What additional thickness of insulation will be necessary to reduce the boil-off rate of liquid nitrogen to one-half of the rate corresponding to Problem 17.23? All values and dimensions in Problem 17.23 apply.

Data From problem 17.23

Liquid nitrogen at 77 K is stored in an insulated spherical container that is vented to the atmosphere. The container is made of a thin-walled material with an outside diameter of 0.5 m; 25 mm of insulation (k = 0.002 W/m · K) covers its outside surface. The latent heat of nitrogen is 200 kJ/kg; its density, in the liquid phase, is 804 kg/m³. For surroundings at 25°C and with a convective coefficient of 18 W/m² · K at the outside surface of the insulation, what will be the rate of liquid nitrogen boil-off?