Thread: Focused Dust Collection

Originally Posted by dmorse

So you're using the Reynolds Number as your measure of turbulence?

It's a indicator of turbulence caused by the air speed in ducts but it doesn't take into account junctions or even minor wall defects which can inject significant (even the majority of) turbulence in DC systems.

Originally Posted by BobL

It's a indicator of turbulence caused by the air speed in ducts but it doesn't take into account junctions or even minor wall defects which can inject significant (even the majority of) turbulence in DC systems.

Of course. That's why we take measurements where there's at five diameters of straight pipe on either side of the measurement point. That way viscosity is able to damp out most of the variation and leave us very close to fully developed flow at the measurement point.

An important use of the Reynolds Number is scaling airflow measurements. For example, a scale model of an airfoil can be tested in a wind tunnel and the dimensionless results like lift coefficient and drag coefficient will be the same for the full size part if velocities are adjusted for the same Reynolds number. Of course a parameter like the loss coefficient for duct components is also a pressure ratio like lift and drag.

BobL --- I have read and re-read your posts on BMH and other topics and it is very interesting and helpful.

I also took note of the tests you did on 90 degree elbows. After the comment about me not seeing much change on a 100 mm due to turbulence, I did some reading about turbulence in the elbows.

I noted in several of the pictures you posted that you had 90 degree elbows very close to the BMH like on the bandsaw pictures. Does the turbulence from the elbow impact the BMH performance? Also, there was a picture of a BMH at a drill press with flex 90 degree bend that would cause turbulence. Are these reducing the performance of the BMH. If so, how would one overcome these issues.

Originally Posted by Maple71

BobL --- I have read and re-read your posts on BMH and other topics and it is very interesting and helpful.

I also took note of the tests you did on 90 degree elbows. After the comment about me not seeing much change on a 100 mm due to turbulence, I did some reading about turbulence in the elbows.

I noted in several of the pictures you posted that you had 90 degree elbows very close to the BMH like on the bandsaw pictures. Does the turbulence from the elbow impact the BMH performance? Also, there was a picture of a BMH at a drill press with flex 90 degree bend that would cause turbulence. Are these reducing the performance of the BMH. If so, how would one overcome these issues.

Ideally one would not use any elbows or flexy on machinery but if straight ducting was used in those positions it would seriously impact on an operators working position and lead to safety issues so the elbows and flex are necessary. On a BS one needs to be easily able to open the doors and flexy just make this easier. Using BMHs at best recovers some of these losses.

On something like a DP the position of the work position changes often so flexy is used to move the dust collection point to get it as close as possible to the work.

On some machinery I like to be able to vacuum up escaped chips after cutting so I deliberately use extra flexy on one of the collection points so I can easily disconnect (I rarely if ever make my connections permanent) that collection point and use the flex as a vacuum cleaner.

Where I have space I set up seperate collection points, one for the machine and a seperate hose for vacuuming up chips.
On my TS Over Head guard a short length of flexy (2) is used for he guard itself - it can swing right right out of the way if needed and a separate port (1) is used for the vacuuming.
The main thing the vacuum flexy is used for is just above the saw is a skylight window and if I leave this open leaves can enter.

Gates.jpg
BTW the top of the guard uses 25mm thick acrylic sheet and the collection point has a BMH like curve routed into the acrylic.

Some of my machines make dual use of one collection point. An example of this is my bandsaw and my grinders where I can pull the flexy of the BS under table connection and plug that into my metal duct catcher - the metal dust catcher is moveable because I have 3 grinders.

With elbows, the most restrictive are those with a tight radius of curvature which are much worse than those that have a longer radius of curvature.
He in Australia we typically see elbows with what are called a 1R radius - the radius of curvature is the same as the radius of the pipe itself, and 2R curvatures.
When 6" ducting is used at typical DC air speeds the losses from using a 2R elbow are surprising low. Low enough not to bother with going to 3R or 4R elbows. The are elbows used by the communications industry that are up to 1200 mm available but large radii curves take up a lot of space and are virtually impractical around machinery.

Have you ever done any testing to see how much deterioration in performance there is with an elbow before the BMH?

I look at your BMH setup with the bandsaw and got me wondering. The BMH has advantages but are compromised by the turbulence induced by the elbow before it. I have tried the BMH in several places and one thing causes me problems and that is the size. The BMH is large and prevents one from getting too close. I always believe that collection as close to the source is a critical consideration.

So if I can get significantly closer to the source with a naked tube end, it seems a better solution than farther away with a BMH and elbow. No doubt in some situations like your example of drum sander on a drill press the BMH makes more sense to me.

I am far from an expert - total novice. But absolutely a laminar flow with no turbulence is much much more efficient than one with turbulence. So I would absolutely say the BMH wins versus a duct that wastes the energy near the opening and pulls air from all around the tube

Originally Posted by Maple71

Have you ever done any testing to see how much deterioration in performance there is with an elbow before the BMH?

I assume you are referring to the BHM and elbow combos such as I use in my BS collection ?
In that case, for the reasons I gave last time, I would still be using a naked elbow as part of the collection port on my BS.
So the comparison is then between a naked elbow AND an elbow and a BMH, not between an elbow and BMH, and a straight duct.
I have not tested it this combo compare to a naked elbow along but I am confident there will still be some sort of improvement especially if 2R elbows are used as they have around 2/3rd the resistance of 1R elbows.

I look at your BMH setup with the bandsaw and got me wondering. The BMH has advantages but are compromised by the turbulence induced by the elbow before it. I have tried the BMH in several places and one thing causes me problems and that is the size. The BMH is large and prevents one from getting too close. I always believe that collection as close to the source is a critical consideration.

Collection close to source applies more to low volume flow collection (eg vacuum cleaners) and more to chips than fine dust. Chips have a higher mass to surface are ratio compared to fine dust so immediately fly through the air further than fine dust, which acts more like a gas and is rapidly slowed down by the surrounding air and then rely on convection and diffusion to move around. Fine dust just needs large air flow collection but it does not have to be right up against the source. A 6" duct with a BMH pulling 1250CFM is evacuating a sphere of air about 3.4ft in diameter every second in front of that hood (actually its not really a sphere but something closer to a mushroom/tear drop shape). The important thing is, unlike a naked port which sucks a substantial amount of air in from behind the port, the BMH pulls almost all its air in from in front of the port hopefully where the fine dust is. This sort of more directed air flow will usually overwhelm the fine dust production from most machinery except for machines like large drum sanders etc.

So while BMHs can get in the way, because they improve the directionally of the collection you can afford to site the BMH a little further back. Measurements of the air speeds in front of BMHs showed the generated up to double the air speed at most given points directly in front of an inlet compared to naked hoods. This also means if required you can move them slightly further back and still retain the same air speed as a naked hood but get greater volume collection.

Chips and visible dust are a different story but they are also far less of a health concern - if you can see the dust it's less likely to cause a problem, which is why I don't worry about too much about visible dust. I also never ultimately rely on making changes to DC systems using dust I can see but use a particle counters to asses fine dust movement and capture. This is a very slow and painstaking process and I don't recommend it to anyone who is not prepared to make many hours of measurements

To capture chips the air speed at the collection point should ideally be faster than the chip speed which is hard to do. The initial chip speed from lathes, table saw blades and routers are very difficult to capture because coming from fast moving blades they move real fast and is why they scatter so far. Nevertheless BMHs still useful because of their capture directionality. I haven't verified this quantitatively but chip collection appears increased and chip scatter reduced from my lathe when using a BMH compared to not using a BMH.

So if I can get significantly closer to the source with a naked tube end, it seems a better solution than farther away with a BMH and elbow. No doubt in some situations like your example of drum sander on a drill press the BMH makes more sense to me.

Getting much closer can make a real difference to chip collection but it won't change fine dust collection all that much - to do that you will have to increase the amount of air you collect.

I do not worry about the chips and what does not get sucked up gets swept.

I tried looking for something that showed velocity vectors for a BMH versus a naked pipe but did not find much but understand that the BMH is better.

Every tool such as a router, bandsaw, table saw and lathe each create different field of dust and chips. Each one creates its own unique collection issues including the distribution of particle size, speed and direction. We can agree to disagree about the issue of a BMH versus a naked duct end in terms of the effect of getting closer to the source. There are times when getting closer will work better. For example, I can get very close to the router bit on a router table versus a BMH. If I have time, I will try to get pictures to illustrate.