Bellmouth Outlet on double height top-hat?

[Alan H]

I read in retired2's thread that the reason for the air straighteners was to reduce turbulence in the outlet pipe.  I plan to do the straighteners and have purchased 2 light tubes 1 for T12 and 1 for T8 to use for this part of the project.  However, since my top-hat will be 24" internal with a 10" inlet, making it more that double height, I though that vibration of the outlet pipe especially if only supported at the top could be an issue. This would be affected by it's connection to the impeller housing as well as the long length inside the separator.

To reduce this, I have an idea of stabilizer rings extending down from the top.  The inner surface of the rings would be lined with foam tape to support the outlet tube, which would reduce vibrations and harmonics of the pipe and negate any rattling caused by wood on metal.

This would also still allow removal of the outlet pipe for inspection and cleaning, as well as it would not be permanently attached to these rings so that while I experiment with height of the bell-mouth above the baffle I wouldn't need to completely open up the separator.  Here are some pictures:

This is the whole separator as I have it drawn in SketchUp.  I will be using metal for the tube, but I've made the external part of the metal a blue-glass color so it can be seen through. There will also be vertical supports around the outside, I just haven't drawn them in yet.

Please forgive my crudely drawn inlet.  ::)


Here it is without the metal tube..


This is just with the Top to show how the stabilizer rings will be attached.  As you can see, there is no bell-mouth, as I didn't see the need to take the time to draw that in SketchUp.



This one shows you a different view, with my inlet rings in place, and how I plan to mount the top-hat in the stand that I'm building for the full DC system.  I will also be changing the orientation of the vertical braces so that there isn't one in line with the inlet.


So, what are your thoughts on stabilizing the outlet pipe in this fashion to reduce vibration and noise from harmonics?

[retired2]

Your outlet neck is pretty long so it might need some stabilization, but I can't recall anyone mentioning vibration of the outlet.  The turbulance and noise I was getting until I added the air straighteners was coming from the blower housing.  I'm sure it was the effect of the turbulent air on the blades of the impeller.  I never noticed any vibration or noise from the outlet pipe, but mine is very short compared to yours.  And mine is connected to the blower with a few inches of flex hose, so that should isolate the blower from the separator from the blower or vica versa.

It's possible that your stiffener could act on the spinning air and cause some turbulance that would not be beneficial.  I think I would try to avoid it, but build your separator in a way that allows you to add it later if needed.  Another potential problem is long shavings getting hung up on the stiffener.  My outlet neck has a pretty good layer of fines stuck to it from use over time, so waste does get attached to even a smooth pipe.

[Alan H]

Your outlet neck is pretty long so it might need some stabilization, but I can't recall anyone mentioning vibration of the outlet.  The turbulance and noise I was getting until I added the air straighteners was coming from the blower housing.  I'm sure it was the effect of the turbulent air on the blades of the impeller.  I never noticed any vibration or noise from the outlet pipe, but mine is very short compared to yours.  And mine is connected to the blower with a few inches of flex hose, so that should isolate the blower from the separator from the blower or vica versa.

It's possible that your stiffener could act on the spinning air and cause some turbulance that would not be beneficial.  I think I would try to avoid it, but build your separator in a way that allows you to add it later if needed.  Another potential problem is long shavings getting hung up on the stiffener.  My outlet neck has a pretty good layer of fines stuck to it from use over time, so waste does get attached to even a smooth pipe.

Good points, thanks!

At this time, I plan to have my separator connected with hard pipe, not flex hose.  I don't have any 6" flex, and it doesn't seem feasible to buy some just for a few inches for this connection.  I plan to have slots for the motor housing attachment to the frame that the system will be mounted on.

This will allow for height adjustment of the outlet pipe in the separator and also for disassembly for any needed maintenance or upgrades, such as possibly upgrading the impeller to the rokon 60-200 that many have done to the HF units like I have.

I will build it without that stabilizers to begin with and see how it runs.   I think one thing that might help mine is that the spin in both the impeller and the separator are both in the same direction. 

[Schreck]

I can't recall reading any discussion on the position of the drop slot in relation to the inlet on double-height separators. 

In single height separators, the slot begins 120° after the inlet and continues around until ending just before the inlet.  In a taller separator, the dust will drop to the level of the baffle and slot much later, after spinning around the perimeter of the separator some distance.  Should the position of the solid portion of the baffle be changed based on this? Does it matter where the slot begins in this case or is it enough that it is present to de-couple the air below the baffle from the air above the baffle?

I'm raising this question because your steel sides will not allow you to observe the separator in operation, and if it does matter, it would be good to be able to change the position of the baffle.

[retired2]

Your outlet neck is pretty long so it might need some stabilization, but I can't recall anyone mentioning vibration of the outlet.  The turbulance and noise I was getting until I added the air straighteners was coming from the blower housing.  I'm sure it was the effect of the turbulent air on the blades of the impeller.  I never noticed any vibration or noise from the outlet pipe, but mine is very short compared to yours.  And mine is connected to the blower with a few inches of flex hose, so that should isolate the blower from the separator from the blower or vica versa.

It's possible that your stiffener could act on the spinning air and cause some turbulance that would not be beneficial.  I think I would try to avoid it, but build your separator in a way that allows you to add it later if needed.  Another potential problem is long shavings getting hung up on the stiffener.  My outlet neck has a pretty good layer of fines stuck to it from use over time, so waste does get attached to even a smooth pipe.

Good points, thanks!

At this time, I plan to have my separator connected with hard pipe, not flex hose.  I don't have any 6" flex, and it doesn't seem feasible to buy some just for a few inches for this connection.  I plan to have slots for the motor housing attachment to the frame that the system will be mounted on.

This will allow for height adjustment of the outlet pipe in the separator and also for disassembly for any needed maintenance or upgrades, such as possibly upgrading the impeller to the rokon 60-200 that many have done to the HF units like I have.

I will build it without that stabilizers to begin with and see how it runs.   I think one thing that might help mine is that the spin in both the impeller and the separator are both in the same direction.

Regardless of matching air rotation, you should still use an air straightener.  Spinning air entering the blower reduces fan efficiency, and strangely, air that is spinning in the same direction reduces air flow more than when it is opposing. 

Schreck, good questions.  I've often wondered whether the positioning of a baffle makes any difference in a 2X separator.  After I added the bell mouth to my separator I became more aware of the waste stream circling in somewhat of a sine wave pattern.  I'm not sure why, but that would suggest that the position of the slot is not all that important in a 2X build.  On the other hand I wouldn't want to chance randomly placing the slot in a 1X build. I think the closed area needs to be where the air enters the chamber, and Phil came up with that configuration after a lot of testing so I accept it as gospel.