The overall heat transfer coefficient (U) for a composite wall of thicknesses $$x_1, x_2, x_3$$ and of corresponding thermal conductivities $$k_1, k_2, k_3$$, is given by equation

A

$$\frac{1}{U} = \frac{k_1}{x_1} + \frac{k_2}{x_2} + \frac{k_3}{x_3}$$

B

$$U = k_1x_1 + k_2x_2 + k_3x_3$$

C

$$\frac{1}{U} = \frac{x_1}{k_1} + \frac{x_2}{k_2} + \frac{x_3}{k_3}$$

D

$$\frac{1}{U} = \frac{x_1}{k_1} + \frac{x_2}{k_2} + \frac{k_3}{x_3}$$