Calculation of the overall heat transfer coefficient
Example
The overall heat transfer coefficient k [W/m^{2}·K] for the conditions shown in the figure to the right is calculated as: | |
where λ [W/m·K] is the wall thermal conductivity, α_{1} and α_{2} [W/m^{2}·K] are the respective individual heat transfer coefficients and k [W/m^{2}·K] is the overall heat transfer coefficient. | |
Assuming the following numerical values: α_{1} = 2,000 W/m^{2}·K α_{2} = 20 W/m^{2}·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.
α_{1} = 4,000 W/m^{2}·K
the k value is still 20 W/m^{2}·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 α_{2} = 200 W/m^{2}·K the k value becomes 179 W/m^{2}·K and with α _{2} = 2000 W/m^{2}·K the k value becomes 909 W/m^{2}·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/m^{2}·K. |
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