1. The problem statement, all variables and given/known data An infinitely long cylindrical shell has an inner radius a and outer radius b. If the inside is maintained at a temperature Ta and the outside at a temperature Tb, determine the rate of heat flow per unit length between inner and outer surfaces assuming the shell has a thermal conductivity k. 2. Relevant equations H = -KA((TH-TC)/L) 3. The attempt at a solution 1) I said let TH = Ta and TC = Tb 2) I let L = b-a so my new expression is: H = -KA((Ta-Tb)/(b-a))3) My issue here is I can not figure out what to use for the cross sectional area A. In the example I was using as reference the heat was flowing through the pipe not the outer shell of it so the cross sectional area was easy to calculate. 4) My idea was to use 2πr and multiply it by the length of the shell but since it is an infinitely long cylindrical shell that wouldn't make sense to do.