Consider a 4m diameter spherical tank initially filled with liquid nitrogen at 1 atm and -196°C (density is 810 kg/m^3 at those conditions). The tank is exposed to 20°C ambient air with a heat transfer coefficient of 25 W/m^2°C. The temperature of the thin shelled-spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Disregarding any radiation heat exchange, determine the rate of evaporation of the liquid nitrogen in the tank because of the heat transfer from the ambient air (heat of vaporization is 198 kJ/kg).