ASHRAE Journal - August 2021 - 49

TECHNICAL FEATURE
TABLE 2 Numerical analysis results. Provisioning of CRAH units for cooling.
AIR CONTAINMENT
CRAH UNITS
NET COOLING POWER
(KW)
PROVISIONING (%)
MEAN TEMPERATURE
IN (°C)
MEAN TEMPERATURE
OUT (°C)
223.0
97.0
209.5
91.1
31.09
15.5
15.2
188.5
82.0
29.47
15.1
HOT-AISLE CONTAINMENT (HAC)
COLD-AISLE CONTAINMENT (CAC)
CRAH 1 CRAH 2 CRAH 3 CRAH 4 CRAH 5 CRAH 6 CRAH 1 CRAH 2 CRAH 3 CRAH 4 CRAH 5 CRAH 6
220.0
95.7
166.5
72.4
31.89
15.3
32.12
27.78
15.0
160.1
69.6
27.29
15.0
180 200 220 240 260 280
CRAH Cooling (kW)
CRAH 1
CFD RESULTS
CRAH 2
CRAH 3
CRAH 4
CRAH 5
CRAH 6
Hot-Aisle Containment with Ducted Air Return
modeling is for a given distribution of
heat load as static provisioning. Table
2 shows the results of six provisioned
CRAH units. CRAH units of the HAC
(1,168 kW) and CAC (1,169 kW) are provisioned
to within 15% of their total capacity.
The CAC has the same geometry in
the IT room, but the provisioning is out
of heat balance. On the high-density rack
server zone, the CRAH units (CRAH 1 to CRAH 3) could
be under-provisioned for the CAC. The HAC is in the
more statically balanced provisioned state than the CAC.
This state means that the deviation for cooling energy of
CRAHs is minimized.
As mentioned earlier, any air not used to cool IT equipment
represents wasted energy. Cold air loss decreases
the capacity of the CRAH unit. As more loss occurs, more
CRAH units of the CAC are required to remove the same
amount of heat while maintaining the appropriate IT
inlet air temperature.
ALTERNATIVES
HAC
CAC
The CRAHs (CRAH 4 to CRAH 6) on the mediumdensity
rack server zone are over-provisioned for both
systems and are undersized by 20%. That means this
case of the CAC would not be a good design of airfl ow. It
is very important that the variation of net cooling energy
between coolers be kept to a minimum. Therefore, for
any air containment, for various distributions of heat
load and given cooling systems, one has to determine
the statically balanced provisioned state.10 There may be
CRAH 1
CRAH 2
CRAH 3
Maximum Cooling Capacity
of a CRAH Unit = 230 kW
CRAH 4
CRAH 5
CRAH 6
Cold-Aisle Containment with Flooded Air Return
TABLE 3 Descriptive statistics of IT environment for the HAC and CAC. Inlet air temperature of rack servers.
NO. OF
RACKS
MEAN
(°C)
100 16.503
100 18.480
SE MEAN
(°C)
STDEV
(°C)
MINIMUM
(°C)
Q1
(°C)
MEDIAN
(°C)
Q3
(°C)
MAXIMUM
(°C)
0.129 1.286 15.000 15.143 16.795 17.277 20.570
0.384 3.839 15.000 15.043 16.785 20.775 27.000
opportunity to improve the temperature profi le when
CAC is used by rightsizing the airfl ow to the cold aisles
with higher loads. As an alternate consideration, using
more than 25% free area grates would have made for
better airfl ow to the area with higher loads and provided
less ∆P across the tiles.
For a comparison based on the CFD simulation results,
the IT environments were compared based on the inlet
air temperatures supplied to each rack server from the
HAC and CAC as shown in Table 3. Because the HAC is
less infl uenced by effects of hot air recirculation and
poor provisioning than the CAC, when the cooling
system operated based on the supply air temperature
of 15°C (59°F) the mean inlet air temperatures of the
HAC and CAC were 16.5°C (61.7°F) and 18.5°C (65.3°F),
respectively, exhibiting an approximately 2.0°C (3.6°F)
temperature difference. The maximum inlet air temperatures,
however, were 20.6°C (69.1°F) and 27.0°C
(80.6°F), respectively, exhibiting a larger temperature
difference of 6.4°C (11.5°F) or higher.
A U G U S T 2 0 2 1 ashrae.org ASHRAE JOURNAL
49
228.6
99.4
35.6
18.1
228.6
99.4
37.7
20.2
228.6
99.4
35.1
17.6
189.4
82.3
29.5
15.1
158.8
69.0
27.2
15.0
180 200 220 240 260 280
CRAH Cooling (kW)
134.5
58.5
25.3
15.0
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ASHRAE Journal - August 2021

Table of Contents for the Digital Edition of ASHRAE Journal - August 2021

Contents
ASHRAE Journal - August 2021 - Intro
ASHRAE Journal - August 2021 - Cover1
ASHRAE Journal - August 2021 - Cover2
ASHRAE Journal - August 2021 - 1
ASHRAE Journal - August 2021 - Contents
ASHRAE Journal - August 2021 - 3
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