The easiest way to cool a data center is to install an air exchanger that generates cool air and circulates it in the server room. But if you want to save money, at least in the long run, a better approach might be to install air exchangers on each rack and use those switches to cool the individual server racks.
The latter method, known as "rear rack cooling," offers several advantages over more traditional air cooling systems in data centers, particularly in terms of energy efficiency.
Let's take a look at how rear rack cooling works and why you might want to consider rear rack cooling as a way to improve your cooling methods in your data center.
What is Data Center Post-Rack Cooling?
Rear rack cooling in a data center requires the use of an air exchanger mounted directly behind the server rack to cool the servers in the rack.
In the rear rack cooling system, each server rack is equipped with an air exchanger. While the rack is running, the air exchanger blows in cold air and expels hot air.
Rear rack cooling with CRAC as well as direct chip cooling
Rear rack cooling differs from traditional data center air cooling systems (known as "computer room air conditioning" or "CRAC") in that instead of blowing cool air across the entire server room, rear rack coolers target the equipment that generates the most heat.
Conceptually, rear rack cooling is somewhat similar to a direct chip cooling system, which circulates liquids around the CPU or other heat sources within the server. This is another way to focus heat dissipation on devices that generate most of the heat.
However, direct chip cooling systems are costly because they require special coolant and circulators. If you want to direct cooling resources to the heat source, but don't have the budget for direct chip cooling, then rear rack cooling is a great option for you.
Benefits of rear rack cooling
The main benefit of rear rack cooling is that the cold air can be used more efficiently. In a CRAC system, most of the air blown into the server room is wasted because there is no contact with the equipment that actually generates heat. In contrast, the rear rack cooling system targets servers in a more localized way, resulting in less waste.
Back-rack cooling can also improve data center energy efficiency, which in turn improves PUE metrics by turning down or shutting down air exchangers when servers in a single rack are underloaded or inactive.
This option is not usually available in CRAC systems. If you have any servers running, then the entire CRAC system probably needs to be turned on, even if there is very little heat generated at times.
Finally, rear rack cooling is a cheaper cooling option than liquid cooling systems, at least in terms of upfront cost. However, liquid cooling is less expensive in the long run because it requires less energy.
Challenges posed by rear rack cooling
While there are many advantages to back-rack cooling, there are some potential hurdles when it comes to implementing it in the data center. Challenges posed by rear rack cooling include:
Need for more air exchangers: Since each server rack requires its own air exchanger, you have to install and maintain more switches than a CRAC system.
Lack of backups: In most cases, there is only one switch per rack cooled by the rear switch. If this switch fails, the rack may quickly overheat. CRAC systems are less prone to such failures because they can have multiple air exchangers to keep running in the event of partial equipment failures.
Energy use: While rear rack cooling has some energy-saving benefits, it is less efficient than liquid cooling systems, which require less energy, and rear rack coolers rely on energy-intensive fans.
Space usage: Rear rack coolers consume more data center space than CRAC systems, not only because of the air exchangers that must be connected to each rack, but also because of the pipes that power the switches. As a result, these systems leave less free space for IT equipment.
When should you choose rear rack cooling?
If you're looking for a way to improve your data center's energy efficiency, but don't have the budget or time to implement advanced cooling solutions such as direct chip cooling or immersion cooling, rear rack cooling is a great compromise that is more efficient and effective than traditional air systems that cool entire rooms, and is less expensive and a little simpler to install than liquid cooling.
On the other hand, if you focus on long-term outcomes for at least the next few years, liquid cooling may provide a better ROI than rear rack cooling because direct-to-chip cooling and immersion systems are more efficient.