FOCUS APC COOLING
Issue 13, Dec 10/Jan 11
Since HACS do not deliver any hot air to the outside room, they eliminate the mixing of hot and cold air that leads to inefficiency. And as a row-based cooling approach, HACS can be easily installed in an existing environment without need for specialized duct work and adjustment to the existing HVAC systems to manage elevated temperatures.
With this approach, there is no requirement for a raised floor. The added bonus is higher availability; since HACS only ‘contain’ hot air, the rest of the data hall provides a much larger reservoir of cool air which servers can draw for extended runtime.
FUTURE FLOORING The future is hard floor data centers because legacy cooling solutions are inappropriate for today’s high-density environments, and to provision dynamic power variation. Legacy cooling is inefficient, costly and wasteful from a carbon footprint perspective.
Containment offers cooling systems that can be set to a higher temperature while supplying the load with safe operating temperatures and reduction of humidification/dehumidification costs. Better physical infrastructure utilization also enables right sizing to fit power consumption and cooling needs.
WHAT THE COOLING EXPERTS SAID: Dr Robert Tozer
One of the world’s most preeminant data center cooling experts
Whilst I agree with the end of an era of “open air management” cooling systems (which mostly use raised- floor cooling) I think we will still continue to see raised-floor cooling designs and for valid reasons.
I have been involved with recent designs that addressed dynamic high-density loads, achieved PUEs below 1.3 using free cooling and used raised-floor cooling, BUT used closed-air management i.e. physical containment between hot and cold air streams (excluding cold-aisle containment). Raised-floor cooling could have been avoided but there were reasons not to do so including: floor plenum was used for UPS/PDUs and cable trays, less restrictions on building height than plan area and use of plenum for thermo-syphon air path for fan-less operation.
The theme for the new generation of data-center cooling is contained air management. This is because if you contain the air streams and adequately control the cooling systems you obtain: • No air recirculation, so the CRAH supply air is the same as the air inlet to IT equipment, which can be as high as 27C to comply with ASHRAE’s recommended range.
• The CRAHs circulate only the amount of air actually required by the IT equipment, saving significantly on fan energy.
• With this it is quite difficult not to obtain a PUE of 1.3 in most of the world and definitely below 1.5 in all of the world (if indirect air side adiabatic free cooling is used).
Ideally, rack exhaust containment is used although hot-aisle and cold-aisle containment are also effective to separate hot- and cold- air streams. The reason for this is that all the data centre space where operatives work is cold with the first solution, whereas for the last solution most of the data hall is hot (except in cold aisles).
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Both hot-aisle and cold-aisle containment offer considerable advantages over legacy cooling systems in terms of system efficiency and ability to handle high-density equipment including blade servers.
HACS, with its row-based cooling architecture is more efficient, more flexible, provides better ride through capability and offers a way to better address the need to manage higher density requirements without making the entire data center hotter than necessary. What’s more, it requires no raised floor for effective operation!
Dr Bob Sullivan Originator of hot-aisle/cold-aisle design
I agree with Neil’s premise that raised floors are no longer necessary for cooling a computer room, and in fact they can be a detriment with high-density cabinet power demands. However, though not efficient, the raised floor, in association with air handler units along the walls, can cool high-density loads by installing open grates rather than perforated tiles.
I disagree with his statement that raised floors create a load restriction. It is the structural subfloor that has the load restriction, except if the computer room is built on a ground contact slab. In cases where the structural subfloor cannot support the weight directly, the raised floor can be used to distribute the weight over a larger area.
Other purposes for a raised floor include: •
Flexibility in cabinet placement and relocation. Without a raised floor, overhead cable trays limit movement of cabinets.
• It presents a cleaner looking installation. •
It is easier to isolate hot and cold aisles. With all cabling under the raised floor it is easier to install partitions for isolation.
• Cabling is not subjected to long-term exposure to high hot-aisle temperatures, which can deteriorate the insulation material.
• Supplying chilled water, or refrigerant, to cabinets or local cooling units is much neater with a raised floor and avoids concern of water overhead.
From a cooling standpoint the raised floor is no longer necessary. But an installation can still use flooded cooling, through the computer room walls or from overhead, and still have a lower raised floor to carry the chilled water piping and cabling. It is a neater looking installation and provides added flexibility to the installation.
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