FOCUS APC COOLING
Issue 13, Dec 10/Jan 11
COOLING WITHOUT A RAISED FLOOR
Traditional raised-floor cooling has survived virtually unchanged for more than 40 years. But when it was conceived nobody anticipated today’s ultra-high-density and megawatt data centers. By Neil Rasmussen, Senior VP of Innovation at APC by Schneider Electric
would be taken seriously as a viable method for cooling if its invention was proposed today. Anyone designing a new data center now with raised-floor cooling is being environmentally irresponsible because the method is entirely unable to accommodate changes which are already challenging operations today – not least of which dynamic power variation amongst IT equipment loads.
K Demands that are stressing physical
infrastructure in today’s data centers include the high power and heat density of blade servers, the weight of loaded equipment cabinets, availability expectations, the need for rapid deployment, pressure to reduce energy costs and carbon footprint and, as already highlighted, dynamic power variation which is an increasing factor with the virtualization of data center hardware and software.
Dynamic power variation changes the game because in such an environment, IT equipment power use varies with a changing workload. Since data center servers could be in a state of high utilization or sleep mode or somewhere in between, total power dissipation will change. So will the distribution of power. Also, the average power with dynamic variation will be much lower than the peak power.
With traditional raised-floor cooling local airflow must be able to provide the maximum airflow required. Changes in power distribution are accommodated by physically moving or adding vented tiles; changes in total power are accommodated by turning on and off CRAC (computer room air conditioning) units. Raised-floor approaches are unsuitable for environments with dynamic power variation because we simply cannot move floor tiles in real time.
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nowing what we know about contemporary data center cooling requirements,
you’d have to question whether the raised floor
densities only achieved through the use of customized designs. Additional fan-assisted devices can be used to improve airflow but this further reduces the efficiency of the overall cooling system by increasing overall power consumption and adding heat to the cold air supply.
Neil Rasmussen, APC Schneider Electric
The raised floor is already a major contributor to data center inefficiency through bypass leakage, poor air coupling to the IT load, uneven distribution, hot spots that require a lowered supply temperature, the need to over-provision computer room air handling (CRAH) units, plus obstructions and blockages commonly found under the floor. Newer approaches that eliminate the raised floor and the traditional CRAH unit result in improved Power Useage Effectiveness (PUE) as well as improved compatibility with high-density and dynamic power loads.
LIMITS TO COLD-AISLE CONTAINMENT Containment of the cold aisle is typically retro fitted in traditional perimeter- based data centers. Traditional cooling environments use the entire room as a hot air return plenum and use cold air via the raised-floor plenum to the cold aisles. This allows the rest of the data hall to become a large hot air return plenum while separating the hot and cold air streams.
While a considerable step forward from
legacy cooling solutions, cold-aisle containment does have drawbacks such as the inefficiencies which result from distances and pressures required for adequate air distribution and the requirement to move cold air from perimeter CRACs to the load. A row-based approach brings the source of the cooling in close proximity to the IT equipment so less energy is required to deliver cold air to its destination.
There are also density limitations, on average around 6kW per rack, with higher
Density limitations can be avoided if cold- aisle containment is deployed in a row- based approach. However, since cold aisle containment minimizes the overall pool of cold air available to the server, any loss of power and/or cooling supply will result in more rapid temperature increases in the server load. This will shorten the amount of time available to the data center manager before overheating and thermal shutdown starts to occur.
To realize maximum benefits, all cold aisles in the entire data hall should be contained, otherwise mixing of hot and cold air in uncontained areas
will occur, diminishing expected energy and cost savings.
THE OPEN AND SHUT CASE Hot-aisle containment systems (HACS) collect exhaust air from IT equipment, removing the heat and making cooled air available to the intakes of racked IT equipment.
HACS ensure efficient air distribution by completely separating supply and return of air paths and eliminating the mixing of hot and cold air streams. Hot aisles typically operate +38˚C with high density servers. The efficiency of HACS will be higher because the hot aisle is capable of maintaining higher temperatures. The net effect of elevated return air to the cooling unit means better heat exchange across the cooling coil, better utilization of the cooling equipment and overall higher efficiency.
Another advantage of hot-aisle containment is improved flexibility. HACS create a room- neutral cooling zone that does not impact the temperature of the surrounding room.
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