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The evolution of water cooling for IBM large server systems: Back to the future

This paper provides a technical perspective and review of water cooling technology as implemented through 5 generations of IBM's high performance computing systems from the S360/91 to the recently announced IBM Power 575 supercomputing system. The use of hybrid air-to-water cooling and then indirect (cold plate) water cooling in earlier IBM systems is described. Attention is given to how and why water-cooling was implemented to provide the required cooling capability while maintaining ease of serviceability at the module level. Also discussed is the use of a Cooling Distribution Unit (CDU) to control cooling system water temperature, distribute water to multiple racks and serve as a buffer between system water and customer facility water. Rising microprocessor power dissipation, increased heat loads at the data center level, and demands for increased cooling energy efficiency are presented as driving the need for the reintroduction of water cooling. The introduction of the rear door heat exchanger to respond to the challenge of rising heat loads at the data center level is discussed. Finally, the IBM Power 575 water cooling system is described. Included in the discussion are the coolant flow architecture and the incorporation of Modular Water cooling Units (MWU's) within the server frame replacing the remote CDU concept.