Upping My Dust Collection System Game

[EvilTwin]

Hard to understand any loss in efficiency?  Can you explain?  How does minimizing pressure drop decrease efficiency?  If the duct was so large dust collects due to decreased velocity, I could see that as a potential problem I guess.  My vertical runs up hill are all 6-inches or less.   

I've never installed / used a duct system like this, big learning curve.  I taped every seam with clear gorilla tape, it's air tight.

Pretty much what Fred was saying.  Think about it this way.  To move 10 GPM of water through a pipe an inch in diameter, the flow will be relatively slow, since there is a fairly large area to contain the water.  But if you use a 1/2" pipe, the flow of water will have to be faster and with more pressure to move the same volume through.  

In a DC system, the max size of the pipe is generally the size of the inlet into the collector.  Bigger than that and the velocity can slow down and the chips may fall out of the stream.  Smaller and its like sucking through a small straw, lots of work for not as much volume and the velocity has to increase to compensate.  Somebody with an engineering degree can probably explain it better.  HTH.

[TDKPE]

Yeah the 8-inch section collapses with all the gates closed.  Right now I'm just making sure I leave one open.  It's a problem.

Long sections have no 'reinforcement' (as it were) in the middle, but a series of short sections do.  You'll find that it takes a greater SP difference with a chain of short lengths than long lengths to collapse the ductwork. 

Since you've already built the system and aren't likely to change the duct out now for either shorter lengths or heavier gauge ducting, I'd suggest building a simple pressure limiting damper.  You can make your own out of a short length of duct, or even wood (all my blast gates are wood, and it would be easy to adapt that design to a pressure limiting damper), with a weighted lever arm and weather strip seals.  The damper arm weight is movable to adjust the pressure at which it starts to bleed air.  Shutting all the tool/machine blast gates would merely pull enough vacuum to start bleeding air through the damper to limit the SP in the whole system. 

Adjust it to stay closed with the smallest and/or furthest duct open and working (using a manometer would be helpful here, even just a nylon tube with water, to find the branch that requires the highest SP), but start opening as soon as you slide the blast gate closed just a little.

It doesn't have to be efficient, like a blast gate - it just has to start passing air at the desired pressure, with a steep slope so small increases beyond that cracking pressure opens it substantially more.  And making your own means you can play with the hinge fulcrum location and weight to make it more responsive by trial and error.

These are common on systems with airfoil impellers, by the way, as they become unstable if the air is restricted too much.  Putting such a damper on it prevents the SP (and therefore the air flow rate) from going below some predetermined minimum, without bleeding air into the system otherwise.

[Danny in Houston]

Long sections have no 'reinforcement' (as it were) in the middle, but a series of short sections do.  You'll find that it takes a greater SP difference with a chain of short lengths than long lengths to collapse the ductwork. 

Since you've already built the system and aren't likely to change the duct out now for either shorter lengths or heavier gauge ducting, I'd suggest building a simple pressure limiting damper.  You can make your own out of a short length of duct, or even wood (all my blast gates are wood, and it would be easy to adapt that design to a pressure limiting damper), with a weighted lever arm and weather strip seals.  The damper arm weight is movable to adjust the pressure at which it starts to bleed air.  Shutting all the tool/machine blast gates would merely pull enough vacuum to start bleeding air through the damper to limit the SP in the whole system. 

Adjust it to stay closed with the smallest and/or furthest duct open and working (using a manometer would be helpful here, even just a nylon tube with water, to find the branch that requires the highest SP), but start opening as soon as you slide the blast gate closed just a little.

It doesn't have to be efficient, like a blast gate - it just has to start passing air at the desired pressure, with a steep slope so small increases beyond that cracking pressure opens it substantially more.  And making your own means you can play with the hinge fulcrum location and weight to make it more responsive by trial and error.

These are common on systems with airfoil impellers, by the way, as they become unstable if the air is restricted too much.  Putting such a damper on it prevents the SP (and therefore the air flow rate) from going below some predetermined minimum, without bleeding air into the system otherwise.

I was thinking about the same thing.  The vacuum relief device sounds like fun to figure out and build. 

Great suggestion on the long length versus short length - I think that may also fix the collapse issue.

[Carl10]

Danny,

First rule of dust collection is to never upsize your input duct.  To keep chips airborne you need a velocity of ~3500 FPM.  Looking at the attached chart you will see that an 8" duct needs 1221CFM to maintain that velocity. Your DC looks like my old Jet 1100, which out of the box would be lucky to get 800CFM (The 1100 is not a real world number).  Add ducting and filter build up and that CFM drops significantly. I bought a pitot tube and just did some tests on a 1.5HP Cyclone I got for free. I started off with the 6" inlet and ran 6" duct 8' then dropped using a large radius 90 to a 4" flex and then to my TS. My original 760CFM (unrestricted test pipe per the mfg) dropped to ~550 but the velocity was ~2800.  Well below what I should have to avoid clogs.  I next replaced the 6" with 5" and my CFMs dropped slightly but  my velocity went up to ~3800.  To put it another way if your 8" is good then 12 would be better or 24 or 48! But if you connected your DC to a 48 pipe you would probably hardly feel much air move because it is such a large cavity.  That is why the larger you go in duct size the higher the velocity needed to maintain the same CFM,

As far as the duct collapse, the simplest solution is to make a wooden ring (cut a 8" circle in 3/4 ply) and slide it down the duct (it can also be used as a hanger bracket).  One in the middle might be enough but you may need 2 evenly spaced.  Even easier is to use a 5" duct and you won't even need rings as the smaller diameter is much stiffer.

Hope this helps. Let us know what you do.

Carl

[EightFingers]

I have that same planer and I have no problem using a 4” duct and I plane some pretty wide stuff.

[Dave Diaman]

Danny, as John said I had some of the thin duct work left over from my 2hp dust collector when I installed my new cyclone. My cyclone crushed it like a soda can. I ended up going to a HVAC supply warehouse and getting heavy gauge duct for less than the 30 gauge stuff costs at Home Depot. It basically cut the cost of my ductwork in half.

[Danny in Houston]

Danny, as John said I had some of the thin duct work left over from my 2hp dust collector when I installed my new cyclone. My cyclone crushed it like a soda can. I ended up going to a HVAC supply warehouse and getting heavy gauge duct for less than the 30 gauge stuff costs at Home Depot. It basically cut the cost of my ductwork in half.

Thanks Dave et al, good info from everyone.  And amazingly we have actually had a great discussion without a big argument about dust collection LOL.  

This is my usual path, just dive in and do something, learn, adjust, waste a little money, and eventually wind up in the right place.

[Stwood_]

Danny, as John said I had some of the thin duct work left over from my 2hp dust collector when I installed my new cyclone. My cyclone crushed it like a soda can. I ended up going to a HVAC supply warehouse and getting heavy gauge duct for less than the 30 gauge stuff costs at Home Depot. It basically cut the cost of my ductwork in half.

I can't imagine someone with a cyclone ever attempting using AC/Heat ducting for their piping.

[TDKPE]

I can't imagine someone with a cyclone ever attempting using AC/Heat ducting for their piping.

Unless it's a retasked '2 hp' HFDC blower mounted to a Pentz cyclone.  Highest SP it will pull is 8.0" WG.  Not enough to collapse a paper bag.  

   

   

[Stwood_]

Unless it's a retasked '2 hp' HFDC blower mounted to a Pentz cyclone.  Highest SP it will pull is 8.0" WG.  Not enough to collapse a paper bag.  

Mmmm yea. I didn't think about some of the lower sp home builts.