Need help understanding dust collector pipe size

[Scott Brandstetter]

I know that it is best to run 6 inch pipe and then use a 4 inch flexible pipe at the last spot to connect to each tool. I also understand that it's best not to use 90 degree turns rather use 2 45's with a spacer to allow better air flow. With that said, I just can't wrap my mind around how 6 inch main pipe draws better dust than a smaller 4 inches. The thought I keep having is a water hose. You turn it on full blast and it comes out at a certain rate. If I cover up have of the outlet with my finger, the pressure increases. Maybe a bad analogy but it just seems that the smaller the main pipe, the stronger the airflow would be, back to the dust collector.

One more thought. Has anyone added a blower (squirrel cage type) midway through the pipes to increase the air flow and dust collection?

[Mike Chalmers]

I would like to keep with your water hose analogy. First of all, you have to, as I did, wrap your head around the fact that DC units rely on volume, not power.

Next. As water or air moves through a conduit, be it hose or pipe, the outside of the stream is in contact with the conduit. This creates friction and, therefore, resistance. This slows the stream down. The larger the conduit, the less in percentage will be in contact with the conduit, thus, less overall resistance. This is only good until the available volume to be moved is increased beyond the DC's total volume capacity. When this happens, the volume of air slows down, as the amount to be moved is not restricted. This explains why reducing the size of the pipe at the last possible location is better than further away.

[Justin Ludwig]

I know that it is best to run 6 inch pipe and then use a 4 inch flexible pipe at the last spot to connect to each tool.

Only if you have a DC that can pull the required CFM at the velocity you want.

I dismantled my shopfox 2hp 1550 CFM DC and ported it outside instead of into 2 bags. Its performance was increased dramatically, but not enough to use 6" pipe even in my main duct. I would lose too much velocity. I have a 12' run, 15' run, and 30'+ run all out of 4". I can have 2 gates open at the same time and get good collection. With one gate open it sucks great. Bad news is the bearing started squeaking yesterday when it powers down. My next DC will be 5hp which will x3 my current CFM. My main line will have to upgrade to probably 7" (haven't done the math yet) and I'll be leaving 2 ports open at all times so it gets the air it needs.

[Jim Andrew]

I have all 6" pipe on my DC, and find that if I forget to close all the gates but the one on the machine I am using, it effectively cuts my air flow at that machine in half, if there is one other open.

[John Donhowe]

As others have already noted, you're looking at two factors: air flow (CFM) and air speed (FPM), which are at odds with each other. Larger pipe will decrease resistance, and improve air flow, but air speed will drop; to some extent, you can get better air speed with a smaller pipe, but with the increased resistance performance will really drop. The trick is to have pipe which is large enough to keep resistance down, but keep adequate air speed.

Instead of thinking of holding your finger over half of the garden hose opening, a more appropriate analogy is to think of how much pressure and flow you'd get coming out of a garden hose half the size but the same length as your original hose.

For big wood chips (planer and jointer) you need about 4500 fpm for collection (vertical pipe fighting gravity), and 3800 fpm for smaller dust (saw, etc.), but only 2800 fpm for horizontal runs to keep saw dust particles in motion. That's why you can often get away with 6" main (horizontal) pipes, but need 4" for collection. Since cfm=fpm*area (of your pipe), for a 6" pipe you need need about 550 cfm air flow.

I think putting a squirrel cage blower in your dust collector line is a recipe for disaster. The blades are going to get clogged pretty quickly with dust, making the fan not only useless, but a barrier to air flow. Large chips trapped in the wheel could throw it off balance, and possibly destroy the fan.

[Jim Becker]

I...I just can't wrap my mind around how 6 inch main pipe draws better dust than a smaller 4 inches. .

The answer is actually quite simple: Dust collectors work by moving air at relatively low pressure and is measured in cubic feet per minute. (CFM) The air, in turn, moves the dust and chips. The more air you can move, the more efficiently you can move the dust and chips. Air takes up space. (and it is not compressed in this case) The difference in the amount of air you can fit in a 4 inch duct and the amount of air you can fit in a 6 inch duct is substantially different. So linking that back to the original premise, since you can fit more air in a 6" duct and in turn, move more air through that 6" duct...you're going to be able to move more dust and chips. Ideally, you want to be able to develop at least 600+ CFM, and preferably more, to adequately evacuate dust and chips from a machine. The maximum CFM you can get through a 4" duct is about 450 CFM. The math says 6" duct is the winner since it can pass at least 800 CFM, give or take, depending on the efficiency of the dust collection system being used.

This all passes down to the machine ports, too. While you can reduce at the machine to 4" (which is what you need to do if you are going to us the OEM port on most large woodworking machines that are not bigger things aimed at the higher end market) that restriction does compromise performance. But this is where the tie in to your water hose/nozzle example comes into play...if you make that reduction as close to the port as possible, you at least retain some of the venturi effect at that point to help reduce the performance loss. That really only helps when the port is right at the cutter...it's not going to help on the cabinet of a cabinet saw. Therefore, many folks will take the time and trouble to retrofit their machines with larger dust collection ports when they are able to do so.

My primary machines all have 120mm (not quite 5") dust collection ports and they are serviced by 5" or 6" drops. (the jointer/planer has a 6" drop and needs every bit of it when I'm surfacing a wide board...)

And no, it's not a good idea to put any kind of blower in the middle of your duct work...

[Rick Fisher]

My dust collector uses 6" Pipe and 6" Flex hose. The motor is single phase, 19 amps, 220V ..
With a single gate open, it draws about 18 amps. As the filter gets dirty, the amperage drop down to 17.2 - 17.3 amps.
Open a second gate, and the amperage will jump up to 19.8 - 20.1 Amps. Over current.
With 3 gates open, its at risk to trip the mag starter.

I really believe that Dust collectors 3 hp and up should come with Ammeters built in .. These machines are built to preform with a certain amount of resistance. Take the bag off a 2hp duct collector and connect it to 6" pipe, it will preform amazingly. Check the ammeter and you could be shocked ..

[Rick Potter]

Take a long soda straw and cut 1" off of it. Now breathe for one minute through the long section, then switch to the short section and breathe through it. Now you know what friction loss is.