Data center containment has gone mainstream over the past decade, which has led to widespread implementation and adoption by many sites. In fact, according to a recent survey by the Uptime Institute, 80% of sites have implemented either hot or cold aisle containment. While the survey represented mostly larger facilities, many medium and small facilities have either implemented or are considering some form of containment. However, although containment is experiencing a high rate of adoption, many people are still unaware of the different options and strategies that are available to them.

Hot Aisle Versus Cold Aisle Containment

When it comes to containment strategies, the most common confusion is usually regarding whether it’s better to contain the hot aisle or the cold aisle. One thing to keep in mind, the goal of both hot and cold aisle containment is the same: to separate the flow path of the conditioned air from the exhaust air so that changes can be made to reduce operating cost, reduce PUE, and increase cooling capacity.

Assuming a computer room is configured in such a way that either is viable, hot aisle containment may be seen as the better option because it has some working environment and ride-through advantages. However, because every computer room is unique, there is no one definitive solution.  For this reason, it is important to understand all the differences between hot aisle containment and cold aisle containment so you can make a decision appropriate to a particular architectural environment and business strategy.

For clarification, cold aisle containment involves doors on the ends of the cold aisles and some form of partitions, baffles, or roof over the cold aisle. Hot aisle containment includes doors on the ends of the hot aisle and a configuration of baffles or duct work from the hot aisle to the returns of the cooling units. Drop ceiling plenums are often used as the means to duct the return air back to the cooling units. Both strategies have their benefits as well as their challenges:

Cold Aisle Containment Benefits:

• Often easier to implement; does not require additional architecture to contain exhaust air and return it to the cooling units (drop ceiling, air plenum, etc.).
• Only requires doors at ends of aisles and baffles or roof over the aisle.
• Generally less expensive.
• Cold aisle containment is typically going to be easier to retrofit in an existing data center, particularly when there are overhead obstructions to circumnavigate, such as power and network distribution, ducts, lighting.
• Cold aisle containment doesn’t absolutely need to be on a raised floor, but it typically is because of challenges associated with delivering supply air to the contained space(s).

Cold Aisle Containment Challenges:

• In cold aisle containment, the overall data center becomes the hot aisle, all areas not in the cold aisle are part of the hot aisle. If cold aisle containment is done well the rest of the space could be dramatically hot if the theoretical advantages of containment are fully pursued, i.e., higher supply temperatures and reduction of bypass airflow resulting in very high return air temperatures.
• There may not be any space with a suitable temperature profile for equipment that for whatever reason is not compatible with living in the cold aisle arrangement. This could mean lowering the supply temperature and/or allowing more bypass airflow to accommodate the equipment that doesn’t fit in the cold aisle. This would result in losing some of the economic benefits of full separation.
• Any leakage from the raised floor plenum not in the contained cold aisle (such as from under PDUs), enters the hot area and mixes with the return air, thus lowering the return air temperature. This reduces the efficiency of the system.
• Full cold aisle containment creates what the NFPA codes call a “separate volume”, so there needs to be fire suppression for the overall data center space and then either additional fire suppression for the contained cold aisle or the containment must be connected to the smoke detection system and remove itself as an obstruction on a smoke alarm.

Hot Aisle Containment Benefits:

• The open, larger area of the room, is a cold environment.
• Leakage from raised floor openings in the larger area of the room goes into the cold space.
• Generally easier to implement to maximum efficiency.
• Hot aisle containment will be more forgiving for network racks and stand-alone equipment such as storage cabinets that might have to live outside the containment architecture, i.e., they will live in the lower temperature area of the computer room.
• Hot aisle containment can perform well in a slab environment by merely flooding the data center with an adequate volume of supply air and containing the exhaust air.
• Enables more surface area for “cold sinks” (with or without a raised floor) for ride through in the event of power failure and engine generators not starting.
• Hot aisle containment, by virtue of the containment structures typically abutting the ceiling where fire suppression is installed, is not creating separate volumes, but merely creating obstructions which need to meet clearance requirements from sprinkler heads. With a well-designed space, it is conceivable that a standard grid fire suppression system could be installed around a hot aisle containment array of barriers and meet code.

Hot Aisle Containment Challenges:

• Requires a contained path for air to flow from the hot aisle all the way to cooling units. Often a drop ceiling is used as return air plenum.
• Generally more expensive.
• High temperatures in the hot aisle create uncomfortable conditions for technicians working on IT gear. Note that some server manufacturers are developing and supplying front-serviceable servers, which means that the hot aisle, whether contained or not, would hardly ever need to be entered.

Maximizing the Benefits

Whether your strategy is to contain the hot aisle or the cold aisle, airflow containment solutions are a best practice for managing IT intake temperatures with maximum efficiency. However, even after containment strategies have been implemented, many sites fail to take full advantage of the efficiency and cost saving improvements containment can provide.

So before we go any further, it’s important to note that the first step in maximizing your containment is to understand the utilization of the computer room cooling infrastructure; this is easily done by calculating your computer room’s Cooling Capacity Factor (CCF). CCF is the ratio of total rated cooling capacity of running cooling units to the estimated heat load. Armed with this knowledge, you will be able to make informed decisions about maximizing the benefits of containment solutions in your facility.

In addition, it’s important to consider the effects of containment on bypass airflow. Bypass airflow occurs when the total flow rate of air through all cooling units exceeds the flow rate of air through IT equipment. Implementation of containment alone does not reduce the volumetric flow rate of bypass airflow in a computer room. Instead, if implemented well, containment creates an environment where cooling unit fan speeds can be reduced or cooling units turned off to reduce bypass airflow rates. These adjustments are necessary to realize the benefits of reducing fan energy and increasing return temperatures, which increase the sensible cooling capacity of cooling units and partial economization hours of any free cooling systems.

Other Considerations

Aside from hot aisle versus cold aisle containment, data centers have two other options to consider: full versus partial containment and hard versus soft containment.

Full containment completely encloses the open area of an aisle while partial containment focuses on creating a sufficient barrier, or separation between hot and cold air flows, to enable cooling optimization without causing the fire suppression challenges inherent with full containment. Partial containment has been growing in popularity in recent years for its lower cost, flexible and non-intrusive design, as well as an increasing awareness of airflow management fundamentals.

Hard containment is understood as doors, baffles, partitions, or other rigid components, while soft containment primarily refers to aisle-end or over-cabinet curtains. For a number of reasons such as aesthetics and effectiveness, hard containment tends to be a preference over soft containment.

Facilities should determine which containment options are appropriate depending on the layout of the computer room, available budget, and long-term goals.

Conclusion

The fact of the matter is, each and every computer room is unique. Whether it be hot or cold aisle containment, full or partial containment, or hard or soft containment; each computer room may reveal conditions that will make one strategy/option more suitable over the other. The best thing to do is to first evaluate your site and calculate the CCF of the computer room. Then understand the unique layout and architecture of each computer room to reveal conditions that make certain containment options preferable (full or partial, hard or soft). With this information you will be able to choose the best containment option for your computer room. Once containment is installed, be sure to consider bypass airflow as you optimize for maximum efficiency and cooling capacity benefits.

This post was originally published on Networks Europe. You can view it here.