Stop over-complicating data center airflow management

March 16, 2021

Simplify airflow management by knowing when to use passive versus active air.(By Geist, a division of Vertiv and provider of intelligent power and management solutions for data centers.)

Data center airflow management operates on a simple premise — IT equipment should only ever take in cool air, and CRAC return plenums should only ever take in warm air. Under no circumstances should there be a mixing of cold air and return air.

That’s it.

Yet, many data centers struggle to facilitate this dynamic, and at a high cost to their operations. Data center cooling is infamously expensive, accounting for 40% of annual data center spend by some estimates. Inefficient airflow exacerbates this problem by causing hot spots that are all too often addressed by increased cooling capacity.

We’re here to say, “No more!” Airflow management doesn’t have to be complicated if you adapt containment methods that are uniquely suited to expelling exhaust and keeping the cool aisle cool.

What Is Data Center Airflow Management?

Data center airflow management controls temperatures in and around IT gear to maintain and increase efficiency. Poor airflow can prevent cool air from reaching overheated components or cause warm air to remain trapped in one area. Without proper air circulation, your IT components and heating, ventilation and air conditioning (HVAC) systems will work harder than necessary to maintain ideal conditions. This problem can cause costs to skyrocket and stunt productivity. 

Data center airflow management addresses common problems by implementing solutions that control room temperatures, reduce fan speeds and create ideal air circulation channels. The primary areas of focus for data center airflow management are the floor, racks, rows and the room itself:

  • Floors: Floors present unique challenges for controlling a data center’s temperature. Standard floors often inhibit airflow, while raised floors can leak air in unwanted locations. 
  • Racks: The racks holding your servers and other equipment need room to circulate air, but allowing too much space can create pockets that trap air before it can escape. 
  • Rows: Situating multiple cabinets into rows is ideal for maximizing spatial efficiency, but for optimum temperature control, it’s necessary to separate aisles by hot and cold air. 
  • Room: Actively controlling the temperature in a data center can maximize the effects of airflow management techniques. 

Common Airflow Solutions

Managing data center airflow involves regulating the circulation of both cold and warm air. Airflow management’s primary goal is to contain cold and warm air to desired locations while providing channels for cool air to reach overheating equipment and for warm air to disperse. Floors, cabinets and temperature-separated aisles are all central battlegrounds for both cold and hot air regulation. Here are a few standard tools used to solve airflow problems: 

  • Brush grommets: Circulating cool air over hot equipment is effective, but a lot of air can escape through the crevices that allow cables into the room. Brush grommets enable cables to pass through while sealing cold air where it needs to be. 
  • Curtains: Sealing off hot and cold cabinet aisles will maximize the effects of any airflow regulation technique. Plastic curtains, blankets or other heavy drapery items are easy to install and produce noticeable results. 
  • Rack chimneys: Hot air rises, and without an avenue to escape from, it will sit on top of IT equipment and complicate cooling efforts. Chimney structures provide an escape route for rising warm air, funneling it into HVAC ducts and out of the building. 
  • End of row doors and aisle ceilings: Structures like doors and ceilings enclose hot and cold aisles to contain air in one space, allowing cool air to ease efficiently overheated equipment and hot air to circulate into HVAC ducts in a controlled manner. 
  • Filler panels: When empty spaces along an aisle of cabinets go unfilled, the gaps cause inner-rack recirculation. This process allows temperatures to fluctuate, reducing the effects of containment stand circulation efforts. Filler or blanking panels prevent hot and cold air from transferring to unused spaces, making it easier to manage airflow.

Help your return air rise

When cool air passes through the IT load, a heat exchange occurs. Cool air becomes hot air, which is expelled into the hot aisle behind cabinets. Here, it will ostensibly rise into return plenums to be treated once again by CRACs. Then, it can be cycled back into the cold aisle through perforated tiles and drawn in by server fans — and so it goes.

However, there are two problems with this theory:

  1. It only works if an adequate supply of cool air passes through the IT load, as opposed to passing around it or never reaching it at all.
  2. It operates under the assumption that warm air in the hot aisle will automatically rise to the return plenum. However, that only works if zero pressure is maintained as treated air passes through the IT load. This process isn’t possible if hot air seeps into the cold aisle because it isn’t properly contained, or because there’s a shortage of cool air being taken in by server fans.

Often, traditional hot-aisle, cold-aisle setups will experience hot spots resulting from this combination of problems — bypass airflow and re-circulation air, respectively. This issue is especially true for high-density racks that generate more heat, particularly for servers positioned higher up and therefore farther from the perforated tiles on the ground. Given these circumstances, how do you simultaneously ensure that servers get enough cool air and hot air is expelled?

Airflow management doesn't have to be complicated.Airflow management doesn’t have to be complicated.

Abandon your hot-aisle aspirations and use active containment chambers instead. The most effective way to balance airflow in high-density racks susceptible to hot spots is to directly pipe return air into the ceiling plenum through a containment chamber placed atop each rack. Small fans that respond to pressure changes are embedded within those chambers. For example, if there’s a shortage of cool air reaching the IT load at any given moment, those internal fans increase RPM to maintain optimal air intake and outtake pressure.

It sounds complicated, but the premise is simple. Cool air goes into the server fans, and hot air comes out the other side and is sucked in through the containment chamber. Then, it returns to the ceiling to become cold air again.

Install Fans in Front of Your ToR Switches

“Install fans in front of the switch that will route cool air into the equipment’s air intake.”

It’s quite possible that certain parts of your data center or server room have low-density racks that don’t necessarily warrant an active containment setup. In these cases, passive containment should suffice — with one notable exception. Many top-of-rack network switches are configured backward so connectors face the maintenance aisle. This arrangement can cause airflow reversal — back-to-front as opposed to front-to-back. This problem is compounded when ToR switches are farthest from the cool-air source in a raised-floor data center setup.

Under these circumstances, it might not be worth it to implement active containment for every cabinet. The more cost-effective option is to install fans in front of the switch that draw in cool air and route it into the equipment’s air intake. This setup ensures a steady supply of treated air passes through equipment, regardless of its orientation on the rack or its distance from the cool-air source.

Who said airflow had to be complicated?

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