Importance of cooling
Earlier this year, there was an outage at Amazon Web Services, courtesy of a defective cooling fan at one of its data centres which caused the back-up generator to overheat and shut itself down. The outage brought down a number of social media sites like Pinterest, Quora and HootSuite.
Recently in November, a cooling outage grounded the airline check-in system in Australian airports, preventing airlines from checking customers in, leading to long queues in the terminals and affecting both domestic and international flights.
An unplanned outage causing severe business disruptions is a data centre operator's worst nightmare and one of the key ways to ensure 24x7 availability is a robust and reliable cooling system to combat the heat generated each day.
All data centre cooling systems have two key functions: to provide cooling capacity and to distribute the cool air to the IT loads that need it. The first function is the same no matter what type of cooling method is employed; it's the second that varies.
There are three commonly used cooling methods - room cooling, row cooling and rack cooling.
Room cooling used to be the most widely used cooling method for data centres in the past. With IT equipment now pushing peak power density to 20 kW per rack or more, the room-based systems - which are designed for power densities on the order of 1-2 kw per rack - just can't keep up with the heat output. A room-oriented architecture may consist of one or more air conditioners supplying cool air to the entire room and often no specific attention is paid to the airflow.
Because of this, airflow will be affected by the unique constraints of the room, including the ceiling height, room shape, obstructions above and under the floor, rack layout, Computer Room Air Conditioning (CRAC) location and the distribution of power among the IT loads. As a result, it's difficult to predict performance, particularly as power density increases and the data centre configuration changes. Much of the cooling capacity may be wasted if the cool air never reaches an IT load that needs it. This method is generally used for cooling lower density heat problems.
Row cooling is gaining popularity among data centres. The CRAC units are dedicated to a particular row and are mounted among the IT racks, overhead or under the floor. This results in far shorter, more clearly defined airflow paths as compared to the room systems. In addition, airflows are much more predictable because all of the rated cooling capacity of the CRAC can be directed at IT loads.
The reduction in the airflow path length also reduces the CRAC fan power required, increasing efficiency. It also allows cooling capacity and redundancy to vary by row, with perhaps one row of racks running high density applications such as blade servers, while another row satisfies lower power density applications such as communication enclosures; each gets only the cooling capacity it needs. Same goes for redundancy, with N+1 or N+2 redundancy targeted at specific rows.
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