# Overall Heat Transfer Coefficient

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 Formulas

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

$$\large{ U = \frac{1}{ \frac{1}{h_1} + \frac{l_1}{k_1} + \frac{1}{h_2} } }$$     (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 composite wall, no thermal contact resistance and disregarding radiation)

Symbol English Metric
$$\large{ U }$$  = overall heat transfer coefficient  $$\large{\frac{Btu}{hr-ft^2-F}}$$   $$\large{\frac{W}{m^2-K}}$$
$$\large{ A }$$ = contact area for each fluid side $$\large{ft^2}$$ $$\large{m^2}$$
$$\large{ A_1 }$$ = contact area for one of the fluids $$\large{ft^2}$$ $$\large{m^2}$$
$$\large{ A_2 }$$ = contact area for the other fluid $$\large{ft^2}$$ $$\large{m^2}$$
$$\large{ h_1 }$$ = individual heat transfer convection coefficient for one of the fluids $$\large{\frac{Btu}{hr}}$$ $$\large{W}$$
$$\large{ h_2 }$$ = individual heat transfer convection coefficient for the other fluid $$\large{\frac{Btu}{hr}}$$ $$\large{W}$$
$$\large{ k }$$ = thermal conductivity of the material $$\large{\frac{Btu-ft}{hr-ft^2-F}}$$ $$\large{\frac{W}{m-K}}$$
$$\large{ l_1 }$$ = wall thickness $$\large{in}$$ $$\large{mm}$$
$$\large{ l_2 }$$ = wall thickness $$\large{in}$$ $$\large{mm}$$ 