Problem: Consider a cylindrical Dewar with an inner container 0.220m high and 0.090m in diameter used to store liquid helium at 4.22 K; at that temperature the heat of vaporization of helium is 2.09 × 10⁴ J/kg. Surrounding the metal container are walls maintained at the temperature of liquid nitrogen, 77.3 K, with vacuum between the can and the surrounding walls.

Part A: How much helium is lost per hour? The emissivity of the metal walls of the container is 0.200. The only heat transfer between the inner and outer walls is by radiation.

I know the correct answer for above problem but explain how to get this answer! Thanks.