ASHRAE Journal - August 2021 - 55

TECHNICAL FEATURE
Thermal Performance Evaluation
The IT environments were analyzed
based on the air temperature distributions
in the HAC and CAC as shown
in Table 5. When the SA temperature
was 15.0°C (59°F), the average air
temperature in the cold aisles of the
HAC was 2.5°C (4.5°F) lower than in
the CAC. Furthermore, the average
air temperature in the hot aisles of
the HAC was 3.7°C (6.7°F) lower than
in the CAC. This is because the hot air
that had passed through the IT server
in the CAC was recirculated to the IT
server inlet without returning to the
CRAH units.
When the HAC at an SA temperature
FIGURE 8 Mean outlet air temperature distributions of rack servers. (a) Test Case 1, (b) Test Case 2 and (C) Test Case 3.
Test Case 1: Hot-Aisle Containment (HAC) with Supply Air Temperature of 15.0°C
32
30
28
26
24
22
20
HAC
H-1
B.
of 17.0°C (63°F) was compared to the
CAC at an SA temperature of 15.0°C
(59°F), the average air temperature distributions
of the cold and hot aisles were
similar with a difference of less than
±0.5°C (0.9°F). In other words, when
the SA temperature of the HAC was
raised by 2.0°C (3.6°F), the IT operation
environment was similar to the CAC.
Figure 9 shows the distribution of IT
environment (temperature) for the
CAC and HAC systems at each SA temperature.
The HAC with a SA temperature
of 17.0°C (63°F) showed a very
similar air temperature distribution
to the CAC with a SA temperature of 15.0°C (59°F). This
implies that the HAC has a better thermal performance
than the CAC. This means this case of the CAC would not
be a good design of airfl ow. Therefore, for any air containment
type , for various distributions of heat load and
given cooling systems, one has to determine the statically
balanced provisioned state.
34
33
32
31
30
29
28
27
CAC
H-1
C.
34
32
30
28
26
24
HAC
H-1
H-2
H-3
H-4
H-5
H-6
H-7
H-8
Hot-Aisle Containment (HAC) Measurement Points (in Hot Aisles [H])
TABLE 5 The air temperature distributions of HAC and CAC systems depend on
SA temperature.
HAC System
With SA 15°C
CAC System
With SA 15°C
Conclusions
In this case study, the effects of deploying air containment
confi gurations were studied. Numerical
simulations and fi eld experiments were conducted to
investigate the thermal performance. Many researchers
are beginning to wonder whether it is better to use
HAC and CAC, but no conclusion have been reached.
HAC System
With SA 17°C
AIR CONTAINMENT
Cold Aisle
Hot Aisle
Cold Aisle
Hot Aisle
Cold Aisle
Hot Aisle
MEAN (°C)
20.08
26.38
22.54
30.08
22.12
29.43
MIN (°C)
19.65
26.08
22.19
29.70
21.31
29.01
MAX (°C)
21.69
27.18
23.96
30.47
22.51
29.72
The thermal performance of each air containment
method depends on its cooling conditions and IT
environments. CAC has extra design effort required
A U G U S T 2 0 2 1 ashrae.org ASHRAE JOURNAL
55
H-9
H-10
H-2
H-3
H-4
H-5
H-6
Test Case 3: Hot-Aisle Containment (HAC) with Supply Air Temperature of 17.0°C
H-7
H-8
Cold-Aisle Containment (CAC) Measurement Points (in Hot Aisles [H])
H-9
H-10
H-2
H-3
H-4
H-5
H-6
Test Case 2: Cold-Aisle Containment (CAC) with Supply Air Temperature of 15.0°C
H-7
H-8
Hot-Aisle Containment (HAC) Measurement Points (in Hot Aisles [H])
H-9
H-10
A.
Mean Air Outlet Temperature of IT Servers: 26.38°C
27.13
25.64
Mean Air Outlet Temperature of IT Servers: 30.10°C
30.52
29.66
Mean Air Outlet Temperature of IT Servers: 29.40°C
28.81
27.96
Rack Outlet Temperature (°C)
Rack Outlet Temperature (°C)
Rack Outlet Temperature (°C)
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ASHRAE Journal - August 2021

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