A cryogenic fluid flows in a 20-mm-diameter tube with an outer surface temperature of 75 K and an emissivity of 0.2. A larger tube, having a diameter of 50 mm, is concentric with the smaller one. This larger tube is gray, with ε = 0.05 and its surface temperature is 300 K. The intervening space between the two tubes is evacuated. 

Determine the heat gain by the cryogenic fluid, in watts per meter of tube length.
Evaluate the heat gain per meter of length if there is a thin walled radiation shield placed midway between the two tubes. The shield surfaces may be considered gray and diffuse with an emissivity of 0.04 on both sides