The majority of the industry understands that liquids are used in some or most of the cooling process for most data centers. However there has been some misunderstandings about what is actually liquid cooled. ASHRAE (the American Society of Heating, Refrigeration and Air-conditioning Engineers) released the second edition of Liquid Cooling Guidelines for Datacom Equipment Centers in 2013. As part of the introduction, the book provides definitions of liquid cooling based on the boundaries that the liquid crosses to get closer to the load. Liquid cooling, in general, is where a liquid is used to extract heat instead of air from a data center space. This might mean in-row coolers, adjacent or rear door cooling other other means.
As the liquid is used more closely to the load, other definitions were given that are starting to become standard for the industry:
- Liquid-cooled Rack: a liquid cooled rack is where the liquid is circulated inside a rack or cabinet for cooling;
- Liquid-cooled Equipment: liquid is used inside a server or other datacom equipment to remove heat;
- Liquid-cooled Electronics: the liquid is used to directly cool the heat source without another medium, such as air.
It should be mentioned that these definitions are for cooling inside the data center itself and does not include definitions for liquid cooling equipment such as chillers or cooling towers. While this equipment does do heat transfer via water or another liquid, this is not typically considered to define a data center as liquid cooled.