6" ducts OK with 4" DC port?

[Tim Cleary]

I have been reading extensively on DC both in this forum and in the workshop area. Perhaps I should post this there, but since this is a more general DC question I thought I would start here.

Anyway, I am redesigning / finishing my basement shop, and as framing is getting done need to get to a DC solution. Not to make this too complicated, I have a Delta DC with a 4" intake port. Is it worth running 6" duct instead of 4" if I have to reduce it back to 4" right before it hits the port/impeller?

BTW I am familiar with this thread http://www.sawmillcreek.org/showthread.php?t=7302
where the wyes actually mask a 6" duct. That is not the case with my DC

Thanks

[Chuck Isaacson]

I would say that you are fine doing that. They say that you should run the biggest you can until you get to the machine.

[Tim Cleary]

Thanks - that was my thought as well. It also leaves an upgrade path to a real DC at some point with 6" port options.

[Joe Jensen]

You need to balance the desire to reduce duct resistance with the need to keep air speed up. Most engineering resources I've found say you will get about 350 cfm max through a 4" port with most dust collectors. These same engineering tables say you want 3500-4000 ft per min air speed to keep the dust suspended so it won't settle in the pipe. So, to small a dust intruduces too much resistance, and too large a duct and the air speed will fall below 3500 and you will have dust settling out and building up in the bottom of the duct.

Here is a good source of these enginering tables. Go to AirHandlingSystems.com and they have design resources. From those tables, you will find that the minimum cfm in a 6" duct to keep air speed over 3500 is 700 cfm. I don't think any off the shelf dust collector can pull 700 cfm through a 4" Port. But I could be wrong. Again, I've read in many differnt places that 350-400 cfm is the realistic max for a 4" port.

In analyzing the duct resistance, there are more engineering tables. they tell you how much resistance for each foot of straight pipe, each elbow, and for flex hose.

[Tom Veatch]

.... Is it worth running 6" duct instead of 4" if I have to reduce it back to 4" right before it hits the port/impeller?...

Absolutely.

You're looking for as little pressure drop in the system as possible. The pressure loss across the 4" orifice will, in general, be significantly less than the increased pressure loss from running 4" throughout rather than 6" ducting. Putting it in other words, the pressure loss from 4" ducting throughout will be greater than the pressure loss in that same system with 6" ducting and (effectively) 4" orifices at the duct ends.

[Tim Cleary]

Joe - Thanks for the resources. Although I was hoping the fairy forum godmother would tell me the perfect answer for my situation, not give me homework. But as per most situations, doing the detail work yourself is the best approach. Thanks for the link, and the general advice / guidelines--

Tim

[Tim Cleary]

Wow- I was just complaining that no one has posted the perfect answer and now you have. I appreciate the advice. My only worry is that it will create a situation where I have to explain to my wife why I need a new DC with 6" intake port - but I have plenty of time to think of a reason on that front - I will just cite some stats from Bill Pentz and hopefully be golden.

Seriously, thanks for the advice

Tim

[Mike Cruz]

Yeah, but if you upgrade your DC to one with a 6" port, won't you then need to run 8" pipe for a future DC upgrade? Try explaining THAT one to the wife! Just put in 12" pipe now and be done with it...

[Alan Schaffter]

What model Delta do you have?

You can remove the intake on most blowers and you can also remove the side panel from the blower housing. If you can remove the side panel by removing the dozen or so screws around the perimeter, then you can make a new one with a piece of sheet metal or ply and add you own 6" intake. That is what I would do since the 4" intake kills CFM from the get go so the velocity in the 6" will be really, really low; maybe too low to keep larger dust in suspension. Also, you will likely upgrade the blower soon or later, so you might as well run 6" duct.

[Joe Jensen]

I am an electical engineer who is also only 12 credits shy of a mechanical engineering degree. Dust collection systems are deceptively marketed. If you have a 6" port on the collector, 6" pipe, and a 4" port, you will still only get 350-400 cfm. If you put a 5HP collector with 8" pipe you'd still get about 350-400 cfm. The reason for this is that the collectors are designed for more flow but not more pressure. What we would like to do with bigger collectors is to get more cfm through the ports on our machines. Doubling the flow through a port takes 4X the pressure at the port. But the way dust collectors are designed , the higher the horsepower the more CFM but at no more pressure. This would let you run multiple machines at one time, but if you are a one man shop using only one machine, the increased HP won't help you.

Think about water. If you have 50 psi at the source, and a 3/4" hose hooked to a nozzle and spray and the water goes say 10 ft. Then you put a 2" hose on the nozzle with the same 50 psi and spray. How far will the water go, the same 10 ft. Now if you have like hundreds of feet of hose, the bigger hose will spray a little further than the smaller hose because the losses will add up. But with say a 25 ft hose, you'd get the same distance. The only way to spray further would be a bigger nozzle, or more pressure. Bigger pipe won't matter. Same for dust collection. The 4" port anywhere in the sytem will limit the cfm. If the cfm is too low, the dust won't stay suspended.

Tons of people on the web say to put big pipe on without understand the aerodynamics.

To answer your question.

Most smaller dust collector generate about 10" of pressure with no filter and no pipe. A used bag will use 1-1.5" of pressure. Now you have 8.5-9" of pressure. 50 ft of 4" pipe uses 5.5" of pressure. Now you have 3" of pressure. Each wide sweep elbow is like 6 ft of pipe. 3 elbows would be like another 18 ft or another 2" of pressure . Problem, that collector with that much pipe and elbows can't pull much air because most of the pressure is used up before you get to the port, you only have about 1" left. Getting 350 cfm through that port will take like 3" of pressure. Now, go to a 6" duct. 50 ft of 6" duct at 3500 ft per min speed uses 3.5" of pressure. Each 6" elbow is equal to 12 ft of 6" pipe. 3 elbows would be 36 ft, or 2.5" more pressure. 6" pipe would increase the pressure at the port to 2" which would double the cfm, but still wouldn't get you to 350 cfm. Keep in mind that 6" pipe needs 3500 cfm to keep the dust suspended. To double the 350 cfm flow through a 4" port, you'd have to 4X the pressure. So if it took 3" to get 350 cfm, it would take 12" at the port to get 700 cfm. Not going to happen with real world filters and pipe.

The answer is less pipe. If you have say 20 ft of pipe and 2 elbows;
4" duct nets 5-5.5" of pressure at the port. Should pull 350-400 cfm.
6" duct nets less than .5" more pressure at the port, or 5.5-6". Should pull about the same CFM as the 4" port.

The key with a smaller collector is not to have too much pipe and very few elbows. Rember the 50 ft 3 elbow math, both 4" and 6" duct is too long. With the 20 ft 2 elbow model, both the 4" and 6" are ok. Weird huh?

[Alan Schaffter]

Without knowing which DC you have it is hard to give specifics, but by your description it sound like it is a 1 hp or 1.5 hp unit with a 10" - 11" impeller. As Joe pointed out rather extensively- a piped system with that unit is almost a non-starter with either 4" or 6" ducting.

Each motor/blower unit has a fan curve that plots static pressure (resistance) vs flow (CFM). The CFM and static pressure ratings provided by manufactures are usually based on a unit running without filters, ducting, etc. Go to Oneida's site and pull up a couple of representative fan curves. Use just some of the resistance data Joe provides or go to Bill Pentz site for a longer list. Add up the resistance.

Since on a fan curve CFM is max where static pressure (resistance) is near zero and CFM is zero where static pressure is max, you can see how you can easily run out of one or the other, especially CFM very easily. A DC is designed to work best in the middle range of the fan curve.

[Joe Jensen]

One other thought. I started with the 2HP Oneida and their duct design. I had mostly 5" duct going to machines with 4 or 5" ports and I wasn't happy with the airflow. I ended up upgrading the ports to 5 or 6" or most machines and had to repipe a lot of the shop. If you are going with hard duct, there is a good chance you will end up getting a more powerful collector. If you use the small duct, you'll for sure have to replumb. Maybe use 5" snap lock as a compromise.

[Rich Aldrich]

Tim,

Three years ago, I bought a Clearvue and installed 6" PVC ducting - ASTM 3034 from Menards. Most of my machines had 4" or smaller ports.

I ended up replacing the ports on my tablesaw, jointer and planer by making my own modifications. I was able to make new ports with 3/4" plywood and 6" pvc stubs that attach to the machines where the 4" ports were attached. The 6" hose slip on the pvc with a little bit of heat with a hair dryer.

For the tablesaw, I made a 6" to 5" reducer with a 4" wye. There was a link from either Bill Pentz website or Clearvue that had a few instrutional pages on how to do this. I also made one for the router table.

I still have to make a hood for my SCMS and connections for a few other tools.

[Roger Bean]

Many thanks to Joe and others for answering questions I've had for a while. I'm running 2HP old Grizzly DC (6" outlet with factory dual 4" Y) with about 25 ft of 4" rigid pipe. Then flex to machines. Worked, sort of, kinda, pretty much, but not spectacular. Recently upgraded to a huge filter bag from American Fabric Filter. My flow and performance improved substantially. I'm now no longer planning on upgrading the main line to 6". Actually best thing I did for dust collection was installing a remote so I actually turn it on whenever I use a machine! Comments?

[Nicholas Lingg]

Air flow will not be the major problem, clogging at the 4" reduction will be. I had to do what Alan did and open every thing up to allow the large clumps of chips from the planer and jointer to pass throught.