UNCOVERING THE LAYERS
systems it is always difficult to show that
liquids are more ‘effective’ than air in
removing the heat.
However, the argument for a metric that
assesses the overhead of heat removal
should include all the components
whose function is to remove heat. So, for
centrally pumped coolants in the case
of liquid cooling, the overhead of the
pump power is correctly assigned to the
numerator of the PUE, but this is not the
case for fans inside the IT equipment.
Dr Jon Summers, Scientific Leader in
Data Centres, Research Institutes of
Sweden, SICS North
were able to integrate the energy required
to provide the digital services alongside
the energy needed to maintain systems
within their thermal envelope.
The research involved both air and liquid
cooling, and for the latter we were able to
work with rear door heat-exchangers, on-
chip and immersion systems.
In determining the Power Usage
Effectiveness (PUE) of air versus liquid
www.intelligentdatacentres.com
So what percentage of the critical load do
the fans consume? Here we can do some
simple back of the envelope calculations,
but first we need to understand how air
movers work. The facility fans are usually
large and their electrical power, P e , can be
measured using a power meter.
curves and affinity laws then you may
arrive at 1kW of electrical power per
cubic metre per second of flow rate, V F .
Therefore, with an efficiency, η, of 50%
the flowrate and pressure follow the
simple relationship, ηPe= ∆P V F .
Thus, 1kW of power consumption will
overcome 2,000 pascals of pressure
drop at a flow rate of one cubic metre per
minute. The IT fans are now employed to
take over this volumetric flowrate of air to
overcome the pressure drop across the IT
equipment and exhaust the hot air at the
rear of the IT equipment.
Again, there is literature on the pressure
drop across a server and we calculated
this at Leeds using a generic server wind
This electrical power is converted into a
volumetric flowrate that overcomes the
pressure drop, ∆P, that is caused by ducts,
obstacles, filters, etc. between this facility
fan and the entrance to the IT.
If you look at a variety of different
literature on this subject, such as fan
Data centre module, ICE (Infrastructure
and Cloud research and test Environment
– ice.sics.se)
Issue 02
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