6" Top Hat Build

[Sthutch]

Hello to all!  What a great resource this is!  I have spent the last couple of days reading through the various posts and preparing to construct my Top Hat based on Retired2 design.  I am very grateful for all the hard work all of you have put in on these designs.  I will be putting this "upstream" on my 2hp DC.  I already have a Wynn filter, the DC doesn't come until Friday.   I have tried the search function, but I can't seem to put the correct search terms in to get the results I wanted.My problem is I can't seem to find answer to my last remaining questions.

1. I am planning on having a 6" inlet and outlet, but I am concerned that it is too wide for a 20" trash can. With an inlet that wide, will the 6" outlet pick up the smaller particles, or will they be going so fast that the suction won't pick them out the the incoming stream?

2. Will the above problem be made worse by adding a bellmouth to the outlet?

3. The area of 6" inlet is 28.27sq in.".  Would it work if I used a transition to a 3.24" x 10" (32.4sq in.).  Home Depot carriers this, but it seems like it is to short.  Also, this would raise my Top Hat by 2".

   https://www.homedepot.com/p/Broan-3-1-4-in-x-10-in-Rectangular-to-6-in-Round-Galvanized-Steel-Duct-Transition-411/202913013?MERCH=REC-_-rv_homepage_rr-_-NA-_-202913013-_-N

or this one, 4"x8" to 6" round

   https://www.amazon.com/Master-Flow-Register-Box-Flange/dp/B00FK8TMD8/ref=sr_1_fkmr1_4?s=hi&ie=UTF8&qid=1522716799&sr=1-4-fkmr1&keywords=4%22x8%22+to+6%22+round+duct+transition

Any and all advice is welcome.
I would also like to express my appreciation to Phil and Retired2 for all the work they have done on this project!

Sthutch

[RobHannon]

The bellmouth will make the outlet more likely to pickup small particles. Adding height to the seperator and putting the inlet further down should mitigate that, providing I am understanding all of Retired2's experiments correctly.

I would also consider an air straightener. Couple of different designs on here. I went with the protective tubes that go over flroescent lightbulbs. If I had to do it over again I would still use them, but I would use a silicone caulk to adhere them rather than CA glue. The CA made them very brittle.

I am also using the same 3 1/4" x 10" duct transition, but my setup has a very tall chamber so the height was not a concern for that design. It seems to be working well for me, but I am also still in the test and tweak phase of this project.

[retired2]

The answer is simple, don't use 6" plumbing with top hat separators 20" or smaller in diameter.  when you do, you trade a very small improvement in SP losses for a bunch of other headaches.  And from your questions you obviously know what some of those problems are.

If the piping configuration is the same, a 6" system will have lower velocities than a 5".  The separator actually benefits from higher velocities, and results in less waste by-pass.   A 6" bellmouth is huge, and even with a rectangular inlet it gets close to the inlet air stream.  If you insist on using 6", put the bellmouth on your bandsaw and cut 1" or more of the flange diameter off.  You'll still get most of the benefit.

One other comment, I don't particularly like those HVAC transitions.  They are made for low velocity air handling systems.  I have not seen any data to prove my belief, but I think they are terribly inefficient. And for heavens sake don't throw away all your static pressure by using tight radius ells.  Those are the ones you'll find at HD in the bin right next to that transition piece.  Pay very close attention to the my last post on my build.  What you will notice is a separator imposes a huge loss, any way you choose to measure it, so you can't afford to give up more using inefficient fittings.

[Sthutch]

Thanks for the replies!  Just so I am clear,  a 5" inlet is more desirable, while keeping the 6" outlet?

After looking the HVAC transitions at the local hardware this morning, I didn't like what I saw either. 

After I explained what I was trying to accomplish he proposed taking length of 6" round a squeezing it into an oblong shape.  That seemed too simple, but would it really cause any reduction in performance?  The area is still the same.  If it would work for 6", wouldn't 5" benefit from this as well?  By doing so, that would really keep the air to the outside of the separator.

Phil's thread on using an inline separator does come to mine.  It seems to me that if air is slower on the outside of a tube, being round keeps the faster moving air centered.  How would this be effected by an oblong shaped tube?  Slower (and thicker)  air at the top and bottom, while the taller sides have "thinner" slow air?  Just a thought. 

My knowledge of air movement an ducting come for what I have learned in the last week.  So if I make an obvious error, I am sorry, LoL.

Thanks,
Sthutch

[retired2]

The 6" outlet is more the problem than the inlet.  The object is to keep the diameter of the outlet, whether it be a simple pipe or a bellmouth, from approaching the incoming airstream.  a rectangular inlet is ideal for two reasons.  First brings the waste stream into the separator over the drop slot where you want it.  Secondly, it keeps the waste stream further away from the outlet pipe than a simple round inlet would.  So, the outlet is more of a problem because it's shape is fixed.

Better check your math on what happens when you beat a round pipe into a rectangular shape.  The circumference will stay the same, but not the area.  Ideally, you want the area of your rectangle to be equal or greater than the round pipe because rectangular duct is not as efficient as round.  And if you are using snap lock pipe, good luck beating that into a rectangular shape.   

i posted a link to an article several times that makes the case that 5" plumbing is the sweet spot for 1-1/2 and 2 hp dust collection systems.  6" connections on your blower don't mean you must use 6" pipe, just use a 6"x5" reducer.

[Sthutch]

Retired2,

Thanks for the advice!  In the past I have tried to shape round ducting, not much success, and it looks crappy.   I will use a 5" inlet.   I got my bases and rings rouged in tonight.  I plan removing the insides tomorrow and, hopefully, get it together. 

Thanks for the tip on locator pins, it made the job so much easier!

Sthutch

[retired2]

Retired2,

Thanks for the advice!  In the past I have tried to shape round ducting, not much success, and it looks crappy.   I will use a 5" inlet.   I got my bases and rings rouged in tonight.  I plan removing the insides tomorrow and, hopefully, get it together. 

Thanks for the tip on locator pins, it made the job so much easier!

Sthutch

From the one photo, it looks like nice work so far.

[Sthutch]

Update on build:  Based on Retired2's advice, I spent the last few days going through past posts and low and behold I found my answer.  I came across RonS's post AND his PDFs plans for making a 5" round transition to a rectangle.

I have a roll of 24" wide flashing, so I printed out the plans and followed the directions. It came out pretty good. Now that the transition issue is addressed, I can get back to the top hat.

[dabullseye]

i use 4" in and out of my top hat along with a homemade rd to rectangular transition and no bell-mouth and my outlet pipe is flush to the top.

as the air enters the top hat and starts its spiral to the drop slot it might help out to offset the outlet just a small amount so the bell-mouth will be further away from the incoming and by the time the dust has traveled 3/4 around the chamber it has dropped somewhat to be out of the vacuum from the bell-mouth.

just a thought

thats why i dont use a bell-mouth and my outlet is flush with the top   

[retired2]

i use 4" in and out of my top hat along with a homemade rd to rectangular transition and no bell-mouth and my outlet pipe is flush to the top.

as the air enters the top hat and starts its spiral to the drop slot it might help out to offset the outlet just a small amount so the bell-mouth will be further away from the incoming and by the time the dust has traveled 3/4 around the chamber it has dropped somewhat to be out of the vacuum from the bell-mouth.

just a thought

thats why i dont use a bell-mouth and my outlet is flush with the top   

In an article I posted several times, and I don't even remember what the main story was about, but there was an inset that talked about the benefits of a simple roundover of air ports.  Maybe you've already done it, but your flush outlet might benefit from well radiused corners.  If your metal pipe comes all the way through, you could use a double thick collar like mine.  Then back the pipe up and either route the corner with the biggest radius router bit you have, or simply take an aggressive wood rasp and have at it.

[Sthutch]

Well, the Top Hat build is done! I have it temporarily routed to my dust collector with a couple of tight turns, but it works fantastic.  It is better than I imagined. I will post some pics and, hopefully a video later on.

Thank you Retired2 and Phil!

[Sthutch]

Here are some pictures.  I have quite a bit of work ahead of me, gotta get the blower locate directly above the Top Hat, but I just had to play with it some. ;D

[retired2]

Nice work Sthutch!  Very neat.

[dabullseye]

i see you have a rod inside the chamber that supports the drop slot baffle.if you are going to hook this up to a planer or jointer Id keep an eye on that rod and make sure it don't catch shavings and start a birds nest.i don't know what you used for the baffle but most people use 1/4" hardboard and don't use a support. if your rod is a threaded rod i would cover it with a piece of pvc or something that is really smooth and debur the cut edges.

[retired2]

I have a threaded rod in mine and it has never caused a birds nest.  Even when I plugged up my separator planing very wet cypress, the plug started at the end of the drop slot and backed up till the rod became involved, but the rod did not start the problem.  The drop slot just could not handle 6" long wet shavings.  Once in a great while I will see a single piece of waste hanging on the rod, but it does not stay there for more than a few revolutions.

Bottom line, the rod is not a problem for normal wood working waste.