Electronic circuit boards are arranged as respective parallel pairs defining a narrow gap there between. One or more such pairs of boards are supported within a hermitically sealable housing and cooled by way of spraying an atomized liquid coolant from a plurality of nozzles into each narrow gap. Transfer of heat from the circuit boards results in vaporization of at least some of the atomized liquid within the narrow gap. The housing further serves to guide a circulation of vapors out of each narrow gap, back toward the nozzles, and back into each narrow gap. A heat exchanger exhausts heat from the housing and overall system, wherein vapor is condensed back to liquid phase during contact and heat transfer therewith. Condensed liquid is collected and re-pressurized for delivery back to the nozzles such that a sustained cooling operation is performed.