Top Hat re-do - question

[Howard In Toronto]

It's been a while!

I'm about to re-do my Top Hat Thien Separator and hav a couple of questions.

Almost 100% of my output is MDF so capturing the super-fine dust is my biggest issue.
My setup is - Thien Top Hat - 6" hose - 30-gallon fibre drum with 19" OD - Delta 50-850

To re-do my Top Hat Separator using a Rubbermaid Roughneck, my questions are -
1 - seeing my biggest issue is capturing MDF's fine dust is the 44 gallons unit's 24" OD TOO big
(keeping in mind 6" hose and the 50-850)
2 - what is the best slot width in a 24" OD (22" ID) situation for fine dust
3 - the 32 gallon's OD is 21"- does anyone have any experience with this unit's walls collapsing
4 - seeing I use 6" hose - how tall do you think I can make the Separator's walls - 8"? 9" just over the 6"?

I appreciate your help.

Howard in Toronto

[phil (admin)]

It has been a while.  Hope things are going well.

(1) Nope, not too big.

(2) For fine dust you can tighten the slot's width up some.  I'd try 1/2" to to 3/4" to start.  That is provided you are ONLY going to feed fine dust.  I've had others say stuff larger gets caught above the baffle after they swore they were only going to run fine dust.

(3) No experience there.  I really like the plastic drums over the plastic garbage cans, though.  The plastic drums are guaranteed not to collapse.  They're also much stronger.  You fill one of those Roughnecks anywhere near full with fine MDF dust and it is going to be pretty darn heavy.

(4) Anything in the 8-9" is going to be about the same.  If you make the separator tall enough (think a two-story separator) so you can extend the outlet tube below the bottom of the inlet, you will get better fines separation.  Obviously the unit will be larger and heavier, though.

[retired2]

It has been a while.  Hope things are going well.

(1) Nope, not too big.

(2) For fine dust you can tighten the slot's width up some.  I'd try 1/2" to to 3/4" to start.  That is provided you are ONLY going to feed fine dust.  I've had others say stuff larger gets caught above the baffle after they swore they were only going to run fine dust.

(3) No experience there.  I really like the plastic drums over the plastic garbage cans, though.  The plastic drums are guaranteed not to collapse.  They're also much stronger.  You fill one of those Roughnecks anywhere near full with fine MDF dust and it is going to be pretty darn heavy.

(4) Anything in the 8-9" is going to be about the same.  If you make the separator tall enough (think a two-story separator) so you can extend the outlet tube below the bottom of the inlet, you will get better fines separation.  Obviously the unit will be larger and heavier, though.

Phil,

Now that my filter bag has developed some cake, I am noticing a fall-off in performance, especially at pick-up ports.  I must now push floor waste deeper into the throat of my floor sweep in order for it to be captured.  None of this comes as a great surprise since I have run a lot of fines recently, and of course some of that has by-passed the separation and added to the filter cake.

I am thinking about replacing my canvas filter bag with a Wynn filter.  Based on the testing I did with a clean filter bag versus no filter bag at all, I do not expect the Wynn filter will result in performance that is any better than a clean canvas filter bag.  However, I do expect it would filter out finer particles due to its specs, and I hope it will can go longer between cleanings simply because of the increased surface area.

I would consider building a new separator if I thought I could make an improvement in SP loss.  The obvious variables are the height and diameter.  I am intrigued by your comment about a two story separator.  You say that configuration would lead to better separation of fines.  If so, that should result in less cake and longer run times between filter cleanings.  But is there any test data that shows a taller or larger diameter separator yields lower SP losses, i.e. all other things being equal? 

[phil (admin)]

I don't have any data, but it would have to lower resistance because:  (1) Resistance is proportional to the airspeed squared.  If the separator is taller, the volume above the baffle is greater, therefor the air speed is lower, and the resistance is thereby reduced.

Now, the problem is that "airspeed squared" (specifically the squared part).  We don't know where we are on the curve, right?  So if we're already way up on the curve, then the extra volume may not reduce airspeed enough to see a substantial reduction in resistance.

And I suspect that is where we are on the curve.  Not only with my separator, but all separators.  That is the reason we pay such a dear penalty of tight radius elbows.  That is, the air is moving so fast that any turning takes its toll on resistance.

So how do you like that answer?  Pretty worthless, eh?  Yes, resistance would be reduced.  But it may not be reduced enough to make it worth your while.

There is an added component, though.  A two-story benefits from the fact that the revolving airstream is being drawn DOWN further.  Which means that there is less collision with the incoming airstream.  Which means less of a hit on resistance there, too (and that is also one reason separation improves).

[retired2]

I don't have any data, but it would have to lower resistance because:  (1) Resistance is proportional to the airspeed squared.  If the separator is taller, the volume above the baffle is greater, therefor the air speed is lower, and the resistance is thereby reduced.

Now, the problem is that "airspeed squared" (specifically the squared part).  We don't know where we are on the curve, right?  So if we're already way up on the curve, then the extra volume may not reduce airspeed enough to see a substantial reduction in resistance.

And I suspect that is where we are on the curve.  Not only with my separator, but all separators.  That is the reason we pay such a dear penalty of tight radius elbows.  That is, the air is moving so fast that any turning takes its toll on resistance.

So how do you like that answer?  Pretty worthless, eh?  Yes, resistance would be reduced.  But it may not be reduced enough to make it worth your while.

There is an added component, though.  A two-story benefits from the fact that the revolving airstream is being drawn DOWN further.  Which means that there is less collision with the incoming airstream.  Which means less of a hit on resistance there, too (and that is also one reason separation improves).

Well Phil I can't say your comments have given me reason to go rushing back into the shop to build a new separator.  However, if I were just starting I would probably build it higher and larger in diameter. 

If there are parallels to the losses encountered with ells then we may indeed be at the point where there is very little to be gained by building larger.  The loss curve for ells is asymptotic.  The losses are very high at 1D, but improve dramatically only to about 2.5D.  By 3.0D the curve is nearly flat and larger diameter bends yield almost no further gains.

Sometimes the laws of physics can be a pain in the butt!