New question about my DC, everybody recommends 6" ducting, will 8" be too big. I have a good friend that is the general manager of a large commercial HVAC company, we spoke today and kind of drew out my new system, he can get me the parts for nearly nothing and will help install them as well, I suggested to him that maybe we run 8" pipe for the little price difference there is, he said to do a little more reasearch it may too big, anyone know if 6" will suffice, of if I would benefit with 8"
Member
Depends on the DC. What ya' got or getting?
Opps, forgot to post the obligatory link.
http://www.billpentz.com/woodworking...Conversion.cfm
Regards,
Chas
Regards,
Chas
Ignorance is curable, stupidity if forever.
Member
There are two contradictory requirements on every dust collector duct installation.
1) They have to have low flow resistance to move the greatest amount of air - use large pipes to do that.
2) The have to have enough flow velocity in the pipes to keep the solid particles entrained in the air stream - use small pipes to do that.
It's a balancing act to use ducts large enough not to choke the flow with high pressure losses and small enough to keep the velocity high enough to move the sawdust through the system.
Is 8" diameter too large? Probably not, but that depends on how much air your DC motor/impeller can move.
To get a ballpark estimate, divide the estimated CFM of your impeller by the cross section area in square feet of your ducts. That will give you a rough idea of the upper limit on flow velocity you can expect to see. Your actual velocity will be less, and it may well be considerably less than that calculated value.
My "back of the envelope" calculations say that in 8" diameter ducts, you can't expect to see more than about 2800 feet per minute flow velocity for every 1000 CFM of air moved. For 6" diameter ducts, the velocity would be in the neighborhood of 5000 feet per minute for every 1000 CFM, and for 4" diameter, 11400 feet per minute.
From that, it looks like the smaller the better. And it would be if flow velocity were the only consideration. But don't forget that 1) requirement above - the smaller the pipe the higher the flow resistance. With a given impeller, you won't move nearly as many CFM with the smaller pipe as you will with the larger pipe. The impeller will spend it's available power fighting the much higher flow resistance instead of moving air.
Conventional wisdom recommends about 4500 feet per minute. If your calculations don't suggest a velocity greater than that, the duct is too large.
Tom Veatch
Wichita, KS
USA
Member
Agree with what Tom said. My system with 1800 or so cfm with one gate open has duct speeds around 8000fpm with 6" ducts so Tom's numbers sound right to me. I'd love to have an 8" main and would have done it that way in the first place if I had only known.To late now.
t
[SIGPIC][/SIGPIC]
so is it a good idea to go with an 8" trunk on a 2HP HF DC? or will 6" suffice?
Member
I reckon you need to re-think it. The main duct size needs to be maintained all the way to the machine, besides that 8'' is way too big for a 2hp machine. You cannot decide to reduce the duct size, no matter what that is without airflow penalty. Bill Pentz is the guru but it really is common sense.
Chris
Everything I like is either illegal, immoral or fattening
Contributor
You need to calculate your desired CFM first, then design your duct layout so you can calculate your frictional losses.Originally Posted by Ramsey Ramco
Once you have your desired CFM, and your frictional loss, you can compare them to your fan curve to see if your dust collector can actually move that volume of air at that static pressure.
I seriously doubt if a 2HP HF dust collector is large enough for an 8" duct.
You also need to keep your duct airspeed in the 4,000 feet per minute range to keep the dust entrained in the air stream.
An 8" duct needs approximately 1,400 CFM to keep the dust entrained.
This is getting large, my home shop only needs 700 to 800 CFM at the machine (my worst machine is a saw with cabinet and overhead dust collection).
So I used 6" duct which results in 750CFM at 4,000 feet per minute.
I have a 5" duct to the 12" jointer/planer and a 4" and 3" duct to the saw, obviously only one machine working at once.
Once you obtain the fan curves for your dust collector and design your duct, I predict that you won't be using larger than 6" duct.
Regards, Rod.
Contributor
A quick and non-scientific method is to look at the inlet size of the dust collector. The HF does not have an 8" inlet so there is zero chance of it working. The 5HP Oneida machines (meant for multiple tools running at once) have 8" inlets. Bill Pentz's site is meant to optimize the problem for 1 person & 1 tool running at once (so he runs 6" to each machine off a 6" main line with blast gates).
I have a 5HP Bill Pentz design and it works extremely well.
Moderator
Remember, we are mostly a group of hobbyists! 6" will work great.
Contributor
When I put in a dust collector in my shop I did as many others and used the calculators to help. I am using a Jet with the pleated canister on top that provides about 1200 cfm. When I looked at installing the piping, I kind of went the other direction - what would work and what is economical. I use only one tool at a time and my main branch is 4 inches which seems small based upon the previous posts. However, I am happy with it as it works great with a cabinet saw, router table, sander, and planer. I have never had a clog even though I am using the 4" PVC pipe and fittings. The other advantage of the smaller pipe size is being able to fit it into a smaller shop.
It sounds like 6" may be the way to go, Next question though is about grounding. How many/ how often, and where do they need to be grounded? Basic knowledge would tell me in 1 place since they are all piped together, but based on the way every other one of my assumptions have been a little off, I'm left just a little bit curious.
Member
Larry, the difference is what and how much is really picked up at the machine. Your DC may pick up chips and most of the visible dust, but, the machine CFM requirements provided by DC and machine manufactures, etc. were computed to collect what in essence is the mostly invisible dust ejected by the machine in an expanding sphere around it. While dust can easily be collected immediately in front of an open 4" or 6" DC duct (an area defined by pi * r sqrd), it is not so easy to collect from that sphere (a volume defined by 4/3 pi * r cubed) - you need A LOT of CFM to do that. To demonstrate, have a smoker blow a cloud of smoke, then ask him to suck it all back up- impossible. That sphere of dust is also right where the operator stands and breathes.
Bottom line is what a person wants a DC to do- collect chips OR collect chips and as much dust as possible, including the invisible, dangerous, .5 - 10 micron stuff.
Last edited by Alan Schaffter; 03-30-2010 at 11:25 PM.
Member
There are lots of posts already here on that and you would be well advised to do a search. In fact grounding is a bit of a fallacy in a pvc system, something peculiar to PVC and its electrical conductivity properties.
Chris
Everything I like is either illegal, immoral or fattening
Contributor
Remember also that if you run 6" pipe through your shop as your main line, then reduce to 4" at a machine, you may not have the proper air velocity to keep dust/chips moving quickly enough. So, to keep enough air moving, if you have 4" ports on your machines, you may need to keep two gates open to keep up the movement.
I agree that the HF DC will likely not keep up with 8" pipe. 6" may even be pushing it, but I'm not sure what the CFM rating is on that. I'm pretty sure it has only a 5" port on it, if you are going to put 6" piping in, you'll have to make a modification on your DC to accept 6" pipe. Don't just put a reducer on it, you'll defeat the purpose of putting in 6" pipe.
I drink, therefore I am.
[QUOTE=I agree that the HF DC will likely not keep up with 8" pipe. 6" may even be pushing it, but I'm not sure what the CFM rating is on that. I'm pretty sure it has only a 5" port on it, if you are going to put 6" piping in, you'll have to make a modification on your DC to accept 6" pipe. Don't just put a reducer on it, you'll defeat the purpose of putting in 6" pipe.[/QUOTE]
You are correct My neighbor is a welder, he is making me a 6" OD receiver and we are going to modify the plate.
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