As we look back on the data center environment of the past 10 to 15 years, the power densities of server racks have remained stable at 3 to 5 kW. During this period, air-cooled data centers using chillers and computer room air conditioning (CRAC) units were sufficient to overcome the heat dissipation from the servers, keeping the facilities, and the CPUs under their roofs, below their maximum temperatures. This was possible as the CPUs did not produce more than 130 W of heat.
Data centers utilized raised floor systems with hot aisles and cold aisles as the main cooling method. Cold air from the CRAC and computer room air handler (CRAH) units was distributed to the space below the raised floor, and then through the perforated tiles it went to the main space in front of the servers. This method was simple and most common for years, and although improved cooling methods have since gradually taken over, it is still employed today.
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