Re: Pressure Drop Across Blow-Through Valve
Dr. Tanase
I assume that you are referring to a drop through valve and a blow through valve as a pneumatic conveying feeding point, consisting of a rotary valve with the pneumatic conveying pipe underneath.
In a blow through valve the material mixing length is usually equal to the length of the rotary valve
Assuming that the blow through valve has a pipe piece connected before and after the mixing zone.
In that case the gas acceleration is not part of the pressure drop, which you want to know.
Then the mixing volume underneath the rotary valve gas pressure drop has 3 zones:
-Cross section increase causing a pressure drop
-Channel gas resistance, where a part of the velocity pressure is converted back into static pressure
-Cross section increase causing a pressure drop
-Conveying pipe gas resistance, where a part of the static pressure is converted into velocity pressure
All these pressure drops can be calculated with Darcy Weisbach equation.
https://en.wikipedia.org/wiki/Darcy%...sbachequation
The pressure drop of the presence of product, causing a gas pressure drop, is more complicated, as the acceleration of the product + the product resistance must be calculated in the time domain.
(The gas velocity and the product velocity changes over the considered conveying length and is also depending on the solid loading ratio).
Considering that the blow through pressure drop is always very small to the total pressure drop, it is sufficient to consider the blow trough valve just as a part of the conveying pipe where the gas + material pressure drop start
Another factor is that the pressure drop over the blow through valve is also depending on the absolute pressure underneath the valve.
Let us forget about the rotary valve leakage.
Measuring is always a good thing to get an idea of the real world.
Then you also know, whether it is worthwhile to go on on. ■
Teus
Re: Pressure Drop Across Blow-Through Valve
Dear Mr. Teus,
Thank you very much for your answer. As always, very detailed and reach in information.
1. Yes, you are right, it is the feeding point of an overpressure pneumatic conveying system. Sorry for to have missed this specification.
2. gas acceleration: yes, right, the gas is accelerated just before entering the blower and in the pipe between the blower and the valve
In my opinion, this is one part of the problem, actually the gas is continuing accelerating till the end point, product does the same, because if not = no pneumatic conveying
3. Darcy-Weisbach: i use it to calculate the linear pressure loss. I considered the valve as local resistance. I can easily convert the (lambdaxD/L) into a coefficient for the valve. Otherwise, in my opinion, for the very turbulent regime inside the valve is difficult
to calculate a lambda friction
4. presence of product: thank you for comments and clarifications, really difficult but having measurements for both states (without/with product) it would give a good information about the additional pressure loss in the valve due to the product
5. considering the valve as part of the pipe: i do the same, but taking the (lamdaxD/L) as 1,6 (just a number deducted from lambda=0,02 ; diam. 125 mm and specific length of 1 m and multiplying it by 10 for the turbulence). For an air speed of 17,5 m/s in the compressed state as 1:1,4 it gives a pressure loss across the valve of about 260 Pa (for clean air). Still looks a little bit underestimated, in my opinion.
6. Absolute pressure under the valve (in the mixing chamber): i iterated it as 1.400 mbar, that looked ok for my case of transport
I also understand that there are no available measurements or research papers on this subject.
Thank you very much one more time for your support. Wish you a pleasant week-end.
Sincerely yours,
Tanase TANASE ■
Re: Pressure Drop Across Blow-Through Valve
Dear Dr. Tanase,
I believe that your calculated figures are quite realistic.
2. gas acceleration: yes, right, the gas is accelerated just before entering the blower and in the pipe between the blower and the valve
In my opinion, this is one part of the problem, actually the gas is continuing accelerating till the end point, product does the same, because if not = no pneumatic conveying
I would not call it a part of the problem, that is how physics work.
The continuous acceleration along the pipeline can only be solved by numeric integration in the time domain and by iteration to find the solution.
This is because there are more variables than equations.
Your calculated figures show that it is not really worthwhile to pursue the pressure drop over the blow through valve as the value << total pressure drop and a deviation in that pressure drop fall within the total expected accuracy of the calculations. ■
Teus
Re: Pressure Drop Across Blow-Through Valve
Dear Mr. Teus,
Thank you one more time for the comments and clarifications.
For sure it is a beautiful complicated problem, having less equations than variables and trying the linearization of it, of course on reasonable small domains.
As you already developed the procedure, it seems quite natural to have this approach, making assumptions and going step by step.
Still some equations could be included in the model, as the one of Euler for example for the acceleration of an incompresible fluid, but including the density variation as a function of the pressure gradient it makes it valid for compressible fluids.
I tried to find out some details about this pressure drop since it is very simple: I want to know !
Not because it is very important, but because is there and we are engineers.
With great respect,
Tanase TANASE ■
Re: Pressure Drop Across Blow-Through Valve
Dear Dr. Tanase,
The Euler equation actually describes the scalar conservation of energy in space.
That makes the equation useful for computed fluid dynamics (CFD).
That requires a very precise spatial frame and software that can handle matrixes.
For a practical approach for designing a practical system, this is too much work and in a 2-phase flow, the interaction between a compressible gas and particles becomes a lifetime effort to describe in a computer algorithm.
For daily engineering purposes, this is (for me anyway) too much mathematics.
Probably, the reason that there are no pneumatic conveying design computer programs using the CFD approach.
Although the CFD calculations can generate beautifully visualize what happens.
As you state, “we want to know” is quite true”.
Once we know and we cannot practically use it, we let it go. (at least that counts for me) ■
Teus
Pressure drop across blow-through valve
Dear All,
I tried to find out a way for to assess the pressure loss across a blow-through valve. Not drop-through, but blow-through.
Of course, classic way is to consider the dynamic pressure and multiply it by valve coefficient/factor. Or course, should be a function of Reynolds.
I tried to find a research paper focused on this matter but i find nothing.
To measure the pressure drop would be definitely a good solution, but implies air flow measurements, considering only-air situation.
Having product dosing, it will give some false indications since some of the pressure drop will be due to product acceleration.
Is there any paper related to this subject of valve resistance coefficient/factor ?
Any opinion or personal consideration/experience would be appreciate.
Thank you all and,
Happy pneumatic conveying.
Tanase TANASE ■