# Is 4 inch dust ports too small?



## DPJeansonne

I have been reading all the confusing discussions on dust collection piping designs. I understand that having the larger 5 or 6" ports would be optimum but most of my tools are 4" and are not easily modified. My 18" drum sander being the main collection problem. I am purchasing a 3hp cyclone to improve my collection and the supplier says for me to try it with existing piping because I will see much improvement over 1.5hp single stage collector. 

I have read that it doesn't help to run 6" pipe with a 4" port as the port will limit the flow no matter what. That is my question. Maybe trial and error will tell.

Does a 4" port limit CFM and installing larger collection piping does no good?


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## rrbrown

DPJeansonne said:


> I have been reading all the confusing discussions on dust collection piping designs. I understand that having the larger 5 or 6" ports would be optimum but most of my tools are 4" and are not easily modified. My 18" drum sander being the main collection problem. I am purchasing a 3hp cyclone to improve my collection and the supplier says for me to try it with existing piping because I will see much improvement over 1.5hp single stage collector.
> 
> I have read that it doesn't help to run 6" pipe with a 4" port as the port will limit the flow no matter what. That is my question. Maybe trial and error will tell.
> 
> Does a 4" port limit CFM and installing larger collection piping does no good?


It does benefit you to run 6" duct and then reduce to 4" at the machine. The longer the run of 4" duct the more the air slows and starves the DC. Where as by running larger duct and reducing down it increases the velocity of the air to try and keep up with the larger duct size.

Sure your 4" duct will work better but the larger DC is going to starve and you loose the full potential of the larger DC system.

By increasing the duct to 6" as I suggested to you my system effiency increased by 50% with a 1.5 hp DC.


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## TomC

I believe you will improve air flow with the larger piping(5 or 6") due to less pressure drop in the main header runs. The 4" connection will be a restriction but not the same as having all 4" piping. I agree you should see an improvement with just the addition HP. I plan on upgrading my DC system within the next 6 months. What supplier are you going with.
Tom


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## Dave Paine

+1 for what Richard said.

I have a 16in drum sander with a 4in port. I am presently running a 1.5HP Jet cannister dust collector. I do not observe any issues with my drum sander. Very little dust remains.

Some people use a larger duct size for the main lines. This reduces pressure drop in the main lines, so actually improves performance.

Your 3HP dust collector will really benefit from a 5 or 6in main duct size, as would the present 1.5HP machine.

My system is adequate for my present needs. I know I could reduce losses with a 4in metal system with large radius fittings, or with larger line size. I am not ready to spend the money when I am not experiencing dust issues with my machines.


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## Fred Hargis

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.


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## Leo G

Just about all my machine ports are 4". All my drops are 5" and I have a tapered reducer at the machine to adapt it to 4". It's a lot better than a long 4" line but the restriction shows. If you pull the reducer off the 5" flex pipe and then just put it on (while the DC is on) it will get sucked onto and held on the hose very well. Proving that the reducer has a pretty good restriction.


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## jigs-n-fixtures

rrbrown said:


> It does benefit you to run 6" duct and then reduce to 4" at the machine. The longer the run of 4" duct the more the air slows and starves the DC. Where as by running larger duct and reducing down it increases the velocity of the air to try and keep up with the larger duct size.


Ok this seems a bit confused. From physics and fluid dynamics I know that Q=v*a, where Q is the volume, v is the mean flow velocity, and a is the cross sectional area of the duct. This means the velocity is roughly twice as high in a 4-inch duct as it is in a 6-inch. 

Friction loss is given by the duct equation, which I'm not going to try and do on the iPhone, but there are several online calculators. 

Assuming an air volume of 2800-cfm, the 4-inch will have almost 8 times as much head loss as a 6-inch, and 33 times as much as an 8-inch inch. 

The 4-inch connection to the machines will restrict flow somewhat, but since the run length is relatively short the increased head loss will not be a major loss, and the increased velocity will help lift large particles out of the machine and into the larger main run. 

The carrying velocity for chips and large sawdust is about 4000-feet per minute. Which means for a dc pulling 2800-cfm, that a 10-inch duct will give the least head loss, while maintaining sufficient velocity to carry any sawdust or chips you would ever produce, in a horizontal run.

Sent from my iPhone using Wood Forum


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## rrbrown

Originally posted by rrbrown

It does benefit you to run 6" duct and then reduce to 4" at the machine. The longer the run of 4" duct the more the air slows and starves the DC. Where as by running larger duct and reducing down it increases the velocity of the air to try and keep up with the larger duct size.



jigs-n-fixtures said:


> Ok this seems a bit confused. From physics and fluid dynamics I know that Q=v*a, where Q is the volume, v is the mean flow velocity, and a is the cross sectional area of the duct. This means the velocity is roughly twice as high in a 4-inch duct as it is in a 6-inch.
> 
> Friction loss is given by the duct equation, which I'm not going to try and do on the iPhone, but there are several online calculators.
> 
> Assuming an air volume of 2800-cfm, the 4-inch will have almost 8 times as much head loss as a 6-inch, and 33 times as much as an 8-inch inch.
> 
> The 4-inch connection to the machines will restrict flow somewhat, but since the run length is relatively short the increased head loss will not be a major loss, and the increased velocity will help lift large particles out of the machine and into the larger main run.
> 
> The carrying velocity for chips and large sawdust is about 4000-feet per minute. Which means for a dc pulling 2800-cfm, that a 10-inch duct will give the least head loss, while maintaining sufficient velocity to carry any sawdust or chips you would ever produce, in a horizontal run.
> 
> Sent from my iPhone using Wood Forum


I think you just said the same thing as I did but you confused the hell out of me with all your technical info.:confused1::confused1::confused1:

I'm a simple man and I hate he technical explanations.:laughing:


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## DPJeansonne

Tom C
I went with Oneida V series.


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## DPJeansonne

*port sizing issues*

Oneida says the 4" port will limit the cfm to about 350 - 400 and 4000 fpm no matter what the duct is. I had heard this in other postings but can't recall where.
The cfm then will slow the fpm flow in a larger diameter pipe. If that cfm doesn't go back up and I don't know why it would then there isn't and benefit and a potential of going way below 4000 in the 6 inch line.

I think it is not a problem with larger chips but fine sanding dust it maybe another problem.

With all that said what you all are telling me is that from real world experience you don't agree.

I can see the static losses go down on 6" duct but at the lower flow the velocity is taking a hit. I really boils down to does the cfm go back up from the restriction caused by the tool port.


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## DPJeansonne

*sander hood mod*



Fred Hargis said:


> 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.


That is the same sander I have. I really like the hood you made.
What hp collector where you using before and after the mod?


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## dgoodyear

The problem with the above calculation is that you assume that the duct can support 2800 CFM which it will not. For static pressures involved in dust collection for the home shop, the air that we are moving is under fairly low pressure. The air acts as in incompressible fluid and therefore your smallest duct will determine your CFM. A 4" duct will move about 400 CFM regardless of the size of the dust collector. If the impeller is larger, the CFM will increase a bit as the air becomes compressed a little. The 4" ducts will starve the dust collector regardless of how long the main header duct is. If your collector has a published fan curve, you will see what static pressure will give you 800-1000CFM. Design your duct system with 6" while trying to keep static pressures within the range that gives you the CFM fan curve. Typically a 6" duct will give you this air flow. You need to take the 6" duct right to the machine. Fine dust is the enemy and is hardest to capture. Once it gets beyond the influence of the negative pressure well created by the dust port, it can escape into the shop, floating around for hours, even days. This is why you need a large volume of air. What you need to do is read Bill Pentz site. He has a static pressure calculator that is quite useful for duct design.


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## Gary Beasley

If you realize the 4" port is starving the system you need to crack open a nearby port to act as a bleeder and raise the airflow volume in the main lines. It probably won't need to be all the way open, just enough to get the DC breathing right.


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## TomC

DPJeansonne said:


> Oneida says the 4" port will limit the cfm to about 350 - 400 and 4000 fpm no matter what the duct is. I had heard this in other postings but can't recall where.
> The cfm then will slow the fpm flow in a larger diameter pipe. If that cfm doesn't go back up and I don't know why it would then there isn't and benefit and a potential of going way below 4000 in the 6 inch line.
> 
> I can see the static losses go down on 6" duct but at the lower flow the velocity is taking a hit. I really boils down to does the cfm go back up from the restriction caused by the tool port.


The cfm is only going to be what is coming thru the 4" opening unless you are also pulling on another machine at the same time or if air is some how leaking into the system. Basically air in equals air out.
Tom


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## dgoodyear

If you plan on operating more than 1 machine at a time then anything more than 4" drops will not work. However 4" line at a tool port will draw 400 CFM, not much more regardless the size of the dust collector. if you are going to run 1 machine at a time then running your 6" line directly to the machine will ensure that you capture the stray dust that gets away momentarily from the machine. You need volume for that and a 4" line will not suffice. You shouldn't be worried about the big stuff. It's the small stuff that you can't see that is the problem.

I agree that opening another blast gate in the system will increase the velocity in the main so that chips and dust stay in suspension in the airstream. However, that doesn't benefit the tool port you're working on. When I installed my collector I researched it to death. 7" mains and 6" drops to my machines is the best thing I ever done. If you want to see my setup you can look at my review on lumberjacks:http://lumberjocks.com/reviews/2869

I measured the fan curve for my system ala Bill Pentz methods and designed my duct system around that. 6" is the way to go. Regardless of the length of pipe, a 4" orifice on the end of a 7" pipe gives about 400-500 CFM and static pressures on the order of 10-11 for a 14" impeller. A cyclone itself has a certain amount of static pressure inherent in it's design. These measurements take this into account.

Really you need the fan curve for your system and a method to estimate static pressure for you duct design (Bill Pentz spread sheet). This will give you a better idea of how much air you will be moving at the tool port for you real world situation.

Keep in mind that without a manufacturers fan curve, the reported values of CFM and static pressure are useless. They typically measure CFM at an unconstrained open inlet with maximum air flow and measure the static pressure by closing off the inlet to measure the "vacuum" pressure of the system. This basically gives you 2 points on teh fan curve: the beginning (where static pressure is so low that it doesn't correspond to any real world DC application for moving air) and the end (where there is no air moving at all).

hope this helps

DJG


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## Fred Hargis

DPJeansonne said:


> That is the same sander I have. I really like the hood you made.
> What hp collector where you using before and after the mod?


The only DC I've had hooked to this sander is the one I now have: an Oneida SDG. It started life as a 2 HP units, and has since had the motor upsized to a 5 HP (very long story, don't ask). But with the setup I now have I estimate I'm getting very close to the 800-900 CFM at the hood, that's likely reduced if the table is very close to the drums (sanding 1/4" thick pieces, for example).


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## DPJeansonne

*piping losses*



dgoodyear said:


> The problem with the above calculation is that you assume that the duct can support 2800 CFM which it will not. For static pressures involved in dust collection for the home shop, the air that we are moving is under fairly low pressure. The air acts as in incompressible fluid and therefore your smallest duct will determine your CFM. A 4" duct will move about 400 CFM regardless of the size of the dust collector. If the impeller is larger, the CFM will increase a bit as the air becomes compressed a little. The 4" ducts will starve the dust collector regardless of how long the main header duct is. If your collector has a published fan curve, you will see what static pressure will give you 800-1000CFM. Design your duct system with 6" while trying to keep static pressures within the range that gives you the CFM fan curve. Typically a 6" duct will give you this air flow. You need to take the 6" duct right to the machine. Fine dust is the enemy and is hardest to capture. Once it gets beyond the influence of the negative pressure well created by the dust port, it can escape into the shop, floating around for hours, even days. This is why you need a large volume of air. What you need to do is read Bill Pentz site. He has a static pressure calculator that is quite useful for duct design.


I looked on Bill Pentz's site at FAQs and found this response---- ( only a portion of response included)

Does a 4" connection at the machine negate the benefit of the 6" duct going right to that machine? Yes, it kills the dust collection performance. At typical airspeeds and pressures for dust collection, air is virtually incompressible. Air can speedup some to get around a short obstruction, but just like a water valve, closing down the opening greatly restricts flow. The standard 4" connections on our larger hobbyist machines kill the CFM below what we need to collect the fine dust. We pretty much have to replace all the 4" ports on our larger machines if we are going to collect the fine dust at the source. 

The other part of your question is what is the impact on airflow when using a 4" drop attached to a 6" line? My engineer friends at Dwyer Instruments that build most of the air measurement meters say roughly 10 diameters of pipe will both stabilize the airflow and set that airflow to about the same duct speed as your main. Most air engineers that are just interested in getting sawdust build systems targeted to get an airspeed of 4000 FPM in the main. That 4000 FPM when pulled through more than about 40” of 4” diameter duct will end up with a total air volume of 350 CFM. That is plenty for good chip collection at most small shop stationary machines, but far short of the 1000 CFM I recommend for good fine dust collection. The bad news is that roughly 350 CFM ends up with our main only having an total airflow of about 1782 FPM. That is way short of the minimum 2800 FPM needed to keep a horizontal main clear. The result is the main ends up building up first the larger chips then finer dust. It will continue to build up this dust until the duct is sufficiently restricted that the airflow is again fast enough to keep the remaining area clear. 
************ check his site FAQ for entire response ************** 

_________________________________ 


Bill says that you should only have a 1 inch difference in main to drop size. I don't know if the 6 to 4 will kill it since it seems alot of people do that.
I have done a little with his calc sheet trying to understand how it works. I think it is setup for a single size duct at a time since it has one cfm input cell. I would like to see if I can have the two different sized ducts input together to see the overall impact.

I guess I need to see what ports I can modify realistically. I didn't say but I only operate one machine at a time.


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## dgoodyear

As Bill said, 4" drops kills the airflow. There are two downsides to this: First plugging in the larger mains and second, killing the airflow necessary to collect the stray fine dust that gets beyond the influence of the dust ports. I put alot of time and effort into designing my own system. I use a 7" main throughout and step down to 6" at each tool. The pipe crossection is about 36% larger in teh 7" duct but my dust collector gives me about 1000 CFM (measured using a pitot and dwyer digital manometer) so that equates to about 3800 fpm which should be OK to prevent the ducts from plugging. I also only operate 1 tool at a time so I ran the 6 inch duct to the machine knowing I really wanted to get excellent dust collection including the fine rogue dust that attempts to make its way out of the machine during cutting. I think I have achieved my goal but I really won't be able to tell until I get my Dylos particle counter.

As for the static calculator. It assumes a single duct size. What I did is use the calculator to try to minimize my static pressure in the main and minimize the length of 6 inch duct and 6" flex to connect to the machine. I then measured the static pressure/CFM in my duct system using the tools I mentioned above in order to give me a better idea of my real world scenario.

I can say as cost was rising I felt that I was getting in too deep. However, I am now happy. My dust collection is the best it's ever been thanks to Bills work.

DJG


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## rrbrown

DPJeansonne said:


> I looked on Bill Pentz's site at FAQs and found this response---- ( only a portion of response included)
> 
> Does a 4" connection at the machine negate the benefit of the 6" duct going right to that machine? Yes, it kills the dust collection performance. At typical airspeeds and pressures for dust collection, air is virtually incompressible. Air can speedup some to get around a short obstruction, but just like a water valve, closing down the opening greatly restricts flow. The standard 4" connections on our larger hobbyist machines kill the CFM below what we need to collect the fine dust. We pretty much have to replace all the 4" ports on our larger machines if we are going to collect the fine dust at the source.
> 
> The other part of your question is what is the impact on airflow when using a 4" drop attached to a 6" line? My engineer friends at Dwyer Instruments that build most of the air measurement meters say roughly 10 diameters of pipe will both stabilize the airflow and set that airflow to about the same duct speed as your main. Most air engineers that are just interested in getting sawdust build systems targeted to get an airspeed of 4000 FPM in the main. That 4000 FPM when pulled through more than about 40” of 4” diameter duct will end up with a total air volume of 350 CFM. That is plenty for good chip collection at most small shop stationary machines, but far short of the 1000 CFM I recommend for good fine dust collection. The bad news is that roughly 350 CFM ends up with our main only having an total airflow of about 1782 FPM. That is way short of the minimum 2800 FPM needed to keep a horizontal main clear. The result is the main ends up building up first the larger chips then finer dust. It will continue to build up this dust until the duct is sufficiently restricted that the airflow is again fast enough to keep the remaining area clear.
> ************ check his site FAQ for entire response **************
> 
> _________________________________
> 
> 
> Bill says that you should only have a 1 inch difference in main to drop size. I don't know if the 6 to 4 will kill it since it seems alot of people do that.
> I have done a little with his calc sheet trying to understand how it works. I think it is setup for a single size duct at a time since it has one cfm input cell. I would like to see if I can have the two different sized ducts input together to see the overall impact.
> 
> I guess I need to see what ports I can modify realistically. I didn't say but I only operate one machine at a time.





dgoodyear said:


> As Bill said, 4" drops kills the airflow. There are two downsides to this: First plugging in the larger mains and second, killing the airflow necessary to collect the stray fine dust that gets beyond the influence of the dust ports. I put alot of time and effort into designing my own system. I use a 7" main throughout and step down to 6" at each tool. The pipe crossection is about 36% larger in teh 7" duct but my dust collector gives me about 1000 CFM (measured using a pitot and dwyer digital manometer) so that equates to about 3800 fpm which should be OK to prevent the ducts from plugging. I also only operate 1 tool at a time so I ran the 6 inch duct to the machine knowing I really wanted to get excellent dust collection including the fine rogue dust that attempts to make its way out of the machine during cutting. I think I have achieved my goal but I really won't be able to tell until I get my Dylos particle counter.
> 
> As for the static calculator. It assumes a single duct size. What I did is use the calculator to try to minimize my static pressure in the main and minimize the length of 6 inch duct and 6" flex to connect to the machine. I then measured the static pressure/CFM in my duct system using the tools I mentioned above in order to give me a better idea of my real world scenario.
> 
> I can say as cost was rising I felt that I was getting in too deep. However, I am now happy. My dust collection is the best it's ever been thanks to Bills work.
> 
> DJG


I understand where you gettting your ino but I tested my system made changes one at a time and tested again. Using just 4" duct and ports first.with a 1.5 hp DC and a standard crappy 30 micron bag filter. 

I then changed to the Wynn canister filter leaving everything else the same. I recorded a 50% increase in velocity and the suction pulled 50 % harder.

I then changed out the duct to 6" all the way to each machine reducing down to 4" immediately at the machine or within a few feet. However I tested in the exact location as before which had a reducer at that point. Again I recorded another 50% increase in velocity and suction. That's a total of 125% increase from wher I started.

The problem with Bill's analysis if I remember correctly is it's based on agree systems I think 5hp which most home shops don't have. I also am a firm believer in results. My results are impossible according to his theory.

Architects and deign engineers design stuff all the time that I theory work but in reality it does not. 

By using 6" main you move more air in the main line at lower velocity. When you reduce down to 4" the air velocity has to increase to keep up with the air volume in the larger duct. This will not work if you use a larger duct then what the DC inlet is.


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## dgoodyear

rrbrown said:


> I understand where you gettting your ino but I tested my system made changes one at a time and tested again. Using just 4" duct and ports first.with a 1.5 hp DC and a standard crappy 30 micron bag filter.
> 
> I then changed to the Wynn canister filter leaving everything else the same. I recorded a 50% increase in velocity and the suction pulled 50 % harder.
> 
> I then changed out the duct to 6" all the way to each machine reducing down to 4" immediately at the machine or within a few feet. However I tested in the exact location as before which had a reducer at that point. Again I recorded another 50% increase in velocity and suction. That's a total of 125% increase from wher I started.
> 
> The problem with Bill's analysis if I remember correctly is it's based on agree systems I think 5hp which most home shops don't have. I also am a firm believer in results. My results are impossible according to his theory.
> 
> Architects and deign engineers design stuff all the time that I theory work but in reality it does not.
> 
> By using 6" main you move more air in the main line at lower velocity. When you reduce down to 4" the air velocity has to increase to keep up with the air volume in the larger duct. This will not work if you use a larger duct then what the DC inlet is.


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...


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## rrbrown

dgoodyear said:


> 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. :laughing:

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.:laughing:

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.


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## DPJeansonne

*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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## dgoodyear

rrbrown said:


> 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.:laughing:
> 
> 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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## rrbrown

dgoodyear said:


> 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. :thumbsup::laughing:


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## dgoodyear

rrbrown said:


> 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. :thumbsup::laughing:


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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## WillemJM

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) / (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


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## woodnthings

*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?

:blink: bill


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## dgoodyear

DPJeansonne said:


> 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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## dgoodyear

woodnthings said:


> 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?
> 
> :blink: bill


You've nailed it on the head.


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## dgoodyear

woodnthings said:


> 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?
> 
> :blink: 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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## dgoodyear

If anybody would like to see testing of my DC have a look on lumberjocks: http://lumberjocks.com/reviews/2869


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## rrbrown

dgoodyear said:


> 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. :laughing:

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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## ChipperOfWood

Fred Hargis said:


> 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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