Adaptable duct sizes?

[Steve B]

I am thinking about making a separator to use with both my DC and a shopvac.  I would build it with 4 inch fittings on the separator for the DC, and use reducer/diffuser adapters to attach 2 1/4 inch shopvac hose.  Has anyone else done this?  Are there potential problems I should worry about?  I realize there will be a sudden velocity changes where the 2 1/4 increases to 4 at the inlet and again where it decreases at the outlet.  Will that interfere with the swirling motion needed for separation?  Will a shop vac pull enough air through such a rig for it to work at all?

Thanks in advance for any thoughts.

Steve

[lifesaver2000]

If I understand your question, I believe I have just completed a similar separator.

I built a "top hat" design with side entry.  The internal height is 6 inches, with an internal diameter of 17 inches.  The inlet and outlet are both four inch closet flanges (a four inch fitting designed to be placed in the floor of a bathroom for installing a toilet), and connected to this is a 4 inch by 2 inch fernco flexible coupling.  The 2 inch end fits a 2.5 inch shop-vac coupling.  The inlet area is designed so that the air is directed into a sort of slot area along the wall that is about 3 inches wide.  It currently sits on top of a 20 gallon trash can.

My plan for this unit was to use it now with the shop vac, but also be able to use it later with a real dust collector.  My hope was that 17 inches would be not too big for the vac, and not too small for the 4 inch dust collector I hope to get.

I just started using this unit yesterday.  So far there is about 3 gallon worth of sawdust in the bottom of the can.  When I took the lid off of the vac, I was able to find about a teaspoon worth of sawdust in the shop vac filter.

I just hope it will work this well if I upgrade to a real dust collector down the road.