Fan size/Hp to run 10" duct?

[Larry Edgerton]

I found a screaming deal on some 10 inch stainless pipe and all the fittings that I would need to build a under floor dust collection system in my new shop, but......

I read a bunch this morning in Bill Pentz's site, but the Cincinnati Fan site I could not open because of my junk computer, and I could not find the fan size/hp needed to keep airflow up to speed with 10" pipe. Anyone know how to figure that?

I don't want to have to run too big of a motor just for economic reasons, but don't want to pass up a deal if I can make it work either.

Thanks, Larry

[Greg Peterson]

10" stainless. Wow and wow. Be curious to see how much fan this stuff needs. Going to guess it will be a very large motor. Does the Pentz site mention branch circuit size in relation to main run size? Wondering what size your branch circuits should be since the main run is 10".

[Scott T Smith]

Larry, just thinking out loud you will most likely be in the 5 - 7 hp range.

Keep in mind that with a system this large you are moving beyond a "homeowner" sized system and up into the ranks of a commercial one. The typical hobby woodworker sized 3 hp system will not pull more than 1500 cfm through the ducts.

Once you start exceeding 1500 cfm through your ducting, as I understand it the potential for a dust explosion is increased, and system grounding becomes an important issue.

[Dan Hintz]

Once you start exceeding 1500 cfm through your ducting, as I understand it the potential for a dust explosion is increased, and system grounding becomes an important issue.
CFM is not the measure, it's dust-to-air ratio. I could pull a tornado through my system, but if there's little dust there's no explosion potential.

[David Kumm]

The fan will have to be 16-18 inches and the motor 5-7.5 hp. The cyclone will have to be sized accordingly. Less flow will work on the drops but the shavings will go too slow in the big pipe. You may find the cost of the other stuff too high in spite of your pipe savings. Dave

[Anthony Whitesell]

I thought Bill's spreadsheet would tell us. But it only correlates CFM-FPM and Duct Size. But for all those that are curious, 10" with the "nominal" main speed of 4000FPM results in 2200CFM.

[Ole Anderson]

Which means you would need at least a couple of 6" feeds open to provide enough cfm to keep the chips moving in a 10" duct. Now if you could get the same deal on some 7" ss duct... The money you save on duct will be more than offset on blower and motor, not to mention the electric bill for running a 5-7 hp motor.

You said under floor duct, in a crawlspace or under a concrete slab? That will affect your choice of material. Under a slab, I would go with PVC.

[Larry Edgerton]

The deal was free. A friend of mine bought it for his log mill but it was too small. It has 2.7 times the volume per foot, so I assume it needs 2.7 times the cfm to achieve the same speed. This is for a commercial shop, small but commercial, so it may be running more than one machine at a time and the machines are fairly large.

Someone mentioned something about not wanting to use stainless underground. Care to elaborate, I'm curious why. I was actually planning on running tandem 6" PVC lines under the floor converging at the wide belt, but the free stainless has its draw.

[Joe Angrisani]

.....Someone mentioned something about not wanting to use stainless underground. Care to elaborate, I'm curious why.....

Corrosion. Rust. Breakdown. Return to Mother Earth.

[Larry Edgerton]

Corrosion. Rust. Breakdown. Return to Mother Earth.

I wouldn't think stainless would break down? It would be welded with stainless rod before it was dropped in the hole. I was wondering about condensation though......

[Scott T Smith]

CFM is not the measure, it's dust-to-air ratio. I could pull a tornado through my system, but if there's little dust there's no explosion potential.

Dan, I should have been more clear on my response. As was explained to me, the CFM is required in order to create enough joules of energy to cause the combustion to occur. If you don't have enough CFM through the duct, you don't have one of the necessary ingrediants for an explosion and the dust to air ratio becomes a non-issue.

As you mentioned, you also need the right ratio (and size) of dust, and I believe a certain RH% range.

[Joe Angrisani]

I wouldn't think stainless would break down? It would be welded with stainless rod before it was dropped in the hole. I was wondering about condensation though......

Stainless Steel isn't Perfect Steel. It's just Less-Prone-To-Rusting Steel. And there's all kinds of stainless. Buried in dirt under a slab I'd guess it shows rust in a year or two, and is having serious breakdown problems in ten.

PVC. That's the way to go if you're burying.

[Dan Hintz]

Dan, I should have been more clear on my response. As was explained to me, the CFM is required in order to create enough joules of energy to cause the combustion to occur. If you don't have enough CFM through the duct, you don't have one of the necessary ingrediants for an explosion and the dust to air ratio becomes a non-issue.

As you mentioned, you also need the right ratio (and size) of dust, and I believe a certain RH% range.
That is also incorrect... stagnant air is just as susceptible to explosion with the same ratio of dust to air. It's not the motion of the air at all that causes the problem, it's the ability of a DC to "atomize" the dust particles into a suspension, allowing for that perfect mix in a large volume. The moving air itself imparts zero energy to the system/explosion, so whomever told you high CFM creates "joules of energy" is talking out of a lower body part.

Old elementary school science fair trick (no longer allowed in the classroom due to a very litigious society): place a tube filled with a dusting of flour inside a large upside-down can (even coffee cans are made from plastic these days, so you'll need to find something other than Folgers), aimed at an open candle flame. Blow into the tube. BOOM! At least when you get the amount of flour correct. I guarantee the CFM coming from your lungs isn't 1/100th that coming from a DC. It's the ratio, plain and simple.

Size of the particle is a factor, as you mentioned. RH% not nearly as much, but it may play a very small role... high humidity environments won't allow for very small particles size. Anything dryer than that, though, and it's go time... no such thing as too dry, so not a range, per se.

The amount of dust necessary for the typical home DC setup is pretty immense. I cannot place the article offhand, but they ran some tests to see how much wood it was necessary to remove via sander to reach that perfect ratio with the lower CFMs typically seen in home shops... it was something insane, like 3/8" removal per minute over a 2'x2' board with 1,200 CFM. I don't know any home shops that can sand that fast, and if they can, they're likely using a much beefier DC than a 2hp unit... which then means more CFM, and the required amount of dust increases, as well. It's just not going to happen.

[Greg Peterson]

Every now and then you read of grain mills or sugar refineries exploding. Cause is the proper ratio of fine (dust sized - whatever that is) particles suspended in the air. One spark and Dan's elementary school experiment goes real world in a big, dangerous way.

Anything will burn. Getting the conditions correct for the burn is another matter. But something as easily ignitable as wood dust requires some pretty extreme conditions that I've never come close to approaching in my shop.

[Scott T Smith]

That is also incorrect... stagnant air is just as susceptible to explosion with the same ratio of dust to air. It's not the motion of the air at all that causes the problem, it's the ability of a DC to "atomize" the dust particles into a suspension, allowing for that perfect mix in a large volume. The moving air itself imparts zero energy to the system/explosion, so whomever told you high CFM creates "joules of energy" is talking out of a lower body part.

Old elementary school science fair trick (no longer allowed in the classroom due to a very litigious society): place a tube filled with a dusting of flour inside a large upside-down can (even coffee cans are made from plastic these days, so you'll need to find something other than Folgers), aimed at an open candle flame. Blow into the tube. BOOM! At least when you get the amount of flour correct. I guarantee the CFM coming from your lungs isn't 1/100th that coming from a DC. It's the ratio, plain and simple.

Size of the particle is a factor, as you mentioned. RH% not nearly as much, but it may play a very small role... high humidity environments won't allow for very small particles size. Anything dryer than that, though, and it's go time... no such thing as too dry, so not a range, per se.

The amount of dust necessary for the typical home DC setup is pretty immense. I cannot place the article offhand, but they ran some tests to see how much wood it was necessary to remove via sander to reach that perfect ratio with the lower CFMs typically seen in home shops... it was something insane, like 3/8" removal per minute over a 2'x2' board with 1,200 CFM. I don't know any home shops that can sand that fast, and if they can, they're likely using a much beefier DC than a 2hp unit... which then means more CFM, and the required amount of dust increases, as well. It's just not going to happen.


Dan, I think that we're trying to say two different things. I fully concur with your comments about the ratio of particles required in order to combust. If that ratio is not present, then there is no danger.

However, where I'm coming from is the source of the "spark" required to initiate a static electricity caused combustion of your particle ratio. In your classroom example, a flame provided the spark.

Please correct me if I'm wrong, but if environmental conditions exist that will result in a buildup of static electricity on dust collection ducting, then all things being equal, if the airflow through the ducting is increased, the amout of static electricity (ie joules of energy) is also increased.

I used to have a link to a theoretical paper by an engineer on this topic, but it was lost in last years computer crash.

[Dan Hintz]

Please correct me if I'm wrong, but if environmental conditions exist that will result in a buildup of static electricity on dust collection ducting, then all things being equal, if the airflow through the ducting is increased, the amout of static electricity (ie joules of energy) is also increased.
Sure, I was speaking in terms of increased airflow alone not making a difference. Once you qualify it with interaction with the ducting wall creating static, then speed makes a difference and will create more energy along the walls. But again, more airflow (and therefore energy) does not have an effect on the chances for an explosion.

I could hook up a spark generator to the end of the pipe, but if the air-to-dust ratio is not correct, no explosion. So, run the air as fast as you want to , but until that ratio is correct, nothing. 10kV spark or a 100kV spark, it's irrelevant... it all comes back to the ratio.

[Larry Edgerton]

Every now and then you read of grain mills or sugar refineries exploding. Cause is the proper ratio of fine (dust sized - whatever that is) particles suspended in the air. One spark and Dan's elementary school experiment goes real world in a big, dangerous way.

Anything will burn. Getting the conditions correct for the burn is another matter. But something as easily ignitable as wood dust requires some pretty extreme conditions that I've never come close to approaching in my shop.

I read a study done by the insurance industry a while ago, and one thing that I found interesting is that it is never the first explosion in a wood shop that blows the walls out. It is on average the third explosion. These explosions happen so rapidly that you can not tell that they are not just one explosion. First say a dust collector situation causes an explosion, whos shock wave raises more dust off of other surfaces in the shop and that explodes using the first explosion as the igniter, then this one raises more dust off of such things as rafters and so forth, and this explodes using the second explosion as an igniter, and so forth until enough force is developed to blow out the walls.

It was an interesting read, and it made me attack my shop a couple of times a year to clean all dust catchers. I had high post and beam trusses in the last shop, and after reading the article I checked, and there was an inch of very fine dust up there.

[Don McManus]

Stainless Steel isn't Perfect Steel. It's just Less-Prone-To-Rusting Steel. And there's all kinds of stainless. .

+1 on that. I used to work with stainless transducers. The 303 and 304 types would rust after only a few hours in our humidity chamber, especially near welds or labels(laser marked). The 316 was more corrosion resistant, IIRC.

[Scott T Smith]

Sure, I was speaking in terms of increased airflow alone not making a difference. Once you qualify it with interaction with the ducting wall creating static, then speed makes a difference and will create more energy along the walls. But again, more airflow (and therefore energy) does not have an effect on the chances for an explosion.

I could hook up a spark generator to the end of the pipe, but if the air-to-dust ratio is not correct, no explosion. So, run the air as fast as you want to , but until that ratio is correct, nothing. 10kV spark or a 100kV spark, it's irrelevant... it all comes back to the ratio.

True... but if you have the correct ratio but no spark - then no explosion.

It's really a "which came first - the chicken or the egg" type of scenario. At the end of the day, you need both a combustible mixture as well as an ignition source in order to have an explosion.

Where I'm coming from is the perspective of what is within my control, versus what isn't. When I researched the issues associated with designing my shop dust collection system a couple of years ago, amongst my production equipment are both a wide belt sander as well as a horizontal resaw bandsaw (along with jointer/planers that produce larger shavings). Because of the fine dust produced by the WBS and HR, I knew that I would have the potential to have a combustible mixture of fine sawdust flowing through my ducts.

If I wanted to use the WBS with very fine sandpaper, there wasn't much that I could do to prevent fine dust in the ducts. Therefore, I looked at what was within my control, and that turned out to be minimizing the potential of an explosion by preventing the creation of an ignition source. So, I opted to install two different dust collection systems - a high flow and a low flow system. The dust collection system that services the WBS and Resaw is a 3 hp system that pulls less than 1500 cfm (plus it has grounded metal ducting). This same system also services my personal woodshop equipment such as the table saw, etc.

The jointer/planer, moulder, straight line rip saw (which produce larger shavings or course dust) are all connected to a 25 hp system (which pulls a heck of alot more than 1500 cfm!).

As usual, I should have taken the time to be more detailed on my original response. Since Larry is considering a 10 hp system, to me the static energy potential becomes a concern that should be evaluated. If he's not generating fine dust, then no problem. If he is though, then it would be wise for him to research further.

Somehow, I can't shake the feeling that I've just lived up to Abe Lincoln's great quote... “Better to remain silent and be thought a fool than to speak out and remove all doubt.” <grin>

[Larry Edgerton]

Somehow, I can't shake the feeling that I've just lived up to Abe Lincoln's great quote... “Better to remain silent and be thought a fool than to speak out and remove all doubt.” <grin>

Nah, I'm listening, and it was my question after all.;) Different views are good, it leads to more questions, which will lead to more answers and a better understanding.

[Larry Edgerton]

I think I am going to go back to my original plan of a split system with two 1500cfm fans and two 6"PVC lines buried converging at the WBS. That will give me 3000 cfm at the WBS, and 1500 everywhere else, and keep my speed up. I decided to scrap the 10". Two sixes will give me close to as much and I will not have to worry about corrosion of condensation. Free stuff has its draw, but in the words of Forrest Gump, "Stupid is as stupid does.", and I have no interest in shooting myself in the foot on something I have only one chance to do right.

Thanks for all of the comments, you have helped me make a decision.

Larry

[Dan Hintz]

If I wanted to use the WBS with very fine sandpaper, there wasn't much that I could do to prevent fine dust in the ducts. Therefore, I looked at what was within my control, and that turned out to be minimizing the potential of an explosion by preventing the creation of an ignition source. So, I opted to install two different dust collection systems - a high flow and a low flow system. The dust collection system that services the WBS and Resaw is a 3 hp system that pulls less than 1500 cfm (plus it has grounded metal ducting). This same system also services my personal woodshop equipment such as the table saw, etc.

The jointer/planer, moulder, straight line rip saw (which produce larger shavings or course dust) are all connected to a 25 hp system (which pulls a heck of alot more than 1500 cfm!).

As usual, I should have taken the time to be more detailed on my original response. Since Larry is considering a 10 hp system, to me the static energy potential becomes a concern that should be evaluated. If he's not generating fine dust, then no problem. If he is though, then it would be wise for him to research further.
Scott,

I think you've missed my final point... a spark is a spark. It doesn't matter if it's a 10kV spark created by a 1000CFM airflow or a 100kV spark created at 5000CFM airflow, it's a source of ignition. Knowing that the spark is going to happen regardless means you don't try to prevent it by decreasing airflow. Just the opposite, you know it's going to happen and increase the airflow, reducing the chances for the air-to-dust ratio from hitting the magic mark.

Since the standard shop is well below the amount of dust needed to go boom, the best idea is to increase airflow even more as dust production increases. You seem to be going to opposite way, which is more dust means you slow things down. If anything, you've increased your chances of going boom as you're pushing closer to the magic ratio, but you haven't removed the cause of ignition, the spark.




And if you follow Abe's advice, you'll never have an opportunity to view things from a different viewpoint. Always question what you know, and be willing to stand up for what you believe... you'll either come out the victor and (hopefully) change someone else's mind for the better, or you'll learn something new and change your own mind.

[Larry Edgerton]

On the explosion thing, I have seen what can happen.

On big houses I will set up a complete shop on site with shaper, saws, drum sander and so forth in the garage. At one such job I had the dust collector outside a garage door and one of my guys was cleaning up the shop area with the DC hose. A Bic lighter fell out of his shirt pocket and got sucked up into the DC, fit the fan and blew the whole thing up. It blew the garage door all to heck, made a flame ball that could be seen from the roof where we were working, 45' above where the DC was located at the basement level. The garage door was bent, the dust bags were junk, but I was lucky, no one was hurt and it just cost me a new garage door. I take the possibility of an explosion seriously.

None of us were anywhere close to the explosion except for the kid on the other side of the garage door, but it could have been bad. I made a hardware cloth screen for the cleanup foot after that. And no lighters! This is what got me researching why dust explosions happen.

[Joe Angrisani]

On the explosion thing, I have seen what can happen.....I had the dust collector outside a garage door and one of my guys was cleaning up the shop area with the DC hose. A Bic lighter fell out of his shirt pocket and got sucked up into the DC, hit the fan and blew the whole thing up.....This is what got me researching why dust explosions happen.

Sounds like a butane explosion, not a dust explosion.

[Scott T Smith]

Scott,

I think you've missed my final point... a spark is a spark. It doesn't matter if it's a 10kV spark created by a 1000CFM airflow or a 100kV spark created at 5000CFM airflow, it's a source of ignition. Knowing that the spark is going to happen regardless means you don't try to prevent it by decreasing airflow. Just the opposite, you know it's going to happen and increase the airflow, reducing the chances for the air-to-dust ratio from hitting the magic mark.

Since the standard shop is well below the amount of dust needed to go boom, the best idea is to increase airflow even more as dust production increases. You seem to be going to opposite way, which is more dust means you slow things down. If anything, you've increased your chances of going boom as you're pushing closer to the magic ratio, but you haven't removed the cause of ignition, the spark.




And if you follow Abe's advice, you'll never have an opportunity to view things from a different viewpoint. Always question what you know, and be willing to stand up for what you believe... you'll either come out the victor and (hopefully) change someone else's mind for the better, or you'll learn something new and change your own mind.


Dan, the "light bulb" just went off - I had not considered the dilution of the air-sawdust ratio that resulted from the increased airflow. That's a good point.

Regarding the strength of the spark, according to the research paper that I read a few years ago, the strength of the potential sparks generated at the lower air flow was insufficient to ignite the dust/air mixture. The comparison here is that an automotive coil that gets too weak will not generate a strong enough spark to ignite the gas/air mixture in a cylinder, even when that mixture is at the ideal 14:1 ratio.

[Dan Hintz]

The comparison here is that an automotive coil that gets too weak will not generate a strong enough spark to ignite the gas/air mixture in a cylinder, even when that mixture is at the ideal 14:1 ratio.
Reasonable comparison, but the engine cylinder charge is under extreme pressure (high pressure means harder to ignite)... that changes things quite a bit. At ambient pressure, size of spark don't mean much...

[Alan Schaffter]

Sounds like a mind-meld now!:D

It is possible to do the math (I'm not inclined to do so) but you can get a pretty darn good idea what it would take to achieve the necessary fuel-to-air ratio for a DC fire or explosion. I had the particle size and density numbers somewhere but can't find them now.

You need to do a little fudging to figure the total dust volume, since in any given shop sample, a certain amount of dust will be too large for this type of combustion. I'm not sure, but also think I read that a higher than optimum density will reduce the chances of fire or explosion. Anyway, once you have the total density needed, multiply that by your system CFM. That will tell you the total amount of dust that must be created (per minute) to achieve the required density. If you can find a number that relates the weight or volume of dust to solid wood (remember there are interstitial spaces) you can compute how much solid wood must be converted to dust (per minute).

I frankly think that number is unachievable in a home or even most commercial shops (factories are a different story). And of course the higher the CFM, the more dust you must generate (per minute) to achieve the required density. Since most home DCs draw from only one machine at time and commercial shops have far greater CFM for multiple machines, the required density may never be reached . . . . except . . . . .

A. If you are using the DC to QUICKLY vacuum up a large pile of sawdust from the floor.

B. If dust is discharged into a bag house or other large enclosure where the air mass is no longer moving fast, the density can possibly build to dangerous levels. You still need a spark. I have not read any grain mill explosion reports, but if I were a betting man, I bet they occur in conditions of high density but relatively stagnant air mass in a mostly empty grain elevator.

And, again, even if you momentarily achieve the correct fuel air ratio, you still need a spark at the right place and the right time.

[Dan Hintz]

I frankly think that number is unachievable in a home or even most commercial shops (factories are a different story). And of course the higher the CFM, the more dust you must generate (per minute) to achieve the required density. Since most home DCs draw from only one machine at time and commercial shops have far greater CFM for multiple machines, the required density may never be reached . . . . except . . . . .

A. If you are using the DC to vacuum up a large pile of sawdust from the floor.

B. If dust is discharged into a bag house or other large enclosure where the air mass is no longer moving fast, the density can possibly build to dangerous levels. You still need a spark. I have not read any grain mill explosion reports, but if I were a betting man, I bet they occur in conditions of high density but relatively stagnant air mass in a mostly empty grain elevator.
Well that paragraph pretty much sums up what I've been saying over, what, 5 or 6 posts ;) I like to call it "in-depth" details :p