The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h₀ = heat transfer coefficient at the outer surface)

ANS:B - 2K/h₀

The critical radius of insulation for a spherical shell represents the point at which the heat transfer rate through the insulation becomes minimized. It occurs when the heat transfer by conduction through the insulation is equal to the heat transfer by convection from the outer surface of the insulation. The critical radius, critrcrit​, for a spherical shell insulation can be calculated using the formula: crit=ℎ0rcrit​=h0​K​ Where:

• K is the thermal conductivity of the insulating material, and
• ℎ0h0​ is the heat transfer coefficient at the outer surface.