# Overall Heat Transfer Coefficient

Written by Jerry Ratzlaff on . Posted in Thermodynamics

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.

### Overall Heat Transfer Coefficient Formula

$$\large{ \frac{ 1 } { U \; A } = \frac{ 1 } { h_1 \; A_1 } + \frac{ l_1 } { k \; A } + \frac{ 1 } { h_2 \; A_2 } }$$

$$\large{ U = \frac{1}{ \frac{1}{h_1} + \frac{l_1}{k_1} + \frac{1}{h_2} } }$$     (heat transfer through the plane wall and disregarding radiation)

$$\large{ U = \frac{1}{ \frac{1}{h_1} + \frac{l_1}{k_1} + \frac{l_2}{k_2} + \frac{1}{h_2} } }$$     (heat transfer through the plane composit wall, no thermal contact resistance and disregarding radiation)

Where:

$$\large{ U }$$  = overall heat transfer coefficient

$$\large{ A }$$ = contact area for each fluid side

$$\large{ A_1 }$$ = contact area for one of the fluids

$$\large{ A_2 }$$ = contact area for the other fluid

$$\large{ h_1 }$$ = individual heat transfer convection coefficient for one of the fluids

$$\large{ h_2 }$$ = individual heat transfer convection coefficient for the other fluid

$$\large{ k }$$ = thermal conductivity of the material

$$\large{ l_1 }$$ = wall thickness

$$\large{ l_2 }$$ = wall thickness