Hyperbolic intake will pull the most air and if you have enough CFM (> 650) a 6 inch pipe. Here is a link to a previous discussion:
Dust collector size
Hi, I am just finishing up a a new shop I'm building on my house! It is a glorious and luxurious 270 feet! I can now have an actual table saw. I'm just sooo happy. In any case, dust collection comes to mind as this shop actually has walls and a roof. I have a few questions. 1) Do people...www.aawforum.org
Agreed Steve, the elliptical shaped bellmouth is the the most efficient port design for fine dust capture and closeup to the work piece works best, provided that there is adequate air flow as specified by Bill Pentz.
Woodturning is one of the worst culprits in generating and broadcasting fine dust while sanding and the larger the diameter of the piece the greater the risk of that.
To summarise the research finding on bellmouths that Steve referred to...
The best bellmouths are those that are short and fat with the elliptical profile the winner
Since getting a meter for measuring the fine dust (PM2.5μ/m3) levels in my workshop I have abandoned the large dust enclosures that I have used in the past that I found were less effective than a strategically placed bellmouth.
As I only have one lathe I just turned my bellmouth out of wood. If I had more than one lathe or multiple other woodworking machines I might have used Steve's method of making a wooden mould and make multiple bellmouths from PVC ducting with that.