Calculation of the overall heat transfer coefficient


Example

The overall heat transfer coefficient k [W/m2·K] for the conditions shown in the figure to the right is calculated as: Overall heat transfer coefficient example
k value equation
where λ [W/m·K] is the wall thermal conductivity, α1 and α2 [W/m2·K] are the respective individual heat transfer coefficients and k [W/m2·K] is the overall heat transfer coefficient.

Assuming the following numerical values:

α1 = 2,000 W/m2·K
α2 = 20 W/m2·K
w   = 2 mm
λ   = 20 W/m·K

the result is:
k value calculation
i.e. the overall heat transfer coefficient is almost equal to the individual heat transfer coefficient on side 2. Even with a 100% increase on side 1, i.e. α1 = 4,000 W/m2·K the k value is still 20 W/m2·K. This is a case which might be at hand when the fluid on side 2 is extremely viscous thus giving very bad heat transfer conditions.

If instead α2 = 200 W/m2·K the k value becomes 179 W/m2·K and with α 2 = 2000 W/m2·K the k value becomes 909 W/m2·K. If in this case the thickness of the wall is decreased by 50%, i.e. to 1 mm instead of 2 mm, the k value becomes 952 W/m2·K.


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