# 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

$$1 \;/\; U \; A = (1 \;/\; h_1 \; A_1) + ( l_1 \;/\; k \; A) + (1 \;/\; h_2 \; A_2)$$

$$U = 1 \;/\; (1 \;/\; h_1) + ( l_1 \;/\; k_1) + (1 \;/\; h_2 )$$     (heat transfer through the plane wall and disregarding radiation)

$$U = 1 \;/\; (1 \;/\; h_1) + ( l_1 \;/\; k_1) + ( l_2 \;/\; k_2) + (1 \;/\; h_2)$$     (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 $$in$$ $$mm$$