Dilute Phase Conveying System Design And Bends
Dear Anthony,
Answers to your questions are given below:
1. Replacing the horizontal and vertical lines by a sloped line is not recommended, especially if the slope from the horizontal is more than 15 degrees. Sharper slopes will cause conveying problems because of the resulting reflux.
2. The present plugging problem may be a result of increasing the pipe diameter from 8 to 12 inches two times. Your proposal to use only an 8 inch diameter line will solve this problem because the conveying velocity will not decrease along the line.
3. You can use "lined" braided metal hose instead of bends provided you have spare pressure available from the blower. Pressure drop in these kinds of hoses is higher than in metal pipe.
Regards,
Amrit T. Agarwal
Consulting Engineer
Pneumatic Conveying Consulting Services
Email: polypcc@aol.com
Ph and Fax: 304 346 5125 ■
Alternate Idea
Amrit,
Do you think it would be better to just replace everything with two 90 degree bends? Horizontal to vertical all the way down then horizontal over to the collector using 8 inch pipe all the way. Should I use larger radius or smaller? I also noticed literature on Tee-Bends. Any info on that?
I realize that the changing in diameter is causing problems and I am planning on fixing that. I have just read that a sloping line is better then a horizontal line.
Thanks also for sending the PDF on your dilute phase theory a couple of weeks ago. It was very helpful.
Anthony ■
Short System
There is no downside to a downward slope in the short system that you described. Also, because the pressure is low there is no compelling reason to step the conveying pipe.
Although in short systems liberties can be taken with layout, two important conveying parameters can not be ignored, i.e. solids loading and conveying velocity.
You did not mention the solids conveying rate but in view of the tendency of this material to pack I would recommend a solids loading of 2.0 or less. Then in a straight 8" system you could expect a conveying capacity of 15,000 to 20,000 lb/hr at a terminal velocity of approximately 5,000 fpm.
This expectation is based on very limited knowledge of your material.
Dennis Hauch, PE ■
Thanks Dennis
Dennis,
The solid loading is < 1 and we are only looking for a rate of around 400 kg/hr roughly 880 lb/hr (due to other parts in the system). The velocity expected is to be around 15 m/s (roughly 3000 fpm). We are dealing with a limited compressor and I'm trying to make the best of it (conveying with nitrogen not air).
Thanks a lot,
Anthony ■
Angled Pipe
To all readers,
I've been reading negative comments on using downward sloped lines but before I found this site I read this article saying it was better to have angled lines (last paragraph in article). If required velocity is max during horizontal runs and min during vertical runs then one would think that anlged would put you in the middle. Also does conveying upward or downward make a big differenece with using sloped lines. I've read the negative comments for using sloped lines to convey upwards. My situation is forced to convey downwards.
http://www.chemicalprocessing.com/ar.../2005/454.html
Anthony Davis ■
Response
The article says that horizontal conveying requires the highest velocity. As long as the conveying layout includes a significant horizontal component you must convey at the horizontal conveying (saltation) velocity as a minimum.
There is a difference in conveying sloped upward or sloped downward, the first should be completely avoided, the second is not the preferred choice but is possible in a very short system like you have.
Dennis Hauch, PE ■
Thanks Dennis
Do you think that it would be better to just go vertically down then horizontal over or horizontal first then down or sloped? I only mentioned having a sloped line becuase of a short distance and figured the required velocity would be less, solving our lack of a proper compressor.
Thanks again,
Anthony ■
Reply
My suggestion:
1) Calculate the saltation velocity for your conveying application. Add a small safety margin to the saltation velocity, say, 5%.
2) Size the conveying pipe based on the conveying velocity obtained above, and in accordance with the available blower capacity (volume).
3) Run the conveying pipe any way you like. My only suggestion here would be to use hard pipe as opposed to flexible pipe.
Dennis Hauch, PE ■
Dilute Phase Pneumatic Conveying design and bends
updated 8-11-05
The current system that is installed consists of a horizontal 0.5 meter line, 90-degree bend to vertical for roughly 1 meter, 45-degree turn to a 1.5 meter pipe then finally a 45 bend to a 1 meter horizontal (conveying downward). The product being conveyed is a powder substance with a bulk density of around 320 kg/m^3 and can compact very easily once a plug occurs.
There must be a foot long horizontal inlet to the pipe and outlet of the pipe. This can not be changed but everything else can. I was wondering if changing to a straight angled line directly from the inlet horizontal to the outlet horizontal would help throughput and stop plugging. This would shorten the horizontal lengths and lessen the number of bends but would get rid of the 1 meter vertical line. Is this a good idea? Also the pipe configuration now changes diameters a couple of times ranging from 8 inch to 12 inch. We are choosing an 8 inch diameter pipe the whole way except for the 12 inch outlet, once again, is fixed and can't be changed.
One last thing, the two bends that will be used, we are thinking of making those steel flex hose because of the tight area we're working with, ease of cleaning out, and need to relieve tension/weight in the line. Is this a bad idea? I have read that flex hose used for bends should be avoided. Because the product being used essentially coats whatever line it is conveying through, I thought the flex hose would act as a smooth pipe once the coating was made.
attached is a picture for visual aid
Thanks,
Anthony
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