reducing turbulent flow around an elbow 9

[silverlilly1]

I am currently looking at the gas flow into the top of a large vertical cylinder. There is an elbow in the pipe directly before the inlet. Because of the turbulent flow produced as the gas goes around the bend, uneven buildup is accumulating in the cylinder. Does anyone have any suggestions for companies that produce devices to decrease the effect of the turbulence while the gas in in the pipe?
Any help would be appreciated. Thank you.

 

[PeterAB]

You'll get turbulent gas flow at any economic velocity. You cannot reduce that without increasing pipe sizes to some uneconomic size.

The usual problem caused by bends is UNEVEN gas flow DISTRIBUTION. For this you can buy gas straightening devices (turning vanes).

However, you spoke of buildup - presumably solids carried in the gas stream. Turning vanes may just be another place for solids to build up.

To minimize solids buildup on turning vane, consider installing staggered vertical blades only.

 

[silverlilly1]

The gas is entering a decomposer unit where it becomes CO and solid matter. The inlet is gas cooled to prevent buildup and the inlet gas is at a low enough temperature not to build up inside the pipe. I am looking for a number of different models of those turning vanes, or alternately some other way to stabilize the flow, such as creating circular movement of the gas. My online search for companies producing these types of devices has turned up only 2-3 companies. Basically I'd like to see if anyone else has had experience with them and could recommend a company or two.

 

[CARF]

Hi,

We had a similar problem two years ago. CFD Engineers (Computational Fluid Dynamics) promised to make a model so we could judge how to place large plates (vanes) to equalize the flow homogenuously across the complete cross sectional area. Not so easy, but it gave some good info
(and of course nice and colorful CFD pictures with the predicted velocity profiles).

In the end we balanced the tower by trial and error. We did a lot of measurements with a "propellor air speed meter". With this small device you can measure gas speeds and direction at any position in the pipe.

Best solution still is to ensure you have a straight (decomposer) inlet of a least 10 times the pipe diameter.

Regards,
CARF

 

[25362]

If the gas were clean, a perforated plate distributor with a pressure drop of at least 100 times the velocity head in the inlet pipe, ie, >100 V[sup]2[/sup]/2g to give an even velocity distribution over the cylinder cross sectional area, has been recommended by D. R. Richardson: How to design fluid flow distributorsChem. Eng., 68 No.9, 83-86 (1961).

 

[davefitz]

I recall about 15 yrs ago there was sold a product called a "Chen preswirler" to be installed upstreamof elbows to provie a pre-swril so as to reduce turbulence and pressure loss across the elbow. I am not sure if it applies to reducing build up , though

 

[steveen]

Did you check with tray manufacturers? there may be some internals to properly distribute the gas across the cylinder.
And consider the use of a long radius ell, if space permits.

 

[m777182]

If you use an elbow where the cross section changes smoothly from a circle at the entrance point through an elliptical cross section at the mid point and again to the circular cross section at the end there will be much less turbulence. It is obvious from the coefficients of friction loss for both elbows. Regretfully almost NO MECHANICAL enginneer would EVER use this solution.
m777182

 

[controlnovice]

I'm not sure if I have the name right, I think Davefitz is talking about the same thing. We called it the 'chen (or cheng)rotation vane' that is inserted just before the elbow. This helps efficiency also.

 

[StoneCold]

m777182
I have never seen an elbow like this. Do you have the name of a vendor or a link?

Thanks
StoneCold

 

[silverlilly1]

If you were asking me that question, it is just a standard elbow, but there is not sufficient space to run a long enough length of straight pipe between the elbow and the inlet so I am looking for other ways to even out the velocity profile.

 

[silverlilly1]

To m777182,

I am curious to hear why a mechanical engineer would not be inclined to use your solution of an elliptical cross-section at the center of the elbow.

 

[controlnovice]

It looks like this is proprietary (damn I wish there was a spell check here) but it's the first thing that popped up on google. It's a good explanation of what it does.

http://www.chengfluid.com/crv_info.htm

 

[btrueblood]

m777182,

Do you have any links to test data, or plots of the loss coefficient for the elliptical section elbow you describe? I can think of a couple of ways to fabricate such an elbow, and this is one mechanical engineer who'd love to use it if it works...

Ben T.

 

[ApC2Kp]

To btrueblood,
The elliptical section elbow might be possible for low pressure duct work, but there are concerns for moderate to high pressure service along with thermal expansion. There might not be much difference for smaller elbows (<12 inch?) but for larger size elliptical sections I could imagine the ellipse deforminig with pressure toward a circular cross section. The Bourdon effect would cause the angle of the elbow to open up. The flexibility and stresses could be higher - a designer would need to qualify the component for ASME B31 since it is not a referenced standard elbow.
To m177128
Does the elliptical section have to maintain the same perimeter, or does the perimeter increase to maintain the same cross section area? To be effective does the ellipse need to be at least a 2:1 aspect ratio?
Duct work designs have turning vanes to change flow direction and smooth out turbulence. Internal flow vanes like the Cheng flow conditioner would probably be easier to implement in an existing system. If everything had to fit inside an existing elbow, a flat plate in the plane of the elbow would provide some benefit, but not as much benefit as would a curved plate following the centerline radius of the elbow. I could visualize an internal half size / half pipe elbow fit inside existing elbow on centerline radius.

 

[silverlilly1]

I am still looking for more ideas, but for anyone who may find themselves in a similar situation, I have found another product in addition to the Cheng Rotation Vane, called the VOREL 90 degree elbow, which can be found at:

http://www.vortab.com/pdfs/VOREL Elbow.pdf

or the company's main site

http://www.vortab.com/Index.htm

 

[m777182]

To ApC2Kp, Btrueblood and StoneCold:
a good source is a book by Bruno Eck, Technische Stroemungslehre, Band 2 (p55), Springer 1981, ISBN 3-540-03488-9. I think there is a translation into English too.I have copies of data tables, but cannot find them at the moment. I do not know for vendors.
m777182

 

[hacksaw]

less the original question be lost sight of:

the problem is solids build up not turbulence as such

 

[silverlilly1]

Thanks, but the problem is turbulence, not solids buildup.

 

[silverlilly1]

Sorry, let me clarify. The problem of uneven buildup inside the reactor can be solved by removing the turbulence in the inlet. This is why I am working to find ways to remove turbulence.