The air intake found in props_farm, with the bonus of rotating fan blades. This prop uses the original GCF-stored textures of the static version, which explains why the archive contains no vmt/vtf.
Set this as a prop_dynamic and use "spin" (no quotes) as default animation to let the magic happen.
Feel free use in your maps. Credit at your discretion.
Creation process: original version decompiled with MDLdecompiler, deconstructed and reconstructed in XSI, animation frames generated with a Python script.
PS: you can still set "idle" as default animation if you want to turn the fan off. I didn't bother making a transition animation: it's on or off, not in-between. In case you need to know, the spin sequence takes one second and consists in a full 360° rotation.