# Overall heat transfer coefficient for water cooling

• andrejen88
In summary: If you are trying to keep the temperature constant and it is falling anyway, then the heat flow (Q) must be negative (heat flowing out of the fermenter), and that heat flow is going to be Q = UAdT/LMTD. So if you are trying to maintain a constant temperature (dT = 0) and Q is negative, then U must be negative, which would mean the cooling water is actually heating the fermenter instead of cooling it.In summary, the conversation revolved around analyzing a fermenter and its water cooling system. The speaker found the required kW and kWh to cool down the fermenter, but had trouble finding the overall heat transfer coefficient U using the equation Q = A*U*dtm
andrejen88
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 formula to calculate a heat transfer coefficient for water cooling systems, or can it only be applied for heat exchangers?

Hello andrejen,

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 that might help you if you can get hold of it. I think the text is parallel in german and english.

Last edited by a moderator:
andrejen88 said:
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.

## 1. What is the overall heat transfer coefficient for water cooling?

The overall heat transfer coefficient for water cooling is a measure of the rate at which heat is transferred between the cooling water and the surface being cooled. It takes into account the properties of the fluid, the characteristics of the surface, and the flow conditions.

## 2. How is the overall heat transfer coefficient calculated?

The overall heat transfer coefficient is calculated by dividing the total heat transfer rate by the temperature difference between the fluid and the surface. This value is then used to determine the amount of heat that can be transferred between the two mediums.

## 3. What factors affect the overall heat transfer coefficient for water cooling?

There are several factors that can affect the overall heat transfer coefficient for water cooling. These include the flow rate of the water, the temperature difference between the water and the surface, the properties of the fluid, and the characteristics of the surface being cooled.

## 4. How does the flow rate of water impact the overall heat transfer coefficient?

The flow rate of water has a significant impact on the overall heat transfer coefficient. A higher flow rate means that there is more contact between the water and the surface being cooled, resulting in a higher heat transfer rate. However, a very high flow rate can also cause turbulence, which can decrease the overall heat transfer coefficient.

## 5. Why is the overall heat transfer coefficient important in water cooling systems?

The overall heat transfer coefficient is important in water cooling systems because it determines the efficiency of the cooling process. A higher heat transfer coefficient means that more heat can be removed from the system, resulting in better cooling performance. It also helps in selecting appropriate cooling equipment and designing efficient cooling systems.

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