The 360 board has the answer you need. Search "fan". But let me see if I can break it down for you.
Inside the fan room. If you look at your map you'll see a narrow path that is just under the giant fan. There is no floor but there is a grappling hook under there; you just can't see it. What you'll need to do is:
equip the grappling hook. Get close to the entryway, jump down, float your way in, and quickly press R2 when the prompt appears. There you should find another portal doorway.
Expert at nothing. Amateur at everything. Thinking........Thinking......... IDEA NOT FOUND