Marcus,
It depends on the specific projector model. When you ceiling-mount a projector, the top of the projected image will be approximately level with the lens-center. Some projectors have a positive (or negative) throw offset, meaning that the top of the image will be a little below (or above) the lens-center. The amount of throw offset differs from model to model (some have a variable offset, or the "lens-shift" feature), and varies with throw distance.
If you need to mount the projector ~18" below the ceiling (at the lens center) to clear the fan, it follows that you need a projector with zero throw offset, so that the screen top is also ~18" below the ceiling. For a ~40"-tall screen, this will result in a 38" clearance below the screen bottom.
A couple of thoughts here:
(1) I strongly suggest that you also consider placing the equipment rack somewhere else, along the side or back wall -- for example, where the wine storage or the PC is currently sitting. Not only this will remove the restrictions on the screen mounting height, but it may well make the front wall visually more attractive, without all the clutter of the electronics.
(2) Choosing the projector based on the throw offset requirement is, IMO, absurd. You should choose the projector first, and then optimize the geometry for the specific projector.
(3) So, depending on the projector choice, temporarily taking down the fan may be the only feasibly solution.
So, what is the projector(s) you have in your mind now?
