Cooling water be calculated as heat exchanger?

[andrejen88]

I'm doing an analysis of a fermentation process, where cooling water is used to cool down the fermenter, through some cooling coals and a coat.

I've found the Q needed to cool the fermenter: Q = dT*M*Cp

And I've found the overall heat transfer coefficient by using Q = U*A*dTm

During the process the mass flow rate changes, and in my analysis I observe, that U also varies a long with it.

So I have two basic questions: Firstly, is the overall heat transfer coefficient applicable when dealing with a cooling water system?
Secondly, does the overall heat transfer coefficient vary with flow rate, because the mass stream gets more turbulent and thus lowering the film resistance inside the pipe?

Sorry for my English (not originally English-speaking)

 

[andrejen88]

Cooling coils inside the fermenter.

 

[IRstuff]

Assuming consistent turbulence and mass flow, yes. Otherwise, it'll be changing all the time, and your process needs to accommodate that. Depending on mean free path of the heat, you might need to consider some sort of internal circulation or agitation to alter the effective Biot number of the material. Increase the effective contact area would likewise help, so putting fins on the coil might be helpful.

TTFN
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[EdStainless]

Yes, your local U will be changing based on flow and delta T.
This is why most systems like this would be designed using a lumped (average for system) U value.
Fins will only help if the surface resistance values are very low on that side. For liquid to liquid systems fins are of very little use.
My hunch is that your overall heat transfer is limited by the product-to-coil heat transfer. The only way to increase that is to have more velocity across the coils (stir more).
It can help to look at this using a resistance model, each step in heat transfer has a resistance.
You basically have 5 components, heat transfer within the fermenter, product to coil surface, coil wall, inside boundary layer, and heat transfer within the cooling water.
The coil wall will be less than 5%, don't worry about it. Higher cooling water flows will improve the last 2 items. So it really the first 2 that you need to address in order to improve overall heat transfer.

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