screw conveyor that does not convey 3

[sacem1]

I have built and installed a 6" nominal diameter screw conveyor with troughs in U shapped steel body.

Lenght of screw conveyor is 20' and its set at a 60º angle with the floor, has a single screw hanger at midpoint and a flat cover.

The screw has to convey ground to mesh 25 sulfate amonia (the same as used in plant fertilizer) at a rate of 1,000 lbs/hr dosification at the inlet by rotary valve which discharges the ciclone output from the hammer mill, discharge at height is completly open and feeds a huge tank with no air presure as it is an open discharge. The ground product has a density of 1.4/1.6.

The screw is operated by a 2 HP @ 1750 rpm driving the screw now at 175 rpm trough a doble reduction (belt/pulley and chain/sproket)there is no scratching sound working unloaded and the screww turns effortlessly if tried by hand.

Well up to now for specs, the problem is this Conveyor Screw DOES NOT CONVEY, it raises the amonia dust from the loading chute and raises it up just to a point that is about one foot from the hanger and thats it nothing goes further up, if you keep feeding more ground product it will absorb the additional material and afterwards begin raising the feeding chute until it spills over, put does not send the product further up.

The screw was turning at about 88 rpm's we have doubled the screw turning speed (175rpm's) to no avail. I've made a special cover that is like a semicircular shroud that makes the U shape trough function like it was of round section but the only thing we get is that the product packs at the beggining of this shroud and does not raise the product, the shroud is 4' long and was put just before the hanger, the idea was to make the screw force convey like an expeller until the ground product is reached by the second half of the screw conveyor but it did not worked, we moved it down to the first section it did not work either, really I do not know what is happening.

We rechecked the hanger size it is less than 6% of the trough section so with a load that should be less than 25% of actual capacity it should not be the culprit.

PLEASE HELP my customer wants to kill me but in 20+ years of buiding screw conveyors that work even upright I've never been met with a Screw Conveyor that does not convey.

Hope someone has the answer out there.

 

[DReimer]

Is it elevating the product 1 foot past the hanger bearing, or is it stopping short of the hanger bearing?

The problem with hanger bearings is you've got a short section of conveyor with no screw, and thus no driving force. You rely on the material coming in behind to push material in the "dead zone" through to the next screw flight. It seems the material you are conveying is not able to manage the transition. I'm no bulk solids conveying expert, but I suspect it might have to do with a combination of angle of repose and fluidity of the powder.

In hindsight, this might be an application for a flexible screw or some type of tubular drag chain system, or a pnuematic delivery system. Of course that does not solve your problem.

Here is an idea that might work, but at some expense. Could you split the screw into two sections? i.e. the first section, say 12' long discharges into a feed hopper for a second screw, maybe another 12' long that discharges into the tank. It means two troughs, another hopper and motor, and a bunch of design, but I suspect it would work if your hangup is due to the hanger bearing. Or, you could try lining the trough with a tough plastic like Tivar and installing a single screw (or two sections coupled together). The liner would function as your bearing surface.

 

[sacem1]

Thanks for your good intentions but the problem is that the damm dust does not reach the hanger section thats why I'm so baffled by this problem.

Maybe a screw with a short pitch could be the answer but I would like to know if any one can give me an idea of why this is happening.

thanks again

SACEM1

 

[GregLocock]

I bet its got something to do with friction. Can you run it at a reduced angle of lift?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sacem1]

Hello GregLocock

Nope it is a tight fit and can't change that

Cheers

SACEM1

 

[GregLocock]

As a trial, stick sandpaper to the trough or the blades of the auger.

Are these ammonia pellets a product you have experience with? I've only ever done grain and chopped straw and sawdust.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[agthompson]

I have seen the same result with materials that want to stick to the screw conveyor flights--especially when the conveyor is inclined. I am not familiar with sulfate amonia, but reducing the pitch of the flights (and running the conveyor faster) can help--or coating the flights with a lower friction coating.
What is the screw pitch? Is the pitch on the screw uniform and are the flights well formed?

 

[zekeman]

Why don't you run as simple test. Get a crossection of the screw conveyor in sheetmetal and load it up with material. Then start inclining the angle until it starts to either slide on the sheetmetal or lose the compactness due to shear forces exceeding the holding shear in the material. In any event one or both of these will occur. If sliding at the surface occurs first then the coefficient of friction of the material against the surface is equal to the tangent of the inclined angle. Chances are this angle is less ths than the sum of (90-60) degrees+the helix angle (in operation that angle is actually 30+/- the helix angle and I have taken the worstcase +) and steps to increase that coefficient should be taken. At 60 degree incline you must have a coefficient of at least .57. If it is the shear problem then the height of the material in the conveyor would have to be reduced to accommodate this.

 

[arunmrao]

Will it help if you add a vibrator along the flow patth?

 

[sacem1]

To GregLocock: We have raised the whole pellets but not the ground ones which turn into a very fine dust. We have screw conveyors working for more than 20+ years in Peru conveying from lime, to cement, feedstocks, soya beans, corn, milled corn, wheat, flour, powdered milk, and I don't remember what more and never had this problem, the design is standard, we don't hyave to reinvent the wheel to manufacture them but....?

To Agthompson: Our standard screw goes with what is called a square pitch that is the flight diameter and the pitch are equal, the flights are very smooth in transition, the pitch is very exact as we lathe turn the disks inside hole and outside diameter before assembly of the screw conveyor.
As told before we have raised the speed (to 140 rpm right now, originally 70 rpm) but to no result.
I don't undesrstand how coating the surface to REDUCE friction might help I think it will worsen the situation.


To Zekeman: I'm going out to the customer plant to make a test out there by your suggestion and I'm taking with me a short exact piece of the screw conveyor (actually a 2' piece we cut of at manufacture because they changed the length before delivery) to make the tests.

To arunmrao: I think putting ANY vibration device would cause the product to fall back in ANY conveyor because you would be loosing adhesion (friction) on the flight surfaces that do the conveying.

Right now I have just received another feed back from the customer plant: By raising the speed the height that is reached by the product is less than half of what was achieved at the lesser speed, so it has just ocurred to me, that maybe we are dealing of an aereation phenomena, that is the product is fed to the screw and the flights mix it with air so that it behaves like if it was in a fluidized bed, and we know that fluidizing beds can only convey DOWN never UP, I'm going to the plant and opening the whole screw conveyor top and turning by hand (really will be very low speed = no aereation) will see if the product can be conveyed to the top, if so we have the solution, if not . . .

I'll keep you posted

Thanks to all

SACEM1

 

[Metalguy]

Is it possible that your product is "slippery" and is being thrown outward via cent. force, causing it to fall back down the OD of the screw?

 

[griffengm]

Sacem,
You state that the conveyor is running in a U-trough which to me means that there is a fair amount of open space on one side (top?).
You also state that the incline is about 60 deg..
At this combination, what is the angle of pitch on the screw relative to the ground?
I can envision a situation where the material will move a little but then slide from the screw to fall back along the top. Experience with fertilizers and handling systems tends to reinforce the impression.
Try putting the screw in a round tube to reduce bypass? Change the angle of the screw pitch? use a series of conveyors to reduce tha incline?
As a quick test, form some light sheet metal into a u-cap and set it over the lower section to see if the material lifts higher.

Griffy

 

[GregLocock]

SACEM1, while you are at the plant check the angle of repose of the dust, and the coefficient of friction on steel. I'm not surprised that speeding the auger up made things worse.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sacem1]

This a situation report:

I went over to my customer plant and made a test with the top removed from the U shaped trough and hand turning it and found that my theory on posible aeration was completly wrong, the product simply did not try to "foam" or be in a fludized state, it simply STICKS to the flights, Agthompson, you were and are RIGHT the problem is the product sticks to the screw flight and acumulates on top of it forming a continuos cone that starts at practically the edge of the flight and ends at the intersection of the flight above it with the central tube this creates a completly slanted surface that runs in an angle that throws the next product out and backwards to the rotation of the flight thus NO ADVANCE OF PRODUCT, the little material that manages to go through just goes to form the buidup on the next flight, by the time all the screw is loaded with this material (up to now it has never reached that far up but maybe if it is fed enough material for enough time it will) you only find that the material is thrown out and backward.

My initial data was not correct in this the screw conveyor is set at a 41º angle with the floor, the flights make a 29º angle with the floor (horizontal plane) and the surface formed by the material stuck to the flights increases that flight angle to a whooping 63º with the horizontal, if you consider that when the conveyor screw is set vertical the working surface of the flight makes a 19º angle then we know why this Conveyor Screw does not convey.

To keep you all informed the material sticks so quickly because it is milled in a hammer mill and conveyed to the feeding spout of the screw by a neumatic transport system which discharges by a rotary vane valve. (By the way that was not designed or supplied by us we just sold the screw) but now we are going to modify the other supplier mill and conveyor so that it discharges directly at the requiered height and we are taking back the screw conveyor.

We might have made the conveyor work by making another screw at half the diameter pitch and enclosed in a tube rather than in a U shaped trough but this people need to clean all the setup every day an cleaning a tube enclosure is neither easy nor fast so we might say we threw the towel on this one.

Add to our global experience that if you have to convey sticky material you might end up with something I had up to now not found a screw conveyor that does not convey.

Thanks to all.

SACEM1

 

[PeterCharles]

here's my input -
1) you don't run a screw conveyor in a U trough at a 60 degree incline or even a 41 degree incline
2) if you need the conveyor to run at these inclinations you need a tubular housing.
3) you also need the flight pitch less than the diameter.
4) the design of these machine is very specialised and can cause you a lot of heartache if it's the first time you have tried it.
5) I suggest you try the bulk-online web site and use its forums. This site is for bulk handling of materials. Suggest you use the "Ask Lyn" forum or "Solution for Screws...." forum

http://www.bulk-online.com/Forum/index.php

The people who visit this web site are experienced materials handling engineers.

 

[IRstuff]

One reason that some materials work is that flow agents such as silicon dioxide are often added to minimize clumping.

TTFN

 

[sacem1]

Thanks PeterCharles for your post:

Yes, you usually do not use a U shaped trough on an incline put customer asked for it and bought it with out telling us it would be at an incline, we met with that set up when he called for modification of the inlet/outlet ports. We have always given a customer a tubular screw for inclined working units.

We have found that smaller than "square" pitch flights are only needed for certain materials but we have a lot of units working vertically with same pitch as diameter (square) they are conveying feed stuff for small livestock feed mixers, flour mills, etc.

In coveying portland type cement we have found that the pitch at the inlet works better if we use double flights (thats two screws on the same shaft starting at 180º one from the other) but still the conveying is done by square pitch and we work at 60º+ many times specially in small portable loading units for concrete mixer trucks, where you fill the cement from a small bulk silo into the trucks.

We have been manufacturing screw conveyors for more than 20 years now and we think that the fabrication work is up to par with most international suppliers but this was, and is, a particular situation that we had never met with and thats the reason of the inquiry.

However thanks for the forum data I allready registered there but it does not seem to have the activity that a thread get here.

Regards

SACEM1

 

[sacem1]

Here is the final update on this thread:

We had almost surrender to a not conveying screw conveyor because of the sticking problem in the flights of the screw however we made a final test:

The product sticks because it gets humidity from the damp air so why not correct this.

The Amonium Nitrate was being grinded in a hammer mill with a 0.5 mm perforated screen,because of this the product stayed a long time in the hammer mill camera where it received a lot of contact with humid air, so we tried a 3 mm perforated screen wich gives a slightly coarser ground product but fine enough to dissolve quickly, with that ...

In order to increase the rate of elevation that we needed the speed was doubled for the screw, we did not needed to increase the power of the electric motor.

IT WORKS the product passes through the hammer mill at a higher rate of production, which makes it less humid and it goes into the conveyor at a greater volume and gets conveyed up in the 41º inclined slope at the requiered rate and it is working fine with the customer satisfayed,

So the final experience will be:

If it sticks because of getting wet try to reduce the causes of the water absortion

If it gives problems because it is ground to fine try a less fine grinding process

If it gives problems climbing the slope increase the screw turning speed

And hope that it works with that type of conveyed material.

Regards

SACEM1