Square or Rectangle Top Hat Inlet

[ocrrhbow]

I've been planning to build a top hat separator for some time now and have been wondering if anyone has experimented with making the inlet square or rectangular.  I know Bill Pentz and the Clearview cyclones transition from round to rectangular prior to entering the collector in order to reduce turbulence.  Could a similar transition help make a Thien top hat separator more effective?   

[phil (admin)]

If you do a side-inlet, a rectangular inlet would be optimum.  This basically extends the wall of the separator and gives debris more time to hug the side.  But the difference between this and a round inlet would be very small.  It isn't going to make a huge difference in separation.

[bill70j]

In industry we always design the inlets as rectangular.  We also offset the inlet from the body of the cyclone to improve efficiency.  In other words, rather than having the outer wall of the inlet ductwork in the same vertical plane as the cyclone wall, we offset it by a certain amount. This is called a scroll, which helps the dust particles stay closer to the wall and keep them from impinging on, and escaping through the outlet tube. 

The pictures below show what I am trying to say and give an idea of how a scroll will help separation efficiency.  The inlet ductwork starts with an offset, but gradually returns to the same dimension as the cyclone wall through the first 180 deg of sweep.

I built my separator with a top inlet, but am now redoing it with a rectangular side inlet to include the scroll feature.

[WayTooLate]

Bill -
Has this design been compared to a 'traditional', side inlet top-hat design? 
Have any consistent measurements been made? 

I would be very interested in knowing these results.  I am not aware of scroll designs being implemented on the suction side of the system.  They have been in use in blower designs for at least 75 years.  It is a proven way to simultaneously increase both velocity and pressure within the blower housing.  But I have some questions about their effectiveness on the suction side...  Some empirical measurements will be quite enlightening! 

Also, are the data listed in your first image based on the dimensions listed?  That is a HUGE design.  I am not certain how much air would be needed to make a 4' diameter separator work effectively.  I have a 10hp, 4000cfm system and I know it could not drive a 4' diameter Thien separator... 

My largest separator is 24" but all new ones are <16".   I have a 12" unit that separates MDF fines down to <3 microns - sampling shows NO particles 3 microns and larger.  On that unit I paid an extreme amount of detail to edges and corners. 

I will look forward to any measurements that Bill can provide. 

- Jim

[bill70j]

Has this design been compared to a 'traditional', side inlet top-hat design?  
Have any consistent measurements been made?  

I would be very interested in knowing these results.  I am not aware of scroll designs being implemented on the suction side of the system.  They have been in use in blower designs for at least 75 years.  It is a proven way to simultaneously increase both velocity and pressure within the blower housing.  But I have some questions about their effectiveness on the suction side...  Some empirical measurements will be quite enlightening!  

Also, are the data listed in your first image based on the dimensions listed?  That is a HUGE design.  I am not certain how much air would be needed to make a 4' diameter separator work effectively.  I have a 10hp, 4000cfm system and I know it could not drive a 4' diameter Thien separator...  

My largest separator is 24" but all new ones are <16".   I have a 12" unit that separates MDF fines down to <3 microns - sampling shows NO particles 3 microns and larger.  On that unit I paid an extreme amount of detail to edges and corners.  

I will look forward to any measurements that Bill can provide.

Jim:

You are correct.  The cyclone suction drawings and separation results I show in the earlier post are reflective of industrial cyclones operating downstream of blowers in a low positive pressure environment, rather than small cyclones in vacuum service like the separators discussed on this forum.  But I would argue that the separation efficiency is a result of pressure differential and velocity through the device, so absolute pressure in the ranges we're talking about shouldn't be a big factor.  These industrial cyclones are huge, often operate with four or six in parallel, and are driven by large 4000HP motor or turbine driven blowers.

But the industrial designs are based on empirical pilot plant data generated in set-up's not too dissimilar in size from our shop separators.  I will do some digging around and see if I can find the pilot plant data that went into the design of the inlet scroll feature.  It's out there.  I just need to find it.

Bill

[WayTooLate]

Bill -
I hate to nitpick or sound negative, but I have a few more questions...

You are correct.  The cyclone suction drawings and separation results I show in the earlier post are reflective of industrial cyclones operating downstream of blowers in a low positive pressure environment, rather than small cyclones in vacuum service like the separators discussed on this forum.  

I am not aware of any cyclones or other separators that are 'downstream' of blowers.  Generally, you want to remove all the waste in the airstream before it gets to the blower impeller.  In all designs I am aware of, the only thing that is downstream of blowers is 'bags' or 'filters' and they do not want to see the debris that is trying to be separated - let alone have it go through the blower..   

I will do some digging around and see if I can find the pilot plant data that went into the design of the inlet scroll feature.

One thing that occurred to me after my last post was that it appears you are using the 'scroll feature' on the inlet side of the airflow.  As I look at all the blowers I have on my ventilation, HVAC and dust collection systems, the scroll is all on the outlet sides of the blowers.

If you can dig up that information, it will be interesting to see if we can scale it down 2000:1 for the small systems we are using in our shops. 

Hope this helps!
Jim

[phil (admin)]

I am not aware of any cyclones or other separators that are 'downstream' of blowers.

Actually, this is fairly common.  Cyclones can be used in a push or a pull configuration.  When used in a push configuration, you don't need anything on the outlet of the cyclone.  So in manufacturing, they often use a large outdoor cyclone and park a truck directly underneath.  When the truck is full, they can pull it out and put a new truck there.

In hobby workshops, a push configuration allows you to just tie a bad to the bottom of the cyclone.  The running unit will inflate the bag and when the bag is full you can cut the cable tie, and install a new bag.

Of course, everything is going to hit your impeller in this case.  But a steel impeller should handle it no problem.

[bill70j]

Jim:
Yes, the systems I have worked with are somewhat unique.  What I am experienced with is known as the fluidized bed catalytic cracking process, which is used in oil refineries to make gasoline components.  It uses an air blower at the front end and cyclones at the back end to separate contaminants from the air before it re-enters the atmosphere.  

One part of the process takes filtered air at atmospheric pressure up to about 20 psig using a big axial blower.  Then the air is introduced into a vessel operating at 1400 degrees where it mixes with talcum powder-like catalyst that is contaminated with carbon.  The carbon is burned off from the catalyst in this hot vessel.  Then the carbon-free catalyst is separated from the air using cyclones, which are good to about 20 microns, followed by an electrostatic precipitator, which is good to about 5 microns, and finally followed by a water scrubbing process to remove sulfur oxides and the last of the catalyst.

As I mentioned, I think there is a real benefit for designing a scroll at the suction side of of our shop separator top-hats, and that the pilot plant data used in designing these industrial cyclones can be of potential use.  What will be important is how the data were used in the scale-up.
Bill

[retired2]

In industry we always design the inlets as rectangular.  We also offset the inlet from the body of the cyclone to improve efficiency.  In other words, rather than having the outer wall of the inlet ductwork in the same vertical plane as the cyclone wall, we offset it by a certain amount. This is called a scroll, which helps the dust particles stay closer to the wall and keep them from impinging on, and escaping through the outlet tube. 

The pictures below show what I am trying to say and give an idea of how a scroll will help separation efficiency.  The inlet ductwork starts with an offset, but gradually returns to the same dimension as the cyclone wall through the first 180 deg of sweep.

I built my separator with a top inlet, but am now redoing it with a rectangular side inlet to include the scroll feature.

Bill, 

I bookmarked your message some time ago when I wasn't quite ready to start my own separator.  I was intrigued by your comments regarding the rectangular inlet and the scroll feature.  I have a bunch of questions if you don't mind.

Have you started rebuilding your separator? 

Did you incorporate the scroll concept? 

Did you use a rectangular inlet?

How did you determine the aspect ratio of the rectangular inlet?

And now for the big question.  How is it working?

Thanks.

[Don_Z]

I too have been holding off on my own Tophat fabtrication until a little more data/science was introduced. There are some really fantastic builds posted on these pages but I have noticed a few things. There are quite a few that have taken some liberties in design changes to the Thien theory and not because of some new and relevant data, but more out of garage hackery (no offence). I am very interested with this introduction of the "scroll" in the conversation as that it has a history of applied mechanics and science that can back it up. Having just received my new DC unit and Wynn filters, I am not in a big hurry to start slapping something together when a little more research will give me better results with premium performance and a superior build.I have worked in the aerospace research and development industry prior to becoming a professional woodworker and rather enjoy gaining higher performance and results from my equipment and machinery rather than settle for the terrible status-quo that one must suffer from the consumer market.

Regards,
Don

[retired2]

I too have been holding off on my own Tophat fabtrication until a little more data/science was introduced. There are some really fantastic builds posted on these pages but I have noticed a few things. There are quite a few that have taken some liberties in design changes to the Thien theory and not because of some new and relevant data, but more out of garage hackery (no offence). I am very interested with this introduction of the "scroll" in the conversation as that it has a history of applied mechanics and science that can back it up. Having just received my new DC unit and Wynn filters, I am not in a big hurry to start slapping something together when a little more research will give me better results with premium performance and a superior build.I have worked in the aerospace research and development industry prior to becoming a professional woodworker and rather enjoy gaining higher performance and results from my equipment and machinery rather than settle for the terrible status-quo that one must suffer from the consumer market.

Regards,
Don

Don, I think you and I are of the same mind regarding the fabrication of a separator.  However, my DC project has progressed from piping up the most offensive pieces of equipment to optimizing the DC itself - that means building a separator, adding a Wynn filter, and enclosing the DC for noise abatement.

I don't know a whole lot about cyclones, and even less about the engneering theory behind their design, but I am starting to have reservations about whether the "scroll" entry is applicable to a Thien separator.  I am even starting to worry that it might hurt the performance. 

Here's the difference that concerns me.  A cyclone has a long body that is totally concentric, either cylindrical or conical, with the exception of the top where the entry scroll makes it eccentric.  Due to the helical air flow, the air entering a cyclone quickly moves down from the eccentric part of the body to the concentric portion.   The Thien separator, by contrast, has a body length that is not much longer than the height of the inlet port. So, if the separator is built with a scroll, the air must continue to circulate in an eccentric pattern.  That just does not strike me as a more efficient design.

So, unless someone produces some data to suggest a "scroll" improves the performance of a Thien separator, I'm going to build mine with the conventional shape.  I'm still thinking about a rectangular entry port, but I'm not sure I want to spend the money to have a custom transition piece fabricated.

[WayTooLate]

I'm going to build mine with the conventional shape.  I'm still thinking about a rectangular entry port, but I'm not sure I want to spend the money to have a custom transition piece fabricated.

Don -
After spending too many years fabricating sheet metal on job sites, here are some suggestions for a 'Poor Man's' transition... 

For purposes of simplicity, I am going to assume that your rectangle 5"x4" and your round duct is 6".   

Home Depot and other 'big box' stores sell 6>7" round transitions for a few bucks. 

Make a layout sheet on paper/cardboard and draw your 5x4 rectangle.
Draw diagonal lines and extend them a few inches past the opposite corners. 
Use the intersection of the diagonals to draw a 7" circle. 
Center the transition on the circle and mark the diagonal lines onto the transition. 
Copy those marks onto the inside of the transition. 
Take a sheet of plywood or MDF (at least 1/2" thick) and cut out an interior rectangle that is 5"x4". 
Get a stiff laminate sheet or Masonite or equivalent that is less than 1/8" thick. 
Cut the thin sheet into an isosceles triangle that is not as tall as the transition and is at least 7" across the base. 
Drop the rectangular cut-out over the transition as far as is will go and remain perpendicular to the cone.  This is your outer form. 
Align the triangle wedge with the interior marks and push it in snugly tight.  This is your inside form. 
With the wedge snugly in place, push down a little farther on the exterior form (maybe 1/4"). 
Remove the wedge and rotate it to the other diagonal corners.  Press it in snugly and press down on the exterior form. 
Keep rotating the wedge and pressing the exterior form.  If you start pushing too hard or too far at one time, the shape will twist or warp. 
When you get near the final dimension, the metal will nearly fill the corners.  Use a chisel and tap it into a sharp corner. 

Be sure the outer form is square to the transition and draw a line around the top and bottom edge of the form.  Remove the form and use shears to trim on the outer/bottom line to get a finished, rectangular edge.  At each diagonal/corner line, slit the diagonal up to the inner line.  These 'flaps' can be screwed or attached to your the rectangular opening of your Top Hat.

Hope this helps!
Jim

[Don_Z]

I am starting to have reservations about whether the "scroll" entry is applicable to a Thien separator.  I am even starting to worry that it might hurt the performance. 

Here's the difference that concerns me.  A cyclone has a long body that is totally concentric, either cylindrical or conical, with the exception of the top where the entry scroll makes it eccentric.  Due to the helical air flow, the air entering a cyclone quickly moves down from the eccentric part of the body to the concentric portion.   The Thien separator, by contrast, has a body length that is not much longer than the height of the inlet port. So, if the separator is built with a scroll, the air must continue to circulate in an eccentric pattern.  That just does not strike me as a more efficient design.

So, unless someone produces some data to suggest a "scroll" improves the performance of a Thien separator, I'm going to build mine with the conventional shape.

I am thinking that the "scroll" airflow concentration will aid in reducing the turbulence inside the separator and therefore aiding the separation process and therefore reducing any scrubbing that may occur. I believe the square inlet would be the optimum choice but from what I have seen available, the typical HVAC transition that you will find at the HD or Lowes will be sufficient. I think I will start on "Project: Thien Scroll" tomorrow. The worst that can happen is I am out $30.00 in material and a day's worth of labor because it didn't work. But if it works...

[retired2]

I am thinking that the "scroll" airflow concentration will aid in reducing the turbulence inside the separator and therefore aiding the separation process and therefore reducing any scrubbing that may occur. I believe the square inlet would be the optimum choice but from what I have seen available, the typical HVAC transition that you will find at the HD or Lowes will be sufficient. I think I will start on "Project: Thien Scroll" tomorrow. The worst that can happen is I am out $30.00 in material and a day's worth of labor because it didn't work. But if it works...

Don,

I'm sure your build will work.  The question is how will we know if it worked better or not?  Even building two slightly different versions, and testing them side by side, might not reveal any differences.  Nevertheless, I'm very intersted in hearing and seeing the outcome of your effort.

Once I get started I'll post my progress as well.  I'm still leaning toward a concentric separator body with a rectangular inlet.  I will probably pay to have a transition piece fabricated so that I have full control over the shape and dimensions.  I am going to transition from 5" round to approximately 6" x 3.5".  The rectangular area is slightly larger simply to compensate for rectangular duct being less efficient than round.

The transition piece design leads to another question.  Will an eccentric transition work better than a concentric one, i.e. will it do a better job of prepping the air stream by moving the waste against the outside wall as it enters the separator?  Phil seemed to think a correctly placed small turn just prior to entry could be beneficial.  Would an eccentric transition provide that slight turn?  Unfortunately, this is another question for which there is likely no verifiable answer! 

[Don_Z]

Actually the eccentric model is the most efficient in regards to the inlet. Because here is what happens, when the incoming air is introduced into the separator with a round inlet there is something you may have not noticed. Look at the videos of Pitbull's 6" inlet/outlet that he has posted that show him demonstrate his inlet. You will see a "before" video with his inlet protruding just a bit inside the separator and as he feeds it dust, you will see the turbulence as the debris is hitting the incoming air and bouncing back through the inlet. Then watch his "after" video and you will see where he tries to cut back on the inlet material, but again, during his demonstration, there is still the turbulence. The factors involved were twofold.
First, the inlet material was protruding inside the separator causing the air and debris that is being "cycloned" to smash against it.
Second, because of the round inlet being introduced into a "cyclone" environment the outside 2 corners facing the opposite side of the inlet are causing a depression in the cyclone action.
With a eccentric transition into the inlet it prepares the incoming air for the cyclone that is happening inside of the separator by removing the corners and therefore removing the depressions in the cyclone. One of the main reasons I feel that a "scroll" method would benefit in this is because it helps introduce the incoming air and debris into the cyclone with the least amount of turbulence and therefore enhancing separation. My 2 cents.