# Is the overall heat transfer coefficient constant for a certain heat exchanger?

• MarkH748
In summary, the person is asking for help with a counter-flow heat exchanger problem where they need to find one of the outlet temperatures. They are unsure how to calculate it without a value for the overall heat transfer coefficient or surface area. They mention a previous example where they were able to calculate the product of U and A with one given outlet temperature, and they are wondering if they can use this value for the second example. They are also unsure if U is constant in both examples. They express their lack of understanding of U and ask for help.
MarkH748
Hi guys. Without going into too much detail. Basically I have a counter-flow heat exchanger problem where I need to find one of the outlets temps only given the inlet temps of each fluid. I have no idea how to do it though as I'm not given a value for over heat transfer coefficient (U) or surface area (A). However, in a previous example the same heat exchanger was used and one of the outlet temps was given which allowed me to calculate the value of the product of the overall heat transfer coefficient and the surface area (U*A).

I was wondering if I can use this value again for the 2nd example? I know A is probably constant but is U constant too? All that has been changed for the 2nd example is flow rate of one of the fluids (same inlet temps and fluid properties).

Any help would be greatly appreciated as my understanding of U isn't all that great.

Mark.

I guess that is the best you can do.

## 1. What is the overall heat transfer coefficient?

The overall heat transfer coefficient is a measure of the effectiveness of a heat exchanger, which is a device used to transfer heat from one fluid to another. It takes into account the thermal conductivity of the materials involved, the surface area of the heat exchanger, and the flow rates and temperatures of the fluids.

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

The overall heat transfer coefficient is calculated by dividing the overall heat transfer rate by the temperature difference between the two fluids. The overall heat transfer rate is the sum of the convective and conductive heat transfer rates, which are determined by the thermal conductivity and surface area of the heat exchanger.

## 3. Is the overall heat transfer coefficient constant for all heat exchangers?

No, the overall heat transfer coefficient can vary depending on the design, size, and materials of the heat exchanger. It can also change over time due to fouling or degradation of the heat exchanger surfaces.

## 4. How does the overall heat transfer coefficient affect the efficiency of a heat exchanger?

A higher overall heat transfer coefficient means that more heat can be transferred in a shorter period of time, making the heat exchanger more efficient. This is because a higher coefficient indicates a more effective transfer of heat between the two fluids.

## 5. Can the overall heat transfer coefficient be controlled or manipulated?

The overall heat transfer coefficient can be optimized by choosing appropriate materials, increasing the surface area of the heat exchanger, and controlling the flow rates and temperatures of the fluids. However, it cannot be directly controlled or manipulated once the heat exchanger is in operation.

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