Data Center Airflow: Overhead vs. Raised Floor Comparison
When designing a data center, few decisions will have a greater impact on the planning process than determining whether to install an overhead or raised floor air dispersion system. The outcome impacts both short- and long-term costs, construction time, system maintenance, and the overall performance of the data center itself.
Data center best practices dictate that a data center has a clean layout that allows for potential growth, account for the data center's thermal needs, and consume energy efficiently.
Below, is a framework for the decision-making process, including an analysis of the most important factors and ramifications involved with this big choice.
What are overhead and raised floor air dispersion systems?
Briefly, while it seems self-explanatory, it is important to understand the key differences between the two HVAC options. An overhead system refers to installing air ducting above the server cabinets, and routing electrical wiring and cabling either above a drop ceiling or in trays below the real ceiling.
A data center raised floor system involves creating an elevated grid of metalwork on top of the concrete floor, and then installing flooring panels atop the grid. The space between the concrete and the flooring panels is where the cabling and electricity are installed and is also the space through which cooled air is blown.
Both options serve generally the same purpose in that they provide a space to house important elements of a data center’s infrastructure – one below the floor and the other above the cabinets. That being said, they differ greatly in almost every way that matters to engineers and data center owners/operators. With data centers getting larger by the minute, all these factors are being multiplied.
Factors that should drive decision-making
While both system types also account for the placement of electrical conduit and cabling, the largest consideration when deciding between the two approaches is air dispersion. This is because the air dispersion side of things will drive most of the differences in system cost and performance.
1. Efficiency
The number one job of an air dispersion system is to keep the servers cool to ensure equipment stays in good working order and operates at peak performance. In short, the better a system is at placing cool air near servers, the better the servers are going to work and the faster they will perform.
Raised floor systems seal the flooring panels, flood the enclosed space between the “floor” and the concrete base with cooled air, and add perforations in the panels for the cooled air to escape. Frequently, return ducts are installed overhead, which pull warm air out of the environment and take advantage of its natural tendency to rise. There is not directionality or ducting in these types of applications, which can lead to situations where a higher gross volume of cooled air is required to make the system do its job . Airflow can also become restricted by the other elements installed beneath the floor, resulting in some areas becoming difficult to cool.
Overhead air dispersion systems rely on duct work above the ceiling panels to move air around the space. This duct work will then discharge the air at strategic points near the server equipment. The goal of these systems is to move air efficiently and to deposit it exactly where it is needed.
While there is more than one type of overhead system, these installations are generally more efficient in terms of how much cooled air is required to meet the data center’s temperature needs compared to raised floor solutions.
2. Cost
There are multiple variables to consider when calculating the costs of an air dispersion solution, including material costs, installation costs, and performance efficiency. As mentioned above, overhead systems tend to be more precise at delivering cooled air exactly where it is needed. As a result, servers can be kept cool and performant with a lower overall volume of air flooded into the system than raised floor solutions. As a result, utility bills tend to be lower. When it comes to upfront costs (both material- and installation-related), on average overhead solutions are more cost efficient. The metal grid installed below the floor panels in a raised floor air dispersion system is the main driver of the cost differences between the two approaches, both in terms of material cost and installation labor-related expenses.
3. Maintenance
Raised floor systems offer the ability to change floor tiles to increase or decrease airflow in front of equipment to keep up with changes to the space, like changes in configuration or upgrades to the equipment itself.
Overhead systems are often preferred for performing ongoing maintenance to electrical components or wiring, as raised floor systems require technicians to remove floor tiles for access. Again, raised floor systems rely on the sealed floor panels to provide a pressurized environment through which cool air can move. When those panels are removed, a loss of pressurization occurs that compromises the system’s integrity and performance for the duration of the maintenance work. The bigger the maintenance job is, and the greater the number of flooring tiles that need removal, the bigger the impact on system performance and related costs.
Another maintenance-related consideration is swapping out servers themselves. Raised floor systems are often not uniform in height throughout – there are raised walkways that are higher than the flooring below. This can be challenging to navigate with a dolly loaded with servers.
Data Center Air Conditioning System Trends
While considerations related to air dispersion have driven more raised floor data center installations in the past, improvements in technology have led to a recent rise in overhead system popularity.
Historically, the ability of raised floor systems to deliver cold air from beneath the floor and then draw air out of the environment as it warmed was more efficient in certain settings than overhead duct work that needed to push cool air down from above. Advances in airflow solutions for data centers in recent years have flipped that dichotomy, however, and now overhead designs are more efficient in most applications.
Options within overhead
Many of the advances that have pushed overhead systems ahead of raised floor are related to the duct work designs and materials themselves. One of the historical inefficiencies of the metal duct work in traditional overhead systems was the need to use diffusers to deliver large volumes of cooled air down a long vertical pathway to the base of server racks. The problem that arises in these systems is that the air is delivered to its destination at a high velocity, which creates mixing and turbulence in the space. In essence, the air does not stay where it is deposited. Data center barriers and containment systems designed to mitigate this problem are only partially effective and add to a project’s overall cost.
Fabric ducting systems have eliminated this inherent problem with overhead systems through their ability to control air dispersion and directionality with adjustable nozzles – think less mass dumping of cooled air and more a controlled application. As a result, fabric can distribute the same quantity of cooled air as metal duct work, but at a lower velocity to prevent mixing, leading to better efficiency and an advantage for overhead systems over raised floor designs.
Fabric ducting systems have eliminated this inherent problem with overhead systems through their ability to control air dispersion and directionality with adjustable nozzles – think less mass dumping of cooled air and more a controlled application. As a result, fabric can distribute the same quantity of cooled air as metal duct work, but at a lower velocity to prevent mixing, leading to better efficiency and an advantage for overhead systems over raised floor designs.
Conclusion
Raised floor designs have historically comprised many data center installations, driven largely by air dispersion performance and ease-of-access to cabling and electric components. This was particularly true for larger data center footprints, as the physical space limitations and initial costs presented by raised floor projects were mitigated to an extent.
In recent years, however, overhead system designs have become more prevalent , as advances in air dispersion technology have led to more performant systems than raised floor designs, often accompanying significant cost savings. See our projects with Liquid Web, Involta-Cedar Rapids and Involta-Northpointe to learn how fabric air dispersion systems increased temperature efficiency in their data centers.