Calculated power requirement seems too low for straight blade turbine agitator

[towki2495]

I am calculating power requirement of an agitator for a 1000L petroleum jelly vessel of 1.1m diameter at 50°C considering the following parameters:

impeller diameter = 0.44 m
agitator rpm = 15 rpm (customer requirement)
fluid density = 850 kg/m3
viscosity = 10 Pa.s (at 50°C) [unsure]

The general power formula and the laminar flow variant formula are both giving a power < 50 Watts only. This doesn't seem right for agitating a 1000L petroleum jelly vessel. Is the problem too low consideration of impeller RPM value or viscosity in this case? Any help will be appreciated, thank you.

Also we are considering a four straight blade impeller system. I've considered the KL value for it same as that of a six blade system. Or should I use the specific KL value for a 4 blade system?

 

[LittleInch]

Try this?

https://myengineeringtools.com/Piping/Power_Agitator_Calculation.html

"Agitator" is quite vague.
A 4 blade is different to a 6 blade.

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[towki2495]

Thank you for your response.

I worked through the calculation in the linked page. I got a final motor power of 4 Watts. Can you please confirm that the viscosity for Petroleum Jelly at 50°C is 10,000 cp. Should I bump up the RPM even though the customer wanted 15 RPM?

 

[LittleInch]

Don't know, but neither 4 w or 10,000 cP looks right to me.

At that sort of viscosity I don't think you're going to get any agitation and all that will happen is that the fluid will shear around the end of the blades and simply rotate as solid block of material.

You're probably also into non Newtonian fluids here so everything gets very weird.

And 15 rpm wouldn't stir my tea so won't do anything for petroleum jelly.

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[towki2495]

The agitation requirement here is just to stir/mix the jelly in a storage vessel (after mixing) at a very slow speed at 50°C so it doesn't freeze/set before it is discharged to the filling hopper. Hence the low RPM requirement by the customer. The melting point of Petroleum Jelly is reported as 40-70°C. So I'm unsure what to consider as the final viscosity here and whether the low RPM is pulling the final power too low.

 

[geesaman.d]

Using a standard pitched blade turbine seems like a very poor selection. Any non-newtonian behavior or yield stress and it will cavern. If you or the customer don't know actual rheology at 50C then I think you'll be stuck making wild and wasteful guesses.

15rpm might work very well for the right impeller selection.

 

[Latexman]

I'm unsure what to consider as the final viscosity here and whether the low RPM is pulling the final power too low.

In God we trust, all others bring data. You need rheology data.

Good Luck,
Latexman

 

[LittleInch]

A quick google shows a wide range of viscosity vs temperature, but 50C is still quite a low temperature for getting to any sort of reasonable viscosity where mixers re going to do something other than jam up or just go round as a solid block of wax / jelly.

I suspect the mixer calculations for power don't look at shear stress in the "fluid" or are valid for what is basically near solid wax.

I take it you've not really got experience of this?

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Also: If you get a response it's polite to respond to it.

 

[geesaman.d]

What I can say about an optimal solution is that a few companies have excellent impeller designs for these highly viscous and/or non-newtonian semi-fluids. But only one company has really optimized it. I can't say which company because of forum rules. It mixes paper stock well up to 10% consistency. That ideal design will have a very high swept volume fraction (no stagnant zones), moderate operating torque, and very low motor power. Trying to horse it with excess power and expecting the recirculation of a normal PBT isn't going to work.

 

[EdStainless]

If they really want to keep this mixed the mixer would need to be double that size.
Are you sure that they didn't mean 0.44m radius?
Until they measure a real viscosity over the range of temperatures of concern.
A quick web search shows Visc at 100C from 5-16 cSt depending on the manufacture.

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P.E. Metallurgy, consulting work welcomed

 

[towki2495]

Thank you for all the responses.

The petroleum jelly will not exceed 50°C, so I don't think I can use its viscosity at 100°C. The vessel diameter is 1.1 m (1000L) and I've considered a third of that as impeller/swept diameter. As suggested by geesama.d, I can increase this ratio to three quarters which requires a power of 250 Watts as opposed to 50 Watts before for a straight blade impeller.

I am new at this and we have no extant rheological data so far I know. The impeller merely needs to stir the petroleum jelly so it doesn't freeze/set before it is discharged and requires no actual mixing. So I'm hoping an anchor impeller is not required and a straight blade turbine will do.

 

[LittleInch]

I am new at this and we have no extant rheological data so far I know. 

Therein lies your problem,

Highly viscous fluids and non Newtonian fluids have very odd properties and trying to do anything without rheological data is not going to work. You really need to find someone to tell you how this fluid reacts to shear and mixing.

Anything which doesn't sweep the whole volume won't work IMO. Think of an ice cream maker....

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