I am trying to calculate the length of a pipe needed for an air cooling system I have found the following formulas: R=1/(h*a) where h is the convection coefficient and a is the surface area. h=Nu*(k/L) where Nu is the nusult number and k is the thermal conductivity. a=π*d*L where d is the diameter and L is the length. Putting this together I get: R=1/[(Nu*(k/L))*(π*d*L)]=1/(Nu*k*π*d) This seems to imply that the length of the pipe has no impact on the amount of energy lost which I am pretty sure is wrong. Any help would be appreaciated.