ASHRAE Journal - October 2014 - 66

2014 ASHRAE TECHNOLOGY AWARD CASE STUDIES

rise across our current IT equipment was less than 20°F
(11°C). This meant that we needed to increase the airflow
capacity or size of our air handling equipment by 50%
to use full containment. This posed a major dilemma:
redesign the entire data center, including all mechanical,
civil and electrical infrastructures with 50% larger airhandling equipment. This would increase the data center
infrastructure costs substantially as well as delay the project, which this fast-track project could not afford.
We used a very creative approach-un-containment-
to overcome this problem. Instead of redesigning the
entire data center with full containment, we chose to
use only a partial barrier between the cold and hot air,
which allowed some directed mixing of hot air back
into the cold aisles. This permitted us to supply less cold
air to the data center, which after mixing with some of
the recirculated hot air within the data center space,
met the total airflow requirement of the IT equipment.
Therefore, it allowed us to supply lesser quantity of
colder air and still be able use our smaller sized central
air-handling unit and cooling system infrastructure
without redesign and at much lower first cost.
The smaller AHU fans also significantly reduced air
distribution fan power. The cold climate of Utah allowed
us to supply below 59°F (15°C) free cold air for 74% of the
time. For the remaining 26% of the time, only trim cooling was used. This turned out to be a very cost effective
(smaller size AHUs, smaller civil structure and space,
etc.) as well as energy efficient (less airflow with free
cooling for 74% of the time) solution. The low airflow
quantity at lower temperature supply air has helped us
achieve such low operating partial cooling PUEs in cold,
as well as warm but dry weather. The partial uncontainment in a cold aisle is shown in Figure 4.
The supply air temperature was further allowed to float
to below 59°F (15°C) to take advantage of the free cooling
in this cold climate by reducing fan speed and thus fan
energy consumption, and counting on the recirculation
hot air to make up the remaining requirement. As can be
seen in Figure 2, the fan power required for air distribution
in the data hall and the UPS hall are lower when ambient
wet-bulb temperatures are lower and rise slowly as the
wet-bulb temperature increases. Due to some situations
in our controls, we noticed that the amount of required
airflow increased rapidly when supply air temperatures
reached near ~59°F (15°C), or wet-bulb temperatures
reached ~52°F (11°C). Once the trim cooling was turned on
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FIGURE 4 Un-containment air distribution

and the supply air temperature was reduced below ~59°F
(15°C), the fan power reduced again. We are working on
our controls sequences to overcome this quirkiness.
A thermograph of temperature profile at the inlet of
the rack is shown in Figure 5. The cold air is delivered
through overhead duct. Since we have a partial barrier and not a complete isolation between cold supply
and hot return air, hot air is allowed to seep into the
cold aisle, which can be seen in the temperature profile
of inlet air to the IT equipment. A low temperature of
about 60°F (15.5°C) near the bottom of the rack and a
high of about 80°F (27°C) near the top of the rack was
observed.
Figure 6 shows a thermographic image of the inlet and
discharge air to IT equipment. The average DT across this
IT equipment of about 27°F (15°C) is representative of
newer IT equipment. However, the average DT across all
IT equipment, including new and legacy compute, storage and networking IT equipment was about 18°F (10°C).
So we are only supplying about two-thirds of the primary
airflow from the air-handling unit at low temperature
of ~59°F (15°C), and counting on about one-third of the
hot air to recirculate in the cold aisle. This allowed us to
undersize our AHUs and associated infrastructure by onethird. It also reduced the fan power requirement. As mentioned earlier, free cooling is available for more than 74%
of the time in this cold climate and trim cooling is needed
for the remaining time. But the overall operating energy
consumption is quite attractive as shown in Table 1 with
partial cooling-only PUE of less than 1.06 in winter and 1.10
annually.
Trim cooling with chiller. The placement of cooling coils as shown in Figure 3 in the air-handling units

ASHRAE Journal - October 2014

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