I’m still putting off/ figuring out what I’m going to do for dust collection.
The people at Onieda say that 5″ ductwork is needed to move enough air but I’m stuck buying more expensive duct.
In a one man shop that will not be put to high usage why not 4″ pvc?
Replies
David,
Lets see...what business is Oneida in?....that's right, dust control....(sorry, I'm being a wise guy)
http://cnets.net/~eclectic/woodworking/cyclone/index.cfm I think these guys say 6" duct work...
I think it has something to do with volumn of air not force of air. Also, it'll run fine with smaller pipe...just much less efficient..(ie. leaves dust behind)
For sizing your ductwork, it really doesn't matter how many hours a week you'll be using the DC. What matters is can the DC handle the quantity of of dust/chips you generate when you're using your power tools the way you want to use them. If you end up using a sub-optimal feed rate on your power tools to avoid choking the DC, then you're serving the machine instead of it serving you.
All else being equal, a 5" duct will move 56% more air than a 4" duct. What is the price difference between the 4" and the 5"? Maybe it's worth paying that difference to get a 56% performance improvement.
I have a 2 hp dust collector, but put in 4 inch pipe because it was FAR cheaper than 5 or 6 inch. The 4 inch is clearly less than ideal - I have to play with blast gates, but it does the job. (For example I can have two gates open with a saw or sander, but the planer must be the only gate open.)
I spent a lot of extra effort on the bags to make things work. The standard (30 micron) bags just move the dust around. The 0.3 micron bags from the DC vendor were useless - they plugged in no time and nothing worked. The answer was a much larger upper bag, out of the proper material, to do the filtering, and a plastic lower bag to collect the trash. Most of the filtering is done by the "cake" of fine dust in the upper bag, which falls into the lower trash bag when it gets too thick.
You want to move chips not air. The air speed needs to be high enough to carry the chips rather than deposit them on the bottom of the duct. Large ducts have lower air speed than smaller ducts.
I have 4" PVC in my shop. It works for me.
Quick check here - I used to deal with dust collection design daily in a previous life (cement plant engineer).
These numbers are swirling around my brain as being the minimum velocities necessary to keep dust entrained:
Horizontal runs - 18m/sec = approx 60 ft/secVertical runs - 12m/sec = approx 40 ft/sec
so, for a dust collector rated at 1200cfm, (say 1000cfm actual a/c slightly blocked dust bags, fan wear and manufacturer's optimistic testing)
4" diameter pipe = 0.087 sq.ft cross section
1000/0.087 = 11500ft/min = 190 ft/sec = way too fast.
This will just (quickly) wear out the pvc duct at the bends and reduce the efficiency of the system dramatically - it wastes a lot of energy to get air moving that quickly (look at the difference in miles per gallon you get in a car travelling at 60mph vs 100mph). Overall you'll move way less air in a 4" duct with velocities at this speed.
Repeating the above maths for 5" pipe gives:
1000/0.14 = 7300 ft/min = 122 ft/sec = far slower.
Now, what actually happens in real life is that every time air goes around a bend/constriction/velocity change in the piping, it loses a bit of energy (referred to as pressure drop).
As the 5" pipe sees a lot less pressure drop than the 4" because the air velocity is lower in the 5" pipe, I'd go 5" pipes.
You could get away with 4" pipes feeding from some machines into the 5" main line. (ones where you don't need max volume flow)
If in doubt, run these velocities and calcs past the manufacturer's technical people.
Of course, these are simple calculations and they don't take into account pressure drops a/c bends/gates/valves/etc or temperature fluctuations and I have no idea of your system design.
Feel free to refer these comments to the Oneida technical people for review and response.
HOpe that this makes sense and helps.
Happy new year,
eddie
Edited 12/31/2003 8:38:17 PM ET by eddie (aust)
Edited 12/31/2003 9:06:00 PM ET by eddie (aust)
All,
I made an elementary but crucial mistake in my first version of the above, for those that saw it - have corrected it now.
Edited 1/1/2004 12:57:25 AM ET by eddie (aust)
Dear David,
I have to agree with BG. I have an Onieda sysyem that they designed and it works great. Now, for a hard question. You are about to spend, what, $800.00 or $1000.00 on a dust collector, but you want to risk performance based on mabe $100.00 of ductwork. Does that make sense? That's sort of like my niehbor who had a Hitachi 8 1/2" slide saw that he refused to put a decent blade on and then complained about how the saw cut. Let Onieda do thier job. Don't reinvent the wheel. Do you want to woodwork or design dust collectors?
Good Luck!
John
David,
Bigger is bettter as long as the DC can move the required amount of air. This deal really is about moving air, not static pressure or duct speed as long as you can maintain around 4000 fpm to keep the dust moving. Most any DC with a 12" impeller or larger will support 6" ducts just fine. I plumbed mine with 6" S&D PVC. Check your local plumbing supply. Total cost for ductwork in my shop is approx. $400 and I have a bunch of ducts.
Just food for thought. Every person that I know of that has switched from 4" to 6" has been amazed by the performance increase. I have a big cyclone now, but the 6" ducts worked well with my old Griz 1029, just not as well as they do with the cyclone.
Oneida curses PVC, of course they only sell metal. Makes sense to me. Not a bash on Oneida, just the way it is. I know if I were selling metal ductwork, I wouldn't recommend using PVC. Personally, I have been perfectly happy with the PVC. It's cheap. It's easy to work with. It's easy to seal. It's easy to reconfigure if/when you need/want to do so. No static trouble at all for me, but some do have static. A bleed wire wrapped around the ducts and attached to the body of the collector and the body of the tool will bleed off any build up.
More on my site if you are interested.
Terry
Hi Terry, If I use 6" pvc I should be fine. I am planning to route the pvc under the slab to get it in the general area first. The electric for the machines and any future ones will go in the same trenches.
You used the term S&D pvc. I'm not sure what S&D is.
For eliminating the static build up can I just use a #14 bare copper wire wrapped around the pvc and then grounding it to the machines?
I'm not sure how to get to your site. thanks for the heads up.
David,
Woops...guess it would help if I gave you the address to my site. :-) Sorry.
http://www.terryhatfield.com
S&D is sewer and drain. It is much lighter and much cheaper than schedule 40 PVC.
The copper wire wrapped around the ducts like you described should do fine for bleeding off any static.
Terry
Registered Shopaholic
>> S&D is sewer and drain. It is much lighter and much cheaper than schedule 40 PVC.
Called DWV, for Drain, Waste, and Vent, in some parts of the country. Still lighter and cheaper than sch. 40.
Uncle,
Amazing how many different names for the same stuff. It's also called PVC2729 in some parts of the country. Seems we just can't make up our mind. :-)
TerryRegistered Shopaholic
>> PVC2729
Interesting! I wonder if that's a chapter number in some plumbing code, or what.
The S&D that I buy actually says 2729 on it. No idea what it really means.
TerryRegistered Shopaholic
Too easy. I Googled for it. It appears to be an ASTM standard.
David,
I think putting your 6" DWV in the slab is great...done all the time in plumbing. However, I have heard one or too war stories about clogging....may want to include a cleanout or two just in case.
I hear you on that. The runs are going to be pretty short and I'll be carefull as to the size of what I put down it. It's a two car garage with an eleven foot cieling so not have ducts above will help it seem larger in size. I have the tools to do it so it won't be too much trouble.
This forum post is now archived. Commenting has been disabled