Can anyone explain to me, for example, in a plate heat exchanger system, where we want to control a exit temperature of a fluid, what are the reasons to choose between a 2 way or 3 way valve? (the fluid is water)Assuming you mean by a 2 way valve you are considering automating the bypass around the exchanger, as you open the valve and reduce the flow through the exchanger the available dP for the bypass valve also decreases so you can't totally bypass the exchanger (you may be able to bypass enough depending on the requirements of the process). The other problem with a 3 way valve is that you have a pressure drop with both 'paths' so total dP loss through the system is going to be greater.We must also presume that both systems has an adequate steam pressure regulating valve upstream from the application, able to maintain the steam input pressure.Further that the condensate outlet in both cases have an adequate sized steamtrap, letting out condensate created , and keeping steam inside the system.The purpose of a steam heatexchanger is as known to utilize the steam energy in a best possible way. As most of the energy is given by transferring back from gas phase (steam) to liquid phase (condensate) the art is to balance the steamside of the system in such a way that the heat exchange surface is exposed to steam.

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