a) Explain the concept of critical radius of insulation. Discuss why it is only relevant in systems with cylindrical and spherical geometries and irrelevant in systems with Cartesian geometry. b)

For a spherical shell where the inner surface temperature is maintained constant, sketch the heat transfer rate (Watts) as a function of the outer radius of the shell. c) On the graph sketched in b). clearly mark the critical radius of insulation and the outer-shell radius larger than the critical radius at which the heat transfer rate equals the rate that occurs when the shell thicknessis zero.