Wall temperature 1

[masken]

Hello all!
I am intrested to know how to calculate a wall temperature in a heat exchanger. Both temperatures on the different sides of the wall are known. Is it the mean temperature?

Thanks,
Mask

 

[TD2K]

You need to calculate the film coefficients (including the fouling coefficients as applicable) in the exchanger either on an overall basis or for a specific location(s) in the exchanger. Given the amount of heat being transferred either on an overall basis or a specific location, you then can calculate the tube temperature. Basically, you have the overall bulk temperature difference and overall heat transfer rate. Then, you calculate the required dT across each 'resistance' layer (either film, fouling or tube wall though the latter is typically very small) to achieve the same heat transfer as that is constant for each layer.

If the heat transfer coefficient on one side is much larger than the other and is clean (eg. condensing steam inside a coil in a water bath), then the metal temperature would be pretty close to the saturation steam temperature since the condensing steam heat transfer coefficient is much larger than the natural convective heat transfer coefficient from the coil to the water bath.

If you have a copy of the TEMA standards, I believe they go through this calculation.

 

[ardilesd]

In fact, there is a temperature profile within the wall, which you can characterized by both wall surface temperature. Being the heat flux constant across the fluid and wall, one can derive (plane geometry)

h1.A.(T1-Tw1) = k.A/e.(Tw1-Tw2)= h2.A.(Tw2-T2)

Notation seems to be typical....

hence, being

Tw2= h1/h2*(T1-Tw1) + T2,

the expression for Tw1 can be found:

(e/k+1/h2).T1 + T2/h1
Tw1 = ---------------------------
1/h1 + 1/h2 + e/k
So, the wall temperature is higly dependent upon the relative heat transfer coefficient. Here, if the heat transfer coefficient for Side 2 is 100 (water) and 10 for Side 1 (Oil), the wall temperature will be closer to the side 2 Bulk temperature.
Regards