ASHRAE Journal - August 2021 - 48

TECHNICAL FEATURE
FIGURE 4 CFD simulation results. The mean inlet air temperature of rack servers in (a) HAC and (b) CAC system, the pressure contours at the height of 1.0 m in (c) HAC
and (d) CAC system.
A.
ASHRAE Recommended: 18°C to 27°C
ASHRAE Allowable: 15°C to 32°C
B.
ASHRAE Recommended: 18°C to 27°C
ASHRAE Allowable: 15°C to 32°C
Temperature (°C)
Temperature (°C)
15 18 21 24 27 30
(E-1) Max. Inlet Air Temp. of
Rack Server: 20.6°C
C.
4 Pa
Cold Aisle
4 Pa
Cold Aisle
Cold Aisle
Cold Aisle
-3 Pa -3 Pa
Cold Aisle
D.
Cold Aisle
4 Pa
Cold Aisle
4 Pa
Cold Aisle
Cold Aisle
15 21.25 27.5 33.75 40
(B-3) Max. Inlet Air Temp. of
Rack Server: 27.0°C
Cold Aisle
1.5 Pa 1.5 Pa
Cold Aisle
Cold Aisle
Hot Aisle Hot Aisle
0.7 Pa
Pressure (Pa)
at 1.0 m Height
-10 -5
5
10
Hot-Aisle Containment With Ducted Air Return
room is less effective than duct. Recirculation occurred
around the ends of rows in the high-density rack server
zone because of inadequately supplied cold aisles or at
least at the end of the cold aisle. Even when CACs were
attempted, there can be paths for hot air leakage.
Hot air moved through leaks in the cabinets and the
return air path. This had a negative influence not only
on the IT environment but also the provisioning level.
The CAC does not work enough while raising the IT inlet
air temperature. This numerical analysis considered
the HAC and CAC to be sealed at suitable levels. This
assumption allows calculating the maximum efficiency
of the CRAH units and allows for a fair comparison.
However, there is always air leakage with HAC or CAC
requiring that the CRAH fan airflow be greater than the
IT equipment airflow. The balance of airflow must equal
the IT equipment airflow plus the percentage of air leakage
from the containment system such as a raised floor.
When using the HAC and CAC, rack server leakage
can be induced by positive differential pressure.9 As
shown in Figure 4c, the pressure contour boundaries are
48
ASHRAE JOURNAL ashrae.org A U G UST 2021
0.5 Pa
Hot Aisle
Hot Aisle
Hot Aisle
Hot Aisle
0.7 Pa
0.7 Pa
Hot Aisle
-4.5 Pa -11 Pa
Pressure (Pa)
-10 -5
0 5
10
Cold-Aisle Containment With Flooded Air Return
between -10 Pa and 10 Pa (-0.04 in w.g. to 0.04 in. w.g.).
The pressure in the HAC is constant, so the pressure differential
between all the hot and cold aisles are similar
(about ΔP = 4 Pa [0.02 in. w.g.]).
The airflow rate of all servers is constant and close
to free delivery. Unlike the HAC, however, the CAC (in
Figure 4d) has a wide pressure differential distribution
between hot and cold aisles (about ΔP = 4 Pa to 15 Pa
[0.02 in. w.g. to 0.06 in. w.g.]) that results in most pressure
in the cold aisle not being constant. The pressure
closer to the CRAH unit is lower, and this negative pressure
causes insufficient airflow for the rack servers in
rack B-3, especially on the high-density rack server
zone. This means the outlet server temperature becomes
higher because the rack servers in this area are not
drawing their required airflow. The pressure differential
could be reflected in the rack inlet temperature distribution.
The air pressure on the supply and return side of
the HAC is better than on the CAC.
In this numerical simulation, the provisioning level
of the air-conditioning resources through numerical
Hot Aisle
Hot Aisle
Hot Aisle
5.5 Pa 12 Pa
at 1.0 m Height

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ASHRAE Journal - August 2021

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