Contain yourself for more efficient data centre cooling

Data centre containment strategies can greatly improve the predictability and efficiency of traditional data centre cooling systems. In fact, The Green Grid views an air management strategy as “the starting point when implementing a data centre energy savings program”. However, most existing data centres are constrained to certain types of containment strategies. Containment, in general, provides some important benefits for an existing data centre:

Reliability will be increased by preventing hot spots. Containment can prevent the mixing between hot air and cold air, which can provide a lower uniform IT inlet air temperature for IT equipment to reduce hot spots.

Rack power density can be increased by eliminating hot air recirculation. For a traditional, uncontained raised-floor data centre, rack power densities are typically kept below 6kW/rack average to help prevent hot IT exhaust air from re-circulating back into the front of the IT equipment. After containing and sealing the holes to eliminate hot air recirculation paths, rack power densities can increase without the threat of hotspots.

Cooling capacity will be increased by increasing the “deltaT” (i.e. the difference in temperature between the cold supply air and the hot return air) across cooling units. For a traditional, uncontained raised-floor data centre, more than 50% of the cold air supplied from the cooling units will bypass back to these units directly as a result of any leakage paths that exist. After contained, supply air will instead go through IT equipment where it will absorb heat energy and transport it back to the cooling units. The higher exhaust air temperatures will lead to a bigger deltaT across cooling units, which can increase the cooling capacity about 20% or more.

Cooling system energy savings will be increased by being able to shut down cooling units that become redundant as the result of effectively separating hot and cold air streams through the use of an air containment system. Additionally, economizer mode hours are increased. When the outdoor temperature is lower than the indoor temperature, the cooling system compressors don’t need to work to reject heat to the outdoors.
Two ways of approaching containment
Hot air and cold air containment are the two high-level methods for an air management strategy, and both of them provide significant energy savings over traditional, uncontained configurations. Why do we need to decide between hot air and cold air containment? Why not just contain both and run the rest of the room on building air? Containing both air streams provides no significant benefit except in cases where IT cabinets are located in a harsh environment (i.e. manufacturing floor). Containing a single air stream is enough to prevent hot and cold air mixing.

Deciding upon which type of air containment is the best choice for existing data centres is a question that continues to raise a lot of discussions among manufacturers, consultants, and end users. In reality, the best containment type will largely depend on the constraints of the facility. Some IT managers will have a choice between two or more types, while others may be restricted to a single strategy for hot or cold air containment due to the physical constraints of the facility they manage.

Assessing your facility for containment
An assessment of the existing conditions of the facility is an essential start point to choosing the right containment solution for a given data centre. During an assessment, constraints are noted: these are obstacles in the facility that cannot be overcome, or can only be changed at great expense or with unacceptable consequences.
For example, raising ceiling height is not realistic and would be considered as a major constraint. For complex projects, expert review is essential in order to check the cost or other consequences of constraints, which end users may not be aware of. Once the consequences of certain constraints are clear, it is important to review them and determine if they can be refined or adjusted to achieve a better overall result. Examples of constraints may include IT equipment arrangement, ceiling height, raised floor plenum depth, column location, cabling location, air distribution type, lighting considerations, and the effects upon the fire detection and suppression system, as well as what ride-through time in a cooling outage.

A line-up of potential solutions
All containment solutions will enable higher rack power densities, increased cooling system energy savings, eliminate hot spots, and increase cooling unit capacity, which in addition to payback period, should all factor into justifying the cost of a solution.

While it is acceptable to implement both types of cold air containment or multiple types of hot air containment, hot and cold air containment types should never be mixed. Mixing hot and cold air containment solutions is likely to lead to lower cooling system efficiency.

Cold aisle containment system (CACS)
This containment method applies to a raised floor (room-cooled downflow units) cooling distribution system. CACS encloses the cold aisle, allowing the rest of the room to become a large hot-air return plenum. By containing the cold aisle, the hot and cold air streams are separated.

Ducted hot aisle containment system (Ducted HACS)
This form of containment can be used with either a raised floor or a hard floor-based (room- cooled) air distribution system. Ducted HACS encloses the hot aisle, allowing the rest of the data centre to become a large cold-air plenum.

Ducted rack
This containment method best applies to environments with scattered high density racks with front-to-back airflow pattern. With this method, a duct is mounted to the back of the rack to contain the hot exhaust air, and then duct it into the drop ceiling. By containing the exhaust airflow path, the hot and cold air streams are separated.
Row-cooled hot aisle containment system (Row-cooled HACS)
This containment solution applies to data centres with existing row-based cooling units but could also serve as a solution for data centres with perimeter cooling units. For Data centres with existing in-row cooling units, containment is achieved by simply adding ceiling panels over the aisle. For data centres with existing perimeter cooling units, this containment solution adds cooling units in between racks or over hot aisles. This method should be used when high density racks are added in a low density data centre, and when all racks are in some form of hot aisle containment.

Rack air containment system (RACS)
This containment method is an ideal solution for use with very high density racks by integrating rack-based cooling units with the racks, forcing the air to circulate only inside of the containment system. RACS are recommended when there are scattered high density racks, or sound attenuation is required; for complete isolation in cases such as stand-alone open data centre environments, or mixed layouts, or to prevent exposure to hot aisles; in wiring closets that lack any form of cooling, exposing high density equipment to high temperatures.

Selection
Once the constraints of an existing facility are well understood and the containment solutions have been reviewed and compared, the choice between hot and cold air containment will be easier. Most will only have one or two practical containment options left by this stage. The air distribution method and cooling unit position are two major considerations on physical constraints.

Conclusion
Data centre containment strategies can provide great benefits, but the best approach for each specific deployment should be determined by first assessing the facility constraints, reviewing all potential solutions, and selecting the right containment hardware.

Cold aisle containment and ducted hot aisle containment are two most common solutions for data centres with existing perimeter cooling units; cold aisle containment is preferred for the facilities with raised floor as a supply air plenum. Ducted hot aisle containment is preferred for facilities with a drop ceiling as the hot air return plenum. For data centres with existing row-based cooling units, row-cooled hot aisle containment is preferred. White Paper 153 “Implementing Hot and Cold Air Containment in Existing Data Centers” can be downloaded free at www.apc.com/whitepapers