Consider a long cylindrical tube of inner and outer radii,

Consider a long cylindrical tube of inner and outer radii, r_i and r₀, respectively, length L and thermal conductivity, k. Its inner and outer surfaces are maintained at T_i, and T₀, respectively (T_i > T₀). Assuming one dimensional steady state heat conduction in the radial direction, the thermal resistance in the wall of the tube is

A_r = 2πrL
From Fourier ’s Law

q_r = kA_r	dT
	dr

q_r = 2π krL	dT
	dr

Boundary conditions:
T = T_i at r = r_i
T = T₀ at r = r₀

q =	2πkL(T_i - T₀)
	In(r₀/r_i)

R_th =	In(r₀/r_i)
	2πkL