Alternatives to Achieving Data Center Airflow Containment
Data center airflow containment is now almost universally recognized as a baseline best practice for optimizing the effectiveness and maximizing the efficiency of data center thermal management. Containment protects IT equipment from troublesome hotspots. Containment enables energy savings from reduced fan energy, reduced chiller operating costs and increased access to free cooling hours. Besides being recognized as a best practice in all relevant industry standards and design guidelines, containment structures are actually mandated by California’s Title 24 Building Energy Efficiency Standards. While data center airflow containment is understood to include hot aisle containment and cold aisle containment and chimney cabinet containment (or “hot collars” by the lexical innovators at the National Fire Protection Association), there are viable alternatives to these containment strategies that may be preferable for some applications. Spoiler alert: I will not be proposing alternatives to containment; rather I will be reviewing some alternative practices for achieving containment objectives.
Single Row Computer Room
One simple alternative to containment aisles is suitable for smaller data centers or computer rooms wherein all the IT equipment can be arrayed in a single row of server cabinets. In such a room, the precision cooling equipment would be aligned along one wall and the server cabinets would be in a single row in front of the cooling equipment, parallel to the wall or perpendicular to the airflow. The fronts of the server cabinets would be facing away from the wall with the cooling unit(s). Downflow cooling units would deliver cooling air under the floor, under the server cabinets, and into the space in front of those cabinets through perforated floor tiles. Upflow cooling units would deliver cooling air through some system of overhead duct work, over the top of the server cabinets and down into the space in front of the cabinets. The upflow scenario could be a little more problematic to avoid short-cycling bypass return air. You definitely don’t want to use any kind of diffuser that would direct supply air back toward the cabinets, which could easily bypass over the tops of the cabinets. If any flow direction management is applied it should be limited to spreading the delivery along the length of the space in front of the cabinets. The row of server cabinets should be as close as code allows to the cooling units, so the return air is pulled directly back into the cooling unit intakes before it can mix in the room. Such an approach will not meet the letter of the law for Title 24, but it will effectively deliver the performance benefits of containment and meet the spirit of any containment design requirement. Furthermore, while I recommend this design for a small computer room, I suppose it could be equally effective for a particularly long room, as long as there was just one row of server cabinets.
Possible variations of the above single row computer room could be deployed with economizer or central air handlers outside of the white space, as long as the necessary conditions of supply delivery locations and return air removal locations were aligned to establish the containment-mimicking separation. If the fans for such a design are too far removed from the room with the IT equipment, supplemental fans may be required at the intake of the return air ducts to assure an adequate negative pressure differential was present to prevent any hot air re-circulation in the room. A CFD study might be a good idea to test the various design possibilities.
Creating Separate Rooms for the Cold and Hot Aisle
Another variation on the single row of server cabinets involves building a wall into which the server cabinets are integrated which creates separate rooms for the “cold aisle” and the “hot aisle.” I have seen this done in several different ways, but one of the most interesting was a rooftop data center for a major financial company that relied exclusively on free cooling air-side economization. Large fans, controlled by pressure sensor feedback, pulled outside air into space in the cold room. The server fans pulled the air through to the hot room and the hot room was evacuated by the pressure differential between the hot room and the outside ambient barometric pressure. This particular data center was running extra-tall server cabinets with five blade systems and one top of rack switch per cabinet for a total cabinet load around 27-30 kW. David Moss at Dell has been reporting to ASHRAE TC9.9 for a couple years now on his experiments relying on the server fans to do all the air-moving work in containment aisles, though not at such extreme heat density. The results have been encouraging and such an air movement design might be particularly suitable for such a single row free cooling data center with both the cold room and hot room adjacent to outside-facing walls – maybe large filter-walls.
One important caveat about these single-row computer room/data center concepts is that all the IT equipment in the room needs to be integrated into the separation topology. That means that side-breathing switches need to be racked in such a way that the airflow all behaves as if it were front-to-rear breathing equipment. If there is storage equipment that typically breathes from front-to-top, that equipment as well needs to be deployed such that, within the constructs of the space, the airflow behaves like front-to-rear breathing equipment, in order to not violate the integrity of either the physical or virtual separation. Even a very small proportion of the total heat load in a room that lives outside the containment parameters can completely undermine all the design-intent benefits. (See “How IT Decisions Impact Data Center Facilities: The Importance of Mutual Understanding,” pp 8-10)
Another variation on the theme would be partial containment. There are several varieties of partial containment, including the end of row doors with no above the aisle containment, above the aisle containment with no end of row doors, and partial aisle containment with the end of row doors. End of row doors without a completely enclosed hot aisle or cold aisle is sometimes going to be the only practical approach to retrofitting an installed, operational space. Not only will installing this partial containment be less intrusive to the critical mission, it will usually offer an easier path to accommodate overhead obstructions such as data and power pathway, lighting fixtures, ducts, diffusers, and fire suppression hardware. Partial containment effectiveness will be increased to the degree that some degree of a partial barrier will extend from the tops of the cabinets toward the ceiling, without encroaching on the definition of a fire suppression obstruction. The cold aisle can be partially contained when precision cooling units can be located at the end of hot aisles, at a minimal distance from the end of the aisle, on a raised floor. The hot aisle can be contained when there is a means for overhead heat removal, such as a suspended ceiling, with open grates, directly connected to cooling unit return air intakes. With partial containment, it is typically wise to utilize a relatively precise airflow management control system, preferably based on sensing pressure differentials, to be sure there is enough supply volume to discourage any re-circulation through the open areas while minimizing the amount of bypass. Finally, such partial containment architectures may not meet the letter of Title 24 in California, but it can meet the spirit of design intent in many applications.
Rear Door Heat Exchangers
Another alternative to aisle or chimney containment is the rear door heat exchanger. While there are no walls or ducts or partitions in this airflow containment alternative, it is nevertheless effectively containment in the cabinet – all the return air is contained inside the cabinet between the servers and the heat exchanger coil(s) and the rest of the room is essentially a cold aisle. Strangely, this approach does not appear to meet the letter of the Title 24 California requirement for a physical barrier, but it definitely meets and exceeds the spirit of the requirement and all best practices on physical separation.
Hot aisle containment, cold aisle containment, and hot collars constitute data center airflow containment for most of us. However, there are variations on this theme of containment that can solve special application challenges, while delivering most of the benefits of the core concept.
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