Theoretical question regarding dust collection and using a builtin plenum

[Stephen Saar]

I've seen this question raised a few times before on this forum, but I can't seem to find the threads now. Anyway here's the basic idea I had. In buildings they often use the space above a drop ceiling as a return plenum. Could you do something similar with a with a dust collector? i.e. you build your shop with 10' ceilings and then make an enclosed/sealed space 6-8 inches deep, but in essence the size of the shop. Then just drop 6 inch ports off that for each machine you want to use. Obviously whatever you build would have to be pretty substantial, i.e. 3/4 plywood or better, and you would need to seal it pretty well, but it seems like it should work. You could also build it in by doing channels in your slab for the building, but the basic idea is the same. Unfortunately I won't be able to build a shop anytime soon, but I enjoy coming up with ideas/designs as a mental exercise, and I was curious what you guys thought about something like this. The only real problem I can think of would be that possibly as the dust was coming up in to the ceiling it might be thrown out of the air stream and just sit up there, but I'd imagine that only a little bit would, and once any amount of it built up it would be sucked up if you changed ports. I don't have much experience with fluid dynamics so I'm not sure how this would affect the dust collection, but in my very simple understanding it seems like it should work so long as the channel/box is properly sealed.

-Stephen

[Dave MacArthur]

Stephen,
Nice idea, but won't work ;) You hit the nail on the head with the problem too. In order to keep the dust "entrained" in the airflow, so it doesn't drop out and pile up (and create fire hazards too), you need to flow around 3600fpm on the vertical runs, and > 2500 or so on horizontal. The general number you are attempting to hit is 4000feet perminute flow through the pipe which results in 3600 in most areas of the piping (faster flow at center than walls, etc.). THIS NUMBER IS THE DEFINING NUMBER for what size pipe you run on your DC! You take your DC's cubic feet /minute, and divide by the cross sectional area of your pipe (in units of feet) to arrive at airflow Feet/minute. THIS is why 7" pipe for a 1000-1200 CFM DC is optimal, but most go with 6" as it's much cheaper.

Now, if you pull dust suddenly into a giant 20 sq ft area "pipe", airspeed will drop to 5fps, NOTHING will stay entrained, and you'll just fill your plenum space with dust.

I recommend you go to Bill Pentz' site and read on DC, you'll enjoy it!
BillPentz DC overview Solution post (http://www.sawmillcreek.org/showpost.php?p=427717&postcount=10)

I also have some posts on DC where I go over the basic theory and what that results in for duct sizing in numbers, if you search my name... cant find the link now .

[Alan Schaffter]

Dave pretty much nailed it, but there is one other consideration. Friction which causes static pressure resistance is higher in a rectangular duct. Do the math and compare the circumference of a round duct to that of a rectangular duct for the same cross-sectional area. The rectangular duct will have more surface area and therefore more friction and higher static pressure resistance.

Plus you have all those corners where flow is either turbulent or too low.

[John Coloccia]

Which is why the laminar airflow in a semiconductor fab is straight down. Now THAT would be a neat idea. A woodworking shop on grates with airflow down through the floor. It'd be the cleanest shop on the planet.

[Don Alexander]

would be a bit rough on the feet after awhile though i would think :eek:

[Stephen Saar]

Yes and God help you if you dropped a part.

-Stephen

[John Coloccia]

I found out the hard way why you have to wear a hard hat in the lower levels of the fab. Nuts and bolts get going quite fast after they've fallen a couple of stories!

[Gordon Eyre]

Stephen,
Nice idea, but won't work ;) You hit the nail on the head with the problem too. In order to keep the dust "entrained" in the airflow, so it doesn't drop out and pile up (and create fire hazards too), you need to flow around 3600fpm on the vertical runs, and > 2500 or so on horizontal. The general number you are attempting to hit is 4000feet perminute flow through the pipe which results in 3600 in most areas of the piping (faster flow at center than walls, etc.). THIS NUMBER IS THE DEFINING NUMBER for what size pipe you run on your DC! You take your DC's cubic feet /minute, and divide by the cross sectional area of your pipe (in units of feet) to arrive at airflow Feet/minute. THIS is why 7" pipe for a 1000-1200 CFM DC is optimal, but most go with 6" as it's much cheaper.

Now, if you pull dust suddenly into a giant 20 sq ft area "pipe", airspeed will drop to 5fps, NOTHING will stay entrained, and you'll just fill your plenum space with dust.

I recommend you go to Bill Pentz' site and read on DC, you'll enjoy it!
BillPentz DC overview Solution post (http://www.sawmillcreek.org/showpost.php?p=427717&postcount=10)

I also have some posts on DC where I go over the basic theory and what that results in for duct sizing in numbers, if you search my name... cant find the link now .

Took the words right out of my mouth. Good response Dave.

[Steve Milito]

You could get away with rectangular duct work. You could even use MDF if you wanted. You would likely get some drop out in the corners, and you would likely want multiple clean outs. You could make up for the additional resistance by optimizing the dust size since you are not restricted to 4, 6, or 8" equivalents. I think the bigger issue is the rectangular to round transition and the drops to the machines.
I doubt you would save much money and it would take a lot more time to put together.
Frankly, it's not worth the effort to build something that doesn't work as well as PVC and doesn't have an appreciable cost saving.


Dave pretty much nailed it, but there is one other consideration. Friction which causes static pressure resistance is higher in a rectangular duct. Do the math and compare the circumference of a round duct to that of a rectangular duct for the same cross-sectional area. The rectangular duct will have more surface area and therefore more friction and higher static pressure resistance.

Plus you have all those corners where flow is either turbulent or too low.

[Joe Chritz]

I found out the hard way why you have to wear a hard hat in the lower levels of the fab. Nuts and bolts get going quite fast after they've fallen a couple of stories!

Indeed they can. 50 feet will get you around 40 MPH. A 1/2" nut would smart a lot at that speed.

Joe