Re: Airflow Rate Control Via Nozzles
Dear Kevin,
You are referring to sonic chokes, I believe.
Sonic chokes (eductor-venturi) in vacuum are rather complicated solutions and should be avoided in pneumatic conveying.
The sonic choke can, theoretically, used underneath the hopper as well.
Both solutions use excessive energy and only used for small material flows.
A rotary valve is the simplest solution. ■
Teus
Multiple Lines With Sonic Valves
Dear Kevin,
As your system is working in vacuum, the using of CD valve is limited given the operation curve of the ventilator.
Loading the line with product, the added DeltaP is displacing the operation point of the ventilator by decreasing the air flow.
Actually, to overcome the extra pressure required, the fan is converting dynamic pressure in static pressure.
The consequence is that the air flow is decreasing by a ratio larger/smaller than 1 to 1, i.e. for each 1 Pa "conssumed" of dynamic pressure is recovered less than 1 Pa as static pressure.
It works very effective in overpressure systems using either compressor or blower compressor.
In this case, the air flow amount is not affected (or in a very small variation) by the pressure resistance of the system, so you can keep the same air amount doesn't matter the system pressure head variation (within the limits of the system).
Years ago i have designed and applied this solution for 3 lines for to convey the finished products from a flour mill.
And the system works, doesn't matter all 3 three lines are loaded at different capacities or only one line is loaded.
In any of the case, the total amount of the product to be transported is constant.
Returning to your injector, even using sonic valves, as Mr. Teus already pointed out, it will not work/operate efficient, you will need an air-lock.
Regards,
Tanase TANASE ■
Multiple Lines With Sonic Valves
Originally Posted by Tanase
As your system is working in vacuum, the using of CD valve is limited given the operation curve of the ventilator.
Loading the line with product, the added DeltaP is displacing the operation point of the ventilator by decreasing the air flow.
Actually, to overcome the extra pressure required, the fan is converting dynamic pressure in static pressure.
The consequence is that the air flow is decreasing by a ratio larger/smaller than 1 to 1, i.e. for each 1 Pa "conssumed" of dynamic pressure is recovered less than 1 Pa as static pressure.
The idea of this sentence is that:
- having a module of compression/vacuum ratio smaller than 1,4:1,0 you will not reach the sonic regime
- as mentioned here above, decreasing the air flow you are approaching normal vacuum operation system and the sonic valve will not operate at the required level
- due to this, the air flow amount will balance between the lines, according to the pressure head of each line (function of mixing ratio).
- The lighter lines will "steal" more air than provided in the calculation while the heavier lines will have less air flow
- this is because the total air amount of the ventilator is constant
Hope to be helpful.
Have a nice day,
Tanase TANASE ■
Re: Airflow Rate Control Via Nozzles
Dear Dr. Tanase,
Thank you for this second post it really helped me understand what you meant. It was my concern that not reaching sonic velocity would result in some lines "stealing" air from others and ultimately not really controlling conveying velocity. The reducers we have installed for now in some of the lines seem to help as we simply have too much airflow otherwise. However, they might lead to problems in the future because of this concept.
Would you have suggestions on how to control a central vacuum supply? Here is some context on my current situation.
- 6 different blowers in parallel which aim to keep a setpoint a bit further down the common line to a set pressure (currently -5psig). The blowers are on VFDs. The number of blowers running is controlled depending on how hard/easy it is to reach the setpoint (this is my supply)
- Multiple hopper/loaders that open/close a vacuum valve to pull their material from different locations. (this is my demand)
- My central vacuum system (supply) has no knowledge of the demand in the plant it is entirely based on the pressure it sees and tries to maintain the setpoint varying 1 or multiple blowers to reach it.
- As you can imagine this results in multiple problems, mainly erosive wear of bends and piping and a lot of dust as I am not controlling the speed of my conveying because I am not controlling the airflow generated by my blower(s)
As you can see I am a bit in a pickle with this system... The goal is not to control perfectly each conveying line, but at least reduce the speed enough to reduce erosive wear and dust.
I had found something like the Burkert Type 8750 flow rate controller but not much else.
If you'd like I could provide a small schematic if it can help.
Thank you very much! ■
Re: Airflow Rate Control Via Nozzles
Dear Kevin,
As I understand now you made a constant vacuum source at a vacuum of –5 psig.
Then you installed reducers in the separate vacuum lines to control the separate lines airflows.
As it impossible to operate the reducers super critical the airflows are depending on the line pressure drops when conveying.
To control the line pressure drops you must control the separate line feeding by a rotary valve
Then it might be possible to get a more or less stable system. ■
Teus
Re: Airflow Rate Control Via Nozzles
Dear Teus,
Thank you for your feedback,
Keven ■
Re: Airflow Rate Control Via Nozzles
Dear Keven,
You are welkome.
Glad to have helped you abit ■
Teus
Once the line is plugged,…
Once the line is plugged, only solution is to remove the plug. Formation of a plug must be avoided. Plugs are formed due to insufficient conveying velocity at the pick up point. If the conveying line is vertical at the pick up point, a much higher velocity is needed to maintain solids entrainment in the air flow. Thats why a well designed conveying line has a long horizontal segment in the beginning.
■
Airflow Rate Control via Nozzles
Hello all,
There is a paragraph in David Mills', Pneumatic Conveying Design Guide (3rd edition) that I do not quite get. Chapter 10 - Nozzle types, p. 245
The preceding analysis applies to either convergent–divergent or to convergent nozzles. For convergent nozzles, however, the range of operation is limited to downstream pressures less than 52.8% of the upstream pressure, that is, below the critical pressure ratio. With convergent–divergent nozzles, this range can be extended significantly, and for a well-made nozzle, it can be as high as 90% of the upstream pressure, with little deviation from the predicted flow rate.
I do not quite see how a CD nozzle can control with only a pressure drop of 10% across it (or am I reading that wrong). I get that under critical pressure condition (52.8%) the nozzle is choked and that the velocity at the throat reaches maximum velocity (Vsonic) hence limiting the mass flow rate.
If anyone can enlighten me on this or provide additional references.
For a bit of context, I've been tasked to control a central vacuum supply which is completely out of control and destroying conveying lines... We've tested some convergent nozzles which seem to help a lot in reducing conveying speeds. However, my concern is that I am not choking my nozzles while conveying as for example a specific line has a pressure drop of about 3psi from atmospheric pickup to the loader while my vacuum supply is around 5psi. If the line runs without material, then the pressure drop is much less effectively choking the nozzle but this isn't really useful other than limiting the consumption of vacuum if ran empty for some reason.
Thanks a bunch!
Keven. ■