New HF DC modification project about to begin - design questions

[rasp55]

Hi all.  New user here.  I've been lurking a few days and decided to improve my dust collection capabilities.

I've acquired a new HF 2HP dust collector, but do not plan to assemble it per their instructions.  Instead I want to mount the motor/blower directly on top of a Thien separator blowing directly into a Wynn filter I already have. 

Questions:
1.  Has anyone considered using a simple plywood box underneath the Thien baffle to collect the chips?  it seems that this would be an easy way to create something that cannot collapse.  My intuition tells me that the box could be square for easy construction and not affect separation.

2.  Assuming the above is acceptable, is there any reason the top of said box could not be square with the baffle slot routed into it?  To put it another way, instead of creating a separate piece to sit in a round opening, just make the opening in the piece that is the top.

3.  Any reason not to use clear poly carbonate to create a circular separation chamber that sits on top of the chip collection box?  I'm planning a side inlet for this.

4.  I'm going to use either 4" S&D or 6" coming into the separator.  Should the width of the 240 degree slot be wider?

Thanks to all

[phil (admin)]

As long as the separation area is round (as Charlie points out) so the air can spin, the bottom of the container can be square.

Can you elaborate on question #4?

[rasp55]

I've a attached a sketch that might explain better.  The input from the machines would come into the separator on the left side in this image, straight into the circular section.  The HF blower/motor is mounted directly above the central hole (exit) in the separation chamber.  I understand that I'll need to bring a pipe down into the chamber an inch or two, at least that's what I think I've read in other threads.

What I wasn't clear on was the width of the slot where the chips fall into the collection box.  It seemed that 1-1/8" was about right  for a shop vac powered system with 2-1/4" hose.  I'm planning at least 4" ducting from the machines, maybe larger.  Should the slot be wider?  Half the nominal duct size?

[phil (admin)]

What I wasn't clear on was the width of the slot where the chips fall into the collection box.  It seemed that 1-1/8" was about right  for a shop vac powered system with 2-1/4" hose.  I'm planning at least 4" ducting from the machines, maybe larger.  Should the slot be wider?  Half the nominal duct size?

Nope, the size of the slot is determined by the media we're separating.  The 1-1/8" works great for most woodworking.  Some users that plane lots of soft woods (which tend to provide long, stringy shavings that clog things) may need to increase that to 1-1/2", but not much larger than that.

[AlanWS]

I would add that the reason you don't need to increase the size of the slot for a bigger DC is that you don't have or want much air to go through the slot.  Only the sawdust and chips, held to the outside of the round container by centrifugal force, should go through the slot.  The genius of the baffle is that this does work well to prevent the circulating air from stirring up the dust that's already fallen through.  The more smoothly the air can circulate in the upper part, the cleaner will be the air drawn off from the center.

The circulation region should be round, but that does not mean it's not helpful to have a plywood box around it.  If the round portion is strong enough to maintain its shape under vacuum, then you don't need to surround it.  But a much weaker cylinder would become useful if it were surrounded by a strong leakproof box.  And it might be easier to seal a single box well, and to suspend a cylinder within it, than to seal the cylinder top and bottom to other parts.