Chill Out – Battle of Aisle Containment

From time to time I have heard a lot of heated discussions on which aisle containment solution is better. No doubt aisle containment is an effective way to improve airflow management and enhance efficiency. From cold aisle to hot aisle, full to partial containment, passive or active hot chimney, each camp has its own supporters and opponents. With every operating data center having its own constraints, the key is to adopt an approach that find a balance between effectiveness, cost and what can realistically be done in that data center.  Here I outline some common containment strategies for a typical data center white space with raised floor, using downflow air supply, top return with perimeter CRAC/CRAH units.

Hot aisle containment with ceiling return plenum:

The idea is to let the cold air flood the data center space, and let the server fans do its work under no external pressure.  Design ample space for return air plenum to minimize pressure drop of return air path.

Passive/Active Hot chimney:

Another variation is the hot chimney which in principle is really just a hot aisle containment. Hot chimneys are usually packaged product. The return air path is not as generous as hot aisle containment. The only reason for hot chimney over hot aisle containment is not to expose staffs to work at high temperature. When return airflow is a problem exhausts fan are installed in the chimney to make it an ‘active chimney’.

Hot chimney is always more expensive than hot aisle containment, especially active chimney which is really just over engineering by creating one problem to solve another. To solve the problem of high temperature exposure to staffs working in hot aisle, we can always pop open a few floor tiles in the hot aisle to let cold air get in, for the short period of time whenever the staffs need to work in the hot aisle.

Full cold aisle containment with overhead cap:

There are many reasons why people oppose this method, and I am one of the opponents of such design. The most common opposing reason is the problem with fire suppression system. Some way needs to be designed to let the sprinkler heads and fire suppression gas to get into the cold aisle. A lot of manufacturers created the removable cold aisle cap, which either automatically opens up on fire alarm, or fixed in place by some fusible link. To me, this is unnecessary engineering, a band-aid solution when hot aisle containment is not feasible to your data center. But as you will see later in this article, there are other better solutions.

Another opposing reason is pressurization of the cold aisle containment needs to be carefully controlled, which is not an easy or cheap task to achieve. More often than not, more air is supplied to the cold aisle than the server fans can take, causing unwanted positive pressurization of the cold aisle. As the system tries to balance (flow output decrease as DP across the tile decrease due to aisle pressurization), a cycle of pressure fluctuation will occur inside the aisle. Such pressure fluctuation causes air leakage (more than 30% as reported from various sources) through all the gaps that exist in the containment system which really negates the point of having a full aisle containment. In worse case it may even damage the server fan motors.

Full cold aisle containment is not really needed if an effective form of partial cold aisle containment is implemented. The extra cost and problem of full cold aisle containment with overhead cap more than often outweighs the marginally small benefits over its partial cold aisle containment counterpart.

Partial cold aisle containment, the bathtub cold aisle solution:

As the name implies, the bathtub cold aisle solution is a partial cold aisle containment without the overhead cap, with only the end of row doors retaining the cold air inside the aisle in the form of a bathtub holding its water inside. The point is to let the server fans take cold air from the cold aisle and maintain the cold air level not to over spill the ‘bathtub’. The trick is to just supply enough cold air to fill the bathtub while the server fans are taking air from it. Air speed coming out from floor grilles need to be carefully engineered so that it doesn’t ‘overshoot’. There had been proposals from vendors on how to achieve this. One of the more complicated solutions is to utilize temperature sensors to control fan speed. When the bathtub overflows, the sensor on the top of the aisle containment will detect colder temperature, and reduce fan speed. When there is insufficient cold air to fill the bathtub to the desired level, another sensor somewhere close to the top of rack will detect hotter temperature, and increase fan speed. Obviously a lot of thoughtful engineering need to be done.

Another simpler method is to put an extended partition on top of racks, basically making the bathtub deep enough so that there is no spill over. With extension hood placed on top of the CRAC/CRAH units return air, this is a cheap method to achieve very effective containment. Due to hot air natural tendency to rise, an extension hood on top of the CRAC/CRAH top that is taller that the cold aisle containment will effectively minimize hot air recirculating into the cold aisle from the top.

An interesting point in supporting hot aisle over cold aisle containment is in the rare case of catastrophic failure, a data center flooded with cold air provide a longer ride-through time than the lesser amount of cold air retained inside the cold aisle. This is probably the least convincing reason. Another one or two more minutes of cold air buffer probably won’t save a data center experiencing such catastrophic failure. Such failure scenario should have been avoided at all cost in the first place.

Of course existing data center has various constraints and not all containment solution is suitable, either due to existing physical constraints, budget constraints or operation constraints. As a personal preference, one of the following will in most cases be feasible to any existing data center:

If there is ceiling plenum, or ceiling plenum can be installed, use hot aisle containment ducted to return ceiling plenum as the first choice;

If hot aisle containment is not possible, use deep bathtub cold aisle with CRAC return extension hood. No complicated engineering is required.

No overhead cap cold aisle or hot chimney should ever be needed.


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