A B767 Captain called in fatigued after an EQUIPMENT COOLING OVERHEAT warning forced a return to land at the departure airport on the outbound leg of an international trip.

Narrative

I had been up since early morning prior to international trip. The incident was; mentally; quite fatiguing. After takeoff we got EICAS 'EQUIP COOLING OVERHEAT'. [We] ran irregular checklist. Just a few minutes later; the First Officer's (pilot flying) primary altimeter goes inoperative; so I take controls. I told the First Officer the easiest way to communicate with Dispatch and Maintenance was via SATCOM. We set it up. The Relief Pilot could NOT communicate on SATCOM so First Officer did that; I flew and the Relief Pilot worked ATC radio. Several times during SATCOM call the SATCOM would drop off requiring another attempt to hook up and/or ACARS communications (slow; distracting; inefficient). Meanwhile there were other communication obligations with flight attendants to keep them in loop. Also the forward panels in cockpit were very hot. We were not sure if the loss of First Officer's altimeter was related to equipment cooling overheat. Nor were we sure that there would not be additional equipment failures. The Purser reported that the First Class temperature was very warm; to the point they were sweating. We had FINALLY come up with the resolution to return to our departure airport. [We] got rerouted [and] consideration given to 'heavy' landing. It so happened we were below Maximum Landing Weight upon arrival. [We] followed SOPs. Uneventful landing; except brakes were hot from landing weight 318;000 LBS. Then came the additional mental gymnastics of explaining all to mechanics; passengers [and] coordinating with ground personnel. By late evening I realized how exhausted I was. I called Crew Desk and reported too fatigued to continue; which was the plan according to scheduling.