managing airflow at the raised floor level
*This article originally appeared in Mission Critical Magazine as Part Four of our four-part series on Containment Best Practices.

This series combines education, design tips, and overall best practices for aisle containment projects in mission critical spaces.  Each of the three previous articles addressed one of the “4Rs” of airflow management: rackrow, and room. In this article, the spotlight’s on the raised floor.

There are a number of considerations that factor into selecting the proper raised floor system for data centers and other mission critical spaces, including the support structure, the type of panels that will sit on top of that support structure and how they will be constructed, the depth of the subfloor plenum, and the weight load of the equipment that will be housed on the floor.  But, there are still a few more factors that must be considered in order for the floor to play its role in a properly functioning aisle containment design.

Before jumping into cost-effective raised floor suggestions, remember the goal of any airflow management initiative is to improve the intake air temperatures to IT equipment.  More specifically,  reducing the highest intake air temperatures so all intake temperatures are as low and even as possible.  By doing this, temperature set points can increase, fan speed can decrease, and cooling units can sometimes be powered off. Making these changes are key to improving efficiency, increasing capacity, and lowering operating costs. While this article focuses on raised floor best practices, airflow should be managed at all levels in the data center — rack, row, room and raised floor — to fully capitalize on all these benefits.

Block Open Spaces Underneath Cabinets

This row-level airflow management technique also applies to floor-level improvements. Take a close look at the small space between the bottom of an IT rack and the top of the raised floor panels the rack sits on.  Although it’s usually only ½ to 2 inches in size, this space allows IT equipment exhaust air to travel under the rack and, ultimately, back into the IT equipment air inlets.  This air recirculation causes several problems for the data center: increased intake temperatures, hot spots, and the longer-term potential for IT equipment failure. If the air mixing is compounded across multiple rows of racks, more cooling units will have to run at higher fan speeds and lower set points to overcome this issue.  Blocking these open spaces with under-rack panels made of flame-retardant material is an easy and cost-effective way to minimize air recirculation and reduce IT equipment inlet temperatures.

Plug the Holes

Leakage at the rack level occurs when supply air bypasses the IT equipment and returns directly to the cooling unit without being used to cool the IT equipment.  This problem can be quickly fixed by installing blanking panels.  At the floor level, however, bypass airflow or leakage occurs when cold supply air comes through gaps and holes in raised floor panels in areas where it’s not supposed to.  Floor-level leakage can happen when solid panels have cutouts that allow for power and data cabling to enter a rack, if cut outs have been made around piping and conduit that penetrate the raised floor, if gaps have been left around the perimeter of the room (including where the floor panels meet the walls and gaps in the sub-floor perimeter), and when perforated floor panels have been placed incorrectly.

Just as there is a variety of sizes and types of gaps and holes that are found in raised floors, there is also a wide range of products on the market that can address each issue.  Fire-retardant foam blocks can be cut and shaped to fit into tight, oddly shaped gaps, and there are different sized grommets and “pillows” that can fill cut outs used for cable pass-throughs.  A best practice for floor panel cutouts is to standardize on a cut size that is appropriately sized — not too big — for the cabling that must pass through it.  Many grommet manufacturers offer standard sizes and templates for cutting access holes.

Perforated Floor Panels

Correctly implementing airflow management best practices at the rack, row, and raised floor level helps to properly match cooling capacity with IT load. Once that’s in alignment, room level adjustments can be made to fully realize energy efficiency, increased capacity, and other returns on investment.  At the raised floor level, the importance of perforated floor panels and their ability to deliver cold supply air into the cold aisle is high.

Raised floor systems in data centers are designed to work so cooling units pressurize the underfloor plenum with cold air. The amount of cooling and pressure required depends on many factors, but the supply needs to be sufficient so that enough cold air comes up through perforated panels in cold aisles in front of server racks to keep them safely cooled — ideally, without overcooling the entire space.

Data Center Raised Floor PanelsThere are many perforated airflow panel options available on the market today. It’s important to consider rack IT load densities in a given aisle, floor pressure, and the amount and direction of airflow through a given perforated panel design in order to achieve optimal cooling.  Perforated airflow panel variations can range from the standard 25% panel, which, as its name implies, has approximately 25% open space in the panel for air to flow through, to high-performance airflow panels, which allow you to direct more airflow toward the server racks, allowing higher-density racks to be safely cooled.  In addition to airflow performance, considerations for airflow panel selection should also include panel weight ratings, ease of installation into a given floor system, ease of moving panels as changes are made in the data center, and the ability to incorporate dampers to restrict or improve airflow through the panel as conditions change over time.  Not all airflow panels are created equally.

Many factors also come into play when determining the right type and number of airflow panels for a given design.  While a fairly straightforward calculation can be used to determine how much cfm is required to cool the IT equipment in one rack (and is generally a good place to start), real-world application often differs from calculated requirements.  Many factors, like plenum floor pressure, can vary across a room.  Just because an airflow panel is rated to provide a certain amount of cfm at a given pressure does not mean that all of the air coming through the panels necessarily makes it into the server rack to provide cooling.  This can be mitigated in part by containing the cold aisle, which helps reduce bypass cooling and ensures the only way the cold air can leave the aisle is through the server racks.

Underfloor Baffles and Diffusers

If an IT load (equipment rack footprint) sits in a small portion of the overall available whitespace, chances are there’s energy being wasted to pressurize the entire subfloor plenum just to provide cooling to that area. In these cases, you fire-retardant plenum-rated baffles can be attached to raised floor stanchions. This creates channels under the subfloor so the appropriate amount of airflow can be directed to IT equipment racks, and the AC units that were used to pressurize the rest of the space can be turned off or cycled down.

Oftentimes, a higher-density rack sitting near a perimeter a/c unit causes a hot spot.  Many in the industry were once under the impression that putting higher-density racks close to a/c units ensured the best volume and temperature of supply air to that rack. Today, most know that’s not the case.  In fact, the exact opposite typically happens. Airflow coming from that nearby a/c unit moves at such a high velocity that it usually bypasses the perforated panel directly in front of the rack and causes a reverse effect, pulling air back down through the panel rather than blowing pressurized air up through the panel.

If the higher load rack cannot be relocated to an area that can provide the required air volume and temperature, installing a diffuser panel under the floor and in line with the airflow direction from the a/c unit will improve the situation.  Diffuser panels can be mesh panels with varying percentages of free airflow. The panels create some resistance to the airflow, slowing it down and allowing some pressure to build up where the higher-density rack is located.

Monitoring

To truly gauge the effectiveness and efficiency of cooling and containment systems, monitoring solutions with alarm and notification capabilities must be deployed.  Measuring temperatures at the rack level helps data center operators fine-tune the controls to ensure rack temperatures remain safe without overcooling the space.  This should be considered a best practice in the data center space. Monitoring rack level temperatures also provides a good indication that floor pressure is sufficient and the selected airflow panels are providing enough cold air to server rack inlets.  Alarm thresholds should be set so that a rise in temperature can be caught and acted upon to prevent a loss of cooling at the local level, which can be caused by many factors.  Without basic temperature monitoring, it is almost impossible to determine the effectiveness of containment and airflow solutions in the data center space.

Enhanced monitoring options are also a powerful tool for data center operators. In a contained aisle, it can be beneficial to monitor differential pressure between the floor plenum and the contained aisle and/or inside the contained aisle and the rest of the room.  Without adequate pressure, enough cold air may not make it into cold aisle, or warm air can penetrate back into the contained cold aisle, degrading both cooling and efficiency.

In addition to temperature and pressure monitoring, it can also be beneficial to monitor humidity and air velocity in the data center space, along with catastrophic failure monitoring for things like leaks and smoke.  Choosing a monitoring platform that can allow for the flexibility of monitoring diverse applications and growth over time can be extremely beneficial for data center operators.

When selecting a monitoring system, several factors should be taken into consideration, including the ease of deployment, ease of integration to existing BMS or DCIM systems, and the flexibility to add additional types of sensors to the chosen system.  Further considerations include whether a wireless, Wi-Fi, or wired system is the best fit for the facility; the battery life of the wireless and Wi-Fi sensors; communication protocols available for system integration; sensor mounting options; communication range and range extender options; the number of sensors that can be used on a single system; and the upfront and long-term cost implications of the complete system.

Conclusion

Done in conjunction with rack-, row-, and room-level best practices, raised floor airflow management is an important and necessary step to achieve efficiency goals. Raised floor and rack-level tasks should be implemented at the same time, and both should be in place before aisle containment doors or panels are installed. Products that support raised floor airflow management best practices include under rack panels to block open spaces between the floor and the rack; fire-retardant foam, pillows, and grommets to plug holes in raised floor panels and around the perimeter of the floor; high-performance directional airflow panels that deliver the correct volume of air to the contained space; underfloor diffusers and baffles to help build pressure and flow in required areas; and monitoring solutions to send immediate alerts when conditions require attention or maintenance. Using these products together as a complete system will deliver the efficiency results provide peace of mind.