? angled inlet into tall separator

[jdon]

Okay, I'm circling in, so to speak) on my separator design, and am thinking of a tall separator (like the one Bulldog8 has built), with a rectangular inlet (like retired2's).

My question is whether the vertical angle of the inlet has any bearing on separation efficiency. It's my understanding of the postings here that the more rotations of air in the separation chamber, the better collection of fine particles; a tall separator allows more rotations. In that case, it makes sense to me that the best placement of the inlet port is right under the top lid of the separator, and parallel to the lid (horizontal). That way, the air flow, as I understand it, takes a shallow spiral path.

On the other hand, Bill Pentz makes a big deal about having an inlet to his cyclone plan that is angled downward, with the downward flow reinforced by an internal air ramp. The air flow takes a much steeper spiral flow.

I realize that a cyclone with a cone is not directly comparable to a Thien separator, but does angling airflow downward, as Pentz recommends, make any sense in a Thein baffle separator? My sense is that while it might improve air flow through the separator, separation efficiency, esp. fine particles, would suffer, so I'm leaning toward going with a horizontal inlet. Any thoughts?

[alan m]

i cant recall anyone here try it.
your better off trying  it
make the inlet transition so that you can put it at an angle as well

[phil (admin)]

The real reason that taller baffled separators work better (than shorter) is that the outlet tube can extend BELOW the inlet tube, which prevents bypass.  Bypass occurs when incoming air/debris sneaks right out the outlet without entering rotation, and it is probably a major contributor to poor performance where fines are concerned.

The angled inlet would be a hindrance for a baffled separator.  You don't want to aim incoming air at the baffle.

[retired2]

The real reason that taller baffled separators work better (than shorter) is that the outlet tube can extend BELOW the inlet tube, which prevents bypass.  Bypass occurs when incoming air/debris sneaks right out the outlet without entering rotation, and it is probably a major contributor to poor performance where fines are concerned.

The angled inlet would be a hindrance for a baffled separator.  You don't want to aim incoming air at the baffle.

Phil,

I often wondered if there was any real benefit to a double high separator, particularly when a single-high unit performs so well?  I suspect if there was some reliable test data available it might show the single-high to be 93% efficient and the double-high 98%.  Maybe it would be a bigger difference if the waste being separated was very fine dust.


However, your comment about the benefit coming from the placement of the outlet pipe relative to the inlet sent me back to look at my test data where I measured CFM changes with the outlet pipe at different positions.  Also, at one point in my testing the outlet pipe accidentally slipped below the lowest intended setting.  I didn't notice this initially because it did not seem to significantly affect the performance.

After reviewing my data, I discovered something interesting that I didn't give much thought to at the time.  What the data shows is that a straight mouthed outlet pipe, with or without an air straightener, shows no change in CFM's when its position is changed.  On the other hand, the bell mouth outlet, with or without an air straightener, clearly shows a slight improvement in air flow as the outlet flange is moved lower, i.e. closer to the baffle.  Admittedly, the amount is small, but it is clear from the data that it is not an aberration.

So, what my data may be showing is that with a bell-mouth outlet the flange placement can be lower than the generally accepted practice for positioning the outlet pipe.  A lower position might give a single-high unit a slightly higher separating efficiency, and at the same time provide a slight improvement in air flow. 

Sorry to the OP for high-jacking your thread.

[phil (admin)]

As always, that is good data, retired2.

My observations in regards to extra-tall versions comes from when I was experimenting with a way to make a kit for a unit that would sit on top of a flat lid added to the drum of a user's choice.

With my original test model which I guess we can call the 1H unit, chalk dust sucked into the separator left a trail that clearly indicated a certain percentage of the incoming air mass was bypassing.  You could see a pink chalk trail including the bottom edge of the outlet tube closest to the inlet.

Later I tried a unit that was about 2.5H because I wanted to see what would happen to resistance.  I didn't see a large (about 2-3% improvement, but this was with a shop vac so not the best test in this regard).  But the surprise came when testing with more chalk (blue this time).  There was blue covering the top of the outlet tube, but barely any closer to the bottom.  The shop vac hose had absolutely less blue chalk on it.  In fact, at this point, there was more pink chalk inside the hose than blue, and it had been run quite a while after the pink test w/ just regular chips.

So I can't quantify the results in terms of separation, but it was quite remarkable.

Also, that 2-3% may become an 8-9% increase in velocity if using a conventional DC blower.  I think there is less air mass collision and a decrease in velocity with the taller units which translates into less resistance.

[jdon]

Thanks, Phil, for the explanation and clarification. If I understand yours and retired2's comments, a tall separator with a bellmouth outlet placed low (close to the baffle) should decrease bypass, while maintaining (or perhaps improving!) air flow.

And retired2,

Sorry to the OP for high-jacking your thread.

absolutely no apology is warranted. The information  you provide is, as Mastercard says, priceless.

I went back to your rectangular inlet/bellmouth thread, and dug down to your table showing amperage (and dB) relative to various "loads" and outlet position. Have you collected any further data regarding outlet position with a bellmouth, to determine whether there is a maximum air flow position? That is, there must be a point, as you further lower the bellmouth, where the gap between it and the baffle is so small that it impedes air flow. Also, I assume that you are relating air flow to amperage, or have you done any direct measurements of air flow. If so, how do you measure it (and do you know of any DIY (i.e. cheap) methods of direct air flow measurement?

Thanks to both of you, and others, for sharing your wisdom, and the patience and willingness to share it with newbies such as myself.

[retired2]

Jdon,

The biggest problem with my thread is that it is a rambling discussion that followed the progress of my build.  Some of my early statements later had to be amended.  Consequently, you have to read the entire tedious thread to get everything.

It appears you did not read far enough.  The first test data I posted included amps and decibels only.  Further on is another table of test data that includes CFM AND FPM data.

[retired2]

As always, that is good data, retired2.

My observations in regards to extra-tall versions comes from when I was experimenting with a way to make a kit for a unit that would sit on top of a flat lid added to the drum of a user's choice.

With my original test model which I guess we can call the 1H unit, chalk dust sucked into the separator left a trail that clearly indicated a certain percentage of the incoming air mass was bypassing.  You could see a pink chalk trail including the bottom edge of the outlet tube closest to the inlet.

Later I tried a unit that was about 2.5H because I wanted to see what would happen to resistance.  I didn't see a large (about 2-3% improvement, but this was with a shop vac so not the best test in this regard).  But the surprise came when testing with more chalk (blue this time).  There was blue covering the top of the outlet tube, but barely any closer to the bottom.  The shop vac hose had absolutely less blue chalk on it.  In fact, at this point, there was more pink chalk inside the hose than blue, and it had been run quite a while after the pink test w/ just regular chips.

So I can't quantify the results in terms of separation, but it was quite remarkable.

Also, that 2-3% may become an 8-9% increase in velocity if using a conventional DC blower.  I think there is less air mass collision and a decrease in velocity with the taller units which translates into less resistance.

Phil,

The colored chalk is a pretty clever idea.  If I had not permanently fixed the location of my outlet port, I would be testing this new observation a little more.

[Schreck]

So, what my data may be showing is that with a bell-mouth outlet the flange placement can be lower than the generally accepted practice for positioning the outlet pipe.  A lower position might give a single-high unit a slightly higher separating efficiency, and at the same time provide a slight improvement in air flow. 

Sorry to the OP for high-jacking your thread.

This is something that I have thought might be significant, but I can't recall it being discussed explicitly ( bell mouth distance from baffle should be different than a plain pipe end).  In ported loudspeaker designs that use round ducts, the box resonance is tuned by adjusting the length of the duct so it resonates at a desired frequency.  The duct length that achieves the desired resonance is slightly less than the calculated "ideal" length due to end effects associated with the plain pipe end of the duct.  This indicates that the effective length of the plain pipe is longer than the physical length. 

The end effects associated with a plain outlet pipe may behave in the same way, so the distance between the effective end of the pipe and the baffle is shorter than the distance from the baffle to the end of the pipe that we measure.  When fitted with a bell mouth, the pipe is extended and a structure is provided to guide the air into the outlet with less turbulence.  So I would expect the ideal distance from the baffle to the bell mouth to be shorter than with a plain pipe.  Most of the design guidance has been given assuming a plain pipe.

In one of the bell mouth threads there was a link to a paper that had great images of the fluid dynamics of a plain pipe vs. a bell mouth.   It's worth looking at the images on the second page of the linked PDF and comparing the end of the physical pipe or bell mouth with the velocity flow profiles.
http://www.jpthien.com/smf/index.php?topic=550.msg4477#msg4477

[jdon]

retired2: You're right- I just needed to read further into your thread for the CFM data.

Thanks, John