solid-solid mixing

[Paola1983]

Hello, I have some problem with an estimating for a power requirement.
How can I calculate the power requirement for an agitator in a solid-solid mixing tank as a coal storage tank?
thanks

 

[MJCronin]

"solid-solid mixing tank as a coal storage tank?"

Are you mixing and blending different types of coal ?

The dry mixing of solids is rarely done with an agitator.

It can be done several ways, depending on the size of particles and the degree of mixing.

A simple screw conveyor feeding onto a belt conveyor can do a pretty good job....

 

[MikeHalloran]

More important than having enough power to drive the agitator is having a sturdy enough scattershield around the tank to deal with the inevitable COAL DUST EXPLOSIONS. Search on that.


Mike Halloran
Pembroke Pines, FL, USA

 

[Paola1983]

The tank is a simple silo to storage the coal (size 0 ÷ 0,2 mm (95% < 90 µm)) and the agitation is necessary to permit a perfect distribution of the coal to the n rotary valves placed beside the silo to feed the lances of kiln.
To reduce the oxygen content into the silo/tank in order to prevent or inhibit deflagration process the silo is inertised with nitrogen...
Thanks for the answer...

 

[MikeHalloran]

Ah. So you're having distribution problems in an inerted powdered coal manifold.

What measures do you take to measure/ monitor/ control the moisture/ humidity in the manifold?

Why are the rotary valves 'beside' the silo, not 'under' it?
Never mind; too expensive to change, right?

... and that's why you need an agitator.
... which you're trying to improve?

Because of the small particle size, I'm guessing the agitator comprises a blade riding just barely above the floor of the silo.

I'd guess the normal power use could be estimated by assuming that the blade supports a column of powder, or maybe an inverted cone of it, and has to lift the column or cone in order to turn. That weight and the tangent of the blade pitch would give you a tangential force. That force on the blade length gives you a torque. The torque and the agitator speed gives you a power.

... all of which are WAG estimates, based on a lot of unfounded assumptions.

... but if the agitator blades run at a nominally small clearance to the floor o the silo, there exists the possibility that the weight of powder bearing down on an agitator blade deflects the blade enough to take up that clearance, causing the blade to drag on the silo floor. ... which increases the shaft power required.

Does that resemble the problem you're actually trying to solve?



Mike Halloran
Pembroke Pines, FL, USA

 

[gr2vessels]

Paola,
The nitrogen blanketing is most likely an illusion of safety, since the coal powder is self igniting / self oxidizing due to the oxygen trapped in coal structure. If you add more energy to the already sensitized coal powder with that mixer, the coal will ignite and start burning in the silo. Once it started, there is no way to extinguish the fire. You might try to use some gas jets in the cone to fluidize the coal, but even that is questionable. You will need some serious filter arrangement in top to allow the gas to escape, a good moisture control, lots of safety arrangements.
Are you trying to invent something from the scratch or you have some plan you need/want to work on.
Cheers,
gr2vessels

 

[Paola1983]

Sorry, I thought to other things and I wrote beside instead of under. Obviously the rotary valves are under the silo.
I have already try to assume the total amount of coal in the silo. To estimate the power I assume the coal as a punctual weight on the blade as per file attached but the power results too high. I've seen in field that this kind of agitator can work with a motor of 5.5 kW installed....
What do you think?

 

[LittleInch]

From what I could see from this sheet you seem to have assumed blades over the full cross section??. Also the rotational speed was quite low? However if you only have blades covering half of the circular area then you might get closer to your 5 KW.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way