Overall Heat Transfer Coefficient Formulas |
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\( \dfrac{ 1 }{ U \cdot A } \;=\; \dfrac{1 }{ h_1 \cdot A_1} + \dfrac{ l_1 }{ k \cdot A} + \dfrac{1 }{ h_2 \cdot A_2} \) \( U \;=\; \dfrac{1 }{ \dfrac{1 }{ h_1} } + \dfrac{ l_1 }{ k_1} + \dfrac{1 }{ h_2 } \) (heat transfer through the plane wall and disregarding radiation) \( U \;=\; \dfrac{1 }{ \dfrac{1 }{ h_1} } + \dfrac{ l_1 }{ k_1} + \dfrac{ l_2 }{ k_2} + \dfrac{1 }{ h_2} \) (heat transfer through the plane composite wall, no thermal contact resistance and disregarding radiation) |
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| Symbol | English | Metric |
| \( U \) = overall heat transfer coefficient | \(Btu \;/\; hr-ft^2-F\) | \(W \;/\; m^2-K\) |
| \( A \) = contact area for each fluid side | \(ft^2\) | \(m^2\) |
| \( A_1 \) = contact area for one of the fluids | \(ft^2\) | \(m^2\) |
| \( A_2 \) = contact area for the other fluid | \(ft^2\) | \(m^2\) |
| \( h_1 \) = individual heat transfer convection coefficient for one of the fluids | \(Btu \;/\; hr\) | \(W\) |
| \( h_2 \) = individual heat transfer convection coefficient for the other fluid | \(Btu \;/\; hr\) | \(W\) |
| \( k \) = material thermal conductivity | \(Btu-ft \;/\; hr-ft^2-F\) | \(W \;/\; m-K\) |
| \( l_1 \) = wall thickness | \(in\) | \(mm\) |
| \( l_2 \) = wall thickness | \(in\) | \(mm\) |
Overall heat transfer coefficient, abbreviated as U, is the heat transfer between items like walls in buildings or across heat exchangers for the conduction within materials.
