Should I keep this DC or go Cyclone?

[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?

[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.