ASHRAE Journal - August 2021 - 56

TECHNICAL FEATURE
because each aisle width needs to be customized to the
expected load per aisle. This is not an issue with HAC,
and the greater simplicity of design would make it the
preferable solution. While both approaches minimize
the mixing of hot and cold air, practical differences
exist in implementation and operation that have significant
effects on the amount of leakage and heat balance.
The thermofluids behavior is very complex for a
high-power density data center, and the actual provisioning
depends on the location of racks, air paths and
the heat load distribution.
In conclusion, specific to underfloor air distribution,
the HAC has a higher cooling efficiency than
the CAC. The air containment is considered one of
the most effective techniques for IT environment
control. However, there is a limit to how safe we can
make the IT environment, given that there is a bypass
of cold air, a recirculation of hot air between cold
aisle and hot aisle, as well as air leakage challenges.
Investigation into the potential for air mixing within
data centers using the HAC or CAC system has shown
that there may not be enough airtightness to maintain
proper IT environments. This result isn't absolutely
definitive; it's not to say that the type of air containment
isn't important, but the actual root of thermal
performance derives from the air path, which is specific
for each and every configuration.
References
1. Niemann, J., K. Brown, V. Avelar. 2011. " Hot-aisle vs. coldaisle
containment for data centers, APC White Paper 135, rev02. "
Schneider Electric.
2. Beaty, D.L. 2013. " Internal IT load profile variability. " ASHRAE
Journal 55(2):72 -74.
3. Cram, M. 2017. " The Power of Hyperscale Computing. " White
Paper. Server Technology.
4. Cho, J., J. Yang, C. Lee, J. Lee. 2015. " Development of an energy
evaluation and design tool for dedicated cooling systems of data
centers: Sensing data center cooling energy efficiency. " Energy and
Buildings 96:357- 372.
5. Lin, P., V. Avelar, J. Niemann. 2013. " Implementing hot and
cold air containment in existing data centers, APC White Paper
153. " Schneider Electric.
6. Dunlap, K. 2014. " Cooling audit for identifying potential
cooling problems in data centers, APC White Paper 40, rev03. "
Schneider Electric.
7. Herrlin, M.K. 2005. " Rack cooling effectiveness in data centers
and telecom central offices: the rack cooling index (RCI). " ASHRAE
Transactions 111(2):476 -484.
8. ASHRAE. 2015. Thermal Guideline for Data Processing Environments,
4th Edition. Atlanta: ASHRAE.
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ASHRAE JOURNAL ashrae.org A U G UST 2021
FIGURE 9 The distribution of air temperature for the CAC and HAC at each SA temperature.
(a) Test Case 1, (b) Test Case 2 and (C) Test Case 3.
A. Test Case 1
CAC with SA Temperature of 15.0°C
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Cold Aisle
Hot Aisle
Cold Aisle
Hot Aisle
Mean (°C) StDev (°C) N
20.08 0.4785 151
26.38 0.2461 151
18 20 22 24 26 28 30
Air Temperature (°C)
B. Test Case 2
CAC with SA Temperature of 15.0°C
2.5
2.0
1.5
1.0
0.5
0.0 18 20 22 24 26 28 30
Cold Aisle
Hot Aisle
Air Temperature (°C)
C. Test Case 3
HAC with SA Temperature of 17.0°C
4.0
3.0
2.0
1.0
0.0
Cold Aisle
Hot Aisle
Cold Aisle
Hot Aisle
Mean (°C) StDev (°C) N
22.12 0.1493 532
29.43 0.1087 532
Cold Aisle
Hot Aisle
Mean (°C) StDev (°C) N
22.54 0.1790 696
30.08 0.1621 696
18 20 22 24 26 28 30
Air Temperature (°C)
9. Nemati, K., H.A. Alissa, B.T. Murray, B. Sammakia. 2016.
" Steady-state and transient comparison of cold and hot-aisle
containment and chimney. " Proceedings of 15th IEEE Intersociety
Conference on Thermal and Thermomechanical Phenomena in Electronic
Systems (ITherm) 1435- 1443.
10. Patel, C.D., C.E. Bash, R. Sharma, M. Beitelmal, R.
Friedrich. 2003. " Smart Cooling of Data Centers. " Proceedings of
IPACK'03 International Electronic Packaging Technical Conference and
Exhibition 1 - 9.
Frequency Density
Frequency Density
Frequency Density
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