Passive two-phase cooling of air circuit breakers in data center power distribution systems

This work presents an experimental investigation of a two-phase looped thermosyphon design for cooling Air Circuit Breakers (ACBs) in switchgear used in data center power distribution systems. The FC72 charged thermosyphon is comprised of a bespoke evaporator that is fixed tightly to the copper adaptor of the ACB. Dielectric tubing connects the evaporator to a remote naturally aspirated and thin form factor condenser. Since both the working fluid and the connection tubing are dielectric, the thermosyphon provides electrical isolation between the ACB and the remote heat sink. The condenser is a thin serpentine channel with surface extensions and is cooled by natural convection and radiation to the ambient environment. Benchtop thermal performance tests were performed for increasing load power for two scenarios: for the stand-alone looped thermosyphon and for the case where copper bus bars are also fixed to the ACB adapter. For 60 W thermal power dissipation, a nominal operational condition, the thermal resistance of the looped thermosyphon system was determined to be 0.55 K/W without bus bars attached. With bus bars attached, this decreases to 0.42 K/W which results in a breaker to ambient temperature rise of about 25 K, which is significantly lower than operational limits of 85 K temperature rise. In-situ tests were then performed on a live ACB system carrying a 2000 A load and showed that the looped thermosyphon system was capable of decreasing the ambient temperature rise by 26 K, which is significant. The low thermal resistance of this passive cooling technology opens the opportunity for increased electrical service per breaker and/or a significant reduction in the volume of copper bus bars used with associated cost reduction of ACB technologies.