Perimeter Cooling vs. In-Row Cooling

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Perimeter cooling vs. In-row coolingDespite what many predicted, the adoption of cloud computing has not eliminated the need to maximize rack densities and pack as much computing power as possible into small footprints. In many data centers, the compute power demand per rack is growing, not shrinking, and the cooling demand is increasing. 

Many facilities are aging buildings with data centers that were built into space originally designed for another purpose.  These retrofitted spaces present extra challenges when trying to cool data centers to ideal temperatures. As data center operators evaluate perimeter cooling vs in-row cooling when making changes in their space, it’s worth considering whether the existing cooling infrastructure is still appropriate. In this article, we will consider the utilization of perimeter room cooling versus in-row cooling and a combination of the two methods.

Perimeter Cooling

Simply put, perimeter cooling is just as it sounds in that cool air comes from a single CRAC installed against a perimeter wall of the room. These data center air conditioners are strategically placed to blow cold air down the data center rows that have the IT equipment intakes – or cold aisles.  The cold air then blows through the racks and exhausts out the back of the enclosures to return to the CRAC intake for re-cooling – hot aisles.  This Hot Aisle/Cold AIsle data center design has been the gold standard for years and is moderately efficient relying on airflow to keep IT equipment operating at an ideal temperature.

Perimeter cooling designs use a single, stand-alone unit which has fewer parts than a data center configuration with multiple in-row units. Therefore the ongoing cost of maintenance may be less for a perimeter cooled data center versus a data center design that includes several in-rows.  However, the overall energy cost of perimeter cooling may be higher because of the lower thermostat setting that may be required to circulate enough cool air throughout the entire data center space.

CRACs used in perimeter cooling are often oversized because the initial installation needs to account for future expansion of the data center.  This can cause some inefficiencies for improperly sized units.  Similarly, perimeter cooling can cause an uneven distribution of cool air, which can be exacerbated as rack height and/or server density increases. This leads to hot spots in the data center. The effect of this uneven temperature regulation is that cooling must be set at a much lower temperature, increasing your energy costs. 

While perimeter cooling might have drawbacks in a traditional data center, it is the preferred cooling system for IT equipment in a gray space environment where a Rugged UPS, telecom racks with DC plants, or other temperature sensitive equipment may be installed.

Benefits of Perimeter Cooling

  • Ideal for gray space environments and other non-traditional data center space
  • Perimeter systems that use a cold-aisle/hot-aisle orientation to direct airflow can be moderately efficient
  • Replacing an aging perimeter cooling unit with a like-for-like replacement can re-use most of the existing cooling infrastructure, where converting to an in-row cooling design can result in significant installation costs as piping and outdoor units need to be re-configured and moved.


In-Row cooling

In-row cooling systems place the AC units between the data center racks which brings the cold air closer to the IT equipment intake and reduces the need to cool the entire data center room. This increases the overall efficiency of the cooling system. Ideal for data centers of all sizes, in-row cooling units allow for higher rack density and boast a smaller overall footprint than perimeter cooling units.  This leaves more room in the data center for IT equipment which is a big plus for edge computing or micro data centers.

In-row cooling designs generally have fewer hotspots than perimeter cooled data centers because in-row units can be placed just about anywhere in a given data center row.  However, as IT equipment gets moved around in a data center, hot spots can result if the adjacent in-row unit is over taxed by high density racks that weren’t part of the original data center design.

Cooling redundancy is also easier to achieve since in-row data center cooling designs generally include multiple units.  If a single unit goes down, there are others in the room generating cold air which will slow the overheating of a data center.  The only redundancy option for perimeter cooling designs is another CRAC for 2N which is expensive to procure and maintain.

Similarly, data centers that use in-row cooling are more scalable as additional in-row units can be added as data center racks or even rows are installed.

Benefit of In-Row Cooling

  • Cools more reliably and consistently than perimeter cooling systems 
  • Requires less space than perimeter cooling
  • Greater flexibility for placement of the condensing unit 
  • Scalable and redundant allowing data centers to add more racks or increase density
  • Increased energy efficiency saves on energy costs

Existing data centers may have specific constraints, such as a unique configuration of the data center floor plan, a current cooling infrastructure that is struggling to maintain an ideal temperature, or a lack of extra floor space to grow the data center. If your current perimeter cooling system is working too hard to cool your space, consider adding in-row cooling in the areas that are hardest to cool. By using a system that combines both perimeter and in-row cooling, you may find that your data center cooling is much more efficient.

The shift to higher density in small to midsize data centers makes in-row cooling options much more attractive. With a compact and versatile footprint, improved cooling efficiency, resiliency and reduced energy costs, in-row cooling solutions are ideal for the data centers of today.  If you currently have a perimeter cooling unit that is undersized or a data center hot spot that you just can’t get enough cold air to, consider adding an auxiliary in-row cooling unit to supplement the cooling from the CRAC.  Our engineers are able to perform a data center cooling analysis using Computational Fluid Dynamics (CFD) to analyze the airflow and temperature patterns of your existing data center as well as assist you with designing the next iteration of your data center cooling infrastructure.

For more information about choosing the
best cooling system for your facility,
call 800-876-9373 or email [email protected].