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Currently, muffin fans, like you find in computer power supplies and mounted in computer cases as well as other electronic gear, are manufactured in a manner that makes them unserviceable. They eventually get fouled with dust and other airborne particulates which cause the fan to "gum up" and stop working. If they could be cleaned, their lifetime would be considerably longer. If you try to take one apart they simply break. If you try to clean them by soaking in even with the most exotic solvents, they don't get clean enough to work very long (assuming they get any cleaner at all). They just plain are not cleanable and they must be replaced.
If a muffin fan were manufactured such that it could be disassembled and re-assembled without destroying it, its life could be greatly extended. Probably won't happen because the muffin fan makers would suffer reduced sales. However, a muffin fan cost me $22 to replace about ten years ago, while today it cost me $13. I would gladly pay more for one that I could clean and get more lifetime out of it.