In vacuum insulated cryogenic vessels (Dewars), the major source of heat transferred to the inner container is by radiation through the vacuum jacket. A technique for reducing this is to place "heat shields" in the vacuum space between the inner and outer containers. Idealize this situation by considering two infinite sheets with emissivly = 1 separated by a vacuum space. The temperatures of the sheets are T 1 and T 2 (T 2 > T 1 ). Calculate the energy flux (at equilibrium) between them. Consider a third sheet (the heat shield) placed between the two which has a reflectivity of R. Find the equilibrium temperature of this sheet. Calculate the energy flux from sheet 2 to sheet 1 when this heat shield is in place.

For T 2 = room temperature, T 1 = liquid its temperature (4.2 K) find the temperature of a heat shield that has a reflectivity of 95%. Compare the energy flux with and without this heat shield.

(σ = 0.55 × 10 –7 watts/m 2 K)