1. The problem statement, all variables and given/known data This is more like a design problem, I'm to evaporate water at 20°C and 2000 psi(Tsat=335.472°C), I have the heat flux the water is going to absorb during heating, and If that flux remains constant during all the length then, how can I find the surface temperature for the part where the water change phases? Right now I have the conditions during heating; for the heat flux constant, but when steam appears I don't know how to proceed, what eqs should I use? Does the pipe reamins with a T=constant? Reasoning tells me that the pipe must always be at the same temperature along his lenght. See, The pipes abosrb heat through turbulent external forced convection, and the water absorbs the heat through internal forced convection. And ultimatly I need to find the temperatures of the hot gas that is going to serve as the heat source. Any Ideas? Was I clear? Im so confused right now...sorry. 2. Relevant equations Q'=Cp*m'*ΔT Q'=h*As*ΔT q'=h*ΔT q=hfg 3. The attempt at a solution Since I know all the heat transfer I need through all the pipe, I was thinking, that h=q'*ΔT, where I will force the Temperatures and see how much mass flow I need. I did a quick calculation and I got 8 kg/s of hot gas (combustion products) that starts at Ti=1227 °C and ends at To=350°C but i don't think this approach is correct, also it seems a lot ok kg/s of hot gas. Any Ideas? thanks!