A server rack sub-assembly, comprising: a tray configured for slidable insertion into a server rack; a motherboard mounted on the tray and laying in a plane, wherein an underside of the motherboard is separated from the tray by a gap; a heat-generating device mounted on a top side of the motherboard; a thermosiphon system including: an evaporator supported on the heating generating device, the evaporator having a bottom surface positioned adjacent and thermally conductively coupled to the heat-generating electronic device, a condenser supported on the tray, the condenser comprising a body comprising a condensate collector positioned above the plane of the motherboard and a plurality of heat conducting fins extending downwardly from the condensate collector below the plane of the motherboard, wherein the body includes: a cavity, and a plurality of walls that divide the cavity into a central channel that extends through the body along a first axis and a plurality of parallel chambers extending laterally away from the central channel along a second axis perpendicular to the first axis, each chamber having a length along the second axis greater than a width along the first axis and a height along a vertical axis greater than the width along the first axis; a condensate line fluidically coupling the central channel of the condenser to the evaporator, the condensate line extending laterally from the condenser body and the central channel and perpendicular to the chambers; and a fan mounted on the tray and oriented to generate an airflow over the motherboard and between said heat conducting fins. a tray configured for slidable insertion into a server rack; a motherboard mounted on the tray and laying in a plane, wherein an underside of the motherboard is separated from the tray by a gap; a heat-generating device mounted on a top side of the motherboard; a thermosiphon system including: an evaporator supported on the heating generating device, the evaporator having a bottom surface positioned adjacent and thermally conductively coupled to the heat-generating electronic device, a condenser supported on the tray, the condenser comprising a body comprising a condensate collector positioned above the plane of the motherboard and a plurality of heat conducting fins extending downwardly from the condensate collector below the plane of the motherboard, wherein the body includes: a cavity, and a plurality of walls that divide the cavity into a central channel that extends through the body along a first axis and a plurality of parallel chambers extending laterally away from the central channel along a second axis perpendicular to the first axis, each chamber having a length along the second axis greater than a width along the first axis and a height along a vertical axis greater than the width along the first axis; an evaporator supported on the heating generating device, the evaporator having a bottom surface positioned adjacent and thermally conductively coupled to the heat-generating electronic device, a condenser supported on the tray, the condenser comprising a body comprising a condensate collector positioned above the plane of the motherboard and a plurality of heat conducting fins extending downwardly from the condensate collector below the plane of the motherboard, wherein the body includes: a cavity, and a plurality of walls that divide the cavity into a central channel that extends through the body along a first axis and a plurality of parallel chambers extending laterally away from the central channel along a second axis perpendicular to the first axis, each chamber having a length along the second axis greater than a width along the first axis and a height along a vertical axis greater than the width along the first axis; a cavity, and a plurality of walls that divide the cavity into a central channel that extends through the body along a first axis and a plurality of parallel chambers extending laterally away from the central channel along a second axis perpendicular to the first axis, each chamber having a length along the second axis greater than a width along the first axis and a height along a vertical axis greater than the width along the first axis; a condensate line fluidically coupling the central channel of the condenser to the evaporator, the condensate line extending laterally from the condenser body and the central channel and perpendicular to the chambers; and a fan mounted on the tray and oriented to generate an airflow over the motherboard and between said heat conducting fins.