions under the floor (often by moving power and data cabling to overhead trays) is fundamental to raised-floor AFM best practices. Sealing cable openings and relocating perforated tiles is a simple and inexpensive way to maximize airflow delivery through the perforated tiles. In locations with high-density cabinets, high-flow perforated tiles may be necessary to supply sufficient volumes of conditioned air.
Above the raised floor however, raised and slab floors share nearly the same AFM issues. The installation of blanking panels in racks where no equipment exists is necessary to ensure that supply air is directed only to operating equipment. Hot-aisle and cold-aisle containment can further separate hot and cold airstreams and help entire rows maintain a consistent temperature.
A very important advantage of raised-floor design is related to the concept of uninterruptable cooling. Although uninterruptable power is a mandatory feature of any data center, many sites are just beginning to implement uninterruptable cooling. In all data centers, the UPS will keep IT equipment operating (and generating heat) when utility power fails. Although most sites have no cooling during this time, until engine-generators come online to provide backup power to the cooling system, there are rarely problems because the time is usually just a few seconds. However when engine-generators fail to start the servers and their fans continue to run, but with no cooling systems running, servers recirculate their own exhaust heat and can overheat quickly – even with moderate equipment density. With no cooling, IT equipment can begin shutting off due to overheating in just ten minutes.
To avoid server overheating during utility power failures, sophisticated raised-floor sites place cooling unit fans on a separate circuit that is fed from the UPS system. With this configuration, cooling unit fans continue to push air through the raised floor plenum and out through the perforated tiles. The air passing through the plenum passes over the sub floor concrete slab where it is cooled by the thermal mass of the slab. Studies of rooms with this configuration have shown that after 15 minutes at 40 watts per ft2, the IT intake temperatures only rose 5°F. However, in the same room without air flowing through the raised floor plenum, the IT equipment temperatures rose 23°F in just ten minutes – creating the risk of heat-related server failure. A data center manager of a Fortune 50 company who had studied this at his own site said he would never consider a slab floor design for this reason alone. While each model has its benefits, the raised-floor obviously has a clear advantage in terms of uninterruptable cooling.
Although the slab floor model presents some advantages over raised floors (at times prompting some to question whether the raised floor is dead), raised floors have the advantage of facilitating easy and inexpensive airflow redirection by moving perforated and solid tiles when racks are added or moved. For both types of floors, precautions, adjustments and forward thinking can overcome nearly any AFM challenge. Best practices in cabling, airflow management and rack/row configuration will ensure that IT equipment is protected, the needs of clients are met, and computer rooms maintain optimal airflow for the best performance possible.
About the Author
Lars Strong, P.E. thought leader and recognized expert on Data Center Optimization, leads Upsite Technologies’ EnergyLok Cooling Science Services, which originated in 2001 to optimize data center operations. He is a certified US Department of Energy Data Centre Energy Practitioner (DCEP) HVAC Specialist. Lars has, and continues to deliver value added services to domestic and international Fortune 100 companies through the identification and remediation of dilemmas associated with the fluid mechanics and thermodynamics of their data center cooling infrastructure. Lars brings his knowledge and ability to teach the fundamentals of cooling science to numerous U.S. and international, private and public speaking events annually.