32
AISLE CONTAINMENT
normally lined up in rows to form pods. The hot air is ducted
away either via a plenum space in the ceiling, or using large
ducts connected to the CRAH unit. This configuration can
lend itself well to large purpose built data centres employing
adiabatic cooling, for example. The aisle containment will
typically need to be extended upwards to connect with the
overhead plenum/ducting, or ‘chimney’ cabinets can be used.
One popular alternative to this approach is known as in-row
cooling, where CRAC units are built into the cabinet rows.
The hot aisle will then typically look identical to a cold aisle
with a flat aisle roof, and there’s no requirement for a hot air
plenum. This provides a more granular cooling as in-row CRAC
fan speeds are controlled individually.
Fire suppression
One consideration when designing an aisle containment
system is fire suppression. When an aisle is created, it can
present a barrier to overhead sprinklers. But there are several
ways to overcome this. Sprinklers can be relocated within
the aisle, although this can be problematic if future changes
are envisaged. The most popular and cost-effective solution
is to use temperature fusible links, which enable lightweight
canopy panels to drop into the aisle in the event of fire and
allow the sprinklers to do their job. Heavier glass panels
require other methods eg. to link the panels into the fire
control system via actuators which will raise the panels in the
event of a fire.
Hot vs cold aisle containment
Data centres are agile environments, frequently changing
with business needs. Consequently heat load distribution
changes, as does the physical infrastructure – with cabinets
and associated hardware being replaced or moved. This
is particularly true of colocation facilities, which form the
majority of data centre space in the UK, where multiple
companies may be hosted in each facility. It’s also true to say
that the majority of existing facilities are traditional raised
floor data halls with edge CRAH units. A raised floor is not
only practical to route power and data to cabinets, but also
provides a natural plenum for cold air to be pushed by the
CRAH units under the data centre floor. Grills are then placed
within the cold aisle through which the cold air can pass to be
drawn into the intake side of the cabinets.
As computing power becomes denser, the need for cooling
increases in importance as the power consumed per rack
space goes up. The substantial cost of building new data
centres means our existing UK facilities aren’t going to be
replaced but will be upgraded. Without an overhead plenum,
cold aisle containment is a logical choice. Perhaps, most
importantly for data centre operators, it’s much easier to
re-configure a cold aisle. This leaves water-cooling to the
rack level via in row cooling, which has gained popularity
and is undoubtedly a more efficient way to remove heat.
Typically, it’s used in a hot aisle containment configuration.
However, it doesn’t lend itself so well to these agile facilities
where cabinets may need to be added or moved. These
systems also generally represent more points of failure, and
the incremental capital cost of adding computer load in new
cabinets is increased. However, where space is at a premium
and with high computing densities, in-row cooled hot aisles
can be the right solution.
Data centres tend to be populated over a period of time so
there’s been a tendency to defer the aisle until the pod is fully
populated. The benefits of the aisle can be obtained from day
one by installing low cost temporary vertical sacrificial panels
in the place of unpopulated cabinet positions.
Dynamic control
While hot and cold aisle containment are undoubtedly a
cost-effective way to optimise cooling and make energy
savings, they’re static measures and as already mentioned a
data centre is a dynamic environment with varying IT load. To
monitor the effects of changes, plan for capacity and provide
more agility in the control of inlet/exhaust temperatures,
airflow and air pressure. Some form of DCIM is required to
gain granular control throughout the data centre. Nowadays
there are many well-proven and comprehensive DCIM
solutions on the market. n
www.networkseuropemagazine.com