why shop vac has more “suction” than DC
OK, so I have an HVAC contractor friend who is installing some ductwork for a woodworker. He gets done installing everything…believe it is all hooked up to a cyclone the guy bought…and the guy scoff’s a bit, saying that the Dust collector/cyclone doesn’t feel as though it has any more “suction” than his small shop vac. Now, I know from hearing bits and pieces of the science behind this…but can someone respond with a quick summary of why it is an unfair comparison between a shop vac and dust collector? I believe it has something to do w/ air flow being an entirely different animal from “suction.” Hopefully, you guys get the picture. We were having this discussion and I realized how bad I was at articulating the whole subject and by the time I was done, I wasn’t even sure I knew what the heck I was trying to say.
thanks.
Replies
Dust collection relies on moving a volume of air (CFM), not suction. It's air speed that moves a volume of air and keeps dust in suspension. A vacuum cleaner is optimized for suction and has a low CFM.
First of all, you must realize that from a physicist's point of view, there is no such thing as suction. What we call suction is merely less pressure than atmospheric pressure and air will move from the more positive pressure to the less positive pressure. Sort of like a stream of water. Points A and B are both above sea level (or zero pressure) but water will always run down hill from point A to a lower elevation point B.
If we could attain a total vacuum, which is impossible, we would eliminate all air particles and we would have zero air pressure. A vacuum cleaner would normally attain a lower static pressure that than could be attained with a dust collector but if hooked up to an eight inch duct like a dust collector might be hooked to, it would quickly lose that pressure differential because it simply doesn't have that capacity. If on the other hand, the dust collector was hooked up to a vacuum hose, it would have less pressure differential that the vacuum cleaner and it would move less dust. You have to realize that it is not the vacuum per se that picks up and moves the dust. It is the speed of air caused by the differential pressures.
Sorry, but I think I understand the principals better than I can explain them.
Edited 5/1/2007 9:08 pm ET by tinkerer2
Compare a garden hose with a nozzle and the Mississippi River. With the nozzle set at a small opening, water can shoot out of the hose at a speed faster than the Mississippi River flows. But the Mississippi River will still be delivering more water per minute.
For dust collection, what is needed is moving a large volume of air, like the Mississippi River.
Edited 5/1/2007 4:59 am ET by wilburpan
If he wanted to make a fair comparison he should have hooked up the shop vac to a few feet of 6" duct and then felt the air flow at the open end of the pipe, the shop vac wouldn't seem so powerful in that test.
Another point to be made about DC versus shop vacs is how each responds to a restricted air flow.
If you try to get a DC system to draw through a shop vac hose, even just a short length going from the large ducts to the dust port on a machine the performance is miserable, the low "suction", high flow design of the DC's impeller won't pull air through a restriction. For smaller shop machines, like table saws with shrouded blades and 2 1/2" ports, you are better off connecting them to a shop vac than trying to hook them up to a large ducted system.
John White, Shop Manager, Fine Woodworking Magazine
Hi John,
SO if my only power tools are a router and a bandsaw, then the vac is the answer?
I also have a lathe, but mask for that (not well enough)
Dave
A shop vac will work as well as or better than a central DC for a router or a bandsaw. The limitation on the effectiveness of dust pick up on either machine is usually how well the dust ports are located rather than the air flow.
John W.
"The limitation on the effectiveness of dust pick up on either machine is usually how well the dust ports are located"
And the size of the ports. IME, smaller ports, such as on bandsaws and router tables, work better with a shop vac.
Mike HennessyPittsburgh, PA
Morning Mike...
I agree with John on the location and I agree with you about smaller port size working better with a Shop Vac as the velocity is more centered to a specific point.
But... a Shop Vac only can only concentrate on one point. I got great results with my BS where the port is located for the dust being forced down by the action of the down-stroking teeth. And I drilled extra holes in my ZCTP to enhance that action.
But... that is only one of two main points of concern as there will be dust that will escape to points other than directly down, espcially in re-saw where must dust is generated.
Having just installed a cyclone, I used that knowledge to my benefit with a 6" pipe going to my BS. The 6" was Y'ed just before point of entry. 4" still goes to the 4" port and 2 1/2" got diverted to that second point of concern to try to catch escaped dust that will spread to the table surface.. floor and airborne. That set-up has succeeded in making my BS about 98% efficeint on dust collection which one smaller port could not do alone. And that makes John's point of location very valid in my mind.
Regards...
Sarge.. jt
True enough. My point was that DC's don't do as well as a vac if you try to reduce the DC duct down to 1-1/2 inches, which is the size of the dust port on both my small-ish bandsaw and the fence on my router table. Now you have a REAL bandsaw. ;-) With a real dust port. In your case, the vac would be a poor second choice.
Mike HennessyPittsburgh, PA
Morning Mike...
I believe that you, John and I are on the same page of our "hymnals" once our thoughts are tied together. We were all just singing a different verse of the same song! ha.. ha...
Regards...
Sarge.. jt
Edited 5/3/2007 1:53 am ET by SARGEgrinder47
Moke,
Exactly right, large DC systems "choke" when you try to reduce them down to much smaller diameters, anything under 4" diameter generally isn't worth the trouble, at that point you are better off going to a shop vac.
John W.
John, and the rest,
Thanks for your prompt response. My dust collection is currently limited to duct taping the vac hose just below the bottom guides on the band saw and I am yet to build the router table (I generally use hand tools so this is a bit of a black art).
The problem with the band saw (16" chinese thing with a solid cast frame that does a perfectly adequate job for the amount it cost and what I ask) is that it has no built-it provision for dust collection and I have been loath to cut the steel covers until I had worked out a solution with pretty good success chance.
The front cover extends to half the side (as does the back cover) so a side mounted port would require mods to both covers. I can easily do this, but there is the unstable area of the join (no overlap of the plates and about a loose 1/8 gap). I thought to mount a takeoff on the front of the cover right at the top, but am unsure if I should rig some sort of ramp to stream the airflow (would need a blade slot to create a bit of a box) Newer saws seem to be like this, and pretty easy with a few pop rivets, but they all come in from the side)
Additional comments appreciated. BTW the vac is a festool thing I bought to go with a sander for surface prep in the house before painting - no complaints for that task.
Thanks
dave
PS Sarge - thought of you the other day when I parked next to an orginal condition old corvette. Not a common car over here.
Morning Patto...
We're building "Smokey and the Bandit" Trans Ams at the moment. That has generated a lot of interest and a million calls a day with the same question ask... "what's it going to cost"?
Burt Reynolds has even got involved, even though I much prefer "Sally Fields" showing up myself. ha.. ha.. ha..ha..ha..
BTW... Cleve that owns "Ponti-World" down in Sydney has some nice GTO's, but I bet a Corvette in your neck of the world is a rare breed indeed!
You can't by chance post a picture of your band-saw.. one with a shot of the guts and one for the exterior? I would like a look at it as I haven't seen anything yet that can't be modified to get you where you need to be with a little effort.
Regards...
Sarge.. jt
Thanks, went out and took some photos last night and it is interesting how taking photos seeking help also clarified my own thoughts (also got a bit distracted because an echidna had decided to take up residence in the backyard and the kids wanted photos).
Might take a couple of days to get them off the camera and onto this though, chat soon.
Dave
Hi, John. If I'm reading this right, I think it reinforces what I'm thinking about doing for DC on my 14" (whimpy, LOL) bandsaw, which would be to run a 4" hose to it, then split it off to 3 hoses: one under the table, one to the lower wheel compartment, and one above the table, attached to this:View Image
The DC is the Jet 1100 CFM canister version. If all the hoses are 2.5", do you think this would allow enough air flow to keep things moving?forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Hi Forestgirl,
The perfect place to collect the dust would be a fraction of an inch under the table surface right where the saw dust starts spilling out of the blade's gullets. Unfortunately that area is cluttered up with trunnions and blade guides so you have to do the best you can.
Working with a group of new 14" band saws we are setting up for testing, the most effective dust pick up seems to be with a 4" opening right below the blade guide assembly. The best set up has the port cast right into the back side of the bottom casting on the Delta style saws. The current Powermatic and Jet saws have a port in that location, you might be able to see it in photos of their machines. Adding a port to an older machine would be difficult, though it would be possible to add one to the door.
This set up works well because the large port pulls a lot of air through it and the door fits well, which means all of the air is coming in around the open area at the blade guides and has enough volume and velocity to capture almost all of the dust.
I think you would be better off trying to get one large fitting below the table rather than several small ones which will have less air flow because central DC's don't draw well through small hoses. A pick up above the table probably won't catch much dust because the blade is carrying all of it downward.
Let me know what brand and model of saw you have I might be able to give you some more specific advice.
Hope this helps, John W.
Hi, John. Mine is a Grizzly 1019Z, the older version of the 14" saw. Looks alot like the current G0555 in the body. I've actually found that dust chute above the table to be very helpful. Of course, that might be because not much is getting sucked below the table, LOL.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
FG,
If you could follow it, you would probably find the dust on the table is coming up from inside of the lower wheel housing, mostly swept up by the blade as it goes upward in the rear blade guard. Some of it will blow out through the open side of the guard and some will follow the blade and go all the way around through the upper housing before it falls on the table.
Ridgid band saws have a fairly effective 2 1/2 inch dust port that bolts onto the front top edge of the saw's lower door. I found that it will fit most other Asian band saws with little or no modification to the door. Hooked up to a shop vac, it does a good job of catching the dust close to the point where it is generated. The chute can be purchased as a repair part from Ridgid, it shouldn't be too expensive.
John W.
Thanks for the tip on the Ridgid accessory, John. Hmmmmm, wonder if purpleheart glows in a black light...think I could make a movie of that dust travelling around? Never would have thought about that kinda travelling distance.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Hitting on what John White mentioned, someone in a forum (here, somewhere else?) posted a photo of a small enclosure they made to cover the lower guide assembly and which has a hose mount. I don't know if it is worth the effort, and of course it has to be removed for every blade change or guide adjustment, and can't be used when table is tilted. But it was an interesting idea and would be good for a saw that is dedicated to a primary use (resawing for example).
JH
Morning FG...
John is correct IMO that most dust that ends up on the table top rides the blade down and around the blade and back up over and deposits on top as it re-enters out of the top wheel case. Your over-head won't hurt as it picks up a portion of that re-entry dust that made it though the main dust chute on the wheel. Consequently, that's why you find a trace of saw-dust on the back of your blades and the tops of the tires that should be removed often to avoid problems with tracking.
Incidentally... my Steel City did an excellent job with just the 4" port as SC sealed that puppy well. Yesterday I was going to open the lower case door to put my hand on the inside port opening to see how much suction I was getting with the 4" Y and the secondary 2 1/2" that runs up and around to the top as I currently have it set up.
Well.. upon trying to open the door, it wouldn't open with the Cyclone running. I though maybe I turned the release knob the wrong direction to open so I reversed direction. Nope... still wouldn't open. I pulled as hard as I could and then braced a foot on the BS base and tugged with full body force.
Still wouldn't open... well........ till I turned off the cyclone and the it opened with ease as it should. Yep... those "tool guys" down at Steel City" sealed that saw well as they told me they did before I purchased it thinking I would need to Y it as most BS's I've seen. ha.. ha... ha..ha..
Regards...
Sarge.. jt
Morning Jamie...
Got to playing around with the idea John W. mentioned about a port just underneath the table-top itself. After fiddling with it for several hours, I came up with a 99.9% method. By golly, I got lucky again as this whole thing rocks for dust pick-up on that BS.
Normally I would not split the reductions more than the drive line (6"), but a gamble with tinkering proved to be a better way in this case. I wouldn't' recommend this for a long run, but in this isolated case under the circumstances I have, it has paid a premium dividend!
Regards...
Sarge.. jt
"After fiddling with it for several hours, I came up with a 99.9% method."
Might have been the development of a subliminally inspired idea:
Do I get any brownie points?? ;-)
Question: On the main split-off that goes under the table, there are 2 mid-hose tape wraps. What are those doing?forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Afternoon FG...
You get the brownie points. Put the magnet idea on your list of "most clever feats". ha.. ha... ha..ha..ha..
You should try on the split you refer to Jamie. The worst scenario is you don't have enough air flow to pull it off, so you might test with pieces in a stationary, but un-temporary position till you know for sure. Then if a "green light" pops up, you can lock it all down.
The 3 tape wraps on the 3" line coming left as you saw them are:
Plumbers 4"-3" reduction boot.. clamp.. 3" flex pipe....
(1) Wrap where 3" flex pipe meets plastic L on 4" main side..
(2) Wrap on opposite end of L...
(3) Wrap where 3" flex pipe meets white "sewer thingy-ma-jig" to insure air tight as there was about a 1/32" gap where it slid in!
Wherever that white thing is, the local plumbing distributor has got a million little, marvelous contraptions of all natures that with a little though can become part of a giant erector set. :>)
Regards...
Sarge.. jt
Hey Sarge,Warning -- openly participating in hijacking this thread. :-)I noticed that you're putting copper wire on your DC ducts now, or at least on the ABS tying back to the flex hose. Are you just being cautious, or were you getting shocked by the stray cat picker-upper?Glen
Morning Glen...
I was being cautious more than having recieved a major shock. After having run the cyclone just a few minutes, my lovely wandered through on her way to the library. I had just handled the pipe making an adjustment. She kissed me good-bye and that was one "electrically charged smack on the lips". Come to think of it, leaving the pipe un-grounded might not be a bad deal if you could find a way to harness it as it could add "spark to your life". ha.. ha... ha..ha..ha..
After cutting PVC on my smaller 12" BS, you realize just how much static electricity that stuff can carry. The fine particles are difficult to get even with a shop evac. So... having copper wire laying around scrap, I stripped it and bought several 25' rolls of 18 gauge copper wire at Lowe's that I doubled together in a strand.
A box of pan head sheet metal screws which I keep in-stock and a few hours of time got me grounded from cyclone to machine. I connected the wire in the flex pipe at each union with PVC and tapped a small hole in the cyclone to ground the pipe there. And then tapped a small hole in each machine to connect the spiral wire of the the flex directly to the machine by splicing a 12 gauge copper wire to it using male and female connectors.
I was going to leave it just to see what happens.. but the kiss was a wake-up call. I don't think I would experience more extreme results you might find in colder climates as opposed to here in Georgia, but it cost me under $6 to ground with mostly what I keep on hand. Why not as I have never been accused of not having enough energy to "go the extra mile". :>)
Regards...
Sarge.. jt
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