Overall heat transfer coefficient for water cooling

Hi all!

I'm analyzing a fermenter and the water cooling system that cools it down.

In analyzing this I've found kW and kWh required to cool down the fermenter using Q = dt*cp*m

However when I try to find the overall heat transfer coefficient U using the data, I find that it varies with either Q or M.

I use the equation: Q = A*U*dtm

At first Q is high due to an extreme cooling from 121 C to 35 C. After that during bacterial fermentation the temperature should stay the same. However this second stage gives me a lower U-value (twice) than during the extreme cooling in the beginning. Does any of you have a suggestion to why this is happening?

Can you at all use this formular to calculate a heat transfer coefficient for water cooling systems, or can it only be applied for heat exchangers?
 

BvU

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Hello andrejen, :welcome:

Could it be the bacterial fermentation itself produces heat ?

You want to be a bit clearer in your typing: m means the same as M (I think) and dtm is not dt times m but the mean delta t.

Could also be that mean delta t is not good for the expression and you need the log mean delta t (aka LMTD).

Furthermore, U doesn't have to be an all-over constant and in your case depends on variables that vary.

There is a very thorough book by the VDI : http://link.springer.com/referencework/10.1007%2F978-3-540-32218-4 [Broken]that might help you if you can get hold of it. I think the text is parallel in german and english.
 
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CWatters

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At first Q is high due to an extreme cooling from 121 C to 35 C. After that during bacterial fermentation the temperature should stay the same. However this second stage gives me a lower U-value (twice) than during the extreme cooling in the beginning. Does any of you have a suggestion to why this is happening?
+1 to what BvU said about the fermentation producing heat. If you want the temperature to remain constant it shouldn't need any cooling unless there is a heat source.
 

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