1 A 12 mm outside diameter pipe carries a cryogenic fluid at 90 K . Another pipe of 15 mm outside diameter and at 290 K surrounds it coaxially and the space between the pipes is completely evacuated. (a) Determine the radiant heat flow for 3.5 m length of pipe if time surface emissivity for both surfaces is 0.25 . (b) Calculate the % reduction in heat flow if a shield of 13.5 mm diameter and 0.06 surface emissivity is placed between pipes.