ASHRAE Journal - January 2023 - 8

LETTERS
column is more focused on where to select what in terms
of free cooling technologies considering some important
points.
The column says that MLC/ELC per ASHRAE Standard
90.4 is one of the most important criteria but not always
the only deciding factor for choosing a particular cooling
technology over others. I agree that wherever we
use water-based cooling technology, water availability
and WUE will also be an important metric besides PUE,
which is common to all.
In most cases, the technology that results in reduced
compressor hours is considered as free cooling technology,
which may or may not mix two free cooling technologies.
In a DX air-cooled cooling system, air economizer
is the free cooling, which results in reduced compressor
hours. Along with this air economizer, we can mix
another free cooling technology like pumped refrigerant,
which will also result in reduced compressor hours.
But if it will be a viable solution from how much free
cooling we are actually adding by this mixing is subject
to analysis.
There are other cooling technologies where the system
does not use compressors (if we ignore trim systems in
them that can have compressors/chilled water supply
from a chiller having compressors), but still it is considered
as free cooling like in a direct/indirect evaporative
system, which actually depends on fresh air intake/
weather conditions.
In the case of free cooling with dual coil DX water
cooler, the column says, " If a cooling tower is used, then
water treatment is required. " Dry coolers in most cases
are not provided with adiabatic systems. Hence the column
ignored these special cases, but yes, you are right-
if dry coolers have adiabatic systems, water treatment is
needed. Legionella is covered under water treatment.
The reference to multiple dry coolers was regarding
one CRAC/one condenser vs. multiple CRAC/few condensers
(or common cooling tower). I agree that the
quantity of condensers (or cooling towers) will depend
upon the desired tier requirement of the data center as
N + 1, 2N, N + 2, etc., and the optimum unit sizes to result
in a more energy efficient system depending upon the
anticipated partial load on the units. It may result in
higher capital cost but will be good for better operation
cost.
We believe you will appreciate that it is not possible to
make the list all inclusive. Hence the column introduction
says, " Some of the main free cooling technologies
available for data centers currently are detailed below.
This is not an all-inclusive list. " I agree that energy
wheels/heat exchangers also play an important role, and
the source of free cooling like geothermal energy will
also be interesting to consider wherever it is applicable.
Mohammad Khurram Saeed, Associate Member ASHRAE, Sydney, Australia
Correction
November's Letter's section omitted the writers of the
letter regarding " Airborne Disease Transmission Risk and
Energy Impact of HVAC Mitigation Strategies. " The letter
was written by W.K. Chow, Ph.D., Fellow ASHRAE, and
C.L. Chow, Ph.D., from Hong Kong. You can read it at
https://tinyurl.com/2s49znwr.
ASHRAE Journal welcomes letters to the editor. The
letters should be no more than 250 words and must
relate to an article published in ASHRAE Journal.
Please send your letters to rebecca@ashrae.org.
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ASHRAE Journal - January 2023

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