R&M Data Center Handbook

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Higher temperatures may be selected today than were previously, thanks to the improved manufacturer quality of
silicon chips. Preset temperatures of 24° C to 27° C and relative humidities of about 60% are currently state of the
art.
In the closed-circuit air conditioning principle, the air cooled by the air conditioning system circulates to IT
components, takes in heat and the warmed air then reaches the air conditioning system again in the form of return
air that is to be re-cooled. Only a small amount of outside air is introduced into the room that is to be airconditioned
and used for air exchange. Optimal conditions with respect to temperature and relative humidity can
only be achieved with closed-circuit air-conditioning units, or so-called precision air-conditioning units. The energy
used in these systems is better utilized, i.e. reducing the temperature of return air is the first priority.
These units are contrasted with comfort air-conditioning units used for residential and office spaces, such as split
air-conditioning units, which continuously use a large portion of the energy they consume to dehumidify recirculated
air. This can lead not only to critical room conditions but also to significantly higher operating costs, which is why
their use is not economically feasible in data centers.
Rack arrangement and air flow are both crucial factors in the performance of closed-circuit air conditioning systems.
This is why 19’’ cabinets, in particular, are currently arranged in accordance with the so-called hot aisle/cold aisle
principle, so as to reproduce, as best as possible, the horizontal air flow required for IT and network components.
In this arrangement, the air flow is forced to take in the heat from the active components on its path from the
raised floor back to the air-conditioning. This process differentiates between self-contained duct systems (Plenum
Feed, Plenum Return) and via room air (Room Feed, Room Return).
One must make sure that the 19” cabinets
correspond to the circulation principle that is
selected. In “Room Feed, Room Return”,
cabinet doors (if they exist) must be perforated
at least 60% in order to permit air
circulation. The floor of the cabinet should be
sealed to the raised floor at least on the
warm side, in order to avoid mixing air
currents. A cooling principle of this type is
suited for power up to approximately 10 kW
per cabinet.
Plenum Feed, Plenum Return (Source: Conteg)
In “Plenum Feed, Plenum Return”, it is absolutely
necessary that the cabinet has doors
and that they can be sealed airtight. By
contrast, the floor must be sealed on the
warm side and be open on the cold side.
These solutions are suitable for power of
approximately 15 kW.
Within the cabinet no openings may exist between the cold side and the warm side, to ensure that mixtures and
microcirculations are avoided. Unused height units and feedthroughs should be closed with blind panels and seals.
Performance can be further optimized by containment of the cold or warm aisle (Contained Cold/Warm Aisle).
Of crucial importance is the height of the raised floor, over which the area of the data center is to be provided with
cold air. Supply air comes out via slot plates or grids in the cold aisle. After it is warmed by the IT devices, the
return air reaches the air conditioning system again to be re-cooled.
Direct-Cooling Principle – Water-Cooled Server Rack
Direct cooling of racks must be implemented when heat loads exceed 10 to 15 kW
per cabinet. This is realized via a heat exchanger installed in the immediate vicinity
of the servers. These are usually heat exchangers cooled with cold water, that are
arranged either below or next to the 19” fixtures. Up to 40 kW of heat per rack can
be drawn off in this way. A cold-water infrastructure must be provided in the rack
area for this method.
The water-cooled racks ensure the proper climatic conditions for their server cabinets,
and are therefore self-sufficient with respect to the room air-conditioning system. In
buildings with a low distance between floors, water-cooled server racks are a good
option for drawing off high heat loads safely without having to use a raised floor. The
high-performance cooling system required for higher power also includes a cold and
hot aisle containment.
Page 70 of 156 © 08/2011 Reichle & De-Massari AG R&M Data Center Handbook V2.0