The overall heat transfer coefficient (U) for a composite wall of thicknesses x1, x2, x3 and of corresponding thermal conductivities k1, k2, k3 is given by equation

• A. \(\frac{1}{U} = \frac{{{k_1}}}{{{x_1}}} + \frac{{{k_2}}}{{{x_2}}} + \frac{{{k_3}}}{{{x_3}}}\)
• B. \(U = {k_1}{x_1} + {k_2}{x_2} + {k_3}{x_3}\)
• C. \(\frac{1}{U} = \frac{{{x_1}}}{{{k_1}}} + \frac{{{x_2}}}{{{k_2}}} + \frac{{{x_3}}}{{{k_3}}}\)
• D. \(\frac{1}{U} = \frac{{{x_1}}}{{{k_1}}} + \frac{{{x_2}}}{{{k_2}}} + \frac{{{k_3}}}{{{x_3}}}\)