air handling units ahu | data center cooling mission critical

7/21/2019 Air Handling Units AHU | Data Center Cooling Mission Critical

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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.



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.

https://www.greenhousedata.com/



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



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.



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

http://www.emersonnetworkpower.com/en-US/Pages/Default.aspx



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].

mailto:[email protected]?subject=Air-Handling%20Units%20for%20Data%20Centers

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air handling units ahu | data center cooling mission critical

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