DATA CENTRES OVERCOOLING
fl oor reduce the static pressure, resulting in a reduced volume of air delivered through the fl oor grilles. This may mean that insuffi cient air is provided to meet the local cooling demand. This can easily be checked on site by placing a sheet of paper above the fl oor grilles and observing how high this fl oats or whether it is drawn down on to the fl oor grilles. Figure 2, on the facing page, illustrates
Air performance metrics are a useful tool to assist operators in their understanding of the effectiveness of their data centre cooling system
the behaviour of the air pressures below the fl oor in a cold aisle at increasing distances from the cooling unit and the impact on velocities, volumes and temperatures. At the fi rst fl oor grille, closest to the cooling unit, velocity pressure is very high and static pressure is negative, causing air to be drawn down into the fl oor. Resulting air supply temperatures are high, as these are from recirculated warm air. Velocity and velocity pressure reduce as the distance from the cooling unit increases, hence static pressure and air volume increase, delivering air at the design supply temperature. High dynamic pressures can be reduced
by decreasing the air velocity, for example by reducing the fl ow rate, redistributing fl ows, installing baffl e plates and removing restrictions such as cable trays.
By understanding ‘air management performance’ in data centres, ineffi cient operation can be identifi ed, quantifi ed and targeted
Metrics Air performance metrics are a useful tool to assist operators in their understanding of the effectiveness of their data centre cooling system. They can be used to quantify ineffi ciencies and to benchmark improvements and may be considered as a fi rst step towards optimisation. The box on page 44 sets out the metrics
that characterise the air fl ows within a data centre, and how well these serve the cooling demand. The metrics may be determined by making a series of temperature measurements in various locations within the space. Measured performance can be compared with optimal/ideal performance and action taken – such as installation of air containment – to improve the effectiveness of cooling delivery.
Figure 1a: Data hall air bypass
Overcooling Traditionally, a data centre cooling system design comprises computer room air conditioning and/or computer room air handling (CRAC/CRAH) units located around the perimeter of the data hall. The units supply cooled air under the raised fl oor. This air emerges from open fl oor grilles placed across the fl oor area. Each cooling unit contains one or more cooling coils/fans. The compressor may be in the unit in the case of a direct expansion (DX) system or found in a chiller located externally to the data hall in the case of a chilled water system. The cooling units typically control to a set
Figure 1b: Vertical recirculation of air
point of around 22C, 50% relative humidity (RH) on return temperature to the unit, at high level. The use of narrow control bands led to the use of the term ‘close control
40 CIBSE Journal April 2012
www.cibsejournal.com
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