A thermosiphon cooling assembly includes a plurality of hairpin condenser tubes that extend into openings of a boiling chamber for collection of refrigerant vapor from a boiler housing. The distal ends of the tubes extend into the chamber from a cover the least distance midway between the ends of the boiler housing and increase in distance in opposite directions toward the ends. The hairpin tubes are in groups with each tube in a group having the same length and the groups having respectively different lengths and at least one group includes more than one tube.