What is water-steam equilibrium?
Under atmospheric pressure (1000 hPa or 1 bar), pure water boils at 100°C and changes into steam. On top of the Mont-Blanc (Alt. 4807 m) where atmospheric pressure is 530 hPa it will boil at 83°C only. On the other hand in a sealed pressure cooker water must be heated to 120°C to start boiling, at that time internal pressure will be around 2000 hPa. For each pressure there is one boiling water temperature: when water is at the temperature corresponding to the pressure there is an equilibrium between water and steam. An evaporator consists of several cells each working at different temperatures. Each of them is in an equilibrium state between steam and water. The lower the temperature, the lower the pressure. The coldest cells have a temperature in the range of 45°C and therefore a pressure of 100 hPa (which means a vacuum of -0,9 bar).
