Advanced Technology for Server Cooling

This paper reports work on advanced cooling technology for servers. The air cooling load on the rack may be enhanced using highly compact Copper/Brass “flat tube” water-cooled heat exchangers that are integrated into the rack frame. The cooling water is supplied by a water chiller. Analysis shows that it is possible to provide cooling (UA/A_fr) in the range of 670–1000 W/m²-K, where the m² is the core frontal area. Also analyzed and compared are advanced technology CPU heat sinks — a thermo-syphon concept and a liquid micro-channel heat sink. The thermo-syphon may be used in a compact thermal-bus concept for heat removal from multiple CPUs. Used with boiling on an enhanced copper boiling surface in a thermo-syphon, heat loads in excess of 75 W/m²-K are possible. The heat removed at each CPU in the chassis is rejected to water flow in a compact water cooled condenser. Performance results of the thermo-syphon concept are predicted, obstacles associated with increasing performance are discussed, and possible solutions are proposed. Performance predictions were also made for water cooled: (1) Copper micro-channels as an attached external sink and (2) Silicon micro-channels integral to the silicon CPU die. It is shown that micro-channels integrated into the silicon die do not offer significant advantage over copper micro-channels.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.