A 4 m diameter thin-shelled spherical tank, full of liquid nitrogen at 1 atm and −196 °C (h_{fg} = 198 kJ/kg), is exposed to ambient air at 20 °C with h = 25 W/m^{2}·°C. The tank wall is practically at the nitrogen temperature. Disregarding radiation, determine (a) the rate of heat gain (answer in kW) and (b) the rate of evaporation of nitrogen (answer in kg/s).