Designing a spiral stirrer 1

[zedman]

I have problem in designing a spiral stirrer. I've made a spiral stirrer for my tanks, dia. 1076mm and height 1615mm.
The problem is the stirrer can't create a vortex.
What should I consider in design a spiral stirrer?
I

 

[CorBlimeyLimey]

"The problem is the stirrer can't create a vortex."
Do you mean your stirrer design doesn't create a vortex (which it should), or that the stirrer is not allowed/supposed to create a vortex.

 

[zedman]

Yes. It doesn't create vortex. I made 2 stirrers before (different tank diameter) and it works. So I wonder if the diameter of the stirrer is small compared to the tank dia., or the position of the stirrer (offset 1/4 od tank d) which is same as 2 previous tanks.
The gearmotor for all these 3 tanks are running at 145rpm, 2.2kW.
Thanks

 

[CorBlimeyLimey]

Same fluid viscosity?
Same stirrer blade angle?
Same immersion depth?
How do the various stirrer diameters compare to the tank diameters.

 

[zedman]

Hi.
Liquid viscosity & blade angle are same for all the tanks.
Immersion depth is defferent. Is it the possible cause of my prob?
For the 1st 2 tanks, the spiral is only 2 rev. The 3rd tank, previously 2 rev. then modified to have 3 rev.

 

[vpl]

More questions than answers:

Is your stirrer not creating a vortex at all, or is the vortex too localized?

Is the issue really the size of the vortex or ensuring that tank contents are well mixed?

What is the tank diameter for this tank as compared to the other ones? Do you have the same proportions (stirrer blade width versus tank diameter)?

What is the immersion depth in this situation versus the other ones? Again, are the proportions the same?

Assuming that the issue is you want a well-mixed fluid and are using the stirrer to achieve that (and that the vortex is how you're ensuring the mixing is going on): Both the stirrer diameter and length as compared to the tank diameter and length are affecting this situation. The key is not to look so much at total diameter and length as the change in proportions between the setup that worked and the one that didn't.


Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[zedman]

Thanks for all your replies.

I can say that the stirring is really an issue as the mixture doesn't mixed well compared to the 1st 2 tanks.
For the 1st 2 tanks, as we pumped in the liquid inside tanks which will create bubbles and after we pour in chemical and start stirring. Within a minute the bubble will cleared. This is our process.

But for the new tank, the bubbles won't clear and this will cause the chemical particles to stick on the bubbles.

1st 2 tanks diameter: 900mm
3rd tank dia:1076mm
Stirrer blade width different between 2 tanks as the ratio to the tank dia. is different.

Now, should I change the stirrer or the germotor to get the stirring that I need. I may undersize the gearmotor as the tank is bigger than the 1st 2 but I use the same motor speed and power?

 

[FACS]

So if I understand correctly, the only difference here is the diameter of the tank.

With a larger tank, there is going to be more surface area clinging to the material, and resisting movement. To get the vortex, you need to overcome the resistance, and get all the material moving fast enough to whirlpool.

So, it probably is not a linear relationship between diameter and mixer dimeter; but you need to get the material moving, and I would suspect a larger diameter spiral is needed, and perhaps a bit more speed.

If not a larger diameter spiral, more spiral surface is needed.

Think about a glass of water; Put in your finger and stir, you make a vortex. Now get a larger glass, and you either have to stir faster or you have to use two fingers. Now use a swimming pool, and your fingers are no longer large enough, you have to put in people and have them run around in a circle. The closer to the edge of the tank you are the better chance you have.


Charlie

http://www.facsco.com

 

[zedman]

Thanks Facs.

 

[zedman]

Facs,

I have made a new stirrer with bigger diameter (bigger by 50mm each side). Speed is not an issue at this moment as we can achieve the speed that we need. The problem is, stil doesn't create vortex. The stirrer does not stir but more to agitate that cause turbulance. I've reduce the speed but looks like no different.

 

[rb1957]

in an earlier post i think you said the stirrer was lower in the tank now than earlier tests ... this could be a factor ?

 

[scastillo]

One possible option, which may or may not work for your application, is to install a recirculation loop in the process. It would draw from the bottom outlet valve, thru a pump and back into the tank thru a sparger that is angled to create a vortex. Make sure to position the outfeed of the sparger below the liquid level to eliminate any bubbles.

If that fails you can always use FACs suggestion of having people running around in a circle inside the tank. Just remember to set the stirrer speed to its lowest setting.

 

[zedman]

Another question:

Is the shape of the stirrer one of the factor creating vortex? Meaning concave stirrer plate vs converge stirrer plate.