R&M Data Center Handbook

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3.5.2. Cooling, Hot and Cold Aisles
Every kilowatt (kW) of electrical power that is used by IT devices is later released as heat. This heat must be
drawn away from the device, cabinet and room so that operating temperatures are kept constant.
Air conditioning systems, that operate in a variety of ways and have different levels of performance, are used to
draw away heat. Providing air conditioning to IT systems is crucial for their availability and security. The increasing
integration and packing densities of processors and computer/server systems causes a level of waste heat that
was unimaginable in such a limited space only a few years ago.
Various solutions for air conditioning exist on the market and are based on power and dissipation loss, in other
words the waste heat from the IT components in use. With more than 130 W/cm² per CPU – this corresponds to
two standard electric light bulbs per square centimeter – the job of “data center air conditioning” has been gaining
in importance. This power density results in heat loads that are much greater than 1 kW per square meter.
According to actual measurements and practical experience, dissipation loss of up to 8 kW can still prevail in a
rack or a housing that uses a conventional air conditioning system that is implemented in a raised floor and
realized through cooling air, as still exists in many data centers. However, the air flow system of the raised floor
implemented in conventional mainframe data centers can no longer meet today’s in some instances extremely
high requirements.
Although a refrigerating capacity of 1 to 3 kW per 19” cabinet was sufficient for decades, current cooling capacities
must be increased significantly per rack. Current devices installed in a 19” cabinet with 42 height units can take in
over 30 kW of electrical power and therefore emit over 30 kW of heat. A further increase in cooling capacity can
be expected as device performance continues to increase while device sizes become smaller.
In order to improve the performance of existing air conditioning solutions that use raised floors, containment is
currently provided for active components, which are arranged in accordance with the cold aisle/hot aisle principle.
Cold and hot aisles are sometimes enclosed in order to enable higher heat emissions per rack.
A cabling solution that is installed in a raised floor, and that is well-arranged and professionally optimized in accordance
with space requirements, can likewise contribute to improved air conditioning.
The decision criteria for an air conditioning solution include, among other things, maximum expected dissipation
loss, operating costs, acquisition costs, installation conditions, expansion costs, guaranteed future and costs for
downtimes and for physical safety.
There are basically two common air conditioning types:
• Closed-circuit air
conditioning
• Direct cooling
Solution per the cold aisle/hot aisle
principle – closed-circuit air
conditioning (Source: BITKOM)
Closed-Circuit Air Conditioning Operation
In the past, room conditions for data center air conditioning were limited to room temperatures of 20° C to 25° C
and relative humidity of 40% to 55% (RH). Now requirements for supply and exhaust air are also being addressed
because of the cold aisle/hot aisle principle, since room conditions in the actual sense are no longer found in the
entire space to be air conditioned.
Conditions for supply air in the cold aisle should be between 18° C and 27° C, depending upon the appl ication –
relative supply air humidity should be between 40% and 60% RH. Lower humidity leads to electrostatic charging,
high humidity to corrosion of electrical and electronic components. Operating conditions with extremely cold
temperatures of under 18° C and high humidity, whic h lead to formation of condensation on IT devices, must
always be avoided. A maximum temperature fluctuation of 5° C per hour must also be taken into considera tion.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 69 of 156