Data centers must be cooled, but designing a new facility or changing an existing one to maximize cooling efficiency can be a mammoth task. Any number of different design strategy or floor layout variations can affect the results, thereby changing efficiency, creating hotspots or altering the amount of infrastructure required for the design. Computational fluid dynamics (CFD) offers a method of evaluating new designs or alterations to existing designs before they are implemented. As with any simulation tool, however, maximizing the benefits requires a careful balance of budget, effort and understanding of the process.
What Is CFD?
Air, like water, is a fluid that moves in response to temperature and pressure gradients. Obstructions of various types (walls, floors and ceilings, server racks and so on) change the way the air flows, and as the number and complexity of these obstructions increases, so does the complexity of the analysis of how air flows. Add to that heat sinks (air conditioners, for instance) and heat sources (servers) and you have a very complicated problem that you can’t analyze using pencil and paper.
If your data center relies mainly on air to cool IT equipment, you’ve probably struggled with “hot spots” and other air flow problems as you try to ensure that your servers don’t overheat. In an existing data center, identifying regions that are insufficiently cooled is as simple as installing temperature sensors and monitoring the data. But what if you want to know what cooling problems might crop up in a new design or a reconfiguration of an existing data center? That’s where CFD comes in.
CFD takes a computer model of a data center and, using numerical techniques, calculates a steady-state airflow “map” of the facility, revealing locations that are insufficiently cooled or that are contributing to cooling inefficiency. Thus, CFD provides insights into challenges that might be posed by a new data center design, a rearrangement of a facility’s layout or another major change.
So, When Do We Start?
On its face, CFD sounds like an indubitably helpful tool, but as you might expect, there are qualifications. Obtaining a CFD analysis—whether on your own computers using purchased software or by way of an analysis firm—costs money, and the potential benefits should not be outweighed by the investment. Here are some essential considerations if you’re looking into CFD as a method of refining your data center design.
- Outsourcing or do-it-yourself—The first thing you must decide is whether you want to tackle the analysis yourself or hire a professional consultancy to do it for you. CFD involves a steep learning curve that may cost you heavily in terms of both time and results. Numerical techniques are fraught with pitfalls that can greatly affect the accuracy of results, and unless you plan to use CFD extensively (i.e., more than just once), you are likely better served by hiring a professional to do the work for you.
- Budget—Depending on whether you’ve decided on a consultancy or an in-house analysis, your costs will vary. Obviously, a consultancy will charge you for its expertise as well as to amortize its own costs (software, computers and so on), and you receive only results for a single design. (Some consultancies will work with you to some extent to optimize the design or to otherwise perform several analyses, but ultimately, you are paying to get just one set of results.) For an in-house analysis, the obvious cost is the software, which can vary depending on number of licenses and so on. Furthermore, you can either buy the software outright or, in some cases, use a cloud-based “pay-as-you-go” model. Less obvious costs are computer usage and employee time—the learning curve has its own price!
- Return on investment—CFD is not a solution for everybody. If you just want to find hot spots in your existing data center, you don’t need CFD, you need data center monitoring equipment. CFD, like any computational (and, particularly, numerical) technique, can suffer from any number of inaccuracies, and it is best used as a predictive tool when actual measurements are impractical. Be sure that investing in a CFD analysis—whether in-house or through a consultancy—has enough potential returns to justify the costs.
Some Pitfalls of CFD
If you’re convinced CFD is right for your data center project, you must be aware of several pitfalls that can crop up in a CFD analysis. First, you’re dealing with a numerical technique, not an analytical one (i.e., CFD doesn’t find the “equation of your data center”—it breaks your data center into tiny pieces and analyzes each piece in light of the pieces around it). Furthermore, the accuracy of the results is limited by the accuracy of the model you supply: every unfilled cable hole or obstruction in an aisle can affect airflow, so the more details you provide, the better your chances of receiving helpful results.
Second, if you’re using software on your own, you shouldn’t just trust everything the software tells you. One of your most important tasks is to gain some insight into how the algorithm calculates results, as this will help you interpret the analysis correctly. (Avoiding such “extra work” is one of the advantages of hiring a professional CFD firm.) Details of your model that seem perfectly reasonable to you could cause the software to return strange results, simply because of quirks in the algorithm.
Third, CFD is attempting to solve an extremely complex problem in a manner that simply cannot take into account every variable, so regardless of whether you perform the analysis in house or hire a consultancy, you should take the results with a grain of salt. This is not to say that CFD analysis is inaccurate, but not every detail of the analysis will correspond with the reality of your newly built facility.
As data centers consume more energy (which is converted to heat that must be removed from the facility) and energy prices continue to rise, the potential return on a CFD analysis increases. Long-term energy savings can greatly outweigh the costs of the analysis, whether through a consultancy or via the in-house route. For instance, if you are able to identify potential hot spots and take steps to eliminate them in your design, you can reduce the required cooling infrastructure, saving capital cost. Furthermore, your facility can operate at a higher temperature, saving operating costs. And when your data center operates at a higher temperature, you increase your opportunities for free cooling, which bypass traditional mechanical cooling methods and save even more cost. The key to successfully using CFD is understanding the purpose and limitations of the technology as well as your own company’s needs.
Photo courtesy of Rob Bulmahn
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