Thien Separator

[Kenny Marjerrison]

Hello guys,

I am working on my dust collection system for my basement shop. I have done a ton of research on the Thien separators and have a few ideas of my own that I am going to try to implement. At this time unfortunately I am restricted to using 4" pipe and I am not making the piping permanent. The basement is only a temporary until I build my new shop (couple years probably). So the system I am working on is this, I picked up an old 3hp system that is identical to the grizzly g1030z2, it is also rated at 2300 CFM. I was able to beat him down on the price due to the fact that it is missing the bags, clamps, and upright supports for the bags (I am guessing someone pitched them not knowing what they were). Works out because I am not going to use the bags anyway. What I am planning to do is build a Thien separator that the motor/impeller will attach directly to the top of and vent the excess outside (I have 23 acres, the nearest neighbor is a half mile away). Now, I am aware of the CO issue with venting outdoors, not to concerned about that, I always have a window or door open. My connection to the tools (tablesaw, 24" drum sander, Dewalt 13" planer) is going to be made with a 10' piece of 4" flex hose going to the separator. The motor will be hooked to the separator with a 6" piece of flex about 12" long and the exhaust vent will be 4" rigid snap together piping. I am looking for info from those who have made a Thien style separator, will I have a problem with the 4" hose to the separator but having a 6" connection to the impeller? I am just wondering if the larger diameter pipe to the impeller will change how the separator works and not allow the material to "settle" the way it should. I would hate to neck that part down if I didn't have to but I also don't want to have to rebuild it after it is put together! If my explaining is confusing the hell out of everyone, my setup will be similar to Alan's in this post http://www.sawmillcreek.org/showthre...tor-with-HF-DC but venting outside instead of into the bags/filters. Thanks guys for your opinions!

KLM

[Phil Thien]

Should be fine.

[Curt Harms]

It sounds like you got a nice blower to do what you're planning to do. If I'm reading this right, you're planning 4" hose from the machine to the separator? I'd use a 6" hose and put a 6 X 4 PVC reducer on the end if you need 4" connections to your machines. my 6" flex fits inside a 6 X 4 PVC reducer nicely. I used some silicone to seal it & help hold it in place. There can be quite a bit of turbulence/resistance in flex hoses. I don't have valid number but I'd suspect that 6" flex would carry about the same CFM or perhaps a bit more than smooth wall 4", especially with a bend or two in it. Your blower has plenty of capacity and I have 6" PVC pipe and 6" hose with the 4" reducer where it attaches to the machines. It seems to work pretty well.

[Kenny Marjerrison]

Thanks for the replies. Curt, The adapter that I have that goes from the separator to the machines is actually a 5", maybe a 5" hose to the machines then a 5-4 reducer? I don't really want to have to make a 6" adapter for the collector as I have a 5" that should work. Let me know your opinion, I am mainly needing it for the 24" drum sander so I want it to be adequate for that, but at any case it will be one machine at a time. One other question, the pickup that goes from the separator to the blower, I have read that it should be around 3-4" from the bottom of the separator inside, does that sound correct? Thanks for the replies!

[Rod Sheridan]

You're going to run a 24" drum sander with one piece of 4" hose?

Aren't you going to be lacking air flow?

Regards, Rod.

[Kenny Marjerrison]

You're going to run a 24" drum sander with one piece of 4" hose?

Aren't you going to be lacking air flow?

Regards, Rod.

Rod,

I don't know thats why I am asking! I bought the sander used and the guy that had it used it hooked to a 4" line with a much smaller DC than the one I am building and said it worked great. But that is the main reason I asked the question, although grizzly says the collector has to have 500 CFM to pull it efficiently, the collector that I have is rated at (yes I know how deceiving the ratings can be) 2300 CFM so even if I lost half on one piece of 4" hose I would still be at over double what grizzly recommends. Correct me if I am wrong. This whole dust collection thing is new to me and there is a lot more to it than one thinks starting out. The other thing that I don't understand is that if 4" is so inadequate for dust collection why does the vast majority of machines have 4" ports? I don't get it. I know that a ton of flow is lost with flex hoses as well but if that is such a huge issue why is there so much more 4" hose/fittings and machines than anything else?

[Phil Thien]

You're going to run a 24" drum sander with one piece of 4" hose?

Aren't you going to be lacking air flow?

Regards, Rod.

A 4" hose moving 350-CFM is evacuating a sphere with a diameter of about 2.2 feet, every single second.

A 6" hose moving 700-CFM would evacuate a sphere with a diameter of about 2.7 feet in that same second.

An increase of 6" (or 3" on either side) in what I call the evacuation sphere is not going to make an enormous difference.

If you want to double that 2.2-foot sphere to 4.4', BTW, you need to pull about 2640-CFM.

IMHO, a 350-CFM 4" hose will work. A 700-CFM 6" hose will work marginally better.

[David Kumm]

I had a performax 25x2 with one 4" port and it will keep the belts from burning up with sawdust but not really keep the air clean so use a respirator and an ambient so your wife doesn't kill you when you open the door and let the dust into the rest of the house. Run the full 5" and reduce down into the port. Your system will do as well as most due to the 3 hp and ducting outside. DC on drum sanders is crappy and the sander would be better with two ports but I would guess you will pull over 6000 fpm through the 4" with a 5" flex. I would run larger than 4" outside to reduce resistance and give you a little more flow. Dave

[Kenny Marjerrison]

Dave,

My grizzly does have 2 4" ports and it came with a wye between them which is where I would be connecting in the line back to the separator. The only reason I am running 4" to the outside is because it is an existing dryer vent out of rigid snap pipe, galvanized. So I appreciate everyone's input and have learned a bunch. I will try everything out with what I have and now I know where to upgrade. Anyone have a lead where I can buy only a foot or so of 6" flex for between the blower and the separator? I would like to not have to buy a 10' piece only for one foot. Thanks everyone!

[Chris Parks]

A 4" hose moving 350-CFM is evacuating a sphere with a diameter of about 2.2 feet, every single second.

A 6" hose moving 700-CFM would evacuate a sphere with a diameter of about 2.7 feet in that same second.

An increase of 6" (or 3" on either side) in what I call the evacuation sphere is not going to make an enormous difference.

If you want to double that 2.2-foot sphere to 4.4', BTW, you need to pull about 2640-CFM.

IMHO, a 350-CFM 4" hose will work. A 700-CFM 6" hose will work marginally better.

Phil, I get to see figures thrown around that agree with what you say about the capture sphere numbers and also disagree with your numbers. Have you got any links that back your numbers as I would like some proof before I start quoting these to others. The other numbers I have seen quote a sphere half the size you are quoting so someone is wrong and I would like to know who.

[Michael W. Clark]

Phil, I get to see figures thrown around that agree with what you say about the capture sphere numbers and also disagree with your numbers. Have you got any links that back your numbers as I would like some proof before I start quoting these to others. The other numbers I have seen quote a sphere half the size you are quoting so someone is wrong and I would like to know who.

The numbers provided for the sphere diameters and flow rates in the 4" and 6" are pretty close. Its based on the volume of a sphere (4/3(pi)r^3).
The sphere example I'm familiar with describes a sphere diameter around an open end duct for a specific capture velocity. Its not based on the sphere volume, but on the sphere surface area, but the volume and area are obviously related. This example is in Chapter 6 of the ACGIH vent manual.

Not to derail this into something theoretical, which I for some reason have a tendency to do, but the dust capture is more a function of the air velocity at the hood face or the open area where the air is being drawn in. In the sander situation, if you double the flow, the velocity through the openings double, which can be a big difference in hood performance. You will improve performance by closing up these openings, but you may also increase the backpressure required to exhaust the air. It takes more power to pull X CFM through a 1/4" slot than it does to pull the same CFM through a 1/2" slot.

Everything has a trade-off and nothing is free, but I would suspect that you would see an increase in capture if you minimize the openings. This would help with the 4" or 6" duct.

[Phil Thien]

The numbers provided for the sphere diameters and flow rates in the 4" and 6" are pretty close. Its based on the volume of a sphere (4/3(pi)r^3).
The sphere example I'm familiar with describes a sphere diameter around an open end duct for a specific capture velocity. Its not based on the sphere volume, but on the sphere surface area, but the volume and area are obviously related. This example is in Chapter 6 of the ACGIH vent manual.

Not to derail this into something theoretical, which I for some reason have a tendency to do, but the dust capture is more a function of the air velocity at the hood face or the open area where the air is being drawn in. In the sander situation, if you double the flow, the velocity through the openings double, which can be a big difference in hood performance. You will improve performance by closing up these openings, but you may also increase the backpressure required to exhaust the air. It takes more power to pull X CFM through a 1/4" slot than it does to pull the same CFM through a 1/2" slot.

Everything has a trade-off and nothing is free, but I would suspect that you would see an increase in capture if you minimize the openings. This would help with the 4" or 6" duct.

Chris, Michael has the correct formula for computing the volume of a sphere.

Michael, for a table saw or shaper (where the tooling is probably throwing chips at 80+ MPH in a fairly narrow beam pattern), velocity is critical. With a sander, where the hood is at least 24" long and maybe 6-10" wide, doubling the velocity is going to be pretty anticlimactic.

Obviously, a 700-CFM flow will outperform a 350-CFM flow. My point is, many assume they will get 2x the collection with 700-CFM, when it just doesn't work like that (IMHO).

[Chris Parks]

I get the following from Bill Pentz and as me and maths are natural enemies I have to ask are the two formulae the same as they appear to me to be different and he seems to quote smaller areas for the catchment sphere or bubble...

At first these larger air volumes do not make sense because we know it takes 50 FPM to move the fine dust and 4000 FPM to move the heavier sawdust and chips. We already know from experience why it takes more air. When we use our shop vacuums they only pickup up right next to the end of the hose. The reason is unlike blown air that holds together for quite a distance, air being pulled or sucked by a vacuum comes from all directions at once. This means that the area being pulled from is roughly a sphere. It also means that our airspeed will fall off at roughly the same rate as that sphere area grows. Airspeed for sucked air falls off at roughly the same rate as the area of a sphere expands given by the formula Area=4*Pi*r^2. Most air engineers target for a duct speed of roughly 4000 FPM because this is what we need to pickup most woodworking dust and keep our vertical ducts from plugging. If we use that 4000 FPM in our air formula where FPM=CFM/Area we can compute how much air is moved in different sized pipes. We can then divide those airflows by 50 FPM to see how big of an area each will cover, translate into square inches then convert to the surface of a sphere. Although the math is fun, the bottom line is airspeed drops below the 50 FPM we need for good fine dust collection very quickly. Our 2” duct at 4000 FPM only supports 87 CFM and that turns into less than 50 FPM at only 4.47 inches from the center of our duct. Our 2.5” standard vacuum hose only supports 136 CFM at 4000 FPM which turns into only 50 FPM at 5.59” from the center of the hose. This explains why we see almost no pickup just 2” inches from the end of our vacuum hoses. A 4” duct at 4000 FPM airspeed only supports 196 CFM which turns into less than 50 FPM at about 6.71” from the center of our hose. A 4” duct that only supports 349 CFM at 4000 FPM only gets 50 FPM about 8.94” from the center of our duct. Many round this to 9” and use this as the standard for 4” duct. Our 5” duct that supports 545 CFM at 4000 FPM only supports 50 FPM out to about 11.18 inches. Our 6” that supports 785 CFM at 4000 FPM only supports 50 FPM out to about 13.42”. And, our 7” duct which supports 1069 CFM at 4000 FPM only gives our needed 50 FPM out to about 15.65”. In short we need to move a lot of air to ensure capturing the fine dust.

[Michael W. Clark]

Chris,
BP is getting his information from ACGIH or a similar source. The formula you give is the surface area of a sphere. The sphere volumes presented by BP are based on a capture velocity of 50 FPM at the edge of that sphere, which may or may not be enough depending on the application (probably not enough in most WW applications). He probably had to pick a value to illustrate the point.

The larger your sphere (farther you are from the hood) the more CFM you need to generate to get the capture velocity at that distance. There are numerous equations relating the required CFM to the capture velocity based on the type of hood you are using. BP's example above applies if you were to remove the top of the drum sander and hang an open-ended duct some distance above the sanding drum in open air. The hood connection acts to direct the airflow into the duct and reduce the wasted amount pulled from behind the duct, i.e. make better use of the CFM you are pulling through the duct. Not sure if this helps..

Mike