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Overall Heat Transfer Coefficient

 

Overall Heat Transfer Coefficient Formulas

\( \dfrac{ 1 }{ U \cdot A }   \;=\;  \dfrac{1 }{ h_1 \cdot A_1}  +  \dfrac{ l_1 }{ k \cdot A}   +  \dfrac{1 }{ h_2 \cdot A_2}  \) 

\( U  \;=\;  \dfrac{1 }{ \dfrac{1 }{ h_1} } + \dfrac{ l_1 }{ k_1} + \dfrac{1 }{ h_2 }  \)     (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

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 \(Btu \;/\; hr\) \(W\)
\( k \) = thermal conductivity of the material \(Btu-ft \;/\; hr-ft^2-F\) \(W \;/\; m-K\)
\( l_1 \) = 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.      

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