(https://www.dropbox.com/s/fji1kfnjyaqv5aq/16gal-filter.png)
The great luck in particle separation afforded by the Thien Separator is well proven, largely through anecdotal evidence of not having to clean the filter for the original dust collection system or shop vac, sometimes for years. What remains to be evidenced of its success is the commensurate redesigning of the air filter. For example, in my 30+-year old Craftsman 16-gallon shop vac, more than 1.3 gallons is occupied by the filter.
So the question that follows: How much of the air filter's design criteria needs to be predicated on solely not creating any back pressure on the blower? Is there a standard equation used in sizing air filters which incorporates, among the various air flow components, a
dirt factor (for lack of a better term)?
Sorry about that - I'm still unclear why my inline images aren't being displayed.
https://www.dropbox.com/s/fji1kfnjyaqv5aq/16gal-filter.png
Then the next question: Where do I begin to look for an air filter that fits my system's configuration?
One place I found (after much searching) was this fantastic tool at K&N Filters:
https://www.knfilters.com/search/universal
It allows you to first select an air filter based on its shape; e.g., round, oval, tapered conical, etc., and then enter minimal constraining dimensions. Leaving some aspect unanswered; e.g., height, allows the returned search results without limit for height. Note that you can order the search results, for example, by height. This is good if your system's design options are flexible in a given direction and less so in another. Finally and most importantly, they also give a cross reference chart of other manufacturers and their respective air filter model numbers.