ASHRAE Journal - July 2022 - 26

COLUMN DATA CENTERS
then shows the two Recommended Envelopes for the
same classes in the Fifth Edition. The single envelope
from the Fourth Edition, with the 60% RH upper limit, is
replaced with two envelopes in the Fifth Edition, with 50%
and 70% RH upper limits, for the different corrosion
conditions (as described in the previous paragraphs).
The Addition of a New High-Density Air-Cooled
Environmental Envelope (H1) Based on IT Technology
The decision to add a new environmental class to
Thermal Guidelines was based on new high-density IT
products. These high-density products use high-powered
components such as CPUs, GPUs and memory.
These components require increased cooling that could
be provided with an increase in heat sink volume/fan
performance. However, server volume limitations do
not permit these performance enhancements within
allowable server sizes using Class A1 cooling temperatures.
For this reason, the new Class H1 was added. The
new Class H1 has more restrictive (lower) recommended
and allowable temperature ranges than the existing
classes A1 through A4. Figure 5 shows the recommended
and allowable ranges for Class H1 for operating environments
with both low and high gaseous pollution levels.
Most of the environmental envelopes shown in the
Thermal Guidelines, including those shown in Figures 4
and 5, are applicable to sea level conditions. At higher
elevations, heat transfer is reduced due to lower air density,
and the temperatures need to be correspondingly
reduced. Figure 6 provides data on the required deratings,
which are signifi cant for all classes. Note that the
guideline threshold is an elevation of 9,840 ft (3,000 m).
Additional information can be found in Appendix G of
the Thermal Guidelines.
Since Class H1 has the most restrictive envelope of
all Thermal Guidelines classes, the addition of this class
of equipment to existing data centers designed for less
restrictive classes requires careful engineering of both
cooling and humidity control systems. Partitioning of
this equipment-with independent cooling systems-
may be needed. The requirement for independent cooling
will likely also be required for new data centers to
maintain as low a PUE as possible.
Liquid-Cooled Class Nomenclature Changes
For liquid cooling, the decision was made to change
the nomenclature for Liquid Cooling Classes, fi rst
26
ASHRAE JOURNAL ashrae.o rg J U LY 2 0 2 2
FIGURE 3 Class A1 to A4 Recommended Envelopes, Fourth Edition of Thermal
Guidelines.2
introduced in the 3rd Edition, to refl ect the water supply
temperature to the ITE. The old classes were W1 through
W5, and the new classes are W17, W27, W32, W40, W45
and W+. The number after the " W " represents the maximum
expected liquid design temperature in degrees
Celsius at the inlet to the ITE. The minimum allowable
temperature for all classes is 35.6°F (2°C).
Care must be taken to ensure compatibility between
the facility's liquid cooling system and the ITE served
regarding working pressures, fl ow rates, differential
pressure and temperature rates of change. Knowledge
of the air temperature range in the data center is also
needed. For all liquid cooling classes, it is essential that
the dew-point temperature of the space be below the
coldest temperature of the recirculating liquid to avoid
localized high relative humidity and/or condensation
issues. Also, the discharge temperature from the ITE will
be higher than the inlet temperature, and heat rejection
equipment must account for this higher temperature
and the appropriate temperature range of the cooling
system.
One of the long-term advantages of liquid cooling
is that there is a greater opportunity for higher water
temperatures than air temperatures due to closer heat
transfer approach temperatures. This increases the percentage
of the year that the facility can be operated in
economizer mode. At the higher temperatures, district
heating is also an opportunity, though provision must
also be made for back-up heat rejection equipment if the
minimum annual district heating demand is less than
the data center's heat rejection requirements. Liquid
cooling is well suited to district heating (relative to an
air-cooled data center) due to the ease and effi ciency
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