DATA CENTRES OVERCOOLING
energy-saving opportunity; 5% of the sample is receiving air hotter than the upper allowable limit, which potentially increases the risk of failures due to overheating. The Rack Cooling Index, published by the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE), provides users with a measure of the spread of server inlet temperatures (
www.ashrae.org).
Conclusions Energy prices are predicted to continue to increase, which puts more pressure on data centre operators to reduce their energy consumption. There are huge energy savings to be made in data centre cooling systems. Data centre cooling design is driven by the
IT hardware it serves. It is difficult to predict how this will change, but trends suggest that densities and exhaust temperatures will continue to increase, resulting in even hotter temperatures in the hot aisle. Air performance metrics are a useful tool
Energy prices are predicted to continue to increase, which puts more pressure on data centre operators to reduce their energy consumption
to assist operators in their understanding of the effectiveness of their data centre cooling system. They can be used to quantify inefficiencies and to benchmark improvements, and may be considered as a first step towards optimisation. CJ
l SOPHIA FLUCKER and ROBERT TOZER work for Operational Intelligence Ltd.
www.dc-oi.com This is an edited version of their paper presented to the CIBSE Technical Symposium.
www.cibse.org Sophia Flucker and David Cameron of OI will be presenting at the CIBSE data centre event on 26th April
Best-practice air management
Where IT equipment receives higher temperature air, a commonly applied solution is to decrease the cooling unit set point. This does not deal with the root cause of the problem and contributes to additional energy wastage, making the overcooling issue worse. By minimising bypass and recirculation, the range of temperatures supplied to the IT equipment is reduced. This can be achieved through separation of hot and cold air streams by using containment. There are various containment types,
including: l Cold aisle containment: This is the most common
42 CIBSE Journal April 2012
type, and is where the cold aisle is closed with a roof and doors, normally fabricated from flame- retardant plastic. The rest of the data hall is at the same temperature as the hot aisle.
l Semi-containment: This is a variation on the above, where curtains (again in flame-retardant material) are fitted above the cold aisle, blocking the air path from the hot aisle. This works well as a retrofit option, particularly where there are cabinets of different heights.
l Hot aisle containment: This ducts the air from the hot aisle back to the
cooling units, usually by way of a ceiling plenum. The rest of the data hall is at the same temperature as the cold aisle. This works best for a new build as it requires coordination with other overhead services, such as cable trunking.
l Direct rack containment: This employs special deeper cabinets, which include a chimney at the back to duct hot air back to the cooling units. This method keeps the hot air outside of the room and may become more widely adopted as hot aisle temperatures increase due to increasing IT equipment exhaust
temperatures and air supply temperatures.
Controlling on-return air temperature at the cooling unit, even with the measures above in place, can still result in a range of temperatures delivered to the IT equipment. This is due to the non-uniform nature of load distribution in most data halls; each cooling unit will deliver a delta T proportional to its load, and thus supply a different temperature. Changing the cooling unit temperature control strategy to supply air control allows this range to be minimised. This can be retrofitted on many cooling units with an additional sensor.
These best practices and others can be found in the EU Code of Conduct for Data Centres, version 3.0.8, 2011.
The improvement of air performance can be quantified by conducting a survey collecting sample temperatures before and after these recommendations have been implemented. The characteristic point on the flow and thermal performance plot should move toward the top right of the chart; with flow and thermal performance values of above 0.9 achievable where best practice measures are fully implemented.
www.cibsejournal.com
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