Cryogenic fluid flows through a tube $30cm$ diameter concentric with a tube

of $90cm$ diameter, surface emissivities of inner and outer tube are $0\mathrm{.}12$ and

$0\mathrm{.}15$ and at temperatures of $-173C$ and $27C$ respectively. Determine $($i$)$ the

heat gained by fluid per meter length of tube and $($ii$)$ percent reduction in

heat gain if the radiation shield with diameter $45mm$ and emissivities $0\mathrm{.}1$

on the inner surface and $0\mathrm{.}05$ on the outer surface is introduced between the

tubes.