Consider the computer room. It needs more precise cooling than mere mortals - and sometimes bosses are more willing to give into the demands of the data center while telling lowly employees to “tough it out.”

Problem is, those data centers are getting even more and more demanding to the point of at least one comfort cooling provider saying typical “room cooling is an ineffective approach for next-generation data centers.”

According to Kevin Dunlap and Neil Rasmussen, “Latest generation high-density and variable-density IT equipment create conditions that room cooling was never intended to address, resulting in cooling systems that are inefficient, unpredictable, and low in power density.”

Dunlap is product line manager for Modular/High Density Cooling Solutions for American Power Conversions (APC), while Rasmussen is a founder and chief technical officer at the APC, which is based in West Kingston, R.I., and provides “protection against some of the leading causes of downtime, data loss, and hardware damage: power problems and temperature.”

The data room cooling technology the authors describe involves what they call “row-oriented and rack-oriented cooling architectures.”

Their research has been reported in a White Paper published by APC.

“The need of next generation data centers to adapt to changing requirements, to reliably support high and variable power density, and to reduce electrical power consumption and other operating costs have directly led to the development of row-and rack-oriented cooling architectures.

“These two architectures are more successful at addressing these needs, particularly at operating densities of 3 kW per rack or greater.

“The legacy room-oriented approach has served the industry well, and remains an effective and practical alternative for lower-density installations and those applications where IT technology changes are minimal.”


To set the stage for their description of the two newer approaches - row and rack - the writers first described in their white paper the more traditional room-oriented architecture.

“The computer room air conditioner (CRAC) units are associated with the room and operate concurrently to address the total heat load of the room. (This approach) may consist of one or more air conditioners supplying cool air completely unrestricted by ducts, dampers, vents, etc. Or the supply and/or return may be partially constrained by a raised floor system or overhead return plenum.”


In the new row approach, the air conditioning systems are specifically integrated with rows of racks or individual racks.

In the row-oriented approach, computer room air conditioning units are mounted among the IT racks. They may be mounted overhead, or they may be mounted under the floor.

“Compared with room-oriented architecture, the airflow paths are shorter and more clearly defined,” said Dunlap and Rasmussen. “In addition, airflows are much more predictable. All of the rated capacity of the CRAC can be utilized and higher power density can be achieved.”

They added, “The reduction in the airflow path length reduces the CRAC fan power required, increasing efficiency. A row-oriented design allows cooling capacity and redundancy to be targeted to the actual needs of specific rows.

“For example, row-oriented architecture allows one row of racks to run high density applications such as blade server, while another row satisfies lower power density applications such as communication enclosures.”

The authors also noted the technology can be achieved with a raised floor that is common in more traditional approaches.


In the rack-oriented approach, the CRAC “are associated with a rack and are assumed to be dedicated to a rack for design purposes.

“The units are directly mounted to or within the IT racks. Compared with a room-oriented or row-oriented architecture, the rack-oriented airflow paths are even shorter and exactly defined, so that airflow is immune to any installation variations or room constraints.”

They added, “A rack-oriented design allows cooling capacity and redundancy to be targeted to the actual needs of specific racks. For example, different power densities for blade servers versus communication enclosures.”

They did say that a potential drawback to the rack approach “is that it requires a large number of air conditioning devices and associated piping when compared to the other approaches, particularly as lower power density.”


It was also noted that the three approaches could be “used together in the same installation. Specifically, a data center operating with a broad spectrum of power densities could benefit from a mix of all three types.”

In fact, the authors’ white paper detailed close to two dozen of what they called “challenges” facing those involved in creating computer room cooling. Depending on the challenge, the writers said rack, row, or room methods could allow for what they called “best performance.” In some challenges, a combination of two options provided best performance.

“It is expected that many data centers will utilize a mixture of the three cooling architectures,” the writers said. “Room-oriented cooling will remain an effective approach for low-density applications and applications where change is infrequent.

“For most users with newer high-density server technologies, row-oriented cooling will provide the best balance of high predictability, high power, density, and adaptability.

“Rack-oriented cooling will find application in situations where extreme densities, high granularity or deployment, or unstructured layout are the key drivers.”

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Publication date:05/14/2007