Questions about a Thien mainly for dust and fine dust

[jnug]

So thought I would try these two questions outside of the larger post I made in the wrong spot....being new here.

I produce only dust and fine dust as it relates to material I want my DC and separator to handle. I have read several posts here from folks suggesting that if your concern is mainly dust one should scrap the standard rule of thumb for the size of the drop (1/2 the size of the inlet) and go to something maybe as small as 0.75". My inlet is 4". So the standard formula would yield a drop 2" across. Wondering whether the recommendation of 0.75" is anecdotal, based on testing.....the opinion of experts, any of the above? So I figured I would ask and see what comes back for a response.

Also I know again from reading here that we want the end of the inlet elbow to come in right at the start of the solid 120" section of the baffle. We don't want it to come in over the drop. As it relates to height above the baffle, do we want the elbow to sit right on the top of the baffle? Do we want it just above of the baffle or does it really not matter as long as the end of the elbow is right where the 120* solid section begins?

[jdon]

Your first question is a little confusing to me, in part because of some imprecise and possibly mixed up terminology. I think that the term "drop" usually refers to the distance between the bottom of your outlet (center) pipe and the surface of the baffle. The rule of thumb for that is 1/2 the diameter of the pipe. So, with a 4" pipe, there should be a 2" gap between the outlet pipe and the baffle. This position is a compromise. With a higher pipe outlet you'll get better air flow, but that's because more air will go straight from the inlet to the outlet, without swirling around the perimeter of the separator, so more dust will escape (bypass). A lower pipe would potentially give better separation, but a narrow gap between the baffle and the outlet will restrict air flow, and result in degradation of performance.

I think you're confusing "drop" with "slot width", which is the width of the opening that arcs 240 degrees around the outer edge of the baffle. The "standard" slot width is about 1-1/2", which is also a compromise. A narrower width (3/4" or 7/8", or so) should give better separation (collect more dust), but it's more likely that larger particles, such as shavings or long strings of wood (planer and jointer) will clog the slot. A wider slot won't clog, but won't separate as well. If you're only generating fine dust, try a narrower slot width.

As I understand it, the orientation of the inlet opening (elbow) to the baffle slot is complicated. My read on it is that you want the air just leaving the elbow (just entering the separator) to "land" on a solid section of the baffle. If the bottom of your inlet is at the same level as the baffle, then the inlet opening should be at the beginning of the solid (120 degree) section of the baffle.

If the inlet opening sits over the slot- or the distance between the inlet and slot is less than 120 degrees, more of the rapidly moving air just entering from the inlet will go down into the waste collection section below the baffle, and will stir up (scrub) and resuspend the dust below, and impair good separation.

With some designs, the separation chamber is taller than the inlet, so there is a vertical distance between the bottom of the inlet and the baffle surface. The air flow entering the separator will spiral downward, and may or may not "land" on the baffle where the solid section of the baffle is located. I dimly recall that Phil Thien posted about using chalk dust to trace air flow. Check out the posts, though, before trying chalk, though- it's bad for cartridge filters.

I'm a relative newbie myself, so some or a lot of the above might be bogus. Maybe wiser heads will correct or corroborate. Just my .02

[jnug]

That was perfect actually. My collection material is always going to be dust or fine dust. So I don't have to worry about larger material. Maybe my width should be less than the standard in order to do a better job of capturing up the dust and fine dust in the separator instead of seeing it go on to the DC.

I will probably end up trying anything that favors dust and fine dust at the expense of larger material. I am going to make sure the inlet finishes above the beginning of the 120* section of the baffle and the inlet will be directed at it and not over the 240* section.

How the dust hits that 120* section might be one of the most interesting aspects of these separator designs. Not sure what is optimal for dust versus larger material. If I had to guess i would guess trying to get the dust to hit the baffle and disrupt its flow is better than having it enter and stay suspended. I am going to try a 6" outlet opening and outlet hose in an effort to encourage the dust to stay in the Separator instead of seeing it going up the hose to the DC. The combination of a baffle and and inlet favoring dust and a 6" outlet may produce a worthwhile result. I just don't know but will very likely try it and see.

Thanks for responding and thanks for the corrections.

[retired2]

http://www.jpthien.com/smf/index.php?topic=1167.0

[sportflyer]

Jnug , adding some wood strips  around the drop zone seem to help with fine dust separation :  see http://www.jpthien.com/smf/index.php?topic=1167.15