Hot-air and cold-air aisle containment is a popular choice for those seeking to find an appropriate cooling system for a data center. Containment as a method of temperature control can increase sustainability and efficiency by keep machines from overheating and eliminating hot spots. Separating the cold air from the warm air is the primary objective when it comes to containment, achieved through modifications to the air flow such as fans, vents or air conditioners. By facing the front of the IT racks toward one another, alternating cold-air and hot-air aisles will be created.

Differences between cold-air and hot-air dominant aisles in the containment setup can mean big savings when it comes to energy costs, with the hot saving up to 40% of the total expense. With hot aisle containment, the savings on energy are a direct result of the increased economizer hours it can provide. An economizer hour is the result of an hour passing in which the system’s chiller was powered off because the rack temperature has remained cool enough. Cooling systems are often required to be set at much lower temperatures in order to prevent hotspots from developing in the room. Hotspots are created when heat is driven upward from the cold air as it passes from the cooling unit to the front of the stacks.

In a cold-air containment system (CACS), the CACS encircles the cold aisle to limit the blending of warm and cool air, creating a hot-air return plenum (an enclosed space for airflow) in the data center. This form of containment is beneficial because the cold air being supplied can be increased while the hot air is returned to the cooling unit, better for both heat exchange and cooling capacity. The cold-air aisles are created through the operation of a chiller, which compresses and expands a refrigerant to reach the desired temperature, in most cases set around 45°F/7°C. By keeping the temperature of the chilled water supply and the temperature of the outside air within a close proximity to each other, economizer hours can be increased as the chiller will not need to run as much. It also allows air supplied by the cooling unit to reach the front of the IT devices without blending into the warm air, creating uniform IT inlet air temperatures. Without the mixing of the two air flows, economizer hours can be increased along with the supply air temperature. Humidification and de-humidification is another area where costs can be reduced when the warm air flow is kept separated from the cool air flow. Costs can be reduced by saving energy and water when the air being supplied to the racks is kept above the dew point, because neither addition nor reduction of humidity is necessary.

Physical infrastructure is another important consideration, especially when it comes to traditional cooling because equipment such as fans is often required to be oversized. It is a vital requirement for effective efficiency and waste reduction to determine the appropriate size to each of the parts of the system. The separation in the air temperature can be enhanced using something as a physical barrier such as a plastic curtain, panel walls, blanking panels or containment strips at the ends of the cold-aisle racks to further guide the cool air from reaching the warm air. Additionally, the use of perforated tiles in raised flooring can offer another physical modifier which can help direct cool air to the determined areas. It is important to keep in mind that the use of something like ventilated tiles can defeat the objective of containment, despite cooling effectively.

On the other end of the spectrum, a hot-aisle containment system (HACS) encloses the hot-air aisle in order to contain the exhaust air from the IT equipment, creating a cold-air return plenum. By establishing a consistent row-oriented (hot-air/cold-air) format, both the HACS and CACS will minimize the mixture of warm air and cool air in the data center. An extra step to control the exhausted warm air from the equipment is to use ducting to connect a computer room air handler (CRAH) or an air conditioner to the HACS.

This row-oriented method is preferred in purpose-built data centers because air-side economizers may already be in use there and they can add to savings on energy costs. Of course, drawbacks exist with this approach, namely that large volume rooms may require greater fan usage and the simple fact that people need to be able to work in the environment. In fact, OSHA regulations require that the room temperature in such an area be kept at a reasonable setting to keep workers there comfortable. Keeping the temperature too high in a HACS hot-air aisle (the back of the IT racks) may require the temperature to be equalized with periodic openings of the aisle to intake cooler airflow. Another consideration when thinking about temperature is the presence of any non-racked IT equipment in the data center that may require special arrangements to be made to keep them cool enough such as ducting. It may also be necessary to think about things like light switches, electrical outlets and fire suppression systems, which also operate in the area and may return less than peak performance if the temperature is displaced too far to one extreme.

This is more of a room-oriented approach, rather than the typical row-oriented containment format, but the end result is relatively similar, except for the potential power increase in fan operation. The room-oriented variation of HACS is considered to be better suited for new buildings or data centers which are especially large, whereas the row-oriented is a better option for existing buildings, retrofits, and smaller data centers.

Each type of containment has their own advantages and drawbacks, but in general, a HACS can save a data center more when it comes to stable efficiency.