Many large-scale data centers take full advantage of the outdoor ambient conditions to provide cooling for servers and other critical network equipment.

Data centers and their cooling systems have been evolving for many years as engineers continually work on reducing energy costs and improving reliability and sustainability.  Advances in the reliability and tolerance to environmental conditions of data center equipment have led to many large-scale data centers taking full advantage of the outdoor ambient conditions to provide cooling for servers and other critical network equipment to lower the PUEs for these facilities.  Systems using air-side economizing and evaporative cooling have consistently delivered PUEs in the 1.14 to 1.3 range.

Data centers are a unique building application for air-side economizing.  The heat created by the data center equipment and the 24/7 operation allow air-side economizing to be the primary cooling source for a significant number of operating hours.  Depending on the location of the data center, air-side economizing can save 30% to 80% on cooling energy versus chiller-based cooling systems.  One problem that data center design engineers face in utilizing air-side economizing is the extreme temperature stratification that can occur in large air-handling unit (AHU) plenums when the warm return air from the hot aisles fails to mix with colder outdoor air that’s being drawn in to provide the correct supply air temperature. 

Temperature stratification can create significant cold spots that can cause freeze concerns for hydronic coils and negatively impact evaporative cooling components. Since many of these units are using large fan arrays, the stratified air can propagate through the supply air into the cold aisle and to critical data center equipment.  Data center design engineers often specify supply air temperature tolerances that cannot be met without proper mixing of the outdoor air and return air in the AHU.  These engineers have discovered that air mixing can significantly reduce temperature stratification and eliminate problems in plenums and in the supply air to the servers.

Static mixers have been used for over 50 years in AHUs with ventilation requirements.  Applications like health care facilities, schools, and entertainment venues use static mixers to avoid cooling coil freeze concerns, eliminate nuisance unit shutdowns caused by freeze stat trips, and extend useable hours of economizer operation to reduce operating costs.  As a static component with no moving parts, they are extremely reliable and made of corrosion-resistant materials.  Figure 1 shows stratified airstreams (represented by red and blue smoke) entering the static mixer. The mixing device creates downstream air turbulence to promote effective mixing and reduce stratification.

Static mixers are downstream of the outdoor air and return air entry points into the AHU mixing box or economizer section, and just upstream of filters and the cooling heat exchanger.  In many AHUs, air mixing devices are installed in a dedicated section.

In recent years, dynamic mixers have been introduced.  Dynamic mixers integrate outdoor air and return air control dampers for economizer control into a partitioning structure that allows mixing of these two airstreams at the point where the outdoor air and return air leave the control dampers.  Dynamic mixers provide the same freeze protection and extended economizer use as static mixers but require minimal downstream distance and eliminate the need for separate outdoor and return air dampers in the AHU.  When plenum distance is critical, these devices can save considerable length. 

Dynamic mixers are typically installed in the mixing box or economizer section of the AHU.  Outdoor air and return air control dampers typically installed in the walls and ceiling of the mixing box are omitted, leaving wall openings that align with outdoor air and return air compartments in the dynamic mixer.  The frame on the dynamic mixer seals against the wall/ceiling and floor of the section to allow all of the entering outdoor air and return air to flow through channels where control dampers direct airflow into the adjacent channel to promote mixing.  AHU mixers can be installed by OEMs in the factory or by technicians in the field.

Whether using static mixers or dynamic mixers, effectively mixing these air streams can ensure an even temperature profile of the supply air discharge from the air-handling unit to the cold aisles, eliminate operational problems associated with stratified airstreams in the AHU (including freezing and heat exchanger performance issues), and  allow full use of air-side economization that reuslts in significant energy savings and lower PUEs.