Bellmouth Outlet on double height top-hat?

[Alan H]

I know the rule of placement for a standard outlet in a single height tophat is 1/2 the height of the inlet centered on the inlet.  How does this change with;

(1) Using a bellmouth
(2) a double height, or higher top-hat separator?

My inlet is a 6" metal duct that transitions to a 4"x10" rectangle.  My outlet is a spun bellmouth that I'm going to have to attach to the outlet pipe, as the bellmouth is only about 4" high anyhow.  The circumference will be about 22" as that's the dimension on the fiber-barrel that I have for my collection bucket.

Let me know if you need any additional information.

[retired2]

There is test data in a table in the thread for my build, just below this post.  It shows slight air flow differences between positions, but not much.  As I recall separation did not change very much.  I think there might be a photo in that thread showing the attachment of the bellmouth to a straight section of pipe.

Ordinarily, I would recomment putting your bellmouth flange at or slightly below the bottom of the inlet.  However, you have a pretty tall inlet so I don't know if that is reasonable.  Is your double high 20" tall?  If so, my advice would be ok.

[alan m]

I use the rule that the outlet should be half the height of the inlet up from the baffle. so it doesn't matter if it is a single ,double or triple height.
I havnt seen any that are more than double height so its hard to say if that introduces any issues.

as for the bellmouth I think it is the same but measured to the bottom rim. so half height up from the baffle  to the rim of the bellmouth

[retired2]

I use the rule that the outlet should be half the height of the inlet up from the baffle. so it doesn't matter if it is a single ,double or triple height.
I havnt seen any that are more than double height so its hard to say if that introduces any issues.

as for the bellmouth I think it is the same but measured to the bottom rim. so half height up from the baffle  to the rim of the bellmouth

I'm not sure where you found the rule you are quoting, but I've never heard it.

[alan m]

not really a hard and fast rule. but it make sense.
as the separator height increases the outlet has to get longer to get the benefit .
so keeping to half the inlet height above the baffle makes sense

[retired2]

not really a hard and fast rule. but it make sense.
as the separator height increases the outlet has to get longer to get the benefit .
so keeping to half the inlet height above the baffle makes sense

It may indeed work just fine, I had just never heard that rule of thumb.  It does seem to produce reasonable locations for all but extreme cases.  In fact, as my tests show the performance is not terribly sensitive to position.  And ironically the best airflow was achieved when the bellmouth was closest to the baffle. It might be that in that position the air flow into the bellmouth was smoothed by the baffle.

[dabullseye]

i tried using the bell mouth at halfway also lower and higher. i don't have any hard data but from my testing other than just my observation but i ended up not using the bell mouth and with the outlet flush with the bottom of the top. i did my testing with the table saw and the drum sander and watched what made it to the bag. six years later i have never had to empty the bag and there is less than a gallon milk jug  of dust in the bag.
i made this crazy thing with a 10" tire inner tube so i could play with setting.its been under the bench for yrs so its kinda dusty

[alan m]

i tried using the bell mouth at halfway also lower and higher. i don't have any hard data but from my testing other than just my observation but i ended up not using the bell mouth and with the outlet flush with the bottom of the top. i did my testing with the table saw and the drum sander and watched what made it to the bag. six years later i have never had to empty the bag and there is less than a gallon milk jug  of dust in the bag.
i made this crazy thing with a 10" tire inner tube so i could play with setting.its been under the bench for yrs so its kinda dusty

i tried using the bell mouth at halfway also lower and higher. i don't have any hard data but from my testing other than just my observation but i ended up not using the bell mouth and with the outlet flush with the bottom of the top. i did my testing with the table saw and the drum sander and watched what made it to the bag. six years later i have never had to empty the bag and there is less than a gallon milk jug  of dust in the bag.
i made this crazy thing with a 10" tire inner tube so i could play with setting.its been under the bench for yrs so its kinda dusty

so your outlet doesn't extend into the separator at all.
I tried that years ago and got a lot of bypass.

I don't think any of us are really qualified to 100% claim anything  too many variable in our shops. what works for you  might not for me.
that's the interesting thing about dust collection.

[retired2]

i tried using the bell mouth at halfway also lower and higher. i don't have any hard data but from my testing other than just my observation but i ended up not using the bell mouth and with the outlet flush with the bottom of the top. i did my testing with the table saw and the drum sander and watched what made it to the bag. six years later i have never had to empty the bag and there is less than a gallon milk jug  of dust in the bag.
i made this crazy thing with a 10" tire inner tube so i could play with setting.its been under the bench for yrs so its kinda dusty

i tried using the bell mouth at halfway also lower and higher. i don't have any hard data but from my testing other than just my observation but i ended up not using the bell mouth and with the outlet flush with the bottom of the top. i did my testing with the table saw and the drum sander and watched what made it to the bag. six years later i have never had to empty the bag and there is less than a gallon milk jug  of dust in the bag.
i made this crazy thing with a 10" tire inner tube so i could play with setting.its been under the bench for yrs so its kinda dusty

so your outlet doesn't extend into the separator at all.
I tried that years ago and got a lot of bypass.

I don't think any of us are really qualified to 100% claim anything  too many variable in our shops. what works for you  might not for me.
that's the interesting thing about dust collection.

It could be the behavior of 2X separators is a little different than 1X.  I have never built or tested the former so I have zero data or observations.

[Alan H]

There is test data in a table in the thread for my build, just below this post.  It shows slight air flow differences between positions, but not much.  As I recall separation did not change very much.  I think there might be a photo in that thread showing the attachment of the bellmouth to a straight section of pipe.

Ordinarily, I would recomment putting your bellmouth flange at or slightly below the bottom of the inlet.  However, you have a pretty tall inlet so I don't know if that is reasonable.  Is your double high 20" tall?  If so, my advice would be ok.

I think I may have seen your chart.. I also seem to recall seeing something about putting it below the inlet.  That seems to make sense to reduce by-pass.

I have metal that I'll be using for the sidewalls. It's 24" x 36" and I have 2 to make sure I have enough to make the top-hat.  I can do up to 24" with this arrangement, and probably will so I don't have to worry about an uneven cut.  I have the room as well, so height is not an issue.

I guess I should have mentioned that I'll be using the HF 2 HP collector and increasing the inlet size to 6" to match the ducting.  I may do the impeller upgrade later, but I'm going to try it as-is to begin with.

[Alan H]

not really a hard and fast rule. but it make sense.
as the separator height increases the outlet has to get longer to get the benefit .
so keeping to half the inlet height above the baffle makes sense

It may indeed work just fine, I had just never heard that rule of thumb.  It does seem to produce reasonable locations for all but extreme cases.  In fact, as my tests show the performance is not terribly sensitive to position.  And ironically the best airflow was achieved when the bellmouth was closest to the baffle. It might be that in that position the air flow into the bellmouth was smoothed by the baffle.

Interesting points made here, I may try this as wel.l.. I'm contemplating making the outlet pipe long enough to experiment with flow, thank you for discussing this in my  thread as it has given me additional food for thought as I proceed with my build.

Thanks all for the feedback and I'll try to keep this thread updated with my progress on the placement.

[Alan H]

So I've been working on this and looking around in my shop and have driven myself to another impasse.

I have a Fiber Barrel that I feel will be sufficient enough in capacity for the amount of woodworking I will be doing, however it is rather narrow in circumference.  It's only 17.5" in diameter.  With this, I've been considering stepping up the diameter of the Thein separator to about 18.75" and using the rings that attach to the barrel lid, to step up in diameter to get to the separator size.  The lid is metal and would be cut to act as the baffle.

My other option is a Rubbermaid Roughneck trash can that would allow me a separator diameter of about 20.5".  My hesitation on this is that I don't want to implode the plastic trash can.  I know it's not likely, but the concern is there, especially if I am careless and forget to open a blast gate before starting the DC system.

Is there a rule of thumb on diameter of the separator to the inlet/outlet sizes?  Both will be 6" on mine.

Any help and opinions are greatly appreciated.

[retired2]

So I've been working on this and looking around in my shop and have driven myself to another impasse.

I have a Fiber Barrel that I feel will be sufficient enough in capacity for the amount of woodworking I will be doing, however it is rather narrow in circumference.  It's only 17.5" in diameter.  With this, I've been considering stepping up the diameter of the Thein separator to about 18.75" and using the rings that attach to the barrel lid, to step up in diameter to get to the separator size.  The lid is metal and would be cut to act as the baffle.

My other option is a Rubbermaid Roughneck trash can that would allow me a separator diameter of about 20.5".  My hesitation on this is that I don't want to implode the plastic trash can.  I know it's not likely, but the concern is there, especially if I am careless and forget to open a blast gate before starting the DC system.

Is there a rule of thumb on diameter of the separator to the inlet/outlet sizes?  Both will be 6" on mine.

Any help and opinions are greatly appreciated.

There are no magical rules about separator diameter that I am aware of.  However, if you go too small, then waste by-pass becomes a problem because the outlet pipe is too close to the separator wall.  Since you are using 6" inlet and outlet, diameter becomes more critical.  I don't remember if you are planning on using a bell-mouth but the makes it much worse because the flange on a 6" is pretty large in diameter.

I'd recommend you go to HD, buy the Rubbermaid Brute.  It is cheap, and you will not collapse it unless you hook up a shop vac to it.  I have never collapsed mine and I dead-head it for a few seconds every time I use my system because I have an intentional motor shut-off delay built in to mine.  So, when I close my blast gate the system continues to run for about 10 seconds.  If you have light gage 6" pipe you have as much chance of collapsing it as you do the brute can.

You need to think about your plans for emptying the drum.  I have to transfer everything to a 33 gal garbage bag and carry it out of my basement shop.  Emptying the drum into a bag can get messy if the drum is too large.  Even with my mid-sized Brute I have spillage that must then be broomed over to my floor sweep where if goes back into the vacuum system.  It is pretty easy, but it creates a very small amount of dust which I've already collected once.

[Alan H]

Thanks for the advice @retired2.

I'll go with the larger Rubbermaid can, but I think I'll only go buy the Brute if the one I have looks like it's going to give me issues. 

The dimensions you show in your thread seem to be about the same, so I'll build mine to work on the bigger diameter.  BTW, yes I do have a bellmouth.

I have plenty of casters around in the shop, so I'll build a platform that the can will sit on and self center, and then a cam-lift to raise it to the separator housing.

[retired2]

Thanks for the advice @retired2.

I'll go with the larger Rubbermaid can, but I think I'll only go buy the Brute if the one I have looks like it's going to give me issues. 

The dimensions you show in your thread seem to be about the same, so I'll build mine to work on the bigger diameter.  BTW, yes I do have a bellmouth.

I have plenty of casters around in the shop, so I'll build a platform that the can will sit on and self center, and then a cam-lift to raise it to the separator housing.

I don't know what your design looks like, or the drum you are planning to use, but if you are going to build it with even a remote possibility of having to change to a different drum, you need to think about how much work will be involved to do that.  It might require you to start over.

I have two Rubbermaid drums in my basement, the Brute, and another one that is a few gallons smaller in size.  The smaller one is a nice can and I keep a 30 gal trash bag in it and use it for
general waste in the shop.  However, I think it might be possible to collapse that one if it were attached to a vacuum system.  It has lighter gage walls and lacks the stacking collar that is built into the Brute.  If your Rubbermaid is larger than the Brute, it may have stiffeners built in that will keep it from collapsing.

If you think the flange diameter on your 6" bellmouth is so big that it is causing by-pass, don't be afraid to put it on a band saw and cut an inch off all the way around.  You will still get nearly all the benefit of the bellmouth. I've posted a chart several times showing different end treatments of pipe and the efficiency of each.  The bellmouth is the best of course, but even a crude flat flange on the end of the pipe makes a big improvement over just straight pipe.