Data centers are projected to consume approximately 73 billion kWh in 2020, according to a U.S. Department of Energy report. Facility managers must continually seek energy efficiency tactics that make the most of that 73 billion kWh and trend toward the desired PUE of 1 — especially with cooling, which can take up nearly 40% of data center energy usage.

Both rear door and traditional methods offer viable cooling solutions for data centers. But when is one better suited for your facility over the other, or when should you employ a hybrid of the two? In this article, we will explain the computer room environment characteristics that facilitate greater success with each cooling system — leading to greater efficiency, reliability, and ROI for your data center.

Definitions of cooling systems

Before we dive into the comparisons of these two systems, first a brief primer of what makes up these data center cooling options.

What is rear door cooling?

Rear door cooling brings the cooling unit directly to the cabinet and cools at the rack level (instead of at a room level, as with CRAC units). The individual cooling unit, also referred to as a rear door heat exchanger, or rear door cooler is installed on the back of the cabinet. Ambient air enters passively or is pulled into the cabinet with mechanical fans, then the hot exhaust air produced by servers passes over a heat exchanger matrix. Heat is then transferred, rejected to coolant and the resulting chilled air re-enters the room at a predetermined ambient temperature — so no hot air ever enters the room.

It should be noted that the capabilities of rear door heat exchangers can vary greatly across manufacturers. For the sake of our discussion today, we’re discussing the specific capabilities of the ColdLogik rear door cooling unit — which handles densities up to 135 kW (others range from merely 35 kW to 75 kw). We find it the most beneficial to reference the most capable iteration of the technology for our discussion comparing rear door cooling to traditional cooling. In the rest of this article whenever we mention rear rack door cooling, we’ll be referring to the ColdLogik rear door cooler.

What is traditional cooling?

Traditional cooling systems seek to better manage airflow (and facilitate an efficient use of CRAC units) through hot aisle / cold aisle containment. Channeling air through the subfloor plenum is often a part of the system if the facility has raised flooring. Doors and panels create aisles and holes are plugged with blanking panels, grommets and gap seals, ensuring cold air swiftly reaches servers and hot air quickly returns to the CRAC unit.

When to use rear door cooling

Here are specific scenarios in which ColdLogik rear door cooling could generate significant results for your facility.

New build data centers

When designing a new data center, engineers and architects typically must consider raised flooring, ceiling configuration and room layouts that will facilitate efficient airflow. However, because rear rack door cooling units are outfitted directly on the cabinet, these complexities may be eliminated, not to mention the cost of sub-plenum air distribution. This reduces overall CAPex and streamlines your construction schedule. In addition, designing a new build data center with a ColdLogik rear rack door cooling system can free up to 60% more computer room square footage — no need to set aside space for one or more CRAC units, depending on redundancies — freeing up more room for servers.

Legacy data centers with areas of high density

In a 2018 survey of data center operators from the Uptime Institute, 20% of respondents said their highest density rack was 30 kW or higher, suggesting that density extremes in facilities are escalating. These high-density racks — such as those holding servers with GPUs or video processing — produce more heat than CRAC units and airflow management can handle. Legacy data center managers can install rear rack door units to spot cool these cabinets and efficiently address the wide range of power densities.

High density data centers

When kW density starts to climb above 20 kW, rear door cooling should absolutely be considered. These units allow up to 135 kW of heat dissipation per cabinet and are built to withstand the demands of high-density cooling.

Need for improved energy efficiency

While ColdLogik rear door units can handle extremely high heat loads, you don’t need to be concerned about a proportionally high power draw to meet those cooling needs. On the contrary, the system operates at an impressive rate of efficiency — as high as 98% energy efficiency. It’s able to achieve this through:

• Higher water temperatures. Up from 72°F, to be exact. The higher water supply temperature ensures operation above dew point, removing the need for condensate trays and pumps and saving further energy. Facility managers could even eliminate chiller use completely, “by having the [rear rack door] device reject heat using indirect evaporative cooling in a cooling tower,” according to the U.S. Department of Energy.
• Liquid heat capacity > air heat capacity. Just by virtue of not having to deal with air circulation in a room. Once heat has been transferred to a liquid, it can be removed from the data center more effectively than if it were transferred to air.

Scalability for future capacity increases

Though rear rack door cooling does function well in high density situations, it can also address concerns for scalability in cooling capacity. As you plan to add more servers to your computer room environment, that will mean adding more cooling capacity. ColdLogik rear door cooling units can be deployed in a staggered format and increased along with capacity. For instance:

• For low duty cooling (up to approximately 5 kW per cabinet), units can be installed every third or fourth cabinet.
• For medium duty cooling (up to approximately 12 kW per cabinet, a unit can be installed every second or third cabinet.
• For high duty cooling (above approximately 12 kW per cabinet) a unit can be installed on each cabinet.

This also saves facility managers the significant cost of deploying more CRAC units before they’re needed and instead scaling up the cooling system as the density grows.

Considerations for rear door cooling

Facility managers should note that to employ ColdLogik rear rack door cooling, it’s best that your facility operates on water-based cooling (and not refrigerant-based cooling). You could ostensibly switch your system over, but the cost of switching your entire data center’s cooling base would likely not justify it.

When to use traditional cooling

Containment and airflow management eliminate bypass airflow and ensure a high degree of efficiency for CRAC units; here is when traditional cooling will work best for your facility.

Legacy data center with success using containment

There’s something to be said for a known quantity with proven results. Containment is a tried-and-true solution for data centers and if it has worked well at your facility, staying the course with equipment you’ve installed and systems you’ve engineered is a viable option. Additionally, containment is a proved method to ensure your CRAC units run with greater efficiency, leading to longer lifespan, greater reliability and energy savings.

A best-fit cooling solution for your facility

At the end of the day, data center cooling is about finding the right solution to fit your facility and address your computer room’s unique needs. You’ve already taken the first step in learning about the advantages of various data center cooling systems — putting you closer to achieving a lower PUE and reducing energy costs. Still have questions about the differences between rear door and traditional cooling, or want to talk with a data center cooling expert about your facility’s specific needs? Start a conversation with a Sealco representative today.

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