Dust collection expert question on port size, cfm, and WG pressure

[Joe Jensen]

Hi all,
I would like to find a table or curves that would show CFM at different WG pressures for 4", 5", and 6" ports. My belief is that collector manufacturers have been balancing their designs to a high CFM and relatively low WG pressure (also static pressure). They do this because customers look at and compare the CFM. If y memory is correct, every time you quadruple the WG pressure at a port, the CFM doubles. Is this correct? I'm sure there are other aerodynamics effects as well, which would impact smaller ports more than large ports.
Why would this be important? I have a 2HP Oneida cyclone. It delivers somewhere around 10" of static pressure and a max CFM of around 1500 or so. I have 6" ports on the machines where I can fit them, 5" on machines where that's the best I can do, and some machines where I can only do 4" ports. If I model the cyclone loss, filter loss, pipe, and port losses, I probably have 2-4" of WG pressure at the ports. If I could quadruple that, I'd be able to pull 2X the CFMs. This would make a 4" port pull as many CFM as a 6" port. Say I have 4" today. To quadruple, I'd need 16". That means my collector would need 12" more static pressure, or about double.
I'd love to find a new fan that when fitted with a 5HP motor, would give me the same CFM and double the static pressure. I think this would let me keep the same cyclone.
To the experts, am I on the right track?

[David Giles]

A confusing topic, but I'll give it a go.

For a give duct work setup, the flow and pressure drop are directly related. Let's assume that you had a big box where the blower is usually located. And we magically keep that box at a constant negative pressure, say minus 6 "WG. So no matter what we do in the ductwork, we always have 6" suction pressure in the box.

Now the flow and pressure drop formula is: 2x the flow = 4x the pressure drop, 3x the flow = 9x the pressure, half the flow = 1/4 the pressure drop. Adding ductwork length or elbows increases pressure drop, so the flow goes down.

A 4" duct has 0.45x the area of a 6" duct. So the flow is 45% through a 4" duct using our magical box.

Now replace the magic box with a motor/blower. At high flow rates, the suction pressure is low. At low flow rates, the suction pressure is high. So we try to calculate the pressure drop through the ductwork and determine the flow from the blower curve. Or we just install 6" ductwork, knowing the flow rate is higher than with 4" ductwork.

In WW shops, it's usually the ductwork that limits the air flow. Bigger diameter duct or removing restrictions (4" machine ports) lowers the pressure drop and increases the air flow. But sometimes the blower becomes the limiting factor. A 1Hp blower on 6" duct work will only pull what the blower will pull. Bigger duct doesn't help because the blower is limited. So big blower emulate our magic box, but small blowers do not.

Blowers also have a top end suction pressure. 5-8" WG is about the limit. Higher pressure blowers are larger diameter wheels which create higher tip speeds. These blowers are high noise and high Hp, often requiring 3Ph power.

[Joe Jensen]

A confusing topic, but I'll give it a go.

For a give duct work setup, the flow and pressure drop are directly related. Let's assume that you had a big box where the blower is usually located. And we magically keep that box at a constant negative pressure, say minus 6 "WG. So no matter what we do in the ductwork, we always have 6" suction pressure in the box.

Now the flow and pressure drop formula is: 2x the flow = 4x the pressure drop, 3x the flow = 9x the pressure, half the flow = 1/4 the pressure drop. Adding ductwork length or elbows increases pressure drop, so the flow goes down.

A 4" duct has 0.45x the area of a 6" duct. So the flow is 45% through a 4" duct using our magical box.

Now replace the magic box with a motor/blower. At high flow rates, the suction pressure is low. At low flow rates, the suction pressure is high. So we try to calculate the pressure drop through the ductwork and determine the flow from the blower curve. Or we just install 6" ductwork, knowing the flow rate is higher than with 4" ductwork.

In WW shops, it's usually the ductwork that limits the air flow. Bigger diameter duct or removing restrictions (4" machine ports) lowers the pressure drop and increases the air flow. But sometimes the blower becomes the limiting factor. A 1Hp blower on 6" duct work will only pull what the blower will pull. Bigger duct doesn't help because the blower is limited. So big blower emulate our magic box, but small blowers do not.

Blowers also have a top end suction pressure. 5-8" WG is about the limit. Higher pressure blowers are larger diameter wheels which create higher tip speeds. These blowers are high noise and high Hp, often requiring 3Ph power.

Thanks David, I'll try to boil my issue down to one example. I have a SawStop with a nice blade shroud inside. It has a 2.5 foot long 4" Diameter flex hose that is tightly twisted. The dust collection in the cabinet is underwhelming. I would like more CFM through that setup. I think the only way to get that is to significantly increase the inches of pressure at the saw. I already have 6" pipe with a limited number of large radius turns. I think my duct static pressure loss is low, and if I went larger, I think I'd have too little linear feet per second of flow and I'd have dust pooling problems.

Felder sells a dust collector for the new Euro health standards that has 25" of static pressure and like 1800 CFM. It's a 5HP machine. I am wondering if it's possible to get a fan for my cyclone that would deliver 20" or more static pressure at the same CFM so I can use the same cyclone. Or, does a machine like the Felder have a 2 stage fan?

[Rick Fisher]

I have that Felder Dust collector. I read for hours before the purchase.

I read Bill Pentz site, cover to cover. I read tonnes of sites.

I use 6" pipe now. The exception is my shaper. It still has 4" flex hose.

So, I read all the theroy on 4" pipe only allowing so much CFM etc. etc.

The Reality is that I replaced a 2hp DC with this Felder DC and with a 4" flex hose, I get every particle of dust. Not only do I get it, It forms a wild looking curved path from the cutter to the hose.

Its like watching a Sci Fi movie. MDF is most noticable.


After hours of reading, I believe that 6" is better than 4". I believe that elbows are not good and flex hose is bad.

I also believe that a huge motor and a huge impellor reverse many sins. My 4" flex hose changes shape when I open the gate. It goes.. oval.


This is a pic of the impeller. The Model I have has an impellor between 17" and 18".




Static pressure may be a part of the success I have had.

I just know that if I hook up a 3" hose to the system, the air makes a screaming sound, like a poorly tuned musical instrument. It will remove 90% of the chips from my Planer with 2 other 6" gates open.

I have to hold the start button for 4 seconds to get the impeller up to speed, which is irritating. So I know the impeller is heavy and large.

I think a 5hp motor with a 18" impeller will make a lot of DC frustration go away..

[David Giles]

I would agree with you and Rick. Higher air flow can be achieved with a bigger dust collector that creates a deeper suction level. The tradeoff is blower horsepower and the price of a new machine.

The alternative is increase the 4" Sawstop port to a 6" port or to add a second 4" port to the cabinet base. I'd suggest this option first. The Sawstop port just connects to the blade shroud. Any dust that escapes the blade shroud settles to the cabinet floor with nowhere to go. This will be the case with any DC connected to the Sawstop, although a bigger machine will collect more of the dust. Provided that you turn it on for every cut (which I don't). The machine needs some way to sweep the dust from the cabinet floor.

Didn't I send you the thread about increasing the Sawstop port size?

Anyway, you appear to be on a right path to achieve your goals. Good luck!

[Joe Jensen]

I would agree with you and Rick. Higher air flow can be achieved with a bigger dust collector that creates a deeper suction level. The tradeoff is blower horsepower and the price of a new machine.

The alternative is increase the 4" Sawstop port to a 6" port or to add a second 4" port to the cabinet base. I'd suggest this option first. The Sawstop port just connects to the blade shroud. Any dust that escapes the blade shroud settles to the cabinet floor with nowhere to go. This will be the case with any DC connected to the Sawstop, although a bigger machine will collect more of the dust. Provided that you turn it on for every cut (which I don't). The machine needs some way to sweep the dust from the cabinet floor.

Didn't I send you the thread about increasing the Sawstop port size?

Anyway, you appear to be on a right path to achieve your goals. Good luck!

I have automatic gates that fire up the collector and open when I start a machine. There isn't enough dust in the cabinet to be worth cutting it up. I have several other machines that cannot accept a larger port. I wonder if the curved fins on the Felder plus more HP are the difference. I don't really have room for a larger cyclone, so a different fan would need to have similar CFM to my existing system or the cyclone wouldn't work.

[David Giles]

Since adding or enlarging ports isn't an option, a blower with more suction is the other option to create more air flow. And so we arrive at the same conclusion, it just took a bit!

Without knowing anything about the Felder, I would guess that the blower blades are "backward inclined" blades. This is a common blade configuration on higher end blowers. It helps prevent the blower from surging at lower flow rates.

Back to your original question about flow vs pressure drop. One of the easier ways to calculate pressure drop is called "velocity head". Essentially, it is the amount of pressure needed to move air from zero velocity to flowing velocity.
For air at standard conditions, one velocity head is
1" WG = 66fps, 3"WG = 115 fps, 5"WG = 148 fps, 9"WG = 198 fps
You can see the pressure ratio = (flow)squared.

Total flow = velocity x pipe area (theoretically).

To determine actual flow, we have to determine entrance and exit losses, pipe friction, bends, etc. When air exits a duct, it loses one velocity head. A sharp edged entrance loss is about 0.5 velocity head. Blah, blah, blah...

But you don't really care about actual flow, you just want more flow. So knowing the new suction pressure vs the current suction pressure should give you a decent estimate of the increase.

Sounds like a cool DC setup. Please post some pictures and results when you finish.

Oh, and if you end up with a spare motor / blower or cyclone, could I have first dibs? Your 2Hp setup would be a nice upgrade to my 1.5Hp Jet.

[Dave MacArthur]

This thread was very interesting, 2 years old but somewhat unique in the velocity head discussion. I'd love to hear more on this real-world pressure loss from David Giles.

Also interested in the larger impellers. I just acquired a 5HP Oneida Pro2000, and was very surprised to find that the impeller is only 15" across, would have thought a 5HP motor would be used to drive a larger impeller.

[Joe Jensen]

It's been 2 years and I still haven't worked on the dust collector. I have switched to a Felder saw/shaper combo and the dust collection is better on that machine than on the Sawstop and on my old PM shaper. I have a PM 24" dual drum sander and the dust collection on that machine is now the weakest in the shop.

Dave, from all the reading I've done, it seems that most manufacturers design the systems to maximize the CFM spec at zero static pressure. This is the spec most users look at when buying, but by itself it's not so useful. IF you had 6" ducts and 6" ports on all machines, and limited turns and limited flex hose, going from 2HP to 5HP with most manufacturers would do very little for you. This is because the 5HP machine only creates a little more suction, but not much more pressure. 5HP designed that was is more about having more than one port open, say a 2 man shop instead of a 1 man shop. The Europeans have implemented tough new "real world" tests of dust collectors that required a dirty filter, more pipe, and they measure the air quality coming out of the machine. From what I can tell, Felder, Al-Ko, and others there are using the 5HP to create more suction. Onieda has higher flow, lower suction, and Felder has lower flow and higher suction. This is ok, because the Felder already has enough flow in a 6" pipe to pull 5000 feet per minute which is the high end of what is recommended for WW. With the Onieda you would have to have like an 8" duct to get that same flow. That's because the Oneida doesn't have as much suction. The Oneida will pull more cfm with say a 10" duct and no ports or turns, but that's not real world.

[FRITZ STOOP]

I moved into an 1800 sf shop and faced the prospect of facing it with a 2hp JDS cyclone left over from my previous shop. When I dissembled the old system, I discovered (at least) two things. The ducting in the old shop was too (no pun intended) much for the 2hp JDS and snap together ducting is three times as much work to take apart and it is to install. After reading (and actually getting to know) Bill Pentz on DC, I made the decision to get Nordfab clamp together ducting and get a big blower (which I made a shed for outside the building).
I haunted the auction sites and in addition to getting some solid SMC machines for $0.05 on the dollar, I also landed a 7.5hp REES blower. Whoa! My days of clogged pipes and dust in my nose were over.
I already had a Minimax CU 300 Smart w/8' slider and 3 or 4 120mm (5") dust outlets and that alone was too much for the JDS (which Bill scoffed at as the WORST machine as far as particle separation there is).
So, for $420 + $350 shipping, I got a $6K sucking machine.
The key is the hp and 3 is minimum, 5 is preferable. I'm fortunate to have light industry 3-phase 240A service on site and that opens up all sorts of possibilities, but the auctions have a fair share of single phase machines too. And the Quick-Fit ducting is insanely simple and adaptable if you need to make change.
Main point, the more power the less engineering and design matter. I actually open unused gates or a floor sweep to INCREASE the suction.
Heavenly.

[Joe Jensen]

I moved into an 1800 sf shop and faced the prospect of facing it with a 2hp JDS cyclone left over from my previous shop. When I dissembled the old system, I discovered (at least) two things. The ducting in the old shop was too (no pun intended) much for the 2hp JDS and snap together ducting is three times as much work to take apart and it is to install. After reading (and actually getting to know) Bill Pentz on DC, I made the decision to get Nordfab clamp together ducting and get a big blower (which I made a shed for outside the building).
I haunted the auction sites and in addition to getting some solid SMC machines for $0.05 on the dollar, I also landed a 7.5hp REES blower. Whoa! My days of clogged pipes and dust in my nose were over.
I already had a Minimax CU 300 Smart w/8' slider and 3 or 4 120mm (5") dust outlets and that alone was too much for the JDS (which Bill scoffed at as the WORST machine as far as particle separation there is).
So, for $420 + $350 shipping, I got a $6K sucking machine.
The key is the hp and 3 is minimum, 5 is preferable. I'm fortunate to have light industry 3-phase 240A service on site and that opens up all sorts of possibilities, but the auctions have a fair share of single phase machines too. And the Quick-Fit ducting is insanely simple and adaptable if you need to make change.
Main point, the more power the less engineering and design matter. I actually open unused gates or a floor sweep to INCREASE the suction.
Heavenly.

I'm glad you ended up with a great solution but I disagree that more HP will automatically solve the problem. You have to have more HP, but it has to be driving a blower designed to generate more pressure, or you won't move a bit more cfm through that 5" port. It sounds like the blower you ended up with is one that generates more pressure than others. Sadly most systems like Griz, and Oneida are designed to increase CFM with the HP and not to generate pressure with the HP.

[Jon van der Linden]

Most of these blowers aren't really "designed." They're just stock designs adjusted for size. If you're looking at commercial DC systems then you'll see companies like Belfab that have different fan designs for the same size system depending on motor speed. Their "Hummer" motor/blower combo is super quiet (I know, I used to have one.) Unfortunately they haven't come up with a fan design that works with the low speed for motors under 10hp. I think that gives an indication of how difficult fan design really is.

The reality is that if you're under 5hp you're best off with a short single connection. Most people cheapen out on dust collection because they don't understand it. It's about health, safety, and tool life. It might not matter much in a garage shop where not that much gets done, but in a commercial shop good dust collection will save money on expensive things like cutter life. Additionally proper dust collection will save time on the amount of work after wood is dimensioned - recirculating chips will dent the wood as it goes through the machine.

As to the question of single vs cyclone, it really depends on what your material is. If it's a wide belt, you have totally different requirements than for a jointer. Dust vs chips, it's not the same. That said, a proper sized single stage will work just fine. (Sizing means flow requirements for each machine, material being evacuated, and distance.) The only place you might find a real difference is if you use a hepa filter. Even the high end European DCs don't filter better than industrial felt, which is what most commercial systems use.