Parametric Golden Spiral Fan Duct v0.2

***Updated Apr 2017***
User PGent fixed the shroud wall generation code back in 2015. I owe him or her a huge apology for not merging the bugfix for a year and a half. Thank you and I am sorry.
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Parametric fan duct derived from http://www.thingiverse.com/thing:139984 incorporating cross-section reduction as an exponential function of the golden ratio conjugate. Default settings result in:

40mm cpu fan form factor
M3 screw bores
approximately 25% r