If you found a way to make a fan that blows air out of potato chips, in principle that would not be patentable, because you just "stuck together" two existing things; fans and potato chips. It doesn't do anything novel.
Funny you should mention fans given Dyson's recent innovations. Dyson had to massage the wording of the patent quite massively to get their patent for the Air-Multiplier fan, not surprising given how it was invented by Toshiba in the 80s
"Massaging"?? Having just read that news report, it seems they just needed to include other inventive features of the design in the patent, which probably just means the claims had to be 'narrowed' to include that feature.
I don't see any issue with that. There is nothing wrong with patenting an improvement to an existing invention as long as it's non-trivial.