Suppose I pass water through a pressure reducing valve, which can lower the pressure enough for the water to boil, will the water fully boil? If so, what would happen if that vapour passed to a tube, for instance, with atmospheric pressure? Will the water return to the liquid state? If it returns back to liquid, how do I sustain the vapour state?
Thanks in advance!
The water might boil if pressure is reduced below the vapor pressure of the water. That means the outlet of the PRV must be at a low pressure.
Boiling water at normal temps requires a very low pressure. Just a reducing valve won't be enough, you will need to induce vacuum at the exit. At room temp, this is around 1/20 of an atmosphere. Doing this through a valve will likely result in freezing; you still have to remove the heat from the liquid, and the most likely mechanism for this is through freezing.
If you mean a momentary flashing to vapor going through a throttle or such, this will likely only be some of the water, and it will immediately condense back to liquid when the high velocity-low pressure condition is no longer there.
Water will boil when the pressure is reduced below its vapor pressure. It is in vapor-liquid equilibrium at that temperature, it is unlikely to be completely vaporized.
