inlet angle question

[bluemil]

I have been looking on this site for quite some time and have had no luck in coming up with an answer to a question.

When building a separator the inlet is set tangentially to the top hat. Does changing the inlet angle axis change the efficiency of the tophat? I know from my experience with Navy aircraft that pre-swirling the air aids in performance of the engine. So I was thinking that angling the inlet to follow the angle at which the dust rotates will increase the overall collection efficiency. What do you all think?

[retired2]

Do you mean tilting the inlet downward slightly?  If so, I think it has been discussed, butI don?t remember anyone posting test data to prove any benefit.  There is a video buried in my build thread that shows the rotating waste stream circles in a bit of an oscillating pattern.  That might suggest there is little or no value to a downward angled inlet.  However, I could see how in a double high design, it might prove to be effective.

I have always been an advocate of rectangular tangential inlets.  Rectangular inlets that are tall and narrow put the waste stream along the outer wall over the drop slot, which is where it should be to optimize separation.  That argument would also seem to support the idea of angling the inlet down forcing the incoming waste stream toward the drop slot. 

Bottom line I doubt that it will provide a significant benefit, but is not likely to be detrimental.  But I think you would get more benefit from a rectangular tangential inlet.

[bluemil]

Thank you very much for your input. I am going to be building mine double high. as well I am going to be adding an inlet that is rectangular I don't have the sketch-up knowledge on how to make a top hat with a rectangular inlet I do have the art skills but not with a computer I will add a scale drawing on what my build entails in future threads.

[retired2]

Thank you very much for your input. I am going to be building mine double high. as well I am going to be adding an inlet that is rectangular I don't have the sketch-up knowledge on how to make a top hat with a rectangular inlet I do have the art skills but not with a computer I will add a scale drawing on what my build entails in future threads.

I think you are on the right track.  Keep the outlet pipe below the inlet to minimize waste by-pass.  I seem to recall some builders experimenting with the outlet height but I don't remember the conclusions, is any.  For sure, you want it below the inlet.

[bluemil]

from my understanding of things it would make the most sense to have the opening height be equal in volume to the hose opening attached to the bell mouth. As I also understand having the bell mouth flare bent as well as keeping the funnel at a less than 15 degree angle is important as well.

[retired2]

I must say I?m confused by all three of your comments.  Height equal to volume?   Bending the flare?  Funnel less than 15 degrees?

[bluemil]

This is what I meant by the funnel size and the flare. As for the opening. It makes sense to me to make the opening of the bell mouth equal distance in area to the area of the opening of the exit orifice.

[retired2]

This is what I meant by the funnel size and the flare. As for the opening. It makes sense to me to make the opening of the bell mouth equal distance in area to the area of the opening of the exit orifice.

I think you are over engineering the bellmouth.  There is a diagram I posted a long time back that showed the benefits of various types of pipe endings.  What it showed is that just a flat flange on the end if the pipe is significantly better than a straight pipe.  It is not as good as a bellmouth, but it is surprisingly close.  What that suggests to me is tweaking the flare on the bellmouth will have negligible effects.

[Schreck]

I think that a 1/2" radius router bit and a 3/4" stock end flange 2" larger in diameter than the outlet would be good enough for our purposes.  Bell mouths that are too large risk pulling in dust from the sides of the drum/tophat, defeating the purpose.  Haven't been tuned in lately to see if anyone has done that, but I can't recall ever seeing it on this forum.  Of course, I am still a newbie...

[retired2]

Schreck,

If anyone has the patience to look for it, I think a long time ago I posted a link to a woodworking magazine article in which the writer was extolling the benefits of radiused pipe ends for all the dust pickups on every tool.  As I recall he fashioned them from glued up blocks of wood and radiused the inlet with a large round over router bit.  The discussion would probably be about the time I suggested bellmouths for use in separators.

I used that writers suggestion on my floor sweep.  I built up the neck with a wood collar, then radiused the underside with the largest bit I had.

[dabullseye]

about this 15 degree slope. you will only be able to get a length of 7.5" before the sides converge to a point and touch each other. so if you were to build a dbl high top hat your outlet will be very small

[Schreck]

Sketches of your intent would help us to understand where the flare would be located.  If located on the outlet as part of the bellmouth, then one of the images in this thread suggests it would be beneficial: http://www.jpthien.com/smf/index.php?topic=550.0

[retired2]

Sketches of your intent would help us to understand where the flare would be located.  If located on the outlet as part of the bellmouth, then one of the images in this thread suggests it would be beneficial: http://www.jpthien.com/smf/index.php?topic=550.0

You found it Schreck!  Unfortunately the article that was attached to that link has been removed by Porter Cable.  I have a vague recollection of someone finding it again somewhere, but that will be looking for another needle in a haystack.

I enjoyed rereading that thread, it brought back not so fond memories of Chuck Lenz who has been banned from just about every woodworking forum on the internet!