Read part two of this tip on overlooked problems with containment solutions.
As power and heat densities continue to rise, cooling hardware becomes more difficult, and the importance of energy efficiency and air containment solutions becomes evident. This tip explains the design issues of typical hot- and cold-aisle approaches, and introduces the important concepts and tradeoffs of containment.
Overcoming hot- and cold-aisle design issues
Hot- and cold-aisle design was developed to lessen the occurrence of hot and cold air mixing. It was a big step forward, allowing air to be directed to and from thermal loads more efficiently and making it possible to cool higher-density loads. But as heat output grew, some of the previous air-mixing problems started creeping back in. The major cause was easy to identify and solve–open spaces.
If there are open spaces, hot air can’t be kept in the hot aisle, and cold air can’t be kept in the cold aisle. Therefore, all the openings in cabinets need to be blocked. Blocking the holes keeps hot air from re-circulating back through the rack spaces between computers and non-contiguous cabinets, and also keeps valuable cold air from bypassing the equipment through those openings. The use of blanking panels in unused rack spaces is still the most ignored principle in the industry, and it leads to a great deal of ineffective data center cooling and energy waste.
But there are two other factors to consider: Heat wants to rise, and fans will take the air they want from wherever they can get it. As heat densities increase, hot air spills over the tops of cabinets and back into the servers. As equipment needs more air, the fans also pull it around the ends of rows and back into the cold aisles. The obvious solution is to put barriers in those pathways as well, block the ends of aisles with walls and doors, and put a ceiling over the aisles at the tops of cabinets.
Voila–the hot-aisle or cold-aisle is now contained! Hot air is now completely trapped in the hot aisle so it can’t escape, and cold air is contained in the cold aisle so none of it is wasted. Seems simple enough, but let’s examine further.
The two containment types
So why do we even need to decide between hot-aisle or cold-aisle? Why not just contain both aisles and run the rest of the room on building air? It has been done, but it creates a lot of unnecessary work and expense. Take the time to decide which method is right for you, understand the potential problems and consider the benefits.
Hot-aisle containment is generally accepted as easier to implement than cold aisle, and it has a small advantage in energy efficiency. Proponents note that the rest of the room has the same comfortable environment as the cold aisle, which actually doesn’t need to be “cold” anymore, and may well be 75 degrees Fahrenheit (23.9 degrees Celsius) or as high as 80.6, according to ASHRAE’s revised Thermal Guidelines for Data Processing Environment.
With so much of the room at a reasonable temperature, equipment fans can draw air from wherever they need it. Therefore, while we should always endeavor to deliver sufficient air to the hardware, it is not as critical to control as it is in cold-aisle containment.
The major drawback usually cited with hot-aisle containment is the working environment within the hot aisle, which can reach 95 degrees Fahrenheit or higher. This is not a comfortable working condition for extended periods of time but, contrary to popular belief, it does not exceed OSHA standards. To temper the heat, some designs actually introduce a little cold air into the contained area to keep the temperature within reasonable limits. Obviously, this offsets some of the efficiency gains of the contained solution, but it is certainly more “worker friendly.”
The ability to reach higher return-air temperatures is actually what improves efficiency. Air conditioner coils deliver more cooling capacity when they are presented with higher-temperature air. Table 1 shows examples of this capacity increase for several common air conditioner sizes. If containment is complete, the hot air can only return to the air conditioners via the physical path that is provided, which maximizes its temperature. This can be accomplished with very large ductwork, but it’s more common to just use the space above the ceiling, known as the “plenum.” A few cautions: The above-ceiling area should be dirt-free, and the ceiling tiles should be sealed on the back so they don’t flake off, or you’ll be changing filters way too often. A number of large grills are required in the hot-aisle ceiling for the air to easily pass through.
Table 1: Typical CRAH capacity ratings with different Return Air Temperatures.
|NOM.||kW CAP||kW CAP||kW CAP|
One of the biggest advantages of cold-aisle containment is that with either under-floor or overhead data center cooling, the aisle tends to fill up with cool air and hot air is prevented from creeping in. This ensures that all available cold air is delivered to the equipment and also minimizes temperature differentials between the upper and lower parts of cabinets. Cold-aisle containment can be particularly advantageous with below-floor air delivery, because cold air falls, making the under-floor air supply fundamentally contrary to the laws of physics.
When cold air is pushed up through floor-tile openings, it will only rise to a certain height unless something propels it higher. The fans in the computer hardware usually pull that air up and in, but it still gets warmer as it rises. However, if we can fully contain the cold aisle, the air within that aisle tends to stabilize much closer to its “delivery” temperature, from floor to ceiling.
As previously noted, the cold aisle doesn’t need to be the chilly 55 degrees Fahrenheit that we’ve seen it at for years. With the ASHRAE upper limit of 80.6 degrees Fahrenheit (27 degrees Celsius), you could pick a very comfortable cold-aisle temperature of around 75 degrees Fahrenheit and be very safe. This temperature allows you to increase the set points on your computer room air conditioning (CRAC) units, which saves a lot of air conditioner energy. Opponents point out that the rest of the room is now essentially a hot aisle, which could be 95 degrees Fahrenheit or more, making everything uncomfortable except the contained cold aisles.
The real challenge of cold-aisle containment is air balance and control. Computer equipment requires a certain amount of air to keep cool. When the only air available is the air delivered to the contained cold aisles, you need to ensure that it’s sufficient by adjusting perforated or grate floor tiles, or overhead grills. This assumes the air conditioners can actually deliver all the cool air the computers need. One could simply “open the flood gates,” and push as much cold air as possible into each cold aisle, but that invites other problems. You may air-starve other areas of your data center by serving some aisles at the expense of others, or you may have to install more air conditioners than you really need to make up the air volume, which is an expensive task that wastes energy. You also might over-pressurize the cold aisle, forcing more air through the computers, as well as through any open spaces between panels and cabinets, which wastes cold air, reduces the hot-aisle temperature and degrades CRAC efficiency. In short, pushing too much of your precious air into the aisle can be counterproductive.
ABOUT THE AUTHOR: Robert McFarlane is a principal in charge of data center design for the international consulting firm Shen Milsom Wilke, McFarlane has spent more than 30 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professional program, is a data center power and cooling expert, is widely published and speaks at many industry seminars.