Re: Variable Feed Rate Problem
Is this a new installation or has this just happened on an old installation.
Is the humidity level where your products are stored constant? Product may be hygroscopic and cause varying flow rates depending on the moisture content. ■
Gary Blenkhorn
President - Bulk Handlng Technology Inc.
Email: garyblenkhorn@gmail.com
Linkedin Profile: http://www.linkedin.com/in/gary-blenkhorn-6286954b
Offering Conveyor Design Services, Conveyor Transfer Design Services and SolidWorks Design Services for equipment layouts.
Re: Variable Feed Rate Problem
The entire process is monitored for humidity, <15% RH, and the conveying gas is Nitrogen. Some of the material is hygroscopic, but final product moiture levels are very low.
This process has been in use for three years now. The first two years the feed rate was a consistent 525 kg/hr. In Dec. of 07 we began seeing the variable rate. Some batches would spike to 700 or 750 kg/hr with little process changes that I know of.
We did add a screen filter in Jan. 08 to the post mill conveying line to capture foreign material, but I was under the impression that this problem must be related to the feeding mechanism.
The feeder screw rpms have been checked and double checked, and the raw material must meet pharma grade specifications, so I am fairly confident the material hasn't changed. ■
Re: Variable Feed Rate Problem
Dear kdsmiller
If a system works well for 2 years and neither material nor settings have changed, then the first thing to check is whether there are some components worn out too far.
As there is a rotary valve in the system, that should be the first item to look at.
The functioning of that rotary lock should also be indicated by the pneumatic conveying pressure.
A malfunctioning rotary lock could show fluctuating pressures.
Specially, when a rotary lock is used with fluidisable materials in combination with a pneumatic pressure conveying system, problems in constant feeding can be expected.
best regards
teus ■
Teus
Re: Variable Feed Rate Problem
The rotary valve and housing on the air lock were inspected and are outside tolerances. Can you explain to me how this increased my feed rates?
I would think the material I'm using would be difficult to fluidize, and want to insure that correcting the air lock will solve this problem. Should I check pressures above the air lock?
Thanks in advance,
Kurt ■
Replace Air Locks And Screw Feeders To Solve Problems
Read this Case History PDF attached to learn how vibratory feeders by JVI can provide reliability and save time and money when replacing rotary air locks and screw conveyors.
See attachment right above my signature at the bottom: PDF: Tube Feeder vs. Rotary Air Lock "info utf vs rotaryal.pdf"
Also read this Case History about the accuracy rate and reliability of vibratory feeders by JVI.
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Teus said: "first thing to check is whether there are some components worn out too far"
You won't have this issue with JVI Vibratory feeders.
JVI Feeders are inherently low maintenance and highly accurate.
Here is a YouTube video demonstrating the accuracy rates of JVI dosing feeders.
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The Dosing Feeder works as well on powders too, see this video for powder materials demonstrated:
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Send your material sample to JVI and we will send you a video like this - we will perform a test on your materials and report the accuracy rates achieved.
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Attachments
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Re: Variable Feed Rate Problem
Dear Kurt,
Looking at your drawing, I understand that there is a screwfeeder, feeding a rotary lock in series.
Underneath the rotary lock exists the conveying pressure.
If the rate increases, then the screw feeder as well as the rotary lock must be delivering more material.
Worn out rotor and screw should reduce the rate.
But, the volumetric efficiency of a rotary lock can be influenced by the conveying pressure. (pressure system)
A higher conveying pressure causes a higher air mass backflow through the rotary valve, preventing enough material to enter the pockets.. Rate reduces
In case the conveying pressure decreases, the volumetric efficiency of the rotary lock increases and the rate increases also.
Therefore I suggested monitoring the conveying pressure and correlating the conveying pressure with the conveying rate.
The next question then would be how can the conveying pressure decrease with a higher conveying rate? As we know that gas pressure, gas flow and conveying rate are related to each other, the pneumatic conveying system has to be investigated.
success
teus ■
Teus
Re: Variable Feed Rate Problem
teus,
I will work on getting the pressure at the airlock for you, but I was wondering if you could explain how the airlock is able to draw more out of the feeder. It doens't seem possible with design the way it is. The feeder operates with a column of material on top of it. I have the feeder set, such that if the material is flowing normally through the feeder I should see a rate of 525 kg/hr. How am I able to get 750 kg/hr. sometimes?
Total cubic meters/min of N2 in the loop: 16.3
Coveying rate of product in kg/hr: between 500 and 750
What pressures would you expect to see at the airlock?
I know the post blender is under vacuum.
Sorry for my ignorance, clearly this problem is out of my field of expertise.
kurt ■
Re: Variable Feed Rate Problem
Dear Kurt,
The 2 feeders are volumetric feeders. (Not mass feeders)
The feeder capacity is given as :
Capacity = displacement * material density * volumetric efficiency
displacement for a screw = rpm * pitch *area
displacement for a rotary lock = rpm * lock volume
material density = material property
volumetric efficiency is a function of
- flow properties of material
- rpm
- airflow due to pressure difference over feeder and clearances (positive or negative)
As you have a closed loop system, the sum of pressure differences around the loop
must be zero. (circle integral=0)
To calculate the pressure at the rotary lock, all the resistances plus the pneumatic conveying pressure drop have to be considered.
To do that requires much more information than you just supplied.
If f.i. the resistance of the nitrogen cooler and the HEPA1 filter is much more than the resistance of the pneumatic conveying line plus dust collector, then a negative pressure underneath the rotary lock is well possible. (May be also increasing the feeding rate)
Is the HEPA1 filter already checked?
Is the nitrogen blower a positive displacement blower or a fan?
Have you already contacted the supplier of the system?
Also look for other operating parameters that have changed since the past.
Diagnosing a system from a distance, based on limited information is always difficult
(but challenging), which of course is understandable.
Success
Teus ■
Teus
Re: Variable Feed Rate Problem
I believe Teus is on the right path with having you check pressure above and below the airlock, especially since you reported the post blender operates under vacuum. Any changes in the vacuum level can dramatically alter the volumetric efficiency of your T60 screw feeder.
However one important issue Teus missed was having you check the airlock venting. Airlocks are commonly installed with insufficient or nonexistent venting. Unless the airlock is properly vented it will simply pass though the vacuum level changes to the feeder.
The T60 feeder has a vertical tube discharge, and should have a removable cap on top of the tube. The next time you have a feedrate fluctuation remove the cap and I betcha the problem goes away.
Semi-related…you can retrofit loadcells into the preblender mounts to convert the T60 into a gravimetric feeder. The weighing resolution won’t be optimal for closed-loop PID control, but will be more accurate than running volumetric. Plus you can then directly datalog the feeding performance.
Regards, Delmar Schmidt
Melfi Technologies Houston
www.melfitechnologies.com ■
Thank You
It appears as if you were correct. The problem in the airlock is transferring pressure to the feeder discharge. Coupling this with a leaking (sucking) product collector gasket was changing the rate the product was going through the feeder. Our feed rate was actually influenced up to 50% by how tight the operators were wrenching the bolts on the product collector.
I still have to run more tests to verify, but preliminary data shows that the two problems combined created the problem. Thanks for your help and giving me insight into our process. I'll report back if anything new arises.
kurt ■
Variable feed rate problem
Hello,
I am a tech at a biopharm mill, we reduce the particle size via pin milling on granular sugar, salt, aminos and vitamins. These components start in a conical blender, then go through a twin screw feeder, a rotary airlock, the pin mill, and are pneumatically conveyed upstairs to a post blender with a filter sock house. The conveying nitrogen is then recirculated back to the mill.
My problem is that I have varying feed rates on different lots of identical components.
Our optimal feed rate would be about 550 kg/hr. We currently vary from 500 to 700 kg/hr. The feeding system is a twin screw feeder with agitator: 60mm diameter, .1dm^3/rev, with a pitch of 1.5mm. The feeder is 23" long. The bulk density of our raw material is .9 kg/L.
I have been asked to solve this problem, but I don't have an engineering background, and have been given no funding or time for an experiment. Any help on possible solutions to this problems or what other data I need to solve this would be greatly appreciated.
Attachments
conmill process drawing (PDF)
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