Well, after 2-3 months worth of full weekends on the project, I finally have a new dust collection system.
It cost a bloody fortune and about a pint of blood, but boy does it suck!
I found a CL deal on a DustHog SC3400 collector. This 700# monster moves an incredible amount of air through two big conical filters via a 7.5HP 3PH motor, but upon getting it home I realized that its catch bin was incredibly small.
So after building an elevated outdoor shed to put the collector (and an air compressor and a phase converter) in, I started thinking about a pre-separation cyclone. This led to Oneida and eventually to Bill Pentz's designs. The big problem is that nearly all the "home" style cyclones are based on much smaller air movers, and they'd be hopelessly overmatched by the 7.5HP motor in my collector.
Oneida sells a standalone industrial cyclone that had the requisite 30" diameter, but it was about $2000 which was more than I could justify. So I scaled up Bill's plans and went to work on the sheetmetal. 20ga sheet is a bear to form and weighs a lot when the assemblies get this big, but necessary with the pressures involved in a system of this size.
Here's the finished exterior of my shop as of this weekend. Note that accessing and installing that exterior corner fitting was somewhat nightmarish when the closest I could get is where that ladder in the photo stands:
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The primary trunk is all ducted in 10" 20ga spiral duct. I ran an 8" flex drop to my big CNC router through a big ball joint, and 6" drops to everything else. After years of MDF dust escaping router projects and covering everything around in a fine, toxic dust I'm so excited I can hardly stand it. My metal working machines are smiling almost as much as I am.