Barry Elliott , C.Eng., provides a technical overview of this important aspect of data centre design.
In trying to design a standardised, modular and upgradeable space for IT and communications equipment much thought needs to be given to rack location and the method of supplying power, communications and refrigerated air to it.
The ‘standard’ model has been defined by TIA 942, ASHRAE and other authoritative sources as being based on a front-to-back cooling regime with rows of racks facing each other. Cold air is supplied to the front of these racks through air vents placed in the raised floor in front of them. The chilled air is fed to these vents from air conditioning units blowing into the plenum space formed by the raised floor. The vented aisle is thus known as the cold aisle and the cold air is drawn through the equipment racks by the IT equipments’ own fans and expelled out of the back into what is now the hot aisle. The rising hot air from this aisle finds its way back to the air conditioning unit to be chilled and then to repeat the cycle.
The fronts of the two facing racks are two whole floor tiles apart and when the depth of the rack and the necessary access clearance space behind it is taken into account we can see that the minimum realistic pitch before the process repeats itself is seven tiles.
Feeding cold air through standard 25 percent open floor vents into a rack with no additional cooling methods normally limits the heat dispersion to about 2 kW per rack, or about five average servers. Other upgrade paths are available to get more air through the rack such as larger tile vents, fan assisted tile vents and fan-assisted racks.
The raised floor will be based on 600 x 600 mm floor tiles with an anti static finish to IEC 61000-4-2 and not less than 300 mm in height. A guide to floor heights, when used as an air distribution plenum, comes from VDI 2054, ‘Air conditioning systems for computer areas’:
||Height of raised floor according to VDI 2054
|200 -500 m2
||Approx 400 mm
||Approx 700 mm
|1000 - 2000 m2
||Approx 800 mm
Other guidance for floor height includes:
• 450 – 600 mm, IBM
• 450mm minimum, 600 is ‘ideal’ according to SUN
• 300 – 600 mm, BS EN 12825
• 300 mm min, TIA 569
The Property Services Agency (PSA) “Method of Building Performance Specification 'Platform Floors (Raised Access Floors)”, MOB PF2 PS, became the de facto industry standard in the UK for about 20 years until the recent arrival of the BS EN 12825:2001 specification.
In July 2001 a European Standard EN 12825 ‘Raised access floors’, was approved by CEN as a voluntary specification for private projects and mandatory for public projects.
For the floor strength the minimum distributed floor-loading capacity shall be 7.2 kPA. The recommended distributed floor loading capacity is 12 kPA (TIA 942). From MOB PF2 PS and BS EN 12825 this means specifying ‘Heavy Duty’ or preferably ‘Extra Heavy Duty’ floor grade.
The plenum area formed under the raised floor must be clean, sealed, dust free, fitted with a vapour barrier and sealed to a level of air permeability of at least 3 m3/h/m2 at 50 Pa (Building Regulations Part F).
The reasons for pressure sealing the plenum area are;
* Chilled conditioned air will be able to escape through poorly finished floor tiles and service penetrations, leading to;
- More electricity consumed to replace that air
- An inability to deliver the volume of chilled air required at the floor vents
- Excessive variation in air pressure across the floor leading to an inability to deliver chilled air at the air vents.
* Unsealed service penetrations (cables/pipes etc) into the plenum area are a fire risk and will allow the spread of fire and smoke into or out of the computer room (Building Regs, part B).
* Gaseous fire suppression systems rely on lowering the level of oxygen available to fires and depend upon a sealed area to work in to prevent oxygen from re-supplying the fire. BS ISO 14520 P1: 2000(E), ‘Gaseous fire-extinguishing systems. Physical properties and system design. General requirements’, requires a pressure test every twelve months.
According to TIA 942 ‘Telecommunications Infrastructure Standard for Data Centers’, an aspirating (early warning) smoke detection system shall be placed in the plenum zone.
Where a need for a fire suppression system in a sub floor space is deemed appropriate, consideration should be given to clean agent systems as a means to accomplish this protection (TIA 942).
The under floor area must not be used for any other purpose other than the supply of air and the distribution of cables. Cables must be fire rated according to the local jurisdiction and must be placed so as not to impede airflow. All redundant cables must be removed (National Electrical Code 2002).
One must also remember that ramps need to be built in somewhere so that people using wheelchairs, and large trolleys of heavy equipment, can gain access to the raised floor from the normal floor level. A maximum slope of 12:1 is called for in the following standards;
Building Regulations Part M and BS 8300 ‘Design of buildings and their approaches to meet the needs of disabled people — Code of practice’.
Barry Elliott , C.Eng, partner, Capitoline LLP, IT physical infrastructure design consultants.
•Date: 26th April 2006 • Region: UK/World • Type: Article •Topic: IT continuity
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