My wife and I stopped at the Heritage museum in Libby Montana and found this old very large air compressor. There was a lot of very cool old machinery associated with mining and logging but this was the only Cat.
I was surprised to recognize a lot of very familiar looking parts on the engine. Even though my little D2 is only 7 years newer there is a lot of the same technology. 
Notice the air compressor has its own radiator 

Nice old unit! That would be a D11000 engine/power unit, same engine as in the Diesel Seventy-Five.

That is a after cooler, to condense as much water out of the air as the ambient temp will allow.

she would sure make a racket building full pressure. i wonder if it is air start capable.