Data Center Flexibility with Good Airflow Management
What does data center flexibility mean? When I think of flexibility, my mind goes down several paths. A child has a flexible structure that allows her to chew on her foot. A balloon is adaptable to change both size and shape. In football, an Offensive Coordinator can be flexible to bring either the running game or the passing game, depending on constraints brought by the opposition. An enterprise can be flexible to change the business plan in time to take advantage of market conditions. It turns out that a flexible data center contains all those elements and solid airflow management disciplines deliver that flexibility. Moreover, data center flexibility is not just a topic of academic amusement: it can be the source of competitive advantage, even survival. To make sure we are all on the same page before delving into some detail on design and operational flexibility, let’s make sure we are speaking the same language about “good airflow management.” For the sake of this discussion, I am talking about maximum separation between supply air and return air so that fundamentally those two air masses are connected only by the path through IT equipment. This separation could be hot aisle containment, cold aisle containment, chimney cabinet containment, reverse plenum containment, rear door heat exchanger containment, or cabinet hot and cold containment – any effective separation will suffice for realizing the benefits of flexibility.
With proper airflow management, it does not matter how the supply air comes into the room, which is the source of all varieties of design and space layout flexibility. Before today’s airflow management disciplines, we were all held hostage by the tyranny of every rack’s heat load’s dependence on what was delivered through the most adjacent perforated floor tile. With proper airflow management, we no longer even need that perforated floor tile or the raised floor. However, even with a raised floor, airflow management facilitates flexibility. For example, standards and best practices insist that precision perimeter cooling units should not be delivering air at right angles to each other; in other words, we do not want these cooling units on adjacent walls. Keeping underfloor airflow parallel prevents supply plumes from colliding and redirecting so return air gets decoupled from its source. This decoupling can cause inefficiencies with low ΔT’s and can create hot spots with return air below-set point resulting in uncooled supply air. Also, these underfloor collisions can also cause vortices, producing low-pressure areas that may fail to deliver required airflow through nearby perforated floor tiles. The data center with good airflow management is much more forgiving by not requiring targeted supply air delivery; as long as the room or the contained cold aisles fill with supply air, all the IT equipment cools adequately. With this degree of flexibility, cooling unit locations can accommodate less expensive plumbing designs or service bay security concerns.
An even more egregious affront to the wisdom of standards and best practices than locating perimeter cooling units on adjacent walls is the practice of establishing cooling, so the underfloor supply delivery is at right angles to the rows of server cabinets. This layout practice is harmful to both the supply and return sides. On the supply side, we increase the odds that we will run out of air before we get to the rows farthest from the cooling units or, conversely, we could underserve the closest row if we have too much velocity and the supply air blows by before developing enough pressure to enter the room. On the return side, we are dragging the return over multiple cold aisles, resulting in re-circulation and hot spots. With good airflow management, the supply will eventually get to where it needs to be, and the return will “bypass” our server inlet fans. Flexibility benefits range from being able to use a conventional mechanical service bay for two different data halls to be able to arrange rows of cabinets and rows of cooling units parallel to the length of a rectangular data hall.
While good airflow management increases the flexibility of deploying many design elements in conjunction with a raised floor, good air separation can also facilitate flexibility in matching air delivery systems with certain economizers without a raised floor. When we are not concerned about re-circulation, bypass or short-cycling, we can concern ourselves with just filling the room (or contained cold aisles) with an adequate volume to maintain a positive pressure. It does not have to be a considerable number. I have run hundreds of controlled data center lab tests with less than a pascal positive pressure differential between the cold aisle and hot aisle and not more than 2˚F inlet differentials within a cabinet or between adjacent high density and low-density cabinets. The point is that the airflow management allows the flexibility of air being delivered overhead or through boundary walls if such delivery topologies facilitated other aspects of the design.
Free cooling is a design element facilitated by the flexibility produced by proper airflow management both architecturally as well as strategically. Architecturally, we frequently see airside economizers or indirect evaporative coolers mounted on building roofs. The complexity of design, not to mention the need to design surplus capacity, is significantly reduced by the elimination of most ductwork when supply air can be forced down directly into the data center and return air pulled straight out of the data center either into the economizer or evacuating the building. Likewise, cooling systems such as re-purposed energy recovery wheels or containerized modular economizers are typically on pads just outside the data center wall or in sizeable mechanical service bays adjacent to the data hall. In either case, associated fan walls eliminate ductwork required for targeted delivery and over-designing capacity to compensate for duct-loss. Strategically, good airflow management allows for aggressive set point management which can make the difference between whether or not the business case could justify investing in free cooling. Table 1 shows how many extra full days of free cooling for a dry bulb system and a wet bulb system would be available for some sample cities with good airflow management in the data center. An additional strategic consideration facilitated by airflow management is site selection for new data centers. With access to well over half the year for free cooling in most cities, climate can now slip down on the priority list on the strategic site selection plan. Furthermore, while Table 1 has all the data centers living inside the ASHRAE recommended environmental envelope, even more, flexibility can be achieved operating the data center within allowable limits. For example, a few years back, Dell Computer announced that their line of Class 3 servers could live happily in 90% of the U.S. cities with data centers without any refrigerant mechanical cooling at all.
|Extra Days of Free Cooling with Good Airflow Management (24 HRS = DAY)|
|Airside Economizer (dry bulb)||Indirect Evaporative Cooling (wet bulb)|
Table 1: Extra 24 Hour Days of Free Cooling with Good Airflow Management
Good airflow management in the data center also supports greater operational flexibility. For example, proper airflow management allows servers, switches, and storage to play nicely together and co-exist in the same space without efficiency or layout compromises. With switches that breathe any way except front to back, deploying them has historically meant compromising the welfare of other IT equipment in the immediate vicinity, wasting energy on significant amounts of over-cooled bypass airflow to compensate for switch exhaust contamination of the rest of the space, or building separate rooms for switches and thereby enhancing the overtime checks of the cabling installers. Likewise, mass storage systems that come in proprietary cabinets with top fan trays are disruptive of the data center and typically require major compensatory temperature and bypass waste or removal to separate spaces. There are viable airflow management solutions to fully integrate switches and storage into the air flow management strategy of any space (see my pieces from January 23, 2018; October 31, 2017; and June 7, 2017). This integration then provides the resultant flexibility to not only locate switches and storage where it makes logical sense for the technological mission of the space but will allow smooth expansion as business needs change.
Good airflow management not only provides flexibility for deploying switches and storage but allows flexibility in general deployment of IT equipment. There was a time not long ago when adding load, whether it was a strategic upgrade or a case of surreptitious density creep, meant re-tuning the whole data center. Sometimes this re-tuning was not just adjusting the mechanical plant to the increased load and moving around perforated floor tiles of different opening ratios, but it could mean having to move the IT load around to spread the load where it could be adequately cooled. With proper airflow management, density creep does not necessitate a kW guard at the door but is instead welcomed by merely rolling out a 5kW cabinet and rolling in a 20kW cabinet. Of course, that new cabinet may ask the mechanical plant to deliver another 2325 CFM of air into the room (CFM = 3.1W÷ΔT). For perspective purposes, if we rolled that 20kW cabinet into a small 500kW room, we would only be asking our cooling to deliver less than 3% additional airflow, which should easily be covered in our redundant reserve. In a 2MW room, that density hike would require less than a 1% increase in airflow volume – for most of us that are within our cautiously conservative margin of error.
Back in the day, we either subjugated all our design and operational decisions to the tyranny of matching each perforated floor tile to the heat load of its adjacent server cabinet or we enriched our electric utility provider while likely continuously fighting a losing battle to hot spots. With good airflow management, we can escape that tyranny and make design decisions that optimize the effectiveness and efficiency of our mechanical plant and make IT equipment deployment decisions based on functional logic. If you’re not there yet, imagine this degree of flexibility. If you have been there for a while, this is just a reminder to not forget about the airflow management experts who helped you get there.
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