Chapter 4, Problem 4.24P

A long power transmission cable is buried at a depth(ground-to-cable-centerline distance) of 2 m. The cableis encased in a thin-walled pipe of 0.1-m diameter, and,to render the cable superconducting (with essentiallyzero power dissipation), the space between the cableand pipe is filled with liquid nitrogen at 77 K. Ifthe pipe is covered with a superinsulator $\left(k_i=0.005\text{W/m}\cdot \text{K}\right)$ of 0.05-m thickness and the surface ofthe earth $\left(k_g=1.2\text{W/m}\cdot \text{K}\right)$ is at 300 K, what is thecooling load (W/m) that must be maintained by a cryogenicrefrigerator per unit pipe length?