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View Full Version : Designing a dust collection system for my shop - air flow question...



DyerWolf
2009-May-18, 02:25 PM
Couldn't think of a better place to ask this: Would there be any downside to using 6" pipe to transport sawdust (rather than 4" pipe)?

I'm buying one of these (link (http://www.jdstools.com/index.asp?PageAction=VIEWCATS&Category=25)), a 1250 CFM 1.5 HP dust collector for my garage wood shop. It uses 4" tubing at the tool & impeller.

Because I have to run at least one length of pipe through the rafters to keep the floor clear, I'm designing the system with one run against the wall of the garage (where there will be two tool ports) and another run up and over the rafters (where there will be another two drops for additional tools). The rafter run will likely be about 30' long. The termination of each run (at the tool side) will have a gate to allow me to shut off airflow to tools I'm not using.

My dad suggested I use a 5" or 6" pipe - rather than 4" pvc for the rafter run.

So the question is: do I lose any efficacy of the system by using a wider pipe?

samkent
2009-May-18, 03:33 PM
Larger pipe means slower air flow. The larger particles may stop moving inside the pipe and build up.

DyerWolf
2009-May-18, 05:47 PM
Another question: static build-up in PVC...

Some folks have suggested I use metal runs vs. PVC to reduce the likelihood of a static-induced dust fire.

Will any metal duct (i.e. aluminum or galvanized duct piping - properly grounded) work, or do I need to look for / avoid certain products?

samkent
2009-May-18, 06:08 PM
Do the professional systems worry about static? We have a small system (2 inch pipe) in our shop for generic dust/debris and ours doesn’t have any static protection.
If I were overly concerned I would drag a bare wire inside the pipe and exit the pipe by the motor and ground it there.

thoth II
2009-May-18, 06:47 PM
Another question: static build-up in PVC...

Some folks have suggested I use metal runs vs. PVC to reduce the likelihood of a static-induced dust fire.




I agree with them. Just use the metal run and ground it and you'll never have a fire, because the electric fields will be continuously depleted before they cause a discharge (a little like the lightning rod principle). And if you ever have a sawdust fire, I would describe it as a fire bomb instead of a fire, you DON'T want that.

kleindoofy
2009-May-18, 06:59 PM
Do the professional systems worry about static? ...
They sure do.

Fire isn't really the problem. A huge explosion is.

The wider the pipe, the more the dust can displace in the air inside. At the right concentration, a static spark can mean big time fireworks. Kaboom.

DyerWolf
2009-May-18, 07:38 PM
Yeah, the dust effectively makes a Fuel Air Explosive, and from what I've seen of military-grade FAE's, an improvised (read: accidental) explosion would be devistating in my little garage...

So, I'm planning grounded metal ducting - but still debating the 6" diameter issue: the collector has a 6" opening & might take the ducting... but I'd still have to choke it down to the manifolds at the tool side.

Swift
2009-May-18, 07:54 PM
If you can, you might also want to put a clean-out port somewhere in your run, such as at a low point, or at bend, where the sawdust might drop out.

apprentice
2009-May-18, 07:55 PM
Hi DyerWolf,

For woodworking applications the minimum duct velocity should be 3500-4000 feet per minute (FPM). Make the pipe as big as possible without going under this. Use galvanize pipe only because it is cheaper than aluminum which is needed only for special purposes. Also, don't use the stuff from Home Depot. This is designed for low pressure systems (up to 1/2") where your system is considered high pressure at 12". Get stove pipe made from 24-22 Ga. galvanized metal.


Grounding is strongly recommended and can be to any grounding point in your garage. The real danger is when a collection system is improperly used to collect different particulate. For example, you can be guaranteed of an explosion (and fire) if a system was used to collect sawdust and steel grindings.

Nice little collector. Good luck with it.

DyerWolf
2009-May-18, 11:47 PM
Thanks for the tip on the stove pipe. I'd have been down to HD as soon as I got my system designed.

I'm now thinking of upgrading to a 3hp system... Decisions, decisions...

pkasper
2009-Dec-07, 01:02 AM
Dyerwolfe, if you're going to go with 6" main pipes you will definitely need 3 hp to keep up the 3500 - 4000 fpm flow rate.