hot and cold aisle containment diagrams showing airflow

Hot and cold aisle containment are popular choices for those seeking to find an appropriate cooling system for a data center. You can increase efficiency and save on energy using either method of containment by eliminating hot spots and preventing IT machines from overheating. The main objective in aisle containment is to separate the mixing of warm and cool air in the data center, specifically in the IT rack area through vents and air conditioners. To keep the hot-air and cold-air aisles apart, containment strategies require that IT racks be arranged with the front of the racks facing each other. This will effectively create alternating aisles of cool and warm air.

The manner by which the containment is established can vary as it’s based on a cold-air preference or a hot air preference. The differences between the two can amount to big savings as far as energy costs are concerned. The savings are due mostly to increased economizer hours which hot-air aisle containment provides. When the rack system’s chiller is not caused to operate because of a raised temperature for the period of an hour, an economizer hour is established. Cooling systems are often required to be set at much lower temperatures in order to prevent hot spots from developing in the room. Hot spots are made as hot air is pushed upward from the cold air passing from the cooling unit to the front of the rack and concentrating there.

Cold-Aisle Containment Systems (CACS)

A cold-aisle containment system (CACS) encloses the cold aisle, separating the hot and cold air streams in the data center, creating an hot-air return plenum, a confined space used for airflow) in the building. Better heat exchange and increased cooling capacity can be attained by using this manner of containment, where the cold air supply can be adjusted while hot air is returned to the cooling unit. The cold-air aisles are created by the operation of a chiller compressing and expanding a refrigerant to maintain a set supply temperature of chilled water, usually 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. Air supplied by the cooling unit, in this setup, will reach the front of the IT rack without mixing in with the hot air, causing uniform IT inlet air temperatures to be reached. When the two air flows are not mixed, the supplied air’s temperature can be raised and economizer hours will be increased.

De-humidification and humidification are additional areas through which costs can be reduced by the effort of separating the cool air from the warm air. When the air being supplied to the racks does not need humidity to be added or removed, such as when the air is above the dew point, costs can be reduced by saving energy and water.

Room using aisle containment and showing airflow diagram

Considering Your Infrastructure

Another important consideration should be physical infrastructure, such as the case of traditional cooling which often requires equipment such as fans to be oversized to cool effectively. Finding the right size for each of the system’s components is critical in raising efficiency and ceasing the waste of energy to cool the area. Physical materials can also be important in the separation of air flow, such as erecting physical barriers like plastic curtains, panel walls, blanking panels or containment strips at the ends of cold-air aisles, for instance. Raised floors are also a physical modifier that can be used which will, through perforated floor tiles, allow cool air to be more evenly distributed in the preferred areas. Despite cooling the hot-air aisle adequately, the use of ventilated floors can work against the goal of containment.

Hot-Aisle Containment Systems (HACS)

The opposite of a CACS, a hot-aisle containment system (HACS) works toward the same effect in reverse, in so much that the heat exhausted by the IT machines is confined to hot aisles, creating a cold-air return plenum. Through a consistent row-oriented (hot-air/cold-air) pattern, the HACS will, similarly to the CACS, keep the warm air from mixing with the cool air. To further control the warm air that is discharged, the HACS can be ducted to a computer room air handler (CRAH) or a remotely located air conditioner.

This method is preferred in purpose-built data centers because of the efficiency benefits through air-side economizers which may already be operating and available for use on site. Just as physical infrastructure is important, such as fan size, it should always be remembered that—more often than not, people will still need to be able to work in the environment. There are actually OSHA regulations that require the room temperature to be kept at a reasonable setting when workers frequent the areas. Decreased efficiency can come as a result of a temperature set too high in a HACS hot-air aisle (the rack backs), as it could require ventilation too occur more frequently.

Heat dissipation beneath floor of contained aisle

Another Consideration

One more consideration when it comes to temperature is all of the non-racked IT equipment present in the data center which may need a cooler running temperature than is present in the HACS and therefore its own duct work or something similar. 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 too far to one side.

This is more of a room-oriented, rather than the row-oriented layout of the CACS, approach but the two methods are not much different in overall theory but more so in physical concerns like fan operation. The room-oriented layout is generally suited better for new data centers or ones that are especially large rather than smaller data centers or retrofits, which may be better suited for a row-oriented approach.

Side by side, HACS takes a slight edge over CACS when it comes to efficiency, but a good data center will be able to utilize both manners of containment to maximize benefits. Back home.