ASHRAE Journal - July 2022 - 25

COLUMN DATA CENTERS
FIGURE 1 Test results for copper: 21°C and 50%, 70% and 80% RH.1
FIGURE 2 Example of copper and silver corrosion
coupons.
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
Combination of Pollutants
21ºC, 50% RH
21ºC, 70% RH
21ºC, 80% RH
investigate the risk of operating at higher levels of moisture
when high levels of gaseous pollutants exist. The
objective was to evaluate the ability of increasing the recommended
moisture level in support of reducing energy
required by data centers.
Five gaseous pollutants were tested under a variety
of temperature and relative humidity (RH) conditions-three
pollutants that are pervasive throughout
the planet (SO2, NO2 and O3) and two catalyst pollutants
(H2S and Cl2). Pollutant levels tested were at or near
the maximum common concentration levels existing
around the world.
The data collected as part of this research led to a fi nding
that relative humidity levels in the range of 50% to
70% RH can contribute to an increased risk of corrosion
when catalyst pollutants are concurrently present.
Figure 1 provides an example of some of the study's
results. In this case, corrosion rates on copper were
measured with eight combinations of fi ve pollutants and
at three different relative humidity levels at a typical ITE
inlet temperature of 69.8°F (21°C). The preponderance
of the data showed increasing corrosion with higher relative
humidity. It also showed signifi cantly higher corrosion
rates when the two catalyst pollutants (H2S and Cl2)
were present.
The Fourth Edition of Thermal Guidelines had a recommended
maximum relative humidity of 60%, without
any qualifi cations as to the corrosivity of the air. The Fifth
Edition, relying on the research from RP-1755, now has
two different maximum recommended RH levels (50%
and 70%), depending on the corrosivity of the environment
as measured empirically using both copper and
silver coupons. Figure 2 shows one of the currently available
corrosion coupons.
For data center environments tested with silver and
copper coupons that are shown to have corrosion levels
less than 300 Å/month for copper and 200 Å/month
for silver, suggesting that only the pervasive pollutants
(SO2, NO2 and O3) may be present, the recommended
moisture limit is raised from 60% to 70% RH. The upper
moisture limit is now 70% RH or 59°F (15°C) dew-point
temperature, whichever has the lowest absolute moisture
content.
For data center environments tested with silver and
copper coupons that are shown to have levels of corrosion
greater than 300 Å/month for copper and/or
200 Å/month for silver, suggesting that Cl2, H2S, and/
or other corrosive catalysts may be present, the recommended
maximum moisture level has been reduced.
The recommended upper moisture limit listed in the
Fifth Edition of Thermal Guidelines is 50% RH or 59°F (15°C)
dew point, whichever has the lowest absolute content.
Chemical fi ltration should also be considered to reduce
levels of corrosive gases.
For reference, Figure 3 shows the Recommended
Envelope for Classes A1 to A4 in the Fourth Edition. Figure 4
J U LY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
25
Corrosion Thickness (Angstrom)
O3
NO2 + O3
SO2 + O3
NO2 + SO2 + O3
NO2 + SO2 + O3 + Cl2
NO2 + SO2 + O3 +H2S
NO2 + SO2 + Cl2 + H2S
NO2 + SO2 + O3 + Cl2 + H2S
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ASHRAE Journal - July 2022

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