Re: Pickup Direction Optimisation
Dear Keven,
As my conveying cycle ends, the plastic pellets in the vertical pipe drop back down in the flexible bend. As the next cycle begins, my system is unable to overcome this plug in the bend and so my hopper overflows.
If you can complete one cycle, it must be possible to complete more cycles.
For now, the system was configured to completely empty the line at each cycle, eliminating this problem, but this wastes conveying time as the emptying of the line is not super efficient from a loading ratio perspective.
I understand that completely emptying the conveying line is working.
The question I have is: Do you use a rotary valve to feed the pipeline?
In that case, the airflow is maintained all the time.
Whether there is material in the hopper or not.
If there is no rotary valve underneath the hopper, it is possible that when the hopper is almost empty a material surge is sucked in to the pipe and an overload of material enters the pipe inlet and creates a too high vacuum and the air mass flow decreases too much and the material cannot be kept in suspension and a plug is formed.
The pressure of our central vacuum supply was also lowered from -4psi to -5psi. This seems to help a bit on starting of the conveying cycle.
As you describe it, the vacuum is INCREASED from -4psi to -5psi.
Without a more precise description of the installation and material it is not possible to give you a better answer that this. ■
Teus
Re: Pickup Direction Optimisation
Dear Teus,
thanks for the quick answer.
If you can complete one cycle, it must be possible to complete more cycles.
It doesn't seem like it. I have tried to illustrate what happens on the picture below if my previous explanation wasn't clear. There is no rotary valve. It is simply a pipe running at the bottom of the hopper. At the start of the second cycle, it is as if the pressure required to pull the extra pellets from the elbow is not enough.
As you describe it, the vacuum is INCREASED from -4psi to -5psi.
Yes I get that 
Terminology... Thanks for correcting me.
Thank you,
Keven. ■
Re: Pickup Direction Optimisation
Dear Keven,
There is no rotary valve.
That means that you are operating an uncontrolled pneumatic conveying system.
Because the feed rate is uncontrolled, the Solid Loading Ratio is uncontrolled, and the vacuum is uncontrolled.
If the Solid Loading Ratio becomes too high, the vacuum becomes too deep and the airflow reduces too much, causing the pellets to drop out of the airflow, forming a plug.
The system is choked.
The only way is a (rpm controlled) rotary valve to maintain constant feeding and constant vacuum. ■
Teus
Re: Pickup Direction Optimisation
Dear Teus,
Thanks for the explanation, it makes sense.
I would have a follow-up question concerning solids loading ratio. How can one control it if pulling bulk material from a box for example using a suction nozzle. We have a lot of those in the plastics industry and the application I presented in this post is very similar to that. We are using Lorenz air flow control to adjust the quantity of air to be drawn. However, the quantity of material cannot be controlled. ■
Re: Pickup Direction Optimisation
Dear Kevin,
Using a suction nozzle is also a possibility, but is considered not as accurate as a rotary valve. ■
Teus
Re: Pickup Direction Optimisation
The system shown is very typical of cheap and cheerful small vacuum conveying systems. Rotary valves are never used it them.
Mostly they are running in low velocity dense phase but can be lean phase.
There are number possible solutions.
1. Add a ball valve with a mesh filter after the hopper. The valve is adjusted so that additional air
can get into the pipe which allows to adjust the loading ratio to maintain a set pressure during conveying.
Lower loading ratio will sort out restarting.
2. Other option is to add an automated valve after the hopper and at the end of the conveying cycle open
the valve to purge the line clear then close it before the next cycle. ■
Re: Pickup Direction Optimisation
Dear Dr. Mantoo,
If I understand you correctly, adding a valve just after the hopper, adding air into the pipeline, is in my opinion a simulation of a suction nozzle
Bypassing air from the intake to dilute the mixture.
Am I correct? ■
Teus
Re: Pickup Direction Optimisation
Dear MR Teus
Suction nozzles have different design some do have bypass and some don't.
Regards ■
Re: Pickup Direction Optimisation
Dr. Mantoo,
thanks for your input.
We do use a similar setup to your "valve with a mesh filter" suggestion: https://lorenzproducts.com/product/air-flow-control/
We have simply increased the volume of air. This reduces our loading ratio hence our conveying capacity, but we still are within our targets.
Thanks again and happy new year!
Keven. ■
Pickup direction optimisation
Hello all, been reading a lot on this forum and other references. Could not find a definitive answer to my question so here it is.
I currently have a hopper with a flexible bend going into vertical piping. This less than ideal configuration is there for space and operational constraints.
As my conveying cycle ends, the plastic pellets in the vertical pipe drop back down in the flexible bend. As the next cycle begins, my system is unable to overcome this plug in the bend and so my hopper overflows.
For now, the system was configured to completely empty the line at each cycle, eliminating this problem, but this wastes conveying time as the emptying of the line is not super efficient from a loading ratio perspective. The pressure of our central vacuum supply was also lowered from -4psi to -5psi. This seems to help a bit on starting of the conveying cycle.
My question to you is, am I wrong in assuming that since Chocking Velocity < Saltation Velocity < Pickup Speed, assuming pickupV is high enough, using a directly vertical pipe (pictured in blue on my picture) would help alleviate this problem as the product wont plug in an inefficient bend? As this is the main acceleration zone for the product, I might be missing something.
fyi, the inside of the flexible pipe is smooth
Please advise.
Thank you!
Keven. ■