more newbie questions

[dragonflight]

Hi all.

I have so... many questions. I want to build a separator for my DC (craftex/grizzzly 2HP) and one for my shopvac.

I have (well will have) rectangular ducting 5"x10-12" (7"-8" equivalent pipe) and looking at 6-7" outlet pipe for the DC.
I currently have a canister filter, but I'm looking at venting outside - despite the fact that I live in Ottawa, Canada and the current temperature is a balmy 24℉ and Saturday is forecast for a high of 10℉! - more on that later.

So first diameter of separator. Bbased on what I have read 17" is too small for 5", vawoodworker84 made a 20" one for a 6" outlet. I am planning on using a 55 gal drum so up to 24" is easy. On the other hand bigger ones need more air, WayTooLate said he needed 1000CFM on a (presumably) 20-22" separator and 6" inlet/outlet (1000cfm is my target, but pushing what I can expect).

Is it really the amount of air or actually the speed? i.e. would I be better off narrowing/shortening the inlet size in order to speed up the air? If I am venting outside (I live in the country) maybe I don't care that much about fine dust but more about less pressure drop?

Next "simple" question is the question of slot width. Phil has said 1 1/8" slot seemed the right compromise. larger slots tend to allow more scrubbing to occur. Has anyone tried vertical baffles under the baffle to limit movement of air underneath the baffle to reduce this scrubbing?

Finally (for this round) has anyone  - and I guess that means Phil - tried a slot in the wall instead of the baffle? I.E. an inside wall with a side slot inside a wall with a bottom slot.

thanks for any and all responses even guesses.

mike

[retired2]

O.K.  one piece at a time. 

Why rectangular duct?  I don't know if I've ever seen anything other than round for conveying applications.

Venting outside eliminates the hassle of filters which is really nice, but I'm sure you know that unless you are working in an unheated space, when you run a DC for any length of time you are going to pump a lot of BTU's to the balmy outdoors.

What's the CFM rating on your Grizz?  I suspect about 1200 CFM.  Unless you have very little fixed plumbing, you will struggle to achieve 1000 CFM.  I'm right around 500 CFM with a 1200 CFM Delta, and that works just fine with a separator that is a fraction of an inch shy of 20" inside.  It is fairly simple to estimate you CFM's with round plumbing, but I wouldn't know where to begin with rectangular.

Speed vs CFM.  Can't change one with out effecting the other.  For most wood working applications you need a minimum of 400 FPM to keep the waste entrained in the air.  Any slower than that and the waste can drop our and cause clogs.  Those comments assume you waste is the normal saw and planer type wood waste.  If you are talking all sanding fines or dust, get ready to be disappointed.

I have been throwing everything under the sun at my 1-1/8" drop slot, and it does really well.  It did not like wet planer shavings from cedar, they were very long and sticky.  I clogged it up numerous times.  Any extremes in the waste stream will be problematic, but sawdust, sanding dust, and planer shavings work fine with the 1-1/8".

I don't think anyone has tried baffles under the slot, but the slot is where all the Thien voodoo occurs, so I wouldn't mess with it.  There are some cyclonic air currents in the waste drum that is confirmed by the waste mounds.

Venting to the side?  That's a new one, but Phil has done some experimenting with a pipe within a pipe as a separator, so he might have some thoughts.

[dragonflight]

So this reply is longer (a lot) than I intended. I hope I get some feedback. If anyone wants to discuss more perhaps we can open some separate topics

O.K.  one piece at a time. 

Why rectangular duct?  I don't know if I've ever seen anything other than round for conveying applications.

expense/convenience. I've spent years thinking about "proper" dust collection, but never really getting there so I purchased the grizzly (actually Craftex - I live in Canada) several years ago to get by until I built my 5HP Bill Pentz super cyclone ...
Of course now that I have a DC the resistance to building the super cyclone has increased!
As elsewhere, in dust collection speed kills. Everyone seems to have there ducts in the ceiling so you have to have vertical shafts that require faster air. In my case I thought all my tool ports are low and the DC intake is low (a small problem for a separator :)), why not build the ducts around the outside walls - all horizontal/down. So less speed - less HP, larger ducts (same amount of air) - less HP. Maybe my little 2HP (220V 9A, 12 3/4" impeller) is enough.
All to say I'm building rectangular ducts from hardboard ≈5x11 which is roughly equivalent to 8" round.

Venting outside eliminates the hassle of filters which is really nice, but I'm sure you know that unless you are working in an unheated space, when you run a DC for any length of time you are going to pump a lot of BTU's to the balmy outdoors.

Surprisingly not. The first time I thought about it I slipped a decimal point and thought it was prohibitive, but on recalculating it a few years ago it isn't. http://www.sawmillcreek.org/showthread.php?131889-Phil-Thien-s-Baffle-and-Bill-Pentz/page2 has a long discussion starting at reply #24. They somewhat discard the idea, but they (at least some of them) missed the point. At 1000CFM and 75℉ temp difference (a lot) it is about 65,000 BTU/hr or 20KW, and the cost is a couple of bucks/hr of running the DC CONTINUOUSLY - which I never do.
The real issue is in a garage shop even running the DC for 25% of the time it maybe hard for a heater to keep up. In my case I am in the basement, live alone in a big house (about 50,000 cu. ft.) and it will probably only mean an air change once every few hours - I may not even have to open a window for makeup air - my fireplace is airtight and I don't have gas.
To further improve things I'm looking at an automatic blast gate that might operate when I'm idling (electronics is one of my other hobbies), and really reduce the time the DC is venting outside.
If all goes well I will retain the option to use my canister filter if I have to.

What's the CFM rating on your Grizz?  I suspect about 1200 CFM.  Unless you have very little fixed plumbing, you will struggle to achieve 1000 CFM.  I'm right around 500 CFM with a 1200 CFM Delta, and that works just fine with a separator that is a fraction of an inch shy of 20" inside.  It is fairly simple to estimate you CFM's with round plumbing, but I wouldn't know where to begin with rectangular.

The impeller is larger (12 3/4")  and  the motor is a "true" 2HP. it is rated at 1700 CFM (I suspect very, very generously). The use of 8" ducting greatly reduces any ducting losses as does a lower speed. I think 1000 CFM is definitely pushing it, so I am very interested in the compromises in baffle/cyclone design.
Not quite sure what you mean about estimating CFM's. For a rectangular duct with a ratio h:w of 5:11 it has to have 4% more area that a round duct to have the same SP loss

Speed vs CFM.  Can't change one with out effecting the other.  For most wood working applications you need a minimum of 400 FPM to keep the waste entrained in the air.  Any slower than that and the waste can drop our and cause clogs.  Those comments assume you waste is the normal saw and planer type wood waste.  If you are talking all sanding fines or dust, get ready to be disappointed.

I know you meant 4000 FPM, but see my earlier comments on what one needs :). My comment on speed vs CFM was w.r.t WayTooLate needing 1000 CFM for his separator to work well. So in his case your right, but in my case (i.e. I can reduce the intake port size). Same(ish) volume, faster speed in the separator, better separation, greater static loss, maybe not so samish volume?
A further thought, we maybe wrong in our comment to Howard about a double height. Maybe the real advantage of a double height (or a 1.x times) is that the air stream gets squashed closer to the wall. It seems that a particle separates if it moves far enough to get to the wall before it goes out the exhaust - if it starts closer ...
So maybe the right answer is to to place the inlet slightly above middle?
As for the fines vs chips, this is another area that puzzles me. If it came down to a choice I can sweep up chips, I can't fines. It appears that you need air volume for fines, speed for chips. Maybe its fine to let chips fill ducts (obviously not FILL) as long as there is some way to clean them out, again pointing to bigger ducts slower air speed.

I have been throwing everything under the sun at my 1-1/8" drop slot, and it does really well.  It did not like wet planer shavings from cedar, they were very long and sticky.  I clogged it up numerous times.  Any extremes in the waste stream will be problematic, but sawdust, sanding dust, and planer shavings work fine with the 1-1/8".

I thought I read that your separator slot was actually 1-3/8" (not that it matters much)

I don't think anyone has tried baffles under the slot, but the slot is where all the Thien voodoo occurs, so I wouldn't mess with it.  There is some cyclonic air currents in the waste drum that is confirmed by the waste mounds.

This thought was in reference to the idea that a bigger slot lets more scrubbing occur hence poorer fines performance - if we could stop the cyclonic action in the waste drum that might reduce scrubbing allowing a bigger slot?

Venting to the side?  That's a new one, but Phil has done some experimenting with a pipe within a pipe as a separator, so he might have some thoughts.

I read that thread, but it seemed to go nowhere.

Hope you sciatica is improving - I suffered for years with back pain - but knock on wood it seems to have gone away!

mike

[retired2]

What I'm learning is if my butt hurts bad enough, my brain stops functioning.

Yes, I was mixing CFM numbers and FPM numbers.  My system is getting away with marginal numbers, approx 400 CFM and 3000 FPM.  The longest run is to bandsaw.  The performance at that machine is not what I would like, a little too much saw dust left on the tool bed.

You apparently have found some data correlating SP losses of rectangular duct to round.  So, I think you could then use tables that are available for calculating SP losses for round duct to calculate your approximate SP and CFM for your longest run.  You will need a fan curve for your Grizz to get improve your calculation, and if you don't have it you can use curves for other fans that might be similar.

However, I'm still scratching my head over the 4% number.  The HVAC industry uses 10% as a rule of thumb.  I just wonder how conveying efficiencies might be different when you are comparing moving just air very air entrained with waste material.  In a round pipe the waste stream spins its way down the pipe, in a rectangular duct, I am not sure what it does, but I can envision a lot of SP robbing turbulance.

Many people put their main conveying ducts down low, which is fine if your shop layout permits it, but if you have adequate FPM the lift to get up to a ceiling run is not a problem.  In fact, calculations for SP losses don't even consider whether duct is vertical or horizontal, just the total length and fitting losses.

[dragonflight]

It makes sense I've been told that was where my head was for years!

http://www.engineeringtoolbox.com/equivalent-diameter-d_205.html has the formula
equivalent diameter = 1.3*(H*W)^0.625/(H+W)^0.25
and http://www.engineeringtoolbox.com/equivalent-diameter-d_443.html some examples.

In the grand scheme of 5% or 10% doesn't make much difference as we are talk 8" vs maybe 6" ducts.

The swirling in a duct is bad and introduces extra losses. Air naturally straightens out over distance in a duct.
Round duct is used because it has less surface area, less weight and cost and in big plants it is custom.

For me rectangular lets me choose arbitrary sizes and is much easier to build custom transitions, corners and hoods.
All my tools have rectangular to round transitions for ducts (except my bandsaw - not sure what I am going to do yet), so even that is easier.

The Cincinatti Fan documentation actually does double the length of vertical sections for SP calculations, but the main reason I mention no vertical ducts is that the target 4000 FPM is based on vertical ducts and a more modest 2500-3000 FPM is generally quotes for horizontal ducts (to keep them clear).

For constant volume the SP loss varies with the 5th power of the diameter of the duct so going from reduces the SP loss greater than 4 times.

Time will tell and who knows the whole thing maybe a bust, but it is pretty easy and cheap to build the ducts and try!

mike

[retired2]

I didn't read every word of the two links in your post, but I get the feeling the equivalency formulas are for air only.  Makes me wonder how applicable they are for pneumatic conveying.

The best advice I can give you is to abandon the idea of rectangular ducts.  There is a reason why all your tools transition from rectangular to round!

But since you seem to have your mind made up, make sure it is airtight because you will probably have more seems to seal unless you are going to bend sheet metal.  And if you are going to bend sheet metal, make it strong because it will be much easier to collapse than round pipe in an equivalent gage.

[dragonflight]

Retired2,

thanks for your replies and thoughts.
Yes I am set on the rectangular ducts, but you are right it is uncommon territory.
I did some searches and though I give the links you probably don't want to read them but in www.hauckburner.com/pdf/pneumatic%20conveying%20%20%20(GJ74).pdf they mention moving sawdust and talk about moving 30lbs/min  in a 6" pipe! I know it would take me a lot longer than a minute (or even a week!) to generate 30lbs of sawdust. I think we are only moving air and not material!
a quote from page 2

Figure 8 shows the friction loss for
various velocities and duct sizes. These resistances are based on standard air. There are no charts
available which list the friction for the various material carrying air flows with varying percentage of
carrying capacities. The best method of determining resistance of air/material mixture is through pilot
plant testing or experimentation. Most writings on this subject however, seem to indicate that selection
based on standard air provides satisfactory performance.

and http://www.sciencedirect.com/science/article/pii/S0301932211002060 talks about simulations and testing on comparing rectangular and circular duct for conveying material

The particle fluctuating velocities in the pipe are higher than in the channel for all situations, yielding mostly higher wall collision frequencies. As a consequence, in all the considered cases, the pressure drop in the pipe is larger than in the channel, especially for high wall roughness.

I honestly did not read the whole thing, so I'm not sure what was kept constant in the simulations (volume, speed, equivalent diameter ) or they were just testing wall resistance (they major cause of the pressure drop), and as before their concern for wall collisions is with a significantly higher proportion of material/air.

I have - in the past - read about rectangular ducts and Bill Pentz does talk about lined particle board to construct ducts, so I'm confident it is worth the risk/effort - as they say the proof will be in the pudding!

I will be sure to let all know how it works - eventually

thanks
mike

[dragonflight]

I just have to add - kids today have no idea how great all this stuff on the internet is. I would have had to spend days looking up stuff in the library to figure this stuff out in my younger days! Now it takes a few tens of minutes to find and read half a dozen references!!!