a explain the concept of critical radius of insulation discuss why it

Question

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.