Read part one of this tip on containment solutions for data center cooling.
In all modern data centers, we want to use variable-speed fans with variable-frequency drives (VFD) and,
The challenge is what to measure to control the VFD’s, and where and how to do it. Logically, you’d monitor the air pressure in the cold and hot aisles, looking for a small differential between the two, and then control the air conditioner outputs to maintain a slightly positive pressure in the cold aisle. This method was tried, but it was found to be more difficult to accomplish than expected.
Highly-sensitive pressure sensors are expensive and difficult to keep in calibration over long periods of time. Therefore, the more accepted method is to create a small opening between the hot and cold aisles and measure the temperature there. If insufficient air is being delivered to the cold aisle, the computer fans will create negative pressure, pulling some hot air back through the opening. Temperature probes will sense the temperature increase and cause the air conditioners to increase output. This method of control makes cold-aisle containment less problematic to implement than it originally was.
This brings us to the two most overlooked aspects of creating a containment environment: lighting and fire protection.
Lighting is often poor in a data center, and creating a contained space can add to the challenge. If the ceiling is the same height as the top of a rack, it’s probably too low for most light fixtures. Raise the ceiling panel and you will need air barriers that extend from the tops of the cabinets. In a retrofit, you will already have lights in the room. Will they be useless, in the way and energy wasters? Will they need to be removed or re-circuited so they can be shut off without darkening other uncontained parts of the room? Some containment solutions have used transparent or translucent ceilings to let in existing light. That saves construction costs, but reduces light level, especially when the panels get dirty. If you need brighter bulbs to make this work, that’s more energy used. And how do you access and re-lamp when the fixtures are above the containment roof? Lighting can be a nuisance to deal with, and perhaps a pricey issue as well, but there’s a much more serious problem -- fire protection.
The two most common fire-protection systems in modern data centers are pre-action sprinklers and inert gas. Sprinklers are almost always required by code and/or insurance, even if you have a good gas-based system. The conventional wisdom is that water protects people and buildings, and gas protects sensitive hardware. Therefore, you’d have either one or two sets of special heads to dispense the fire suppressant into the room. These heads should be carefully located by a licensed fire protection engineer in the original room design to provide proper (and legal) protection. That means not only covering the floor area, but ensuring that the water gets into the aisles between cabinets, and that the gas will very quickly fill every cubic foot of the room.
If I totally enclose the aisles (closing off both ends and using a roof), I’ve actually created a number of small rooms within rooms. Therefore, more heads will be needed within the contained spaces for whichever fire systems are used. In a new facility, there’s a cost associated with this extra piping. In a retrofit (existing aisles are turned into contained aisles), the cost to revamp fire protection can be considerable, but the biggest concern is the danger of doing heavy plumbing work over active computing hardware.
Each contained aisle also needs dedicated detectors in addition to those that cover the rest of the room. Pre-action sprinklers and inert gas systems are normally armed only after two heads of different types have both detected smoke. In a new building, the separations created by containment might be used to create zones within each contained aisle, minimizing the area of water or gas discharge in the event of a fire. The expense to build multiple fire protection zones would be higher, but in a large facility it might be worthwhile. In a retrofit, however, it would likely be cost prohibitive. And if you use an aspirating smoke detection system, such as VESDA, then sniffer tubes also need to be located in each contained aisle. In short, contained aisles each require the same attention to fire protection as they would if they were in separate rooms.
Symplex Isolation Systems’ Airblock Curtain Systems attempts to address the fire protection issue by using flexible containment barriers between the ceiling and the tops of the cabinets. These barriers can be dropped out of the way in the event of a fire. The plastic curtains attach to the ceiling with fusible links that melt in a fire in the same way that sprinkler heads activate, dropping the containment barriers out of the way of the sprinkler spray patterns. This method might be alright with sprinkler-only fire protection, but it is useless with inert gas, which is supposed to extinguish a fire long before it would melt links. And the pressure of a gas release would also blow the curtains out of the way, letting the gas escape and defeating its ability to extinguish the fire.
It has become common to use close-coupled air conditioning in high-density data centers. Containment makes these solutions even more effective. In-row or top-of-cabinet cooling units discharge cold air directly in front of equipment and pull hot air back into the rear returns before it has a chance to disperse. Either of these close-coupled methods can provide the total cooling for a contained group of cabinets, and can be an excellent way to create an area within an existing data center for very high-density computing.
There are other containment solutions, such as chimney cabinets. Chimney cabinets are effectively individual hot-air containment units that trap the hot air within the backs of the cabinets and direct it out of a top-mounted duct, usually into a plenum ceiling that is coupled back to the CRACs.
When considering chimney cabinets, always weigh the issue of fans. Some cabinets are completely passive, relying on convection, the negative pressure created by the CRACs and the positive pressure from the server fans to move the hot air out of the cabinets and back to the air conditioners. Passive cabinets can work very well if you manage the equipment and cable installation inside so as not to impede the air flow. Other systems use fan boosters, but all fans use energy. Therefore, it can be worth examining how much power the cabinets will use, and if they need to be on a UPS to keep air moving in the event of a power failure. With passive units, air will continue to move because of the fans in the servers, which are maintained on a UPS.
In existing data centers, and even in new facilities, implementing partial containment can be a solution. Partial containment simply means that closures may not go all the way from floor to ceiling, and/or the spaces above cabinets may not be fully sealed. A common and simple way to implement partial containment is with heavy plastic curtains, similar to those commonly used to reduce air transfer on loading docks. These thick, overlapping strips are inexpensive and simple to custom cut. The strips are installed in openings of varying sizes, are easy to walk through and don’t even require special doors. Strips can be used on the hot or cold aisle, and some users have reported as high as 80% effectiveness compared with full containment solutions. The cost of using strips is far less than building walls, doors and ceilings, and there is virtually no disruption if it is implemented in an existing facility. Further “partial containment” doesn’t interfere with lighting or fire protection and also minimizes the need for exact air balance in cold-aisle implementations.
In short, “containment” can greatly improve both the effectiveness and efficiency of data center cooling, but it is not as simple as just putting up walls. There are a number of aspects to consider, most importantly the type of containment option used and the manner of installation.
Read part one of this tip on containment solutions for data center cooling..
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 Professionals program, is a data center power and cooling expert, is widely published and speaks at many industry seminars.
This was first published in April 2011