heat transfer

A hollow spherical vessel of ID =19cm and OD=20 cm contains a hot fluid. The fluid is to be cooled by exposing the vessel to a surrounding cold fluid when the outside film coefficient is 10W/m2K. If the vessel is to be lagged by mica sheet (k=0.5815W/mK), determine the thickness of insulation so that the rate of heat transfer from the hot fluid is maximum