UNCOVERING THE LAYERS
power is distributed with the use of power
distribution units (PDUs).
There would be the requirement for
cooling distribution units (CDUs), the
number of which would depend on the
amount of thermal energy to collect
and the volumetric flow rates of the
liquid coolants in the loops. There is
an analogous relationship between the
current (rate of flow of electrons) and the
liquid flow rate (rate of flow of thermal
energy) with electrical and thermal
power being given by multiplying the
flows with the voltage and temperature
differences respectively.
With the above definition of liquid cooling,
there are naturally two approaches. One
where the liquid does not penetrate the
air cooled IT, but does penetrate the IT
racks, which is commonly called Indirect
Liquid Cooling (ILC), and the other in
which liquids penetrate the IT and collect
thermal energy directly from components
inside the IT systems, referred to as Direct
Liquid Cooling (DLC).
64
Issue 07
An example of the former is the rear door
heat exchanger (RDHx), whereas the latter
could be direct to DPU cold plates with
heat sinks in the liquid path.
The case of Total Liquid Cooling is a special
version of Direct Liquid Cooling where all
heat bearing components within the IT
systems are connected to the liquid cooling
loop and there is zero requirement for air
cooling. An example of this category would
be dielectric liquid immersion.
The High Performance Computing (HPC)
solutions are already making use of ILC,
DLC and TLC approaches to manage the
higher heat fluxes and now the Advanced
Cooling Solutions working group of the
Open Compute Project are using similar
distinctions of liquid cooling.
As evidenced by HPC systems and the
adoption of DLC by Google for their
Tensor Processing Units (TPU), the
practice of heat transfer from the DPU
using liquid cooling is well established
and can use one of many approaches,
such as cold plates with single phase or
two-phase liquid flows driven by natural
or forced convection, impinging jets of
fluids with different geometries of heat
exchangers and heat spreaders, different
dielectric liquids providing both direct
and indirect contact.
These methods are all very capable of
managing the kW per square centimetre
heat fluxes from the DPUs and with the
growing heat fluxes in the future with no
anticipated technology shift in the DPU,
there will be a rise in the adoption of
liquid cooling.
The numerous companies that offer liquid
cooled solutions today have collected a
wealth of operational understanding and
so issues of maintenance have matured
and are well established.
Liquid cooling has arrived and data
centres will have to consider how to
integrate liquid distribution into the data
halls to facilitate liquid to the IT rack to
be future ready. ◊
www.intelligentdatacentres.com