Re: Sub-Sonic Or Super-Sonic Eductor
Dear David,
In pneumatic conveying, it is standard to have a constant mass flow of air.
That ensures a stable pneumatic conveying process whereby the pressure is directly related to the SLR (Solid Loading Ratio).
This requirement is met when the pressure ratio over the nozzle throat is super critical (or supersonic).
This is applied in a pressurized air system
When using a dedicated positive displacement blower, the nozzle should be sub sonic, to prevent the blower bleeding off air due to a too high pressure.
1, The air speed after at the throat can be > March 1, so more static pressure is converted to kinetic energy, does this help increase the material/air ratio? or energy efficiency
The material/air ratio (SLR) is the ratio between he mass flow of material and he mass flow of air.
The SLR is, in case of a constant mass flow of air, only depending on the material flow (Feeding rate)
Energy efficiency is standard low in a venturi-eductor system.
2, Since the air speed at feeder throat is higher, breakage of the feeder is less likely to happen?
I do not really understand this question.
It is clear that the acceleration can be quite high, due to the high velocities.
3, material to be conveyed is fine calcined alumina with APS: 1.5 microns, so the wear on the venturi throat and powder degrading will not be a problem?
The nozzle itself is not effected by wear as only clean air passes the nozzle.
The diffusor (where the kinetic energy of the supersonic air is converted in material velocity and conveying pressure) is effected by the material and is therefore often made from wear resistant material and also replaceable.
I think a Roots blower with output pressure about 50kpa gauged and sufficient mass flow rate can speed up the air to March 1.4 at nozzle exit and the static exit pressure is about 45kpa, which is much lower than ambient/back pressure.
This would give a pressure ratio of 1.45/1.5 = 0.967, which is sub-critical.
This example requires an absolute pressure of -0.2 barg at the nozzle exit resulting in a pressure ratio of 0.8/1.5 = 0.528 (critical pressure ratio)
When the exit pressure of the nozzle drops below the critical pressure ratio, the air mass flow does not change anymore.
That is why it is called critical.
1, Is back pressure ambient 101.3k? i think its above ambient becuase the material above the throat and the downstream piping.
The impulse of the air leaving the nozzle is shared with the air itself and the fed material with air.
The velocity of the air is converted into pressure in the diffusor to a level that is matching the conveying pressure of the subsequent piping system for the feed rate at that moment.
A system balancing problem.
2, Is low nozzle static exit pressure a problem? In Mr. David Mills "pneumatic conveying guide" says it should be ambient or just below ambient(conveying nozzle), but for CD nozzle's so called 'design condition', if the exit pressure = the ambient and no shockwaves, the supply side should be very high? So i think the only possibility to use cd nozzle for air conveying application is to let the shock happens outside the nozzle(which might help dispersion of the powder).
Mr. David Mills is right.
Designing a pneumatic conveying system with a venturi-eductor is not simple because the conveying characteristics must match the venturi-eductor characteristics and the compressor characteristics.
Have a nice day ■
Teus
Still Confused, Nozzle & Pd Blower
Dear Teus,
Thank you for your reply, that really helps. Sorry for my late reply, i have to understand something before i can ask something. The most difficult part to me:
First, is to understand how the motive air nozzle works, especially the pure convergent nozzle which, i believed, is used on powder conveying eductors. Unfortunately, most books fail to describe this clearly, expecially the relationship between throat pressure, exit plane pressure, and backpressure.
According to the books, to my understanding, for example, if inlet pressure is fixed at 150Kpa abs, assume backpressure to be atmosphere(101.3 kpa abs), then, a convergent nozzle can never choke whatever how small the throat size is. Because the throat pressure is always equal to backpressure which is the atmosphere pressure, and backpressure/inlet pressure is 101.3kpa abs/150kpa abs=0.675 which is >0.528. If the above is correct, then the nozzle exit pressure will be atmosphere and no negative static pressure is generated. IF the static pressure of the air stream leaving the nozzle is not negative(below 101.3kpa abs), how can the flow entrain powder?
Or the entrainment happens at venturi throat, not by the air stream at nozzle exit?
Second, the PD blower provides a constant flow, the pressure it supplies relies on the nozzle size(system resistant), how can i make sure its 150kpa abs(i'm sure it will not be), then i have to increase or decrease the rotating speed, but the air supply will also change accordingly. There must be some connections nozzle size, static pressure, PD blow flow rate, etc, and I feel that i'm almost there, but finally find that i'm still in a loop.
can anyone give me a standard procedure of developing this kind of system and necessary formulars and parameters such as flow coefficient etc that i can actually follow and get a final solution? If this will affect any vendor's interest, please email me at zche011@hotmail.com.
Many thanks!
David ■
Re: Sub-Sonic Or Super-Sonic Eductor
Dear Teus,
Thank you for your reply, that really helps. Sorry for my late reply, i have to understand something before i can ask something. The most difficult part to me:
First, is to understand how the motive air nozzle works, especially the pure convergent nozzle which, i believed, is used on powder conveying eductors. Unfortunately, most books fail to describe this clearly, expecially the relationship between throat pressure, exit plane pressure, and backpressure.
According to the books, to my understanding, for example, if inlet pressure is fixed at 150Kpa abs, assume backpressure to be atmosphere(101.3 kpa abs), then, a convergent nozzle can never choke whatever how small the throat size is. Because the throat pressure is always equal to backpressure which is the atmosphere pressure, and backpressure/inlet pressure is 101.3kpa abs/150kpa abs=0.675 which is >0.528. If the above is correct, then the nozzle exit pressure will be atmosphere and no negative static pressure is generated. IF the static pressure of the air stream leaving the nozzle is not negative(below 101.3kpa abs), how can the flow entrain powder?
Or the entrainment happens at venturi throat, not by the air stream at nozzle exit?
Second, the PD blower provides a constant flow, the pressure it supplies relies on the nozzle size(system resistant), how can i make sure its 150kpa abs(i'm sure it will not be), then i have to increase or decrease the rotating speed, but the air supply will also change accordingly. There must be some connections nozzle size, static pressure, PD blow flow rate, etc, and I feel that i'm almost there, but finally find that i'm still in a loop.
can anyone give me a standard procedure of developing this kind of system and necessary formulars and parameters such as flow coefficient etc that i can actually follow and get a final solution? If this will affect any vendor's interest, please email me at zche011@hotmail.com. We can buy an eductor from vendors, but the price they quote is enough to buy a car in china.
Many thanks!
David ■
Re: Sub-Sonic Or Super-Sonic Eductor
Dear David,
Have a look at:
https://www.google.nl/search?q=sub+c...sm=93&ie=UTF-8
Success
Teus ■
Teus
Sub-sonic or Super-sonic Eductor
Hi all,
We are to use a venturi feeder under bulk bag unloader.
There are two options of the nozzle: converging only or a laval nozzle.
question is that:
1, The air speed after at the throat can be > March 1, so more static pressure is converted to kinetic energy, does this help increase the material/air ratio? or energy efficiency
2, Since the air speed at feeder throat is higher, brokeage of the feeder is less likely to happen?
3, material to be conveyed is fine calcined alumina with APS: 1.5 microns, so the wear on the venturi throat and powder degrading will not be a problem?
I think a Roots blower with output pressure about 50kpa gauged and sufficient mass flow rate can speed up the air to March 1.4 at nozzle exit and the static exit pressure is about 45kpa, which is much lower than ambient/back pressure.
1, Is back pressure ambient 101.3k? i think its above ambient becuase the material above the throat and the downstream piping.
2, Is low nozzle static exit pressure a problem? In Mr David Mills "pneumatic conveying guide" says it should be ambient or just below ambient(conveging nozzle), but for CD nozzle's so called 'design condition', if the exit pressure = the ambient and no shockwaves, the suppy side should be very very high? So i think the only possibility to use cd nozzle for air conveying application is to let the shock happens outside the nozzle(which might help dispersion of the powder).
These matter really confused me, please help.
Many thanks!
David Chan ■