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COOLING
Direct free cooling
With direct free cooling, filtered outside air is fed directly
into the data centre if the CRAC system determines that it is
at the right temperature. This means that the compressors
do not have to operate as often, saving electrical energy.
However, in urban areas air quality might not be good
enough and is often full of particles, which can be hostile
to modern IT equipment. Therefore, direct free cooling
should only be used where filtering can control the quality
of outdoor air. Also, if the outdoor air is too cold, a certain
proportion of warm air from the data centre must be
mixed in with it to supply controlled, tempered air to the IT
equipment.
Figure 4 - Indirect free cooling with CRAC and dry cooler
Figure 5 - Indirect free cooling with CRAH and chiller
Humidification also has to be controlled and methods
to humidify or dehumidify air can also be expensive and
complex, with large ductwork systems often required. In
addition, large openings in the building fabric are needed
to bring in the outdoor air, plus an equally large ductwork
system and fabric opening for the exhaust air. These
openings and ductwork bring a security risk to the data
centre that has to be considered and in the event of an
external situation such as a fire, the system must have the
ability to operate independently of the air inlet.
Indirect free cooling
With indirect free cooling, no outside air enters the data
centre and CRAC systems can be configured to suit
requirements. However, indirect free cooling is theoretically
less efficient than direct free cooling, as at least one heat
transfer must always take place between the air in the data
centre and the outdoor air.
Single stage indirect free cooling is based on an air/water
heat exchanger placed on the hot water return of the chiller.
Outside air is blown across the heat exchange and helps
removes the heat before it gets to the chiller. Therefore,
with a return water temperature of 20°C, if the outside
temperature is 19°C or less it starts reducing the activity of
the chiller and reduces running costs.
Two stage indirect free cooling systems have a
considerably smaller footprint than their single stage
counterparts, but are less efficient as they function with two
heat transfers. In the first heat exchanger, the heat from
the air in the data centre is transferred to a liquid, normally
glycol. This is then pumped to the outside in relatively small
pipes, where heat from the data centre is rejected to the
outdoor air via a second heat exchanger.
Indirect dynamic free cooling
To further improve the efficiency of indirect free cooling,
dynamic technology controls the mode in accordance with
the current heat load in the data centre, and so increases
the time in free cooling mode. Moreover, it has another
operating mode – extended free cooling – that further
lengthens the operational time and drastically reduces
operating costs by cutting energy intensive compressor
cooling to a minimum.
Liquid asset
Air contains a certain amount of water and the total amount
depends on the air temperature and barometric pressure. As
water absorbs into the air, an adiabatic process takes place
– this means the temperature of the air decreases, while the
energy content of the air remains unchanged.
By using adiabatic assistance in direct free cooling
systems the air that enters the data centre is cooled before
entering. Indirect free cooling and adiabatic assistance
is more conducive however, as the adiabatically assisted
outdoor air never enters the data centre. In single stage
indirect free cooling systems, adiabatic assistance reduces
the temperature of the outdoor air entering the air/air heat
exchanger and reduces the amount of mechanical cooling
required to satisfy the supply air temperature to the IT
equipment. In two-stage indirect free cooling systems, the
warm outdoor air can be cooled by adiabatic assistance
before entering the dry cooler to either increase the hours of
usable free cooling, or ext end the mixed mode at the upper
limit to reduce the use of mechanical cooling.
Evaluating needs and solutions
As the density of installed equipment in the data centre
has risen, so too has the amount of heat generated. While
being able to fit more kit into a smaller space is generally
considered a good thing, the need to control temperature
has led to the growing use of free cooling. It should be
remembered that no free cooling system is perfect though,
and all have limitations. It's therefore important to evaluate
what kind of free cooling is best suited to the application on
a case-by-case basis. n
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