Propeller agitator design

[Nasser D]

Hi dear engineers, I am trying to calculate flow, velocity and discharge pressure of an agitator that we have in our system. I have motor power, diameter and RPM of the impeller. Thanks

 

[LittleInch]

Agitator don't create any pressure or "flow" in a strict sense. They just whirl around and try and "agitate" the contents of a tank to create mixing or prevent stratification. normally you just run them on for a set period of time and they either work or don't work. Not a huge amount of science involved.

And there are so many different types you wouldn't believe. size of tank, type of material, viscosity and density of material, is it Non newtonian?

Any details you need you can find out from the vendor.

Why do you want to know if it's one you already have?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nasser D]

Thanks for the response, this is special design, the function of this agitator is moving molten salt between reactor jacket and salt cooler (a heat exchanger), I need to know how much flow gets into the heat exchanger. I have information of impeller size and electromotor power and RPM.

 

[SuperSalad]

I would agree with LittleInch about how there can be a lot "check and see" methods when it comes to designing agitation, but I would disagree about there not being a huge amount of science involved. Agitator design can be modelled, but obviously it is going to be tough to accurately model the real world because of the huge amount of variables in vessel agitation.

I've used this site: CheCalc to do some basic order of magnitude estimations and the formulas used are pretty robust. You might be able to use the values there to back-calculate what you are looking for with the data you have on hand.

- Andrew

 

[LittleInch]

Well give us a diagram and tell us what the sizes are.

But an agitator still only agitates.

You might need some CFD analysis here to give you the answer as there are so many variable which will affect your "flow"

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nasser D]

Motor power:250 hp
Impeller RPM: 494.4
Impeller type: Propeller 4 blade
Impeller diameter: 32 inch
I need to know P1?

 

[SuperSalad]

You won't be able to estimate P1 reliably without more data. You'll need the diameter or cross section of where the impeller appears to be within, the pipe sizes of the two outlets from that area, the viscosity (and possibly the density) of your liquid, the pitch angle and blade dimensions (not just overall impeller diameter) of your 4 blades.

I think you might be able to estimate it at that point, but with all of that data gathering, you might just be better off installing a flowmeter and pressure gauge to measure it directly.

- Andrew

 

[TiCl4]

the function of this agitator is moving molten salt between reactor jacket and salt cooler (a heat exchanger)

Based on this and the sketch you provided (is the twin-prop blade thing in your sketch the "agitator"?) this sounds more like an axial flow pump. By the way your sketch is pretty much useless. There are no descriptions of what the equipment is. A PFD or P&ID would be more helpful.

Does it look something like this? If this indeed a pump, you can find the nameplate somewhere on it and contact the OEM for the information you need.

 

[Nasser D]

I have most of the other data of size of system and impeller. Could you please let me know how I can calculate P1 with those information that you mentioned?

 

[SuperSalad]

Did you look at the link for the CheCalc agitator page I posted above? If you input "user defined" in the agitator type, you can change the power numbers and flow numbers to match your motor power and speed to get an estimated bulk velocity and agitator pump rate.

If you are confident that all the flow on the downstream of you agitator ends up going into those two outlets and doesn't end up as turbulence in the vessel, split that flow in half for the velocity in your pipe. Then you can try using a Bernoulli equation [ P₁+1/2ρv²₁+ρgh₁ = P₂+1/2ρv²₂+ρgh₂ ] to get the pressure in the pipe.

Keep in mind, I am making a lot of assumptions about your system to even suggest this as plausible.

- Andrew

 

[MJCronin]

Mmmmmmmm...... EXCEL Spreadsheet PIDs !!!! .... With mysterious chunky blocks and circles and Arrows !!!

I forgot how much confusing fun these were !!!!

Even a hand drawn PID sketch would be better that what we have been presented here

https://www.lucidchart.com/pages/tutorial/p-and-id

Oh, one more thing .... How do we know that an "agitator" (or axial flow pump, for that matter) is the best choice to move molten salt ?

How can you have a completed PID design when you have no idea of the flow in the system ????

MJCronin
Sr. Process Engineer

 

[LittleInch]

Nasser D

your diagram makes no real sense as it is not clear at all where this propeller is in the system.

The pressure P is not set by the impellor, but by the downstream system and its pressure drops and friction etc.

All the agitator does is create flow. How much pressure you get is dependant on how much that velocity is impeded and turns into potential energy.

That is quite a healthy sized motor but I don't think there is any way you can calculate pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

Maybe a picture would do it justice. Plus, I've never seen one of those.

Good Luck,
Latexman

 

[Nasser D]

Thanks all for your inputs, let me digest all the ideas and check the resources that you introduced, will be back to you. tnx