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DATA CENTRES OVERCOOLING


l Excess air supply through floor grilles; l Open cable cutouts (in the floor beneath cabinets towards the rear); and


l Gaps in the raised floor (bottom of empty racks, power distribution units).


Bypass can be minimised by ensuring appropriate floor grilles are located where needed and by sealing gaps in the floor. Recirculation flow is where IT equipment


draws in warm air from IT equipment discharge due to lack of cool air being supplied as a result of negative/low static pressure or bypass air. Recirculation flow occurs both inside cabinets (particularly if there are no blanking plates) and outside: over the top of racks and around the end of a row of racks. Solutions include installing blanking plates, replacing solid rack doors with a perforated type and ensuring sufficient cold air is supplied to the IT equipment inlet. Negative pressure flow occurs when high underfloor air velocities cause the Venturi effect and air is drawn down into the raised floor via the floor grilles, rather than the reverse. When total pressure is less than the velocity pressure (proportional to the square of the velocity), static pressure is negative. This may be observed in data halls with high air velocities at the cooling unit supply, with floor grilles placed close to these cooling units. More commonly observed is low pressure, where high velocities under the


www.cibsejournal.com April 2012 CIBSE Journal 39


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