Replies

1. 
   nazard | Nov 27, 2007 01:42am | #1

   Bill,

   You might want to also post over in Braketime.  I seem to recall some recent discussions RE central vac systems.

   -Jerry

2. 
   zolton | Nov 27, 2007 06:17am | #2

   There was an article in Fine Homebuilding some years ago about a guy using a shop vac for a central vac system in a home. I don't recall a mention of using two inline for that purpose though...

   A check in the archives of FWW might bring some information on the subject.

   Zolton 

   * Some people say I have a problem because I drink hydraulic brake fluid. But I can stop any time I want.

3. 
   Dave45 | Nov 27, 2007 06:24pm | #3

   I kinda doubt that connecting two shop vacs in series would do much good.  The second vac would (I suppose) be connected to the exhaust of the first and I can't see how that would improve the performance of either.  I suspect that the second vac would become starved for air as the filter in the first vac got dirtier.  It might be sort of interesting to try, though. - lol

   Do you really need to build a second system at all?  I built my DC system with two gated ports with reducers that allow me to hook up my shop vac hoses.  I use the 2" hose for my router/shaper and oscillating spindle sander.  I use 1.25" hoses with my detail sander, pad sander, biscuit joiner, pocket screw jig, and ROS.

   My shop vac screams like an F-4 on full afterburner, so when it's time to clean up, I use the 2" vac hose and attachments connected to a convenient DC port.

   
1. 
   43Billh | Nov 27, 2007 06:58pm | #4

   Hey Dave,

   I tried Reducing my regular d/c ducting down to a shop vac hose and the suction was lousy. The smaller the hose, the less suction.

    The guy at Oneida told me that would happen but I tried it anyway .

   I just my cobble something together as an experiment. Just need to figure out how to measure the difference. 

1. 
   Dave45 | Nov 27, 2007 07:37pm | #6

   I agree that using a smaller hose cuts down on the suction, but the things I do using the vac hoses don't generate enough "stuff" to really need the suction I can get with a 4" hose.  I don't expect my DC to get everything, but now that I've upgraded to an 1100 cfm DC, I getting most of it.

   Connecting the hand sanders with 1.25" hose gets ~80% - 90% of the dust.  The biscuit joiner gets it all and the pocket screw jig gets 90+%.

   Using the 2" hose with the router/shaper gets most of the dust/shavings.  The "worst" performance comes when I use the oscillating spindle sander.  I mostly use it with the belt sanding attachment and after a longish sanding session, I'll have a pile of sawdust around the large roller.  If I need to, I can unplug the 2" hose from the sander and suck up the pile in a few seconds.

2. 
   saschafer | Nov 27, 2007 07:32pm | #5

   Two shop vacs in series definitely doesn't make much sense. Two shop vacs in parallel, however, would probably be useful. You could have a "trunk" line, with a shop vac at each end, and as many branch lines as you needed in between.

   -Steve

    

1. 
   Dave45 | Nov 28, 2007 12:50am | #7

   I think that putting vacs at each end of a trunk line would be self defeating.  Wouldn't they essentially try to "fight" one another for air resulting in little to no flow in either direction?

   Connecting two vacs to the same end of a trunk might work since they would be drawing air in the same direction, but I still doubt if even two vacs would have the same flow rate as a regular DC.

1. 
   saschafer | Nov 28, 2007 01:22am | #8

   "Wouldn't they essentially try to 'fight' one another for air resulting in little to no flow in either direction?"

   Nope. It doesn't work that way. The air travels the path of least resistance, which is from the inlet, not from either vac. Therefore, you get flow in both directions, away from the inlet and towards the two vacs.

   The advantage of putting one at each end rather than both at one end is that it evens out the pressure drops along the trunk line: As you get further from one vac, and it therefore contributes less suction, you get closer to the other.

   Here's another way to look at it: Take a bucket of water and punch a hole in the bottom near one side, so that the water starts to drain out. Now punch another hole over near the other side. The water doesn't stop draining, unsure of which hole to go down; rather, it drains twice as fast.

   -Steve

    

   Edited 11/27/2007 5:24 pm ET by saschafer

1. 
   Dave45 | Nov 28, 2007 03:43am | #9

   Although it seems counterintuitive, I'll take your word for it.  My DC works well enough, and my backlog is full enough, that I'm not terribly interested in running experiments with shop vacs. - lol

   I gotta wonder, however, why I've never seen (and I've seen a few) any kind of air handling system with fans/blowers on opposite ends of a duct.

2. 
   saschafer | Nov 28, 2007 04:02am | #10

   "I gotta wonder, however, why I've never seen (and I've seen a few) any kind of air handling system with fans/blowers on opposite ends of a duct."

   For a given performance requirement, it's almost always cheaper to have larger diameter ducting and one larger blower at one end than two smaller ones at each end.

   -Steve

    

3. 
   nazard | Nov 28, 2007 05:21am | #11

   Steve,

   What you are saying is that you could boost the performance of a DC system having a long main trunk line by having blowers at each end.  How would that translate into a working CFM measurement: two 650 CFM units in parallel = 1300?  Not to put you on the spot with numbers, but I certianly am curious about the practical application of using multiple blowers.

   Thanks!

   -Jerry

4. 
   saschafer | Nov 28, 2007 05:42am | #12

   Let's say the line is 20' long, with a blower at each end. The connection to your machine as at some point along that 20' stretch, say 5' from the left end.

   Now imagine that we block off the main trunk line just to the right of the machine inlet, effectively disconnecting the right blower. The left blower is still connected, so you're going to get all your suction from that one. Let's say that you get X cfm.

   Now do the same thing the other way around, blocking off the trunk line just to the left of the machine inlet. Now, all of the suction is coming from the right blower. Let's say that you get Y cfm. Since the pipe is three times as long in this case (15' vs. 5'), Y is going to be less than X.

   Now, to a first approximation, what you'll get with both blowers connected is X + Y cfm. But the actual value is going to be a bit less.

   Having said all that, don't forget what I said before: Such a setup is likely to cost a lot more for a given level of performance than a properly designed single-blower system. The only reason it could even possibly make sense with shop vacs is that shop vacs are mass-produced commodity items, so two small shop vacs could indeed cost less than one larger one. That's much less likely to be true with a bona fide dust collector.

   -Steve

    

5. 
   nazard | Nov 28, 2007 05:59am | #13

   Steve,

   Thanks for the swift reply.  As soon as time permits, I'm going to experiment with a couple of 1 hp DC units.  I'll post the results.  Should be fun!

   -Jerry

6. 
   Carya | Nov 28, 2007 05:04pm | #14

   > Now, to a first approximation, what you'll get with both blowers connected is X + Y cfm. But the actual value is going to be a bit less.

   Hmm.  ;)

   Since we're all guessing here, I guess that you will get up to double the airflow with two DCs in parallel but you will not go over the suction of one of them.  If that's true, you would get less than twice the airflow if the ducting is not large enough, including the port on the tool.  Therefore, airflow is not doubled at one tool.  If a DC is designed for a 4" port, two of them probably won't each pull rated airflow through that same 4" port (regardless of restrictions).

   I wouldn't want people to run out and buy a second, inexpensive 1.5HP DC thinking they'll get double the dust collection at one tool (unless they use two ports on that tool).  Otherwise, seems like a good strategy for a shop with multiple tools. 

   Its just a clarification, 'cause I wouldn't want to start an argument, eh... :)

    

   Andy

   (My new tagline: click on my alias, and then click "Ignore Posts".)

    

7. 
   saschafer | Nov 28, 2007 08:50pm | #15

   "Therefore, airflow is not doubled at one tool."

   Not doubled, but still increased, because of the reduced pressure at the inlet to the collection manifold. It really depends on where the bottleneck is. If the bottleneck is in the machine itself, then you're not going to get very much improvement. But if the bottleneck is the dust collector and/or the plumbing, then you should see significant improvement. In small home-size DCs, I think the latter is often the case, especially if you use those small-diameter tubing kits available from Woodcraft, Lee Valley, etc.

   In the long run, using something like the new Steel City mini-collectors as an inline booster may prove to be an optimal strategy for all but the largest machines: Put one of these at each machine (they're only $100 each), hook your regular DC inlet hose to the outlet, and you should be able to get good collection with even a relatively small DC at the far end of the pipe. It doesn't look like they were designed for this arrangement, however, and it may take some experimentation to make them work properly.

   "Its just a clarification, 'cause I wouldn't want to start an argument, eh..."

   No, not us, never.

   -Steve

    

8. 
   Carya | Nov 28, 2007 09:25pm | #16

   > using something like the new Steel City mini-collectors as an inline booster

   Interesting tool.  Looks like it was meant for taking to jobsites.

   > No, not us, never.

   How boring we've become... ;)

    

   Andy

    

4. 
   Samayoa | Feb 18, 2017 12:34am | #17

   Hi as you I had the same idea so I start make researchs but I guess no body have done it before, so I decided to give it a try and guess what actually works, and works great better then what I thought. So it really worth it to try it up.