A long pipe (To=22mm, To=25mm) is made of aluminum ( k= 200 W/mK). Hot gasses (Tg = 800 K, hg = 200 W/mK) flow inside the pipe. On the outside, the pipe is exposed to air ( Ta = 300 k, ha = 10 W/mK). In order to increase heat transfer from this pipe by 200%, it is recommended to use integrally cast annular aluminum fins of height 10 mm and thickness 1 mm. Calculate the spacing of the fins.