ASHRAE Journal - August 2021 - 50

TECHNICAL FEATURE
Figure 5 shows the inlet air temperature
range. The boxplot shows the
distribution of data, such as the maximum,
minimum and median values
as well as quartiles of each alternative,
for a comparison of the two alternatives.
The interval plot of the 95% confidence
interval shows that the HAC
systems maintained uniform inlet air
temperature ranges compared to the
CAC system.
FIGURE 5 Inlet air temperature distributions of 100 rack servers for the HAC and CAC systems. (a) Boxplot and
(b) interval plot.
A.
Experimental Investigation
A comparison of the HAC and CAC
systems of room-based cooling on the
low-density rack server zone was done
through a field measurement. Two
air containment areas were used for the experimental
investigation. One air containment area's dimensions
were 3660 mm (W) × 7320 mm (L) × 2200 mm (H)
(144 in. × 288 in. × 87 in.). The second air containment
area's dimensions were 3660 mm (W) × 10 980 mm (L) ×
2200 mm (H) (144 in. × 432 in. × 87 in.). The HAC has
one hot aisle module, consisting of 19 rack servers in two
rows. The CAC consists of one cold aisle module with
23 racks in two rows. The dimensions of the server rack
are 610 mm (W) × 1100 mm (L) × 2200 mm (H) (24 in. ×
43 in. × 87 in.). The racks were arranged face-to-face.
This reference data center has large dedicated IT
27.5
25.0
22.5
20.0
17.5
15.0
HAC
CAC
rooms and operates room-based cooling systems with
CRAH units. The average IT power of the HAC and CAC
were 52.8 kW and 35.9 kW, respectively. The IT input
power was very constant. This means that the IT cooling
load was also constant, and that there were continuous
heat gains from the IT equipment.
The cooling capacity of the CRAH unit in charge of the
HAC system was 70.3 kW, and the corresponding air
volume was 390 m3/min (13,773 cfm), whereas 48.9 kW
with an air volume of 270 m3/min (9,535 cfm) was
required for the CRAH unit of the CAC. To achieve reliable
and stable experimental data, it was attempted to
provide more uniform airflow.
The supply air system is an underfloor air distribution
(UFAD) system. The height of raised floor is 600 mm
(24 in.), and the perforation percentage of the perforated
tiles is 25%. The return air system for the HAC is
a direct-ducted return air system and for the CAC is a
50
ASHRAE JOURNAL ashrae.org A U G UST 2021
Boxplot
Interval Plot
B.
19.5
19.0
18.5
18.0
17.5
17.0
16.5
16.0
HAC
CAC
Individual Standard Deviations Are Used to
Calculate the Intervals
ceiling plenum return air system. While the raised floor
infrastructure is a common approach used in the cooling
of HAC and CAC, the return path is a different approach.
These test cases were not designed by intuition alone
due to the complex thermofluid behavior. However,
both air containment systems could be compared to see
if there were any differences of thermal performance for
the similar geometry in the IT room.
Figure 6 shows the IT environment measuring points
of inlet air temperature and humidity sensors. The IT
rooms use the most general air distribution method with
a raised floor where the cold air is supplied from the
CRAH unit into the perforated tiles. The temperature
and relative humidity were measured continuously for
a month (from Sept. 18 to Oct. 16, 2019) at 32 points per
floor, comprising 20 points at the top and bottom inlets,
10 points at the top outlet of IT equipment, and two
points at the SA and RA of the CRAH unit.
The measurements were carried out in two scenarios.
Blanking panels are installed in rack servers to maintain
proper airflow. In the test cases, it is very difficult to
measure the leakage rate inside the rack server. Doors,
cable cutouts, floor tiles, fault ceiling and ductwork can
increase the leakage problem-and at the risk of severely
damaging the thermal balance due to under provisioning.
As the CRAHs are sized for the overall load in the IT
room, average 25% over-provisioning is anticipated. But
the actual provisioning depends on the distribution of
airflow devices and the heat load.
The measurements were carried out for three cases for
a month. In Test Case 1, the supply air temperature of
95% CI for the Mean
Inlet Air Temperature of Rack Servers (°C)
Inlet Air Temperature of Rack Servers (°C)
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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
ASHRAE Journal - August 2021 - 4
ASHRAE Journal - August 2021 - 5
ASHRAE Journal - August 2021 - 6
ASHRAE Journal - August 2021 - 7
ASHRAE Journal - August 2021 - 8
ASHRAE Journal - August 2021 - 9
ASHRAE Journal - August 2021 - 10
ASHRAE Journal - August 2021 - 11
ASHRAE Journal - August 2021 - 12
ASHRAE Journal - August 2021 - 13
ASHRAE Journal - August 2021 - 14
ASHRAE Journal - August 2021 - 15
ASHRAE Journal - August 2021 - 16
ASHRAE Journal - August 2021 - 17
ASHRAE Journal - August 2021 - 18
ASHRAE Journal - August 2021 - 19
ASHRAE Journal - August 2021 - 20
ASHRAE Journal - August 2021 - 21
ASHRAE Journal - August 2021 - 22
ASHRAE Journal - August 2021 - 23
ASHRAE Journal - August 2021 - 24
ASHRAE Journal - August 2021 - 25
ASHRAE Journal - August 2021 - 26
ASHRAE Journal - August 2021 - 27
ASHRAE Journal - August 2021 - 28
ASHRAE Journal - August 2021 - 29
ASHRAE Journal - August 2021 - 30
ASHRAE Journal - August 2021 - 31
ASHRAE Journal - August 2021 - 32
ASHRAE Journal - August 2021 - 33
ASHRAE Journal - August 2021 - 34
ASHRAE Journal - August 2021 - 35
ASHRAE Journal - August 2021 - 36
ASHRAE Journal - August 2021 - 37
ASHRAE Journal - August 2021 - 38
ASHRAE Journal - August 2021 - 39
ASHRAE Journal - August 2021 - 40
ASHRAE Journal - August 2021 - 41
ASHRAE Journal - August 2021 - 42
ASHRAE Journal - August 2021 - 43
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ASHRAE Journal - August 2021 - 48
ASHRAE Journal - August 2021 - 49
ASHRAE Journal - August 2021 - 50
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ASHRAE Journal - August 2021 - 80
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