The circular duct is held on by Plastic Clips as opposed to Screws, hidden beneath.
You can remove it with some careful prying giving you direct access to the Fan Blades as well as being able to properly clean the Duct itself.
Also if you're okay with your system being MIA for a few hours while it drys... once you take off the Duct and unscrew the Fan; you can simply simply soak the Heatsink in Warm Water to fully clean it out without compressed air.
I tend to use Compressed Air on areas that are difficult to access or have delicate Electronic Parts.