posted May 02, 2006 07:12 PM            

I see air pockets (bubbles) in the inflow tube from the flowmeter. You can see tiny bubbles sometime going thru the flowmeter. In the infow tube smaller bubbles will sometimes move in the opposite direction of the water flow and merge with other pockets of air to form even larger bubbles and move toward the extruder slower than the greater flow.
I looked up info on fluid dynamics and i have understood how fluids can get wrapped up from flowing smoothly

As i see it. Since the inflow tube is only approximately 3/8 inch in diameter, the air (bubbles) block what little room there is for water to flow without being hindered.
When a part of the mix is dryer as it is pushed against the extruder head, the pressure increases, the ball is pushed out with greater force, the familiar big oblong dryer corn ball emerges and causes the motor amp. meter (amp meter is related to the amount of fine coarce corn grain being extruder fed) to surge above its 70/74 amp setting because of the extra pressure put on the screw and motor.
Also, part of the mix may sometimes be wetter than the norm which will output a smaller ball because of lesser pressure and the amp meter will see the fluctuation in the reverse.

Clearly there is air in the water infow line downstream from the flowmeter. The question is i cannot convince others what i see. You see, the inflow water line used to be copper, so they never noticed the air in the line and so were induced to blaming the grain or the old parts. We normally add a small drip or flow of cottonseed oil to even out the inconsistencies somewhat.
Granted different batches of grain are more difficult and old parts become a issue, but do you tend to agree that a consistent water flow will help output a more consistent ball size even with the notorious single screw chamber inconsistent mixing problems.

Wenger, the company that produces these extruders have been contacted, I am being told that they dont think its a water problem. Wenger has not seen the bubbles that develop in the the line.

posted May 02, 2006 08:07 PM            

I would try to start the machine with some product which was already moistened so a stable flow can be established, and then slowly introduce dry material and water.

A better process design is probably the best solution.

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Best Regards,

Tom Cunningham

posted May 04, 2006 07:34 PM            

As to the bubbles in the line, sometimes some days are worst than others, and different times of the shift more troublesome. You can see the results in the product when multiple bubbles are moving thru the line doing the ballroom dance.
At production start up the line is clear of bubbles because we open up the flowmeter almost full blast to satuate the mixing chamber before the grain so to reduce the event of a lockup should you start too much grain.
The water bubbles in the line later could be due to sanitation using water during their cleaning, the next door plant consuming water, or the overall nature of the water pressure into our plant. But air could be entering into the flowmeter thru the adjusting valve or backing up into the inflow tube from back pressure inside the extruder.
Such a complicated mess.

posted May 04, 2006 09:55 PM            

This is the type of test that is done on mixing processes I've been involved in as a troubleshooting technique.

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Best Regards,

Tom Cunningham

posted May 05, 2006 06:21 AM            

I want to emphasize that the cornballs arnt really that bad as to shock the customer into a fit. We just dont like not having control when the extruder spits out 1 inch long oblong balls and sometimes oval smaller balls intermittently and having to make adjustments all the time to get a imposible happy medium to keep product and packing weights in control.
I work for UTZ Quality foods by the way.

posted May 05, 2006 08:13 AM            

When trying to resolve these problems it is important to think "Inside the extruder" and "Outside the box". Because of the high risk of failure and looking bad, people are unwilling to try new things on extruders. This is unfortunate and messing with extrusion processes is truly for the brave. Hopefully you have some plant management that appreciates taking action, even if it does not work out.

Good luck.

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Best Regards,

Tom Cunningham

posted May 07, 2006 09:04 AM            

I was checking around and i found some useful info i could use to understand that the water may not be all whats causing my problem but you would think that if could help. http://www.clextral.com/763e767f6f93bacbc12567520067ea71-OpenDocument.htm#Q+.How+do+twin

Q .How do twin screw extruders differ from single screw extruders ?
A .The twin screw was developed to overcome the limitations of the single screw extruder, and can be compared to the " next generation " of the single screw unit. With a twin screw extruder, you have the following advantages :

Better control of product flow
In a twin screw extruder, the dual screw action conveys the product through the barrel. In a single screw extruder, friction between the screw and the barrel is used to convey the rpoduct.

Single screw extruders offen experience slippage and surging. Slippage occurs when high pressure in the barrel causes the product to slip between the screws and the barrel wall. The result is that the product does not undergo proper cooking or processing. Surging occurs when the product is held back from proper flow. Presure builds up, and finally, the product bursts uncontrollably through the die. This can result in undercookked and misshapen products.

Better control of processing factors
All the processing parameters : temperature, presure, screw speed, moisture content and flow rate can be varied independently on a TSE. This gives you maximum process control. On single screw extruders, you can adjust the processing factors, but to a much lesser degree.

Better mixing
First, the " figure 8 " mixing style of a TSE offers better mixing than the single twist mixing on a single screw extruder. In addition, TSE's offer a wide range of mixing elements, including intricate mixing discs, reverse screws and compression screws that provide superior mixing. Finally, a TSE can give you more latitude on ingredients than a single screw unit can. A TSE can accept a wide range of particle sizes assuring proper mixing and processing of all.

Twin screw extruders are " self-cleaning "
The dual screws on a twin screw extruder are " self-wiping ", that is, complimentary screw flights from one shaft remove product from the other as they turn. Single screw machines do not have this feature, and are susceptible to dead spots where product not conveyed will adhere to the screws. This " burned on " product can break loose, block the die and plug the barrel.

posted May 07, 2006 09:27 AM            

If designed correctly a single screw or twin screw will run well. If designed poorly, they will run badly.

I like to describe it this way; A single screw is a two stoke engine, simple, cheap to own and run, can give a great bang for the buck, but tempermental. A twin screw is a four stroke engine, more expensive, more complicated, but reliable and dependable.

All depends how you want to run a business. I prefer to work on twin screws, but if I were spending my own money .......

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Best Regards,

Tom Cunningham

posted May 07, 2006 09:32 AM            

Interuptions in flow patterns in a single screw are a great way of getting more mixing. If you want to send me a copy of the screw drawing, I might be able to make some suggestions.

tomcun@ptd.net

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Best Regards,

Tom Cunningham

posted December 13, 2006 02:21 PM            

I’ve been absent for this board but I wanted to give you an update on what has happened.

Last May after consulting with you, the air in the extruder inflow line mysteristly dissipated. I’m happy; the extruder output became more consistent, end of story right?

Through late spring, summer and the fall very little problems visually with air getting into water feed line. The product was very consistently good.

Well, many weeks ago I started noticing the odd behavior of the output again and I noticed more air gradually becoming visual.

Since last spring:
In October a new nose cone screw replaced the supposed worn part and a few days ago they put another Head jacket (over the nose screw), not a brand new one a less worn jacket, because of the awful output we were having. The newer jacket did wonders to the quality of the output probably because of the tighter, better mixing.

Everything is fine. I guess the better mixing and less room for extruciate to slip and surge is reducing the effect of the little missing bits of water entering the mix. But you can still see the effect by observing the Extruder motor amps meters jumping forward, the ball ever so slightly getting larger or inconsistent, then looking toward the inflow water line to see pockets/bubbles of air flowing around and with the flow.

I want to correct and earlier error. I said there was 60 psi of water entering through the flow meter. You said that’s a lot. You’re right. The flow meter indicates generally mili meters, mm`s of water per whatever, seconds maybe, the meter doesn’t say. It may not be accurate and maybe just indicator mark to show where the water is, it can move a few mm`s +/-sometime during production. We generally run from 50mm to 65 mm depending.
Not a lot of water. So I’m thinking if the meter indicates say 60 and the stationary/ moving air bubbles measure as much as the largest occurring, aprox 3 or4mm +/-, then the missing 4mm of water entering the extruder chamber per second causes the mix to be intermittently dryer resulting in the pressure increasing (motor amps surging). Product getting larger.

Now since I noticed the air in the water line dissipating in late spring and increasing again in late fall, could it be the colder water causes the rubber seals or bushings in the flow meter dial knob or whatever to harden and become less flexible thereby sucking in bits of air? We never notice air entering the flowmeter, only exiting.

Well we all have more important things going on this season.
I just wanted to post this.
Thank You
Andrew

posted December 13, 2006 05:11 PM            

Is the water being injectected into the extruder from the top, bottom or side of the barrel?

If the product under pressure where it is being injected?

What is the screw diameter and speed?

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Best Regards,

Tom Cunningham

posted December 13, 2006 07:31 PM            

The corn grits are dropped into the barrel by way of a vibrater pan under grain hopper. Its a consistent flow. Not under pressure.

The screw diameter is aprox 4 inches. There are 3 parts to the screw system seperated by two donuts or spacers which are probably flow interupters all fixed aorund the shaft.
A long screw aprox 1'6" which is the receiver of the grain (aprox 6"downstream water enters), a spacer, another screw aprox. 10', another spacer, then the nose cone screw aprox 10".
So all in all the whole process is about 3 1/2 feet, not much time to throughly mix properly i guess.

The speed i dont know, Rpm i would guess are over 100 per minute.

I dont have diagrams, information on the ancient extruder or am i able to generate images/diagrams of the system.

Just want remind you that i am a lowly operator. The possiblity that management would modify or experiment to improve the efficienty of the old thing is nill.

posted February 15, 2007 07:23 PM            

At 400RPM a short interuption of water flow would likely upset the process. This is something you already know.

By using a nossel at then end of the line, to spray the water, you would likely get a better mix and pressurize the line. Any leaks would result in water going out of the leak and air not being sucked in.

Alternaively you could put a small surge tank in the line and pump water from the bottom of the surge tank, which eliminates bubbles and variations in flow from the plant water line. You might even condition the water temperature while it was in the surge tank.

Good luck.

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Best Regards,

Tom Cunningham