# Closed pressurized water vessel

Suppose we have a closed container not isolated from the external environment so that heat exchanges can occur on the lateral wall (mainly for convection). The water inside is pressurized by a gas (nitrogen for example) starting from a pressure that is not exceeding 20 bar.

As water is drained from the tank I'm expecting that pressure will decrease as a result of the expansion and so will temperature that will lead to the onset of heat exchanges between liquid and gas and internal lateral walls. At this point a question comes: If a thermal control is inserted in the tank so that liquid temperature stays always in the range of 10 - 30 degree Celsius, boiling will occur only if vapor pressure of liquid will equate the pressure exerted by nitrogen on the liquid free surface. Can evaporation take place before reaching this condition? In this case, it is possible to consider that phase equilibrium is present?

I'm saying this since I can expect that some water molecules will have enough energy to escape liquid but will eventually come back due to volume constraint (even in the case of small quantities of vapor).

The same can be said also when boiling happens in such a way that its onset is always accompanied by condensation?

• For plain text formatting by StackExchange markdown, see as inspiration SE meta - formatting // SE meta - tables // Advanced formatting Oct 4, 2023 at 14:28
• If there is an inert gas as nitrogen, the liquid would never boil if there is enough time for evaporation of liquid vapor. Because external pressure would be always bigger than vapor pressure. Oct 4, 2023 at 17:06
• Thanks @Poutnik. Suppose that pressure of nitrogen will lower up to equate vapor pressure of water (around 0.03 bar for a temperature of 25 degree Celsius). Boiling will occur in these conditions? Oct 4, 2023 at 20:00
• Imagine a pressure cooker that contains water and air, that is sealed and slowly heated, within safe margins. The the p = p_vapor + 1 atm (of air). Pressure is always higher than vapor pressure. Boiling is possible only temporary and locally, if water is heated fast enough so gas phase is not about saturated. Oct 4, 2023 at 20:24
• Ok thanks, so if heat exchanges are controlled we could assume phase equilibrium in the tank. An equal amount of water will evaporate while the exact amount will condense. Oct 4, 2023 at 20:36