What part confuses you? You've described it well
The recirc fan(s) is(are) under the cabin floor (actually just behind the rear wall of the forward cargo).
The stale air from the cabin (exiting via the floor grilles) and generally cool air in the cargo sidewalls is drawn through the underfloor hepa filter (s), then to the recirc fan(s). Ducts leading from the fan(s) go up the main cabin sidewall to the overhead cabin distribution ducts. The air is then distributed in the cabin via vents near the overhead luggage compartments and aircraft centreline.
A picture is worth a thousand words. Perhaps someone can provide a schematic diagram from the Maintenance Manual (21-25-00)
Cheers
Last edited by NSEU; 9th Feb 2016 at 05:17.