Before getting on with the subject at hand, I want to take a short step back to provide a little context for this discussion. First, while many of my readers (and perhaps even some non-readers) may have their preferences for hot aisle containment (HAC) or cold aisle containment (CAC), when it comes to efficiency, prior to drilling down into any application-specific variables or constraints, there is no basis for the superiority of one style of containment over the other. Testing conducted by Intel in conjunction with T-Systems in the Data Center 2020 test lab data center in the Euroindustriepark in Munich, Germany produced results clearly revealed by the resultant test report:  “Data Center 2020: Containment Efficiencies Reveal No Significant Differences.” Therefore, our thought process for deciding which containment horse to ride should focus instead on the costs and complexities of integrating into a specific situation.

Furthermore, I would like to make a modest suggestion here for an evolutionary step in the actual terms to which the “CAC” and “HAC” acronyms apply. After all, there is precedent that a universally accepted acronym is not necessarily cast in concrete. For example, DVD was unofficially digital video disk before it became officially digital versatile disk. Likewise, the GAO decided they preferred to be the Government Accountability Office rather than the General Accounting Office and the coinage of RAID for redundant array of inexpensive disks has migrated in the common lexicon to redundant array of independent disks. So in the spirit of such promoting permutations of greater precision, I suggest we talk about HOT AIR CONTAINMENT and COLD AIR CONTAINMENT. What’s the difference you ask? I propose that there is a strategic commonality between a cold aisle containment and server cabinets that allow cold air to enter inside a solid front door for a surrogate contained cold aisle within each cabinet and, therefore, the defining feature is the cold air containment. Likewise, in a subsequent piece, I will be discussing how hot aisle containment, cabinet containment with chimneys (hot air collars for anybody needing to carry on a conversation with the fire marshal) and rear door heat exchangers are sub-categories of the overall class of hot air containment. This lexical correction will remove the frequent awkwardness attendant to either what is actually included in a discussion or presumed to be included in a discussion. With that said, what conditions are most conducive for cold air containment?

Cold air containment is generally going to be happier on a raised access floor. A raised floor is not an absolute prerequisite for cold air containment, but, by definition, that cold supply air needs to find its way into that contained space. I have seen these contained spaces supplied by overhead ducts and even by a fan wall when the server cabinets either directly abut a boundary containing a fan wall or the space between the end of the contained aisle and the boundary wall is bridged with some kind of barrier and that end of the contained aisle therefore lacks a door. While these strategies will allow for a cold air containment structure to be built on a slab floor, when isolating conditions particularly compatible for cold air containment, an existing raised floor should be right at the top of the list.

Another condition particularly conducive to cold air containment is the presence of a fully operational data center. If the scenario is an upgrade of an existing data center rather than a new build, then cold air containment will typically be the most practical containment deployment. While most versions of hot air containment require structures that interface with the ceiling, cold air containment can be built below the upper boundary of an existing architectural space and thereby will not interfere with or be constrained by any overhead ductwork, network or power cable pathways, busway, lighting, or fire suppression grids. Ah yes – fire suppression. Building inspectors have been instructed by NFPA 75 and NFPA 76 to regard these cold air containment structures as “a separate volume,” and to treat them accordingly; that is to say, one fire suppression system for the data center room at large and another fire suppression system for each of the cold air containment spaces.

Another condition conducive to cold air containment is the absence of any compromises on the specification for front-to-rear breathing ICT equipment; in fact, this is more than a conducive condition: front-to-rear breathing equipment is a basic minimum price of entry. Just consider the space. The cold air space is a small portion of the overall space of the room so the effect of contamination by non-compliant hardware airflow patterns is going to be more dramatic. Likewise, equipment that is ingesting air from anywhere other than that contained cold air space is going to be pulling in un-mixed heated return air that could be anywhere from 85˚F to 125˚F, depending on supply set points and equipment delta Ts. So does that automatically exclude any deployment that includes a 1000 pound gorilla’s side-breathing switches or any of the other myriad of non-compliant breathing equipment on the market? Of course not. There are standard equipment cabinets available that redirect the airflow to convert the most popular non-compliant breathing equipment to front-to-rear compliance. For other, less popular equipment with all variety of strange non-compliant breathing architectures, including front-to-side, side to rear, both sides to rear, front to both sides, even back to front, there may be rack-mount boxes available for correcting these airflows and, where there are not product solutions, there are vendors either in the industry or at the local sheet metal shop who are willing and able to solve the problem. Nevertheless, it is not all rainbows and rose petals. There may be equipment that will stubbornly resist such correction, such as front-to-top breathing arrays, that would preclude living in a cold air containment environment. The rule here: cold air containment is going to want to see only front-to-rear breathing ICT equipment, or equipment that can be effectively converted to front-to-rear breathing behavior.

Cold air containment will deliver better efficiency results for data centers that come with some degree of sophistication in managing airflow volumes with pressure sensors. Research conducted by test engineers at Tate Access Floors found that servers themselves are a significant path for bypass airflow and this condition is going to be most flagrant in cold air containment wherein excess airflow volume is confined in a smaller space. A sub-category to this condition would be management and disposition of dead servers. Dead servers are always going to be an open door to bypass airflow in any kind of data center, but that path could be exacerbated by pressure differentials in cold air containment.

Finally, cold air containment is going to be most effective when deployed by data center operators who truly want their space to be more efficient and effective and have therefore taken all the routine steps required to assure the effectiveness of that containment, such as filler panels in unused rack mount spaces, sealing around the perimeter of the equipment mounting area in cabinets, sealing between cabinets, plugging all holes in the raised floor and establishing a general attitude of discipline for maintaining these practices through the normal progression of moves, adds and changes.