air handling units ahu | data center cooling mission critical
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Air Handling Units AHU | Data Center Cooling Mission CriticalTRANSCRIPT
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Air-Handling Units for Data Centers
Tomasz Wyszołmirski/iStock
Typical standard rooftop air-handling units (AHUs) simply are not designed to provide
the temperature control and redundancy demanded by mission-critical data-center
environments. For instance, over a 10-year period, a typical standard rooftop AHU may
run for about 30,000 hours, compared with a data-center cooling system, which may
run for 80,000 hours or more. Also, while typical service response time for traditional
comfort cooling systems is 12 to 24 hours, the criticality of data-center operations
demands a response time of no more than two to four hours.
AHUs designed for data centers, which until recently were available only for large
enterprise and hyperscale operations, offer a level of protection, reliability, efficiency,
and flexibility needed for these unique environments. They provide an opportunity to
reliably and cost-effectively manage heat in a facility.
This article will discuss four of the most common types of air-handling systems for
data centers:
Indirect evaporative free-cooling system.
Direct evaporative free-cooling system.Direct-expansion (DX) system.
Chilled-water system.
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FIGURE 1. Indirect evaporative free-cooling system.
Unit controls operate the scavenger fan according to the unit’s mode of operation,
controlling supply air to a user-adjustable set point. During cold ambient conditions,
when the unit is operated in dry mode, the controller adjusts cooling capacity bymodulating the scavenger fan to the desired leaving-air set point. The scavenger fan is
sped up to increase cooling capacity or slowed down to decrease cooling capacity.
When outdoor temperature rises and the unit is unable to achieve the desired set point
efficiently using only the heat exchanger in dry mode, the unit pumps are activated,
and water is sprayed over the heat exchanger. This increases the capacity of the heat
exchanger by bringing down the outdoor air to near the wet-bulb condition.
While this method requires a constant supply of treated water, no moisture from the
secondary air stream is transferred to the primary air stream. This means air
introduced back into the data center is dry, humidity levels are low, and no outside air
is required for economizing. Additionally, electrical infrastructure can be downsized to
realize a low initial cost per kilowatt.
Green House Data, a cloud-hosting, colocation, and managed-IT provider, recently
deployed four 300-kW indirect evaporative free-cooling air handlers in its facility in
Cheyenne, Wyo. The system uses 90 to 95 percent less energy than conventional
data-center air conditioners and has been instrumental in achieving the target PUE of
1.14.
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Direct Evaporative Free-Cooling System
In a direct evaporative free-cooling system (Figure 2), hot return air and outside air are
mixed by adjusting damper position on the unit controller. The hot, dry mixed air is
pulled through a filter and a water-saturated medium. Heat in the air evaporates the
water. The resulting moist, cool air then is released through a blower back into the data
center.
FIGURE 2. Direct evaporative free-cooling system.
Typically, a direct evaporative free cooler is a large unit deployed outside of a data
center. Often, a full containment system with higher supply and return temperatures is
recommended. A direct evaporative free-cooling system is well-suited for geographic
locations where the outside air is cool, humidity is low, and data-center managers are
willing to operate within wider temperature and humidity ranges. Because there is no
compressor, significantly less energy than with other forms of cooling is required, with
the water pump and fan accounting for most of the consumption. With this design,
electrical infrastructure can be downsized, resulting in kilowatt-per-unit readings lower
than those with other types of air handlers.
A direct evaporative free-cooling unit introduces outside air to a data center and
requires a constant supply of treated water.
DX System
Typically, a DX system (Figure 3) is a roof-mounted unit serving a single- or two-story
building. Often, it requires supply and/or return ductwork for air distribution. In basic
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terms, a DX system is characterized by the use of a thermal-management system
utilizing refrigerant to cool air directly, as opposed to utilizing a chilled-water loop to
facilitate heat removal between refrigerant and the ambient environment.
FIGURE 3. Direct-expansion system.
DX systems, which easily accommodate modular build-outs, rely on three primary
energy-consuming components: compressors, condensers, and evaporator fans. Two
heat exchanges—indoor air to the refrigerant and the refrigerant to outdoor air—take
place to remove heat. As such, this type of design often is capable of achieving a high
level of efficiency with the added benefit of a simplified infrastructure, typically with no
single points of failure.
Because DX systems typically are roof-mounted, a building’s roof structure may need
to be improved. Also, low ambient conditions can cause operation issues with DX units.
Chilled-Water System
A chilled-water system (Figure 4) can be a vertical unit along the perimeter or in the
gallery of a building that discharges into a raised floor or, in the absence of a raisedfloor, a horizontal unit in the gallery or outside of a building. It uses a facility’s chilled-
water systems as the cooling source for around-the-clock operation. The chiller cools
water, which is circulated through coils inside of the chilled-water AHU. As warm air
from the data center runs over the coils, heat is transferred to the water inside of the
coils, and the now-cooler air is returned to the data center. The warmer water inside of
the coils is returned to the chiller, where heat is transferred to a second stream of
water flowing through a cooling tower and expelled to the outside.
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FIGURE 4. Chilled-water air-handling unit.
This design operates with a high sensible heat ratio, ensuring proper humidity is
maintained. It is ideal for precise maintenance of data-center conditions and is very
efficient when paired with higher water-loop temperatures and water-side or air-side
economizers.
Indoor units typically are easy to service and commission. However, there can be a
significant upfront cost associated with the needed chiller plant, and as with direct and
indirect evaporative free-cooling systems, a constant supply of treated water is
needed.
Conclusion
Air-handling technology designed for data centers offers an opportunity to reliably and
cost-effectively manage heat in a facility. It is important that data-center and facility
managers understand the options available to them to ensure they implement the
design that will achieve the desired efficiency and PUE.
As vice president, North America marketing, thermal management, for Emerson
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Network Power , John Peter “JP” Valiulis is responsible for evaluating new technologies
and developing highly efficient and reliable controls and product solutions for mission-
critical applications. Previously, he held positions in strategic planning, new-venture
development, and product marketing with SPX Cooling Technologies, PepsiCo, and
The Walt Disney Co. He has a bachelor’s degree in economics from Georgetown
University and a master’s degree in business administration from The Wharton School,
The University of Pennsylvania.
Did you find this article useful? Send comments and suggestions to Executive Editor
Scott Arnold at [email protected].