The Graetz problem is a convection-diffusion equation in a pipe invariant along a di-6 rection. The contribution of the present work is to propose a mathematical analysis of the Neumann, 7 Robin and periodic boundary condition on the boundary of a semi-infinite pipe. The solution in the 8 3D space of the original problem is reduced to eigenproblems in the 2D section of the pipe. The set of 9 solutions is described, its structure depends on the type of boundary condition and of the sign of the 10 total flow of the fluid. This analysis is the cornerstone of numerical methods to solve Graetz problem 11 in finite pipes, semi infinite pipes and exchangers of arbitrary cross section. Numerical test-cases 12 illustrate the capabilities of these methods to provide solutions in various configurations. 13