Re: Pneumatic Cotton Unloading System
Dear vishal,
Where do we start?
First of all, you need to assess, whether the raw cotton is suitable for pneumatic conveying.
Determine the raw cotton particle shape and size, raw cotton particle density and bulk density.
Also try to find out (or calculate) the suspension velocity (= terminal velocity) of the raw cotton
Are there raw cotton pneumatic conveying systems in use somewhere?
Then start with the simple calculations for orientation.
From the required conveying rate, the minimum pipe diameter can be calculated from the continuity equation.
Volume(bulk) = Pipe area times 1.5 x suspension velocity
Your mobile grain conveying system uses a rotary lock.
Will a rotary lock work for raw cotton and what would be the size?
Will a cyclone work for the separation of raw cotton?
Then a pneumatic conveying calculation could be started, which will be rather complicated, because the shown fan in the mobile grain conveying system will experience a conveying vacuum and a conveying pressure at the same time.
The difference (or sum if you like) between influences the volume flow of the fan.
A high vacuum or a high pressure may reduce the air flow to such an extent that the conveying process stops and the pipeline chokes.
I do not know how to calculate pressure drop, cyclone size, blower size, number of cyclone to use, flexible rubber pipe diameter, type and size of inlet hood etc. also I am confused about which type of cyclone should I use? 1D3D, 2D2D etc.
All the variables, mentioned by you, have to be calculated and made consistent with eachother.
This requires multiple iterative calculations
That is why pneumatic conveying is simple in principle and so difficult to design.
Success ■
Teus
Re: Pneumatic Cotton Unloading System
Originally Posted by Teus Tuinenburg
Where do we start?
First of all, you need to assess, whether the raw cotton is suitable for pneumatic conveying.
Determine the raw cotton particle shape and size, raw cotton particle density and bulk density.
Also try to find out (or calculate) the suspension velocity (= terminal velocity) of the raw cotton
Are there raw cotton pneumatic conveying systems in use somewhere?
Then start with the simple calculations for orientation.
From the required conveying rate, the minimum pipe diameter can be calculated from the continuity equation.
Volume(bulk) = Pipe area times 1.5 x suspension velocity
Your mobile grain conveying system uses a rotary lock.
Will a rotary lock work for raw cotton and what would be the size?
Will a cyclone work for the separation of raw cotton?
Then a pneumatic conveying calculation could be started, which will be rather complicated, because the shown fan in the mobile grain conveying system will experience a conveying vacuum and a conveying pressure at the same time.
The difference (or sum if you like) between influences the volume flow of the fan.
A high vacuum or a high pressure may reduce the air flow to such an extent that the conveying process stops and the pipeline chokes.
All the variables, mentioned by you, have to be calculated and made consistent with eachother.
This requires multiple iterative calculations
That is why pneumatic conveying is simple in principle and so difficult to design.
Success
Hello sir,
Thankyou for replying to my thread.Shape and size of cotton balls I found is 10mm in diameter also I found the conveynig velocity for cotton is 3500-5000 feet/min. and raw cotton weights around 38lb/cu.ft. and requires suction pickup of 2 in w.c .(data is from cotton ginners handbook). raw cotton density is 1.54-1.56 gm/cu. cm. Although there is no present system to unload cotton from trucks, pneumatic conveying systems are vastly used in cotton gins. and about
Will a cyclone work for the separation of raw cotton?
I think cyclone will work because we only need to transport cotton from truck to warehouse(no need to separate seed from raw cotton).
also
the shown fan in the mobile grain conveying system will experience a conveying vacuum and a conveying pressure at the same time.
what if we use another blower to aid the discharge? or whole separate blower for discharge side only.( means two blowers. one for suction only and other for discharge only).
Thank you 
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Re: Pneumatic Cotton Unloading System
Dear vishal,
Dimension conversion of the raw cotton pneumatic conveying properties:
-size = 10 mm
-Bulk density = 609 kg/m3
-Raw cotton particle density = 1545 kg/m3
-conveying velocity = 17.8 to 25 m/sec (Air velocity)
-suction pickup of 2 in w.c = 50 mmWC? (Pick-up pressure drop?)
As the suspension velocity is estimated at approx. 20 m/sec, the minimum conveying velocity should be approx. 2x 20 m/sec = 40 m/sec.
The raw cotton bulk volume flow is calculated as:
12000 kg/hr / 609 kg/m3 = 19.7 m3/hr # 0.0055 m3/sec
At an assumed SLR=20, this results in an airflow of 12000/3600/20/1.10 = approx. 0.15 m3/sec
At a gas velocity of 40 m/sec, this results in a pipe diameter of approx. 3”.
It seems that the installation could be feasible.
The resulting conveying pressure drops (vacuum + pressure) determine the blower pressure.
It might be possible that the pipe diameter must be increased with the blower size in order to reduce the pressure drop over the blower.
The blower should also displace the leakage air through the rotary lock.
(no need to separate seed from raw cotton).
The cyclone must separate the raw cotton and dust from the conveying air in order to have clean air through the blower.
what if we use another blower to aid the discharge? or whole separate blower for discharge side only.( means two blowers. one for suction only and other for discharge only).
This is a possibility, however, the investment for the unit will be higher.
A centrifugal fan is more forgiving to dust than a positive displacement blower (Roots type)
pneumatic conveying systems are vastly used in cotton gins. and about
If you have conveying data from these installations, that could be a source to verify the usual conveying velocities and the raw cotton losses.
Is the possible crushing of raw cotton in the rotary lock a problem?
Success ■
Teus
Re: Pneumatic Cotton Unloading System
Hello sir,
Thank you for reply.
suction pickup of 2 in w.c = 50 mmWC? (Pick-up pressure drop?)
yes. pickup suction of 2in. w.c .
and yes I also think centrifugal fan is most suitable. Unfortunately I am unable to gather any conveying data from installations 
.
Is the possible crushing of raw cotton in the rotary lock a problem?
yes this will be a huge problem. No damage should be done to seeds.
I think we have all the necessary data now where should I begin? my main confusion is, is cyclone dimensions really necessary? what if I make cyclone by assuming its diameter(as all other cyclone dimensions can be calculated from the barrel dia.) but then question arises which type of cyclone should be used? 1D3D, 2D2D etc. and what about pressure drop across the cyclone?
Thanks ■
Re: Pneumatic Cotton Unloading System
Dear vishal,
yes this (crushing of raw cotton in the rotary lock) will be a huge problem. No damage should be done to seeds.
Is a rotary lock then still viable?
The rotary lock is a significant part in the design.
An alternative lock between the vacuum part and the pressure part will change everything.
Maybe a batch system with a valve is an option?
As the raw cotton seed seems to be so friable, is pneumatic conveying at such high velocities still an option?
Start with a preliminary drawing of the installation with a 6”pipe line.
Select a fan blower with the right amount of air (6”- 40 m/sec) and 0.5 bar pressure drop.
Then you have an idea how the installation could look like.
From there, start calculating (estimating) the vacuum and the discharge pressure.
Check the fan airflow against the fan pressure drop (vacuum + pressure).
Fan air flow and fan pressure drop should match with the calculations and the fan curve.
This certainly needs iteration.
Do not forget air leakages and additional pressure drops in the system.
F.i. the rotary leakage is a function of the pressure drop over the rotary lock (vacuum + pressure)
Concerning the cyclone, there are numerous articles about cyclones.
Success ■
Teus
Conveying Cotton
Dear Teus,
I think 40 meters/sec pick-up velocity is too high for cotton. It should be about 20 to 25 ft/sec.
Best regards,
Amrit Agarwal
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Originally Posted by Teus TuinenburgIs a rotary lock then still viable?
The rotary lock is a significant part in the design.
An alternative lock between the vacuum part and the pressure part will change everything.
Maybe a batch system with a valve is an option?
As the raw cotton seed seems to be so friable, is pneumatic conveying at such high velocities still an option?
Start with a preliminary drawing of the installation with a 6”pipe line.
Select a fan blower with the right amount of air (6”- 40 m/sec) and 0.5 bar pressure drop.
Then you have an idea how the installation could look like.
From there, start calculating (estimating) the vacuum and the discharge pressure.
Check the fan airflow against the fan pressure drop (vacuum + pressure).
Fan air flow and fan pressure drop should match with the calculations and the fan curve.
This certainly needs iteration.
Do not forget air leakages and additional pressure drops in the system.
F.i. the rotary leakage is a function of the pressure drop over the rotary lock (vacuum + pressure)
Concerning the cyclone, there are numerous articles about cyclones.
Success
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Re: Pneumatic Cotton Unloading System
Dear Amrit,
I estimated the required gas velocity in relation to the (local) suspension velocity.
The idea is, if the gas velocity equals the suspension velocity of a particle, the particle would not be accelerated in a vertical pipe line.
In this case, the drag force equals the gravity force.
Hence, the gas velocity must be higher than the suspension velocity.
If the gas velocity <= to the suspension velocity, there will be no pneumatic conveying.
The suspension velocity of a particle is calculated as the terminal velocity:
v-suspension=SQRT{(4*d*materialdensity)/(3*dragfactor*gasdensity)}
For the raw cotton, with the given data:
v-suspension=SQRT{(4*0.01*1545)/(3*0.05*1.15)}
v-suspension= 18.9 m/sec
A soybean of 6 mm has a suspension velocity of approx. 12 m/sec.
When was working in the port of Rotterdam with 500 tph. grain unloaders (Bühler Miag), the applied air velocity was between 35 m/sec to 42 m/sec.
When I estimated the suspension velocity of the raw cotton at approx. 16 m/sec, my first perception was: ” this is high”, because I thought that raw cotton was a light product.
However, the particle density is given as 1545 kg/m3.
I agree that this estimated suspension velocity is not more than an estimate. A direct measurement is a better approach.
Best regards ■
Teus
Re: Pneumatic Cotton Unloading System
Originally Posted by Teus TuinenburgI estimated the required gas velocity in relation to the (local) suspension velocity.
The idea is, if the gas velocity equals the suspension velocity of a particle, the particle would not be accelerated in a vertical pipe line.
In this case, the drag force equals the gravity force.
Hence, the gas velocity must be higher than the suspension velocity.
If the gas velocity <= to the suspension velocity, there will be no pneumatic conveying.
The suspension velocity of a particle is calculated as the terminal velocity:
v-suspension=SQRT{(4*d*materialdensity)/(3*dragfactor*gasdensity)}
For the raw cotton, with the given data:
v-suspension=SQRT{(4*0.01*1545)/(3*0.05*1.15)}
v-suspension= 18.9 m/sec
A soybean of 6 mm has a suspension velocity of approx. 12 m/sec.
When was working in the port of Rotterdam with 500 tph. grain unloaders (Bühler Miag), the applied air velocity was between 35 m/sec to 42 m/sec.
When I estimated the suspension velocity of the raw cotton at approx. 16 m/sec, my first perception was: ” this is high”, because I thought that raw cotton was a light product.
However, the particle density is given as 1545 kg/m3.
I agree that this estimated suspension velocity is not more than an estimate. A direct measurement is a better approach.
Best regards
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Dear Teus,
Many thanks for explaining to me.
My experience shows that the required conveying velocity in vertical lines is less than in horizontal lines.
Most of the well-known co-relations for conveying velocity are based on horizontal lines.
I use 20 pipe diameters horizontal section before allowing any vertical runs. This criteria seems to work.
I have not used velocities above 120 ft/sec at the END of the conveying line to minimize particle attrition.
Best regards,
Amrit Agarwal
Pneumatic Conveying Consulting
email: polypcc@aol.com ■
Re: Pneumatic Cotton Unloading System
Dear Amrit,
Thanks for your reply.
Horizontal lines require indeed higher gas velocities.
In vertical lines, the gravity is in line with the drag force. Therefore, the particle is kept in suspension easily.
In horizontal lines, the gravity is acting perpendicular to the drag force and to keep the particle in suspension, other forces are needed to keep the particles in suspension.
To generate those forces, a higher (horizontal) velocity is required.
(Increasing the Magnus effect and turbulence)
As the standard suspension velocity of a particle is based on atmospheric conditions, the pickup velocity should be related to the gas pressure at pick up point and the end velocity to the gas pressure at the end of the pipeline.
In a vacuum system, at the beginning of the pipeline,, this gas pressure is mostly the ambient pressure minus the material column pressure drop outside the nozzle or the pressure drop before f.i. a rotary valve.
In a pressure system, at the end of the pipeline, the gas pressure is mostly approx. atmospheric but in many cases a significant back pressure. (f.i. an BF injection system)
The minimal distance between 2 bends should be related to the acceleration length after the first bend. For fine products (lower suspension velocity), this length is smaller than for course products (higher suspension velocity).
A velocity of 120 ft/sec # 37 m/sec.
The velocities, I mentioned were for vacuum unloaders, where the pipe lines were stepped 3 times (4 different diameters), because of telescopic suction arms.
Indeed, the referenced grain unloaders were less suitable for friable products, s.a. rice, bird seed and corn.
Best regards ■
Teus
Pneumatic cotton unloading system
Hello,
I am mechanical engineering student and I want to do an academic project on transporting raw cotton from truck to gin warehouse(in cotton ginning factory) pneumatically. I researched on internet and found some calculations but still I am not sure about these calculations. I have attached a photo of mobile grain conveying system. I want to use similar principal and want to transport cotton instead of grains. I am confused at what to calculate first? blower size or cyclone size( funny that I do not know how to calculate the size of either). I have necessary equations but I am not sure how to use those equations in my calculations. I do not know how to calculate pressure drop, cyclone size, blower size,number of cyclone to use,flexible rubber pipe diameter, type and size of inlet hood etc. also I am confused about which type of cyclone should I use? 1D3D, 2D2D etc.. here are related data. truck usually contains 10-12ton of densely put raw cotton. so, I need to empty it in say 1hour. also conveying distance is say 200ft. so discharge pipe length is 200ft and inlet pipe length of 10ft would be sufficient. Also blower will be getting power from tractor power take off (tpo) drive.Any help would be much appreciated.
Thanks in advance
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