UNCOVERING THE LAYERS
tunnel. For a 1U server for example the
pressure drop is around 350 pascals, but
this does depend on the components
inside the server.
Fans that sit inside a 1U server are typically
at best 25% efficient and it is a commonly
known fact that smaller fans are less
efficient than larger ones. We can now use
the simple equation, ηP e = ∆P V F , again to
determine the electrical power that small
less efficient fans require to overcome the
server pressure drop at one cubic metre per
second assuming no air has wondered off
somewhere else in the data centre.
This yields an accumulated fan power
of 1.4kW. But just how much thermal
power can these fans remove? For this
answer, we need to employ the steady
state thermodynamic relationship of P T =
ρc p V F ∆T, making use of the density of air,
ρ (=1.22kg/m 3 ), its specific heat capacity
at constant pressure, c p (=1006J/kg/K)
and the temperature increase (delta-T), ∆T,
across the servers. Now we must make a
guess at the delta-T.
64
Issue 02
We can try a range, say five, 10 and 15ºC,
which with the same flow rate of one
cubic metre per second we obtain the
thermal power that is injected in the
airstream in passing through racks of
servers of 6136W, 12273W and 18410W
for the three-respective delta-T values.
It is then easy to see that in complete
ideal conditions, the small server fans
respectively consume 18.6%, 10.3% and
7.1% of the total server power assuming
no losses in the airflow.
Given that these simple equations are
based on a lot of assumptions that would
yield conservative approximations, it is not
unreasonable to say that IT fan power can
consume more than 7% of the power of a
typical 1U server.
It is now very tempting to add all of these
figures together to show how partial PUE
is affected by the rack delta-T. Gains in
reducing end use energy demand of data
centres are clearly best addressed by
analysing the full integrated system. ◊
WE ARE ON A
CRUSADE TO
APPROACH
DATA CENTRES
AS INTEGRATED
SYSTEMS
AND OUR
EXPERIMENTS
ARE GEARED
TO INCLUDE
THE FULL
INFRASTRUCTURE
WHERE THE
FACILITY HAS IT
IN IT.
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