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Citi recently designed an award-winning 100,000 square ft. datacentre that achieves greater than Tier IV reliability and which also aims for a gold accreditation through the US Green Building Council's LEED scheme.

Citi has an ambition to significantly reduce its greenhouse emissions by 2011 and so has designed the new facility to minimise its environmental impact, for example it incorporates an optimised cooling design to produce 8,183 hours of free cooling per annum (93% of the time). Overall, the new facility provides a total saving of 11,700 tonnes of CO2 per annum compared to a typical datacentre.

One of many innovative solutions employed by the design team was the use of the state of the art simulation capabilities of 6Sigma DC and the Virtual Facility© to evaluate and improve the airflow management within the datacentre at each stage from concept design onwards.

Future Facilities Ltd, a UK company with specialised computational fluid dynamics (CFD) expertise for datacentres and mission critical facilities, has applied its extensive knowledge and experience to create 6Sigma DC, a unique virtualisation package specifically for the sector.

6Sigma DC and the Virtual Facility©

The main output of 6Sigma DC is the Virtual Facility©, a full and mathematically precise 3D representation of the datacentre which facilitates an holistic approach to the design and ongoing thermal and asset management of datacentre space. The Virtual Facility© enables the owner/ operator to take a proactive approach to the placement and replacement of equipment within facilities, modelling impacts before any physical deployments are undertaken. 6Sigma DC comprises two components for the design and operations phases of facilities:

In design the Virtual Facility© enables the scientific evaluation and optimisation of decisions including choosing between raised floor or in-row cooling units; locating air handling units; floor void depth; rack design and choice; rack layout; choosing between ceiling plenum or return ducts for hot air return and testing redundancy in cooling and failure scenarios.

In operation the Virtual Facility© enables the owner / operator to walk-through a full 3-Dimensional inventory of all equipment housed within the facility; visualise the temperature distribution and airflow at both room and cabinet level; track historic changes in equipment layout; preview possible future equipment deployment and evaluate the thermal/ environmental impact of these options prior to deployment.

Putting the technology to work at Citi’s new datacentre

Citi applied the Virtual Facility© to make decisions regarding the following aspects of the new datacentre:

1) Under floor airflow management

• Perforated diffusion baffles are attached to floor jacks in front of CRAC units, this converts the high air velocity into an even static pressure in the active area of the data hall.

• Variable speed drive fans automatically maintain the even pressure in the event of CRAC failure.

2) High Level data cabling

• High level data cabling eliminates underfloor cable dams, which cause low air flow and local hot spots in a typical datacentre.

• High level data cabling also removes the need for cable penetrations in the raised floor which unless completely sealed are a cause of waste of cool air and therefore a source of inefficiency.

3) Above floor airflow management

• Open network frames are interleaved with server racks to reduce length of data cabling. A novel Hot-Aisle/ Cold-Aisle/ Cold-Aisle layout overcomes overheating problems in the open frames.

• Hot exhaust air is captured in the ceiling plenum, reducing mixing of hot and cold air and therefore protecting the equipment.

4) Recapturing the hot exhaust air

• Hot exhaust air is captured in a ceiling plenum; reducing mixing of hot and cold air and protecting equipment from high inlet air temperatures.

• A well designed false ceiling provides the effectiveness of high-level return ductwork whilst offering flexibility throughout the life of the facility: ventilation grilles can be simply moved to accommodate changes to rack layout and changing ventilation strategies of free-standing kit, whereas ductwork need to be re-designed for each layout change.

Optimised cooling design creates significant savings

Good airflow management enables the datacentre to be designed to achieve greater than Tier IV reliability despite experiencing internal air temperatures that are considerably higher than in a typical facility.

By modelling operations using the Virtual Facility©, a high internal air temperature can safely be set as follows: CRAC supply at 20˚C (5-7˚C higher than a typical datacentre) and CRAC return between 27-29˚C (also 5-7˚C higher). In turn this leads to high chilled water temperature in the coils: Flow is 10˚C (3˚C higher than typical) and return is 18˚C (6˚C higher than typical).

At Citi the 2 coils in the CRAC units are linked to free cooling heat exchangers providing free cooling and free pre-cooling from 17˚C external WBT, which equals 8,183hrs per annum (93% of the time) or 8,486 MWhr per annum. At the same time the design is highly resilient; 2 coils provide a 2N cooling system at the CRAC (> Tier IV standard.) and free cooling increases 3N system redundancy during the winter period (also exceeding the Tier IV standard).


For more details regarding Future Facilities Ltd., the 6 Sigma DC software suite and the Virtual Facility©, please visit Future Facilities online at or contact Hassan Moezzi on +44 (0) 20 7840 9540 or Damien Wells of Spa Communications on 07900 302102.

Editors Note:

In typical datacentres, cooled air is supplied at between 13°C and 15°C, where conventional wisdom recommends an air intake to racks of 20°C - 25°C. In fact most IT equipment has operating limits of between 30°C and 35°C, so the question arises why supply air need to be at such a low temperature.

Overcooling wastes energy and therefore contributes to higher emissions. In part this is caused by poor design which leads to poor (or no) airflow management and therefore to a mixing of cool supply air and hot exhaust. In order to compensate, ACU control set points are lowered because air supplied at 13°C can mix to a certain extent with the hot exhaust air and still hit equipment inlets in the 20°C - 25°C range.

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