Please allow me to take part in your interesting conversation.
“the “static electricity” theory”
If I may say, this can never happen in a nitrogen-padded fuel tank, in inert atmosphere (no oxygen.) I wrote to Boeing at the time inquiring about the subject, I got no answer, however, I still assume that a nitrogen pad is in place.
With no oxygen in the tank, fuel vapor/nitrogen mixture will never ignite unless the tank ruptures and available sparks through static electricity or faulty wires or any other source.
“Still, I definitely buy the overheated fuel from two hours of running the AC unit (located under the tank) and the spark from the chaffing wires “
The low pressure nitrogen pad, say at a few inches of WC will prevent any sparks from igniting, as I mentioned above.
Flying the plane will cause very cool air (in the summer) at a high velocity to cool the A/C unit and the fuel tank almost immediately when the plain is gaining altitude. Heat input will have no effect on a huge amount of fuel, in a small heat transfer area of the tank.
Commercial aircraft do not have nitrogen inerting systems installed. They’re too costly and heavy to be practical from what I understand.
The AC units are below the center tank, which would prevent a direct heat exchange of the airflow against the bottom of the aircraft and the center tank.
That system was developed at Parker Hannifin in the mid 70s. to the best of my (in this case rather limited) knowledge it was never adopted for commercial airlines.