Bell mouth outlet pipe?

[retired2]

Here's a link to a dust collector test that has been posted here before.  It is a good report on dust collector performance, but it also contains a small inset on the lower left corner of the last page that might get overlooked.  The inset discusses the performance improvements of a bell mouth pick up port vs square cornered pick-ups.  Their testing showed improvements of more than 100cfm and a drop in SP of 1" for a bell mouth shape.  That's pretty significant.

http://www.portercable.com/uploads/PCD/Documents/News/182DustCollectors.pdf

So, the question becomes, would a bell mouth outlet on a Thein separator provide some level of improved performance.  It sure seems like it might be worth a try.

Has anyone done this? 

[Don_Z]

HAHAHAHA You are absolutely killing me here Retired! I knew there was a reason for me to wait until today to start my build. Now you had to go and show me this? Looks like I am going to have to engineer this in my plan as well. Good find.

[retired2]

HAHAHAHA You are absolutely killing me here Retired! I knew there was a reason for me to wait until today to start my build. Now you had to go and show me this? Looks like I am going to have to engineer this in my plan as well. Good find.

Don't rush into this Don.  My goal is to find enough improvements that the Thein separator will work without turning on the dust collector!!  Just kidding of course.

As you're thinking about how you are going to engineer a bell mouth into your design, here are some other options, but none quite as good.

[retired2]

Call me slow and stupid, I don't get the chart. How is there a 90% VP loss from a unflanged straight pipe ? Or even 50% VP loss with it flanged. Where did you find the chart ?

Chuck,

This is engineering data published by Cincinatti Fan.  However, there is no explanation of what VP (velocity pressure) is or how it is measured.  So, without that understanding, I think all we can extract from the chart is there are some real differences in performance between various shaped inlet ports, some very clearly much better than others.

Regards   

[retired2]

WHAT ?  The only thing that I can extract from any of this is that your throwing out information that you have no idea about, but you still want to try to come off like you do know something about it. You have just lost all credibility and patience with me.

Chuck, 

I'm sure the current Thien separator works as well as it does due to the testing of many different ideas without hard data to support the outcome.  That is what this thread is all about.  I threw out an idea without any personal experience to support it, but I provided two sources of data that seemed to support the idea.  I made no claims whatsoever.  As a matter of fact, I titled the thread with a question mark.  To most people that means "I don't know, do you have an answer?"

Now, if you have no value for my posts, that is O.K. - you can simply skip over them, but please don't bother to reply if your only objective is to malign me in a public forum.

[retired2]

Don,

Here's an explanation why a bellmouth inlet is more efficient than a straight pipe inlet.  The source of this information is online course material from the U.S. Government.  Again, this explanation seems to support the idea that a bellmouth pipe might provide an improvement in performance of the Thein separator exit pipe.



Hood Entry Loss

The following discussion on hood inlets will help you understand how hood designs affect the overall pressure drop and fan requirements of a system.

When air enters a duct under suction, the airflow converges as shown in Figure 2.  The area of air convergence upon entering a duct is referred to as vena contracta. The area around the mouth inlet is void of airflow due to the formation of the vena contracta.

As air passes through the vena contracta its velocity increases.  After passing through the vena contracta, the airflow expands to fill the duct.  As the air expands, some of the velocity pressure converts to static pressure.  The hood static pressure and entry loss are related to the size of the vena contracta.

The hood geometry determines the size of the vena contracta by influencing how smoothly the airflow will enter the duct see Figure 3, [Hood Entry Loss Coefficient (Fd) for Various Duct Designs].  Figure 3 shows three different duct designs and their corresponding hood entry loss coefficients.  The duct inlet design illustrated in Figure 3(c) is superior to those in parts (a) and (b) because air enters the duct more smoothly (less vena contracta effect) and air is drawn into the duct primarily from the front where the contaminated air is located.








Here's another hobby that has found an application for bellmouth pipes:

[Vodkaman]

I think that the bell mouth idea is valid input and worth posting. I am assuming that unproven ideas are acceptable here. The bell mouth would definately be on my list of ideas to try. The improved efficiency might even cause more problems than it solves. Certainly the height above the baffle plate would have to be reduced.

Interesting input, thanks for posting.

Dave

[phil (admin)]

Another place they're (bell-mouth) used is porting for low-frequency drivers (woofers).

I haven't done any testing.  Maybe I will give it a shot.

[retired2]

Another place they're (bell-mouth) used is porting for low-frequency drivers (woofers).

I haven't done any testing.  Maybe I will give it a shot.

Phil,

I think the other challenge this idea presents is what is the best way to create the bellmouth.  I am picking one up this week at my local supplier (about $20) who is also building my round to rectangular entry transition piece.  One thing he told me that concerns me a bit is the off-the-shelf bellmouths have a pretty large flare.  I think he said a 5" bellmouth has a flare that is 9" or 10" O.D.  It remains to be seen how that will effect internal air flow dynamics.  I wish I had a way to fabricate something more in the proportions of the carburetor inlet ports posted above. 

I have employed a mini bellmouth on my floorsweep by just hitting the corner with the largest roundover bit I own.  However, it should be larger to get the maximum benefit, but large roundover bits are expensive and they require a large router mounted in a table. 

I think I am going to fabricate my separator top with a bolt-on outlet connection so I can easily test different combinations.  I don't think this idea is going to be as simple as putting a purchased bellmouth in place of a straight pipe.  It will probably require some experimentation, but maybe with several of us trying different combinations we will hit on something worthwhile.  Even if nothing comes of this, it is the challenge of trying that makes it fun. 

 

[Don_Z]

Well done and thanks again Retired. Anything that helps the community is a benefit. It is the naysayers that hinder the concept of progress, so I appreciate what info you do find and contribute. I plan on adding bellmouth adapters at my machine ports using stacked mdf and then shaping by hand using my tried and true Nicholson #50.

[Don_Z]

The concept that I gathered from that article was that as long as there was a decent radius from the inlet port that it will create the vena contracta effect and therefore give an improvement. I am thinking that by opening up the factory 4" DC hole in say, a table saw and if you wanted to end up with a 6" duct connection, then by cutting it to 8" then take 2 pieces of MDF to fit the new hole, cut your 6" hole. Then either rout out the radius or use whatever your preferred method would be ( I am VERY fond of my patternmakers rasps.) continue the radius to suit your vision of a bell mouth opening.

[retired2]

The concept that I gathered from that article was that as long as there was a decent radius from the inlet port that it will create the vena contracta effect and therefore give an improvement. I am thinking that by opening up the factory 4" DC hole in say, a table saw and if you wanted to end up with a 6" duct connection, then by cutting it to 8" then take 2 pieces of MDF to fit the new hole, cut your 6" hole. Then either rout out the radius or use whatever your preferred method would be ( I am VERY fond of my patternmakers rasps.) continue the radius to suit your vision of a bell mouth opening.

Don, I think what you just described actually reduces or eliminates the vena contracta, which is exactly what you want to do.  And I think a filed radius on built-up MDF would work just fine since I don't think the geometry of the flare is all that critical. 

However, I cringe at the thought of cutting holes in my equipment, and I'm not sure I would have the courage to open up a 4" port to an 8" hole.  My cowardice would probably result in me trying to put a collar on the inside the existing port (space permitting) to give it a bellmouth entrance.  But you are right, if your DC is big enough to handle 6" duct then an 8" hole with a bell mouth radius to 6" pipe is probably the ultimate.
 

[Don_Z]

Or there is this...

http://www.pennstateind.com/store/N-BELL06.html

[retired2]

Or there is this...

http://www.pennstateind.com/store/N-BELL06.html

Don, you ain't seen nothin yet!!  Take a look at these links for info on making "cheap" bellmouths.  If I ever tried this in my wife's kitchen, I'll be looking for a new place to sleep.

http://forums.sohc4.net/index.php?topic=57484.0
http://forums.justcommodores.com.au/vy-holden-commodore-2002-2004/92806-cai-alien-design.html

And if you want dimensional information on the off-the-shelf models so that you can build your own, here's another link:

http://www.spiralmfg.com/hoodsf.htm

[Don_Z]

And just when you think the internet CANNOT get any dumber, you have to show me this  :o? This is the stuff comedy gold is made from. My girl would have me outdoors for pulling one of those stunts. But I have to tell ya, that Spiral web site looks like a one stop shopping site for what we are trying to accomplish. I do believe I will have to inquire about pricing. Good find, thanks.