ASHRAE Journal - May 2022 - 55

COLUMN DATA CENTERS
FIGURE 1 Recommended and allowable environmental envelope for Classes A1,
A2, A3, and A4 with low levels of gaseous pollutants.3
FIGURE 2 Recommended and allowable environmental envelope for Class H1 with
low levels of gaseous pollutants.3
Recommended vs. Allowable Envelopes
The thermal guidelines publication has both recommended
and allowable ranges. While other parameters
(such as air pollution and elevation) are also limiting
factors, the primary environmental operating conditions
are related to temperature and humidity (both
relative and absolute) and are displayed on sea level psychrometric
charts in the guidelines.
The recommended range is likely the most important
range for data center designers and operators. Facilities
should be designed and operated to target the recommended
range for most hours of the year.
ITE, on the other hand, should be designed to operate
within the extremes of the applicable allowable environmental
classes. Figure 1 provides an example chart from
the Fifth Edition; it shows both the recommended and
allowable conditions for environmental Classes A1 to A4
under conditions with low gaseous pollutants and corresponding
ITE corrosion potential.3 As a second example,
Figure 2 shows the recommended and allowable envelopes
for the new high-density H1 air-cooled class, again
with low corrosion potential. If the corrosion potential
is high, the maximum recommended relative humidity
(RH) is lowered from 70% to 50%. Similar charts are
available in the Fifth Edition for all classes with both low
and high levels of gaseous pollutants, in both SI and I-P
units.
Environmental Class Defi nitions
As shown in Figures 1 and 2, not all ITE is designed for
the same environmental conditions (i.e., the Allowable
envelopes). Several classes of environmental envelopes
TABLE 1 Thermal Guidelines environmental classes.
FIRST EDITION
FIFTH EDITION
ENVIRONMENTAL
CLASS
1 Renamed
Class A1 in
Third Edition
2 Renamed
Class A2 in
Third Edition
N/A
ENVIRONMENTAL
CLASS
A1
A2
A3
CLASS DESCRIPTION
Typically a data center with tightly
controlled environmental parameters (dew
point, temperature, and RH) and missioncritical
operations
Typically an information technology space
with some control of environmental
parameters
Similar to Class A2, but with a wider
allowable range that will likely allow for
chiller-less cooling with a water-cooled
heat rejection system in most climates
N/A
N/A
A4
H1
Similar to Class A2/A3, but with a wider
allowable range that will likely allow for
chiller-less cooling with an outdoor aircooled
system in most climates
High-density air-cooled servers (see Fifth
Edition, Section 2.2.2 for more information)
are specifi ed in the Thermal Guidelines book. The First
Edition and Second Edition each had four Classes: 1, 2, 3
and 4. Among them, Class 1 had the most restrictive
allowable environmental envelope, while Class 4 had
the widest specifi cation.
In the Third Edition, industry pressure to increase the
effi ciency of data center operation led to the addition
of two new classes, A3 and A4. There was a co-incident
renaming of the original classes with this publication. To
help avoid confusion, Table 1 provides a correspondence
between the class naming convention used in the First
Edition and the Fifth Edition.
From Table 1, the current classes (in the Fifth Edition)
are A1, A2, A3, A4 and H1. Classes A1 and A2 correspond
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ASHRAE Journal - May 2022

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