Consider a cylindrical tube of radius (r_1) covered with an insulating layer up to an outer radius (r_2). The outer surface is exposed to a fluid at temperature (T_\infty) with a convective heat transfer coefficient (h). Let the temperature at the surface of the tube (inner boundary of insulation) be (T_1). Heat flows radially outward through the insulation by conduction and is then dissipated to the surroundings by convection. The total thermal resistance to heat transfer is the sum of conductive resistance of the insulation and convective resistance at the outer surface. The conductive resistance for a hollow cylinder is: [ R_{cond} = \frac{\ln(r_2/r_1)}{2\pi k L} ] where (k) is the thermal conductivity of insulation and (L) is the length of the