Calculation of the overall heat transfer coefficient
|The overall heat transfer coefficient k [W/m2·K] for the conditions shown in the figure to the right is calculated as:|
|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:
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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