Is 4 inch dust ports too small? - Page 2 - Woodworking Talk - Woodworkers Forum
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post #21 of 33 Old 08-22-2012, 01:56 PM
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Originally Posted by dgoodyear View Post
One thing is easy to explain. The Wynn filters have a massive amount of filter area compared to a seasoned bag filter. If the Wynn filter will effectively decrease static pressure in the dust collector allowing more air to flow. You can expect to see a decrease in air flow as the Wynn filter becomes seasoned. Are you saying that initially you were pulling 400 cfm in a 4" duct and then started started pulling 600 cfm? or was it more like 300 then pulled 450 because the two are quite different. I expect the latter to be more realistic since the air is not that compressible. a 4" duct can support about 400 - 500 CFM. Without details about how the measurement was done I cannot provide an explanation.

I do agree that using a duct larger than the inlet on the DC does not make sense. Thats no different than runnign the collection system unconstrained. Not sure how running 6" everywhere except at the machine ports and the DC inlet would increase your air flow in the main duct. Something doesn't add up...
Look your new here, I know nothing about who you are what your trained in (school) and definitely not sure how you support your claims. What testing equipment your using etc. Nothing personal.

As for the Wynn filter yes that is why but it is also the same reason the 6"duct works better. More air flow in the system. The longer the run of 4"duct the slower the air moves because the system is starving. Even with a 4" port at the machine your still moving more air by volume then if it was a 4" duct. Think about it like this if a 4" port was a bad thing why would they put a reducer or "y" right off the impeller and why use 4" ports on the machines. You have to reduce down to increase the velocity needed at the machine. More velocity is needed to pick up debris then is needed to keep it moving inside the system.

As I said I'm a simple guy. I'm a Marine (Engineer) I have worked in construction most of my life including framing/trim carpenter as well as electrician, Hvac etc Although I went to school for some things most everything I learned was done the old fashion way on the job training. I've aced about everything I went to school for but never liked the environment. I'm antsy.

Here is a link to the testing. I warn you the testing equipment is not high tech but none the less they were effective. I do know I tested the port just a few feet from the Dc and then the one used for testing and they were the same even though the one I used for testing was 25 ft away roughly.

Last edited by rrbrown; 08-22-2012 at 02:11 PM.
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post #22 of 33 Old 08-22-2012, 02:13 PM Thread Starter
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4 inch restriction

dgoodyear

I understand that a 4" duct or system will limit the flow to ~ 350 -400 cfm but will a port alone acting like a restriction or orifice react the same? Is it that once you knock the velocity down it can not recover?
I am trying to understand what goes on.
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post #23 of 33 Old 08-22-2012, 02:20 PM
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Originally Posted by rrbrown View Post
Look your new here, I know nothing about who you are what your trained in (school) and definitely not sure how you support your claims. What testing equipment your using etc.

As I said I'm a simple guy. I'm a Marine (Engineer) I have worked in construction most of my life including framing/trim carpenter as well as electrician, Hvac etc Although I went to school for some things most everything I learned was done the old fashion way on the job training. I've aced about everything I went to school for but never liked the environment. I'm antsy.

Here is a link to the testing. I warn you the testing equipment is not high tech but none the less they were effective. I do know I tested the port just a few feet from the Dc and then the one used for testing and they were the same even though the one I used for testing was 25 ft away roughly.
I, like you, am just trying to help. I don't claim to be an expert and my purpose here is not to disprove anyone. I know I am new but I do have some qualifications although I am not an expert. I am a condensed matter physicist (Ph D) by trade working as a medical physicist now. I have some background in theoretical fluid dynamics and engineering. My years of study have provided me with enough knowledge to do my own measurements with equipment that I have purchased for the purposes of testing my own dust collection system. My measurements involve using a custom made needle valve to vary the duct cross-section. I use a dywer instruments mark II digital manometer for measuring both static and velocity pressure using a pitot tube.

A pitot tube is a must for these measurements since it's small cross section leads to minimal perturbation of the air stream and doesn't introduce turbulence in the air flow. The air speed changes as a function of the position of the pitot withing the airflow. If the pitot is in the center, the velocity is higher, if it is at the edge the velocity is slower because of resistance against the wall of the duct. using a pitot you can build an airflow map and determine the CFM although usually if you multiply the center measurement by 0.9 you get the CFM in the duct.

I do not dispute that fact that you had an increase in air flow after you upgraded to the pleated filter. Thats the same reason I ditched my bag filter on my first dust collector. Increasing surface area decreases the static pressure on the DC side, increasing airflow. It just doesn't seem to make sense (based on my measurements) to me that reducing to a 4" port at the DC inlet lead to an increase in air flow when you effectively decreased your main duct size. With my 2HP cyclone I am able to draw about 500 CFM through a 4" duct but thats it...Just trying to understand the setup for your equipment. I'll have a look and post again later....
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post #24 of 33 Old 08-22-2012, 02:37 PM
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Originally Posted by dgoodyear View Post
I, like you, am just trying to help. I don't claim to be an expert and my pose here is not to disprove anyone. I know I am new but I do have some qualifications although I am not an expert. I am a condensed matter physicist (Ph D) by trade working as a medical physicist now. I have some background in theoretical fluid dynamics and engineering. My years of study have provided me with enough knowledge to do my own measurements with equipment that I have purchased for the purposes of testing my own dust collection system. My measurements involve using a custom made needle valve to vary the duct cross-section. I use a dywer instruments mark II digital manometer for measuring both static and velocity pressure using a pitot tube.


I do not dispute that fact that you had an increase in air flow after you upgraded to the pleated filter. Thats the same reason I ditched my bag filter on my first dust collector. Increasing surface area decreases the static pressure on the DC side, increasing airflow. It just doesn't seem to make sense (based on my measurements) to me that reducing to a 4" port at the DC inlet lead to an increase in air flow when you effectively decreased your main duct size. With my 2HP cyclone I am able to draw about 500 CFM through a 4" duct but thats it...Just trying to understand the setup for your equipment. I'll have a look and post again later....
Your over thinking or I'm not getting my point across. When you reduce down the 6" to 4" the air velocity has to increase on the 4" side to keep up with the volume of air on the 6" side. This causes the suction (pull on objects ) to also increase because your moving more air faster to maintain the air at the exhaust port.

Nothing personal I promise and this is probably going to sound bad at first. I have had many fits over designed things or house plans that some architect/design engineer said works in theory. My brother in law is a engineer smart as hell in his profession but worthless mechanically. Not saying or implying in any way that this is you. I'm just
Pointing out that the in theory argument is seriously flawed in my opinion. I rather deal with reality of things.

I hope that didn't come across as bad as it could have.
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post #25 of 33 Old 08-22-2012, 03:52 PM
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Originally Posted by rrbrown View Post
Your over thinking or I'm not getting my point across. When you reduce down the 6" to 4" the air velocity has to increase on the 4" side to keep up with the volume of air on the 6" side. This causes the suction (pull on objects ) to also increase because your moving more air faster to maintain the air at the exhaust port.

Nothing personal I promise and this is probably going to sound bad at first. I have had many fits over designed things or house plans that some architect/design engineer said works in theory. My brother in law is a engineer smart as hell in his profession but worthless mechanically. Not saying or implying in any way that this is you. I'm just
Pointing out that the in theory argument is seriously flawed in my opinion. I rather deal with reality of things.

I hope that didn't come across as bad as it could have.
I take no offense and I hope I haven't offended you or anybody else. Just trying to share my experience. I don't think I am overthinking....My understanding is that you have 4" ports on your machines, 6" duct work, and then reduce to 4" at the DC inlet. is that correct? I agree that this setup reduces static pressure compared to a 4" duct system of the equivalent length since there is inherently less resistance in the 6" duct. If my understanding of your setup is correct then you have created an restrictive orifice in two places ie at the beginning (tool port) and at the end (inlet to DC) both of which are 4". For a given fan diameter there is a fan curve that relates how much CFM changes as a function of static pressure in the duct system. If you run an OPEN ended 6" directly to the DC which has a 4" inlet it will be approximately the same as running the DC with an open unconstrained inlet. Not quite but similar. Therefore you will have the maximum CFM the DC can move. If you replace the inlet with a 4" OPEN ended duct, the inlet is no longer unconstrained and the static pressure has to be higher than it would be for an open inlet because of air resistance in the duct. This means you would move less CFM than an open inlet. Now adding a 6" section of pipe to the 4" on the open end adds more static pressure, then adding a 4" tool port on the end of that adds more static pressure again. There is conservation of volume. The CFM in a 4" pipe is the same as the CFM in a 6" pipe but the velocity is different. the fact of the matter is that adding any duct that will give you a useful air velocity inside the pipe (3800 fpm) to the inlet will increase static pressure and decrease air volume at the tool port. I don't dispute the velocity being higher. To collect all the fine dust, or to be sure you're getting all the fine dust you need a large volume of air which you can not achieve with a 4" tool port. For a constrained tool shroud you can get away with high velocity. Look at the SawStop. The blade guard is very well designed in my opinion and it hooks up to a shop vac!
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post #26 of 33 Old 08-22-2012, 04:17 PM
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I don't agree with Bill Pentz, provided one keeps the connections to the 4" ports as short as possible.

Friction loss = ((Pipe friction factor) x (length of pipe) x (air velocity)) / (2 x (pipe diameter) x (gravitational acceleration))

So, it follow that if the 4" length of pipe is short, as well as the opening the effect of losses is not that great.

It also follows that for a given CFM a 6" line offers roughly 4 times the friction losses compared to a 4" line, meaning this is what is important.

Explained below:

Volume = Velocity x Area

so from 4" to 6" the area is:

4" = 12.6 square inches

6" = 28.3 sqaure inches'

To make things simple lets say the cross sectional area of the 4" pipe is 1/2 that of the 6" pipe. It means the velocity in the 4" pipe will be twice that in the 6" pipe.

In the friction formula above, friction losses are proportional to velocity squared, so it follows that the friction loss in the 4" pipe will be four times that in the 6" pipe and for any substantial length of pipe, that will kill the system

Pure mathematics is, in it's way, the poetry of logical ideas. - Albert Einstein.
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post #27 of 33 Old 08-22-2012, 04:30 PM
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OK, now I'm confused here

Most dust collectors have an inlet port of about 6" like my Jet 1100. They supply a "Y" to give you 2 - 4" ports. Why would anyone with common sense reduce a 6" main line down to 4" at the DC port unless I mis-read the posts above???

I use a 6" to 4" reducer at the DC inlet because I only run 4" to my each of my machines.

So far I got this much from this:
1. GO BIG or GO HOME. 6" main line or better for maximum collection of fine dust... chips don't care, 4" will work OK. Correct?

2. Also, 4" main line which is much more common, will work just not as well as 6". Correct?

3. For a mobile DC unit running to only one tool at a time 4" from DC to machine port will work OK, but 6" is better? Correct?

bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #28 of 33 Old 08-22-2012, 04:32 PM
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Originally Posted by DPJeansonne View Post
dgoodyear

I understand that a 4" duct or system will limit the flow to ~ 350 -400 cfm but will a port alone acting like a restriction or orifice react the same? Is it that once you knock the velocity down it can not recover?
I am trying to understand what goes on.

If you look at the way manufacturers measure fan curves it may give you a better idea of the answer to the question. Any manufacturer that has a fan curve extends the inlet with a length of duct. Adding up to 10 x the diameter of the inlet usually provides an air flow that is not turbulent. I am not sure if they all do this. Some make up a set of donuts to act as orifices to restrict airflow. Unfortunatley using a donut tends to give higher velocity measurements in teh center of the pipe the closer the pitot is situated with respect to the opening. So the Pitot needs to be far from the opening. They place a pitot tube a couple feet from the DC inlet with the velocity probe oriented towards the on comming air flow. The static pressure ports are perpendicular to the air flow. With a fancy pitot you can measure both static and velocity pressure (used to calculate CFM). So for a 6" open pipe they would run the DC, measure the static pressure and velocity pressure. The may then change to a donut that gives an opening of 5". Then measure the static pressure and velocity pressure again. So on and so forth until the pipe is completely closed off. Then you get the maximum vacuum that the DC can create. This number is usually the one that's quoted by the manufactures. So if a manufacturer states 2000 CFM and 12" static pressure it doesn't mean the DC can move 2000 CFM at 12" static pressure because the 12" is measured with the duct closed which means the DC is moving no air!

Then they plot a graph of CFM vs Static pressure. In essence they use a set of predefined orifices sizes to alter the CFM or air flow and then measure the induced static pressure change. The fan curve then tells you about how the impeller moves air over a range of static pressures. There is no recovery since volume of air is conserved. ie the air moving through the orifice needs to equal the volume of air moving in the 6" pipe (as per example). the velocity will decrease as a function of lowering the orifice diameter. Once you have restricted the opening you have a defined static pressure. That static pressure corresponds to some CFM.

As for your question about the the port acting as a restriction or an orifice...I'm not sure...completely. It seems to me that there are several things happening that somebody else may need to correct me on. If you mean a restriction as being a wye or 90 then they increase static pressure...then here it goes. I think the resistance associated with these type of restrictions may due to turbulence induced into the air flow which effectively induces some randomizing currents in the air stream that decreases the air velocity and air in the boundary layers along the pipe induce friction in the air flow reducing the velocity. This is happening continiously while the collector is pulling air so it manifests itself as in increase in static pressure (decrease in CFM) for the system. For a tool port there may be several things happening. Probably some of the first explanation and some physics of the compressibility of air. A tool port which opens into a cabinet will pull air from all directions into a duct of a certain diameter, the air is barely compressed as it enters the duct an therefore a given duct can only move so much air (at a given density).

If anybody else has an explanation please feel free to bash mine! I hope answers your question in the quote above.
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post #29 of 33 Old 08-22-2012, 04:38 PM
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Originally Posted by woodnthings View Post
Most dust collectors have an inlet port of about 6" like my Jet 1100. They supply a "Y" to give you 2 - 4" ports. Why would anyone with common sense reduce a 6" main line down to 4" at the DC port unless I mis-read the posts above???

I use a 6" to 4" reducer at the DC inlet because I only run 4" to my each of my machines.

So far I got this much from this:
1. GO BIG or GO HOME. 6" main line or better for maximum collection of fine dust... chips don't care, 4" will work OK. Correct?

2. Also, 4" main line which is much more common, will work just not as well as 6". Correct?

3. For a mobile DC unit running to only one tool at a time 4" from DC to machine port will work OK, but 6" is better? Correct?

bill
You've nailed it on the head.
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post #30 of 33 Old 08-22-2012, 05:07 PM
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Originally Posted by woodnthings View Post
Most dust collectors have an inlet port of about 6" like my Jet 1100. They supply a "Y" to give you 2 - 4" ports. Why would anyone with common sense reduce a 6" main line down to 4" at the DC port unless I mis-read the posts above???

I use a 6" to 4" reducer at the DC inlet because I only run 4" to my each of my machines.

So far I got this much from this:
1. GO BIG or GO HOME. 6" main line or better for maximum collection of fine dust... chips don't care, 4" will work OK. Correct?

2. Also, 4" main line which is much more common, will work just not as well as 6". Correct?

3. For a mobile DC unit running to only one tool at a time 4" from DC to machine port will work OK, but 6" is better? Correct?

bill
You've hit the nail on the head! Although some reading into this may see that your points 1 2 and 3 each have an answer of 6 which gives 666==Bill Pentz is evil! All joking aside I think BPs stuff is somewhat misunderstood. My understanding is that fine dust is easy to move around. If it gets beyond the influence of the air moving to a collection port, for example spun off at high speed from a router bit it is carried by the air stream created by the spinning tool. Then it gets carried by the air currents around the room where eventually you breath it in deep into your lungs. It may be like this for days in your shop since submicron dust may take days to settle and gets stirred up for more days if you move around in the shop. With a large volume of air, you can recapture the stray stuff that would normal escape the tools (with a properly designed dust hood of course) if it starts its journey away from the tool.
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post #31 of 33 Old 08-22-2012, 05:31 PM
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If anybody would like to see testing of my DC have a look on lumberjocks: http://lumberjocks.com/reviews/2869
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post #32 of 33 Old 08-22-2012, 07:30 PM
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I take no offense and I hope I haven't offended you or anybody else. Just trying to share my experience. I don't think I am overthinking....My understanding is that you have 4" ports on your machines, 6" duct work, and then reduce to 4" at the DC inlet. is that correct? I agree that this setup reduces static pressure compared to a 4" duct system of the equivalent length since there is inherently less resistance in the 6" duct. If my understanding of your setup is correct then you have created an restrictive orifice in two places ie at the beginning (tool port) and at the end (inlet to DC) both of which are 4". For a given fan diameter there is a fan curve that relates how much CFM changes as a function of static pressure in the duct system. If you run an OPEN ended 6" directly to the DC which has a 4" inlet it will be approximately the same as running the DC with an open unconstrained inlet. Not quite but similar. Therefore you will have the maximum CFM the DC can move. If you replace the inlet with a 4" OPEN ended duct, the inlet is no longer unconstrained and the static pressure has to be higher than it would be for an open inlet because of air resistance in the duct. This means you would move less CFM than an open inlet. Now adding a 6" section of pipe to the 4" on the open end adds more static pressure, then adding a 4" tool port on the end of that adds more static pressure again. There is conservation of volume. The CFM in a 4" pipe is the same as the CFM in a 6" pipe but the velocity is different. the fact of the matter is that adding any duct that will give you a useful air velocity inside the pipe (3800 fpm) to the inlet will increase static pressure and decrease air volume at the tool port. I don't dispute the velocity being higher. To collect all the fine dust, or to be sure you're getting all the fine dust you need a large volume of air which you can not achieve with a 4" tool port. For a constrained tool shroud you can get away with high velocity. Look at the SawStop. The blade guard is very well designed in my opinion and it hooks up to a shop vac!
No you got it wrong.

6" from dc all the way to the machine reduce down to 4" blast get then machine.

The blade guard on a SawStop is deigned well but they ave a flaw with the overran dust collectn system they use a single 4" port and split it between the guard and the 4" port on the saw. See design engineers created a good guard and screwed it up with that one fitting. Ideally a larger duct should be used to split as they have it or they should reduce the port on the saw to a smaller size like 3" so that the two split second can support the 4" hose..
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post #33 of 33 Old 10-20-2012, 08:12 PM
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Try thinking outside the box. The pic below is my drum sander, just because I couldn't modify the factory hood. May not be pretty, but works like a charm. Besides, by doing it this way, I still have the intact factory hood should I ever try to sell it.

Really nice set up. I have a very similar situation and after seeing yours I am going to try to do the same thing. Only problem I might have is I plan on only 5" ducting.

It appears that you made your own blast gates. They look great.
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