Should I keep this DC or go Cyclone?

[Darrell Bade]

I am finally getting ready to pipe my DC and tools in the shop. I will have a Tablesaw, Jointer, Planer, Bandsaw, Router, Belt Sander.

I am going to run 6" PVC and drop to 4" right before the tools.

I currently have a Powermatic PM1900 dust collector. The specs on it are 3HP, Static Pressure 11.31" of water, velocity at 4" 5200 fpm, air flow at 4" 234 cu. ft/min, air flow at 8" 1754 cu. ft./min.

This DC should be big enough to get the job done. I have had it for a couple of years and have been just connecting to tools as needed. Now that it is getting down to plumbing time, I am wondering if I should consider selling it and buying a cyclone. My only reason for thinking this is because emptying the bags can be a pain in the butt.

I bought the thing for a good price and would not loose any money on it, but would need to put quite a bit with it to get a cyclone.

Is their any reason for me to consider a cyclone instead of this dust collector other than the ease of dumping a drum rather than emptying the bags?

[Jamie Buxton]

Well, you're either going to empty the bags in your existing DC, or you're going to empty the bin from under a cyclone. That's pretty much a wash, isn't it? Where the cyclone has an advantage is that less dust gets to the filter, so you don't have to clean it as often.

[Dan Friedrichs]

Consider spending some more time reading about the design of good dust collection ductwork, first. Your idea of running 6" duct and reducing to 4" is very, very bad - the 4" restriction will essentially make the entire system perform as though it was plumbed entirely with 4". It's kind of like hooking a fire hose up to a garden hose. If you run 6" ducts, you really need 6" ALL THE WAY to the tool to get the full benefit (and that may mean hacking a larger, 6", dust port into tools which don't have 6" dust ports already).

That DC is plenty powerful for a 1-man shop. If all you want is ease-of-emptying, consider a pre-separator (a la Phil Thien)

[Ole Anderson]

. Your idea of running 6" duct and reducing to 4" is very, very bad - the 4" restriction will essentially make the entire system perform as though it was plumbed entirely with 4".

Not true. Have you ever seen a real shop with a full professionally designed dust collection system? All sorts of duct sizes including 4" short end runs and drops for some of the smaller tools. The closer you are to the cyclone, or bag unit the bigger the pipe. As one example, if you go to the Oneida site, they start with a 7" main collector pipe, go to 6", then go to 5" and end up attaching to a machine with a 4" port. I don't agree with their maximum length on 7" and 6" pipe, but you get the idea.

http://www.oneida-air.com/files/Ductwork%20Design%20&%20Order%20Form%202hp%20&%20Under%2010-06.pdf

One reason to go to a cyclone/filter system is that you can generally get a filter that is able to get the fine dust below 1 micron, whereas the bags stop at 1 micron if you are lucky and have the 1 micron bags. The specs on the Powermatic 3 hp unit look good, presuming you dropped a decimal on the air flow at 4" of suction. Plenty of performance for a shop operating one tool at a time.

[Dan Friedrichs]

Not true. Have you ever seen a real shop with a full professionally designed dust collection system?

The design of ductwork for a commercial shop is entirely different than a 1-man shop, because there is a possibility of having more than one machine running at a time. Not true of a 1-man shop, so it's best to a have a single diameter of ductwork all the way from the blower to the tool.

The disadvantage of reducing duct size is that the airflow decreases. At the relatively low static pressures produced by this type of blower, air does not readily compress, so even a short section of smaller ductwork absolutely limits the airflow of the rest of the system.

I also wouldn't trust that file from Oneida you linked to. The first thing it says is that PVC is bad for ductwork because of dust explosion risks (a myth that has been debunked many times over).

[Joe Cowan]

I recently installed an Oneida cyclone and ran it to my bigger tools. I kept my bag type Penn State DC and use it on the 12" disc sander, and band saw. I also bought a floor sweep tool from Rockler and use the DC to vac the shop floor. With the DC specs asking to not turn on the cyclone over 6 times in one hour, I thought tools like the disc sander would be better off, using the smaller bag type DC.

[david brum]

Darrell,

Your 3 hp DC will likely perform as well a larger cyclone and better than many home size cyclones. A 3 hp DC doesn't have to pull air through a cylone body, so doesn't have nearly as much friction to overcome. If you check some of the WW mag articles on dust collection design, you'll see this confirmed. The downside with a DC is they also aren't nearly as good at separating fine dust, so the filters clog more easily.

I'm using a 3hp DC with Thien separators inside the separator body as well as cartridge filters. They generally work well, although I have clogged them after a day of using a drum sander. I view that as a tolerable tradeoff, since that is a grand or two that I don't have to spend (yet).

For the lower bags, you can get nifty quick release bands from Grizzly for pretty cheap. They are giant hose clamps with a knuckle that you lock down after you've taken up the slack by hand. Quick and easy to use.

BTW, you should at least run 6" to your tablesaw. There is just no way to get enough airflow through 4" ducting to do the job. Consider that 6" will allow more than twice the airflow as 4". The difference is very noticeable and worth the effort.

[Ole Anderson]

I am not concerned with the explosion hazard of static electricity, but the startling shock factor when working with rapidly spinning sharp tools, which is also appropriately listed by Oneida as a plus for metal ductwork. I don't wonder why the NFPA requires metal in commercial shops. I am not saying PVC is not an appropriate material for home shops, I just prefer steel, and that was after considerable deliberation. And it helped that I get a huge discount from my son who manages a HVAC supply shop. My shop has the TS closest to the DC, needing 600-900 cfm for top and bottom suction, therefore I am running 7" from the my 2 hp Oneida upstream past the TS to the next big split, where I start reducing the pipe size in accordance with the flow requirements of each tool, 7" to 6" to 5" to 4" hose for tools like my drill press and BS for which 350-400 cfm is adequate. No hard pipe smaller than 5". To occasionally flush out my 7" I just open several upstream blast gates. Oversize your pipes for small flow tools and you will build up material in the pipes due to inadequate air velocity. Unfortunately, sometimes you need to oversize your very long pipes to minimize pipe friction losses. Now if your big tools are at the end of the run, yes you would hold the diameter constant. Bottom line, there is no simple answer to correct pipe sizing, it depends on your shop layout, your DC capability, your tool requirements and your preferences and capabilities.

[Jeff Monson]

The disadvantage of reducing duct size is that the airflow decreases. At the relatively low static pressures produced by this type of blower, air does not readily compress, so even a short section of smaller ductwork absolutely limits the airflow of the rest of the system.




That is very good advice.

I'd modify your machines if possible to accept a 6" fitting. The increased airflow will also do a much better job of reducing the fine dust....and that IMO is the most important part of dust collection.

[Mark Beall]

Not true. Have you ever seen a real shop with a full professionally designed dust collection system? All sorts of duct sizes including 4" short end runs and drops for some of the smaller tools. The closer you are to the cyclone, or bag unit the bigger the pipe. As one example, if you go to the Oneida site, they start with a 7" main collector pipe, go to 6", then go to 5" and end up attaching to a machine with a 4" port. I don't agree with their maximum length on 7" and 6" pipe, but you get the idea.

http://www.oneida-air.com/files/Ductwork%20Design%20&%20Order%20Form%202hp%20&%20Under%2010-06.pdf



That is the proper design in the case that you are collecting from more than one tool at a time (and have the horsepower to do that). The reason for combining multiple smaller branches into a larger one is to keep the velocity in the main pipes within a reasonable range (fast enough to keep the dust suspended, not so fast that you create too much drag which limits the volume of air you can move). It is not the correct design for cases where you are only collecting from one tool at a time.

mark

[Carl Joseph]

The design of ductwork for a commercial shop is entirely different than a 1-man shop, because there is a possibility of having more than one machine running at a time. Not true of a 1-man shop, so it's best to a have a single diameter of ductwork all the way from the blower to the tool.

I'm not sure how that would make a difference when reducing the duct size at the machine.

If people want a more thorough explanation of cyclones, ducting, etc. then check out Bill Penz's site here ... http://billpentz.com/woodworking/cyclone/ducting.cfm

[John Coloccia]

All you have to do is run a 6" duct, stick your hand in it, and then put a 4" reducer on it and do the same thing. The difference is dramatic, I don't care what any dust collection experts design. This is shouldn't be in question. The questions are:

1) will the reduction in airflow be enough to prevent effective movement of dust through the ductwork, i.e. will the dust simply fall out and collect in the ducts....if so then you need to add more inlets to keep the airflow up.

2) will the airflow at whatever inlet or inlets you select adequately collect the dust you wish to collect. That depends on your tool design, not just the dust collector.

Generally, it's a good idea to take the largest size duct right to the tool, and then if you need to reduce it, reduce it there. That minimizes friction losses, and also gives you the option of upgrading later when you find that airflow to be inadequate.

[richard poitras]

Why not do both, keep your current unit and just add a cyclone body to it in conjunction with the current system.

[david brum]

Why not do both, keep your current unit and just add a cyclone body to it in conjunction with the current system.

Because a 3hp DC relies on it's inherent efficiency to perform well. A 3hp unit has a roughly a 12" impeller. This works great if you're just emptying into a filter or bag. If you add a cyclone body however, the added friction will reduce the airflow significantly. For instance, you might get 1800 cfm in the stock configuration. With a cyclone attached, you might get 1000 cfm (a guess based on a quick look at the 12 1/2" impeller on Grizzly's 1.5hp cyclone). Could be less.

Even then, you can't just use any cyclone body. Unless the body has correctly placed ramps, neutral vane, etc, it won't work any better than a Thien separator. I figure if you're gonna get a decent cyclone body, you would need a bigger impeller, bigger impeller housing, appropriate filters, etc. to make it work properly. All that can be shop built or purchased in the case of an impeller, but it takes some serious shop time and effort. I think the Thien separator/Wynn filter setup works great for minimal effort.

[Philip Rodriquez]

Jamie answered your question. When I went to a cyclone, it was because I was running my thickness sander a lot and I got tired of the cfm loss I experienced with my jet 1100 can unit. If I had your unit, I wouldn't have made the change.

[Jeffrey Makiel]

Your idea of running 6" duct and reducing to 4" is very, very bad - the 4" restriction will essentially make the entire system perform as though it was plumbed entirely with 4".

Actually, it behaves like a reduced 6" system, but certainly not like a system that is wholly made up of 4" duct. That would defy Bernoulli's law and 100+ years of fluid dynamics. But, then again, us woodworkers are capable of anything :) .

Jeff :)

[Van Huskey]

Also don't forget to rule out the best solution first... venting outside. For many it is not possible but if it is that is where you should look first.

[Ole Anderson]

Actually, it behaves like a reduced 6" system, but certainly not like a system that is wholly made up of 4" duct. That would defy Bernoulli's law and 100+ years of fluid dynamics. But, then again, us woodworkers are capable of anything :)

Jeffrey, That was the point I was trying to make. You said it with brevity and tact. Thank you.

[Dan Friedrichs]

Actually, it behaves like a reduced 6" system, but certainly not like a system that is wholly made up of 4" duct. That would defy Bernoulli's law and 100+ years of fluid dynamics. But, then again, us woodworkers are capable of anything :) .

Jeff :)

True :) But I suspect that the CFM present at the tool would be closer to a fully-4" system than a fully-6" system. The only advantage to hooking 6" duct to a 4" port would be the reduced losses from friction. For smooth-inside-wall PVC pipe being run short distances, I doubt those are large losses.

[Jeffrey Makiel]

True :) But I suspect that the CFM present at the tool would be closer to a fully-4" system than a fully-6" system. The only advantage to hooking 6" duct to a 4" port would be the reduced losses from friction. For smooth-inside-wall PVC pipe being run short distances, I doubt those are large losses.

A 6" duct is twice the area of 4" duct which means the velocity is double in a 4" duct as to maintain the same flow. Since pressure loss (w.g.= water gage of static pressure) is the square of velocity, the pressure loss is 4 times greater in a 4" dia duct given the same flow and length.

So...for a more realisitic application: A very short run of 6" duct with an elbow or two, and some flex duct, can easily produce 2" w.g. loss at the high velocities we typcially run. That equates to a loss of 8" w.g. when using a comparable length of 4" duct. This is very significant. However, if we use a 6" duct system with a 4" converging transition fitting at the end, this fitting would not result in 6" loss (8"-2"=6")...more like an inch or so. So, I would say that having a 4" fitting on a 6" duct run is much closer to a wholly 6" duct system than a wholly 4" duct system. And, it's not the such a horrible thing to do as a first reaction may dictate.

Duct material smoothness is irrelevant for the question in hand when similar materials are used in either design.

Jeff :)

[Ole Anderson]

Another analogy is that of a fire hose. A long 2.5" hose is needed to carry, say, 300 gallons per minute, but the 3/4" nozzle at the end still is able to move the same 300 gallons per minute. Yes, without the nozzle, the hose may move 400 gallons per minute, depending on pressure and length.

[John Coloccia]

I disagree that you maintain the same flow when you transition to 4". I don't have empirical evidence of this, but I know that having played with 6" and 4" inlets, 4" inlets are downright anemic compared to a full 6". The problem with applying these equations is they assume an ideal gas, i.e. the molecules are infinitely small for example. It's just not so because if it were then I could then simply make a pinhole and suck 1300CFM through that pinhole at incredible speed. What actually happens is the pinhole sucks an insignificant amount of air.

But like I said before, there's no reason to guess at this when it's so easy to simply test for yourselves like I did. You may come to a different conclusion.

[Dan Friedrichs]

Jeff, I don't know enough about fluid dynamics to argue this point with you convincingly, but I do disagree with you, because based on my extensive reading on dust collection, I know that what you're saying is contrary to what nearly everyone else has concluded. Bill Pentz's website explicitly says:
"Even reducing ducting size right at the machine for the shortest possible distance to a small 4" port will still kill system performance. The smaller ducting, flex hose, and small ports limit the maximum airflow..."

I think your mistake is, as John says, assuming ideal compressible gases. As such low pressures, you will not compress air.

I disagree with your static pressure numbers. Bill Pentz has a static pressure calculator on his website. Using it, I find that in order to get the 2" of water you spec'd, you'd need 47' of 6" ducting. 47' of 4" ducting will only produce (according to the calculator) 3.3" H20, not the 8" you specified. Clearly you are making some simplifying assumption about fluid flow that is incorrect in this application.

[Leo Graywacz]

The design of ductwork for a commercial shop is entirely different than a 1-man shop, because there is a possibility of having more than one machine running at a time. Not true of a 1-man shop, so it's best to a have a single diameter of ductwork all the way from the blower to the tool.



I am a one man shop and I use two tools at the same time all the time. I run the jointer and the tablesaw at the same time. I have a cyclone system that starts out at 7" reduces down to 6" for most of the main trunk and the drops are 5". I did this specifically so I can have more than one gate opened.

Typical machines that I run in parallel are:

Jointer and tablesaw
Jointer and planer
Jointer and edge sander
Cope and stick shapers

Most of my machines have 4" ports so I have a 5" to 4" reducer on the ends of my 5" drops.

[George Bregar]

I am a one man shop and I use two tools at the same time all the time. I run the jointer and the tablesaw at the same time. I have a cyclone system that starts out at 7" reduces down to 6" for most of the main trunk and the drops are 5". I did this specifically so I can have more than one gate opened.

Typical machines that I run in parallel are:

Jointer and tablesaw
Jointer and planer
Jointer and edge sander
Cope and stick shapers

Most of my machines have 4" ports so I have a 5" to 4" reducer on the ends of my 5" drops. You run those combos by yourself at the same time? :confused:

[Leo Graywacz]

If you mean am I at the machines at the same time, of course not. But both machines are running and both gates are open. I joint the board and turn and push it through the tablesaw, pick up another board, joint and turn and run it through the tablesaw.

[Homer Faucett]

Also don't forget to rule out the best solution first... venting outside. For many it is not possible but if it is that is where you should look first.

I'm quoting Van because I don't want his brief, dead-on post to get lost in this sea of static pressure debate. I totally agree with Van, and he has directly answered the original question.

The OP wants to know whether he needs to replace his bag unit with a much more expensive cyclone. As I see it, the only benefit to switching to a cyclone is that it is a prefiltration system that keeps your return air filters from clogging nearly as fast, and it allows for much finer filters to be used on the exhaust port. That does come with some loss of CFM, as I understand it. So, the options appear to be (correct me if you all disagree):

(1) Keep the old unit and live with the lower efficiency of dust capture by the filter bags and the cleaning involved. Drawbacks are obvious, but the system should have plenty of pull when clean.

(2) Direct vent the exhaust outside, eliminating the need for the filter bags. The drawback is that you are pulling air conditioned or heated air from your shop and blowing it outside, and you will have to allow for some makeup air either through planning, or just because your shop isn't very airtight. Also, if you aim the exhaust at your neighbor's kitchen window, you might not be very popular during the homeowner's association meetings.

(3) Use a neutral vein or Phil Thein separator before your bags to help reduce the fine dust making it to the bags. Optionally add canister filters with better filtration than your bags, thereby replacing the bags with a finer filter.

(4) Replace your system with a cyclone.

I'd weigh the options, but wouldn't personally replace with a cyclone until I was sure my old system would not work. It should be fairly easy to swap one for the other with some modification to the piping, assuming you do not have some unusual scenario. Good luck with whatever you decide.

[George Bregar]

(2) Direct vent the exhaust outside, eliminating the need for the filter bags. The drawback...if you aim the exhaust at your neighbor's kitchen window, you might not be very popular during the homeowner's association meetings.

(3) Use a neutral vein or Phil Thein separator before your bags to help reduce the fine dust making it to the bags. Optionally add canister filters with better filtration than your bags, thereby replacing the bags with a finer filter.
You hear this all the time but from everything I have seen if you combine a PT separator with venting outside there is very little dust exhausted. It's unnoticeable. I originally had planned to go with a cyclone but can't see a reason to now. There are reasons not to exhaust outside, but with a separator dust isn't one of them. Removing conditioned air, noise, not wanting or able to make a hole in the wall...

[Homer Faucett]

You hear this all the time but from everything I have seen if you combine a PT separator with venting outside there is very little dust exhausted. It's unnoticeable. I originally had planned to go with a cyclone but can't see a reason to now. There are reasons not to exhaust outside, but with a separator dust isn't one of them. Removing conditioned air, noise, not wanting or able to make a hole in the wall...

On the flip side, I don't know why you would want to combine a PT separator with outside venting. If it really is as efficient as you say, you'd be better off keeping your heated air inside and adding a cartridge/canister filter. It's kinda like recommending a cyclone and then venting the cyclone outside. Why do it? The whole point of the PT separator, bags, filters, cyclone is so that you can exhaust the air back inside without spraying the inside of your shop with sawdust.

The downside to the PT separator, bags, filters, cyclone is that they restrict air flow. As such, I see no good reason to restrict the flow and then vent the filtered air outside.

That's why I did not have that combination of a separator/external discharge as an option, although it sounds to me like you might have tried this (but I can't tell for sure). Aren't you freezing yourself out of your workshop this time of the year in Wisconsin with that combination? We were at -6 F down here this morning. I'd hate to even exhaust 40 degree air outside at those temps. That's why I went with the cyclone when I found a used Oneida unit around these parts.

[George Bregar]

On the flip side, I don't know why you would want to combine a PT separator with outside venting. If it really is as efficient as you say, you'd be better off keeping your heated air inside and adding a cartridge/canister filter. It's kinda like recommending a cyclone and then venting the cyclone outside. Why do it? The whole point of the PT separator, bags, filters, cyclone is so that you can exhaust the air back inside without spraying the inside of your shop with sawdust.

The downside to the PT separator, bags, filters, cyclone is that they restrict air flow. As such, I see no good reason to restrict the flow and then vent the filtered air outside.

That's why I did not have that combination of a separator/external discharge as an option, although it sounds to me like you might have tried this (but I can't tell for sure). Aren't you freezing yourself out of your workshop this time of the year in Wisconsin with that combination? We were at -6 F down here this morning. I'd hate to even exhaust 40 degree air outside at those temps. That's why I went with the cyclone when I found a used Oneida unit around these parts. The PT separator, like a cyclone, removes much of the fine dust, but not all. My point was to your comment of your "neighbors window"...there is an inconsequential amount of fines to worry about...this would be less true without a PT or cyclone and then I wouldn't exhaust outside if that was a worry. And while it is true that both restrict flow, you can upgrade a single stage DC with a PT and get better performance for much less money. A Grizz 2HP cyclone is a grand, a 3HP single stage is $400.

My woodshop is actually in Northern WI at my lake home. I plan on exhausting outside for most of the year but using a filter in the winter for the very reason (heat loss) that you bring up. The PT will reduce the fines getting to the filter, and eliminate the hassle of emptying bags.

[Dan Friedrichs]

I also use a PT separator and vent outside. Living in a very-close-packed suburban neighborhood, I can't just blow all the sawdust outside. So I catch nearly all of it with the PT separator, then blow just the very fine stuff outside. There is absolutely no dust visible outside.

[Homer Faucett]

The PT separator, like a cyclone, removes much of the fine dust, but not all. My point was to your comment of your "neighbors window"...there is an inconsequential amount of fines to worry about

Fair enough. My point was that if the remaining dust was truly inconsequential (of no consenquence), then there would be no need to exhaust outside. I live in the country, and could exhaust outside with no separation, but chose not to do so due to the heat loss issue. However, I'm sure there are those around in subdivisions where houses are close enough that even a separated exhaust would be an issue with a picky housewife doing her dishes while looking out the open kitchen window.


And while it is true that both restrict flow, you can upgrade a single stage DC with a PT and get better performance for much less money. A Grizz 2HP cyclone is a grand, a 3HP single stage is $400.Agreed. That's why I recommended to the OP that he not upgrade until he considers options 1-3. I guess now he also has options 2+3 combined to considered, given your comments.


My woodshop is actually in Northern WI at my lake home. I plan on exhausting outside for most of the year but using a filter in the winter for the very reason (heat loss) that you bring up. The PT will reduce the fines getting to the filter, and eliminate the hassle of emptying bags.Are you exhausting outside to improve air flow in your shop? If the PT design had been around when I was looking at DC for my shop in '05, I probably would have considered using the PT with good canister filters after it. I looked at either direct exhaust or building my own cyclone (seemed like a lot of work, but I didn't want to lose shop heat which is precious during the time of year when I have the most time to do woodworking). Then, I found a good deal on a used cyclone and jumped on it (same price as a Grizzly 1 stage).

Going the extra step of creating some diverter system to vent outside and inside to filters just seems like a lot of trouble to me, and I don't see the extra upside . . . other than perhaps reducing the number of times you have to clean the filters. Then again, I don't like to make things any more complicated than they absolutely have to be. Your solution does sound interesting. Good luck with planning and implementing it, and enjoy the time at the lake house!

[John Coloccia]

The reasons to vent outside are:

1) better performance because you're removing restriction
2) HEPA filters are expensive so you want them to last forever if you can manage it. They certainly don't last forever.

[Van Huskey]

I once had a thought about using a canister filter DC and attaching PVC to the top center with a true union ball valve then venting outside. With the twist of a valve one could convert, not the best solution for a convertible system but easy and cheap.

[George Bregar]

Fair enough. My point was that if the remaining dust was truly inconsequential (of no consenquence), then there would be no need to exhaust outside. I live in the country, and could exhaust outside with no separation, but chose not to do so due to the heat loss issue. However, I'm sure there are those around in subdivisions where houses are close enough that even a separated exhaust would be an issue with a picky housewife doing her dishes while looking out the open kitchen window. I could exhaust outside w/o separation my shop has woods behind it but will probably keep the PT because of ease of emptying...I'm doing a 55 gallon drum conversion...and to accommodate winter when I will be going thru a filter.


Are you exhausting outside to improve air flow in your shop? If the PT design had been around when I was looking at DC for my shop in '05, I probably would have considered using the PT with good canister filters after it. I looked at either direct exhaust or building my own cyclone (seemed like a lot of work, but I didn't want to lose shop heat which is precious during the time of year when I have the most time to do woodworking). Then, I found a good deal on a used cyclone and jumped on it (same price as a Grizzly 1 stage). No, to save wear and tear on and cleaning the the filter


Going the extra step of creating some diverter system to vent outside and inside to filters just seems like a lot of trouble to me, and I don't see the extra upside . . . other than perhaps reducing the number of times you have to clean the filters. Then again, I don't like to make things any more complicated than they absolutely have to be. Your solution does sound interesting. Good luck with planning and implementing it, and enjoy the time at the lake house! I was thinking of doing a diverter but will probably remove the "duct" on the DC and run my own ducting outside in the summer, reinstall in the winter. Can't take more than five minutes. Worth it not to have to clean filters for nine months. My BIL will fab up the "square to round" transition. He works with metal.

[Jeffrey Makiel]

I disagree that you maintain the same flow when you transition to 4".

That was never stated...at least by me. Please re-read my earlier response.

The statement that I was responding to was that "adding a 4" duct at the end of a 6" run makes the run behaves like it is all made up of 4". That is simply not the case whether calculated or properly measured. Rather, it makes the run behave less than an entire 6" duct run but more than an entire 4" duct run.

By the way, air is indeed an ideal gas (PV=nRT), but that's irrelevant in the discussion here. That's because there are no density changes at these low pressure differentials (i.e., systems typically operate with a differential pressure of less than 1 psi) and are therefore considered an incompressible gas at these low pressures (i.e., P & V remain constant). Also, the temperature (T) doesn't change significantly from start to finish. So, nothing changes in the ideal gas equation of PV=nRT and it can be ignored.

Jeff :)

[Steve knight]

bags on most dc's really block a lot of airflow. I had a jet 2hp dc and put the impeller and motor on a clearview and I saw a good increase in airflow. for the most part most dc's don't have enough bag area to really get the airflow they are rated for. my clearview got upgraded to to the 5hp setup and I went with huge bags. I have 5 30"x8' bags and they never fully inflate when in use so they don't restrict airflow. it makes a big difference.

[Jeffrey Makiel]

Jeff, I don't know enough about fluid dynamics to argue this point with you convincingly, but I do disagree with you, because based on my extensive reading on dust collection, I know that what you're saying is contrary to what nearly everyone else has concluded. Bill Pentz's website explicitly says:
"Even reducing ducting size right at the machine for the shortest possible distance to a small 4" port will still kill system performance. The smaller ducting, flex hose, and small ports limit the maximum airflow..."

I think your mistake is, as John says, assuming ideal compressible gases. As such low pressures, you will not compress air.

I disagree with your static pressure numbers. Bill Pentz has a static pressure calculator on his website. Using it, I find that in order to get the 2" of water you spec'd, you'd need 47' of 6" ducting. 47' of 4" ducting will only produce (according to the calculator) 3.3" H20, not the 8" you specified. Clearly you are making some simplifying assumption about fluid flow that is incorrect in this application.

Dan...we are talking about one 4" fitting right at the machine. I'm assuming this was done to accommodate a 4" port on the machine itself. The example you provided from the website talks about including additional 4" fittings along with 4" flex duct, which has a very high loss factor due to its roughness and bends. This is more than just one fitting as the OP discussed.

As for the ideal gas thing... I agree... Ideal gas laws are irrelevant here. I'm not sure how it entered into our discussion. Please see my post directly above this.

Regarding the Bill Pentz calculator, I suspect that your design flow in the 4" duct was less than the 6" duct. Please check to see if the flow thru the 4" duct is less than the 6" duct. This appears to be a neat calculator that includes fan response as well as duct design.

However, I did find one qualitative statement on the Excel calculator chart that helps assert my point: "Pipe loss factor based on 4000 FPM. This loss factor increases rapidly as the airspeed increases." Boy...is that true and possibly understated!

Perhaps my earlier post was overly worded, my apologies. But, if you try to push the same design air volume (in CFM) thru a 4" duct as a 6" duct to satisfy the needs of the machine, the velocity will double and the pressure loss will increase four fold. That's Mother Nature, not theory. In response, the dust collector blower will see the increased pressure and therefore reduce its output until a happy medium between pressure loss and flow capability are met. In the end, the flow thru the 4" duct is reduced. This all happens because of pressure loss, and is worsened because pressure increases as a square of velocity. That's why we get a big bang for the buck when going with a 6" duct vs. a 4" duct when operating a 2 or 3 hp dust collector.

To summarize, a 2" pressure loss thru a length a 6" duct at XX CFM will equate to an 8" loss thru a 4" duct of similar length at the same XX CFM. Therefore, flow must be reduced (high flow at 8" vs. less flow at 3.3") unless you get a much, much bigger dust collector blower to supply 8" at the desired high flow. Sorry, no other way around it! :)

Jeff :)