Example $4\mathrm{.}10$ : The inner sphere of a Diwar flask is $30cm$ diameter and outer sphere is $36cm$ diameter. Both spheres are coated for which emissivity is $0\mathrm{.}05.$ Determine the rate at which liquid oxygen $($latent heat $=21\mathrm{.}44k\frac{J}{k}g)$ would evaporate at $90K(-183C)$ when the outer sphere temperature is $293K(20C).$ Assume that the other modes of heat transfer are absent$.$