text: Conduction through cylindrical shells (e.g. pipes) can be calculated from the internal radius, , the external radius, , the length, , and the temperature difference between the inner and outer wall, .
  The surface area of the cylinder is 
  When Fourier's equation is applied:
  and rearranged:
  then the rate of heat transfer is:
  the thermal resistance is:
  and , where . It is important to note that this is the log-mean radius.