This is just a conceptual think I have been thinking about. Hopefully I am not just missing something obvious. Imagine we have a large vertically oriented cylinder filled completely with a saturated liquid and the surface contacting the liquid is providing a constant heat flux to the fluid. Since the liquid is saturated any heat we add to it will immediately cause it to start boiling. Now from my understanding, as the liquid within the cylinder is boiling the cylinder contains a saturated mixture and it all remains at the same temperature and pressure, until all of the liquid has become a vapor. The vapor rises to the top of the cylinder because its density is smaller than that of the liquid. So my question is this. Since the surface of the cylinder contacting the fluid is providing a constant heat flux, won't the temperature of the vapor at the top of the cylinder begin to rise even though all of the liquid has not boiled? So now there would be superheated vapor at the top and boiling liquid at the bottom? Or would all of the fluid in the cylinder remain at the same temperature and pressure regardless of how the heat is being applied? Hope that made sense.